Understanding Autoimmune Response Mechanisms in Leprosy

Abstract

Leprosy is a persistent granulomatous disease that occurs due to Mycobacterium leprae infection. Leprosy primarily affects peripheral nerves, skin, and mucous membranes. Reactions in leprosy are immunological complications that may occur at any stage of disease progression, irrespective of treatment status. This review explores the potential link between M. leprae infection and autoimmune responses, emphasizing the role of immune dysregulation in leprosy reactions. We have delineated a comprehensive exploration of reactions in leprosy within the framework of autoimmunity, drawing insights from previously documented research. Biotic elements, including bacteria, might be associated with an imbalance in the host’s homeostatic mechanism, leading to an autoimmune response. Mycobacteria have been reported for their potential to modulate host immune responses in both humans and experimental animal models. Pathogens can induce autoimmunity via molecular mimicry during the initiation of the disorder, and they may promote chronic pathologies with inflammation and/or superantigens, prompted by the loss of immunological tolerance to self-antigens, which can lead to systemic or organ-specific damage. Autoimmune manifestations in leprosy are triggered by the impairment of the regulatory mechanisms of the host due to M. leprae infection. Leprosy reactions are influenced by autoimmune processes triggered by M. leprae infection. Understanding the immunological interactions between the pathogen and the host may provide insights into disease management and therapeutic strategies.

Introduction

Leprosy reactions are immunological complications that may manifest before, during, or following treatment, affecting approximately 30–50% of individuals with leprosy.[1,2] Erythematous inflammation of existing lesions, the appearance of new lesions, and the appearance of worsening signs of neuritis are characteristic symptoms of Type 1 reaction (T1R). This is associated with infiltration of lymphocytes, epithelioid cells, Langerhans giant cells, and edema in conjunction with a decrease in bacillary load in the lesions.[3] The upgradation of the cell-mediated immunity (CMI) in T1R is due to the increased reactivity to M. leprae antigenic epitopes, demonstrated by the in vitro lymphocyte transformation test (LTT) and in vivo delayed-type hypersensitivity (DTH) in the skin.[4] This phenomenon may result in a local reduction in bacillary load and an increase in T-cell reactivity, leading to nerve damage.[5] Erythema nodosum leprosum (ENL), also known as Type 2 reaction (T2R), occurs in almost 25% of leprosy cases and is characterized by the emergence of erythematous, inflamed nodules and papules. The pathogenesis of ENL involves immune complex deposition in the lesions with an alteration of CMI. The complement activation and neutrophil relocation,[6] along with the release of tissue-damaging interleukins (ILs) 12 (IL-12) and tumor necrosis factor-alpha (TNF-), are crucial factors in the evolution of the disease.[7,8] Both Type 1 and Type 2 reactional episodes may occur in the same patient, particularly in borderline lepromatous (BL) and subpolar lepromatous leprosy (LL) patients, at different time intervals.[9,10] Despite these findings, the pathogenesis of disease and subsequent reactions resulting in huge tissue damage are not well understood. Therefore, our group conducted an in-depth investigation to establish additional possible mechanisms that could be mediated by a loss of discrimination between self- and non-self-recognition by the immune system of the host and might explain the development of the autoimmune reaction. Such an autoimmune reaction can be initiated by similar antigenic determinants of the pathogen, and in turn, while making an effort to destroy the pathogen, it might cause functional loss of the host tissue and lead to clinical manifestations of tissue damage.

Bacterial persistence and macrophage polarization

Among the cells of the immune system, macrophages are significant host cells needed for the persistence of M. leprae and Mycobacterium tuberculosis (M. tuberculosis).[11] M. leprae is also well known for establishing its survival in the mammalian host by its’ affinity to immunologically privileged sites, essentially the peripheral nerves, and by parasitizing selectively non-professional phagocytic cells like major histocompatibility complex (MHC) class II negative Schwann cells.[12,13] It has been recently confirmed that, through a mechanism regulated by Jagged 1 (JAG1), a protein localized inside the vascular endothelium, endothelial cells persuade monocytes to differentiate into M1 macrophages upon activation with the help of interferon-gamma (IFN-γ), while unstimulated endothelial cells trigger monocytes to turn out to be M2 macrophages.[14]

Tatano et al.[15] have reported the presence of a novel macrophage population that is functionally divergent from M1 and M2 macrophages and expresses unique phenotypes designated as mycobacterial infection-induced suppressor (MIS) macrophages, and played a major role through the production of IL-6 and TGFβ, but not reducing the role of IL-21 and IL-23 as demonstrated both in vitro and in vivo with BALB/c mice. These cells downregulate the expression of Th1 and Th2 cytokines; however, they markedly augment the production of IL-17A and IL-22 by expanding the Th17 cell lineage. It has been mentioned that MIS macrophages play a fundamental role in the enhancement of Th17 polarization and suppressor action against T-cell mitogenesis. The Th17 cells have also been known to play a pathogenic role in autoimmune disease.[16,17] The fate of macrophage polarization toward autoimmunity due to antigens specific to M. leprae is illustrated in Figure 1.

