Binding to Ia protects an immunogenic peptide from proteolytic degradation

A 34 amino acid hen egg-white lysozyme (HEL) peptide was designed and synthesized to investigate if an immunogenic peptide once bound to an Ia molecule becomes proteolytically inaccessible. The determinant recognized by T cells, HEL(52-61) was composed of L-amino acids whereas the 12 amino acid extension on each side of this core were composed of D-epimers. This peptide, HEL(40-73) was resistant to proteolysis, except in the core region, where any cleavage would destroy the determinant. Initially HEL(40-73) was shown to be able to stimulate the HEL specific T cell, 3A9, indicating that an I-Ak molecule can bind and present large peptides that extend beyond the theoretical binding groove. HEL(40-73) was then used to examine the proteolytic sensitivity of determinants recognized by T cells. If HEL(40-73) was treated with chymotrypsin before binding to I-Ak, the determinant was totally destroyed; however, if HEL(40-73) was allowed to first bind to I-Ak, then the determinant became resistant to chymotrypsin cleavage. Thus an Ia molecule can protect a determinant from proteolytic degradation, a finding that has important implications for proposed pathways of Ag processing.