CD4⁺ T cells that enter the draining lymph nodes after antigen injection participate in the primary response and become central–memory cells

We explored the relationship between the time of naive CD4⁺ T cell exposure to antigen in the primary immune response and the quality of the memory cells produced. Naive CD4⁺ T cells that migrated into the skin-draining lymph nodes after subcutaneous antigen injection accounted for about half of the antigen-specific population present at the peak of clonal expansion. These late-arriving T cells divided less and more retained the central–memory marker CD62L than the T cells that resided in the draining lymph nodes at the time of antigen injection. The fewer cell divisions were related to competition with resident T cells that expanded earlier in the response and a reduction in the number of dendritic cells displaying peptide–major histocompatibility complex (MHC) II complexes at later times after antigen injection. The progeny of late-arriving T cells possessed the phenotype of central–memory cells, and proliferated more extensively during the secondary response than the progeny of the resident T cells. The results suggest that late arrival into lymph nodes and exposure to antigen-presenting cells displaying lower numbers of peptide–MHC II complexes in the presence of competing T cells ensures that some antigen-specific CD4⁺ T cells divide less in the primary response and become central–memory cells.