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A peptide binding weakly to the major histocompatibility molecule augments T cell responses

An I‐Ad‐derived peptide PB1 was found to enhance the reactivity of I‐Ad‐restricted T cells. The augmentative effect was not due to the cross‐reactivity of PB1 peptide with antigens. PB1 had no effect on T cells specific for I‐Ab and I‐Ek, nor did PB1 increase the T cell responses to concanavalin A and staphyloccocal enterotoxin B. The strict I‐Ad specificity suggests that PB1 enhances the recognition of antigen‐I‐Ad complex by T cell receptor. PB1 bound to I‐Ad weakly. The augmentative effect could be found on other I‐Ad‐binding peptides in appropriate conditions; however, PB1 was distinct in its prominently augmentative effect on all the I‐Ad‐restricted T cells analyzed. A similar enhancing activity was demonstrated on a synthetic transferrin receptor peptide with minimum affinity for I‐Ad. The unusual enhancing activity of PB1 may thus be attributed to the low I‐Ad binding affinity. It was postulated that the binding of low‐affinity PB1 would not only stabilize I‐Ad structure, but also enhance the binding of other peptides. This was supported by the increased binding of OVA 323‐339 and cI 84–98 to I‐Ad in the presence of PB1. The inclusion of PB1 in the immunization mixture also enhanced T cell responses in vivo, suggesting the possibility of using low‐affinity peptide to promote specific immunity.