The Function of T Cell Immunity in Lymphedema: A Comprehensive Review

Abstract

Lymphedema is a debilitating disease characterized by extremity edema, fibroadipose deposition, impaired lymphangiogenesis, and dysfunctional lymphatics, often with lymphatic injury secondary to the treatment of malignancies. Emerging evidence has shown that immune dysfunction regulated by T cells plays a pivotal role in development of lymphedema. Specifically, Th1, Th2, Treg, and Th17 cells have been identified as critical regulators of pathological changes in lymphedema. In this review, our aim is to provide an overview of the current understanding of the roles of CD4+ T cells, including Th1, Th2, Treg, and Th17 subsets, in the progression of lymphedema and to discuss associated therapies targeting T cell inflammation for management of lymphedema.

Introduction

Lymphedema, a progressive and debilitating disease characterized by swelling of extremities and fat deposition, can arise from congenital anomalies, infections, or lymphatic injury due to tumor treatment, such as lymph node dissection or radiation therapy.^1–4 Current treatments for lymphedema encompass complete decongestive therapy,^5,6 lymphovenous anastomosis,⁷ and lymph node transfer.⁸ However, the efficacy of these treatments is often limited by the severity and complexity of lymphedema, which is rooted in the unknown pathogenetic mechanism.

Through detailed examination of samples from animal models and human lymphedematous patients, it has been revealed that lymphedema progression is associated with inflammation, epidermal thickening, lipid accumulation, fibrosis, and lymphatic dysfunction.^9–11 However, the high interstitial protein concentration resulting from lymph stasis does not directly lead to tissue fibrosis and inflammation,¹² suggesting that lymph stasis may only be the initial trigger in the progression of lymphedema, which likely involves multiple steps.

The T cell-mediated response has been reported to be the hallmark and core feature of inflammation in lymphedema, which actively regulates the development of fibrosis, lymphangiogenesis, and lymphatic dysfunction.^13–16 Intriguingly, different subtypes of T cells, including Th1 cells, Th2 cells, Tregs, and Th17 cells, have shown distinct, but combined, effects in lymphedema. In this comprehensive review, our aim is to elucidate the role of T cells in this prevalent condition caused by lymphatic dysfunction.

We will discuss the function of various T cell subtypes in lymphedema development and how each cell line influences tissue inflammation, lymphangiogenesis, tissue fibrosis, and lymphatic function. Through this review, we hope to provide new insights for future studies to advance our understanding and potentially develop curative interventions for lymphedema.

CD4+ T Cells

Lymph injury activates dendritic cells and initiates CD4+ T cell inflammation

CD4+ T cells not only serve as bystanders but also play a crucial role in secondary lymphedema. Previous studies have utilized mouse tail models¹⁷ or axillary lymph node dissection (ALND)/popliteal lymph node dissection (PLND) models,¹⁸ which are well-established animal models of lymphedema. Flow cytometry results of mouse tail models showed a striking increase in CD4+ T cells (7.5 and 38 times) in lymphedematous tissue at 3 and 6 weeks after tail excision compared with controls.¹⁹ Similarly, ALND also resulted in CD4+ T cell inflammation with an 11- and 30-fold increase at 3 and 6 weeks postoperatively, respectively.¹⁹

Furthermore, samples from the breast cancer-related lymphedema (BCRL) arms exhibited a 2.6-fold increase in CD4+ T cells compared with the control arms. The augmentation in CD4+ T cells was in line with tissue edema in mouse models¹⁹ and the severity of human lymphedema. These findings provide evidence that lymphatic injury triggers an inflammatory response characterized by activation of CD4+ T cells.

How can lymph stasis galvanize CD4+ T cells? This problem was addressed with studies focusing on transferring dendritic cells (DCs, CD45.1+CD11c+ cells) to CD45.2+ CD4KO mice that had gone through PLND.¹⁵ This study showed that circulating DCs were rapidly gratified to where lymph injury occurred and then were transported to lymph nodes to further stimulate CD4+ T cells.

One remaining puzzle here is the antigen or molecules that attract DCs to the lymphatic injury site, the solution to which might be of high importance to the field of lymphedema.

