Effective vaccination of mice against leprosy bacilli with subunits of Mycobacterium leprae

Pathogenesis and Host Immune Response in Leprosy

Leprosy is an ancient insidious disease caused by Mycobacterium leprae, where the skin and peripheral nerves undergo chronic granulomatous infections, leading to sensory and motor impairment with characteristic deformities. Susceptibility to leprosy and its disease state are determined by the manifestation of innate immune resistance mediated by cells of monocyte lineage. Due to insufficient innate resistance, granulomatous infection is established, influencing the specific cellular immunity. The clinical presentation of leprosy ranges between two stable polar forms (tuberculoid to lepromatous) and three unstable borderline forms. The tuberculoid form involves Th1 response, characterized by a well demarcated granuloma, infiltrated by CD4⁺ T lymphocytes, containing epitheloid and multinucleated giant cells. In the lepromatous leprosy, there is no characteristic granuloma but only unstructured accumulation of ineffective macrophages containing engulfed pathogens. Th1 response, characterised by IFN-γ and IL-2 production, activates macrophages in order to kill intracellular pathogens. Conversely, a Th2 response, characterized by the production of IL-4, IL-5 and IL-10, helps in antibody production and consequently downregulates the cell-mediated immunity induced by the Th1 response. M. lepare has a long generation time and its inability to grow in culture under laboratory conditions makes its study challenging. The nine-banded armadillo still remains the best clinical and immunological model to study host-pathogen interaction in leprosy. In this chapter, we present cellular morphology and the genomic uniqueness of M. leprae, and how the pathogen shows tropism for Schwann cells, macrophages and dendritic cells.

Combination chemoprophylaxis and immunoprophylaxis in reducing the incidence of leprosy

Leprosy is a complex infectious disease caused by Mycobacterium leprae that is a leading cause of nontraumatic peripheral neuropathy. Current control strategies, with a goal of early diagnosis and treatment in the form of multidrug therapy, have maintained new case reports at ~225,000 per year. Diagnostic capabilities are limited and even with revisions to multidrug therapy regimen, treatment can still require up to a year of daily drug intake. Although alternate chemotherapies or adjunct immune therapies that could provide shorter or simpler treatment regimen appear possible, only a limited number of trials have been conducted. More proactive strategies appear necessary in the drive to elimination. As a prevention strategy, most chemoprophylaxis campaigns to date have provided about a 2-year protective window. Vaccination, in the form of a single bacillus Calmette–Guérin (BCG) immunization, generally provides ~50% reduction in leprosy cases. Adapting control strategies to provide both chemoprophylaxis and immunoprophylaxis has distinct appeal, with chemoprophylaxis theoretically buttressed by vaccination to generate immediate protection that can be sustained in the long term. We also discuss simple assays measuring biomarkers as surrogates for disease development or replacements for invasive, but not particularly sensitive, direct measures of M. leprae infection. Such assays could facilitate the clinical trials required to develop these new chemoprophylaxis, immunoprophylaxis strategies, and transition into wider use.

Experimental models of leprosy

Leprosy (Hansen’s disease) is a chronic granulomatous bacterial disease which mainly affects skin and peripheral nervous system. Leprosy is caused by the obligate intercellular pathogen known as Mycobacterium leprae. Creating experimental models of leprosy is associated with serious problems due to biological characteristics of the pathogen. Numerous attempts to develop experimental models on different types of animals resulted in a few reproducible models on mice and nine-banded armadillos. Strains of knockout mice with genetic defects caused by site-directed mutagenesis are used as a basis for different leprosy models. Experimental models of leprosy are used for screening of anti-leprosy drugs, detection of drug resistance, studies on the pathogenesis of leprosy, production and evaluation of viability of M. leprae, developing of anti-leprosy vaccines.

Advances and hurdles on the way toward a leprosy vaccine

Prevalence rates for leprosy have declined sharply over the past 20 y, with this decline generally attributed to the WHO multi-drug therapy (MDT) campaign to provide free-of-charge treatment to all diagnosed leprosy patients. The success of this program appears to have reached its nadir, however, as evidenced by the stalled decreases in both global prevalence and new case detection rates of leprosy. Mass BCG vaccination for the prevention of tuberculosis (TB) at national levels has had a positive effect on leprosy decline and is often overlooked as an important factor in current leprosy control programs. Because BCG provides incomplete protection against both TB and leprosy, newer more effective TB vaccines are being developed. The impact that application of these vaccines will have on current leprosy control programs is unclear. In this review, we assess the need for vaccines within leprosy control programs. We summarize and discuss leprosy vaccine strategies that have been deployed previously and discuss those strategies that are currently being developed to augment recent breakthroughs in leprosy control.

