Most Mycobacterium leprae carbohydrate-reactive monoclonal antibodies are directed to lipoarabinomannan

Monoclonal antibodies to lipoarabinomannan/arabinomannan - characteristics and implications for tuberculosis research and diagnostics

Antibodies to the mycobacterial surface lipoglycan lipoarabinomannan (LAM) and its related capsular polysaccharide arabinomannan (AM) are increasingly important for investigations focused on both understanding mechanisms of protection against Mycobacterium tuberculosis (Mtb) and developing next-generation point-of-care tuberculosis (TB) diagnostics. We provide here an overview of the growing pipeline of monoclonal antibodies (mAbs) to LAM/AM. Old and new methodologies for their generation are reviewed and we outline and discuss their glycan epitope specificity and other features with implications for the TB field.

Overcome low levels of detection limit and choice of antibody affects detection of lipoarabinomannan in pediatric tuberculosis

The World Health Organization (WHO) emphasizes that tuberculosis (TB) in children and adolescents is often overlooked by healthcare providers and difficult to diagnose. As childhood TB cases rise, finding a diagnostic high in sensitivity and specificity is critical. In this study 91 urine samples from children aged 1–10 years were analyzed for tuberculostearic acid (TBSA) by gas chromatography/mass spectrometry (GC/MS) and capture ELISA (C-ELISA). In C-ELISA the CS35/A194-01 antibody performed very poorly with both curve-based and model-based cutoffs. The area under the ROC curve (AUC) of the CS35 OD₄₅₀ values was only 0.60. Replacing the capture antibody with BJ76 gave a better performance in both sensitivity and specificity (AUC = 0.95). When these samples were analyzed by GC/MS, 41 classified as ‘probable/possible’ for TB were distinctly TBSA positive with ten samples having <3 ng/mL LAM. However, from the 50 samples with ‘unlikely’ TB classification, 36 were negative but 7 had >3 ng/mL and were designated as LAM positive. This experimental assay assessment study signifies that i) the antibody pair CS35/A194-01 that has been successful for adult active TB diagnosis is not adequate when LAM level is low as in pediatric TB; ii) no one mAb appears to recognize all TB-specific LAM epitopes.

Immobilization of Proteinase K for urine pretreatment to improve diagnostic accuracy of active tuberculosis

The World Health Organization (WHO) calls for the development of a rapid, biomarker-based, non-sputum test capable of detecting all forms of tuberculosis (TB) at the point-of-care to enable immediate treatment initiation. Lipoarabinomannan (LAM) is the only WHO-endorsed TB biomarker that can be detected in urine, an easily collected sample matrix. For obtaining optimal sensitivity, we and others have shown that some form of sample pretreatment is necessary to remove background from patient urine samples. A number of systems are paper-based often destined for resource limited settings. Our current work presents incorporation of one such sample pretreatment, proteinase K (ProK) immobilized on paper (IPK) and test its performance in comparison to standard proteinase K (SPK) treatment that involves addition and deactivation at high temperature prior to performing a capture ELISA. Herein, a simple and economical method was developed for using ProK immobilized strips to pretreat urine samples. Simplification and cost reduction of the proposed pretreatment strip were achieved by using Whatman no.1 paper and by minimizing the concentration of ProK (an expensive but necessary reagent) used to pretreat the clinical samples prior to ELISA. To test the applicability of IPK, capture ELISA was carried out on either LAM-spiked urine or the clinical samples after pretreatment with ProK at 400 μg/mL for 30 minutes at room temperature. The optimal conditions and stability of the IPK were tested and validation was performed on a set of 25 previously analyzed archived clinical urine samples with known TB and HIV status. The results of IPK and SPK treated samples were in agreement showing that the urine LAM test currently under development has the potential to reach adult and pediatric patients regardless of HIV status or site of infection, and to facilitate global TB control to improve assay performance and ultimately treatment outcomes.

Urine lipoarabinomannan in HIV uninfected, smear negative, symptomatic TB patients: effective sample pretreatment for a sensitive immunoassay and mass spectrometry

Our study sought to determine whether urine lipoarabinomannan (LAM) could be validated in a sample cohort that consisted mainly of HIV uninfected individuals that presented with tuberculosis symptoms. We evaluated two tests developed in our laboratory, and used them on clinical samples from Lima, Peru where incidence of HIV is low. ELISA analysis was performed on 160 samples (from 140 adult culture-confirmed TB cases and 20 symptomatic TB-negative child controls) using 100 μL of urine after pretreatment with Proteinase K. Two different mouse monoclonal antibodies-CS35 and CHCS9-08 were used individually for capture of urine LAM. Among cases, optical density (OD450) values had a positive association with higher bacillary loads. The 20 controls had negative values (below the limit of detection). The assay correctly identified all samples (97-100% accuracy confidence interval). For an alternate validation of the ELISA results, we analyzed all 160 urine samples using an antibody independent chemoanalytical approach. Samples were called positive only when LAM surrogates-tuberculostearic acid (TBSA) and D-arabinose (D-ara)-were found to be present in similar amounts. All TB cases, including the 40 with a negative sputum smear had LAM in detectable quantities in urine. None of the controls had detectable amounts of LAM. Our study shows that urinary LAM detection is feasible in HIV uninfected, smear negative TB patients.

Urine lipoarabinomannan as a marker for low-risk of NTM infection in the CF airway

BACKGROUND

Individuals with Cystic fibrosis (CF) are the most vulnerable population for pulmonary infection with nontuberculous mycobacteria (NTM). Screening, diagnosis, and assessment of treatment response currently depend on traditional culture techniques, but sputum analysis for NTM in CF is challenging, and associated with a low sensitivity. The cell wall lipoarabinomannan (LAM), a lipoglycan found in all mycobacterial species, and has been validated as a biomarker in urine for active Mycobacterium tuberculosis infection.

METHODS

Urine from a CF cohort (n = 44) well-characterized for NTM infection status by airway cultures was analyzed for LAM by gas chromatography/mass spectrometry. All subjects with positive sputum cultures for NTM had varying amounts of LAM in their urine. No LAM was detected in subjects who never had a positive culture (14/45). One individual initially classified as NTM sputum negative subsequently developed NTM disease 657 days after the initial urine LAM testing. Repeat urine LAM testing turned positive, correlating to her positive NTM status. Subjects infected with subspecies of M. abscessus had greater LAM quantities than those infected with M. avium complex (MAC). There was no correlation with disease activity or treatment status and LAM quantity. A TB Capture ELISA using anti-LAM antibodies demonstrated very poor sensitivity in identifying individuals with positive NTM sputum cultures.

CONCLUSION

These findings support the conclusion that urine LAM related to NTM infection may be a useful screening test to determine patients at low risk for having a positive NTM sputum culture, as part of a lifetime screening strategy in the CF population.

