Constraints in antigen processing result in unresponsiveness to a T cell epitope of hen egg lysozyme in C57BL/6 mice

T cell hybridoma clones were derived after fusion of BW-5147 parent cells with lymphocytes from C57BL/6 mice injected with phosphorylcholine (PC)-hen egg lysozyme (HEL) conjugates. Several T cell hybridomas were preferentially reactive with PC-HEL over unconjugated HEL, and a particular clone (PC-H4.1) was further analyzed. This T cell hybridoma clone could respond to its maximal level toward unconjugated HEL only when the dose of HEL was increased to 5-10-fold of the PC-HEL concentration. Interestingly, this clone was not stimulated by unfolded HEL (by S-carboxymethylation) to the level of PC-HEL. A synthetic peptide representing the amino acid position 47-61 of HEL, which is known to be non-immunogenic upon HEL injection in C57BL/6 mice, was able to stimulate the hybridoma only to a level comparable to that induced by unconjugated HEL. The T cell response to this synthetic peptide required an additional antigen-processing step, based on its inability to stimulate T cells after treatment of antigen-presenting cells with leupeptin, chloroquine or paraformaldehyde. Deletion of a single C-terminal amino acid residue of HEL 47-61 (arginine) significantly enhanced (10-100-fold of HEL 47-61) the T cell reactivity and abrogated the necessity of further antigen processing. These results suggest that the lack of a T cell response to a certain epitope may not be due to the lack of a T cell repertoire reactive to the epitope. In some cases, the unresponsiveness may be due to the difficulty in generating the particular epitopes. Taken together, modification of the lysozyme molecule with PC conjugation may facilitate further antigen processing of HEL to generate an optimal epitope for the nonresponder mice.