T-cell exhaustion in leprosy: molecular mechanisms and therapeutic implications

1 Introduction

Leprosy, which is mainly caused by Mycobacterium leprae and less commonly by Mycobacterium lepromatosis, continues to be one of the major chronic skin and peripheral nerve diseases (1–3). Today, multidrug therapy (MDT) is in widespread use, but transmission remains in many endemic settings, such as in India, Brazil, and Indonesia (4–8). The disease shows a very wide spectrum, ranging from tuberculoid leprosy (TT) with strong cell-mediated immune responses and little bacillary growth to lepromatous leprosy (LL) in which the cell-mediated immune responses are ineffective and bacillary growth is uncontrolled (9, 10). This wide spectrum of immunity underlies the importance of host immunity in disease outcome and explains why leprosy is a good human model to investigate immune regulation in chronic infection. Immune cells are continuously stimulated by their encounter with the antigen from M. leprae, which may favor progressive impairment of T-cell responses (11).

Long-term exposure to antigens and immune activation can induce a unique form of T-cell dysfunction called exhaustion. Exhausted T-cells have a dampened proliferative response, decreased anti-microbial function, decreased production of pro-inflammatory cytokines such as Interferon gamma (IFN-γ), Tumor Necrosis Factor alpha (TNF-α), and Interleukin-2 (IL-2), and an increased expression of inhibitory immune checkpoint receptors, including PD-1, CTLA-4, LAG-3, TIM-3, and TIGIT (12, 13). T-cell exhaustion was first recognized in chronic viral infections, such as Lymphocytic Choriomeningitis (LCMV), lately in Human Immunodeficiency Virus (HIV) and cancer, but is now becoming recognized in chronic bacterial infections such as tuberculosis and leprosy (14–17).

Moreover, M. leprae is armed with several conserved virulence determinants that facilitate its intracellular survival, immune modulation, and chronic persistence. Immune evasion and stress tolerance, along with long-term survival in macrophages and Schwann cells, are achieved through secretion systems such as Accessory Secretion Factor 2 (secA2), secretion system 3 (ESX-3) and secretion system 5 (ESX-5), and genes associated with lipids namely Phenolphthiocerol Polyketide Synthase C, D and E (ppsC, ppsD, ppsE), Mycocerosic Acid Synthase (mas), Polyketide Synthase 15/1 (pks15/1), Mycobacterial Membrane Protein Large 3 and 10 (mmpl3, and mmpl10) (18–24). These determinants can lead to chronic antigen persistence that can constantly activate T-cell receptors and eventually result in immune exhaustion (Figure 1).

The purpose of this review is to discuss the current scenario of T-cell exhaustion in leprosy, the immunological mechanisms, inhibitory receptor pathways, metabolic and epigenetic changes involved, and the correlation with the disease severity and treatment outcome. In addition, the article discusses the possibilities of biomarkers associated with exhaustion as emerging strategies to enhance host-directed treatment in leprosy.

2 Materials and methods

A thorough literature search was done with PubMed, Google Scholar, and Ovid databases related to T-cell exhaustion in Leprosy. Medical Subject Headings (MeSH) and free-text words were combined together using Boolean operators (AND/OR). The most common search terms were: “T-cell” and “immune exhaustion” and “leprosy”, “T-cell exhaustion” and “leprosy”, “immune checkpoint molecules” and “leprosy”, “PD-1” and “leprosy”, “PD-L1” and “leprosy”, “CTLA-4” and “leprosy”, “LAG-3” and “leprosy”, “TIM-3” and “lepromatous leprosy”, and “Hansen’s disease” and “T-cell dysfunction”. Articles published in English, original research articles, reviews, and related experimental studies were considered for inclusion. For this review, immunological dysfunction, checkpoint pathways, chronic antigen stimulation, and host immune responses in leprosy were prioritized.

3 Explaining T-cell exhaustion

T cell exhaustion is a gradual deterioration of T-cell function that arises during chronic antigen exposure, such as that in persistent infection and cancer (12–14). This differentiation stage is different from the functional effector or memory T-cells, and is actually an adaptive response to chronic stimulation, not a terminal failure. Exhaustion has certain key features, such as loss of function, persistent expression of inhibitory receptors, changes in transcriptional programs, metabolic changes, and loss of memory properties. Exhausted T-cells have an early defect in proliferation and production of IL-2, followed by impaired secretion of TNF-α and IFN-γ, and then severe functional defects. Multiple immune checkpoint molecules such as PD-1, CTLA-4, LAG-3, TIM-3, TIGIT, 2B4 (CD244), and CD160. are co-expressed on exhausted cells. The number of inhibitory receptors expressed simultaneously is correlated with the level of exhaustion (25–29). Exhausted T-cells have a distinct transcriptional signature, which is controlled by specific transcription factors, including Thymocyte selection-associated high mobility group box protein (TOX), Nuclear factor of activated T-cells (NFAT), T-cell factor 1 (TCF-1), Basic leucine zipper transcription factor, ATF-like (BATF), Interferon regulatory factor 4 (IRF4), as well as by altered levels of T-bet and Eomesodermin. This program is epigenetically maintained by changes in DNA methylation and chromatin accessibility (30–33). Bioenergetic insufficiency or dysregulated metabolic programs are different in these cells compared to effector and memory T-cells. Exhausted T-cells have no antigen-independent homeostatic maintenance and rely on continuous antigen for survival, unlike memory T-cells, which can be sustained by IL-7/IL-15 in the absence of antigen (30).