Comparative Structural Study of Terminal Ends of Lipoarabinomannan from Mice Infected Lung Tissues and Urine of a Tuberculosis Positive Patient

Mycobacterium tuberculosis lipoarabinomannan (LAM) is a biomarker for active tuberculosis (TB) disease. The presence of LAM in the urine of TB patients, whether HIV positive or negative, has been validated by a gas chromatography/mass spectral method with good specificity (84%) and sensitivity (99%). However, point-of-care (POC) methods to detect TB LAM in urine using immunoassays have poor sensitivity and are limited to only HIV coinfected TB diagnosis. We hypothesized that these disappointing results with the POC methods may be due to the antibodies used in the immunoassays as there could be structural differences between LAM in vivo and LAM in vitro. To address this issue, we infected C3HeB/FeJ mice with M.tb W. Beijing SA161 and purified LAM from the lung. Analysis of these sources of LAM using a panel of existing mAbs revealed differences in epitope patterns. Conventionally, the non-reducing termini of LAM are identified by their release with endoarabinanase. These epitopes correspond to linear tetra-(Ara₄), branched hexa-(Ara₆) arabinofuranosides, and their mannose-capped versions. We discovered two distinct epitopes. In the first case, it was found that the non-reducing termini of LAM from M.tb strain SA161 are highly succinylated, especially when the LAM was isolated from the mouse lungs. In the second case, it was found that Cellulomonas endoarabinanase digestion of LAM from both SA161 and LAM from a TB+ HIV- patient's urine yielded epitopes based on 5 arabinoses as major components and a profound lack of Ara₆. The epitopes based on 5 arabinoses from M.tb SA161 and from the LAM in human urine must result from underlying structural and thus epitope differences. These results suggest approaches to develop specific antibodies for POC tests for LAM in the urine of suspected TB patients.

Mycobacterial mannose-capped lipoarabinomannan: a modulator bridging innate and adaptive immunity

Mannose-capped lipoarabinomannan (ManLAM) is a high molecular mass amphipathic lipoglycan identified in pathogenic Mycobacterium tuberculosis (M. tb) and M. bovis Bacillus Calmette-Guérin (BCG). ManLAM, serves as both an immunogen and a modulator of the host immune system, and its critical role in mycobacterial survival during infection has been well-characterized. ManLAM can be recognized by various types of receptors on both innate and adaptive immune cells, including macrophages, dendritic cells (DCs), neutrophils, natural killer T (NKT) cells, T cells and B cells. MamLAM has been shown to affect phagocytosis, cytokine production, antigen presentation, T cell activation and polarization, as well as antibody production. Exploring the mechanisms underlying the roles of ManLAM during mycobacterial infection will aid in improving tuberculosis (TB) prevention, diagnosis and treatment interventions. In this review, we highlight the interaction between ManLAM and receptors, intracellular signalling pathways triggered by ManLAM and its roles in both innate and adaptive immune responses.

Detection of lipoarabinomannan in urine and serum of HIV-positive and HIV-negative TB suspects using an improved capture-enzyme linked immuno absorbent assay and gas chromatography/mass spectrometry

TB diagnosis and treatment monitoring in resource limited regions rely heavily on serial sputum smear microscopy and bacterial culture. These microbiological methods are time-consuming, expensive and lack adequate sensitivity. The WHO states that improved TB diagnosis and treatment is imperative to achieve an end to the TB epidemic by 2030. Commercially available lipoarabinomannan (LAM) detection tools perform at low sensitivity that are highly dependent on the underlying immunological status of the patient; those with advanced HIV infection perform well. In this study, we have applied two novel strategies towards the sensitive diagnosis of TB infection based on LAM: Capture ELISA to detect LAM in paired urine and serum samples using murine and human monoclonal antibodies, essentially relying on LAM as an 'immuno-marker'; and, secondly, detection of α-d-arabinofuranose and tuberculostearic acid (TBSA)- 'chemical-markers' unique to mycobacterial cell wall polysaccharides/lipoglycans by our recently developed gas chromatography/mass spectrometry (GC/MS) method. Blinded urine specimens, with microbiologically confirmed active pulmonary TB or non TB (HIV+/HIV-) were tested by the aforementioned assays. LAM in patient urine was detected in a concentration range of 3-28 ng/mL based on GC/MS detection of the two LAM-surrogates, d-arabinose and tuberculostearic acid (TBSA) correctly classifying TB status with sensitivity > 99% and specificity = 84%. The ELISA assay had high sensitivity (98%) and specificity (92%) and the results were in agreement with GC/MS analysis. Both tests performed well in their present form particularly for HIV-negative/TB-positive urine samples. Among the HIV+/TB+ samples, 52% were found to have >10 ng/mL urinary LAM. The detected amounts of LAM present in the urine samples also appears to be associated with the gradation of the sputum smear, linking elevated LAM levels with higher mycobacterial burden (odds ratio = 1.08-1.43; p = 0.002). In this small set, ELISA was also applied to parallel serum samples confirming that serum could be an additional reservoir for developing a LAM-based immunoassay for diagnosis of TB.

Characterization of the Antigenic Heterogeneity of Lipoarabinomannan, the Major Surface Glycolipid of Mycobacterium tuberculosis, and Complexity of Antibody Specificities toward This Antigen

Lipoarabinomannan (LAM), the major antigenic glycolipid of Mycobacterium tuberculosis, is an important immunodiagnostic target for detecting tuberculosis (TB) infection in HIV-1–coinfected patients, and is believed to mediate a number of functions that promote infection and disease development. To probe the human humoral response against LAM during TB infection, several novel LAM-specific human mAbs were molecularly cloned from memory B cells isolated from infected patients and grown in vitro. The fine epitope specificities of these Abs, along with those of a panel of previously described murine and phage-derived LAM-specific mAbs, were mapped using binding assays against LAM Ags from several mycobacterial species and a panel of synthetic glycans and glycoconjugates that represented diverse carbohydrate structures present in LAM. Multiple reactivity patterns were seen that differed in their specificity for LAM from different species, as well as in their dependence on arabinofuranoside branching and nature of capping at the nonreducing termini. Competition studies with mAbs and soluble glycans further defined these epitope specificities and guided the design of highly sensitive immunodetection assays capable of detecting LAM in urine of TB patients, even in the absence of HIV-1 coinfection. These results highlighted the complexity of the antigenic structure of LAM and the diversity of the natural Ab response against this target. The information and novel reagents described in this study will allow further optimization of diagnostic assays for LAM and may facilitate the development of potential immunotherapeutic approaches to inhibit the functional activities of specific structural motifs in LAM.

