The Role of Virus Infection in Deregulating the Cytokine Response to Secondary Bacterial Infection

Introduction

Infection of the host by multiple pathogens is known to occur with increased incidences of microbial infections arising during an ongoing viral infection (Sethi 2002; McCullers and Bartmess 2003; Griffiths and others 2011). This phenomenon may be linked to the ability of the primary viral infection to impact the immune response to a secondary infection (McNamee and Harmsen 2006). This is typically seen when a viral infection, such as human immunodeficiency virus (HIV), directly depletes the helper CD4⁺ T-cell population, thus increasing the susceptibility to opportunistic bacterial infections or even to other secondary viral infections (Nicholson and others 1985; McDougal and others 1986; Prince and others 1986; Flórido and others 2013; Santos and others 2013).

Disease symptoms associated with either viral or bacterial infection alone are usually less severe than the symptoms observed during concomitant infection (McCullers and Rehg 2002; Alonso and others 2003; McCullers and Bartmess 2003; Okamoto and others 2003). Dual infections may result in a potentially lethal disease with an eventual systemic inflammatory response leading to sepsis (McCullers and Rehg 2002; Alonso and others 2003; McCullers and Bartmess 2003; Okamoto and others 2003). Consequently, there is a need to understand the immunological and pathological causes of the lethal synergism associated with concomitant viral and bacterial infections, which may help in developing novel immunotherapeutics. The following sections are organized according to the different suppressive mechanisms where diverse viruses can influence the ability of the host to respond to secondary bacterial infections. The deregulation of the cytokine networks by viral infection represents a major part of this section.

Role of Proinflammatory Cytokines in the Immune Response to Pathogens

Inflammation is part of the nonspecific immune response to pathogens (Ferrero‐Miliani and others 2007), which is mediated by proinflammatory cytokines (Dinarello 2000). Proinflammatory cytokines are produced by activated immune cells such as macrophages and dendritic cells (DCs) and are needed to stimulate cells of the adaptive immunity such as the T helper (Th) cells, which in turn produce more proinflammatory cytokines (Parker 2006). For example, interleukin (IL)-23 and IL-12 are proinflammatory cytokines, which belong to the IL-12 family (Gee and others 2009), and are secreted by activated macrophages and DCs (Oppmann and others 2000). Several reports suggest a role for IL-23 in promoting the inflammatory response to certain infections such as Listeria monocytogenes (Happel and others 2005; Kohyama and others 2007; Meeks and others 2009), by supporting the differentiation of Th17 cells leading to the expression of the key proinflammatory cytokine IL-17 (McGeachy and others 2009; Sutton and others 2009).

Other cytokines produced by macrophages and DCs such as interferon (IFN)-γ, IL-18, and tumor necrosis factor (TNF)-α promote many functions critical for clearance of bacterial infections (Skoskiewicz and others 1985; Micallef and others 1996; Standiford and others 1999). For example, IFN-γ upregulates macrophage MHC class II expression, thus promoting antigen presentation and interaction with CD4⁺ T cells at sites of inflammation (Skoskiewicz and others 1985). On the other hand, IL-18 synergistically enhances IL-12-induced IFN-γ production by CD4⁺ T cell (Micallef and others 1996). In addition, IL-18 enhances proliferation and cytotoxicity of T and NK cells and stimulates them to produce proinflammatory cytokines, including TNF-α, IL-1, IL-2, IL-6, and granulocyte–macrophage colony-stimulating factor (Micallef and others 1996; Kohno and others 1997; Puren and others 1998; Takeda and others 1998). Further in vivo studies have shown the importance of IL-18 in the protective immune response to a number of bacterial infections, including Salmonella, Yersinia, Chlamydiae, and Shigella (Bohn and others 1998; Mastroeni and others 1999; Lu and others 2000; Sansonetti and others 2000).

Similarly, TNF-α is an integral component of effective innate immunity and has been reported to provide antimicrobial resistance in many studies, for instance, by increasing the neutrophil count during a bacterial infection (Takashima and others 1997; Standiford and others 1999; Kielian and others 2004). Specifically, TNF-α has been shown to activate macrophage phagocytosis and microbicidal activity (Nacy and others 1991; Michlewska and others 2009). Therefore, proinflammatory cytokines play an indispensable role for the clearance of microbial infections.