Exhausted T-cells are heterogeneous, comprising of at least three functionally distinct subsets: i.e., progenitor exhausted cells, intermediate or transitional exhausted cells, and terminally exhausted cells (16, 17, 34). Progenitor exhausted cells (T-bet ^high, PD-1^intermediate, TCF-1+) retain proliferative potential and capacity for self-renewal. They are sensitive to PD-1 blockade and are able to give rise to terminal progeny (35). Intermediate exhausted T cells are partially functional cells that are a transition stage between progenitor (stem-like) and terminally exhausted cells. They exhibit more and more inhibitory receptors (e.g., PD-1) and start to express markers (e.g., TIM-3, TOX) and keep some proliferative and cytokine-producing ability. In terms of their function, they have moderate effector function, which over time becomes fully exhausted in the presence of chronic antigen exposure. Terminally exhausted cells (Eomes^high, PD-1^high, TCF-1-) express higher levels of multiple inhibitory receptors, produce fewer cytokines but retain some cytolytic capacity, and have limited proliferative ability (16, 17, 34). Exhaustion is at least partially reversible; PD-1 pathway blockade can transiently reinvigorate function by expanding progenitor subsets, though it does not reverse the underlying epigenetic state (34).

T-cell exhaustion is different from other dysfunctional states, like anergy, which results from suboptimal priming in the absence of co-stimulation, and from senescence, which is driven by progressive telomere shortening and replicative arrest (13, 31).

3.1 Molecular mechanisms of T cell exhaustion in leprosy

Persistent T-cell receptor (TCR) signaling during chronic antigen exposure to M. leprae, which results in continuous T-cell receptor engagement, is the main driver of the exhaustion process (Figure 1) (36, 37). The severity of exhaustion is directly related to the intensity and duration of the antigen load. In LL patients, having a high bacterial index (BI), T-cells are unable to enter the quiescent state characteristic of memory T-cells, and instead move towards exhaustion, and express several inhibitory receptors: PD-1, LAG-3, TIM-3, TIGIT, CTLA-4, and CD160 (31). Engagement of PD-1 with PD-L1 recruits Src homology region 2 domain-containing phosphatase-1,2 (SHP-1, 2) to the TCR complex, which dampens TCR signaling in the Phosphatidylinositol 3-kinase- Protein Kinase B (PI3K-AKT) and Rat Sarcoma virus (RAS) pathways. This pathway also results in cell cycle arrest through upregulation of p27 and p15. LAG-3 binds to the Major Histocompatibility Complex (MHC) class II molecules and further weakens the TCR signaling. The frequency of co-expression of inhibitory receptors is associated with the level of exhaustion, like triple or quadruple positivity (such as PD-1+LAG-3+TIM-3+) defines the highest level of exhaustion (38).

LAG-3 binds to MHC class II molecules and other ligands such as fibrinogen-like protein 1 (FGL-1) and triggers inhibitory signals through its intracellular KIEELE motif (KIEELE refers to a highly conserved, 6-amino-acid sequence found within LAG-3), which inhibit TCR signaling, calcium mobilization, and downstream activation of NFAT, AP-1, and NF-κB, leading to diminished T-cell proliferation and cytokine production (39). CTLA-4 exerts inhibitory effects on T-cell activation by competitively binding to CD80 and CD86 of antigen-presenting cells (APCs) and by recruiting phosphatases like PP2A and SHP-2 that suppress PI3K-AKT and Mitogen-Activated Protein Kinase (MAPK) signaling pathways, respectively, resulting in less co-stimulation, IL-2 production and less T-cell expansion (40). TIM-3 binding to its ligands such as galectin-9, Carcinoembryonic Antigen-related Cell Adhesion Molecule 1 (CEACAM1), phosphatidylserine and High Mobility Group Box 1 (HMGB1) leads to the dissociation of the adaptor protein HLA-B-associated transcript 3 (BAT3) and the recruitment of inhibitory signaling molecules, which suppress TCR-mediated activation, inhibit phosphorylation of Zeta-chain-associated protein kinase 70 (ZAP-70) and Linker for Activation of T cells (LAT), reduce activation of PI3K-AKT and Extracellular signal-Regulated Kinase pathway (ERK)signaling cascades, and decrease the production of effector cytokines including IFN-γ and TNF-α (41). TIGIT binds to both APCs and target cells through Immunoreceptor Tyrosine-based Inhibitory Motif (ITIM) and Immunoglobulin Tyrosine Tail (ITT)-like motifs, recruiting adaptor proteins and the phosphatase SHIP-1, it also inhibits the PI3K-AKT and MAPK pathways, suppresses the secretion of cytokines, and competes with the activating receptor CD226 for ligand binding. Further, TIGIT signals help to induce IL-10 and inhibit IL-12 secretion by APCs, establishing an immunosuppressive state (39). CD160 binding to MHC class I molecules and herpesvirus entry mediator (HVEM) reduces AKT-dependent signaling pathways and suppresses the proliferation, cytokine production, and cytotoxic activity of T-cells (42). These inhibitory receptors converge on shared signaling pathways, dampen the activation of the TCR, diminish effector functions, shift cell metabolism, and induce a transcriptional program involving factors like TOX, NR4A and BATF, resulting in the generation and persistence of exhausted T cells in chronic infections and cancer.

