Antigen-specific CD4+ T-cells are major regulators of adaptive immune responses. Interactions between TCR and peptide-MHCII complexes are critical for the selection and expansion of these cells. The role of peptide-MHCII stability in CD4+ T-cell selection, however, has not been reported yet. We used altered peptide ligands (APL) which displayed different stabilities in complex with MHCII. Mice were immunized with the APLs and draining lymph nodes were analyzed. Although the accumulation of antigen-specific CD4+ T-cells was similar, we found that the repertoire of T-cells selected was different among the APLs. Single cell sorting of antigen-specific CD4+ T-cells and sequencing of their TCR alpha- and beta-chains revealed that high stability complexes selected a broad and mainly private repertoire of CD4+ T-cells. Low stability complexes on the other hand selected a more focused and public repertoire. Our findings suggest that peptide-MHCII stability is a critical parameter in the selection of CD4+ T-cells during adaptive immune responses.

This study is partly supported by FWF - Austrian Science Fund (Erwin Schroedinger Fellowship).