Because proinflammatory cytokines are important for microbial clearance, their deregulation can significantly impact the effectiveness of the immune response to secondary bacterial infections (summarized in Table 1 and Fig. 1). In the following sections, we will discuss 3 well-studied viral models in the mouse and human fields, lymphocytic choriomeningitis virus (LCMV), influenza A virus (IAV), and HIV, which have been used to conduct viral/bacterial coinfection studies.

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FIG. 1.

Deregulated immune responses during viral/bacterial coinfection. In a bacterial infection alone (top part), the immune system, particularly antigen-presenting cells, responds to the bacterial infection with an efficient production of proinflammatory cytokines. The induction of an inflammatory response leads to clearance of the bacterial infection as described in the review. Conversely, if viral infection occurs before the bacterial exposure (bottom part), the host becomes more susceptible to the secondary bacterial infection due to a deregulated immune response. Depending on what cytokines become affected, which may be dependent on the type of viral infection, an inefficient bacterial clearance can be the ultimate outcome.

LCMV is a natural mouse pathogen (Khanolkar and others 2002), which has proved to be a robust model to study host–pathogen interactions in the mouse model (Khanolkar and others 2002; Lapošová and others 2012). LCMV can also be associated with human disease in immunosuppressed individuals such as organ transplant patients (Fischer and others 2006) and the course of disease resembles viral hemorrhagic fever caused by the highly pathogenic Lassa virus (de La Torre 2009). Although wild house mice are the natural reservoir for the virus, hamsters and other pet rodents can acquire the virus through exposure to infected mice and become a prevalent source of human exposure (Emonet and others 2007).

IAV causes epidemic disease in humans on an annual basis (McCullers 2006), and clinical data have shown that the majority of deaths following IAV infection are due to secondary bacterial pneumonia superinfection during influenza pandemics (McCullers 2006). Therefore, it is important to understand the mechanism for the increased susceptibility to secondary bacterial infection associated with influenza infections. Another virus model commonly associated with viral/bacterial coinfection is HIV. Since HIV infects CD4⁺ T cells, monocytes, and macrophages, it interferes with the key immune responses (Gartner and others 1986), which can facilitate a secondary opportunistic infection (Pasman 2012; Chang and others 2013).

Type I IFN Responses in LCMV Coinfection Models

Studies using LCMV have uncovered many key concepts in viral immunology, which were consequently extended to other models of human viruses (Zhou and others 2012), including its ability to effect the production of type I IFN (Biron 1998; Malmgaard and others 2002; Navarini and others 2006). Type I IFNs are the key cytokines essential for the antiviral immune response as they induce an antiviral state in target cells by limiting virus replication (Samuel 1991; Moltedo and others 2011). All type I IFNs bind to the IFN receptor (IFNAR) that consists of IFNAR1 and IFNAR2 chains (Uze and others 2007). Type I IFN binding to its receptor complex results in the activation of signaling pathways, which induces and activates intrinsic antiviral factors such as RNA-activated protein kinase, the 2-5A system, and the Mx proteins (Jacobs and Langland 1996; Samuel 1998). Furthermore, by acting directly or indirectly on NK cells, T cells, B cells, DCs, and phagocytic cells, IFNs effectively regulate both innate and adaptive immune responses (Prchal and others 2009).

Viral-induced type I IFN can make the host susceptible to secondary bacterial infections. For instance, LCMV-induced type I IFNs (Biron 1998; Malmgaard and others 2002; Navarini and others 2006) can cause apoptosis in bone marrow granulocytes (Navarini and others 2006), important for the elimination of bacterial infections (Xu and others 2008). The outcome in this case would be reduced granulocyte infiltrates at the site of bacterial infection, thus enhancing susceptibility to subsequent bacterial superinfection (Navarini and others 2006).