Vaccination with the ML0276 antigen reduces local inflammation but not bacterial burden during experimental Mycobacterium leprae infection

Leprosy elimination has been a goal of the WHO for the past 15 years. Widespread BCG vaccination and multidrug therapy have dramatically reduced worldwide leprosy prevalence, but new case detection rates have remained relatively constant. These data suggest that additional control strategies, such as a subunit vaccine, are required to block transmission and to improve leprosy control. We recently identified several Mycobacterium leprae antigens that stimulate gamma interferon (IFN-gamma) secretion upon incubation with blood from paucibacillary leprosy patients, a group who limit M. leprae growth and dissemination. In this study, we demonstrate that M. leprae-specific mouse T-cell lines recognize several of these antigens, with the ML0276 protein stimulating the most IFN-gamma secretion. We then examined if the ML0276 protein could be used in a subunit vaccine to provide protection against experimental M. leprae infection. Our data demonstrate that combining ML0276 with either a Toll-like receptor 4 (TLR4) (EM005), TLR7 (imiquimod), or TLR9 (CpG DNA) agonist during immunization induces Th1 responses that limit local inflammation upon experimental M. leprae infection. Our data indicate that only the ML0276/EM005 regimen is able to elicit a response that is transferable to recipient mice. Despite the potent Th1 response induced by this regimen, it could not provide protection in terms of limiting bacterial growth. We conclude that EM005 is the most potent adjuvant for stimulating a Th1 response and indicate that while a subunit vaccine containing the ML0276 protein may be useful for the prevention of immune pathology during leprosy, it will not control bacterial burden and is therefore unlikely to interrupt disease transmission.

Effect of vaccination with refined components of the organism on infection of mice with Mycobacterium leprae

Only native products of Mycobacterium leprae, whether cell wall, cytosol, or membrane derived, can confer protective immunity against challenge in the mouse footpad. Previously, recombinant proteins were shown to be ineffective. The cell wall skeleton-the mycolyl-arabinogalactan-peptidoglycan complex-devoid of proteins is not protective.

Protection against virulent Mycobacterium avium infection following DNA vaccination with the 35-kilodalton antigen is accompanied by induction of gamma interferon-secreting CD4(+) T cells

Mycobacterium avium is an opportunistic pathogen that primarily infects immunocompromised individuals, although the frequency of M. avium infection is also increasing in the immunocompetent population. The antigen repertoire of M. avium varies from that of Mycobacterium tuberculosis, with the immunodominant 35-kDa protein being present in M. avium and Mycobacterium leprae but not in members of the M. tuberculosis complex. Here we show that a DNA vector encoding this M. avium 35-kDa antigen (DNA-35) induces protective immunity against virulent M. avium infection, and this protective effect persists over 14 weeks of infection. In C57BL/6 mice, DNA vaccines expressing the 35-kDa protein as a cytoplasmic or secreted protein, both induced strong T-cell gamma interferon (IFN-gamma) and humoral immune responses. Furthermore, the antibody response was to conformational determinants, confirming that the vector-encoded protein had adopted the native conformation. DNA-35 immunization resulted in an increased activated/memory CD4(+) T-cell response, with an accumulation of CD4(+) CD44(hi) CD45RB(lo) T cells and an increase in antigen-specific IFN-gamma production. The protective effect of the DNA-35 vectors against M. avium infection was comparable to that of vaccination with Mycobacterium bovis BCG and significantly greater than that for previous treated infection with M. avium. These results illustrate the importance of the 35-kDa protein in the protective response to M. avium infection and indicate that DNA vaccination successfully promotes a sustained level of protection during chronic M. avium infection.