CD4+ T cells are necessary for lymphedema

Abnormal storage of interstitial proteins induced by lymph stasis has been identified as the initial step in chronic lymphedema.¹² Subsequent studies in rodent lymphedema models using nude mice (with natural T cell deficiency), CD4KO mice (with CD4 depletion), and rodents with antibody-caused CD4 depletion have demonstrated the indispensable role of CD4+ T cells.

Nude mice and CD4KO mice experienced significantly dwindled tail volume and subcutaneous thickness in the mouse tail model.¹⁹ Similarly, depletion of CD4 cells using antibodies after lymphatic stasis contributed to less inflammation, minor subcutaneous edema, and reduced fat deposition.¹⁴ CD4+ T cells acted as a critical regulator for tissue inflammation since CD4+ T cell depletion downregulated the number of CD4+ T cells in mouse tail models with lower levels of CD45+ leukocytes and macrophages.¹⁴

Furthermore, the fact that immunosuppressive patients still develop lymphedema despite having lower levels of CD4+ T cells further underpins the theory that a small number of CD4+ T cells could induce secondary lymphedema.²⁰ These proofs reinforce the essential role of CD4+ T cells in lymphedema.

CD4+ T cells are significant for fibrosis

Fibrosis, characterized by accumulation of the extracellular matrix (ECM),²¹ plays a central role in development of lymphedema. CD4+ T cells have been implicated in fibrosis in various organs, including the liver,²² lungs,²³ and skin.²⁴ ECM deposition and activation of the SMAD-TGF-β pathway (SMAD, homologues of the Drosophila protein, mothers against decapentaplegic and the Caenorhabditis elegans protein; TGF-β, transforming growth factor-beta) have been observed in both murine and human lymphedema.^17,25 Fibrosis consequently results in impaired lymphatic function.^25–27

Depletion of CD4 using antibodies significantly decreased the fibrotic hallmarks such as type I and type III collagen deposition and the ratio of type I and III collagen; alpha-smooth muscle actin (α-SMA); E-cadherin; phosphorylated SMAD-3; and TGF-β1 in mouse tail models.¹⁴ These findings highlight the critical role of CD4+ T cells in lymphedema-associated fibrosis.

CD4+ T cells significantly regulate lymphangiogenesis and lymphatic function

Recent data have elucidated the inhibitory role of CD4+ T cells in lymphangiogenesis. Zampell et al.¹⁴ found that the blockage of CD4+ cells led to increased lymphatic vessel endothelial receptor-1 (LYVE-1)+ vessels and podoplanin+ vessels in tail lymphedema models after 6 weeks. Besides, CD4 blockage also enhanced the lymphatic transport capacity, as evidenced by increased tracer uptake in draining lymph nodes, and accelerated lymphatic transport in CD4-inhibited animals.^14,15 These results collectively highlight the negative impact of CD4+ T cells on both lymphatic formation and function.

In a study by Gousopoulos et al.,²⁸ the number and morphology of lymphatic vessels were investigated in a mouse tail model. The authors observed that the lymphatic vessel accounted for 13.3% of the total area at 6 weeks in the mouse tail model of lymphedema in comparison with 1.6% of the total tissue area in the control tissue, indicating proliferation of lymphatic endothelial cells (LECs). However, despite this increased lymphangiogenic phase, the lymphatic transport of the tracer remained delayed.

In particular, the presence of CD4+ cells increased during the lymphangiogenic phase, suggesting a correlation between CD4+ T cells and early lymphatic expansion with impaired transport functionality. Similar results were reported by Ogata et al.,²⁹ who observed that early lymphangiogenesis manifested itself as sprouting of and leaky lymphatics. Lymphatic vessels in the early phase of lymphangiogenesis might not be able to transport lymph fluids, but might function as a facilitator of acute inflammation.^30–32

It is reasonable to infer that the newly formed lymph vessels initiated by lymph stasis do not repair injured lymphatics, but induce further inflammation.