The role of type I IFN in causing apoptosis in bone marrow granulocytes was observed by employing IFNAR^−/− mice infected with a high dose of LCMV-WE followed 2 days later by a coinfection with L. monocytogenes. In this study, wild-type mice exhibited 1,000-fold higher bacterial titers than IFNAR^−/− mice in the spleen, and ∼20-fold higher titers in the liver. This phenomenon was further linked to lower granulocyte infiltration in LCMV-infected wild-type mice compared to IFNAR^−/− mice (Navarini and others 2006). These findings suggest that virus-induced granulocytopenia due to type I IFN induction may critically contribute to bacterial coinfections (Table 1) (Navarini and others 2006). Understanding the kinetics of the virus infection is critical in this model, because type I IFNs are typically produced in high amounts early after viral infection (Lee and others 2009). Thus, the risk for a superinfection may be higher when the host is exposed to the bacteria soon after the viral infection has ensued.

Influenza-Induced Type I IFN Deregulates Immune Responses Against Bacterial Infections

In another infection model of IAV, the influence of type I IFN on pulmonary host defense against bacteria was studied. Shahangian and his colleagues established a model of sequential IAV and pneumococcal lung infection using IFNAR^−/− mice. The IFNAR^−/− mice were infected with influenza (PR8 strain) and challenged with Streptococcus pneumoniae (Shahangian and others 2009). They found that bacterial clearance in IFNAR^−/− mice increased 4-fold compared to wild-type mice. The less efficient bacterial clearance in wild-type IAV-infected mice was attributed to impaired production of neutrophil chemoattractants such as macrophage inflammatory protein (MIP2 or CXCL2) and keratinocyte chemoattractant (KC or IL-8) from alveolar macrophages (AMs) following secondary challenge with S. pneumoniae (Shahangian and others 2009). As a result, an inadequate neutrophil response during the early phase of host defense against secondary bacterial infection was evident (Shahangian and others 2009). This defect was overcome by administration of exogenous Mip2-α and KC, which helped in appropriate neutrophil recruitment during the bacterial infection, whereas blocking CXCR2 (common receptor for Mip2 and KC) rendered the mice more susceptible to the bacterial infection in IFNAR^−/− mice, confirming that type I IFN-mediated suppression of KC and Mip2 was the major mechanism underlying the enhanced susceptibility of wild-type mice to secondary bacterial infection (Shahangian and others 2009).

More recently, the protein lysine methyltransferase Setdb2 was identified as an IFN-stimulating gene induced during influenza infection, which repressed expression of the gene encoding the neutrophil attractant CXCL1 and other genes that are targets of the transcription factor nuclear factor-κB (NF-κB) (Schliehe and others 2014). Mice with a gene deficiency in Setdb2, infected with influenza followed by S. penumoniae 5 days later, demonstrated increased CXCL1, increased neutrophil recruitment, and lower bacterial burden compared to wild-type mice (Schliehe and others 2014). These data suggest that type I IFN produced after IAV infection can weaken innate immune responses against secondary bacterial challenge by impairing chemotaxis of neutrophils.

A similar function for type I IFN was reported in a mouse model of upper respiratory tract pneumococcal colonization. In this infection model, the synergistic increase in IFN-β expression during coinfection was associated with an inhibition of chemokine C-C motif ligand-2 that promotes recruitment of macrophages, but not neutrophils, to the infected upper respiratory tract (Nakamura and others 2011). Nakamura and others (2011) reported that the dominant pathway for pneumococcal induction of IFN-β required sensing by the pattern recognition receptor Nod2-dependent pathway. The increased colonization associated with concurrent influenza virus infection was not observed in mice lacking Nod2 or the type I IFN receptor (Nakamura and others 2011). Therefore, the Nod2-dependent pathway activated by bacterial infection is involved in the synergistic increase in IFN-β expression during concurrent viral infection.

A novel mechanism that virus-induced type I IFN mediates inhibition of Th17 pathway was reported by Kudva and others (2011). It has been shown by several groups (Ye and others 2001; Kohyama and others 2007; Ma and others 2008; O'Quinn and others 2008) that Th17 cells have an important role in host defense against viral and bacterial infections as they help in neutrophil recruitment to infected tissues by production of IL-17 (Jeffrey and others 2007; Pelletier and others 2010; Griffin and others 2012). An important role of type I IFN in inhibiting neutrophil recruitment through suppressing the Th17 pathway has been reported.

