The Role of Natural Killer Cells and Regulatory T Cells While Aging with Human Immunodeficiency Virus

Abstract

Combined antiretroviral therapy (cART) has increased the quality of life of people living with HIV (PLHIV). Consequently, the number of PLHIV >50 years is increasing worldwide. Patients on cART are known to remain in a proinflammatory state. The latter is linked to the development of non-AIDS-related chronic conditions. Although the number of aging PLHIV is increasing, the effect of HIV infection on the process of aging is not fully understood. Understanding the complexity of aging with HIV by investigating the effect of the latter on different components of the innate and adaptive immune systems is important to reduce the impact of these comorbid conditions and improve the quality of life of PLHIV. The role of killer immunoglobulin receptors (KIRs), expressed on the surface of natural killer (NK) cells, and their human leukocyte antigen (HLA) ligands in the clearance, susceptibility to or disease progression following HIV infection is well established. However, data on the effect of KIR–HLA interaction in aging HIV-infected population and the development of non-AIDS-related comorbid conditions are lacking. Moreover, conflicting data exist on the role of regulatory T cells (Tregs) during HIV infection. The purpose of this review is to advance the current knowledge on the role of NK cells and Tregs while aging with HIV infection.

Introduction

The use of combined antiretroviral therapy (cART) has improved the survival rate of human immunodeficiency virus (HIV)-infected individuals. cART contributed to an increase in the life expectancy of people living with HIV (PLHIV) approaching that of HIV-negative individuals.^1,2 Globally, access to antiretroviral therapy (ART) increased from 8 million in 2010 to 21.7 million in 2017 with 59% of all PLHIV in 2017 accessing treatment (2018 Global HIV statistics, fact sheet. https://unaids.org/en/resources/fact-sheet). In the United States, ∼50% of PLHIV are 50 years of age and older (Centers for Disease Control and Prevention [CDC], https://cdc.gov/hiv/group/age/olderamericans/index.html). Despite the fact that new cases within PLHIV more than 50 years of age are declining, the CDC reported one in six HIV diagnoses in 2016 in this group. The global proportion of PLHIV 50 years of age or older is expected to reach an estimated 21% by 2020.³ Eighty percent of this population lived in low- and middle-income countries in 2016 and the percentage of PLHIV50⁺ is likely to increase in high- and low-income countries.

Despite the success of cART, the rate of non-acquired immunodeficiency syndrome (AIDS) comorbidities are increasing in treated PLHIV leading to an increased number of deaths exceeding those of AIDS deaths.^4

–7 These comorbidities include cardiovascular disease,^8

–13 liver disease,^14,15 renal disease,^9,11,16 diabetes,^{8
–10,12,16,17} neurocognitive abnormalities,^18,19 as well as non-AIDS-defining malignancies, including liver, brain, anal, and lung cancers.^20,21 Importantly, as PLHIV age, the number of medications to treat comorbid conditions increases leading to an increase in drug-to-drug interaction, adverse drug effects, increased hospitalization, and lower adherence to ART.²²

Overlapping with chronic diseases of aging, these non-AIDS comorbidities have been associated with residual and continuous immune activation (IA) maintained in ART-treated HIV-infected individuals.^{6,13,19,23,24} The persistence of HIV in infected individuals results in increasing levels of proinflammatory mediators [tumor necrosis factor (TNF)-α, transforming growth factor (TGF)-β, interferon (IFN)-α, interleukin (IL)-1β, IL-12, IL-6, IL-8, sCD14, sCD163, macrophage inflammatory protein (MIP)-1α, MIP-1β, RANTES, interferon-induced protein 10 (IP-10)],^23,25 C-reactive protein and D-dimers contributing to IA.²⁴ HIV-associated IA is also reported to affect the thymus, despite partial regain of functionality following cART, leading to defective development of regulatory T cells (Tregs). Moreover, microbial translocation and loss of T-helper 17 (Th17) were also suggested to contribute to IA.^24,26 While many of these biomarkers have been proposed as predictors of serious non-AIDS-related pathologies, the association with and the ability to predict poor clinical prognosis was not evident.²⁷ Nevertheless, accelerated IA stimulates viral replication resulting in replicative senescence of T cells and premature immune aging.²³

A number of “immune parallels” has been advanced between HIV infection and aging.^23,24 The following major factors have been suggested: (i) the accumulation of CD28⁻CD8⁺ T cells leading to the generation of highly differentiated senescent cells expressing CD57 as well as shortening of telomeres in CD8⁺ T cells, another marker of senescence; (ii) the decline of naive CD4⁺ and CD8⁺ T cell counts to levels comparable to 85-year-old uninfected individuals. Moreover, constant inflammation is maintained among HIV-infected individuals coinfected with cytomegalovirus (CMV), hepatitis C virus (HCV), hepatitis B virus (HBV), and Epstein Barr virus and may lead to higher risk of development of non-AIDS comorbidities.²⁴

As the number of older adults with HIV is increasing, the HIV and Aging Consensus Project has published on the specific risk factors affecting older adults with HIV infection.²⁸ The group recommended screening HIV-infected individuals for diabetes, kidney functions, hypertension (salt intake, weight, exercise), as well as cognitive impairment and depressive disorders. Moreover, assessment of the Framingham Risk Score was also recommended along with cholesterol and blood pressure. Although biological aging in HIV-infected individuals is believed to start earlier compared with uninfected healthy subjects (55 vs. 65 years, respectively),²⁹ the impact of aging with HIV infection and subsequent pathways leading to disease manifestation is not fully understood. With the absence of a chronic care model especially in limited resource countries, these non-AIDS morbidities pose a major risk on treated HIV-infected people progressing into an older population.

Understanding the complexity of aging with HIV is important to develop and prioritize interventions leading to reduced immune dysfunction, the development of comorbid conditions and improving the quality of life of PLHIV. The purpose of this review is to advance the current knowledge on natural killer (NK) cells, their human leukocyte antigen (HLA) ligands, as well as Tregs in people living and aging with HIV and their potential role in the development of non-AIDS comorbidities.

NK Cells, HIV, and Aging

NK cells and killer immunoglobulin receptors

NK cells constitute an important component of the innate immune system involved in immunosurveillance and the regulation of the crosstalk between the innate and the adaptive immune responses through secretion of chemokines/cytokines.^30,31 NK cells, in contrast to B and T cells, lack the expression of cluster of differentiation 3 (CD3), as well as antigen-specific receptors, such as B cell receptors (BCR) or T cell receptors (TCR). NK cells express CD56 and/or CD16; thus they are phenotypically defined as CD3⁻CD56⁺CD16⁺.^30,32 On the basis of the density of CD56, NK cells are divided into three subsets: terminally differentiated cytotoxic CD56^dim/CD16⁺ cells, less differentiated cytokine-producing CD56^bright CD16⁻ cells, and a minor CD56⁻CD16⁺ subset characterized by poor antiviral activity.^31
–33 In healthy individuals, CD56^brightCD16⁻ and CD56^dimCD16⁺ subsets constitute 5%–10% and up to 90% of the peripheral NK cell population, respectively.³⁴ CD56^bright NK cells are immature peripheral cells characterized by longer telomeres compared with the more mature CD56^dim NK cells.^35,36 CD57, present on terminally differentiated CD4⁺ and CD8⁺ T cells, is also expressed on different subsets of NK cells.³⁷ This marker is expressed on CD56^dimCD16⁺ (terminally differentiated cytotoxic) and CD56⁻CD16⁺ (inflammatory) NK cells. The cytokine-producing and regulatory CD56^bright CD16⁻ do not express CD57. Consequently, CD57⁺ NK cells are mainly cytotoxic yet poorly responsive to cytokine stimulation.^38,39

NK cells play a crucial role in early responses to infections; these cells kill target cells (infected or transformed) through: membrane disruption of target cell through secretion of perforin and granzymes; caspase-dependent apoptosis through the engagement of death receptors on target cells with their ligands on NK cells (e.g., Fas-ligand or tumor necrosis factor-related apoptosis-inducing ligand); and antibody-dependent cellular cytotoxicity (ADCC) through the binding of Fcγ receptor III (FcγRIII or CD16) on NK cells to the Fc portion of the antibody opsonizing the target cell.^30,31 Importantly, the activation of NK cells contribute to adaptive immunity following exposure to type I IFNs namely IFN-α and IFN-β secreted by dendritic cells (DCs) as well as proinflammatory cytokines, such as IL-12, IL-15, and IL-18.^40,41 Activated NK cells mainly secrete IFN-γ, which in turn enhances the maturation and effector functions of DCs, macrophages, and granulocytes⁴⁰ and shapes T cell responses in lymph nodes.⁴¹ They also produce proinflammatory cytokines (TNF-α, IL-10, IL-3, IL-6, granulocyte macrophage colony-stimulating factor and granulocyte colony-stimulating factor).^41,42 Chemokines, including C–C motif-chemokine ligand (CCL)-2, CCL-3, CCL-4, and CCL-5, chemokine (C motif) ligand XCL-1 (lymphotactin), and chemokine (C–X–C motif) ligand CXCL-8 (IL-8) are also produced by NK cells. These chemokines play an important role in the colocalization of NK cells to inflamed sites along with other innate immune cells (i.e., DCs).

NK cells express distinct and broad range of germline-encoded receptors that are either inhibitory or activating. These receptors include leukocyte immunoglobulin-like receptors (LIR), natural cytotoxicity receptors (e.g., NKp30, NKp44, NKp46), C-lectin receptors (e.g., NKG2A, NKG2C, NKG2D), killer cell lectin-like receptor (KLR) (e.g., KLRG1), signaling lymphocyte activation family, Fcγ receptor CD16, and most importantly killer immunoglobulin-like receptors (KIRs).^42
–44 The effector function of NK cells depends on the balance between inhibitory and activating signals. The inhibitory NK cell receptors (KIRs, LIRs, and NKG2A) function to inhibit NK cells from killing normal self-cells by recognizing major histocompatibility complex (MHC) class I, expressed on the surface of normal nucleated cells in humans. Tumor cells and pathogen-infected cells often downregulate MHC class-I expression to evade recognition by T cells. The downregulation of MHC-I is detected by these inhibitory receptors resulting in lower inhibitory signals and higher activating signals driving a potent NK cell response. KLRG1, another inhibitory cell receptor,⁴⁵ is expressed on almost 60% of human NK cells specifically the mature CD56^dimCD16⁺ NK cell subset.^46,47 The function of KLRG1 is still not fully understood.