Th1 Cells Coregulate Fibrosis and Lymphangiogenesis

Th1 cells, a prominent subtype of CD4+ T helper cells, are stimulated by interleukin (IL)-12 to undergo differentiation and secrete significant amounts of interferon (IFN)-γ, tumor necrosis factor-α, and IL-12, whereas IFN-γ could activate macrophages.³³ Evidence demonstrated that lymph stasis-induced inflammation was characterized by a mixed Th1 and Th2 response in both animal models³⁴ and human tissue.¹⁹ Mehrara's research team reported that Th1-deficient mice had the same severity of lymphedema. On the contrary, Th2-deficient mice showed improved outcomes with less fibrosis, more lymphangiogenesis, and enhanced lymphatic function.³⁴

Although it has been acknowledged that Th2 is the key driver in tissue fibrosis by triggering collagen accumulation, the Th1 response counterbalanced this process. In a murine model of lower limb lymphedema, Cho et al.¹³ observed that injection of hyaluronidase significantly reduced the volume of the lymphedematous lower limbs, the dermal thickness, and the fibrotic areas of lymphedematous tissue compared with the phosphate buffer solution-treated lymphedema group.

This effect was attributed to Th1 cell-secreted cytokines, namely IFN-γ and IL-12, suggesting the involvement of Th1 cells in antifibrotic mechanisms in lymphedema. The molecular data from hyaluronidase-treated mice also underpinned the theory that hyaluronidase curbed the fibrogenesis response since the expression levels of matrix metalloproteinases, fibronectin, and α-SMA plummeted following hyaluronidase injection, a process that was regulated by higher levels of IFN-γ and IL-12 (Fig. 1).

FIG. 1.

Overview of the function of CD4+ T cell subtypes in lymphedema. Th1 cells inhibited lymphatic formation in vitro through activation of IFN-γ. Activation of Th1 cells also contributed to restoration of skin fibrosis. Th2 cells inhibited lymphangiogenesis and enhanced fibrosis through the IL-4 and IL-13 pathway, which could be counterbalanced by antibodies of IL-4 and IL-13. Together with Th1 cells, Th17 could induce lymphangiogenesis by activating macrophages to release VEGF-C. Tregs in lymphedematous tissue could counterbalance the effect of Th1 and Th2 responses. Adoptive transfer of Tregs into murine models of lymphedema resulted in stimulation of lymphangiogenesis, highlighting the potential therapeutic role of Tregs in promoting lymphatic vessel formation. IFN, interferon; IL, interleukin; VEGF, vascularized endothelial growth factor.

However, it should be noted that all the data in Cho's study¹³ were obtained within the first 7 days of murine lymphedema, and the more prolonged effect of Th1 cells in chronic lymphedema remained to be elucidated. While these findings do not establish causality regarding the antifibrotic function of Th1 cells in lymphedema, they provide valuable information for future studies.

The secretion of IFN-γ by Th1 cells plays a crucial role in inhibiting the formation of lymphatic vessels.^35,36 When LECs were cocultured with IFN-γ, the LEC migration rate and proliferation dropped drastically and apoptosis was induced.³⁶ Additionally, IFN-γ significantly downregulated prospero homeobox protein-1(Prox-1), and LYVE-1 in LECs, which are important markers for LECs, leading to suppression of lymphatic formation in vitro (Fig. 1).³⁵

However, coculture of CD11b+ macrophages and CD4+ T cells promoted the expression of vascular endothelial growth factor-C (VEGF-C) in macrophages as well as protein levels of VEGF-C in the medium. VEGF-C is a well-known prolymphangiogenic factor that promotes lymphatic vessel growth. This suggests that there was a prolymphangiogenic microenvironment in the initial stages of lymphatic stasis.²⁹

Adding antibodies against IFN-γ and IL-17, interestingly, inhibited the production of VEGF-C in vitro and in lymphedematous mice, suggesting that Th1 cells, Th17 cells, and macrophages collaborated to induce lymphatic formation (Fig. 1). The cross talk among Th1/Th17/macrophages in lymphangiogenesis deserves further exploration in lymphedema models.

Th2 Cells Promote Fibrosis, Inhibit Lymphangiogenesis, and Diminish Lymphatic Function in Lymphedema

Th2 cells play a pivotal role in various inflammatory diseases characterized by dysregulated lymphangiogenesis³⁷ and fibrosis,³⁸ including asthma, by specifically secreting IL-4, IL-5, and IL-13. The Th2 response has a leading role in all pathological changes of lymphedema, including tissue fibrosis, impaired lymphangiogenesis, and compromised lymphatic function.