The Th17-induced cytokines—IL-17, IL-22, and IL-23—were observed to be decreased following coinfection with influenza virus and Staphylococcus aureus. This decrease in IL-17, IL-22, and IL-23 was linked to the preexisting IAV-induced type I IFN suppressing production of IL-23 by DCs. The suppression of IL-23 is critical in this model as IL-23 activates STAT3 for Th17 differentiation and production of the proinflammatory cytokines (IL-17, IL-21, and IL-22) by Th17 cells (Kudva and others 2011). Furthermore, overexpression of IL-23 during influenza led to a markedly improved bacterial clearance (Kudva and others 2011). Similarly, Ivanovand and others (2013) recently reported that IL-22-deficient mice were significantly more susceptible to pneumococcal infection following influenza. These findings indicate a novel mechanism by which IAV-induced type I IFN inhibits Th17 immunity through decreased IL-23 production, ultimately resulting in increased susceptibility to secondary bacterial pneumonia.

It has been found that in other infection models, IL-17-producing γδ T cells can be particularly suppressed by type I IFN (Henry and others 2010), and indeed, this has been reported to occur during secondary pneumococcal infections following influenza (Li and others 2012). In the latter study, the mice were challenged intranasally with S. pneumoniae after primary aerosol sublethal dose of IAV (Li and others 2012). Those mice that were coinfected with IAV and bacteria showed higher mortality and increased S. pneumoniae replication in their lungs compared to mice infected with bacteria alone (Li and others 2012). This high mortality rate in coinfected mice was linked to viral induction of type I IFNs (Li and others 2012). It is interesting that γδ T cells were the primary source of the rapid production of IL-17 in the coinfection model and it is likely that type I IFNs can inhibit IL-17-mediated neutrophil recruitment, possibly by suppressing γδ T cells. These findings highlight the important role of Th17 in protective antimicrobial immune responses, which may be altered due to production of type I IFN after primary viral infections.

In summary, it appears that although LCMV and influenza can downregulate the ability of neutrophils to clear a bacterial infection through type I IFN induction, their mechanisms of action are distinct. LCMV-induced type I IFN causes apoptosis of bone marrow granulocytes (Navarini and others 2006), which include the largest reserve of neutrophil granulocytes (Babior and Golde 2001). On the other hand, influenza-induced type I IFN inhibits the production of neutrophil chemoattractants leading to attenuated neutrophil responses during secondary pneumococcal pneumonia (Shahangian and others 2009). Influenza virus-induced type I IFN can also affect IL-17-mediated neutrophil recruitment possibly by suppressing Th17 and γδ T cells. The inconsistencies in outcomes from these various laboratories could be due to numerous factors. This can include alterations in the days intervened between initial virus infection and/or secondary bacterial challenge, differences in doses of virus and bacteria used, and disparities in the combinations of virus and bacterial strains studied. The effect of virus infection is not only limited to deregulating function of type I IFN but also includes deregulating key immune cells and proinflammatory cytokines.

Mechanisms of Suppression of Cytokines in Influenza/Bacterial Coinfection

In other infection models, it has been shown that prior virus infection can also lead to suppression of DC, natural killer, and Th17 cell cytokine induction. While investigating concurrent pulmonary infections with IAV and mycobacteria in vivo, it was noted that coinfected mice exhibited a decreased MHC class II and class I expression on DCs compared to mice infected only with mycobacteria. As a result of the reduced MHC expression, the authors found less activation of mycobacteria-specific CD4⁺ and CD8⁺ T cells. Coinfection also resulted in a decreased magnitude of BCG-specific CD4 and CD8 T-cell IFN-γ-secreting responses, resulting in the impaired clearance of mycobacteria in the coinfected mice (Flórido and others 2013). This is not surprising since DCs play a major role in regulating the adaptive immune response, and therefore, the reduced MHC expression on DCs will eventually influence the efficiency, by which the adaptive immune system clears secondary bacterial infections.