Detection of the tuberculosis antigenic marker mannose-capped lipoarabinomannan in pretreated serum by surface-enhanced Raman scattering

The ability to detect tuberculosis (TB) continues to be a global health care priority. This paper describes the development and preliminary assessment of the clinical accuracy of a heterogeneous immunoassay that integrates a serum pretreatment process with readout by surface-enhanced Raman scattering (SERS) for the low-level detection of mannose-capped lipoarabinomannan (ManLAM). ManLAM is a major virulence factor in the infectious pathology of Mycobacterium tuberculosis (Mtb) that has been found in the serum and other body fluids of infected patients. The effectiveness of ManLAM as a TB diagnostic marker, however, remains unproven for reasons not yet well understood. As reported herein, we have found that (1) ManLAM complexes with proteins and possibly other components in serum; (2) these complexes have a strongly detrimental impact on the ability to detect ManLAM using an immunoassay; (3) a simple pretreatment step can disrupt this complexation; and (4) disruption by pretreatment improves detection by 250×. We also describe the results from a preliminary assessment on the utility of serum pretreatment by running immunoassays on archived specimens from 24 TB-positive patients and 10 healthy controls. ManLAM was measurable in 21 of the 24 TB-positive specimens, but not in any of the 10 control specimens. These findings, albeit for a very small specimen set, translate to a clinical sensitivity of 87.5% and a clinical specificity of 100%. Together, these results both provide much needed evidence for the clinical utility of ManLAM as a TB marker, and demonstrate the potential utility of our overall approach to serve as a new strategy for the development of diagnostic tests for this disease.

Epitope-specific antibody levels in tuberculosis: biomarkers of protection, disease, and response to treatment

Monoclonal antibodies restricted to Mycobacterium tuberculosis can measure epitope-specific antibody levels in a competition assay. Immunodominant epitopes were defined from clinical samples and related to the clinical spectrum of disease. Antibody to the immunodominant epitopes was associated with HLA-DR15. Occupational exposure showed a different response and was consistent with recognition of dormancy-related proteins and protection despite exposure to tuberculosis (TB). Studies in leprosy revealed the importance of immune deviation and the relationships between T and B cell epitopes. During treatment, antibody levels increased, epitope spreading occurred, but the affinity constants remained the same after further antigen exposure, suggesting constraints on the process of epitope selection. Epitope-specific antibody levels have a potential role as biomarkers for new vaccines which might prevent the progression of latent to active TB and as tools to measure treatment effects on subpopulations of tubercle bacilli.

Design of a complement mannose-binding lectin pathway-specific activation system applicable at low serum dilutions

Recently we showed that alternative pathway (AP) amplification was responsible for more than 80% of specific classical pathway-induced terminal pathway activation under physiological conditions. The present study aimed to design a system for specific lectin pathway (LP) activation applicable at low serum dilutions with a fully functional AP. Comparison between activation of normal human serum (NHS), a mannose-binding lectin (MBL) homozygous D/D-deficient serum, and sera deficient in C1q and C2, all diluted 1 : 2, was essential to document optimal conditions for LP specificity. Mannan on the solid phase of enzyme-linked immunosorbent assay (ELISA) plates was used for activation, showing 0.5 microg mannan/well to give optimal conditions because at this concentration a good signal was preserved for C4 and TCC deposition in NHS, whereas the C3 deposition observed in C2-deficient serum at higher mannan concentrations reached nadir at 0.5 microg/well, indicating a lack of direct AP activation under these conditions. Pooled NHS and C1q-deficient serum gave the same degree of C4 and terminal complement complex (TCC) deposition, whereas deposition of these products was not obtained with MBL-deficient serum. Reconstitution with purified MBL, however, restored the depositions. A blocking anti-MBL monoclonal antibody (mAb) completely abolished the complement deposition, in contrast to a non-inhibiting anti-MBL mAb. Activation of C2-deficient serum induced C4 deposition similar to NHS, but negligible deposition of C3 and TCC, confirming the lack of direct activation of AP. Thus, this assay is unique in being LP-specific at low serum dilution and thus particularly suitable to study LP activation mechanisms and the role of AP amplification under physiological conditions.

Epitope-specific antibody levels demonstrate recognition of new epitopes and changes in titer but not affinity during treatment of tuberculosis

Antibody levels rise during treatment of tuberculosis. This study examined when this rise occurred, whether there was recognition of new antigen binding sites (epitopes) on the same or different antigens, and how long specific antibody persisted. Forty patients with smear-positive pulmonary tuberculosis provided serum before and during treatment. Antibody levels were measured using a monoclonal antibody competition assay to epitopes restricted to the Mycobacterium tuberculosis complex and an enzyme-linked immunosorbent assay for lipoarabinomannan. Significant increases in antibody levels were apparent after 7 days of treatment. Five samples (12.5%) had positive titers to all epitopes at the start of treatment, and this increased to 23 (58%) during treatment. Antibody to epitopes with the poorest sensitivity (the TB23 epitope of the 19-kDa antigen and the TB78 epitope of hsp65) showed the greatest increases after treatment. Antibody to these two epitopes was also absent in some patients with relapsed tuberculosis until after treatment. Antibody titers showed a biphasic response, with a fall at 2 to 3 months of treatment. Sera from two patients showed changes in the affinity of epitope-specific antibody during treatment, whereas the majority did not. Those infected with isoniazid-resistant strains of M. tuberculosis showed a late rise in antibody. Antibody to the TB68 epitope of the 16-kDa alpha-crystallin homolog was short-lived, but it recurred with bacteriological relapse during treatment. Positive antibody titers persisted for at least 3 to 18 months after treatment. Diagnostic tests for tuberculosis should be evaluated using only pretreatment sera. Delayed antigenic recognition could be due to active suppression and/or failure to engage internal antigens of M. tuberculosis.

Characterization of the epitope of anti-lipoarabinomannan antibodies as the terminal hexaarabinofuranosyl motif of mycobacterial arabinans