Th2 cells as the leading facilitators of fibrosis

In the study conducted by Mehrara and colleagues,¹⁹ it was observed that fibrosis resulting from ALND was significantly reduced in CD4KO or nude mice, suggesting the profibrotic role of CD4+ T cells. Further investigation revealed that Th2 cells, a main subtype of CD4+ cells, are the main drivers of skin thickening and collagen deposition.

Administration of the IL-4 monoclonal antibody (IL-4mAb) or IL-13 monoclonal antibody (IL-13mAb) in wild-type mice resulted in a marked reduction in adipose and collagen deposition, showing that Th2 response could trigger fibrosis (Fig. 1). Additional evidence showed that fibrosis in lymphedema was associated with dermal type I collagen deposition.³⁹

Intriguingly, improved data on type I collagen deposition and subcutaneous tissue thickness were also noticed when IL-4mAb was administered for 3 weeks in mice with chronic tail lymphedema, further supporting the crucial role of the Th2 response in chronic fibrosis. Besides, the assessment of lymphatic function using 99mTc uptake and lymphatic fluorescence also yielded better results in IL-4mAb-treated lymphedematous mice.

However, these results were insufficient as they only indicated the function of IL-4 in lymphedema; therefore, the same research team conducted additional experiments using Th2-deficient mice (STAT6 knockout, STAT6KO) and proved that STAT6KO mice exhibited less fibroadipose thickness and less type I collagen 6 weeks after tail surgery compared with wild-type mice and Th1-deficient mice.³⁴ The above findings highlight the role of Th2 cells and Th2 cytokines in fibrosis.

Th2 inhibits lymphangiogenesis in lymphedema

Th2 cells have been identified as regulators of lymphatic formation through secretion of IL-4 and IL-10. Recent studies by Savetsky et al.⁴⁰ have demonstrated the strong antilymphangiogenic effect of IL-4 and IL-13 in vitro and in a lymphangiogenic corneal model. Prox-1 has proven to be a core regulator of proliferation and maintenance of LECs.⁴¹

Treatment with recombinant IL-4 or IL-13 decreased the mRNA expression of Prox-1 and lymphatic vessel endothelial hyaluronan receptor-1 (LYVE-1) in mice and human LECs.^37,40 Treatment with IL-4Ab and IL-13Ab restored the number of LYVE-1-positive lymphatics in inflammatory corneal neovascularization (Fig. 1). In an LEC culture system,³⁷ the coculture of LECs with IL-4 or IL-13 inhibited lymphatic tube formation through IL-4- and IL-13-dependent pathways (Fig. 1).

Furthermore, in an allergic asthma model, IL-4mAb and IL-13mAb elevated the density of lymphatic vessels and lymphatics. Collectively, these studies suggest that Th2 cytokines exert a negative influence on lymphangiogenesis by targeting LECs. Histological analysis of a mouse tail lymphedema model at 6 weeks postsurgery revealed that Th2-deficient mice had more lymphangiogenesis at the wound sites compared with wide-type mice, indicating the antilymphangiogenic role of Th2 response in chronic lymphedema. However, how specific cytokines from Th2 cells influence lymphangiogenesis and the underlying mechanisms remain unclear.

Additionally, both asthma and lymphedema show Th2-type inflammation^34,42 and lymphatic transportation insufficiency of antigens³⁷ despite lymphatic hyperplasia^43,44 and end up in fibrosis of tissue.^19,45 Therefore, we should reevaluate the drugs of asthma for their potentials in treating lymphedema.

Th2 causes lymphatic dysfunction in lymphedema

Th2 cells decrease the density of lymph vessels and curb their function, which has been supported by antigen clearance data in an asthma model.³⁷ Lymphatic collecting vessel pumping, which accounts for two-thirds of the lymphatic transport function, is estimated to be compromised in PLND lymphatic insufficiency.

However, CD4KO and STAT6KO mice outperformed wild-type mice in growth of more collateral lymphatic vessels and more frequent lymphatic contractions, as demonstrated by indocyanine green (ICG) transportation data.³⁴ These results further underpin the involvement of Th2 cells in lymphangiogenesis and lymphatic function.