KIRs are highly polymorphic type 1 transmembrane glycoproteins composed of two (2D) or three (3D) extracellular immunoglobulin domains as well as short (S) or long (L) cytoplasmic tails^42,48
–50 determining the inhibitory or activating functions of KIRs. Genes encoding KIRs are located in the leukocyte receptor complex on chromosome 19q13.4.⁵¹ Sixteen genes encode KIR, of which seven are activating (KIR2DS1, KIR2DS2, KIR2DS3, KIR2DS4, KIR2DS5A, KIR2DS5B, and KIR3DS1), eight are inhibitory (KIR2DL1, KIR2DL2, KIR2DL3, KIR2DL5A, KIR2DL5B, KIR3DL1, KIR3DL2, and KIR3DL3), and one (KIR2DL4) predominantly activating but can transmit inhibitory signals (The Allele Frequency Net Database [AFND]; http://allelefrequencies.net and Takeshita et al.⁵²). Two KIR haplotypes are defined in humans: haplotype A and haplotype B.^49,53 Haplotype A encodes inhibitory receptors (KIR2DL1, KIR2DL3, KIR3DL1, KIR3DL2, KIR3DL3, KIR2DP1, KIR3DP1, and KIR2DL4) and one activating receptor KIR2DS4. Haplotype B on the other hand carries a variety of gene combinations and encodes more activating receptors as compared with haplotype A; these include: KIR3DL3, KIR2DS2, KIR2DL2, KIR2DL5B (inhibitory) KIR2DS3, KIR2DP1, KIR2DL1, KIR3DP1, KIR2DL4, KIR3DS1, KIR2DL5A (inhibitory), KIR2DS5, KIR2DS1, and KIR3DL2.⁵³ The balance between the inhibitory and activating signals regulates the function of NK cells. Despite the high allelic polymorphism of KIR genes, almost all individuals possess the following genes referred to as framework loci: KIR3DL3, KIR3DP1, KIR2DL4, and KIR3DL2.^49,53

NK cell, KIR, and HLA ligands during HIV infection

KIRs bind to MHC class I molecules; the latter includes the classical HLA: HLA-A, HLA-B, and HLA-C. The majority of HLA-I ligands to activating KIRs are still not identified, except for KIR2DS4: HLA-A3/11 and HLA-C (C1 or C2). In contrast, the inhibitory KIR2DL1, KIR2DL3, KIR3DL1, and KIR3DL2 bind to HLA-C2, HLA-C1, HLA-Bw4, and HLA-A3/A11, respectively. A number of KIR-HLA pairs has been implicated in the control or the progression of several human viral diseases, including human influenza virus, viral hepatitis (HCV and HBV), human cytomegalovirus (HCMV), and HIV.^51,54 Moreover, the frequency of expression of certain KIRs and the interaction with their HLA ligands were implicated in controlling HIV disease progression in HIV-infected subjects.³¹ Below we summarize the available literature on the effect of HIV infection on NK cell distribution and KIR expression.

It is well established that HIV infection results in the accumulation of phenotypically and functionally aberrant NK cells resulting from alterations in the distribution of NK cell subsets (CD56⁻, CD56^dim, and CD56^bright).⁴³ The weakly functional CD56⁻ NK cells, rarely present in healthy individuals, increase during chronic HIV infection, chronic HCV infection,⁵⁵ and in response to HCMV infection.⁵⁶ Earlier studies reported a significant increase in CD56⁻ NK cell subset in untreated viremic HIV-infected individuals.⁵⁷ However, the expression of CD56 on NK cells was similar to that of healthy donors following successful ART for >2 years, as previously reported by the same group.⁵⁸ CD56⁻ NK cells isolated from the same individuals expressed higher levels of inhibitory KIRs (KIR2DL2) and LIR1 suggesting lower cytolytic activity.

Recently, the expansion of different NK cell subsets was studied in ART-naive viremic controllers (viral load of 40–2,000 copies/mL of plasma), viremic noncontrollers (VNC) (viral load of >5,000 copies/mL of plasma), and elite controllers (EC) (viral load of <40 copies/mL of plasma). The results showed the expansion of CD56⁻CD16⁺CD7⁺ NK cells in VNC compared with HIV-negative controls.⁵⁹ Importantly, CD56^dimCD16⁺ NK cells with increased expression of sialic acid-binding immunoglobulin-like lectin 7 (Siglec-7), were abundant in EC and healthy controls. The frequency of these cells was associated with levels of HIV DNA, suggesting the role of these cells in immune control of HIV. The loss of Siglec-7 is suggested to be associated with dysfunctional NK cells occurring during HIV infection⁶⁰ as well as HCV infection.⁶¹ The expression of Siglec-7 was also reported to decrease on NK cells of chronically HIV-infected individuals.^59,62 NK cells from healthy donors expressing Siglec-7 have higher functional activity defined by an increased expression of CD107a and higher IFN-γ production.⁶³ Importantly, the expression of Siglec-7 on NK cells of HIV-infected individuals was restored following 18 months of successful ART.⁶² Collectively, these results suggest that high HIV viral load is associated with the expansion of phenotypically and functionally aberrant NK cell subsets.

The CD56^dim CD16⁺ subset, which constitutes the majority of circulating NK cells in healthy subjects, was relatively reduced in treatment-naive HIV-infected subjects with a parallel increase in the percentage of CD56⁻CD16⁺ subset.⁶⁴ Recent studies also reported the reduction in CD56^dim NK cells in treatment-naive (median age 25 years) and treated HIV patients (median age 30 years) compared with uninfected subjects.⁶⁵ In a prospective randomized clinical trial evaluating the effect of cART on the adaptive and innate immunity following primary HIV infection, a decrease in CD56^dimCD16⁺ subset and a reciprocal increase in CD56^bright subset were detected.⁶⁶ Similarly, in nonresponders (NRs) (median age 45 years) on highly active ART (HAART) for a median of 36 months, CD56^bright NK cells were higher in frequency and absolute number when compared with responders and healthy subjects; moreover, this subset inversely correlated with CD4⁺ T cell count.⁶⁷ The NRs were defined as HIV-infected patients on HAART with CD4⁺ cell count of <350 cells/μL compared with >500 CD4⁺ cells/μL in responders. These results suggest that alterations in the NK cell subsets could contribute to HIV disease progression.

The expression of CD57, another predominant marker on NK cells linked to senescence of the latter as well as T cells,⁶⁸ on CD56^dim CD16⁺ NK cells was also studied in a cohort of ART-naive HIV-infected individuals.⁶⁹ The results of this study showed an increased expression of CD57 on CD56^dim NK cells with a relative increase in the frequency of the CD57^bright CD56^dimCD16⁺ NK cells among HIV seropositives compared with the CD57^dimCD56^dimCD16⁺ NK subset. Similar to previous reports,^64,70 this study confirmed the significant loss of CD56^dimCD16⁺ NK cells during HIV infection. Importantly, CD57-expressing CD56^dimCD16⁺ NK cells were characterized by an increasing prevalence of KIR⁺ and granzyme B⁺ cells, that is, a mature phenotype. This differentiated subset is less functional as indicated by the decreased expression of the proliferation marker Ki-67 and the degranulation marker CD107a.⁶⁹ Recently, Gondois-Rey et al. ⁷¹ reported on the alterations of NK cell subsets during primary HIV infection. A mature NK cell profile, that is, patients with higher frequency of CD57⁺CD56^dim NK cells compared with patients with higher frequencies of CD56^dim NKG2A⁺ or CD56^dim NKG2A⁻NKG2C⁻CD57⁻ subpopulations, was associated with better response to cART and significant decrease in viral load (<50 copies/mL) 3 months following initiation of cART. Despite the small sample size, the authors advanced the potential significance of assessing the role of mature NK cells during the course of HIV infection and its impact on disease prognosis.

The majority (90%) of PLHIV and 50% of healthy individuals are seropositive for CMV.⁷² Consequently, few studies attempted to study the phenotypes of NK cells among the former. CD57, suggested to expand in response to CMV,⁷³ was reported among treated older (50–70 years) HIV⁺ individuals, whether CMV⁺ or CMV⁻. Despite the success of cART, HCMV infection/reactivation in PLHIV was reported to contribute to HIV disease progression, immune senescence, and phenotypical and functional alterations in NK cells.^56,74 While CD57⁺ NK cells were higher among HIV-infected individuals compared with healthy controls⁶⁹ and associated with reduced viral load,⁷¹ the status of HCMV infection among study participants was not assessed. More studies are needed to elucidate the confounding role exerted by HCMV on NK cell subsets and the potential role of the latter in disease progression and control among HIV-positive individuals.

KIR/HLA-I interaction during HIV infection

It is clear that the interaction between KIRs and their corresponding HLA ligands is important in differential responses to many viral infections, such as HCV,⁷⁵ HBV⁷⁶ and other disease conditions.^77

–80 Specifically, KIR2DL2 and/or KIR2DL3 along with HLA-C1 were associated with severe influenza infection,⁵⁴ whereas KIR2DS2 and KIR2DS3 were suggested to be associated with susceptibility to chronic HBV infection.⁸¹ Similarly, KIR2DL3/HLA-C1 and KIR2DS3/HLA-C2 ligands were described as protective and risk factors for the development of chronic HCV infection, respectively.⁷⁵ Moreover, KIR/HLA interaction was also advanced as potentially involved in preventing the generation of protective immune responses against HCMV in patients undergoing kidney transplant or hematopoietic stem cell transplantation.⁸²

Ample data exist on the interaction between specific KIRs and their HLA-I ligands on target cells and their impact on the clinical outcome during the course of HIV infection.³¹ HLA-B alleles, with the Bw4 epitope, including HLA-B*57 and HLA-B*27, are associated with the control of HIV replication and consequently delaying progression to AIDS.⁸³ HLA-Bw4-80I are ligands for KIR3DL1 (inhibitory receptor) and putative ligands for the NK cell-activating receptor KIR3DS1; these KIR alleles in combination with the former are associated with protection from severe disease outcome among HIV-infected individuals.^54,84

–87 When compared with other alleles, KIR3DL1*004 allele interacts with HLA-Bw4-80I and significantly slows the progression to AIDS.⁸⁷ Recently, and while comparing long-term nonprogressors (LTNP) to progressors, increased frequencies of KIR3DL1-Bw4T80, KIR3DL1-Bw4/Aw4, and KIR3DL1-Aw4 were reported in the former in a South Indian Cohort.⁸⁸ Progressors on the other hand expressed KIR3DL1 with homozygous Bw6. Several early studies also reported on the interaction between KIR3DS1 and its putative ligand HLA-Bw4-80I resulting in delay of disease progression and protection from opportunistic infections.^84,89