mAb CS-35 is representative of a large group of antibodies with similar binding specificities that were generated against the Mycobacterium leprae lipopolysaccharide, lipoarabinomannan (LAM), and which cross-reacted extensively with LAMs from Mycobacterium tuberculosis and other mycobacteria. That this antibody also cross-reacts with the arabinogalactan (AG) of the mycobacterial cell wall, suggesting that it recognizes a common arabinofuranosyl (Araf)-containing sequence in AG and LAM, is demonstrated. The antibody reacted more avidly with 'AraLAM' (LAM with naked Araf termini) compared to 'ManLAM' (in which many Araf termini are capped with mannose residues) and mycolylarabinogalactan-peptidoglycan complex (in which the terminal Araf units are substituted with mycolic acids). Neither did the antibody bind to AG from emb knock-out mutants deficient in the branched hexa-Araf termini of AG. These results indicate that the terminal Araf residues of mycobacterial arabinan are essential for binding. Competitive ELISA using synthetic oligosaccharides showed that the branched hexa-Araf methyl glycoside [beta-D-Araf-(1-->2)-alpha-D-Araf-(1-)(2)-(3 and 5)-alpha-D-Araf-(1-->5)-alpha-D-Araf-OCH(3)] was the best competitor among those tested. The related linear methyl glycoside, beta-D-Araf-(1-->2)-alpha-D-Araf-(1-->5)-alpha-D-Araf-(1-->5)-alpha-D-Araf-OCH(3), representing one linear segment of the branched hexa-Araf, was less effective and the other linear tetrasaccharide, beta-D-Araf-(1-->2)-alpha-D-Araf-(1-->3)-alpha-D-Araf-(1-->5)-alpha-D-Araf-OCH(3), was ineffective. The combined results suggest that the minimal epitope recognized by antibody CS-35 encompasses the beta-D-Araf-(1-->2)-alpha-D-Araf-(1-->5)-alpha-D-Araf-(1-->5)-alpha-D-Araf within the branched hexa-Araf motif of mycobacterial arabinans, whether present in LAM or AG.

A novel lipoarabinomannan from the equine pathogen Rhodococcus equi. Structure and effect on macrophage cytokine production

Rhodococcus equi is a major cause of foal morbidity and mortality. We have investigated the presence of lipoglycan in this organism as closely related bacteria, notably Mycobacterium tuberculosis, produce lipoarabinomannans (LAM) that may play multiple roles as virulence determinants. The lipoglycan was structurally characterized by gas chromatography-mass spectrometry following permethylation, capillary electrophoresis after chemical degradation, and (1)H and (31)P and two-dimensional heteronuclear nuclear magnetic resonance studies. Key structural features of the lipoglycan are a linear alpha-1,6-mannan with side chains containing one 2-linked alpha-d-Manp residue. This polysaccharidic backbone is linked to a phosphatidylinositol mannosyl anchor. In contrast to mycobacterial LAM, there are no extensive arabinan domains but single terminal alpha-d-Araf residue capping the 2-linked alpha-d-Manp. The lipoglycan binds concanavalin A and mannose-binding protein consistent with the presence of t-alpha-d-Manp residues. We studied the ability of the lipoglycans to induce cytokines from equine macrophages, in comparison to whole cells of R. equi. These data revealed patterns of cytokine mRNA induction that suggest that the lipoglycan is involved in much of the early macrophage cytokine response to R. equi infection. These studies identify a novel LAM variant that may contribute to the pathogenesis of disease caused by R. equi.

Expression of a Mycobacterium tuberculosis arabinomannan antigen in vitro and in vivo

The outermost layer of Mycobacterium tuberculosis contains two major polysaccharides, arabinomannan (AM) and glucan (GC). We studied the in vitro and in vivo expression of an M. tuberculosis AM antigen using monoclonal antibody (MAb) 9d8 (2a), an isotype-switched variant of the immunoglobulin G3 (IgG3) MAb 9d8. MAb 9d8 had been previously shown to bind M. tuberculosis AM and the M. tuberculosis surface. Our in vitro experiments showed that MAb 9d8(2a) bound strongly to whole-cell M. tuberculosis Erdman but not to the CDC 1551 strain grown in medium for an extended period. However, AM antigen was detected in the culture supernatant of both strains, and its concentration increased in a time-dependent manner. The detection of AM antigen from both strains was decreased in the presence of Tween 80. In mice infected with M. tuberculosis Erdman, AM antigen accumulated in organ homogenates concomitant to an increase in bacterial organ burden and an increase in IgG and IgM titer to AM. These results (i) indicate that the surface expression of AM during in vitro growth changes with culture age, is strain dependent, and is affected by the presence of Tween 80 in the culture media; (ii) show that AM is produced by bacteria growth in vivo; and (iii) demonstrate that the amount of in vivo-detected AM can be dependent on the number of bacteria in the infected organ.

Evidence for human CD4+ T cells in the CD1-restricted repertoire: derivation of mycobacteria-reactive T cells from leprosy lesions

Both the CD4-CD8- (double negative) and CD4-CD8+ T cell lineages have been shown to contain T cells which recognize microbial lipid and glycolipid Ags in the context of human CD1 molecules. To determine whether T cells expressing the CD4 coreceptor could recognize Ag in the context of CD1, we derived CD4+ T cell lines from the lesions of leprosy patients. We identified three CD4+ Mycobacterium leprae-reactive, CD1-restricted T cell lines: two CD1b restricted and one CD1c restricted. These T cell lines recognize mycobacterial Ags, one of which has not been previously described for CD1-restricted T cells. The response of CD4+ CD1-restricted T cells, unlike MHC class II-restricted T cells, was not inhibited by anti-CD4 mAb, suggesting that the CD4 coreceptor does not impact positive or negative selection of CD1-restricted T cells. The CD4+ CD1-restricted T cell lines produced IFN-gamma and GM-CSF, the Th1 pattern of cytokines required for cell-mediated immunity against intracellular pathogens, but no detectable IL-4. The existence of CD4+ CD1-restricted T cells that produce a Th1 cytokine pattern suggests a contributory role in immunity to mycobacterial infection.

Mycobacterial lipoarabinomannan: an extraordinary lipoheteroglycan with profound physiological effects

Detailed structural and functional studies over the last decade have led to current recognition of the mycobacterial lipoarabinomannan (LAM) as a phosphatidylinositol anchored lipoglycan with diverse biological activities. Fatty acylation has been demonstrated to be essential for LAM to maintain its functional integrity although the focus has largely been on the arabinan motifs and the terminal capping function. It has recently been shown that the mannose caps may be involved not only in attenuating host immune response, but also in mediating the binding of mycobacteria to and subsequent entry into macrophages. This may further be linked to an intracellular trafficking pathway through which LAM is thought to be presented by CD1 to subsets of T-cells. The implication of LAM as major histocompatibility complex (MHC)-independent T-cell epitope and the ensuing immune response is an area of intensive studies. Another recent focus of research is the biosynthesis of arabinan which has been shown to be inhibitable by the anti-tuberculosis drug, ethambutol. The phenomenon of truncated LAM as synthesized by ethambutol resistant strains provides an invaluable handle for dissecting the array of arabinosyltransferases involved, as well as generating much needed structural variants for further structural and functional studies. It is hoped that with more systematic investigations based on clinical isolates and human cell lines, the true significance of LAM in the immunopathogenesis of tuberculosis and leprosy can eventually be explained.

