Recognition of mycobacterial antigens by sera from patients with leprosy

Structure, stability and chaperone function of Mycobacterium leprae Heat Shock Protein 18 are differentially affected upon interaction with gold and silver nanoparticles

Gold nanoparticles (AuNPs) and silver nanoparticles (AgNPs) have several biomedical applications. However, the effective usage of these two nanoparticles is impeded due to limited understanding of their interaction with proteins including small heat shock proteins (sHSPs). Specifically, no evidences of interaction of these two nanoparticles with HSP18 (an antigenic protein) which is an important factor for the growth and survival of M. leprae (the causative organism of leprosy) are available in the literature. Here, we report for the first time evidences of "HSP18-AuNPs/AgNPs interaction" and its impact on the structure and chaperone function of HSP18. Interaction of citrate-capped AuNPs/AgNPs (~20 nm diameter) to HSP18 alters the secondary and tertiary structure of HSP18 in a distinctly opposite manner; while "HSP18-AuNPs interaction" leads to oligomeric association, "HSP18-AgNPs interaction" results in oligomeric dissociation of the protein. Surface hydrophobicity, thermal stability, chaperone function of HSP18 and survival of thermally stressed E. coli harbouring HSP18 are enhanced upon AuNPs interaction, while all of them are reduced upon interaction with AgNPs. Altogether, our study reveals that HSP18 is an important drug target in leprosy and its chaperone function may possibly plays a vital role in the growth and survival of M. leprae pathogen in infected hosts.

Role of Subunit Exchange and Electrostatic Interactions on the Chaperone Activity of Mycobacterium leprae HSP18

Mycobacterium leprae HSP18, a major immunodominant antigen of M. leprae pathogen, is a small heat shock protein. Previously, we reported that HSP18 is a molecular chaperone that prevents aggregation of different chemically and thermally stressed client proteins and assists refolding of denatured enzyme at normal temperature. We also demonstrated that it can efficiently prevent the thermal killing of E. coli at higher temperature. However, molecular mechanism behind the chaperone function of HSP18 is still unclear. Therefore, we studied the structure and chaperone function of HSP18 at normal temperature (25°C) as well as at higher temperatures (31–43°C). Our study revealed that the chaperone function of HSP18 is enhanced significantly with increasing temperature. Far- and near-UV CD experiments suggested that its secondary and tertiary structure remain intact in this temperature range (25–43°C). Besides, temperature has no effect on the static oligomeric size of this protein. Subunit exchange study demonstrated that subunits of HSP18 exchange at 25°C with a rate constant of 0.018 min^-1. Both rate of subunit exchange and chaperone activity of HSP18 is found to increase with rise in temperature. However, the surface hydrophobicity of HSP18 decreases markedly upon heating and has no correlation with its chaperone function in this temperature range. Furthermore, we observed that HSP18 exhibits diminished chaperone function in the presence of NaCl at 25°C. At elevated temperatures, weakening of interactions between HSP18 and stressed client proteins in the presence of NaCl results in greater reduction of its chaperone function. The oligomeric size, rate of subunit exchange and structural stability of HSP18 were also found to decrease when electrostatic interactions were weakened. These results clearly indicated that subunit exchange and electrostatic interactions play a major role in the chaperone function of HSP18.

Predominant recognition of species-specific determinants of the GroES homologues from Mycobacterium leprae and M. tuberculosis

The Mycobacterium leprae and M. tuberculosis 10,000 MW heat-shock protein homologues of GroES have previously been identified as major immunogens for human T cells. We used synthetic peptides to characterize the determinants recognized by murine T cells. The findings suggest that, despite 90% sequence identity between these two proteins, T cells recognize prominently the species-specific determinants localized within amino acid residues 21-40 and 49-72. Analysis of the molecular determinants of species-specificity for the M. leprae GroES sequence 25-40, using T-cell hybridomas and major histocompatibility complex (MHC)-binding assays, led to the identification of epitope cores and critical residues. Interestingly, closely overlapping epitope cores were found to be restricted by either H-2Ad (24-34) or H-2Ed (28-34). Furthermore, the site recognized by the M. leprae-specific monoclonal antibodies ML06 and ML10 was also localized in the overlapping sequences 25-31 and 25-29. In conclusion, we demonstrated that immunodominant species-specific T- and B-cell epitopes can be found in a mycobacterial heat-shock protein despite its highly conserved amino acid sequence. This finding suggests the feasibility of identifying a sufficient number of M. leprae-specific determinants for a composite T-cell immunodiagnostic reagent for tuberculoid leprosy.