–92 Alter et al. were the first to show in vitro that NK cells control the replication of HIV in a receptor ligand-specific manner.⁸⁴ In this study, the level of HIV replication in vitro using a p24 Gag ELISA was determined in infected CD4⁺ (iCD4⁺) T cells coincubated with autologous NK cells. Both NK and CD4⁺ T cells were isolated from HIV-negative individuals (N = 36) who possessed KIR3DS1/Bw4-80I genes (N = 8), KIR3DS1 gene alone (N = 8), HLA-Bw4-80I alone (N = 10), or neither KIR3DS1 nor HLA-Bw4-80I genes (N = 10). Results showed a significant reduction in HIV replication in vitro when iCD4⁺ T cells were co incubated with NK cells isolated from individuals expressing both KIR3DS1 and its ligand HLA-Bw4-80I. This result was compared with those of cells isolated from other individuals who either lack the expression of these genotypes or express either KIR3DS1 or HLA-Bw4-80I. NK cells expressing KIR3DL1 were also able to reduce HIV replication in vitro, but to a lesser extent than NK cells expressing KIR3DS1.⁹²

Upon coculture with iCD4⁺ T cells, NK cells isolated from individuals expressing KIR3DL1*h/*y-HLA-B*57 or KIR3DS1-HLA-Bw4-80I combinations secreted higher levels of chemokines (CCL3, CCL4, and CCL5) than NK cells from individuals with KIR3DL1*l/*x-HLA-B*57 genotypes or from those who are Bw6 homozygote. Based on the expression on NK cell surface, KIR3DL1 genotype is divided into high (*h), low/intermediate (*l), and null (*004) unexpressed alleles. Homozygous KIR3DL1 can be divided into two groups: *h/*y (with *y being either *h allele or *004) with no *l alleles, and *l/*x (with *x being another *h, *l, or *004) with at least one *l allele.^87,93 Similarly, a higher intracellular expression of these chemokines as well as IFN-γ and the activation marker CD107a were detected in these NK cells suggesting a higher functional potential of these cells upon stimulation with autologous iCD4⁺ T cells and higher ability to inhibit HIV replication in infected CD4⁺ T cells. Recently, the interaction reported between KIR2DL2/S2/and HLA-C*1202 and KIR2DL2/S2/HLA-C*14:03 in treatment-naive HIV-infected individuals exerted a protective effect through suppression of viral replication in vitro in a Japanese cohort.⁹⁴

Moreover, Alter et al. were the first to report on the impact of HLA-I ligands on clonal expansion and KIR expression on NK cells during acute HIV infection.⁸⁵ The levels of KIR3DS1 on NK cells of individuals with acute HIV infection and expressing HLA-B Bw4-80I was significantly higher than those expressing HLA-B Bw6 and those who are HIV negative. The levels of KIR3DL1 expression showed a similar pattern to KIR3DS1. These results demonstrate the dependent expansion of KIR3DS1 and KIR3DL1 in the presence of their putative ligand.

NK Cells and KIRs in Aging

Phenotypic and functional alterations have been reported in NK cells due to aging,^{32,38,95
–97} whereas scarce data exist on the effect of aging on KIR expression on NK cells. Below we summarize data on the impact of aging on the number, phenotype, and function of NK cells.

Overall, a decrease in the immature CD56^bright NK cell subset accompanied by an increase of the mature CD56^dim NK cell subset is observed suggesting an expansion in NK cell frequency with age due to the latter.^38,98 Consequently, NK cell subsets are remodeled in aging with a shift in balance toward the more mature CD56^dim subset. Despite this change, the production of IFN-γ is maintained in the elderly by the CD56^bright cell subset suggesting a compensatory mechanism for cytokine production and immune regulation in this group.⁹⁹ Significantly lower CD56^bright NK cell number was detected among subjects more than 60 years old compared with those 20–40 years of age.^100
–102 Similar results were recently reported in individuals >60 years old compared with younger subjects.^101,102 Consequently, an expansion and accumulation of the mature NK cell subsets is clearly detected in agreement with the findings of Le Garff-Tavernier et al., whereby the frequency and the absolute number of CD56^bright subset gradually decreased with age along with increased proportion of CD56^dim/CD56^bright and thus higher frequency of mature NK subset in elderly individuals (>60 years).⁹⁵ Importantly, Le Garff-Tavernier et al. ⁹⁵ extensively analyzed the phenotypic and functional characteristics of NK cells from healthy subjects of different ages: newborn (cord blood), adult (18–60 years), old (60–80 years), and very old (80–100 years). IFN-γ was produced following treatment of NK cells from different age groups with IL-2 and IL-18. Significantly higher levels of IFN-γ were detected in cord blood as compared with other age groups. NK cells isolated from very old subjects exerted low cytolytic activity ex vivo that was reversed following IL-2 activation suggesting the potential recovery of age-related changes along with an increase in the frequency of CD56^dim NK cell subset expressing the terminal differentiation marker CD57. The latter is associated with a mature NK cell phenotype with increased cytotoxic capacity and decreased response to cytokines.^38,95,97 Importantly, CD57⁺ NK cells are characterized by a strong ADCC.⁴⁷

CMV infection was associated with the expansion of terminally differentiated effector memory CD8⁺ T cells, while aging is associated with the decline of naive CD8⁺ T cell proportions¹⁰³; moreover, it was associated with the expression of CD57⁺NKG2C⁺ NK cells;¹⁰⁴ the latter was reported to play a role in the control of infection.^104

–107 Recently, the effect of HCMV seropositivity and aging on NK cells was evaluated in young (18–35 years) and old (>70 years) healthy individuals.¹⁰³ CD57⁺ NK cells, CD56^dimCD16⁺CD57⁺, and CD56⁻CD16⁺CD57⁺ NK cells from young CMV seropositives as well as CD56⁻CD16⁺CD57⁺ NK cells from elderly expressed higher levels of CD94/NKG2C compared with CMV seronegatives. Importantly, the expression of CD57 on CD56⁻CD16⁺ NK cell subpopulation was higher among the older CMV-seropositive compared with younger CMV-seropositive individuals. Moreover, the immature CD56^bright NK cells decreased among elderly and expressed higher levels of granzyme A, that is, differentiated, compared with the younger group. This study highlighted the role of CMV in shaping NK cell subsets in different age groups and suggests the significance of determining the status of CMV infection especially in people aging with HIV.

Variable results have been reported regarding the expression of KIRs as well as other NK cell receptors. The expression of KLRG1 appears to be reduced in elderly.^108,109 Interestingly, a lower frequency of KLRG1⁺CD57⁻NK cells was reported among CMV⁺ young individuals compared with higher frequency of KLRG1⁻CD57⁺NK cells leading authors to advance the role of CMV rather than aging in shaping NK cell phenotypes.¹¹⁰ The expression of NKG2A was reported to either decrease or remain the same in elderly (reviewed in refs.^38,98). Interestingly, the age-related changes on NK cells have been proposed to contribute to the increased frequency of senescent cells detected in aged tissues¹¹¹ as well as low immune responses in older subjects.³⁸ In this study, the surface expression of the inhibitory receptor CD94/NKG2A was reported to be significantly lower in all age groups compared with newborns similar to a previous report.⁹⁹ Moreover, NKp30 and NKp46 (activating NK cell receptors) are significantly lower in adults, old, and very old individuals as compared with newborns. The frequency of NK cells expressing these receptors was previously reported to decrease in elderly subjects (>60 years) compared with younger adults (18–59 years).¹⁰¹ Le Garff-Tavernier et al. ⁹⁵ reported the increased expression of the following in adult controls compared with cord blood with similar levels detected in other age groups: KIR2DL1/DS1 (bind group 2 HLA-Cw), KIR2DL2/DL3/DS2/(bind HLA-C3 group 1), KIR3DL1/DS1 (HLA-Bw4 ligand). Moreover, an increase in the expression of the inhibitory receptor LIR-1/ILT-2 was detected in very old subjects.

As previously discussed, during the course of untreated and uncontrolled HIV infection, the frequency of CD56^dim NK cells decline, whereas CD56^bright NK cells reciprocally increase, which is not in parallel with the impact of aging on these subsets. Large studies are needed to investigate the dynamics affecting NK cell subsets, KIR expression and the interaction with their HLA-I ligands among PLHIV above 50 years during the course of HIV infection. Elucidating these mechanisms would have implications in developing therapeutic approaches that could enhance NK cell activity through exploitation of KIRs and KIR-HLA ligand combination aiming at reducing the incidence of comorbidities and deaths due to them.

Tregs in HIV and Aging

Regulatory T cells

Tregs, a subset of CD4⁺ T cells, play a major role in maintaining immune homeostasis. These cells are characterized by their ability to maintain immune tolerance to both self-antigens in autoimmune disorders and non-self-antigens in various infectious diseases.^112

–115 Based on their site of differentiation, Tregs are classified into thymic (tTregs) and peripheral Tregs (pTregs).^{116

–122} The development and maturation of tTregs are mediated by the interaction between B7 molecules expressed on antigen-presenting cells, CD28 expressed on thymocytes and IL-2, respectively.^{117,121
–123} pTregs are derived from naive CD4⁺ T helper cells in the peripheral lymphoid organs. Peripherally induced Tregs are generated following weak TCR and CD28 stimulation/or exposure to subimmunogenic peptides for a long period of time resulting in expression of forkhead box P3 transcription factor (FoxP3). Tregs are phenotypically defined by the expression of FoxP3.¹¹⁷ FoxP3 binds to nuclear factor of activated T cells to form a complex, modulating the suppressive function of Tregs. The latter is mediated through increasing the expression of T cell activation markers (CTLA-4 and CD25) on CD4⁺ T cells and inhibiting the transcription of IL-2.^124

–128 Moreover, TGF-β plays an important role in the generation of pTregs in vitro and in vivo.^{116,121
–123}

CD4⁺CD25⁺FoxP3⁺ Tregs constitute the most investigated immunophenotype. This phenotype plays a vital role in attenuating cell-mediated immunity and suppressing activated CD4⁺ T cell function.^{112,129

–133} Tregs suppress the function and proliferation of Th1, Th2, and Th17 CD4⁺ and CD8⁺ T cells.^{112
–114,134} Tregs exert their immunosuppressive function mainly through: (i) release of anti-inflammatory mediators, including IL-10, IL-35, and TGF-β^135,136; (ii) CTLA-4 expressed on Tregs blocking the CD28 costimulation^137,138; and (iii) binding to IL-2 and thus competing with non-Tregs and inhibiting activation.^{115,116,120,127,132,139

–143}

It is well established that an increased frequency of Tregs is detected in aged mice and humans and thus contributing to age-associated immunosuppression (reviewed in refs.^123,131,144). Briefly, animal studies suggested the accumulation of Tregs in lymphoid organs during aging leading to increased susceptibility to infection and reduced vaccine responses. For example, influenza infection induced antigen-specific FoxP3⁺ Tregs as well as increased levels of memory Tregs following secondary infection.^145,146 Recently, vaccine-induced Tregs suppressed anti-influenza immune responses in vitro and in vivo in the mice model.¹⁴⁷ When addressing the impact of aging on vaccine responses, Van Der Geest et al. ¹⁴⁸ reported a decline in naive Tregs and increased ratios of Tregs and T effector (T_eff) cells in the CD4⁺ T cell memory compartment of healthy elderly volunteers. These data suggest a bias toward Tregs in the elderly, which was also shown to inversely correlate with immune responses following influenza vaccine. Similarly, a significant increase in FoxP3⁺CD4⁺ T cells was detected in the skin of older individuals infected with Varicella Zoster virus above 60 years of age.¹⁴⁹ CD4⁺CD25⁺FoxP3⁺ Tregs were also higher among individuals with lung cancer compared with healthy controls.¹⁵⁰ Importantly, these cells as well as FoxP3 mRNA also increased with aging in healthy people.¹⁵¹ Along the same lines, aging has been recognized to program the generation of a suppressive immune milieu affecting DCs and T cells in mice¹⁵² and consequently loss of their function in tumor-bearing mice.¹⁵³

Overall, the process of aging is characterized by a decline of the thymus function and its involution. Despite the decline in tTreg and pTreg differentiation with age, older individuals accumulate mature Tregs in blood and secondary lymphoid organs. This phenomenon would lead to suppression of immune responses and thus enhancement of immunosenescence.