Figure 1.

The effect of the stimulus on the development of the macrophage lineage

Trigger for autoimmune manifestations

Autoimmunity can be defined as the consequence of a loss of control in the balance of homeostatic mechanisms, leading to a defect in “self” recognition. All healthy individuals have autoreactive B and T cells, meaning that the intra-thymic negative selection is still not well defined. All organs have an extensive spectrum of major autoantigens, which are recognized by peripheral physiological autoreactive T cells and are widely acknowledged as targets for numerous autoimmune diseases.[18] An essential characteristic of the immune system is tolerance. Tolerance involves the ability of the immune system to distinguish between self and non-self-antigens and allow it to detect and react to foreign antigens but not to self-antigens.[19] The critical steps in the pathogenesis of autoimmune diseases are the activation and clonal expansion of autoreactive lymphocytes. Numerous mechanisms can be employed to explain the modalities in T- and B-cell activation, which ultimately break down tolerance. Among the major proposed mechanisms, polyclonal lymphocyte activation and inflammation induced by an infection, which subsequently leads to an elevation in the immunogenicity of organ-specific autoantigens, are prominent in the breakdown of immune tolerance. Activation of autoreactive T cells through up-regulation of Th-1 cytokines and/or other designated molecules (MHC, B7.1/CD28), infection or destruction of the CD4+ T-cell subset in a preferential manner, and virus-encoded superantigens released de novo by self-epitopes secondary to a virus-specific response can induce autoimmune disease triggered by infectious agents. Moreover, T-cell-mediated damage (epitope spreading) and cross-reactivity with viral or pathogen epitopes (molecular mimicry) are also reasons for the development of autoimmunity.[20] Damian first coined the term “molecular mimicry” in 1964 to specify antigens that are common to the host and its parasites, whether they are of bacterial, viral, or eukaryotic origin.[21] The finding that the measles virus phosphoprotein and a herpes simplex virus Type 1 protein cross-reacted with an intermediate filament protein of human cells, while generating monoclonal antibodies, offered the first indications that molecular mimicry might be involved in autoimmune diseases.[22] Epidemiological, clinical, and experimental findings have established the association between infectious agents and autoimmune disease, indicating the existence of cross-reactivity between host “self” antigens and microbial determinants. Molecular mimicry can be defined as similar structures (either having linear or conformational epitopes) shared by molecules from divergent genes or their protein products.

Evidence of autoimmunity in leprosy

The imbalance in the host’s homeostatic mechanism may be caused by biotic components of the ecosystem, such as bacteria, which subsequently result in the development of an autoimmune response. It has been described that mycobacteria modulate host immune responses in humans and experimental animals.[23] The clinical manifestations of leprosy are primarily governed by the host’s immunity to M. leprae. About 95% to 99% of individuals infected with the bacterium do not develop overt disease,[24,25] due to the presence of adequate protective immunity. An epidemiological study of comparative genomics and single-nucleotide polymorphisms on M. leprae genome information reported on a molecular basis revealed that M. leprae isolates exhibit very limited genetic diversity. This study established that the pathogen lacking minimal genetic diversity has continuously infected the host since ancient times.[26] It has been anticipated that infection may stimulate tolerance breakdown through several nonspecific mechanisms, leading to the development of autoimmunity.[27] Furthermore, it has been found that numerous chronic diseases, including tuberculosis and leprosy, have been noted to disturb the balance of the homeostatic mechanism, resulting in an autoimmune response.[28,29,30] It has been postulated that hosts’ immune responses against M. leprae antigens are responsible for tissue damage and neural impairment in leprosy patients since M. leprae is remarkably non-toxic in nature.[31] Furthermore, in 1987, a classical experimental model of adjuvant arthritis was demonstrated by Van Eden et al.,[32] which was produced by a single immunization of Lewis rats with a variant of Freund’s complete adjuvant to establish antigenic mimicry between mycobacteria and cartilage proteoglycans.

