Processing and reactivity of T cell epitopes containing two cysteine residues from hen egg-white lysozyme (HEL74-90)

The Ag processing and structural requirements involved in the generation of a major T cell epitope from the hen egg-white lysozyme protein (HEL74-88), containing two cysteine residues at positions 76 and 80, were investigated . Several T cell hybridomas derived from both low responder (I-A(b)) and hi gh responder (I-A(k)) mice recognize this region. These hybridomas are stro ngly responsive to native HEL, hut unresponsive to the reduced and carboxym ethylated protein. Air-oxidized HEL74-88 peptide was unable to bind I-A(k) molecules and failed to stimulate T cells in the absence of intracellular A g processing. Further functional competition assays showed that alkylation of cysteine residues with bulky methyl groups interferes with the contacts for the MHC class II molecules (I-A(k)) of high responder mice and the I-A( b)-restricted TCR of low responder mice. Serine substitutions of the cystei ne residues of HEL74-88 either enhanced or abrogated T cell stimulation by the peptides without significant alterations in the class II binding. These results suggest that the cysteine residues of peptides must be free from d isulfide bonding for efficient stimulation of T cells and yet frequently us ed modifications of cysteine residues may not be suitable for peptide-based vaccine development.