Serological distinction of integral plasma membrane proteins as a class of mycobacterial antigens and their relevance for human T cell activation

This study pertains to classification and antigenic analysis of mycobacterial plasma membrane proteins in relation to human T cell proliferative responses, using a 'fast grower' Mycobacterium fortuitum as model. Membrane vesicles, prepared by sonication and differential centrifugation, were subjected to biphasic Triton X-114 extraction for isolation of integral peripheral (aqueous phase) proteins. Neither protein pool showed any appreciable overlap serologically. SDS-PAGE showed five prominent bands in peripheral and three in the integral protein pool, whereas immunoblotting with rabbit antisera identified only two major antigens (60 and 67 kD) in the former and five (24, 34, 42, 51 and 54 kD) in the latter. ELISA with a panel of anti-mycobacterial MoAbs revealed that nine out of 12 previously known antigens were present in the peripheral protein pool. Only two of them (33 and 40 kD) were additionally detected amongst integral proteins. The membrane-associated immunosuppressive moiety lipoarabinomannan was semiquantitatively located in aqueous phase. In bulk T cell proliferation assays, seven out of 10 subjects belonging to a 'responder' background (BT-BB leprosy patients and healthy contacts) showed high responses for Myco. fortuitum antigens. Proliferative response with integral proteins was comparable to that with whole membrane, but it was significantly higher (P < 0.0005) than the response with peripheral proteins. The distinction and relevance of integral membrane proteins as a class of mycobacterial antigens make them worthy of consideration in a subunit vaccine design.

Characterization of the gene encoding the immunodominant 35 kDa protein of Mycobacterium leprae

Analysis of the interaction between the host immune system and the intracellular parasite Mycobacterium leprae has identified a 35 kDa protein as a dominant antigen. The native 35 kDa protein was purified from the membrane fraction of M. leprae and termed MMPI (major membrane protein I). As the purified protein was not amenable to N-terminal sequencing, partial proteolysis was used to establish the sequences of 21 peptides. A fragment of the 35 kDa protein-encoding gene was amplified by the polymerase chain reaction from M. leprae chromosomal DNA with oligonucleotide primers derived from internal peptide sequences and the whole gene was subsequently isolated from a M. leprae cosmid library. The nucleotide sequence of the gene revealed an open reading frame of 307 amino acids containing most of the peptide sequences derived from the native 35 kDa protein. The calculated subunit mass was 33.7 kDa, but the native protein exists as a multimer of 950 kDa. Database searches revealed no identity between the 35 kDa antigen and known protein sequences. The gene was expressed in Mycobacterium smegmatis under the control of its own promoter or at a higher level using an 'up-regulated' promoter derived from Mycobacterium fortuitum. The gene product reacted with monoclonal antibodies raised to the native protein. Using the bacterial alkaline phosphatase reporter system, we observed that the 35 kDa protein was unable to be exported across the membrane of recombinant M. smegmatis. The 35 kDa protein-encoding gene is absent from members of the Mycobacterium tuberculosis complex, but homologous sequences were detected in Mycobacterium avium, Mycobacterium haemophilum and M. smegmatis. The availability of the recombinant 35 kDa protein will permit dissection of both antibody- and T-cell-mediated immune responses in leprosy patients.

Chemotherapy of lepromatous leprosy: recent developments and prospects for the future

Leprosy is a major debilitating infectious disease, primarily of the developing world. In this paper the current status and future prospects of antimicrobial therapy of the severe anergic lepromatous form of the disease are reviewed. Until the last few years only dapsone, rifampicin, clofazimine and ethionamide have had practical application in its therapy, and only rifampicin was bactericidal. Recently, antibiotics from three different classes have been found to be bactericidal in lepromatous patients: a tetracycline (minocycline), a macrolide (clarithromycin), and several fluoroquinolones (including pefloxacin, ofloxacin and sparfloxacin). Against a background of drug resistance and bacterial persistence, recommendations for multidrug therapy and the means to devise rationally based therapy for the future are discussed.

Vaccination with pure Mycobacterium leprae proteins inhibits M. leprae multiplication in mouse footpads