Tregs, HIV infection, and aging

The role of Tregs in HIV and simian immunodeficiency virus (SIV) infections has been recently reviewed and is gaining attention in an attempt to manipulate these cells for the development of HIV therapeutic strategies.^117,154,155 However, the debate about the beneficial or detrimental role of Tregs during HIV infection is still ongoing. Importantly, naive Tregs are potential targets for HIV infection due to the increased expression of HIV coreceptors, CCR5 and CXCR4.^{117,120,127,143,154,156,157} The infection of Tregs in vitro with HIV (X4) was reported to result in downregulation of FoxP3 expression, decreased production of TGF-β and increased production of IL-4; suggesting loss of the suppressive function of HIV-infected Tregs.¹⁵⁸

The interaction between IL-2 and IL-2Rα receptor (CD25) expressed on the surface of Tregs has been linked to the homeostatic role exerted by these cells.^159,160 IL-2 facilitates the TGF-β-mediated transformation of naive T cells to FoxP3⁺-expressing Tregs. Consequently, high levels of IL-2 produced by activated T cells are suggested to result in the expansion of Tregs. This homeostatic balance, previously described in autoimmune diseases,¹⁶⁰ is disturbed in HIV-infected individuals due to virus-mediated alterations of Tregs phenotype. The latter is defined by the loss of IL-2R-α on Tregs, thus, decreasing their suppressive capacity.¹⁶¹ Previous reports confirmed the loss of Tregs suppressive function following loss of CD25 expression on the surface of Tregs isolated from healthy individuals.¹⁶² Further studies are needed to investigate the role of these balances in HIV infection and, therefore, therapeutic agents targeting Tregs and IL-2.

Importantly, Tregs suppress HIV-specific CD8⁺ cytotoxic T cell responses resulting in an increased viral load¹⁶³ and contributing to viral persistence and disease progression.^{120,121,127,164,165} The latter was confirmed ex vivo among controllers expressing protective HLA-B*27 and HLA-B*57 alleles. The HLA-B*27 or HLA-B*57-restricted CD8⁺ T cells were able to escape the suppressive mechanism of Tregs and retain their proliferative capacity.^119,166 These restricted CD8⁺ T cells were able to upregulate granzyme B production upon antigenic stimulation and consequently becoming less susceptible to suppression by Tregs. These results, in addition to the low frequency of Tregs, are suggested to contribute to robust HIV-specific CD8⁺ T cell responses in controllers.

Similarly, the role of Tregs was further studied among HIV-exposed seronegative (HESN) individuals.^167,168 HESN individuals constitute an important model to study resistance and/or control of HIV infection. Briefly, peripheral blood mononuclear cells (PBMCs) from HESN adults from HIV serodiscordant cohort were used to assess the function of Tregs and protection from HIV.¹⁶⁸ Treg-depleted PBMCs were stimulated in vitro with Gag, Pol, Env, and Vpu peptide pools. Two subsets of Tregs were detected: a subset suppressing HIV-specific CD4⁺ T cell proliferation and another one unable to suppress the latter. When compared with cells able to suppress HIV-specific CD4⁺ T cell proliferation, Tregs unable to suppress the latter produced MIP-1β and showed increased T cell activation and frequencies of Tregs. The role of Tregs in immunological reconstitution was also studied among treated HIV-infected individuals.¹⁶⁹ The percentages of peripheral and lymphoid FoxP3⁺ Tregs were assessed in immunological nonresponders (INR) (CD4 T cells <200 cells/μL), intermediate responders (CD4 T cells 200–500 cells/μL), and immunological responders (CD4 T cells <500 cells/μL) following cART. INR had significantly higher percentages of Tregs than other groups, including controls with higher percentages producing IL-10. Moreover, this group showed a higher percentage of activated Tregs. HIV controllers showed lower frequencies and counts of Tregs, compared with uninfected subjects, thus potentially leading to stronger HIV-specific T cell responses as well as T cell activation.¹⁷⁰ Similarly, many studies reported higher frequencies of Tregs among untreated HIV-viremic individuals compared with elite controllers, LTNP as well as HIV-infected individuals, on treatment.^{165,170

–173} The lower frequencies of Tregs reported in these groups were similar to those reported in healthy controls.

In an attempt to analyze immunological markers potentially contributing to comorbidities, treated HIV-infected individuals older than 45 years of the AGE_hIV Cohort Study with undetectable levels of viremia, showed high percentages of Tregs associated with increased IA,¹⁷⁴ the hallmark of HIV infection.¹⁷⁵ Increased levels of IA were confirmed in this group by detection of increased percentages of CD38⁺ HLA-DR⁺ CD4⁺ and CD8⁺ T cells, Tregs, and PD-1-expressing CD4⁺ T cells. These results confirmed previous reports.^176,177 The role of Tregs was further evaluated in nonprogressing HIV infection. The percentages of activated Tregs in LTNPs (with ongoing viral replication) and EC (with low-to-undetectable levels of viral replication) were found to be higher than those detected among progressors while percentages of resting Tregs were lower,^169,178 suggesting the beneficial role of these cells in immune reconstitution and nonprogression.

Recently, lower levels of HIV DNA and viral mRNA were detected in FoxP3-expressing CD4⁺ T cells.¹⁷⁹ The latter were either infected in vitro and are in agreement with previously reported data¹⁸⁰ or isolated from HIV-infected individuals and tested ex vivo.¹⁷⁹ Tregs were also suggested to play a role in inhibiting HIV expression in neighboring CD4⁺ T cells. Briefly, Tregs from HIV-infected individuals were cocultured with conventional and memory CD4⁺ T cells. Antibody-mediated blocking of Tregs' inhibitory surface markers (CTLA-4, PD-1, and GARP) increased HIV DNA integration and viral mRNA expression in neighboring CD4⁺ T cells; suggesting contact with cell surface molecules as the mechanism behind potentially protecting neighboring CD4⁺ T cells from HIV infection and inhibiting viral replication.¹⁷⁹

An emerging body of evidence further suggests the detrimental role of Tregs in inhibiting HIV-specific immune responses. Following treatment interruption, as expected, HIV RNA in plasma and HIV-DNA in PBMCs increased, and CD4⁺ T cell counts decreased. The proportion of HLADR⁺ CD38⁺ CD8⁺ T cells (assessing IA) was associated with the expansion of Tregs in HIV-infected individuals undergoing treatment interruption.¹⁸¹ Similar results were observed in the gastrointestinal tract, whereby the frequency of mucosal Tregs increased during chronic HIV infection among noncontrollers compared with controllers and uninfected controls and was associated with viral load and IA.¹⁶⁵ These results suggested yet again the role of Tregs in promoting viral persistence and HIV disease progression.

It is clear that conflicting data exist regarding the impact of Tregs on either enhancing or reducing HIV disease progression^{182

–187} as well as controlling IA. A significant complication exerted by Tregs lies in being recognized as a major reservoir for SIV/HIV (reviewed in refs.^115,117). pTregs were described as latently infected with HIV without addressing tissue Tregs in humans. Importantly, increased numbers of SIV-infected Tregs were detected in gut-associated lymphoid tissue of untreated macaques.¹⁸⁸ Further investigations are needed to understand the persistence of HIV in tissue Tregs compared with pTregs and consequently understand their role in immunosenescence.

In summary, the “split personality of Tregs”,^126,154 with a detrimental role contributing to disease progression through suppression of HIV-specific T cell responses and a beneficial one resulting in eliminating viral spread, reestablishing immune constitution at low CD4⁺ T cell counts, and preventing immune hyperactivation among infected subjects, requires further investigation for clearer conclusions on disease outcome in HIV-infected individuals.

Limited data exist on the impact of aging on Tregs during HIV infection. Tenorio et al. ¹²⁸ studied the impact of aging on Tregs in treatment-naive young (18–30 years) and older (≥45 years) HIV⁺ individuals. The results of this study indicated significantly higher total percentages of Tregs among older HIV positives; the latter were compared with younger seropositives as well as healthy age-matched controls. Moreover, a positive correlation was shown between age of HIV-infected subjects and percentages of Tregs while a negative one was reported among HIV-negative subjects. These results were reported in a small sample size and consequently should be cautiously interpreted. Tregs have been suggested to exert a detrimental role contributing to disease progression through suppression of HIV-specific T cell responses; consequently, the increase in their percentages during HIV infection as well as their expansion in aging healthy individuals might contribute to an accelerated disease progression in aging HIV-infected subjects.¹²⁸ In an attempt to understand the changes on NK cells phenotypes and frequency (summarized above) among INR, Giuliani et al. ⁶⁷ reported an overall decrease in the number of Tregs. Currently, the consensus on the fate of Tregs during HIV infection is still controversial especially among NRs, whereby similar,¹⁸⁹ lower¹⁹⁰ and higher^{177,178,191,192} frequencies were reported when compared with responders. This study also reported an inverse correlation between reduced Tregs' count and CD56^bright NK cells among NRs. The expansion of the latter was suggested to shift the homeostatic control of Tregs leading to reduced immune reconstitution in this group. It is worth mentioning that in vitro studies in humans as well as studies in mice model suggest the ability of Tregs to suppress the cytolytic activity and proliferation of NK cells.^193

–196

Conclusion

HIV-infected individuals receiving ART are known to have a shorter life expectancy and higher rate of age-associated diseases compared with age-matched uninfected individuals.^9
–11 While comorbidities are associated with the natural aging process, the use of ART, IA, and chronic inflammation due to HIV are linked to an increased risk of comorbidities in older HIV patients.^9,11,197 Importantly, HIV infection and aging exert similar impact on NK cell subsets and Tregs. Consequently, new treatment challenges are faced to control the concomitant effect of aging with HIV on immune reconstitution. Further studies are needed to understand the role of NK cells, the KIR–HLA interaction and Tregs in older HIV-positive individuals with comorbidities and potentially targeting these for the development of innovative and effective therapeutic interventions.