Exhaustion is regulated by the context-specific activity of transcription factors in the transcriptional regulatory network. TOX is a master epigenetic remodeling regulator and sustains the expression of inhibitory receptors (37). NFAT without Activator protein 1 (AP-1) triggers an exhaustion program; NFAT alone binds to the Pdcd1 (PD-1) promoter and results in transcription, while NFAT-AP-1 heterodimers induce an effector function (33). High expression of T-bet and intermediate expression of PD-1 is found in progenitor exhausted cells (T-bet^hi PD-1^int), while high expression of Eomes is a feature of terminally exhausted cells (Eomes ^hi PD-1^hi) (16, 17, 34). B lymphocyte-induced maturation protein-1 (Blimp-1) has the ability to induce expression of a number of inhibitory receptors in chronic infection, but inhibits PD-1 in acute episodes, indicating functional duality. BATF is induced by PD-1 signaling and functions as a transcriptional repressor that displaces Jun from AP-1 dimers and inhibits effector genes such as Ifng. Forkhead box protein O1 (FoxO1) nuclear accumulation occurs during chronic stimulation that leads to increased Pdcd1 promoter activity (31).

Exhausted T-cells undergo stable epigenetic reprogramming marked by DNA demethylation of the Pdcd1 locus, which persists even after antigen clearance or reduced PD-1 expression. This is a stable chromatin signature that is indicative of the exhausted state of T-cells, rather than their short-term activation. Checkpoint blockade is able to partially reverse T-cell function, but it does not completely reset the exhausted epigenetic landscape. In addition, exhausted T-cells are highly metabolically abnormal, such as impaired glycolysis, impaired mitochondrial respiration, and NAD+/NADH metabolic imbalance. Such bioenergetic abnormalities limit ATP production, resulting in limited proliferation, cytokine secretion, and effector functions (30–32, 38).

Table 1 summarizes the available studies that support T-cell exhaustion in leprosy, and highlights findings from experiment-based studies on inhibitory receptor expression, impaired cytokine responses, reduced proliferative capacity, and associations with disease severity. It compiles data demonstrating that exhausted immune phenotypes are particularly prominent in multibacillary/lepromatous forms of the disease that may contribute to ineffective control of Mycobacterium leprae infection (25–30, 43, 44).

Table 1. Key studies demonstrating T-cell exhaustion in leprosy.

TT, Tuberculoid; BT, Borderline Tuberculoid; BL, Borderline Lepromatous; LL, Lepromatous Leprosy; HC, Healthy Control(s); PBMCs, Peripheral Blood Mononuclear Cells; IHC, Immunohistochemistry; ELISA, Enzyme-Linked Immunosorbent Assay; CTLA-4, Cytotoxic T-Lymphocyte–Associated Protein 4 Tregs- Regulatory T Cells; PD-1, Programmed Cell Death Protein 1; PD-L1, Programmed Death-Ligand 1; APCs, Antigen-Presenting Cells; FOXP3, Forkhead Box P3; NK cells, Natural Killer Cells; NKT cells, Natural Killer T Cells; scRNA-seq, Single-Cell RNA Sequencing; TIGIT, T Cell Immunoreceptor with Ig and ITIM Domains; LAG-3, Lymphocyte Activation Gene 3; HAVCR2, Hepatitis A Virus Cellular Receptor 2; APOE, Apolipoprotein E; GWAS, Genome-Wide Association Study; scTCR/BCR-seq, Single-Cell T-Cell Receptor / B-Cell Receptor Sequencing. Bold values are names of Techniques/Procedures.

