Epitope specificity of the T cell proliferative response to lysozyme: proliferative T cells react predominantly to different determinants from those recognized by B cells.

The fine specificity of murine B 10.A/SgSn (B 10.A) T cells reactive with hen egg-white lysozyme (HEL) has been studied through the use of reduced, carboxymethylated HEL, a set of peptides encompassing the entire molecule, and a set of variant lysozymes from other species. Cells were taken from the lymph nodes draining the site of immunization at the base of the tail, and were restimulated in vitro with immunogen or analogue to measure T cell reactivity. Unlike B cell reactivity, which we have shown to be mainly associated with an epitope preserved in the N-C peptide (residues 1--17, Cys6--Cys 127, 120--129) most T cell reactivity appears to be directed towards a limited number of determinants on cyanogen bromide cleavage fragment II of HEL (LII) (13--105). This was confirmed by a cell-dilution assay in which antigen-reactive units are measured; reactivity was highest to LII, intermediate to N-C, and low but significant to cyanogen bromide cleavage fragment III (LIII) (106--129). Furthermore, priming with LII is as effective as immunization with HEL and results in the same extensive cross-reactivities to variant lysozymes. Although LII reactivity predominates in the response to HEL, injection of LIII and N-C reveals sizeable reactivity to the homologous peptides and to HEL. By cross-stimulation studies, specific epitopes could be defined in certain regions of HEL. B 10.A is clearly responsive to the overlap between N-C and LII (residues 13--17), and to an epitope in the region 106--121, but is poorly responsive to the C-terminal portion (120--129). The response to 106--121 is characterized by an exquisite specificity in which as little as a single amino acid substitution (Asn for Gln) is recognized.