T cell recognition of a highly conserved epitope in heat shock protein 60: self-tolerance maintained by TCR distinguishing between asparagine and aspartic acid

Cross-reactive T cell recognition of self-heat shock proteins (hsp) has been ascribed a regulatory role in inflammatory arthritis in both animal models and human disease. The previous work implies that a repertoire for epitopes in self-hsp60 should exist in normal subjects. Accordingly, we sought to generate self-hsp60-reactive T cell clones from a healthy individual using a highly purified preparation of recombinant human (Hu) hsp60. Epitope mapping using synthetic peptides and truncated constructs indicated that the T cell clones obtained actually recognized hsp60 derived from Escherichia coli. Using a series of alanine-substituted peptides and additional appropriate synthetic peptides, it was demonstrated that the clones maintain self-tolerance because of their sensitivity to an asparagine to aspartic acid sequence difference between E. coli and HuHsp60 in the epitope-containing peptide. In addition, despite substantial conservation of sequence, the homologous peptide from HuHsp60 did not compete with the E. coli-derived peptide for recognition or antagonize responses by acting as an altered peptide ligand. The results suggest that, even when the immune system targets a highly conserved epitope in bacterial hsp60, self-tolerance is maintained. Furthermore, the finding that T cell clones specific for minor contaminant proteins in HuHsp60 preparations can readily be isolated raises the possibility that the HuHsp60 facilitates presentation of antigenic proteins to the immune system.