Cytometric detection of mycobacterial surface antigens: exposure of mannosyl epitopes and of the arabinan segment of arabinomannans

The physical arrangement of cell envelope components leads to the exposure of selected structural motifs which in turn may influence host-parasite interactions. To gain insight into the exposed epitopes, the present study describes a flow cytometric method designed to probe defined molecules on dispersed mycobacteria. The hydrophobic fluorophore N-hexadecanoyl aminofluorescein inserted in the mycobacterial cell envelope permitted focusing of fluorescence-activated cell sorter analysis on cells that were further labeled with defined monoclonal antibodies and fluorochrome-coupled streptavidin. The use of antibodies directed against the lipooligosaccharide of Mycobacterium tuberculosis demonstrated the specific detection of the antigen on the cell surface of a Canetti-like strain of M. tuberculosis, and not on those of mycobacterial strains that were devoid of the glycolipid. Thus, the method was applied to investigate the relative amounts of surface-exposed mannosylated compounds and D-arabinan-containing substances of different strains of the tubercle bacillus and a strain of the rapidly growing nonpathogenic species Mycobacterium smegmatis. Both M. tuberculosis and M. smegmatis are endowed with mannosyl and arabinan epitopes on their surfaces, although there are many differences in terms of exposed mannosyl epitopes between the various strains of the tubercle bacillus examined. These differences are correlated with the amounts of terminal mannosyl residues that cap the surface-exposed arabinomannans (A. Ortalo-Magné, A. B. Andersen, and M. Daffé, Microbiology 142:927-935, 1996) but not with the degrees of virulence of the strains. This novel approach could provide new insights into the distribution of defined surface-exposed antigens and thereby into the architecture of the cell envelopes.

Mannosylated lipoarabinomannan interacts with phagocytes

Infection by Mycobacterium tuberculosis first involves its adhesion to mononuclear host phagocytes. Various macrophage opsonic and non-opsonic receptors are known to mediate this adhesion, with some specificity of mannosyl receptors for the more virulent strains. Mannosylated lipoarabinomannan, a major component of cell walls from M. tuberculosis and Mycobacterium bovis BCG, is endowed with mannooligosaccharide units that could mediate its binding to these latter receptors. To explore its interaction with murine immune cells by flow cytometry, we report a new procedure to fluorescently tag the polysaccharide molecules. We covalently labeled mannosylated lipoarabinomannan from M. bovis BCG with biotin, allowing formation of stable complexes with streptavidin coupled to a fluorochrome. In this work, we demonstrated that this major carbohydrate antigen interacts selectively with murine phagocytes, i.e. granulocytes and macrophages. This binding was affected by temperature and was serum- and divalent-cation-dependent. It also appears to involve a metabolically recycling protein receptor on the phagocyte surface and mannosyl aggretopes on the mannosylated lipoarabinomannan molecule. Thus, the latter may provide a means for mycobacteria to bind to and invade their host phagocytes. This molecule could constitute one of the early factors of mycobacterial virulence.

Age-dependent humoral responses of children to mycobacterial antigens

In the United States, disseminated infection with environmental mycobacteria, including the Mycobacterium avium complex, is the most common opportunistic bacterial infection seen in AIDS patients. However, the source and relative degree of exposure to environmental mycobacteria during childhood are unknown. To examine the age-related exposure to mycobacteria, we obtained serum samples from 150 children ranging in age from 6 months to 18 years. Each sample was tested against both M. avium (serovar 1) sonic extracts and mycobacterial lipoarabinomannan, using an enzyme-linked immunosorbent assay (ELISA). All serum samples were also subjected to immunoblot analysis with the sonic extract antigen. These studies established that elevated ELISA values (P < 0.0001) and increased immunoblot reactivity (P < 0.0001) against mycobacterial antigens were both associated with increasing age. The seroreactivity differences were most striking when comparing the age groups of children below the age of 6 with the older age groups. Our results suggest that the development of humoral immune responses to mycobacterial antigens in children correlates with increasing age and that there may be an environmental factor predisposing to mycobacterial exposure which is related to advancing age.

Opposing effects of interferon-gamma on iNOS and interleukin-10 expression in lipopolysaccharide- and mycobacterial lipoarabinomannan-stimulated macrophages

Previous studies have demonstrated that, like bacterial lipopolysaccharide (LPS), arabinofuranosyl-terminated lipoarabinomannan (AraLAM) from an attenuated strain of Mycobacterium induces potent early gene (c-fos, KC, JE and TNF-alpha) responses in murine macrophages, whereas extensively alpha-Manp capped LAM (ManLAM) from virulent M. tuberculosis do not. In this study we have extended analysis of the influence of mycobacterial LAM on macrophage function by demonstrating that AraLAM (but not ManLAM), like bacterial LPS, is a potent stimulator of inducible nitric oxide synthase (iNOS) expression independent of the autocrine activity of co-stimulated tumour necrosis factor-alpha (TNF-alpha) release. The inability of ManLAM to induce iNOS expression was not due to induction of the 'deactivating' cytokine interleukin-10 (IL-10). Indeed, like LPS, AraLAM was also a potent inducer of IL-10 expression. However, analysis of AraLAM- or LPS-induced responses in the presence of interferon-gamma (IFN-gamma) showed that, whereas IFN-gamma acts as a potent co-stimulus for iNOS, it completely inhibits the IL-10 response. Hence, the presence of IFN-gamma early in infection will have an important immunomodulatory role in determining the macrophage response. These results have important implications for the pathogenesis of virulent and avirulent mycobacteria in vivo.

Soluble Mycobacterium bovis protein antigens: studies on their purification and immunological evaluation

The eradication of bovine tuberculosis is an ultimate aim of the beef industry and the development of accurate diagnostic tests will expedite eradication. Characterization of Mycobacterium bovis antigens, and a detailed understanding of their immune reactivity will aid in the development of more specific and sensitive diagnostic tests. With this aim, studies were conducted which have resulted in the purification and immunological characterization of the major soluble M. bovis antigens. The purified antigens were found to contain cross-reactive epitopes and immunological responses to these proteins varied among individual animals. Thus if more specific diagnostic tests are to be formulated, they will have to be at the epitope level, using only species-specific epitopes and not whole proteins. Due to the genetic diversity of the response of infected cattle to individual epitopes, a large cocktail of such epitopes will be necessary for the development of a sensitive test.

