The nature of cryptic epitopes within the self-antigen myelin basic protein

Mechanisms that allow potentially autoreactive T cells to escape central tolerance and persist in the peripheral lymphoid organs of healthy individuals are poorly defined. It has been proposed that such cells are specific for epitopes which normally are not well presented to the immune system or, in other words, are cryptic. We have used synthetic peptides to define potential T cell epitopes within the N-terminal portion of myelin basic protein (MBP). These were defined in terms of their relative affinity for the MHC-restriction element I-Au and their ability to activate T cells in mice of the H-2(u) haplotype. Three epitopes were identified, one of which corresponded to the known dominant N-terminal epitope (Ac1-9). The other two epitopes (9-20 and 5-20) bound to their MHC-restriction element with relatively high affinity but were cryptic, as defined by the poor response to these epitopes following immunization with intact MBP. Even the longer of these two epitopes did not induce autoimmune encephalomyelitis in H-2(u) mice. These results demonstrate that antigen processing can control both the induction of and effector function of autoreactive T cells, and is therefore a principal mechanism involved in limiting the autoreactive T cell repertoire.