TCR repertoire restriction as a consequence of postthymic selection of human naive CD4⁺ T cells. Polyclonal CD31^{+ thymic}naive CD4⁺ T cells undergo selection in the periphery, depending on TCR affinity for self-peptide/MHC II complexes presented by APC (specificities A and E). T cells with other TCR specificities (specificities B, C, D, and F) that do not receive sufficient survival signals may finally disappear from the peripheral repertoire.