Footnotes

Acknowledgments

The authors would like to thank Antonious Fakhoury and Kamil Kadi for their technical assistance.

Author Disclosure Statement

No competing financial interests exist.

Funding Information

No funding was received for this article.

References

Kaplan-Lewis

, Aberg

, Lee

: Aging with HIV in the ART era. Semin Diagn Pathol, 2017; 34:384–397.

Nakagawa

, May

, Phillips

: Life expectancy living with HIV: Recent estimates and future implications. Curr Opin Infect Dis, 2013; 26:17–25.

Autenrieth

, Beck

, Stelzle

, Mallouris

, Mahy

, Ghys

: Global and regional trends of people living with HIV aged 50 and over: Estimates and projections for 2000–2020. PLoS One, 2018; 13:e0207005.

Calcagno

, Nozza

, Muss

, et al.: Ageing with HIV: A multidisciplinary review. Infection, 2015; 43:509–522.

Escota

, O'Halloran

, Powderly

, Presti

: Understanding mechanisms to promote successful aging in persons living with HIV. Int J Infect Dis, 2018; 66:56–64.

Hunt

: HIV and aging: Emerging research issues. Current Opin HIV AIDS, 2014; 9:302–308.

Smit

, Brinkman

, Geerlings

, et al.: Future challenges for clinical care of an ageing population infected with HIV: A modelling study. Lancet Infect Dis, 2015; 15:810–818.

Chu

, Umanski

, Blank

, Meissner

, Grossberg

, Selwyn

: Comorbidity-related treatment outcomes among HIV-infected adults in the Bronx, NY. J Urban Health, 2011; 88:507–516.

Guaraldi

, Orlando

, Zona

, et al.: Premature age-related comorbidities among HIV-infected persons compared with the general population. Clin Infect Dis, 2011; 53:1120–1126.

10.

Hasse

, Ledergerber

, Furrer

, et al.: Morbidity and aging in HIV-infected persons: The Swiss HIV cohort study. Clin Infect Dis, 2011; 53:1130–1139.

11.

Schouten

, Wit

, Stolte

, et al.: Cross-sectional comparison of the prevalence of age-associated comorbidities and their risk factors between HIV-infected and uninfected individuals: The AGEhIV cohort study. Clin Infect Dis, 2014; 59:1787–1797.

12.

Torres

, Cardoso

, Velasque Lde

, et al.: Aging with HIV: An overview of an urban cohort in Rio de Janeiro (Brazil) across decades of life. Braz J Infect Dis, 2013; 17:324–331.

13.

Longenecker

, Sullivan

, Baker

: Immune activation and cardiovascular disease in chronic HIV infection. Curr Opin HIV AIDS, 2016; 11:216–225.

14.

Rockstroh

, Mohr

, Behrens

, Spengler

: Liver fibrosis in HIV: Which role does HIV itself, long-term drug toxicities and metabolic changes play?. Curr Opin HIV AIDS, 2014; 9:365–370.

15.

Sherman

, Rockstroh

, Thomas

: Human immunodeficiency virus and liver disease: An update. Hepatology, 2015; 62:1871–1882.

16.

Vance

, Mugavero

, Willig

, Raper

, Saag

: Aging with HIV: A cross-sectional study of comorbidity prevalence and clinical characteristics across decades of life. J Assoc Nurses AIDS Care, 2011; 22:17–25.

17.

, Chen

, Sheng

, et al.: Risk factors for incident diabetes mellitus among HIV-infected patients receiving combination antiretroviral therapy in Taiwan: A case–control study. HIV Med, 2009; 10:302–309.

18.

Rosenthal

, Tyor

: Aging, comorbidities, and the importance of finding biomarkers for HIV-associated neurocognitive disorders. J Neurovirol, 2019; [Epub ahead of print]; DOI: 10.1007/s13365-019-00735-0.

19.

Spudich

: Immune activation in the central nervous system throughout the course of HIV infection. Curr Opin HIV AIDS, 2016; 11:226–233.

20.

Deeks

, Lewin

, Havlir

: The end of AIDS: HIV infection as a chronic disease. Lancet, 2013; 382:1525–1533.

21.

Mpondo

: HIV infection in the elderly: Arising challenges. J Aging Res, 2016; 2016:2404857.

22.

Gleason

, Luque

, Shah

: Polypharmacy in the HIV-infected older adult population. Clin Interv Aging, 2013; 8:749–763.

23.

Appay

, Kelleher

: Immune activation and immune aging in HIV infection. Curr Opin HIV AIDS, 2016; 11:242–249.

24.

Zicari

, Sessa

, Cotugno

, et al.: Immune activation, inflammation, and non-AIDS co-morbidities in HIV-infected patients under long-term ART. Viruses, 2019; 11:200.

25.

Sandler

, Douek

: Microbial translocation in HIV infection: Causes, consequences and treatment opportunities. Nat Rev Microbiol, 2012; 10:655.

26.

Zevin

, McKinnon

, Burgener

, Klatt

: Microbial translocation and microbiome dysbiosis in HIV-associated immune activation. Curr Opin HIV AIDS, 2016; 11:182–190.

27.

Murray

, Suzuki

, Law

, et al.: Circulating microRNAs in sera correlate with soluble biomarkers of immune activation but do not predict mortality in ART treated individuals with HIV-1 infection: A case control study. PLoS One, 2015; 10:e0139981.

28.

Work Group for the HIV, Aging Consensus Project: Summary Report from the Human Immunodeficiency Virus and Aging Consensus Project: Treatment strategies for clinicians managing older individuals with the human immunodeficiency virus. J Am Geriatr Soc, 2012; 60:974–979.

29.

Fitch

, Feldpausch

, Looby

: Biomarkers and clinical indices of aging with HIV. Interdiscip Top Gerontol Geriatr, 2017; 42:47–58.

30.

Mandal

, Viswanathan

: Natural killer cells: In health and disease. Hematol Oncol Stem Cell Ther, 2015; 8:47–55.

31.

Mikulak

, Oriolo

, Zaghi

, Di Vito

, Mavilio

: Natural killer cells in HIV-1 infection and therapy. AIDS, 2017; 31:2317–2330.

32.

Camous

, Pera

, Solana

, Larbi

: NK cells in healthy aging and age-associated diseases. J Biomed Biotechnol, 2012; 2012:195956.

33.

Ansari

, Ahmad

, Meyer-Olson

, Kamarulzaman

, Jacobs

, Schmidt

: Natural killer cell heterogeneity: Cellular dysfunction and significance in HIV-1 immuno-pathogenesis. Cell Mol Life Sci, 2015; 72:3037–3049.

34.

Poli

, Michel

, Theresine

, Andres

, Hentges

, Zimmer

: CD56bright natural killer (NK) cells: An important NK cell subset. Immunology, 2009; 126:458–465.

35.

Ouyang

, Baerlocher

, Vulto

, Lansdorp

: Telomere length in human natural killer cell subsets. Ann N Y Acad Sci, 2007; 1106:240–252.

36.

Romagnani

, Juelke

, Falco

, et al.: CD56brightCD16- killer Ig-like receptor-NK cells display longer telomeres and acquire features of CD56dim NK cells upon activation. J Immunol, 2007; 178:4947–4955.

37.

Kared

, Martelli

, Ng

, Pender

, Larbi

: CD57 in human natural killer cells and T-lymphocytes. Cancer Immunol Immunother, 2016; 65:441–452.

38.

Solana

, Tarazona

, Gayoso

, Lesur

, Dupuis

, Fulop

: Innate immunosenescence: Effect of aging on cells and receptors of the innate immune system in humans. Semin Immunol, 2012; 24:331–341.

39.

Luetke-Eversloh

, Killig

, Romagnani

: Signatures of human NK cell development and terminal differentiation. Front Immunol, 2013; 4:499.

40.

Lanier

: Evolutionary struggles between NK cells and viruses. Nat Rev Immunol, 2008; 8:259–268.

41.

Vivier

, Raulet

, Moretta

, et al.: Innate or adaptive immunity? The example of natural killer cells. Science, 2011; 331:44–49.

42.

Martinet

, Smyth

: Balancing natural killer cell activation through paired receptors. Nat Rev Immunol, 2015; 15:243–254.

43.

Scully

, Alter

: NK cells in HIV disease. Current HIV/AIDS Rep, 2016; 13:85–94.

44.

Voehringer

, Koschella

, Pircher

: Lack of proliferative capacity of human effector and memory T cells expressing killer cell lectinlike receptor G1 (KLRG1). Blood, 2002; 100:3698–3702.

45.

Tessmer

, Fugere

, Stevenaert

, et al.: KLRG1 binds cadherins and preferentially associates with SHIP-1. Int Immunol, 2007; 19:391–400.

46.

Ito

, Maruyama

, Saito

, Koganei

, Yamamoto

, Matsumoto

: Killer cell lectin-like receptor G1 binds three members of the classical cadherin family to inhibit NK cell cytotoxicity. J Exp Med, 2006; 203:289–295.

47.

Lopez-Vergès

, Milush

, Pandey

, et al.: CD57 defines a functionally distinct population of mature NK cells in the human CD56^dimCD16⁺ NK-cell subset. Blood, 2010; 116:3865–3874.

48.

Campbell

, Purdy

: Structure/function of human killer cell immunoglobulin-like receptors: Lessons from polymorphisms, evolution, crystal structures and mutations. Immunology, 2011; 132:315–325.

49.

Rajalingam

: Diversity of killer cell immunoglobulin-like receptors and disease. Clin Lab Med, 2018; 38:637–653.

50.

Saunders

, Vivian

, O'Connor

, et al.: A bird's eye view of NK cell receptor interactions with their MHC class I ligands. Immunol Rev, 2015; 267:148–166.

51.

Ivarsson

, Michaëlsson

, Fauriat

: Activating killer cell Ig-like receptors in health and disease. Front Immunol, 2014; 5:184.

52.

Takeshita

LYC

, Gonzalez-Galarza

, dos Santos

EJM

, et al.: A database for curating the associations between killer cell immunoglobulin-like receptors and diseases in worldwide populations. Database (Oxford), 2013; 2013:bat021.

53.

Bashirova

, Thomas

, Carrington

: HLA/KIR restraint of HIV: Surviving the fittest. Annu Rev Immunol, 2011; 29:295–317.

54.

Jost

, Altfeld

: Control of human viral infections by natural killer cells. Annu Rev Immunol, 2013; 31:163–194.

55.

Bjorkstrom

, Ljunggren

, Sandberg

: CD56 negative NK cells: Origin, function, and role in chronic viral disease. Trends Immunol, 2010; 31:401–406.