Molecular mimicry and autoantibodies in leprosy

In several studies, it has been established that the pathogen may circumvent host immunity’s detection and destruction by exhibiting antigenic or molecular mimicry with the host tissue.[33] The escape of M. leprae from host immune surveillance may be possible due to antigenic mimicry, which could also be responsible for the long incubation period before the development of clinical manifestations of the disease. The aforementioned antigenic mimicry may enable M. leprae to survive in the human host in a condition when the antigens are not recognized as “non-self,” a characteristic environment noted in LL, where no protective immunity could be found even after the patient’s tissues are loaded with bacteria. Furthermore, the antigenic mimicry between M. leprae and the host may initiate an autoimmune response against the host’s own antigens. Monoclonal antibodies against M. leprae have been reported to cross-react with human nerve and skin cells in earlier studies. It has been proposed that the emergence of autoimmune clinical manifestations of leprosy may be triggered by this antigenic similarity.[34,35] Human epidermal cytokeratin 1/2 and mycobacterial 65-kDa heat-shock proteins have been found to share a carboxy-terminal epitope.[36] Recently, significantly higher positive rates of autoantibodies such as anticardiolipin antibody (ACA), anti-nuclear antibody (ANA), extractable nuclear antigen antibody (ENA), anti-streptolysin O (ASO), and rheumatoid factor (RF) in cured leprosy patients compared to controls have been reported, and it has been suggested that autoimmune responses may influence the development and prognosis of leprosy.[37]

Collectively, these findings suggest that autoantibodies against keratin might be one of the reasons for the autoimmune responses in leprosy patients. This is supported by the fact that most leprosy lesions are observed on the skin, and follicular keratosis is a common clinical manifestation, particularly during the T1R in tuberculoid leprosy. Moreover, Lazarine leprosy, a rare and atypical manifestation of leprosy primarily occurring within the borderline spectrum, is characterized by spontaneous ulcerative T1R arising from an exaggerated hypersensitivity response.[38] This condition may represent tissue damage mediated by autoantibodies. Borderline tuberculoid (BT) patients with T1R have been reported to show a consistent presence of immune response-associated (Ia) antigens on all keratinocytes, while lepromatous patients undergoing ENL reactions showed only a patchy distribution.[39,40] These research findings suggest that the activation of local T-cells leads to the production of terminal lymphokines, IFNγ, with subsequent production of epidermal growth factors and induction of Ia on epidermal cells, which may be an important event in reactional leprosy states. Additionally, it is intriguing that individuals previously categorized as “anergic” due to lepromatous leprosy can experience a temporary restoration of T-cell reactivity as the disease progresses naturally.[39] Earlier studies have suggested that the immunological cascade involved in leprosy also involves an autoimmune response against nerve antigens,[41] leading to demyelination. This may be caused by molecular mimicry between mycobacterial proteins (such as ferredoxin-NADP-reductase and a conserved mycobacterial membrane protein) and myelin P0, which has a major role in compacting myelin through homotypic interactions, as suggested by Vardhini and associates.[42] Myelin basic protein (MBP) has also been identified as an antigen in circulating immune complexes in patients with LL. It has been suggested that MBP released from damaged nerves after infection with M. leprae evokes an immune response that elicits anti-MBP antibodies that target MBP in peripheral nerves. The demyelination and nerve degeneration seen in leprosy patients may be mediated by this mechanism.[43] Additionally, earlier studies have suggested a connection between MBP antibodies and neurodegeneration in leprosy patients, either directly or indirectly.[41,44] The humoral immune system experiences substantial changes because of infection with M. leprae, which includes anomalous responses that are often linked to the autoimmune syndrome. The survival of M. leprae and its ability to evade the host immune response may be aided by the similarities between the stress proteins of the host and M. leprae antigens. A study revealed that the levels of antibodies against the hosts’ cytoskeletal and stress proteins were found to differ between multibacillary and paucibacillary cases of leprosy, with higher levels associated with LL, especially in patients who were relapsing with the disease. They concluded that this finding raises the possibility that the conserved proteins of this group are involved in molecular mimicry reactions with M. leprae antigens or stress proteins.[45] It has been recently demonstrated by Kotb et al.[46] that LL patients have a higher risk of developing autoimmune disease than healthy individuals. The group identified that a CD5+ B cell and CD19+ (total) B cells are significantly increased in LL patients, and they hypothesized that these B lymphocytes may play a role in altering patients’ protective immune responses to autoimmune responses. Additionally, a considerable percentage of CD19+ (total) B cells may contribute to the emergence of eye complications in LL patients.