In this study, we evaluated vaccination with a number of purified, as well as recombinant, Mycobacterium leprae proteins for protective efficacy in mice. BALB/c mice were immunized intradermally with various native somatic (purified) or recombinant M. leprae proteins and their synthetic polypeptides emulsified in Freund's incomplete adjuvant. The protective efficacy of these preparations was assessed by enumeration of bacilli in the footpads of mice challenged with viable M. leprae 1 to 2 months following immunization. Protection was afforded by the purified and recombinant 10-kDa M. leprae cytoplasmic heat shock protein, the recombinant cell wall-associated 65-kDa M. leprae heat shock protein, and to a lesser extent, the purified 28-kDa M. leprae cytoplasmic protein (superoxide dismutase). Vaccination with either the purified or recombinant 35-kDa M. leprae cell membrane protein, the synthetic 27-amino-acid N-terminal peptide of the 10-kDa protein, the recombinant 18-kDa M. leprae protein, or the purified 22-kDa cell membrane protein was ineffective. When the interval between immunization and challenge was increased to 6 months, the purified 10-kDa M. leprae protein and the recombinant 65-kDa M. leprae protein lost vaccine efficacy, while a sodium dodecyl sulfate-soluble protein fraction of the M. leprae cell wall (soluble proteins), as had been found previously, continued to protect, suggesting that multiple M. leprae protein epitopes are critical for solid vaccine protection.

Isolation of Brucella abortus ssb and uvrA genes from a genomic library by use of lymphocytes as probes

Brucella abortus proteins from virulent S2308 expressed from a pBluescript II SK- genomic library stimulated peripheral blood mononuclear (PBM) cell proliferation from cattle vaccinated with B. abortus S19. The method described here permits a rapid and directed approach to isolate genes encoding antigens of B. abortus that interact with lymphocytes primed to the living bacterium. The supernatants from the bacterial host JM109 (DE3) were cultured with freshly isolated bovine PBM cells. A total of 300 clones were evaluated. Ten clones were identified that stimulated T-lymphocyte proliferation. Among them, one clone with a 2.5-kb insert stimulated T-lymphocyte proliferation in all three animals, suggesting that the proteins encoded by genes within this fragment may represent immunodominant antigens. DNA sequencing of this clone reveals two large open reading frames (ORFs). ORF II has a high degree of similarity to the Escherichia coli ssb gene, which codes for the single-stranded DNA binding protein. ORF I, in the opposite direction to ORF II, shows similarity to the N terminus of the E. coli uvrA gene, which codes for one of the three subunits of the E. coli ABC excision nuclease. The observation that the PBM cells recognized and proliferated in response to proteins expressed from single clones provides a novel strategy to select bacterial antigens that may prove useful in designing alternative vaccines against brucellosis.

Host response to Mycobacterial cell wall subunit during experimental tuberculosis in mice

The cell-wall protein-peptidoglycan complex (CW-PPC) of Mycobacterium tuberculosis, an immunologically potent component, was used to study the correlation between immune response and in vivo bacterial multiplication in the course of experimental tuberculosis infection in mice. Antibodies to CW-PPC were detected only after seven weeks of infection with M. tuberculosis H37Rv and afterwards no significant change was seen throughout the experiment. Delayed type hypersensitivity (DTH) to CW-PPC showed a gradual increase from the fifth week onward with a maximum during the 12th week after infection (p.in.) which did not change significantly afterward. The increased immune response in the course of infection correlated well with the multiplication rate of bacilli in the lungs. These results indicate a role of CW-PPC in antituberculous immunity.

Hansen's disease

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Protective efficacy of different cell-wall fractions of Mycobacterium tuberculosis

Immunization with various cell-wall fractions of M. tuberculosis H37Ra, progressively depleted of lipids (cell-wall-insoluble fraction; CWIF), soluble proteins (cell-wall core; CWC), mycolic acids and arabinogalactans (cell-wall-protein-peptidoglycan complex; CW-PPC) elicited significant levels of both humoral and cell-mediated immune response. Mice immunized with these fractions, when challenged with an LD50 dose of M. tuberculosis H37Rv, exhibited significant protection as revealed by high survival rates and decreased bacterial load in lungs, liver and spleen, as compared to nonimmunized animals.

Cellular immune response to the cell walls of Mycobacterium leprae in leprosy patients and healthy subjects exposed to leprosy

Cell walls of M. leprae consist of complex arrangements of carbohydrate, lipid, peptidoglycan and protein molecules. Recently, extractable proteins of a wide range of molecular weights were identified as components of the cell wall. We have examined the cellular immune responses of Nepali leprosy patients to a cell wall preparation of M. leprae enriched for these proteins. Strong lymphocyte proliferative responses to the antigens were present in half of the paucibacillary leprosy patients and in the majority of healthy control subjects with occupational exposure to leprosy. Patients with multibacillary disease responded poorly and patients with tuberculosis had intermediate responses. Proliferative responses to the cell wall protein fraction were strongly correlated to the proliferative responses to sonicates of the whole leprosy bacillus. Immunization of mice with cell wall proteins resulted in inhibition of growth of M. leprae following foot-pad inoculation with viable organisms. Therefore cell-mediated immune responses to the extractable proteins of the cell wall may play a role in protective immunity against M. leprae infection.