Subsequent experiments revealed a significant decrease in perilymphatic inducible nitric oxide synthase (iNOS)+ cells and α-SMA+ cells near the lymphatics at 4 weeks following PLND in Th2-deficient rodents, which explained at least in part that Th2 cells inhibited lymphatic function by regulating perilymphatic fibrosis and inducing an iNOS-producing pathway. However, how Th2 cells induced more NOS remains to be elucidated.

Notably, the core pathological process of filarial lymphedema equals Th2-featured inflammation as well.⁴⁶ Furlong-Silva et al.⁴⁷ demonstrated in a murine hindlimb model of filarial infection that early-stage filarial lymphatics exhibit a Th2 immune response, with the major subtypes of infiltrating CD4+ T cells secreting IL-4 and IL-13.

ICG data showed that no severe lymphatic remodeling or dysfunction was identified in IL-4R-deficient mice following filarial infection, indicating an enhanced Th2 immune response in filarial lymphedema, which consequently led to activation of macrophages and further development of lymphatic dysfunction.

Treg Cells Mitigate Lymphatic Function in Secondary Lymphedema

Tregs exert immunosuppressive effects through various mechanisms, including secretion of inhibitory cytokines such as TGF-β and IL-10, induction of apoptosis in specific immune cells, and modulation of IL-2 signaling.^48–50 Tregs characteristically express CD25 and rely on forkhead box P-3 (Foxp3), which serves as their most specific marker and transcription factor, for their development and function.^51,52

Generally, Tregs can be subdivided into two groups: thymic Tregs (tTregs), which arise in the neonatal period,⁵³ and peripherally induced Tregs (pTregs or iTregs), which are CD4+CD25+Foxp3+ and are transformed from naive CD4+CD25‒ T cells by T cell receptor costimulation and TGF-β.⁵⁴

Treatments targeting Tregs have shown promise in managing inflammatory diseases. For instance, adipose-derived stem cell (ADSC)-secreted exosomes have been demonstrated to ameliorate acute colitis by elevating the abundance of Treg cells in lymph nodes and promoting the production of Treg-related cytokines such as TGF-β and IL-10.⁵⁵

Ex vivo expanded regulatory T cells harbor the ability to suppress lupus progression in lupus-prone mice.⁵⁶ Preliminary results from mouse models also point to the potential of Tregs in multiple sclerosis⁵⁷ and asthma.⁵⁸

However, Tregs have recently received attention in the field of lymphedema. Studies by Gousopoulos have shown that upregulated infiltration of Tregs was identified in mouse and human lymphedematous tissues.¹⁶ The findings of these results provide evidence that Tregs are prominent regulators of lymphatic function and lymphedema.

Depletion of Tregs in mice with lymphedema exacerbated the manifestations of the condition, as evidenced by higher levels of IFN-γ, IL-13, IL-4, and TGF-β1, along with elevated macrophage infiltration. Conversely, Treg transfer to mice with established lymphedema significantly restores lymphedema pathology, as shown by less lymphatic dilation, reduced collagen deposition, and improved lymphatic function (Fig. 1).

Further analysis of Treg infiltration in the lymphedema area showed that the proliferating Tregs were nTregs.⁵⁹ Accumulation of nTregs following ALND in mice led to decreased inflammation.⁵⁹ Treg-depleted mice also showed higher levels of DCs in the ALND-treated ipsilateral forelimb than in wild-type mice, suggesting that Tregs also suppressed DC activation.

Besides, since lymphedema is primarily driven by DC-activated CD4+ T cells in regional lymph nodes,¹⁵ it is plausible that Tregs regulate lymphatic function by lowering other CD4+ cells in the ipsilateral limb through their effects on DCs. Whether Tregs directly influence LECs to promote lymphatic vessel formation, akin to the role of Th17 cells in this process, remains an intriguing question.⁶⁰

Tregs are involved in filarial lymphedema⁶¹ in addition to mouse tail lymphedema. Treg cell markers, including Foxp3 and TGF-β, were found to be significantly diminished in patients with lymphedema, indicating that failure to induce the nTreg reaction was the hallmark of filarial lymphedema.