A western blot characterization of Mycobacterium bovis antigens recognized by cattle sera

The immune response to Mycobacterium bovis in cattle was assessed by Western blot. The antibody recognition pattern to M. bovis whole cell extracts and culture supernatant antigens was studied by using sera from M. bovis-infected (n = 62) and healthy (n = 38) cattle. Although the recognition patterns were highly variable, some proteins were regularly detected, mainly those with molecular masses of 17, 23, 28, 42, 66, 71 and 80 kDa in cellular extracts, and with molecular masses of 23 and 33 kDa in supernatants. Whole cell extract antigens were more frequently recognized than culture supernatant antigens. Healthy controls produced only a weak antibody response. The antibody response was variable, depending on tuberculosis stage. In early stages very few antibodies were detected. A response against the 66-kDa stress protein was mounted in intermediate tuberculosis and remained stable in more advanced disease. In late diseases, the preferentially recognized antigens were a 28-kDa cellular protein and supernatant antigens. The 28-kDa protein was studied in some detail. As determined by using monoclonal antibodies, the 28-kDa protein is different from superoxide dismutase. This protein aggregated in stored cell extracts and was not totally transferred to nitrocellulose. The principal conclusions of this work are: (i) whole cell extract proteins are more frequently recognized than the secreted proteins and (ii) a 28-kDa protein is a major antigen in late disease.

Sequence and immunological characterization of a serine-rich antigen from Mycobacterium leprae

Sera from lepromatous leprosy patients were used to screen a Mycobacterium leprae lambda gt11 library. Three positive plaques were picked, and lysogens were constructed. Immunoblot analysis showed that all of the lysogens expressed an apparently identical beta-galactosidase fusion protein which reacted strongly with the sera. The 1.7-kbp insert from one clone was subcloned into the lacZ gene in pUR290; sequence analysis of the end fused to lacZ revealed an open reading frame with no significant homology to previously published sequences. The insert was used to screen an M. leprae cosmid library, and five clones were isolated. The insert was also found to hybridize to clones expressing the M. leprae antigen which had previously been designated class III and 25L. A 1.8-kbp HindIII fragment was subcloned from one of the cosmids and sequenced. The sequence revealed a 1,227-bp open reading frame, encoding a 408-amino-acid protein with a predicted molecular mass of 42,466 Da. The protein contains amino- and carboxy-terminal hydrophobic domains and a hydrophilic central domain; the amino-terminal domain shows some homology to a 51-kDa hypothetical antigen of Mycobacterium tuberculosis, while the hydrophilic region contains a high proportion of serine residues, and we have therefore designated the protein serine-rich antigen (Sra). Some repeated motifs are present in the protein, but their significance is unknown. Seventy-eight percent of serum samples from multibacillary leprosy patients and 68% of serum samples from paucibacillary leprosy patients recognized the fusion protein, showing that this is a major M. leprae antigen. In contrast, all serum samples from endemic controls were negative, while 26% of serum samples from tuberculosis patients were weakly positive.

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.

High antibody levels to the mycobacterial fibronectin-binding antigen of 30-31 kD in tuberculosis and lepromatous leprosy