3.2 Comparison of T-cell exhaustion in leprosy with other chronic infections

T-cell exhaustion is a hallmark of many chronic infectious diseases in which persistent antigen exposure drives progressive dysfunction of effector T cells (14, 45). In chronic viral infections such as HIV (46, 47), Hepatitis B, Hepatitis C (48), and LCMV (49), exhausted T-cells exhibit sustained expression of inhibitory receptors including PD-1, LAG-3, TIM-3, and TIGIT, accompanied by hierarchical loss of cytokine production, reduced proliferative capacity, and impaired cytotoxicity (15, 50). Similar observations have been reported in chronic intracellular bacterial and parasitic infections. In Tuberculosis, PD-1/PD-L1 blockade enhances Mtb-specific IFN-γ⁺TNF-α⁺ polyfunctional T-cell responses, indicating reversible checkpoint-mediated dysfunction (51). T-cell exhaustion is one of the immunological hallmarks of chronic leishmaniasis, especially in the visceral and diffuse cutaneous types. Chronic exposure to leishmania antigens results in ongoing expression of inhibitory receptors, including PD-1, CTLA-4, TIM-3, and LAG-3, on T cells. The dysfunctional exhausted T-cells have reduced capacity to the activate macrophages contributing to their decreased ability to kill intracellular parasites, which leads to parasite persistence. Blocking immune checkpoint pathways, such as PD-1/PD-L1, has been demonstrated to enable control of parasites and restore T-cell function in both animal models and human studies (52, 53). Likewise, in Malaria, inhibition of PD-1 and LAG-3 pathways restores T-cell activity and improves pathogen control (54). These findings collectively demonstrate that persistent antigenic stimulation across chronic infections drives convergent exhaustion characterized by inhibitory receptor upregulation and impaired effector function.

However, T-cell exhaustion in leprosy appears particularly profound and distinct compared with other chronic infections. LL patients exhibit marked co-expression of PD-1, LAG-3, and TIGIT on CD8⁺ T- cells, often at higher frequencies than observed in tuberculosis or chronic viral infections, suggesting deeper exhaustion states (25, 27). Unlike many systemic infections, leprosy demonstrates pronounced tissue-specific exhaustion, with inhibitory receptor expression being more prominent in dermal lesions than in peripheral blood (27). Chronic exposure to non-cultivable M. leprae, prolonged intracellular persistence within Schwann cells and macrophages, and continuous neural antigen stimulation likely contribute to sustained immune dysfunction. In addition, lipid-rich cell wall components of M. leprae promote tolerogenic macrophage phenotypes, while elevated toll-like receptor-4 (TLR4) signaling positively correlates with PD-1 expression, linking innate immune activation to adaptive T-cell exhaustion (25). Unique immunometabolic features, including APOE-associated MHC-II downregulation in macrophages and altered PD-1 function on Tregs, further distinguish leprosy from viral infections and cancer-associated exhaustion (27). These findings position leprosy as an archetypal human model of intracellular bacteria-induced T cell exhaustion.

3.2.1 Immune checkpoint inhibitors in chronic viral infections

Checkpoint inhibitors (CPIs) have emerged as promising immunotherapeutic strategies in chronic viral infections (including HIV, Hepatitis B, and Hepatitis C). Persistent antigen exposure in these chronic viral infections drives T-cell exhaustion, these virus-specific exhausted T-cells progressively lose their proliferative capacity, cytokine production, and cytotoxic function. Blockade of inhibitory pathways, particularly the PD-1/PD-L1 axis, can partially restore antiviral immunity by enhancing T-cell proliferation, increasing IFN-γ and TNF-α secretion, and improving viral control. Preclinical studies in LCMV infection demonstrated that PD-1 blockade reverses T-cell dysfunction and reduces viral burden, providing the foundation for subsequent human studies (49). In HIV infection, anti-PD-1 antibodies have been shown to increase HIV-specific CD8⁺ T-cell responses and may contribute to reversal of viral latency (50). In chronic hepatitis B, PD-1 blockade enhances HBV-specific T-cell activity and has occasionally been associated with reductions in hepatitis B surface antigen levels (55). However, clinical benefits remain inconsistent, and immune-related adverse events, including excessive inflammation and tissue damage, remain important concerns. Current research is focused on combining checkpoint inhibitors with therapeutic vaccines, antiviral therapy, cytokine modulation, and adoptive cell therapies to achieve durable immune restoration while minimizing immunopathology.

4 Implications for treatment

4.1 Inhibitory checkpoint blockade

Blockade of PD-1 and PD-L1 is the most widely studied immunotherapeutic approach. Anti-PD-1 and anti-PD-L1 antibodies have been shown to restore the production of IFN-γ and TNF-α ex vivo by T-lymphocytes from LL patients (25). Currently, nivolumab, pembrolizumab, and atezolizumab are FDA-approved drugs for multiple cancers, and repurposing these agents for refractory leprosy cases, particularly high-BI cases that are unresponsive to MDT, should be explored (56). Co-blocking of PD-1 and LAG-3 in chronic LCMV infection indicates similar approaches to leprosy (14). The combination of PD-1+ and LAG-3+ targeting results in additive restoration of polyfunctional T-cells (IFN-γ+/TNF-α+) in tuberculosis and malaria models, and may be applicable to leprosy. Theoretically, multi-receptor blockade regimens designed to address the specific exhaustion types of each individual patient may have a great impact (57).