The effects of concurrent or successive IAV/pneumococcal coinfection on cytokine production by human monocyte-derived DCs were studied using live IAV and heat-killed pneumococcus (Wu and others 2011). The authors of the study reported that successive challenge of influenza virus and pneumococcus on DCs generally promoted a synergistic inflammatory response, but the different time intervals between the challenges of the 2 pathogens was a critical factor (Wu and others 2011). However, the interpretation of this data should be evaluated with care as the DC cytokine responses against live or heat-killed S. pneumoniae can differ significantly with distinct patterns of Th1- and Th17-associated cytokines in response to heat-killed and live pneumococci when cocultured with CD4⁺ T cells (Olliver and others 2011).

Functionality of NK cells is also affected by primary virus infection. It has been demonstrated that IAV infection in a murine model predisposed the host to S. aureus superinfection in the lung by impairing NK cell responses (Small and others 2010). The mechanism involved an IAV-induced decreased production of TNF-α from NK cells, which led to reduced phagocytosis of S. aureus by AMs compared to naive wild-type NK cells (Small and others 2010). When compared to naive NK cells, adoptive transfer of TNF-α-deficient NK cells to the airway of flu-infected mice failed to restore flu-impaired antibacterial host defense. In contrast to WT naive NK cells, NK cells from flu-infected lung or TNF-α-deficient mice failed to enhance antibacterial activities of AMs against S. aureus (Small and others 2010). Conclusively, primary IAV infection reduces antistaphylococcal host defense by inhibiting NK cell-induced TNF-α production and crippling NK cell ability to stimulate antibacterial activities of AMs.

Didierlaurent and others (2008) reported suppression of neutrophil recruitment in response to bacterial infection weeks after the resolution of influenza infection. Mice infected with S. pneumoniae after 2–6 weeks of influenza infection, resulted in lower numbers of neutrophils recruited to the airways (Didierlaurent and others 2008). This decrease in the number of neutrophils was linked to long-term desensitization of lung AMs to the bacterial Toll-like receptors (TLR) flagellin, lipopolysaccharides (LPS), and lipoteichoic acid (Didierlaurent and others 2008). In addition, AMs isolated after the resolution of lung influenza infection had impaired nuclear translocation of the p65 subunit of NF-κB upon TLR-5 (flagellin) ligation (Didierlaurent and others 2008).

In a more recent study, it was shown that prior influenza infection does not inhibit S. aureus binding and uptake by phagocytic cells. Rather, it inhibits S. aureus-induced Th17-associated antimicrobial peptides (AMPs) necessary for bacterial clearance in the lung (Robinson and others 2013b). Importantly, exogenous lipocalin 2 (AMP) improved S. aureus clearance in mice coinfected with S. aureus and influenza (Robinson and others 2013b). These findings indicate a novel mechanism by which influenza A inhibits Th17 immunity and increases susceptibility to secondary bacterial pneumonia.

In addition, a study has further shown that IL-1β (influences polarization of Th17 cells) was inhibited during IAV/S. aureus coinfection. This led to attenuation of Th17 immune functions and increased susceptibility to secondary bacterial infection (Robinson and others 2013a). Remarkably, in this coinfection model, prior influenza infection did not attenuate S. aureus-induced inflammasome activation (Robinson and others 2013a). Instead, an early suppression of NF-κB activation was observed, suggesting an inhibition of NF-κB-dependent transcription of pro-IL-1β (Robinson and others 2013a). These data support a mechanism by which preceding influenza A infection attenuates S. aureus-induced NF-κB activation, leading to inhibition of IL-1β production and subsequent attenuation of the Th17 response (Robinson and others 2013a).

To further elucidate the mechanisms involved in the inhibition of Th17 responses in influenza, this group extended their study to investigate known cytokines involved in Th17 regulation such as IL-27. IL-27 is known to attenuate Th17 differentiation through STAT-1 and has been shown to play a critical role in viral and bacterial coinfection (Robinson and others 2015). IL-27 receptor α-knockout mice coinfected with influenza and S. aureus had improved bacterial clearance compared to wild-type controls (Robinson and others 2015). However, there was little impact of IL-27 signaling on host defense against S. aureus in the absence of preceding influenza (Robinson and others 2015).

