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Molecular cloning, purification, and serological characterization of MPT63, a novel antigen secreted by Mycobacterium tuberculosis.

ABSTRACT: Proteins that are actively secreted by Mycobacterium tuberculosis generate immune responses in the infected host. This has prompted the characterization of protein components of mycobacterial culture filtrates to develop subunit vaccines and immunodiagnostic reagents. Fractionation of filtrates of M. tuberculosis cultures has yielded an abundant protein called MPT63, which has an apparent molecular mass of 18 kDa. We report the molecular cloning and nucleotide sequence of the mpt63 gene, purification of recombinant MPT63 antigen from Escherichia coli cells, and serological characterization of MPT63. Nucleotide sequence analysis of mpt63 identified an open reading frame encoding a protein of 159 amino acids (aa) consisting of a 29-aa secretion signal peptide and a 130-aa mature MPT63 protein. Recombinant MPT63 protein, purified from E. coli cells, and native MPT63, purified from M. tuberculosis culture filtrates, were indistinguishable in serological assays. Thus, the recombinant protein constitutes a valuable reagent for immunological studies. MPT63 evoked humoral immune responses in guinea pigs infected with virulent M. tuberculosis by the aerosol route. The mpt63 gene is found only in species of the M. tuberculosis complex, as shown by DNA hybridization experiments. Moreover, polyclonal antibody against MPT63 does not cross-react with proteins of a common environmental mycobacterial species, Mycobacterium avium. The absence of cross-reactive epitopes makes MPT63 an attractive candidate as an M. tuberculosis complex-specific diagnostic reagent. In particular, evaluation of MPT63 as an M. tuberculosis complex-specific reagent for diagnostic skin testing is under way.

SUBMITTER: Manca C

PROVIDER: S-EPMC174551 | biostudies-other | 1997 Jan

REPOSITORIES: biostudies-other

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Molecular cloning, purification, and serological characterization of MPT63, a novel antigen secreted by Mycobacterium tuberculosis.

Manca C C Lyashchenko K K Wiker H G HG Usai D D Colangeli R R Gennaro M L ML

Infection and immunity 19970101 1

Proteins that are actively secreted by Mycobacterium tuberculosis generate immune responses in the infected host. This has prompted the characterization of protein components of mycobacterial culture filtrates to develop subunit vaccines and immunodiagnostic reagents. Fractionation of filtrates of M. tuberculosis cultures has yielded an abundant protein called MPT63, which has an apparent molecular mass of 18 kDa. We report the molecular cloning and nucleotide sequence of the mpt63 gene, purific ...[more]

PMID: 8975887

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Project description:A novel immunogenic antigen, the 6-kDa early secretory antigenic target (ESAT-6), from short-term culture filtrates of Mycobacterium tuberculosis was purified by hydrophobic interaction chromatography and anion-exchange chromatography by use of fast protein liquid chromatography. The antigen focused at two different pIs of 4.0 and 4.5 during isoelectric focusing, and each of these components separated into three spots ranging from 4 to 6 kDa during two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The apparent differences in molecular masses or pIs of these isoforms were not due to posttranslational glycosylation. The molecular weight of the purified native protein was determined by applying gel filtration and nondenaturing polyacrylamide gel electrophoresis and found to be 24 kDa. ESAT-6 is recognized by the murine monoclonal antibody HYB 76-8, which was used to screen a recombinant lambda gt11 M. tuberculosis DNA library. A phage expressing a gene product recognized by HYB 76-8 was isolated, and a 1.7-kbp fragment of the mycobacterial DNA insert was sequenced. The structural gene of ESAT-6 was identified as the sequence encoding a polypeptide of 95 amino acids. The N terminus of the deduced sequence could be aligned with the 10 amino-terminal amino acids derived from sequence analyses of the native protein. N-terminal sequence analysis showed that the purified antigen was essentially free from contaminants, and the amino acid analysis of the antigen was in good agreement with the DNA sequence-deduced amino acid composition. Thus, the heterogeneities observed in the pI and molecular weight of the purified antigen do not derive from contaminating proteins but are most likely due to heterogeneity of the antigen itself. Native and recombinant ESAT-6 are immunologically active in that both elicited a high release of gamma interferon from T cells isolated from memory-immune mice challenged with M. tuberculosis. Analyses of subcellular fractions of M. tuberculosis showed the presence of ESAT-6 in cytosol- and cell wall-containing fractions. Interspecies analyses showed the presence of ESAT-6 in filtrates from M. tuberculosis complex species. Among filtrates from mycobacteria not belonging to the M. tuberculosis complex, reactivity was observed in Mycobacterium kansasii, Mycobacterium szulgai, and Mycobacterium marinum.

Project description:Increasing evidence demonstrates that antigen-specific cellular and humoral immunity plays an indispensable role in protection against Mycobacterium tuberculosis infection. Antigen is a key element in the development of a successful diagnostic method and vaccine. However, few antigens are available, and a systemic study on M. tuberculosis ORFeome-based antigen screening is still lacking. In the current study, a genome-wide examination was conducted on high-throughput M. tuberculosis encoding proteins and novel antigens were identified via a comprehensive investigation of serological and antigen-specific cellular responses. The serological immunoglobulin G level of each protein was detected in pooled sera from 200 pulmonary tuberculosis patients by means of semi-quantitative Western blot. Of the 1,250 detected proteins, 29 were present at a higher level relative to the commercialized 38-kDa protein. Furthermore, the top 12 of the 29 proteins had not been previously reported, and their antigenicity was validated in serum from each individual patient. Results confirmed that the 12 proteins displayed nearly identical immunoglobulin G antibody levels in patients with pulmonary and extrapulmonary tuberculosis. Antigen-specific cellular interferon-γ secretion was also evaluated using a cell-based ELISPOT assay. Thirty-four of the proteins were able to induce positive interferon-γ production by peripheral blood mononuclear cells from pulmonary tuberculosis patients as judged by positive (commercial ESAT-6 antigen) and negative controls. The top 4 candidates out of the 34 proteins displayed good accuracy ranging from 50% to 80% compared with the commercial ESAT-6 antigen. Subsequent epitope examination confirmed that a pool of peptides, including a 25aa peptide from Rv1198, demonstrated significant tuberculosis-specific cellular interferon-γ production. Overall, the current study draws significant attention to novel M. tuberculosis antigens, many of which have not been previously reported. This discovery provides a large amount of useful information for the diagnosis of tuberculosis and the development of vaccines to provide protection against tuberculosis.

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Molecular cloning, purification, and serological characterization of MPT63, a novel antigen secreted by Mycobacterium tuberculosis.

Publications

Molecular cloning, purification, and serological characterization of MPT63, a novel antigen secreted by Mycobacterium tuberculosis.

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