The expression of CTLA-4, a competitor for binding to CD80/86, is increased on T-regulatory cells (Tregs) and conventional T-cells in LL patients. Blocking CTLA-4 alone is only marginally effective in the ex vivo treatment of leprosy, but the combination of anti-CTLA-4 and anti-PD-1 has proved to be more effective in the treatment of cancers and may shift the balance towards effector responses in LL patients by not only inhibiting Treg suppression but also reversing T cell exhaustion (38, 43, 45). Checkpoint blockade could be used as a complementary treatment to the usual MDT (rifampin, dapsone, clofazimine). MDT, when combined with PD-1 inhibitors may promote bacterial clearance by enhanced Th1 cytokine production and macrophage activation, prevent relapse by maintaining long-term memory responses, and limit reactional episodes due to smoldering bacterial levels that cause immune dysregulation (45).

A few drugs that block PD-1 can cause immune disorders that resemble autoimmune diseases, such as pneumonitis, colitis, and hepatitis, in 10–30% of cancer patients (15, 32, 36). The primary concerns in leprosy are Type 1 (T1R) and Type 2 Reactions (T2R), i.e., life-threatening situations that require very high doses of corticosteroids. Leprosy reactions are acute inflammatory episodes that may occur before, during, or after multidrug therapy and are a major cause of nerve damage and disability (10, 58). T1R, also known as reversal reaction, occurs mainly in borderline forms of leprosy (BT, BB, and BL) and results from an increase in cell-mediated immunity (CMI) against M. leprae. It is a Type IV hypersensitivity reaction characterized by activation of Th1 immune responses and increased production of cytokines such as IFN-γ and IL-15. Clinically, existing skin lesions become erythematous, swollen, and tender, and patients frequently develop acute neuritis with painful nerve enlargement, sensory loss, and motor weakness. In contrast, T2R, also known as erythema nodosum leprosum (ENL), occurs predominantly in BL and LL patients with high bacillary loads. It is an immune complex-mediated (Type III hypersensitivity) systemic reaction associated with complement activation, neutrophilic inflammation, and elevated levels of pro-inflammatory cytokines such as TNF-α and IL-10. Patients develop crops of painful erythematous nodules accompanied by systemic manifestations including fever, malaise, arthritis, lymphadenitis, orchitis, and ocular inflammation (10, 58). Close monitoring during treatment (BI, nerve function assessment) and pre-treatment risk stratification are essential. Immune checkpoint blockade therapy earlier, after 1–2 months of MDT, when bacterial load is reduced, may lower the risk of reactions and may reverse T-cell exhaustion (59).

Recent advances in HIV and cancer immunotherapy have identified several innovative strategies that could potentially be translated to leprosy, particularly for patients with persistent infection, treatment non-responsiveness, and immune exhaustion. Beyond conventional PD-1 blockade, dual checkpoint inhibition targeting PD-1 together with LAG-3, TIM-3, or TIGIT has shown superior restoration of exhausted T-cell function and may overcome compensatory inhibitory pathways that limit immune recovery (50). Emerging evidence highlights the importance of stem-like TCF1⁺ progenitor exhausted T-cells, which serve as the major responders to checkpoint blockade and may represent valuable biomarkers and therapeutic targets in leprosy. Similarly, the transcription factor TOX has been identified as a master regulator of exhaustion-associated epigenetic programming, making TOX-directed interventions a promising future approach (46, 47, 50).

4.2 Co-stimulatory molecules

The positive costimulatory pathways, including OX40 (TNFRSF4/CD134), Importin beta-binding, 4-1BB (TNFRSF9/CD137), and glucocorticoid-induced tumor necrosis factor receptor (GITR), act together by both enhancing TCR signals and relieving inhibition (60, 61). OX40 and 4-1BB are co-stimulatory receptors belonging to the TNF receptor superfamily that play crucial roles in regulating T-cell immunity. OX40 is primarily expressed on activated CD4+ T-cells and promotes T-cell proliferation, survival, cytokine production, and memory formation (60). In contrast, 4-1BB is predominantly expressed on activated CD8+ T-cells and natural killer cells, enhancing cytotoxic activity, cell survival, and effector functions (61). Both molecules strengthen immune responses and have emerged as promising targets for immunotherapy in cancer and chronic infections. Their ability to modulate T-cell function makes them potential candidates for host-directed therapeutic strategies. OX40-targeted therapies have been most extensively investigated in cancer, including melanoma, non-small cell lung cancer, head and neck cancer, and ovarian cancer, where OX40 agonists are used to strengthen anti-tumor immune responses (60, 61). In autoimmune and inflammatory disorders such as atopic dermatitis, asthma, rheumatoid arthritis, and systemic lupus erythematosus, strategies aimed at blocking the OX40–OX40L pathway are being explored to reduce excessive T-cell-mediated inflammation (60). OX40 modulation has also been studied in chronic infectious diseases, including tuberculosis, HIV, and chronic viral hepatitis, where enhancing T-cell function may improve pathogen control (38, 60). In chronic LCMV infection, co‐administration of pro‐4‐1BB with PD‐1 blockade led to increased CD8+ T-cell proliferation, cytotoxicity, and viral clearance compared to single-agent therapy. Although not yet studied in leprosy, both 4-1BB and OX40 are found on activated leprosy-specific T-cells and are therefore considered therapeutic targets for leprosy (14, 38).