Mechanisms of stimulation of interleukin-1 beta and tumor necrosis factor-alpha by Mycobacterium tuberculosis components

The granulomatous immune response in tuberculosis is characterized by delayed hypersensitivity and is mediated by various cytokines released by the stimulated mononuclear phagocytes, including tumor necrosis factor-alpha (TNF alpha) and IL-1 beta. We have demonstrated that Mycobacterium tuberculosis cell wall component lipoarabinomannan (LAM), mycobacterial heat shock protein-65 kD, and M. tuberculosis culture filtrate, devoid of LPS as assessed by the Amebocyte Lysate assay, stimulate the production of TNF alpha and IL-1 beta proteins and mRNA from mononuclear phagocytes (THP-1 cells). The effect of LAM on the release of these cytokines was specific, as only LAM stimulation was inhibited by anti-LAM monoclonal antibody. Interestingly, we found that LAM and Gram-negative bacterial cell wall-associated endotoxin LPS may share a similar mechanism in their stimulatory action as demonstrated by inhibition of TNF alpha and IL-1 beta release by monoclonal antibodies to CD14. Anti-CD14 monoclonal antibody MY4 inhibited both TNF alpha and IL-1 beta release with LAM and LPS but no effect was observed with other mycobacterial proteins. An isotype antibody control did not inhibit release of cytokines under the same experimental conditions. M. tuberculosis and its components upregulated IL-1 beta and TNF alpha mRNAs in THP-1 cells. Nuclear run-on assay for IL-1 beta demonstrated that LAM increased the transcription rate. The induction of IL-1 beta was regulated at the transcriptional level, in which these stimuli acted through cis-acting element(s) on the 5' flanking region of the IL-1 beta genomic DNA. M. tuberculosis cell wall component LAM acts similarly to LPS in activating mononuclear phagocyte cytokine TNF alpha and IL-1 beta release through CD14 and synthesis at the transcriptional level; both cytokines are key participants in the host immune response to tuberculosis.

Results of the third immunology of leprosy/immunology of tuberculosis antimycobacterial monoclonal antibody workshop

An international workshop was sponsored by the World Health organization to screen new antimycobacterial monoclonal antibodies and to identify antibodies which could be recommended as standard reagents giving consistent results under differing assay conditions. Fifty-eight antibodies were submitted to the workshop by eight independent laboratories. Nineteen of the antibodies recognized antigens distinct from those identified in earlier workshops, defining at least 10 new protein antigens. Monoclonal antibodies characterized in the workshop provide a set of convenient reagents for further characterization of mycobacterial antigens.

Specificity of IgG subclass antibodies in different clinical manifestations of leprosy

We analysed specific IgG subclasses levels to Mycobacterium leprae sonicate extract (MSE), lipoarabinomannan B (LAM) and phenolic glycolipid I (PGL-I) in the sera of leprosy patients with different clinical manifestations. IgG2 was found to be the predominant antibody to MSE regardless of clinical manifestations, and IgG1 response was mostly seen in lepromatous patients. IgG3 reacted only rarely but IgG4 reacted relatively more in certain clinical groups such as borderline lepromatous and lepromatous with erythema nodosum leprosum (ENL) reaction. Most of the IgG subclass responses to MSE could be accounted for reactivity with LAM, suggesting that LAM is the major immunogen involved in the pathogenesis of leprosy. In contrast to LAM, PGL-I antigen showed considerably lower reactivities for IgG subclasses. An association between IgG subclass responses and clinical manifestations of leprosy was also seen. Whereas borderline lepromatous patients were found to have significantly higher levels of IgG2 and IgG4 to MSE, lepromatous patients had elevated levels of IgG1 and lower levels of IgG2. An interesting observation, however, was the significantly higher levels of IgG2 to LAM in the pure neuritic leprosy patients.

Phenolic glycolipid-1 of Mycobacterium leprae binds complement component C3 in serum and mediates phagocytosis by human monocytes

Previous studies from this laboratory have demonstrated that Mycobacterium leprae, an obligate intracellular bacterial parasite, enters human mononuclear phagocytes via complement receptors on these host cells and bacterium-bound C3. The present study investigates the role of M. leprae surface molecules in C3 fixation and phagocytosis. By enzyme-linked immunosorbent assay, C3 binds selectively to phenolic glycolipid-1 (PGL-1), a major surface molecule of the leprosy bacillus. C3 fixation to PGL-1 is serum concentration dependent and is abolished in heat-inactivated serum or serum containing ethylenediaminetetraacetic acid. C3 fixation is also abolished in serum containing ethyleneglycol-bis (beta-aminoethyl ether)N,N,N'-tetraacetic acid and MgCl2 indicating that isolated PGL-1 fixes C3 via the classical complement pathway. The capacity of PGL-1 to fix C3 is dependent upon its terminal trisaccharide since sequential removal of monosaccharide units of the trisaccharide results in a stepwise reduction in C3 fixation. Deacylation of PGL-1 also abolishes C3 fixation. C3 fixes to the trisaccharide of PGL-1 that is chemically linked to bovine serum albumin via the chemical carrier, 8-methoxycarbonyloctanol. PGL-1 mediates C3 fixation to polystyrene microspheres, and PGL-1 and C3 together mediate ingestion of polystyrene microspheres by human monocytes, wherein these inert test particles reside in membrane-bound phagosomes. Thus, complement receptors on mononuclear phagocytes, complement component C3, and PGL-1 comprise a three-component receptor-ligand-acceptor molecule system for mediating phagocytosis of M. leprae.

Antigens of the Mycobacterium avium, Mycobacterium intracellulare complex

In the past decade, the clinical significance of the Mycobacterium avium, Myobacterium intracellulare complex (MAC) has increased dramatically primarily because of the association between the MAC and the acquired immunodeficiency syndrome (AIDS). Recent hospital reports have suggested that about one-half of AIDS patients in the United States are infected with the MAC. The resulting myobacteremia is a primary cause of mortality in 5-10% of these patients. This increased clinical importance of the MAC has generated renewed interest in MAC immunobiology. In this review, recent immunological and biochemical characterizations of four classes of dominant myobacterial antigens - glycopeptidolipids, arabinogalactan, lipoarabinomannan and MAC proteins - is examined. In addition, future prospects for improved diagnosis of MAC disease using defined monospecific antigens is discussed.

Inhibition of interferon-gamma-mediated activation in mouse macrophages treated with lipoarabinomannan

Lipoarabinomannan (LAM), purified from the cell walls of Mycobacterium leprae and M. tuberculosis, is a potent inhibitor of interferon-gamma (IFN-gamma) mediated activation of macrophages. The capability of LAM to inhibit IFN-gamma activation of macrophages in vitro was dose dependent and required a 24-h pre-exposure. Defective activation was evident as a block in IFN-gamma-induced cytocidal activity for tumour cell targets and microbicidal capacity for intracellular Toxoplasma gondii. Additionally, LAM treatment blocked the induction of surface Ia antigens on peritoneal macrophages by IFN-gamma. The requirement for pretreatment with LAM was further substantiated by the finding that peritoneal macrophages that were activated in vivo were not affected by LAM treatments and retained full microbicidal function. However, once inhibited by LAM treatment in vitro, macrophages remained fully refractory to IFN-gamma activation for up to 5 days in culture. Inhibition of IFN-gamma activation in macrophages treated with LAM was not overcome by 100-fold increases in the dose of IFN-gamma used or by a constant dose of IFN-gamma in combination with 100-fold increases in the level of endotoxin used to trigger cytotoxic activity. The defect in IFN-gamma unresponsiveness was not due to altered receptor function, as control and LAM-treated macrophages showed similar capacity to bind, internalize, and digest radiolabelled IFN-gamma. Based on the in vitro findings reported here, the inhibition of IFN-gamma-mediated macrophage activation by exposure to LAM may contribute to defective macrophage function observed in lepromatous granulomas and thus constitutes an important aspect of pathogenesis in mycobacterioses.

