High frequency of autoreactive myelin proteolipid protein-specific T cells in the periphery of naive mice: mechanisms of selection of the self-reactive repertoire.

The autoreactive T cells that escape central tolerance and form the peripheral self-reactive repertoire determine both susceptibility to autoimmune disease and the epitope dominance of a specific autoantigen. SJL (H-2(s)) mice are highly susceptible to the induction of experimental autoimmune encephalomyelitis (EAE) with myelin proteolipid protein (PLP). The two major encephalitogenic epitopes of PLP (PLP 139-151 and PLP 178-191) bind to IA(s) with similar affinity; however, the immune response to the PLP 139-151 epitope is always dominant. The immunodominance of the PLP 139-151 epitope in SJL mice appears to be due to the presence of expanded numbers of T cells (frequency of 1/20,000 CD4(+) cells) reactive to PLP 139-151 in the peripheral repertoire of naive mice. Neither the PLP autoantigen nor infectious environmental agents appear to be responsible for this expanded repertoire, as endogenous PLP 139-151 reactivity is found in both PLP-deficient and germ-free mice. The high frequency of PLP 139-151-reactive T cells in SJL mice is partly due to lack of thymic deletion to PLP 139-151, as the DM20 isoform of PLP (which lacks residues 116-150) is more abundantly expressed in the thymus than full-length PLP. Reexpression of PLP 139-151 in the embryonic thymus results in a significant reduction of PLP 139-151-reactive precursors in naive mice. Thus, escape from central tolerance, combined with peripheral expansion by cross-reactive antigen(s), appears to be responsible for the high frequency of PLP 139-151-reactive T cells.