4.3 Cytokine inhibition

Cytokine-mediated immunosuppression plays a central role in the development and maintenance of T-cell exhaustion in leprosy. Among the immunoregulatory cytokines, interleukin-10 (IL-10) and transforming growth factor-β (TGF-β) are the most prominent suppressive mediators, particularly in LL cases (62, 63). IL-10 is produced by Tregs, tolerogenic macrophages, and, in some instances, exhausted T-cells themselves. Through activation of the Signal Transducer and Activator of Transcription 3 (STAT3) pathway, IL-10 suppresses Th1 cytokine production, inhibits macrophage activation, reduces antigen presentation, and promotes the intracellular persistence of M. leprae. Similarly, TGF-β, secreted by Tregs, macrophages, and other immune cells within leprosy lesions, contributes to immune tolerance by inhibiting T-cell proliferation, suppressing macrophage microbicidal activity, and promoting the differentiation and maintenance of Tregs (62, 63). Elevated expression of TGF-β in LL lesions has been associated with impaired cell-mediated immunity and reduced mycobacterial clearance. Together, IL-10 and TGF-β establish an immunosuppressive microenvironment that favors bacillary survival and progressive T-cell dysfunction. Experimental studies in chronic infectious diseases have demonstrated that simultaneous blockade of IL-10 and PD-1 pathways can synergistically restore T-cell function, highlighting that these pathways operate through complementary and non-redundant mechanisms (62, 63). While IL-10 blockade primarily enhances APCs activation and pro-inflammatory cytokine production, PD-1 blockade directly reinvigorates exhausted T-cell effector responses. However, the interaction between PD-1 signaling and Tregs introduces an additional layer of complexity. PD-1 is expressed on activated T-cells, including Tregs, where it regulates their proliferation, survival, and suppressive capacity. Although PD-1 inhibition is generally intended to restore antimicrobial immunity, evidence from cancer and chronic infection models suggests that PD-1 blockade can paradoxically enhance the proliferation and suppressive activity of specific Treg subsets, resulting in increased secretion of IL-10 and, potentially, TGF-β (62, 63). In the context of leprosy, particularly LL, where Treg-derived immunosuppressive cytokines already contribute significantly to immune dysfunction, this phenomenon could counteract the beneficial effects of checkpoint inhibition. Consequently, the therapeutic outcome of PD-1-targeted immunotherapy may depend on the balance between effector T-cell reactivation and Treg-mediated immunoregulation. These observations support the hypothesis that selective depletion or modulation of Tregs, for example through anti-CD25 antibodies, before immune checkpoint blockade may improve therapeutic efficacy by removing a major source of suppressive cytokines. Understanding the molecular interplay among PD-1 signaling, Treg biology, IL-10 and TGF-β production, and T-cell exhaustion is therefore essential for the rational development of host-directed immunotherapies in leprosy while minimizing the risk of excessive inflammation and disease reactions (11, 64).

4.4 Metabolic reprogramming

Metabolic reprogramming has been identified as a promising approach to overcome T-cell exhaustion and improve the success of immune checkpoint therapies in chronic infectious diseases. Exposure to M. leprae antigens may trigger profound metabolic rewiring of T-cells, resulting in impaired glycolysis, mitochondrial dysfunction, increased oxidative stress and decreased bioenergetic fitness, which all lead to impaired effector function and sustained expression of inhibitory receptors like PD-1 (29, 31). Metformin, a common, widely used antidiabetic drug, has been gaining considerable popularity as an immunometabolic adjuvant because of its impact on cell metabolism and enhancing immune responses. Metformin does this by activating AMP-activated protein kinase (AMPK), which helps maintain the health of mitochondria, increases fatty acid oxidation, and decreases excessive inflammatory and oxidative stress pathways. All these effects can contribute to enhancing metabolic fitness, facilitating the generation of long-lived memory-like T-cells, and partially reinstating effector function compromised in chronic antigen stimulation in T-cells (65). In recent years, metformin has been shown to act synergistically with PD-1 blockade in models of cancer and chronic infection. The mechanism of this is largely attributed to metabolic reprogramming to restore T-cell dysfunction. On one hand, PD-1 inhibition directly shifts exhausted T-cell programs back to their effector functions, while on the other hand, metformin enhances the metabolic capacity necessary to sustain their functions. This combination has been associated with enhanced cytokine production, increased proliferation of antigen-specific T-cells, improved mitochondrial function, and superior pathogen or tumor control compared with either intervention alone (66). In the context of leprosy, particularly multibacillary leprosy cases, where persistent bacillary antigen exposure may drive both immune exhaustion and metabolic impairment, metformin could potentially augment the effectiveness of PD-1-targeted immunotherapy. Furthermore, metformin has been reported to enhance autophagy and intracellular antimicrobial mechanisms in mycobacterial infections, suggesting additional benefits beyond T-cell modulation (67). While there is still little evidence in leprosy, these results suggest that a new approach to host-directed therapy might be a combination of metabolic reprogramming strategies like metformin and immune checkpoint blockade, which could lead to better clearance of mycobacteria and recovery of protective cellular immunity. To establish the best time, dose, and safety of such combination therapies in leprosy, further mechanistic and clinical studies are needed (29, 31).