The level of autoantibodies is a hallmark of autoimmune diseases, and these immunological abnormalities have been reported in many chronic infections. Numerous autoantibodies have been associated with leprosy, particularly LL. These autoantibodies may be organ-specific, like those that target the thyroid, nerves, testes, and gastric mucosa. On the other hand non-specific autoantibodies, like RF,[47] anti-neutrophil cytoplasmic autoantibodies (ANCA),[48] ANA,[49] and anti-phospholipids,[50] particularly in the Lucio phenomenon, as well as cryoglobulins, have been reported.[51] In addition to the autoantibodies previously mentioned, patients with leprosy have been found to have other autoantibodies, including anti-mitochondrial,[52] anti-actin and anti-myosin,[45] and anti-endothelial cell antibodies.[53] These antibodies have been shown to trigger apoptosis in endothelial cells and may be involved in the pathogenesis of leprosy.[54] The major autoantibodies reported in leprosy are summarized in Figure 2.

Figure 2.

Autoantibodies in leprosy

Numerous studies have reported on the widely variable prevalence of autoantibodies in leprosy. In most studies, positivity for RF and ANA has been reported more frequently in LL patients compared to those with BT, varying from 0% to 30% of cases.[55] The presence of ANAs might have been due to the possibility of cross-reactivity between mycobacterial antigens and human DNA. The presence of ANCAs (cytoplasmic ANCA and perinuclear ANCA) has been reported in a wide spectrum of diseases, such as rheumatoid arthritis, systemic lupus erythematosus, ulcerative colitis, subacute bacterial endocarditis, tuberculosis, human immunodeficiency virus infection, and leprosy. The ANCA prevalence has been reported to be higher in BL/LL cases compared to tuberculoid (TT)/ BT leprosy cases. Medina et al.[48] noticed myeloperoxidase (MPO)-related perinuclear ANCA in LL patients and suggested that cross-reactivity to heat-shock proteins may have contributed to the origin of ANCA in leprosy and other infectious diseases. The likelihood of ANCA in leprosy patients is due to polyclonal B-cell activation by M. leprae antigens, with the participation of T cells. In another study, 62.5% of patients were found to have the presence of cytoplasmic ANCA, especially in lepromatous patients. In a cohort study, the frequency of anti-beta-2 glycoprotein I (β2-GPI) and anti-cardiolipin antibodies were found to be increased in BL/LL patients compared to healthy controls.[56] The frequency of positivity of autoantibodies across the leprosy spectrum is summarized in Figure 3.

Figure 3.

Frequency of positivity of autoantibodies across the leprosy spectrum

It has also been shown that anti-endothelial cell antibodies, present in leprosy patients, reacted with calreticulin, vimentin, tubulin, and heat-shock protein 70.[53] It has been mentioned that the antigenic determinants in normal human epidermis and dermis are recognized by monoclonal antibodies against M. leprae. These antibodies have been reported to react with 12 kDa, 18 kDa, and 65 kDa antigens of M. leprae on western blot immunoassays.[34] Recent studies on leprosy patients with T1R have identified five salivary proteins, including S100-A9, 35.3 kDa, and 41.5 kDa proteins, serpin peptidase inhibitor (clade A), and cystatin SA-III, as well as four slit skin scraping proteins, namely 41.4 kDa protein, alpha-1 antitrypsin, vimentin, and keratin 1, as cross-reactive antigens by western blot with purified IgG from M. leprae soluble antigen hyperimmunized rabbit sera. Additionally, the study predicted a total of 22 B-cell epitopes that could be further validated to predict T1R in leprosy.[57]

Mimicking epitopes of M. leprae antigens evokes an autoimmune response against the host’s tissues and could elucidate “the onset of reversal reaction” in tuberculoid leprosy, wherein extensive granuloma formation with edema is noted without the presence of M. leprae.[34] Singh et al.[58] observed that leprosy patients have elevated levels of anti-keratin antibodies (AkAbs) compared to healthy individuals. The highest levels of AkAbs were found in patients with T1R, followed by LL, BL, ENL, and TT/BT leprosy. Additionally, the study found a clinical correlation between the level of AkAbs and the number of lesions present in leprosy patients, which is noteworthy. A phylogenetically conserved family of proteins, known as heat-shock proteins (HSPs), is expressed during chronic inflammation and other types of physiological stress conditions that trigger both humoral and cellular immune responses against microorganisms.[59] In vivo, autoimmune phenomena may also be evoked by HSPs during persistent mycobacterial infections. A study identified seven B cell epitopes of cytokeratin-10 of keratin (host) and 65 kDa HSP (groEL2) of M. leprae that were noted to share mimicking epitopes that augment molecular mimicry.[58]