Significant increased Th17 responses and decreased IL-10 production in IL-27 receptor α-knockout mice were also observed (Robinson and others 2015). IL-10 is known to contribute to enhanced susceptibility to secondary bacterial pneumonia (van der Sluijs and others 2004). Therefore, IL-27 induced during influenza infection plays a role in the exacerbation of S. aureus infection by suppressing Th17 immunity and inducing IL-10 production (Robinson and others 2015). These studies indicate the role of primary virus infection in inhibiting Th17 immunity as a mechanism of predisposing host to secondary bacterial coinfections. Aside from the Th17 lineage, there exist other CD4⁺ T helper cell types that are regulated by virus/bacterial coinfection.

Depletion of CD4⁺ T-Cell Populations by HIV Results in a Shift in T-Cell-Regulated Cytokine Production

HIV-seropositive patients have been shown to acquire secondary infections such as Mycobacterium tuberculosis, Mycobacterium avium, and S. pneumoniae (Källenius and others 1992; Archibald and others 1998) due to the near loss of CD4⁺ T-cell populations (Imami and others 1999; Geldmacher and others 2012; Doitsh and others 2014) and associated cytokine network deregulation (Keating and others 2012). The lethal combination of HIV and M. tuberculosis has been thoroughly reviewed by Shankar and others (2013). The following sections will focus on the impact on cytokine production and regulation in this detrimental disease combination.

As a result of HIV infection, HIV-positive patients' IFN-γ levels were observed to be lower compared to healthy controls, but this defect in cytokine production was significantly improved by applying highly active antiretroviral therapy (HAART) (Hodsdon and others 2001; Yao and others 2013). Specifically, HAART has been shown to restore CD4⁺ (total), CD4⁺ CD45RA⁺ CD62L⁺ (naive), and CD4⁺ CD45RO⁺ (memory) helper T cells, which was positively correlated with increased plasma IL-12, the cytokine required for IFN-γ production, as well as IFN-γ production in CD4⁺ T cells to aid in disease control (Yao and others 2013). It is important to note that, in coinfected individuals with secondary bacterial infections such as M. tuberculosis, M. avium, and S. pneumoniae, there is a consistent decrease in IFN-γ production (Sousa and others 1998; Elliott and others 1999; Bal and others 2005).

Indeed, many studies have shown that the production of the Th1-mediated cytokine IFN-γ is significantly impaired in HIV/bacterial coinfection, which may contribute to enhanced disease progression (Hodsdon and others 2001; Schluger and others 2002; Bal and others 2005; Tadokera and others 2011; Buldeo and others 2012; Yao and others 2013). The downregulation of IFN-γ in coinfection has been attributed to the elimination of CD4⁺ T cells as Th1 cells are the major source of IFN-γ (Hodsdon and others 2001). Th1 responses are vital to clearance of bacterial infection as IFN-γ stimulates macrophages to produce reactive oxygen species, which are involved in the clearance of M. tuberculosis (Skinner and others 1997); the decrease in IFN-γ production then leads to diminished immunity to coinfection as well as progression to a more robust tuberculosis infection (Elliott and others 1999; Buldeo and others 2012).

Recently, Quiroga and others (2012) found that there is a significant increase in CD25⁺/FoxP3⁺ PBMC, also known as regulatory T cells (Treg), in HIV/M. tuberculosis coinfection patients compared to HIV alone or healthy donors. Treg cells typically produce anti-inflammatory cytokine IL-10 to downregulate CD4⁺ T-cell effector functions to regulate immune functions. The increased numbers of Treg cells in coinfected individuals would suggest an anti-inflammatory mechanism, further inhibiting Th1-mediated IFN-γ production. The shift in the helper T-cell balance changes the proinflammatory cytokine production in response to HIV/M. tuberculosis.