Antigens of Brucella abortus S19 immunodominant for bovine lymphocytes as identified by one- and two-dimensional cellular immunoblotting

Cellular immune responses are influential for protection against intracellular bacteria such as brucellae. Therefore, identification of Brucella abortus antigens that activate primed bovine lymphocytes is fundamental for discerning the breadth of cellular response in bovine brucellosis. Potentially antigenic components of B. abortus S19 were isolated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by nitrocellulose blotting. Specific one-dimensional blot segments induced proliferation of peripheral blood lymphocytes from all 25 of the vaccinated cattle tested and were defined as immunodominant. Individual proteins that stimulated lymphocyte proliferation were further characterized by two-dimensional cellular immunoblotting by two different approaches. Individual one-dimensional stimulatory blot segments were eluted, concentrated, and then subjected to two-dimensional cellular immunoblotting. Alternatively, entire two-dimensional gels containing all of the B. abortus components were blotted and nitrocellulose sections containing individual proteins were assayed for lymphocyte activation. Thirty-eight Brucella proteins that induced lymphocyte proliferation were resolved by both procedures. Phenotypic analysis of the proliferating cell population demonstrated the presence of CD4+, CD8+, and immunoglobulin M+ lymphocytes. Two immunogenic proteins, 12 and 31 kDa, identified by two-dimensional cellular immunoblotting, were subjected to partial N-terminal amino acid analysis. The 12-kDa protein was within the area of greatest lymphocyte proliferation, while the 31-kDa protein was chosen for comparison with a 31-kDa protein previously reported by others. A search of the National Biomedical Research Foundation protein data bank showed that the sequences were not homologous with other known proteins. Identification of Brucella proteins immunogenic for bovine lymphocytes provides an important step in distinguishing the various proteins involved in pathogenicity and/or disease resistance.

Vaccination of mice with a soluble protein fraction of Mycobacterium leprae provides consistent and long-term protection against M. leprae infection

Groups of BALB/c mice were vaccinated intradermally with either Freund's incomplete adjuvant (FIA) alone, 10(7) heat-killed Mycobacterium leprae organisms in FIA, or a number of fractions of M. leprae containing soluble and/or cell wall components. At 1, 3, 6, 9, and 12 months later, vaccinated mice were challenged in the right hind footpad with 5,000 live M. leprae organisms, and vaccine protection was assessed 6 to 8 months later, at the peak of M. leprae multiplication in the negative control (FIA alone), by the two-sample rank-sum test. In these studies, a cell wall fraction rich in peptidoglycan was consistently ineffective. Both heat-killed M. leprae and a fraction containing cell wall and fixed proteins generally protected when the interval between vaccination and challenge was 1 or 3 months but not subsequently. On the other hand, soluble proteins of M. leprae alone or in combination (with cell wall fractions) consistently (14 of 14 instances) afforded highly significant protection (P less than or equal to 0.01) at all challenge intervals up to 1 year after vaccination. These results suggest that the soluble protein fraction of M. leprae offers promise for a vaccine against leprosy.

Current concepts in the pathogenesis of leprosy. Clinical, pathological, immunological and chemotherapeutic aspects