56.

Peppa

: Natural killer cells in human immunodeficiency virus-1 infection: Spotlight on the impact of human cytomegalovirus. Front Immunol, 2017; 8:1322.

57.

Mavilio

, Lombardo

, Benjamin

, et al.: Characterization of CD56−/CD16+ natural killer (NK) cells: A highly dysfunctional NK subset expanded in HIV-infected viremic individuals. Proc Natl Acad Sci U S A, 2005; 102:2886–2891.

58.

Mavilio

, Benjamin

, Daucher

, et al.: Natural killer cells in HIV-1 infection: Dichotomous effects of viremia on inhibitory and activating receptors and their functional correlates. Proc Natl Acad Sci U S A, 2003; 100:15011–15016.

59.

Pohlmeyer

, Gonzalez

, Irrinki

, et al.: Identification of NK cell subpopulations that differentiate HIV-infected subject cohorts with diverse levels of virus control. J Virol, 2019; 93:pii:e01790-18.

60.

Marras

, Nicco

, Bozzano

, et al.: Natural killer cells in HIV controller patients express an activated effector phenotype and do not up-regulate NKp44 on IL-2 stimulation. Proc Natl Acad Sci U S A, 2013; 110:11970.

61.

Varchetta

, Mele

, Lombardi

, et al.: Lack of Siglec-7 expression identifies a dysfunctional natural killer cell subset associated with liver inflammation and fibrosis in chronic HCV infection. Gut, 2016; 65:1998–2006.

62.

Brunetta

, Fogli

, Varchetta

, et al.: The decreased expression of Siglec-7 represents an early marker of dysfunctional natural killer-cell subsets associated with high levels of HIV-1 viremia. Blood, 2009; 114:3822–3830.

63.

Shao

, Yin

, Zhang

, et al.: Siglec-7 defines a highly functional natural killer cell subset and inhibits cell-mediated activities. Scand J Immunol, 2016; 84:182–190.

64.

Tarazona

, Casado

, Delarosa

, et al.: Selective depletion of CD56(dim) NK cell subsets and maintenance of CD56(bright) NK cells in treatment-naive HIV-1-seropositive individuals. J Clin Immunol, 2002; 22:176–183.

65.

Jiang

, Yang

, Sun

, et al.: IL-10(+) NK and TGF-beta(+) NK cells play negative regulatory roles in HIV infection. BMC Infect Dis, 2018; 18:80.

66.

Ripa

, Pogliaghi

, Chiappetta

, et al.: Dynamics of adaptive and innate immunity in patients treated during primary human immunodeficiency virus infection: Results from Maraviroc in HIV acute infection (MAIN) randomized clinical trial. Clin Microbiol Infect, 2015; 21:876 e871–e874.

67.

Giuliani

, Vassena

, Di Cesare

, et al.: NK cells of HIV-1-infected patients with poor CD4(+) T-cell reconstitution despite suppressive HAART show reduced IFN-gamma production and high frequency of autoreactive CD56(bright) cells. Immunol Lett, 2017; 190:185–193.

68.

Brenchley

, Karandikar

, Betts

, Ambrozak

, Hill

, Crotty

, et al.: Expression of CD57 defines replicative senescence and antigen-induced apoptotic death of CD8+ T cells. Blood, 2003; 101:2711–2720.

69.

Hong

, Eberhard

, Keudel

, Bollmann

, Ballmaier

, Bhatnagar

, et al.: HIV infection is associated with a preferential decline in less-differentiated CD56dim CD16+ NK cells. J Virol, 2010; 84:1183–1188.

70.

Alter

, Teigen

, Davis

, et al.: Sequential deregulation of NK cell subset distribution and function starting in acute HIV-1 infection. Blood, 2005; 106:3366–3369.

71.

Gondois-Rey

, Cheret

, Mallet

, et al.: A mature NK profile at the time of HIV primary infection is associated with an early response to cART. Front Immunol, 2017; 8:54.

72.

Affandi

, Montgomery

, Lee

, Price

: HIV patients stable on ART retain evidence of a high CMV load but changes to natural killer cell phenotypes reflect both HIV and CMV. AIDS Res Ther, 2015; 12:41.

73.

Hendricks

, Balfour

Jr. , Dunmire

, Schmeling

, Hogquist

, Lanier

: Cutting edge: NKG2C(hi)CD57+ NK cells respond specifically to acute infection with cytomegalovirus and not Epstein-Barr virus. J Immunol, 2014; 192:4492–4496.

74.

Peppa

, Pedroza-Pacheco

, Pellegrino

, Williams

, Maini

, Borrow

: Adaptive reconfiguration of natural killer cells in HIV-1 infection. Front Immunol, 2018; 9:474.

75.

Gardiner

: NK cell function and receptor diversity in the context of HCV infection. Front Microbiol, 2015; 6:1061.

76.

Di Bona

, Aiello

, Colomba

, et al.: KIR2DL3 and the KIR ligand groups HLA-A-Bw4 and HLA-C2 predict the outcome of hepatitis B virus infection. J Viral Hepat, 2017; 24:768–775.

77.

Boyton

, Altmann

: Natural killer cells, killer immunoglobulin-like receptors and human leucocyte antigen class I in disease. Clin Exp Immunol, 2007; 149:1–8.

78.

Carrington

, Wang

, Martin

, et al.: Hierarchy of resistance to cervical neoplasia mediated by combinations of killer immunoglobulin-like receptor and human leukocyte antigen loci. J Exp Med, 2005; 201:1069–1075.

79.

Hirayasu

, Ohashi

, Kashiwase

, et al.: Significant association of KIR2DL3-HLA-C1 combination with cerebral malaria and implications for co-evolution of KIR and HLA. PLoS Pathog, 2012; 8:e1002565.

80.

Jobim

, Jobim

, Salim

, et al.: Analysis of KIR gene frequencies and HLA class I genotypes in breast cancer and control group. Hum Immunol, 2013; 74:1130–1133.

81.

Zhi-Ming

, Yu-Lian

, Zhao-Lei

, et al.: Polymorphisms of killer cell immunoglobulin-like receptor gene: Possible association with susceptibility to or clearance of hepatitis B virus infection in Chinese Han population. Croat Med J, 2007; 48:800–806.

82.

Aiello

, Accardi

, Candore

, et al.: Role of immunogenetics in the outcome of HCMV infection: Implications for ageing. Int J Mol Sci, 2019; 20:pii:E685.

83.

Martin

, Carrington

: Immunogenetics of viral infections. Curr Opin Immunol, 2005; 17:510–516.

84.

Alter

, Martin

, Teigen

, et al.: Differential natural killer cell-mediated inhibition of HIV-1 replication based on distinct KIR/HLA subtypes. J Exp Med, 2007; 204:3027–3036.

85.

Alter

, Rihn

, Walter

, et al.: HLA class I subtype-dependent expansion of KIR3DS1+ and KIR3DL1+ NK cells during acute human immunodeficiency virus type 1 infection. J Virol, 2009; 83:6798–6805.

86.

Altfeld

, Fadda

, Frleta

, Bhardwaj

: DCs and NK cells: Critical effectors in the immune response to HIV-1. Nat Rev Immunol, 2011; 11:176–186.

87.

Martin

, Qi

, Gao

, et al.: Innate partnership of HLA-B and KIR3DL1 subtypes against HIV-1. Nat Genet, 2007; 39:733–740.

88.

Maruthamuthu

, Rajalingam

, Pandian

, et al.: Inhibitory natural killer cell receptor KIR3DL1 with its ligand Bw4 constraints HIV-1 disease among South Indians. AIDS, 2018; 32:2679–2688.

89.

Boulet

, Song

, Kamya

, et al.: HIV protective KIR3DL1 and HLA-B genotypes influence NK cell function following stimulation with HLA-devoid cells. J Immunol, 2010; 184:2057–2064.

90.

Martin

, Gao

, Lee

, et al.: Epistatic interaction between KIR3DS1 and HLA-B delays the progression to AIDS. Nat Genet, 2002; 31:429–434.

91.

, Martin

, Gao

, et al.: KIR/HLA pleiotropism: Protection against both HIV and opportunistic infections. PLoS Pathog, 2006; 2:e79.

92.

Song

, Lisovsky

, Lebouché

, Routy

J-P

, Bruneau

, Bernard

: HIV protective KIR3DL1/S1-HLA-B genotypes influence NK cell-mediated inhibition of HIV replication in autologous CD4 targets. PLoS Pathog, 2014; 10:e1003867.

93.

Yawata

, Yawata

, Draghi

, Little

, Partheniou

, Parham

: Roles for HLA and KIR polymorphisms in natural killer cell repertoire selection and modulation of effector function. J Exp Med, 2006; 203:633–645.

94.

Lin

, Kuroki

, Kuse

, et al.: HIV-1 control by NK Cells via reduced interaction between KIR2DL2 and HLA-C(*)12:02/C(*)14:03. Cell Rep, 2016; 17:2210–2220.

95.

Le Garff-Tavernier

, Beziat

, Decocq

, et al.: Human NK cells display major phenotypic and functional changes over the life span. Aging Cell, 2010; 9:527–535.

96.

Valiathan

, Ashman

, Asthana

: Effects of ageing on the immune system: Infants to elderly. Scand J Immunol, 2016; 83:255–266.

97.

Solana

, Campos

, Pera

, Tarazona

: Shaping of NK cell subsets by aging. Curr Opin Immunol, 2014; 29:56–61.

98.

Manser

, Uhrberg

: Age-related changes in natural killer cell repertoires: Impact on NK cell function and immune surveillance. Cancer Immunol Immunother, 2016; 65:417–426.

99.

Hayhoe

RPG

, Henson

, Akbar

, Palmer

: Variation of human natural killer cell phenotypes with age: Identification of a unique KLRG1-negative subset. Hum Immunol, 2010; 71:676–681.

100.

Chidrawar

, Khan

, Chan

, Nayak

, Moss

: Ageing is associated with a decline in peripheral blood CD56bright NK cells. Immun Ageing, 2006; 3:10.

101.

Almeida-Oliveira

, Smith-Carvalho

, Porto

, et al.: Age-related changes in natural killer cell receptors from childhood through old age. Hum Immunol, 2011; 72:319–329.

102.

Gounder

, Abdullah

BJJ

, Radzuanb

, et al.: Effect of aging on NK cell population and their proliferation at ex vivo culture condition. Anal Cell Pathol (Amst), 2018; 2018:7871814.

103.

Campos

, Pera

, Sanchez-Correa

, et al.: Effect of age and CMV on NK cell subpopulations. Exp Gerontol, 2014; 54:130–137.

104.

Guma

, Angulo

, Vilches

, Gomez-Lozano

, Malats

, Lopez-Botet

: Imprint of human cytomegalovirus infection on the NK cell receptor repertoire. Blood, 2004; 104:3664–3671.

