T cell activity and MHC II binding capacity of main-chain modified peptides
摘要
An abundant network of hydrogen bonds between the peptide backbone and conserved residues of the MHC II molecules plays a major interacting role and contributes to the ability of the MHC II molecules to bind a wide range of peptides. As a result, we observed that retro-inverso analogs of T cell peptides in which the amide bond was reversed while the side chains retained a correct orientation, did not show any binding capacity. Neither did they elicit T cells nor produce specific antibodies. However, we also demonstrated that punctual modification into the peptide backbone did not necessarily lead to inactive peptides. These observations have been made using the peptide 24-36, a snake toxin fragment which binds to I-Ed and stimulates specific T cells. We have systematically replaced the CO-NH linkage in the central region of the 24-36 peptide by reduced Ψ[CH2-NH] and N-methylated Ψ[CO-NMe] peptide bonds in order to assess the contribution of respectively the carbonyl and the NH group. As revealed by I-Ed binding assays, pseudopeptides bound with various efficacy. Depending on its location into the peptide sequence, the introduced modification made the corresponding peptide less potent or unable to compete with the reference peptide or in contrary did not affect its binding capacity. Weak binders to I-Ed generally stimulated a 24-36 T cell hybridoma with low, if any, efficiency. However, among the good binders, some were as T cell stimulating as the native peptide. Interestingly, others did not yield to T cell stimulation suggesting they formed differently recognized complexes as compared to the 24-36 peptide. These results confirm the major binding contribution arising from the peptide backbone. They also show that particular positions in the peptide tolerated N-methylation or reduction without dramatic effects on binding and thus may be used to produce agonist or antagonist peptides with increasing stability toward proteases.