Proinflammatory cytokines such as TNF-α and IL-2 are inhibited during HIV/bacterial coinfection. To exemplify the effect of coinfection, first, HIV-negative/M. tuberculosis had no significant differences in TNF-α and IL-2 production in mitogen-stimulated PBMC supernatants (Bal and others 2005). Meanwhile, in ex vivo HIV/M. tuberculosis-coinfected PBMC or in vitro HIV-infected U1 macrophages infected with M. tuberculosis, there was a significant suppression of TNF-α production as well as a reduction in Th1 proinflammatory cytokine IL-2, compared to M. tuberculosis infection alone (Bal and others 2005; Patel and others 2007; Anandaiah and others 2013).

Decreased TNF-α production may be attributed to decreased M. tuberculosis-induced TLR signaling, as reduced IL-1 receptor-associated kinase-1 (IRAK-1) phosphorylation and NF-κB nuclear translocation were observed upon M. tuberculosis infection in HIV-positive U1 macrophages compared to HIV-negative cells (Patel and others 2007). Furthermore, M. tuberculosis-specific IL-2-secreting CD4⁺ T cells collected from coinfected patients showed that IL-2 production is inversely proportional with HIV viral load (Day and others 2008); this suggests that with an increased HIV viral load and, in turn, fewer TNF-α- or IL-2-secreting CD4⁺ T cells to fight infection, the immune system would be more susceptible to secondary bacterial infections.

Interestingly, M. tuberculosis enhances HIV replication in PBMC indicating a possible role for M. tuberculosis-induced cytokines in upregulating HIV viral replication (Goletti and others 1998). This study showed that upon inhibition of endogenous TNF-α or IL-2 during M. tuberculosis infection using soluble TNF-α or IL-2 receptor antagonists or by the addition of exogenous IL-10 and transforming growth factor (TGF)-β, there was also a significant decrease in HIV replication measured by the reverse transcriptase activity (Goletti and others 1998).

Moreover, anti-TNF-α antibodies have been shown to reduce LPS-induced expression of CXCR4 and CCR5 HIV coreceptors in CD4⁺ T cells and also attenuated HIV infectivity due to increases in CCR5 ligand chemokines, RANTES and MIP-1β (Juffermans and others 2000). In addition, HIV-infected-monocyte-derived macrophages (MDM) treated with TNF-α has been documented to increase HIV-1 expression and intensify M. tuberculosis growth compared to the bacterial growth in uninfected MDMs (Imperiali and others 2001). Overall, proinflammatory cytokines such as TNF-α, IL-2, and IFN-γ play a role in the viral and/or bacterial replication in coinfection models, signifying a possible mechanism for the lethal synergism associated with concomitant HIV/M. tuberculosis through the dysregulation of these cytokines upon primary infection with HIV.

Conclusion and Future Directions

Viral and bacterial coinfection has been affiliated with numerous diseases, particularly with infections of the upper respiratory tract, like pneumonia, which are of pressing human concern. This review showed how preexisting virus infection deregulates cytokine and immune cell function predisposing the host to secondary bacterial infections with key findings summarized in Table 1. As shown in Fig. 1, the inhibition of secondary bacterial clearance can be due to virus-induced suppression of cytokine or chemokine production by antigen-presenting cells such as DCs or macrophages, as well as helper T cells. Several research groups have particularly identified a key role of virus-induced type I IFN in increasing susceptibility to secondary bacterial infections. Virus-induced type I IFN has been shown to play a widespread role in suppressing the function of important immune cells such as granulocytes, neutrophils, macrophages, DCs, and γδ T and Th17 cells.

Although the impact of the virus infection in deregulating immunity against bacterial infections appears clear, not all the molecular mechanisms accounting for this are well-defined. Continued research should elucidate the intracellular signaling pathways/molecules involved in cytokine inhibition and the cell types which are critically affected during this suppression. It also appears that the harmful impact of virus infection on the host response to an established bacterial infection might be only apparent, clinically, a long period after coinfection ensues.

The present studies highlight the need for further investigation into the interactions between host and virus together with bacterial infections in both experimental and clinical settings. The identification of the mechanisms of how a virus alters the host defense may lead to novel hot spots for therapeutic immune targets. Further studies will continue to fill in the gaps in pathways used by viruses to modify the immune system, impacting the host's ability to control and clear secondary bacterial infections.