Immunoblot assays showed that mycobacterial fibronectin-binding antigens are important targets of the humoral immune response in tuberculosis and leprosy. Using culture filtrate antigens of Mycobacterium tuberculosis, strong reactivity with the fibronectin-binding of 30-31 kD (Fn 30-31) was demonstrated in 55.9% of tuberculosis sera and in 56.5% of lepromatous leprosy sera. Sera from patients with tuberculoid leprosy and control sera gave very weak binding. Reactivity of tuberculosis and lepromatous leprosy sera with the fibronectin-binding antigen of 58-60 kD (Fn 58-60) was less conspicuous. The ability to react with fibronectin of the antigens of 58-60 and 30-31 kD was demonstrated by parallel labelling with a fibronectin-biotin conjugate. Fn 30-31 was purified to homogeneity by a two-step procedure and used for ELISA. Positive titres were found in 63% out of 65 tuberculosis sera and in 60.5% out of 43 lepromatous leprosy sera. Antibody titres in lepromatous leprosy sera were higher than in tuberculosis sera. Our observations indicate indirectly that M. leprae possess a highly immunogenic molecule homologous to M. tuberculosis Fn 30-31, which elicits a high antibody response in lepromatous leprosy but not in tuberculoid leprosy. In this investigation, direct evidence for the presence of this antigen in M. leprae was obtained by immunochemistry of lepromatous leprosy lesions with a monospecific antibody raised against M. tuberculosis Fn 30-31.

Major proteins of mycobacterial strain ICRC and Mycobacterium leprae, identified by antibodies in sera from leprosy patients and their contacts

Sera from leprosy patients across the clinical spectrum, healthy contacts, tuberculosis patients, and healthy donors were tested for their reactivity with antigens of mycobacterial strain ICRC (a cultivable mycobacterium) and Mycobacterium leprae by immunoprecipitation technique. Using M. leprae antigens, it was not possible to distinguish between reactivities of sera from lepromatous, borderline lepromatous, borderline tuberculoid, and tuberculoid leprosy patients. All these sera identified M. antigens with molecular masses of 47, 36, 21, and 14 kDa. When the same sera were tested for their reactivities with antigens of mycobacterial strain ICRC, several differences were observed. The 21-kDa antigen of mycobacterial strain ICRC was exclusively precipitated by sera from all lepromatous leprosy patients and from those undergoing erythema nodosum leprosum reaction. Sera from all the other donors tested failed to identify the 21-kDa antigen of mycobacterial strain ICRC. The 14-kDa protein of mycobacterial strain ICRC was identified by sera from a few lepromatous leprosy patients (5 of 26) and all their contacts. Our studies indicate that antigens present on cultivable mycobacteria rather than species-specific antigens may prove to be useful in the serodiagnosis of leprosy.

Mycobacterium avium complex disease in patients with AIDS: seroreactivity to native and recombinant mycobacterial antigens

Antibodies to Mycobacterium avium complex (MAC) antigens were measured by enzyme-linked immunosorbent assays and immunoblot analyses in sera from 20 patients with AIDS and disseminated MAC disease, 5 human immunodeficiency virus-seronegative patients with pulmonary MAC infections, and 20 healthy controls. Whereas enzyme-linked immunosorbent assay titers for healthy controls and patients with AIDS and MAC disease were comparable, human immunodeficiency virus-seronegative patients with MAC disease had higher anti-MAC antibody titers (P less than 0.01). Immunoblot analysis with the same sonic extracts indicated that each of the three groups had a limited heterogeneous response to M. avium antigens. No significant differences in immunoblot reactivities were detected. However, immunoblot studies with recombinant nontuberculous mycobacterial antigens revealed that sera from over 90% of the patients with MAC disease and only 25% of controls recognized a recombinant protein derived from a 35-kDa mycobacterial antigen. Although sonic extracts did not permit adequate discrimination of antibody reactivity in patients with MAC disease, recombinant antigens may be useful as indicators of disease.

T cell response to purified filtrate antigen 85 from Mycobacterium bovis Bacilli Calmette-Guérin (BCG) in leprosy patients

T cell proliferation and IFN-gamma production of peripheral blood mononuclear cells from 25 healthy controls and 39 leprosy patients were tested against BCG-bacilli and culture filtrate. Mycobacterium leprae and purified antigen 85 (the major secreted 30-32 kD protein antigen) from M. bovis strain BCG. In lepromin negative healthy controls, blastogenesis was low to M. leprae and completely negative to antigen 85. IFN-gamma levels were very low, close to detection limits. In all lepromin positive controls, significant proliferation and IFN-gamma secretion was found in response to M. leprae and antigen 85. In the group of lepromatous leprosy (LL) patients, 25/29 of patients (with either positive (13) or negative (12) lymphoproliferative response to BCG) were unreactive to M. leprae or to antigen 85. Four LL patients with positive T cell response to BCG responded with detectable lymphoproliferative response and IFN-gamma secretion to antigen 85. All tuberculoid (TT) leprosy patients responded to BCG, M. leprae and antigen 85. Hence, T cells from leprosy patients and controls demonstrate a marked parallelism of responsiveness towards whole M. leprae and purified antigen 85 from M. bovis BCG, suggesting strong cross-reactivity between the two species and underlining the biological importance of such secreted antigens.