4.5 T-reg targeting

Tregs are key mediators of immune regulation in leprosy and have been implicated in determining the clinical spectrum of disease as well as the development of reactional states. Classical studies have demonstrated that Tregs, characterized by the expression of CD4, CD25, and Forkhead box P3 (FOXP3), are more abundant and functionally active in multibacillary forms of leprosy, particularly LL, where they contribute to suppression of protective CMI (26, 43). Through the secretion of immunoregulatory cytokines such as IL-10 and TGF-β, Tregs inhibit effector T-cell responses, reduce macrophage activation, and promote bacillary persistence. Furthermore, alterations in Treg frequency and function have been reported during reactional states, suggesting that an imbalance between regulatory and effector immune responses may contribute to the immunopathology of both T1R and T2R (10, 64). Tregs represent an attractive target for host-directed immunotherapy in leprosy. Evidence from chronic infections and cancer has shown that excessive Treg activity can contribute to T-cell exhaustion and limit the effectiveness of immune checkpoint blockade. Importantly, PD-1 signaling is also involved in regulating Treg function, and PD-1 blockade may paradoxically enhance the suppressive activity of certain Treg subsets, including increased production of IL-10 (11). Consequently, strategies aimed at selectively modulating or depleting Tregs, such as anti-CD25-based approaches or therapies targeting Treg-associated signaling pathways, may enhance the efficacy of checkpoint inhibitors by reducing immunosuppressive constraints on effector T-cells. Transient Treg depletion using denileukin diftitox (IL-2 diphtheria toxin conjugate) or low-dose cyclophosphamide prior to checkpoint blockade could transiently lift suppression, allowing effector T-cell expansion (11, 64). However, because Tregs also play a critical role in maintaining immune homeostasis and preventing excessive inflammation, indiscriminate depletion may increase the risk of immune-mediated tissue damage and leprosy reactions. Therefore, future therapeutic approaches should focus on achieving a balanced modulation of Treg activity that restores antimicrobial immunity while preserving immunological tolerance (10, 26, 43).

4.6 Therapeutic vaccination along with immune checkpoint blockade

Vaccination as a therapeutic measure in addition to immune checkpoint blockade could be a potential host-directed therapy for leprosy, which is still in the speculative stage. The rationale is that therapeutic vaccines can stimulate antigen-specific immune responses by increasing Mycobacterium leprae-specific T-cells, while immune checkpoint blockade, like PD-1 inhibitors, can rejuvenate the function of exhausted cells and boost their effector activity. Therapeutic vaccination along with a checkpoint inhibitor has proven to be more effective than either approach alone in preclinical and clinical studies of cancer and chronic viral infections (68, 69). However, there have been no clinical trials comparing therapeutic vaccination and immune checkpoint blockade in leprosy so far. More work needs to be done in the preclinical setting to clearly demonstrate the ability of therapeutic vaccines, such as mycobacterial vaccine candidates or immunotherapeutic formulations, to safely complement checkpoint inhibitor therapy to improve antimycobacterial immunity without the development of excessive inflammation or leprosy reactions. This method should therefore be considered as a potential future treatment option (15, 17, 32).

5 Discussion

The past five years have solidified a compelling body of evidence that the T-cell exhaustion is a key and actionable mechanism responsible for profound immunosuppression seen in LL (12, 13, 31, 70–72). In leprosy, a hierarchy of programs is integrated to drive T-cell exhaustion, which includes prolonged TCR signaling due to high burden of M. leprae antigens, increased expression of immune checkpoint molecules on both CD4⁺ and CD8⁺ T-cells (PD-1, LAG-3, TIM-3, TIGIT), and transcriptional/epigenetic repression (TOX and NFAT-driven networks) (12, 13, 31). This program leads to a gradual decrease in cytokines, impaired proliferation, and impaired metabolism, which together make the exhausted T-cells incapable of controlling the intracellular bacilli (29, 73). Independent studies from India, Brazil, and China have demonstrated increased expression of immune checkpoint molecules in LL patients (25–27, 29, 30, 44). Indian studies have primarily reported elevated PD-1 expression on circulating T-cells (25, 26, 44), whereas studies from Brazil and China have shown significantly increased expression of both PD-1 and LAG-3 on circulating as well as lesional T-cells, suggesting a more profound state of T-cell exhaustion in patients with lepromatous disease (27, 29, 30). This evidence now supports progression to clinical trials that combine PD-1 and/or LAG-3 blockade as an adjunctive therapy in patients with high bacterial burdens or persistent immune dysfunction after MDT. New therapies are in development, such as dual checkpoint blockade, costimulatory agonists, cytokine modulation, and Treg-targeted therapies (32 ,45).