105.

Lopez-Verges

, Milush

, Schwartz

, et al.: Expansion of a unique CD57(+)NKG2Chi natural killer cell subset during acute human cytomegalovirus infection. Proc Natl Acad Sci U S A, 2011; 108:14725–14732.

106.

, Sinzger

, Frascaroli

, et al.: Human cytomegalovirus-induced NKG2C(hi) CD57(hi) natural killer cells are effectors dependent on humoral antiviral immunity. J Virol, 2013; 87:7717–7725.

107.

Muntasell

, Vilches

, Angulo

, Lopez-Botet

: Adaptive reconfiguration of the human NK-cell compartment in response to cytomegalovirus: A different perspective of the host–pathogen interaction. Eur J Immunol, 2013; 43:1133–1141.

108.

Hazeldine

, Lord

: The impact of ageing on natural killer cell function and potential consequences for health in older adults. Ageing Res Rev, 2013; 12:1069–1078.

109.

Müller-Durovic

, Lanna

, Covre

, Mills

, Henson

, Akbar

: Killer cell lectin-like receptor G1 inhibits NK cell function through activation of adenosine 5′-monophosphate-activated protein kinase. J Immunol, 2016; 197:2891–2899.

110.

Bigley

, Lowder

, Spielmann

, et al.: NK-cells have an impaired response to acute exercise and a lower expression of the inhibitory receptors KLRG1 and CD158a in humans with latent cytomegalovirus infection. Brain Behav Immun, 2012; 26:177–186.

111.

Sagiv

, Biran

, Yon

, Simon

, Lowe

, Krizhanovsky

: Granule exocytosis mediates immune surveillance of senescent cells. Oncogene, 2013; 32:1971–1977.

112.

, Cloyd

: HIV-1 binding to CD4 on CD4+CD25+ regulatory T cells enhances their suppressive function and induces them to home to, and accumulate in, peripheral and mucosal lymphoid tissues: An additional mechanism of immunosuppression. Int Immunol, 2009; 21:283–294.

113.

Jiao

, Fu

, Xing

, et al.: The decrease of regulatory T cells correlates with excessive activation and apoptosis of CD8+ T cells in HIV-1-infected typical progressors, but not in long-term non-progressors. Immunology, 2009; 128(1 Suppl):e366–e375.

114.

Moreno-Fernandez

, Zapata

, Blackard

, Franchini

, Chougnet

: Human regulatory T cells are targets for human immunodeficiency virus (HIV) infection, and their susceptibility differs depending on the HIV type 1 strain. J Virol, 2009; 83:12925–12933.

115.

Rocco

, Mellors

, Macatangay

: Regulatory T cells: The ultimate HIV reservoir?. J Virus Erad, 2018; 4:209–214.

116.

Bocian

, Kiernozek

, Domagala-Kulawik

, Korczak-Kowalska

, Stelmaszczyk-Emmel

, Drela

: Correction to: Expanding diversity and common goal of regulatory T and B cells. I: Origin, phenotype, mechanisms. Arch Immunol Ther Exp, 2017; 65:521.

117.

Kleinman

, Sivanandham

, Pandrea

, Chougnet

, Apetrei

: Regulatory T cells as potential targets for HIV cure research. Front Immunol, 2018; 9:734.

118.

Lee

, Lichterfeld

: Diversity of HIV-1 reservoirs in CD4+ T-cell subpopulations. Curr Opin HIV AIDS, 2016; 11:383–387.

119.

Moreno-Fernandez

, Presicce

, Chougnet

: Homeostasis and function of regulatory T cells in HIV/SIV infection. J Virol, 2012; 86:10262–10269.

120.

Phetsouphanh

, Xu

, Zaunders

: CD4 T cells mediate both positive and negative regulation of the immune response to HIV infection: Complex role of T follicular helper cells and regulatory T Cells in pathogenesis. Front Immunol, 2014; 5:681.

121.

Simonetta

, Bourgeois

: CD4+FOXP3+ regulatory T-cell subsets in human immunodeficiency virus infection. Front Immunol, 2013; 4:215.

122.

Tao

, Cheng

, Tang

, Liu

, Pan

, Li

: Foxp3, regulatory T cell, and autoimmune diseases. Inflammation, 2017; 40:328–339.

123.

Darrigues

, van Meerwijk

JPM

, Romagnoli

: Age-dependent changes in regulatory T lymphocyte development and function: A mini-review. Gerontology, 2018; 64:28–35.

124.

Chaudhary

, Abd Al Samid

, al-Ramadi

, Elkord

: Phenotypic alterations, clinical impact and therapeutic potential of regulatory T cells in cancer. Expert Opin Biol Ther, 2014; 14:931–945.

125.

Elkord

, Abd Al Samid

, Chaudhary

: Helios, and not FoxP3, is the marker of activated Tregs expressing GARP/LAP. Oncotarget, 2015; 6:20026–20036.

126.

Hasenkrug

, Chougnet

, Dittmer

: Regulatory T cells in retroviral infections. PLoS Pathog, 2018; 14:e1006776.

127.

Lopez-Abente

, Correa-Rocha

, Pion

: Functional mechanisms of Treg in the context of HIV infection and the janus face of immune suppression. Front Immunol, 2016; 7:192.

128.

Tenorio

, Spritzler

, Martinson

, et al.: The effect of aging on T-regulatory cell frequency in HIV infection. Clin Immunol, 2009; 130:298–303.

129.

Dejaco

, Duftner

, Schirmer

: Are regulatory T-cells linked with aging?. Exp Gerontol, 2006; 41:339–345.

130.

Liu

, Gong

, Huang

, Reynolds

, Mosley

, Gendelman

: Neuromodulatory activities of CD4+CD25+ regulatory T cells in a murine model of HIV-1-associated neurodegeneration. J Immunol, 2009; 182:3855–3865.

131.

Raynor

, Lages

, Shehata

, Hildeman

, Chougnet

: Homeostasis and function of regulatory T cells in aging. Curr Opin Immunol, 2012; 24:482–487.

132.

Wang

: Functional control of regulatory T cells and cancer immunotherapy. Semin Cancer Biol, 2006; 16:106–114.

133.

Zhang

, Zhang

, Ji

, Jiao

, Wu

: Longitudinal changes of peripheral blood DC subsets and regulatory T cells in Chinese chronic HIV-1-infected patients during antiretroviral therapy. PLoS One, 2012; 7:e37966.

134.

Campbell

, Koch

: Phenotypical and functional specialization of FOXP3+ regulatory T cells. Nat Rev Immunol, 2011; 11:119–130.

135.

Collison

, Pillai

, Chaturvedi

, Vignali

DAA

: Regulatory T cell suppression is potentiated by target T cells in a cell contact, IL-35- and IL-10-dependent manner. J Immunol, 2009; 182:6121–6128.

136.

Strauss

, Bergmann

, Szczepanski

, Gooding

, Johnson

, Whiteside

: A unique subset of CD4+CD25highFoxp3+ T cells secreting interleukin-10 and transforming growth factor-β1 mediates suppression in the tumor microenvironment. Clin Cancer Res, 2007; 13:4345–4354.

137.

Ito

, Hanabuchi

, Wang

Y-H

, et al.: Two functional subsets of FOXP3+ regulatory T cells in human thymus and periphery. Immunity, 2008; 28:870–880.

138.

Mandelbrot

, McAdam

, Sharpe

: B7-1 or B7-2 is required to produce the lymphoproliferative phenotype in mice lacking cytotoxic T lymphocyte-associated antigen 4 (CTLA-4). J Exp Med, 1999; 189:435–440.

139.

Chevalier

, Didier

, Petitjean

, et al.: Phenotype alterations in regulatory T-cell subsets in primary HIV infection and identification of Tr1-like cells as the main interleukin 10-producing CD4+ T cells. J Infect Dis, 2015; 211:769–779.

140.

Jenabian

, Ancuta

, Gilmore

, Routy

: Regulatory T cells in HIV infection: Can immunotherapy regulate the regulator?. Clin Dev Immunol, 2012; 2012:908314.

141.

Jenabian

, Seddiki

, Yatim

, et al.: Regulatory T cells negatively affect IL-2 production of effector T cells through CD39/adenosine pathway in HIV infection. PLoS Pathog, 2013; 9:e1003319.

142.

Pandiyan

, Zheng

, Ishihara

, Reed

, Lenardo

: CD4+CD25+Foxp3+ regulatory T cells induce cytokine deprivation–mediated apoptosis of effector CD4+ T cells. Nat Immunol, 2007; 8:1353.

143.

Valverde-Villegas

, Matte

, de Medeiros

, Chies

: New insights about Treg and Th17 cells in HIV infection and disease progression. J Immunol Res, 2015; 2015:647916.

144.

Fessler

, Felber

, Duftner

, Dejaco

: The impact of aging on regulatory T-cells. Front Immunol, 2013; 4:231.

145.

Brincks

, Roberts

, Cookenham

, et al.: Antigen-specific memory regulatory CD4+Foxp3+ T cells control memory responses to influenza virus infection. J Immunol, 2013; 190:3438–3446.

146.

Sanchez

, Zhu

, Huang

, Yang

: The development and function of memory regulatory T cells after acute viral infections. J Immunol, 2012; 189:2805–2814.

147.

Lin

P-H

, Wong

W-I

, Wang

Y-L

, et al.: Vaccine-induced antigen-specific regulatory T cells attenuate the antiviral immunity against acute influenza virus infection. Mucosal Immunol, 2018; 11:1239–1253.

148.

van der Geest

, Abdulahad

, Tete

, Lorencetti

, Horst

, Bos

, et al.: Aging disturbs the balance between effector and regulatory CD4+ T cells. Exp Gerontol, 2014; 60:190–196.

149.

Vukmanovic-Stejic

, Sandhu

, Seidel

, et al.: The characterization of varicella zoster virus-specificT cells in skin and blood during aging. J Invest Dermatol, 2015; 135:1752–1762.

150.

Hou

P-F

, Zhu

L-J

, Chen

X-Y

, Qiu

Z-Q

: Age-related changes in CD4+CD25+FOXP3+ regulatory T cells and their relationship with lung cancer. PLoS one, 2017; 12:e0173048.

151.

Rosenkranz

, Weyer

, Tolosa

, et al.: Higher frequency of regulatory T cells in the elderly and increased suppressive activity in neurodegeneration. J Neuroimmunol, 2007; 188:117–127.

152.

Garg

, Delaney

, Toubai

, et al.: Aging is associated with increased regulatory T-cell function. Aging Cell, 2014; 13:441–448.

153.

Gardner

, Mamotte

CDS

, Jackaman

, Nelson

: Modulation of dendritic cell and T cell cross-talk during aging: The potential role of checkpoint inhibitory molecules. Ageing Res Rev, 2017; 38:40–51.