An intriguing finding emerged from the results: Tregs and Th17 cells enjoyed a shared differentiation pathway. When stimulated by TGF-β plus IL-2, naïve CD4+ T cells differentiate toward Foxp3+ Tregs, whereas TGF-β plus IL-6 induces the Th17 lineage.^62,63 Solid proofs have been accumulated showing that a lower Treg infiltration frequency and impaired Treg function were deemed as the shared pathways of a variety of autoimmune diseases, such as multiple sclerosis,⁶⁴ psoriasis,⁶⁵ rheumatoid arthritis,⁶⁶ and inflammatory bowel disease.⁶³

The delicate balance between Th17 and Treg cells plays an essential role in lymphedema and studies targeting this balance may hold promise for lymphedema research. Recent studies have demonstrated that ADSCs exert immunomodulatory effects on various diseases, such as lupus⁶⁷ and atopic dermatitis,⁶⁸ by targeting the balance of Th17/Tregs.

It could be inferred that ADSCs may exert immunoregulation properties on Th17/Treg balance to ameliorate lymphedema; however, the exact underlying mechanism requires further experimental exploration.

Th17 Cells Could Exacerbate Lymphedema by Regulating Lymphangiogenesis and Lymphatic Function

Th17 cells, another subtype of CD4+ immune cells, are induced by IL-6 and TGF-β and expanded by IL-23. They are known to produce various cytokines, including IL-17, IL-21, and IL-22,^69,70 and have been implicated in the pathogenesis of autoimmune diseases such as Crohn's disease,⁷¹ rheumatoid arthritis,⁷² and psoriasis.⁷³

Recent evidence has shed light on their role in regulating lymphangiogenesis through VEGF signaling pathways. Th17 cells have been shown to induce lung cancer cells to release VEGF-C, resulting in further novel lymphangiogenesis.⁷⁴ In a mouse corneal model,⁷⁵ Th17-induced lymphangiogenesis was found to depend on VEGF-D.

Interestingly, lymphedematous tissue was found to be enriched in IL-17-producing cells in both mouse lymphedema models and clinical samples.⁷⁶ The transcription factor, RAR-related orphan receptor C (RORC), which is essential for Th17 differentiation, was increased on day 7 after lymphatic obstruction in animal models, indicating the potential role of Th17 in lymphedema.²⁹

Mechanically, Th17 with Th1 could induce lymphangiogenesis by activating macrophages to release VEGF-C. Moreover, coinjection of anti-IFN-γ and anti-IL-17 antibodies into mice after lymphatic obstruction significantly suppressed lymphangiogenesis in the early phase of lymphatic injury and prevented chronic lymphedema (Fig. 1).²⁹

Particularly, the lymphatics formed as a result of IL-17-mediated lymphangiogenesis in the first week after lymphatic obstruction were dysfunctional and leaky, providing no relief, but rather accelerating lymphedema.²⁹ Besides, Wang observed that the Th17 response was particularly enhanced and parallel to marked epidermal hyperplasia in elephantiasis, a severe presentation of lymphedema.⁷⁶

However, two questions remained unanswered: whether Th17 cells always induce the formation of novel lymphatic vessels and whether Th17 immunity participates in the chronic phase of lymphedema (i.e., more than 6 weeks in the mouse tail model).

There is some conflicting evidence regarding the prolymphangiogenic role of Th17 cells. In a study by Park et al.,⁶⁰ LECs were found to respond directly to IL-17A through cell membrane receptors. IL-17A significantly downregulated the protein levels of Prox-1 and LYVE-1 and tube formation of LECs (Fig. 1).⁶⁰ In animal models, intranasal treatment with CTO, which is cholera toxin mixed with the ovalbumin peptide, induced an immune response featuring Th17 inflammation within the first week,⁷⁷ which faded after 3 weeks following CTO treatment.

Consistent with this, novel and enlarged chaotic lymphatics formed in the first week and regressed thereafter.²⁹ Markedly, Th17 cells consistently produce a significant amount of IL-17A, either in the initial or regression phase of this process. Blockade of IL-17A using antibodies increased lymphatic density and elevated lymphatic function.⁶⁰

Therefore, neolymphangiogenesis in the early stage following lymph stasis could be reviewed as a simple, acute, inflammation-associated lymphangiogenic response corresponding to higher levels of VEGF-C secretion from a variety of immune cells and epithelial cells.^78,79 However, Th17 cells were found to prohibit LECs from tube formation by conjugation of IL-17 and IL-17A receptors.