In recent years there have been notable advances in the laboratory investigation and field management of leprosy. Progress, however, continues to be hindered by the lack of efficient methods for early diagnosis and implementation of control and treatment measures. Diagnosis is still made on the same principles as a century ago (clinical and histopathological findings), and only 1 in 3 known patients worldwide receives optimal chemotherapy. In 1988, nearly 1 in 10 newly diagnosed patients already had debilitating deformities. Contributing factors are operational, administrative and financial difficulties in implementing multidrug therapeutic regimens, inadequately trained personnel, and lack of priority and political commitment to leprosy control. The formulation and implementation of multidrug therapy is the most important development in leprosy in the past 10 years. Dapsone monotherapy was the mainstay for treatment and control for approximately 40 years, but secondary dapsone-resistant strains, first noted in 1964, now infect as many as 50% of all new patients. Multidrug regimens recommended by the WHO consist of various combinations of therapy using dapsone, rifampicin, clofazimine and a thionamide. Duration of therapy is limited to 6 months for paucibacillary and 2 years or more for multibacillary patients; relapse rates thus far are low. The average cost of treatment worldwide, including the cost of drugs, is estimated at $US150 per patient. The recent annual drop of nearly 8% in newly registered patients may be due to the implementation of these therapeutic regimens. Newer drugs that may be introduced into these regimens include fluoroquinolones, minocycline and clarithromycin. While knowledge of the microbiology of the leprosy bacillus and host response has advanced remarkably, there is little improvement in the understanding or amelioration of social aspects of leprosy. Better treatment and control reduces the stigma, but improvements in the attitudes of patients and society towards leprosy are as important as advances in medical science in achieving ultimate eradication of the disease.

Leprosy vaccines

Leprosy is the clinical manifestation of chronic infection with Mycobacterium leprae, an intracellular parasite with a predilection for skin and nerves. Disabilities and mutilations associated with this disease, which are attributable primarily to nerve involvement, have made leprosy among the most feared and stigmatizing of all diseases. It is still widespread in the warmer regions of the globe, including southern Europe, southern USA and most of the developing countries. Though widespread, the distribution of the disease in endemic regions is sparse (a prevalence rate of 1 per 1000 is high) and predominantly rural, for reasons which are not understood, but which add to the difficulty of providing effective disease control.

Protein antigens of Mycobacterium leprae

Protein antigens of Mycobacterium leprae have been identified by screening the lambda gt11, pYA626 and pHC79::M. leprae genomic libraries with pooled sera from leprosy patients and with antiserum to M. leprae cell wall protein (CWP) aggregate. Immunological screening of the lambda gt11 library with pooled sera from 21 lepromatous (LL) leprosy patients resulted in the identification of 19 antigens that are apparently different from previously identified M. leprae antigens. Five additional antigens were identified by screening the lambda gt11 library with pooled sera from 30 borderline tuberculoid or tuberculoid patients. Four other antigens were identified by screening the lambda gt11 library with anti-CWP. Two groups of recombinant cosmids were identified by screening the pHC79 library with LL patients' sera: one group specified proteins that reacted with monoclonal antibodies (mAb) against the 65-kDa protein and against the 18-kDa protein; the other group specified a 15-kDa protein that did not react with any of the mAb that were tested. One pYA626 clone also specified a 15-kDa protein that reacted with LL patients' sera, but did not react with any mAb. Genes specifying several of these antigens have been subcloned into the Asd+ plasmid vector pYA292 and have been introduced into a delta cya delta crp delta asd Salmonella typhimurium strain to evaluate the ability of individual M. leprae proteins to elicit immune responses against M. leprae infection.

Identification of an immunostimulating protein from Mycobacterium leprae

Despite the recent identification of a number of Mycobacterium leprae proteins, the major immunogenic determinants of this organism remain obscure. We isolated from M. leprae a potent immunostimulatory preparation, designated the MLP fraction, which contains a major protein of 35 kilodaltons (kDa). This protein was precipitated by monoclonal antibody ML03-A1, which recognizes a 35-kDa protein of M. leprae, and by sera obtained from patients with lepromatous leprosy. Neither sera from healthy controls nor sera from patients with pulmonary tuberculosis recognized the 35-kDa protein, and only one of four serum samples from patients with borderline tuberculoid leprosy reacted with this protein. The MLP fraction stimulated T-cell proliferation in patients with leprosy whose T cells proliferate in response to whole M. leprae cells. Apparently, the T-cell epitope associated with MLP is also expressed on M. tuberculosis and M. bovis BCG, since patients with pulmonary tuberculosis and BCG-vaccinated individuals demonstrated significant responses to the MLP fraction. The 35-kDa M. leprae protein, purified to homogeneity in the laboratory of P. J. Brennan, stimulated T-cell proliferative responses in all MLP-responsive subjects. These findings suggest that the 35-kDa protein present in MLP is an immunostimulatory component of M. leprae. In addition to serving as a useful probe for study of the T-cell anergy associated with lepromatous disease, this protein may ultimately be useful as a component of a vaccine designed to provide protection against infection with M. leprae.