Immune exhaustion is now recognized as a target and potentially reversible approach and has to be carefully researched for efficacy, safety, and patient selection in leprosy research. The understanding of the tissue-centric exhaustion and Treg-mediated suppression in leprosy, alongside TLR–checkpoint crosstalk, provides a broader understanding of the immune regulation in tuberculosis, chronic viral infections, and other mycobacterial diseases. Leprosy is a neglected tropical disease that encompasses a worldwide burden, offers a unique opportunity to develop immune checkpoint–based therapies, and demands global capacity building.

Despite their promising therapeutic potential, immune checkpoint inhibitors face several important limitations that currently restrict their application in chronic infectious diseases, including leprosy, HIV, and hepatitis B virus (HBV) infection (12, 14). First, T-cell exhaustion is a complex and multifactorial process involving the coordinated action of multiple inhibitory receptors, transcriptional programs, metabolic alterations, and immunosuppressive cytokines (15). Consequently, blockade of a single pathway, such as PD-1/PD-L1, often results in only partial restoration of T-cell function because alternative inhibitory receptors, including TIM-3, TIGIT, LAG-3, CTLA-4, and CD160, may remain active and continue to suppress immune responses. This compensatory regulation has been observed in both chronic viral infections and cancer, and may similarly limit therapeutic efficacy in leprosy (12, 14, 15, 17, 31).

Second, immune restoration following checkpoint blockade carries the risk of excessive inflammation and immunopathology. In HIV and HBV infection, enhanced immune activation may contribute to tissue damage, immune reconstitution inflammatory syndromes, or exacerbation of underlying inflammatory processes. Similar concerns exist in leprosy, where restoration of T-cell function could potentially precipitate or worsen leprosy reactions, including T1R and T2R, both of which are associated with significant morbidity and risk of nerve injury (10, 72, 74–76). Third, the durability of immune restoration remains uncertain, as exhausted T-cells may exhibit fixed epigenetic and transcriptional alterations that are not completely reversible through checkpoint inhibition alone (12, 31).

Additional limitations include the lack of validated biomarkers for identifying patients most likely to benefit from treatment, uncertainty regarding optimal timing and duration of therapy, limited understanding of long-term safety, and the absence of clinical trials evaluating checkpoint inhibitors in leprosy (74). These observations suggest that future therapeutic strategies may require combination approaches involving immune checkpoint inhibitors together with therapeutic vaccination/cytokine modulation/metabolic reprogramming/co-stimulatory receptor agonists. Therefore, while immune checkpoint inhibitors represent a promising avenue for host-directed therapy, substantial mechanistic, translational, and clinical research is still required before their routine use in leprosy can be considered (12, 31, 74).

6 Conclusion

Despite significant advances in understanding T-cell exhaustion in leprosy, several critical research gaps remain unaddressed. To date, no published human clinical trials have evaluated PD-1 or multi-checkpoint blockade in leprosy patients. Most evidence comes from ex vivo studies of patient samples. While PD-1 blockade transiently restores effector function, the underlying epigenetic demethylation of the Pdcd1 locus persists. Whether combinations with epigenetic modulators (DNA methyltransferase inhibitors, histone deacetylases) can durably reprogram exhaustion remains unexplored in leprosy. Tissue-specific signs of T-cell exhaustion are more pronounced in LL lesions and less in the peripheral blood, indicating that immune dysfunction is more distinct in the lesion site. This underscores the promise of intralesional checkpoint inhibitor therapy to improve local immune activation while minimizing systemic toxicity, which is being tested in melanoma. Recent work has identified progenitor-like (TCF-1+, PD-1^intermediate), intermediate/transitional exhausted, and terminally differentiated (TCF-1-, PD-1^high) CD8 T-cell subsets within exhausted pools. Characterization of leprosy-exhausted T-cells, their response to checkpoint blockade, and prospective randomized trials that combine checkpoint inhibitors with MDT are critical to understand the therapeutic effect, optimal dose, timing, and safety.

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Footnotes

Edited by: Emmanuel Siddig, University of Khartoum, Sudan

Reviewed by: Katherine Kelda Castro, Oswaldo Cruz Foundation (FIOCRUZ), Brazil

Ayman Azhary, Omdurman Islamic University, Sudan