MHC class II peptide flanking residues of exogenous antigens influence recognition by autoreactive T cells.

Molecular mimicry between exogenous microbial antigens and self-epitopes has been proposed as a triggering mechanism for autoimmune diseases for many years. We reported that a peptide from a protein specific to Chlamydia pneumoniae (Cpn0483) which shares a motif with the dominant encephalitogenic epitope of the self-antigen, rat myelin basic protein (rat68-86), elicits experimental autoimmune encephalomyelitis (EAE) in Lewis rats. We recently observed that rat68-86 utilizes aspartic acid (D) and arginine (R) in the common motif as primary and secondary TCR contacts, respectively. In contrast, the encephalitogenic activity of Cpn0483 is dependent on R and the C-terminal asparagine (N), which flanks the MHC class II-P9 anchor residue. Thus, rat68-86 and Cpn0483 share a common motif, are encephalitogenic and are both restricted by MHC class II RT1.B(l). T cells from rats immunized with the encephalitogenic Cpn0483 peptide proliferated to the priming peptide as well as to the non-encephalitogenic CpnN>A analog. However, CpnN>A-primed T cells did not respond to the native Cpn0483 peptide. We conclude that the MHC-flanking C-terminal asparagine residue markedly influences T cell recognition by the chlamydial peptide.