154.

Chevalier

, Weiss

: The split personality of regulatory T cells in HIV infection. Blood, 2013; 121:29–37.

155.

Hubert

, Seddiki

: Regulatory T cells (Tregs): A major immune checkpoint to consider in combinatorial therapeutic HIV-1 vaccines. Hum Vaccin Immunother, 2018; 14:1432–1437.

156.

Ambada

, Ntsama

, Nji

, et al.: Phenotypic characterization of regulatory T cells from antiretroviral-naive HIV-1-infected people. Immunology, 2017; 151:405–416.

157.

Matavele Chissumba

, Namalango

, Maphossa

, et al.: Helios + regulatory T cell frequencies are correlated with control of viral replication and recovery of absolute CD4 T cells counts in early HIV-1 infection. BMC Immunol, 2017; 18:50.

158.

Pion

, Jaramillo-Ruiz

, Martinez

, Munoz-Fernandez

, Correa-Rocha

: HIV infection of human regulatory T cells downregulates Foxp3 expression by increasing DNMT3b levels and DNA methylation in the FOXP3 gene. AIDS, 2013; 27:2019–2029.

159.

Almeida

ARM

, Zaragoza

, Freitas

: Indexation as a novel mechanism of lymphocyte homeostasis: The number of CD4+CD25+ regulatory T cells is indexed to the number of IL-2-producing cells. J Immunol, 2006; 177:192–200.

160.

Sakaguchi

, Yamaguchi

, Nomura

, Ono

: Regulatory T cells and immune tolerance. Cell, 2008; 133:775–787.

161.

Mendez-Lagares

, Jaramillo-Ruiz

, Pion

, et al.: HIV infection deregulates the balance between regulatory T cells and IL-2-producing CD4 T cells by decreasing the expression of the IL-2 receptor in Treg. J Acquir Immune Defic Syndr, 2014; 65:278–282.

162.

Miyara

, Yoshioka

, Kitoh

, et al.: Functional delineation and differentiation dynamics of human CD4+ T cells expressing the FoxP3 transcription factor. Immunity, 2009; 30:899–911.

163.

Kinter

, Horak

, Sion

, et al.: CD25+ regulatory T cells isolated from HIV-infected individuals suppress the cytolytic and nonlytic antiviral activity of HIV-specific CD8+ T cells in vitro. AIDS Res Hum Retroviruses, 2007; 23:438–450.

164.

Nobrega

, Horta

, Coutinho-Teixeira

, et al.: Longitudinal evaluation of regulatory T-cell dynamics on HIV-infected individuals during the first 2 years of therapy. AIDS, 2016; 30:1175–1186.

165.

Shaw

, Hunt

, Critchfield

, et al.: Increased frequency of regulatory T cells accompanies increased immune activation in rectal mucosae of HIV-positive noncontrollers. J Virol, 2011; 85:11422–11434.

166.

Elahi

, Dinges

, Lejarcegui

, et al.: Protective HIV-specific CD8+ T cells evade Treg cell suppression. Nat Med, 2011; 17:989–995.

167.

Legrand

, Nixon

, Loo

, et al.: Strong HIV-1-specific T cell responses in HIV-1-exposed uninfected infants and neonates revealed after regulatory T cell removal. PLoS One, 2006; 1:e102.

168.

Pattacini

, Murnane

, Kahle

, et al.: Differential regulatory T cell activity in HIV type 1-exposed seronegative individuals. AIDS Res Hum Retroviruses, 2013; 29:1321–1329.

169.

Gaardbo

, Ronit

, Hartling

, et al.: Immunoregulatory T cells may be involved in preserving CD4 T cell counts in HIV-infected long-term nonprogressors and controllers. J Acquir Immune Defic Syndr, 2014; 65:10–18.

170.

Hunt

, Landay

, Sinclair

, et al.: A low T regulatory cell response may contribute to both viral control and generalized immune activation in HIV controllers. PLoS One, 2011; 6:e15924.

171.

Brandt

, Benfield

, Mens

, et al.: Low level of regulatory T cells and maintenance of balance between regulatory T cells and TH17 cells in HIV-1-infected elite controllers. J Acquir Immune Defic Syndr, 2011; 57:101–108.

172.

Rios

, Velilla

, Rugeles

: Chronically HIV-1 infected patients exhibit low frequencies of CD25+ regulatory T cells. Open Virol J, 2012; 6:49–58.

173.

Rueda

, Velilla

, Chougnet

, Rugeles

: Incomplete normalization of regulatory t-cell frequency in the gut mucosa of Colombian HIV-infected patients receiving long-term antiretroviral treatment. PLoS One, 2013; 8:e71062.

174.

Group

AGS

: T-cell activation independently associates with immune senescence in HIV-infected recipients of long-term antiretroviral treatment. J Infect Dis, 2016; 214:216–225.

175.

Hileman

, Funderburg

: Inflammation, immune activation, and antiretroviral therapy in HIV. Curr HIV/AIDS Rep, 2017; 14:93–100.

176.

Lim

, Tan

, Price

, et al.: Proportions of circulating T cells with a regulatory cell phenotype increase with HIV-associated immune activation and remain high on antiretroviral therapy. AIDS, 2007; 21:1525–1534.

177.

Piconi

, Trabattoni

, Gori

, et al.: Immune activation, apoptosis, and Treg activity are associated with persistently reduced CD4+ T-cell counts during antiretroviral therapy. AIDS, 2010; 24:1991–2000.

178.

Gaardbo

, Hartling

, Ronit

, et al.: Regulatory T cells in HIV-infected immunological nonresponders are increased in blood but depleted in lymphoid tissue and predict immunological reconstitution. J Acquir Immune Defic Syndr, 2014; 66:349–357.

179.

Zhang

, Robinson

, Duverger

, Kutsch

, Heath

, Cron

: Regulatory CD4 T cells inhibit HIV-1 expression of other CD4 T cell subsets via interactions with cell surface regulatory proteins. Virology, 2018; 516:21–29.

180.

Selliah

, Zhang

, White

, et al.: FOXP3 inhibits HIV-1 infection of CD4 T-cells via inhibition of LTR transcriptional activity. Virology, 2008; 381:161–167.

181.

Weiss

, Piketty

, Assoumou

, et al.: Relationship between regulatory T cells and immune activation in human immunodeficiency virus-infected patients interrupting antiretroviral therapy. PLoS One, 2010; 5:e11659.

182.

Andersson

, Boasso

, Nilsson

, et al.: Cutting edge: The prevalence of regulatory T cells in lymphoid tissue is correlated with viral load in HIV-infected patients. J Immunol, 2005; 174:3143–3147.

183.

Apoil

, Puissant

, Roubinet

, Abbal

, Massip

, Blancher

: FOXP3 mRNA levels are decreased in peripheral blood CD4+ lymphocytes from HIV-positive patients. J Acquir Immun Defic Syndr, 2005; 39:381–385.

184.

Epple

H-J

, Loddenkemper

, Kunkel

, et al.: Mucosal but not peripheral FOXP3+ regulatory T cells are highly increased in untreated HIV infection and normalize after suppressive HAART. Blood, 2006; 108:3072–3078.

185.

Kinter

, Hennessey

, Bell

, et al.: CD25(+)CD4(+) regulatory T cells from the peripheral blood of asymptomatic HIV-infected individuals regulate CD4(+) and CD8(+) HIV-specific T cell immune responses in vitro and are associated with favorable clinical markers of disease status. J Exp Med, 2004; 200:331–343.

186.

Nilsson

, Boasso

, Velilla

, et al.: HIV-1-driven regulatory T-cell accumulation in lymphoid tissues is associated with disease progression in HIV/AIDS. Blood, 2006; 108:3808–3817.

187.

Tsunemi

, Iwasaki

, Imado

, et al.: Relationship of CD4+CD25+ regulatory T cells to immune status in HIV-infected patients. AIDS, 2005; 19:879–886.

188.

Allers

, Loddenkemper

, Hofmann

, et al.: Gut mucosal FOXP3+ regulatory CD4+ T cells and nonregulatory CD4+ T cells are differentially affected by simian immunodeficiency virus infection in Rhesus macaques . J Virol, 2010; 84:3259–3269.

189.

Horta

, Nobrega

, Amorim-Machado

, et al.: Poor immune reconstitution in HIV-infected patients associates with high percentage of regulatory CD4+ T cells. PLoS One, 2013; 8:e57336.

190.

Marziali

, De Santis

, Carello

, et al.: T-cell homeostasis alteration in HIV-1 infected subjects with low CD4 T-cell count despite undetectable virus load during HAART. AIDS, 2006; 20:2033–2041.

191.

Mendez-Lagares

, Pozo-Balado

, Genebat

, Garcia Perganeda

, Leal

, Pacheco

: Severe immune dysregulation affects CD4(+)CD25(hi)FoxP3(+) regulatory T cells in HIV-infected patients with low-level CD4 T-cell repopulation despite suppressive highly active antiretroviral therapy. J Infect Dis, 2012; 205:1501–1509.

192.

Saison

, Demaret

, Venet

, et al.: CD4+CD25+CD127− assessment as a surrogate phenotype for FOXP3+ regulatory T cells in HIV-1 infected viremic and aviremic subjects. Cytometry B Clin Cytom, 2013; 84:50–54.

193.

Gasteiger

, Hemmers

, Bos

, Sun

, Rudensky

: IL-2–dependent adaptive control of NK cell homeostasis. J Exp Med, 2013; 210:1179.

194.

Ghiringhelli

, Ménard

, Terme

, Flament

, Taieb

, Chaput

, et al.: CD4+ CD25+ regulatory T cells inhibit natural killer cell functions in a transforming growth factor–β–dependent manner. J Exp Med, 2005; 202:1075–1085.

195.

Romagnani

, Della Chiesa

, Kohler

, Moewes

, Radbruch

, Moretta

, et al.: Activation of human NK cells by plasmacytoid dendritic cells and its modulation by CD4+ T helper cells and CD4+ CD25hi T regulatory cells. Euro J Immunol, 2005; 35:2452–2458.

196.

Trzonkowski

, Szmit

, Myśliwska

, Dobyszuk

, Myśliwski

: CD4+CD25+ T regulatory cells inhibit cytotoxic activity of T CD8+ and NK lymphocytes in the direct cell-to-cell interaction. Clin Immunol, 2004; 112:258–267.

197.

Strategies for Management of Antiretroviral Therapy (SMART) Study Group, Lundgren

, Babiker

, El-Sadr

, et al.: Inferior clinical outcome of the CD4+ cell count-guided antiretroviral treatment interruption strategy in the SMART study: Role of CD4+ cell counts and HIV RNA levels during follow-up. J Infect Dis, 2008; 197:1145–1155.