Identification of human T cell epitopes in the Mycobacterium leprae heat shock protein 70-kD antigen

Abstract

In a number of pathogens, heat shock proteins (hsp) stimulate humoral and cellular immune responses despite significant sequence identity with host hsp. The 70-kD hsp of Mycobacterium leprae, which shares 47% identity with human hsp70 at the protein level, elicited a T cell response in most Myco. bovis (bacille Calmette-Guérin (BCG)) vaccinees as well as leprosy and tuberculosis patients and their contacts. In order to locate T cell epitopes, DNA fragments encoding portions of the 70-kD hsp were expressed in the vector pGEX-2T and tested for T cell reactivity in an in vitro proliferative assay. Cultures of peripheral blood mononuclear cells (PBMC) from BCG vaccinees indicated that the C-terminal half of the molecule contained multiple T cell epitopes, as the T cells from a majority of Myco. leprae hsp70-reactive individuals responded to C-344. Lower proportions of patients with paucibacillary leprosy (36%) and tuberculosis patients (16%) responded to C-344. The smaller C-142 fragment which includes the terminal 70 residues unique to Myco. leprae and is the target for the human antibody response elicited a cellular response in few patients and no vaccinees. In order to map T cell epitopes, two series of synthetic peptides encompassing the region 278-502 were prepared. Using overlapping 12mer and 20mer peptides, this region of the molecule was found to contain several potential T cell epitopes. The longer peptides gave a clearer indication of reactive sequences including regions of the molecule which were not identified with the 12mer peptides. Fine mapping of reactive peptide pools using the 12mer peptides identified two T cell epitopes. Although both were located in regions of the molecule shared with Myco. tuberculosis, one appeared to be cross-reactive with the equivalent human sequence, and thus has the potential to initiate autoimmune responses.