Antigen-experienced CD4 T cells display a reduced capacity for clonal expansion in vivo that is imposed by factors present in the immune host

It is thought that protective immunity is mediated in part by Ag-experienced T cells that respond more quickly and vigorously than naive T cells. Using adoptive transfer of OVA-specific CD4 T cells from TCR transgenic mice as a model system, we show that Ag-experienced CD4 T cells accumulate in lymphnodes more rapidly than naive T cells after in vivo challenge with Ag, However, the magnitude of clonal expansion by Ag-experienced T cells Was much less than that of naive T cells, particularly at early times after primary immunization. Ag-experienced CD4 T cells quickly reverted to the slower but more robust clonal expansion behavior of naive T cells after transfer into a naive environment. Conversely, the capacity for rapid clonal expansion was acquired by naive CD4 T cells after transfer into passively immunized recipients, These results indicate that rapid in vivo response by Ag-experienced T cells is facilitated by Ag-specific Abs, whereas the limited capacity for clonal expansion is imposed by some other factor in the immune environment, perhaps residual Ag, The Journal of Immunology, 2000, 164: 4551-4557.