These findings also provide clinical evidence of the role of AkAbs in the development of T1R. Degenerative changes in muscles like “leprous myositis”[60] and “leprous neuromyositis”[61] have also been reported. Singh et al.[62] have recently reported the presence of molecular mimicry between tropomyosin and the probable ATP-dependent Clp protease ATP-binding subunit of M. leprae. This mimicry might be responsible for “leprous myositis” and muscular weakness. Moreover, the humoral and CMI responses against 8 B cell epitopes and 5 T-cell epitopes, respectively, have been observed to be significantly higher in T1R in comparison with non-reactional leprosy patients.[63] A comparative view of the level of autoantibodies against MBP, keratin, and myosin is shown in Figure 4.

Figure 4.

A comparative view of the level of autoantibodies against the host-specific proteins across T1R and Non-reactional TT/BT leprosy patients

The presence of the neural antigen, MBP, in circulating immune complexes in LL patients was reported and indicated that it could be associated with the pathogenesis of leprosy. The release of MBP after M. leprae nerve damage could evoke anti-MBP antibodies, which could react with MBP in the peripheral nerve and subsequently induce circulating immune complex formation.[43] According to Singh et al.[64] leprosy patients across the spectrum have elevated levels of anti-MBP antibodies. They also reported that four B cell epitopes, myelin A1 and M. leprae proteins, 50S ribosomal L2, and lysyl tRNA synthetase, are cross-reactive. Earlier studies have proven that M. leprae binds to a nerve protein, myelin protein zero (P0), which is exclusively found in peripheral nerves and may be a crucial step in M. leprae’s binding and invasion of Schwann cells. According to a study conducted by Vardhini et al. in 2004, a comparison between myelin P0 and M. leprae proteins identified two proteins[42] that exhibited similar characteristics. These two proteins were ferredoxin nicotinamide adenine dinucleotide phosphate (NADP) reductase and a conserved membrane protein that shared similarities with the query sequence. The functional importance of these proteins could potentially involve molecular mimicry, receptor binding, and cell signaling events that contribute to neurodegeneration. The demyelination of nerves during the progression of the disease may be significantly influenced by antigenic mimicry, according to the preceding findings. Many autoimmune diseases are marked by the production of autoantibodies that target host proteins or form part of immune complexes deposited in tissues. These autoantibodies can activate the complement system, leading to tissue damage and systemic inflammation.[65] In leprosy, deposits of the membrane attack complex or the soluble terminal complement complex have been identified in association with nerve damage.[66] Additionally, elevated serum levels of terminal complement complex have been linked to leprosy reactions and have been proposed as a valuable diagnostic marker for identifying patients at risk of reactional episodes.[67]

Furthermore, the transgenic model has been useful in investigating the mechanisms underlying autoimmune diseases and in developing therapeutic approaches to manage them. Additionally, this model has been effective in identifying self-determinants that are associated with or may augment the autoimmune response.[20] In leprosy patients, the similarity between host proteins and mycobacterial antigens may be at the level of peptide sequence or the structural level of proteins; however, the exact mechanism has not been fully understood. The wide-ranging autoantibody profile observed in LL could be attributed to several mechanisms, including polyclonal B-cell activation, deficiencies in suppressor T cells, and molecular mimicry between microbial and host components. These mechanisms can cause cross-reactions, resulting in autoimmune phenomena.

Conclusion

The present review highlights the initiation and promotion of autoimmune manifestations by mycobacteria, with special reference to leprosy, which is caused by infections, in addition to the genetic predisposition of the host. The pathogen can trigger autoimmunity through molecular mimicry during the initiation of disease. It can promote chronic pathologies by inducing inflammation and/or modulating immunological tolerance to self-antigens, which can cause systemic or organ-specific damage. A better understanding of the mechanisms operating between the host and pathogen may lead to the development of strategic therapies to prevent pathological changes during the disease.

Ethics approval

Ethics approval was waived for this study because no patients’ data were reported.

Conflicts of interest

There are no conflicts of interest.

Use of artificial intelligence (AI)

The preparation of this manuscript was carried out entirely by the author without the use of artificial intelligence technologies.

Funding Statement

Nil.