It would have been the late phase when Th17 cells came into play and inhibited lymphatic formation. Further investigation using well-established mouse tail lymphedema models may help clarify this discrepancy.

Dysregulation of Th17 differentiation has been implicated as a potential predisposing factor for both primary and secondary lymphedema. The RORC gene, which encodes the transcription factor for Th17, has been linked to lymphedema susceptibility in patients with primary lymphedema⁸⁰ and BCRL.⁸¹ Filarial lymphedema shares similarities with BCRL as only a small percentage of patients with filariasis develop lymphedema,⁸² suggesting that the specific immune response of the host to lymphatic injury may play a role.

An interesting fact is that nude mice presented a less severe pathological change of lymphatic injury in filarial lymphedema, which could be strengthened instead with T cell transfer.^83,84 Moreover, filarial lymphedema was also characterized by a Th17 immune response.^61,85 The analysis of peripheral blood mononuclear cells from chronic filarial lymphedema patients showed a drastic increase in the mRNA of Th17-related cytokines.⁶¹

Compared with asymptomatic filaria-infected patients, individuals with filarial lymphedema showed significantly higher frequencies of Th17 cells; this tendency remained the same when CD4+ T cells were cocultured with filarial antigens.⁸⁵ In vitro studies further demonstrated that blocking IL-1R and IL-23R led to dwindling frequencies of Th17 cells, indicating that the Th17 response in lymphedema was dependent on IL-1 and IL-23 signaling.⁸⁵

These findings provide important clues indicating that dysregulated Th17 differentiation may exacerbate lymphedema, although further investigation is warranted to elucidate the nuanced details of this complex immune response in the context of lymphedema pathogenesis.

Concluding Remarks

This review discusses the current status of T cell immunity in lymphedema. Through investigations in both human patients and murine models with lymphedema, it has been observed that lymphedematous tissue is enriched in CD4+ T cells, including Th1, Th2, Treg, and Th17 cells. This unique immune environment plays an essential role in tissue fibrosis, impaired lymphatic function, and distorted lymphatic formation.

Activation of CD4+ T cells by DCs and differentiation into Th2 responses have been implicated in driving the hallmark pathological changes in lymphedema. However, the precise mechanical details underlying these processes require further clarification. In vitro studies have revealed that IFN-γ-secreting Th1 cells and IL-17A-secreting Th17 cells directly diminish LEC proliferation and lymphatic tube formation.

It is noteworthy that during the early phase after lymph stasis, Th1, Th17, and macrophages cooperatively regulate lymphatic formation without improving lymph transportation, ultimately promoting tissue edema in the chronic phase. However, studies focusing on Th1 and Th17 responses in chronic lymphedema are scarce. Tregs have emerged as a clinically significant T cell subtype in the context of lymphedema as adoptive transfer of Tregs has been shown to limit the pathological process of lymphedema.

Targeting the immune response of CD4+ T cells provides insights into therapeutic management of lymphedema. For example, anti-Th2 immunotherapy may hold promise for improving skin stiffness and quality of life for women with BCRL.⁸⁶

Recent evidence shows that ADSCs may modulate the balance between Tregs and Th17 cells, potentially playing a critical role in the interaction between ADSCs and lymphedema. Therefore, further research targeting various T cell subtypes and their respective treatments may offer significant potential in the field of lymphedema management in the future.

Footnotes

Data Availability

All data are included in the article.

Ethical Statement

This work is a review-type research. There is no need for IRB approval for human subjects research or from the Institutional Animal Care and Use Committee (IACUC).

Author Disclosure Statement

No competing financial interests exist.

Authors' Contributions

A.F. was involved in conceptualization (supporting); writing—original draft (lead); formal analysis (lead); writing—review and editing (equal); and visualization. C.L. was involved in conceptualization (lead); writing—review and editing (equal); methodology (lead); writing-original draft (supporting); writing—review and editing (equal); and study supervision.

Funding Information

This work was supported by a grant from the Chinese Academy of Medical Sciences (CAMS) Innovation Fund for Medical Sciences (CIFMS) (No. 2020-I2M-C&T-B-082) to C.L. and a grant from the Scientific Research Fund of Plastic Surgery Hospital, Chinese Academy of Medical Sciences (YS202016), to C.L.

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