Enzyme-linked immunosorbent assay for distinguishing serological responses of lepromatous and tuberculoid leprosies to the 29/33-kilodalton doublet and 64-kilodalton antigens of Mycobacterium tuberculosis

Immunoblot assays for the antibodies to Mycobacterium tuberculosis sonic extracts showed that all serum specimens of 40 lepromatous and of 28 tuberculoid leprosy patients reacted in a significant manner to 29/33-kilodalton (kDa) doublet and 64-kDa antigens, respectively. By using an enzyme-linked immunosorbent assay, we observed a significantly high immunoglobulin G antibody titer to the purified M. tuberculosis 29/33-kDa doublet and 64-kDa antigens in lepromatous and tuberculoid leprosy patients, respectively, as compared with normal subjects and tuberculosis patients. This enzyme-linked immunosorbent assay serology may be useful for distinguishing two polar types of leprosy and for diagnosing leprosy in general.

The carbohydrate- and lipid-containing cell wall of mycobacteria, phenolic glycolipids: structure and immunological properties

Phenolic glycolipids were first discovered as cell-wall constituents of M. bovis, M. bovis BCG, M. marinum, and M. kansasii. Recently, such compounds were also isolated from M. leprae and have been shown to be specific-species serological markers. Moreover, they seem to be involved, in the case of lepromatous leprosy, in the stimulation of the suppressor T-cells. The functional activities of these phenolic glycolipids over the immune cells stimulation emphasized the role played by these molecules in the mycobacteria pathogenicity. Phenolic glycolipids have also been found in M. gastri and M. tuberculosis strain Canetti. From a structural point of view, these glycolipids contain the same aglycon moiety mainly assigned to phenolphthiocerol diester while the sugar part structure confers to some of these glycolipids their antigenic specificity. The search of immunoreactive glycolipids and their function analysis remain a challenge for chemists and immunologists for the understanding of the mycobacteria pathogenicity.

Blocking of Mycobacterium leprae adherence to dissociated Schwann cells by anti-mycobacterial antibodies

The association of Mycobacterium leprae with Schwann cells may represent an early crucial step in M. leprae pathogenesis. Using a dissociated Schwann-cell system and anti-mycobacterial monoclonal and polyclonal antibodies directed against surface and cytoplasmic components, we investigated the nature of M. leprae epitopes that mediate cytadhesion. Antibodies to polysaccharide and lipid components of M. leprae cell wall inhibited cytadhesion, whereas those directed against both surface and cytoplasmic protein epitopes did not show any such effect. No synergistic or antagonistic activity in inhibiting cytadhesion was observed when antibodies were used in combination. Thus, the association of M. leprae with Schwann cells may be mediated collectively by more than one of its lipid/polysaccharide epitopes. Also, a role for humoral immunity in intervention in the initial steps of M. leprae pathogenesis needs to be considered.

Generation of monoclonal antibodies to the specific sugar epitopes of Mycobacterium avium complex serovars

Monoclonal antibodies have been generated to the unique distal sugar epitopes on the oligosaccharide haptens of the glycopeptidolipid antigens of clinically prominent members of the Mycobacterium avium serocomplex. Thus, antibodies are described that recognize the distal O-acetyl-alpha-L-rhamnopyranosyl residue of the specific glycopeptidolipid of M. avium serovar 1, the 4-O-acetyl-2,3-di-O-methyl-alpha-L-fucopyranose of serovar 2, the 4-O-methyl-alpha-L-rhamnopyranosyl-(1----4)-2-O-methyl-alpha-L- fucopyranosyl unit of serovar 4, the 4,6-(1'-carboxyethylidene)-3-O-methyl-beta-D-glucopyranosyl unit of serovar 8 [and the 4,6-(1'-carboxyethylidene)-beta-D-glucopyranosyl residue of serovar 21], and the 4-O-acetyl-2,3-di-O-methyl-alpha-L-fucopyranosyl-(1----4)-beta-D- glucuronopyranosyl unit of serovar 9. Epitope definition was arrived at through use of the pure, chemically defined glycopeptidolipid antigens and neoglycoproteins containing the chemically synthesized distal sugars of some select serovars. These monoclonal antibodies combined with the already published information on the structure of the antigen determinants and the tools used to arrive at these structures provide powerful means for fundamental studies on the role of these antigens in immunopathogenesis and for the precise mapping of the epidemiology of opportunistic infections caused by M. avium.

Identification, isolation and partial characterization of Mycobacterium tuberculosis glycoprotein antigens

In Mycobacterium tuberculosis culture filtrates, three concanavalin A (ConA)-binding bands of 55, 50 and 38 kilodaltons (kD) were identified by labelling blotted proteins with a ConA-peroxidase conjugate. Binding was inhibited by the competitor sugar alpha-methyl mannoside and by reduction with sodium m-periodate. Bands of 55, 50 and 38 kD stained with Coomasie blue were sensitive to digestion with proteases, thus indicating that they are proteins. Glycoproteins were isolated by lectin affinity chromatography or by elution from nitrocellulose membranes. On the isolated form, the 55-50 kD doublet glycoprotein was 65.4% protein and 34.6% sugar. The purified 38 kD molecule was 74.3% protein and 25.7% carbohydrate. By immunoblot, antibodies against mycobacterial glycoproteins were demonstrated in immunized rabbits and in patients with pulmonary tuberculosis, but not in healthy individuals. Treatment with sodium m-periodate abolished binding of rabbit antibodies to the 38 kD glycoprotein. Reactivity of the 55-50 kD doublet glycoprotein was not altered by reduction. By immunoblot with monoclonal antibodies TB71 and TB72, a carbohydrate-dependent and a carbohydrate-independent epitope could be identified on the 38 kD glycoprotein.

