The Proliferative Capacity of Individual Naive CD4 ⁺ T Cells Is Amplified by Prolonged T Cell Antigen Receptor Triggering

Strong antigenic encounter by T cells rapidly induces immunological synapse formation and surface T cell receptor (TCR) downregulation. Although surface TCR expression can remain low for several days, T cells can still sustain antigenic signaling. It has been unclear whether prolonged antigenic signaling occurs in the absence of surface TCR replenishment, being maintained by a few “nondownregulatable” surface TCRs that might reside in a synaptosomal structure. Alternatively, the low surface TCR level induced by antigen might represent a dynamic state of expression involving continual surface TCR replenishment, reengagement by antigen, and ongoing downregulation. To resolve this issue, we studied in vivo–generated, dual-specificity primary naive CD4⁺ T cells. On these cells, antigenic stimulus exclusively downregulated antigen-specific, but not antigen-nonspecific, TCRs. In addition to providing a means to track TCR engagement, this also allowed us to use the antigen nonspecific TCR to track TCR expression in isolation from TCR engagement by antigen. Surface TCR replenishment began within the first day of stimulation, and occurred synchronously with continuous antigen-specific TCR engagement and downregulation. Furthermore, by enhancing CD25 expression, extended signaling through surface-replenishing TCRs significantly amplified the number of daughter cells generated by naive CD4⁺ T cells that had already committed to proliferate. This effect required TCR engagement and could not be substituted for by interleukin 2. These data demonstrate that TCR triggering and consumption can occur over an extended period of time, with a significant impact on the effector responses evoked from naive CD4⁺ T cells.