Lipoprotein antigens of Mycobacterium tuberculosis

Detergent phase separation and metabolic labelling have been used to screen for the presence of lipoproteins amongst the antigens of Mycobacterium tuberculosis. Both techniques indicated that four antigens, with subunit molecular weights of 19, 26, 27 and 38 kilodaltons (kDa), are lipoproteins. This finding is consistent with the presence of conserved cysteine residues characteristic of other bacterial lipoproteins within the amino terminal sequences of the 38 kDa and 19 kDa proteins. It is proposed that lipoproteins are involved in the induction of humoral and cellular immune responses to mycobacteria and have a functional role in the transport of nutrients through the mycobacterial cell wall.

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 and characterization of antigenic determinants of Mycobacterium leprae that react with antibodies in sera of leprosy patients

Antigenic determinants of Mycobacterium leprae were identified by screening a lambda gt11::M. leprae genomic library with two separate pools of sera from leprosy patients. A total of 45 recombinant clones were detected with pooled sera from 21 lepromatous (LL) leprosy patients and 5 additional clones specified polypeptides that reacted with antibodies in pooled sera from 30 borderline tuberculoid or tuberculoid leprosy patients. The recombinant clones that specified antigenic determinants that reacted with sera from LL patients were condensed into eight groups on the basis of DNA hybridization experiments among the M. leprae DNA insert fragments. In addition, 11 of the 45 recombinant clones did not hybridize to members of the eight groups nor to one another; these represent unique recombinant clones. None of the recombinant clones identified by screening with sera from tuberculoid leprosy patients hybridized to each other or to any of the 45 LL recombinant clones. The polypeptides specified by the recombinant clones were usually fusion proteins with beta-galactosidase, ranging in size from 117 to 175 kilodaltons (kDa). Members of hybridization group III specified nonfusion proteins of 45 kDa. Only members of hybridization group I reacted with any of 30 monoclonal antibodies prepared against M. leprae proteins; recombinant proteins from these clones reacted with a single monoclonal antibody directed against the M. leprae 65-kDa protein. Thus, at least 22 new antigenic determinants of M. leprae have been identified on the basis of their reactivity to antibodies in sera from LL patients or sera from tuberculoid leprosy patients or both.

Cellular immune responses of leprosy contacts to fractionated Mycobacterium leprae antigens

Antigens of armadillo-derived Mycobacterium leprae sonic extract were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and blotted onto a nitrocellulose membrane, and the unstained blot was converted into 20 fractions of antigen-bearing particles. These were tested in cellular proliferation assays, and reproducible results were obtained between batches of fractions. Peripheral blood mononuclear cells from healthy contacts of leprosy patients (presumed to have protective immunity) were tested with the fractions to investigate which antigens they recognized. A small group of tuberculoid leprosy patients were also tested. Both groups showed a wide range of responses. Almost every fraction stimulated proliferation with at least one donor, yet none was clearly immunodominant or inhibitory in either group. Thus, protective immunity did not appear to be associated with proliferation caused by any single fraction.

High level expression of genes cloned in phage lambda gt11

Plasmid cloning vectors have been constructed which allow genes originally cloned in lambda gt11 to be expressed at a high level in Escherichia coli. They are based on the pEMBL and pUC vectors, with the genes transcribed from the lac promoter. The EcoRI site in the vector has been altered to be in the same reading frame as the site used for cloning in lambda gt11. Cloned proteins are expressed fused to a 2-kDa leader sequence containing a run of six Aparagine residues which considerably improves the stability of the recombinant proteins, but does not interfere with immunological assays. Using these vectors, the Mycobacterium leprae 18-kDa protein was expressed at 20 mg per litre of culture and constituted 15% of total cell protein.