Lipoarabinomannan from Mycobacterium tuberculosis induces the production of tumour necrosis factor from human and murine macrophages

We show here that purified lipoarabinomannan (LAM) from Mycobacterium tuberculosis can cause the release of tumour necrosis factor (TNF) in vitro from human blood monocytes and activated mouse peritoneal macrophages, and the production of TNF in vivo in mice pretreated with Propionibacterium acnes, with a potency comparable to that of lipopolysaccharide (LPS) from Gram negative bacteria. Like LPS, LAM binds to polymyxin B. We confirmed that its activity was distinct from any contaminating LPS and was associated with the antigenic activity by affinity chromatography, using a monoclonal antibody specific for LAM. Treatment with dilute alkali greatly diminished the TNF-inducing activity, suggesting that omicron-acyl groups may be involved. When LAM was fractionated by electrophoresis on SDS-Page and blotted on nitrocellulose, most TNF-inducing capacity coincided with the bulk of the LAM, as estimated by molecular weight and antigenic activity. This modification of the Western blotting technique may be generally useful for the study of macrophage-triggering molecules. The ability of LAM to cause the release of TNF may be responsible for some of the characteristics of tuberculosis, such as fever, weight loss, raised acute phase reactants and necrosis that can be mediated by this cytokine.

The use of antigen-bearing nitrocellulose particles derived from Western blots to study proliferative responses to 27 antigenic fractions from Mycobacterium leprae in patients and controls

Antigens present in sonicates of Mycobacterium leprae were separated by SDS-PAGE, blotted electrophoretically on to nitrocellulose, and visualized with a colloidal gold stain. Six bands identified by existing monoclonal antibodies, and a further 21 bands not previously studied, were converted into antigen-bearing nitrocellulose particles for use in vitro lympho-proliferation studies. Controls (putative non-contacts) responded poorly to the antigenic fractions presented in this way. Contacts responded variably to a wide range of the antigens, and most frequently (23%) to the 18,000 MW fraction. Responses to this, and to several other low molecular weight antigens, were not seen in non contacts, and were very rare in all patient groups, which tended to respond to high molecular weight components. The most interesting individual band was at 36,000 MW. This caused significant stimulation of cells from 25% of tuberculoid donors, but never stimulated the cells from lepromatous cases. Indeed this fraction significantly suppressed the background proliferation of the cells from 30% of the lepromatous cases, though the significance of this observation is unclear. Responses to the 65,000 MW heat-shock protein did not differ significantly between the donor groups. Overall the results suggest that the spectrum of clinical leprosy may not be determined by the response to any one antigen. However, this study can not rule out the possibility that the response to one or a few antigens determines the outcome during the first few days after infection.

The inhibitory effects of mycobacterial lipoarabinomannan and polysaccharides upon polyclonal and monoclonal human T cell proliferation

Lipoarabinomannan from Mycobacterium tuberculosis was able to inhibit antigen induced T cell proliferation of human CD4+ T cell clones specific for influenza virus. The inhibitory effect was also present when peripheral human T cells were stimulated with crude mycobacterial antigen extracts. Non-specific T cell stimulation, i.e. IL-2, PHA and anti-CD3 antibodies coupled to beads, was not affected. The inhibitory property was also found when arabinomannan and arabinogalactan of mycobacterial origin were tested but not with other unrelated polysaccharides used as controls. The effect appears to be related to the processing of the antigen by the antigen-presenting cells, since it was evident when T cell clones were stimulated with whole virus, whereas stimulation with a synthetic peptide containing the relevant epitope was not inhibitable.

Serological responses of patients with lepromatous and tuberculoid leprosy to 30-, 31-, and 32-kilodalton antigens of Mycobacterium tuberculosis

Sera from patients with lepromatous and tuberculoid leprosy were examined in immunoblot assays for antibodies to Mycobacterium tuberculosis culture filtrate antigens. Antibodies to 30- and 31-kilodalton proteins were present in 88 and 81%, respectively, of 16 patients with lepromatous disease and absent in 16 patients with tuberculoid disease. Antibodies to a 32-kilodalton protein were found in 12 and 38% of lepromatous and tuberculoid patients, respectively. These reactivities may be useful for distinguishing lepromatous and tuberculoid leprosy.

Leprosy

Leprosy affects over 10 million people in the world. The disease is a model of graded cell-mediated immunity, in this case to the causative organism, Mycobacterium leprae. The clinical manifestations are due to (i) bacterial progression, (ii) immunologic responses of the host, (iii) peripheral nerve damage due to either or both bacterial progression and immunologic responses of the host, and (iv) preventable secondary deformities following nerve damage, which account for most of the stigma of the disease. Treatment modalities are now available to control or minimize the effects of bacterial progression, harmful immunologic responses of the host, peripheral nerve damage, and secondary deformities. Unique biochemical characteristics of M. leprae reside in the cell wall and associated macromolecules. Some of these molecules are potent immunogens in humans, while others constitute the structural integrity of the bacillus. Proteins of M. leprae are currently under intensive investigation as a result of deoxyribonucleic acid cloning of M. leprae genes. Structure-function and antigenic relationships of M. leprae proteins should become available by using recombinant deoxyribonucleic acid procedures coupled with T- and B-cell cloning to advance our understanding of the immunologic reactions encountered in Hansen's disease. Until recently, the study of the immunology of leprosy has been stymied by the lack of immunologically specific M. leprae antigens. The definition of specific antigens and production of recombinant and synthetic immunologic reagents have fostered state-of-the-art research efforts into new immunodiagnostic procedures and development of a leprosy vaccine. Also discussed is progress in understanding of the mechanism(s) underlying the M. leprae-specific immunodeficiency associated with lepromatous leprosy, including the role of suppressor T cells and defective macrophage function. Metabolic studies of M. leprae suggest intact catabolic pathways and energy generation with purine bases and catalase as possible growth factors. Special attention may also need to be given to biophysical parameters for eventual in vitro cultivation. Rapid in vitro systems, using quantitation of bacillary metabolic activity, may soon replace the lengthy mouse footpad test for determining the viability and drug susceptibility of the leprosy bacillus.

Mycobacterial lipoarabinomannan inhibits gamma interferon-mediated activation of macrophages

The principal efferent role of the macrophage in acquired resistance to intracellular pathogens depends on activation by T-cell lymphokines, primarily gamma interferon (IFN-gamma). However, mouse macrophages that are heavily burdened with Mycobacterium leprae are refractory to activation by IFN-gamma and are thus severely compromised in their capacity for both enhanced microbicidal and tumoricidal activities. We report here that lipoarabinomannan (LAM), a highly immunogenic lipopolysaccharide that is a prominent component of the cell walls of M. leprae and M. tuberculosis, was a potent inhibitor of IFN-gamma-mediated activation of mouse macrophages in vitro. Inhibition of macrophage activation by LAM required preincubation for approximately 24 h, resulting in uptake of LAM into cytoplasmic vacuoles of macrophages. Intact LAM was necessary to inhibit IFN-gamma-mediated activation, as this property was lost when the acyl side chains were removed from LAM by mild alkaline hydrolysis. In addition, LAM was an abundant constituent of macrophages isolated from lepromatous granulomas of M. leprae-infected nude mice and likely contributed to the defective activation of granuloma macrophages by IFN-gamma.