Absence of preferential high-affinity TCR expansion during CD4 T cell responses

Abstract Current views assume higher-affinity T cells, as identified by pMHCII tetramers, possess enhanced capabilities while lower-affinity T cells are inconsequential to the immune response. We set out to formally address the contribution of low-affinity T cells to the expanded repertoire. To perform this study we employed a micropipette based assay and T cell receptor signaling functional reporter to quantify the total number of antigen-specific CD4 T cells from naïve precursors through their expansion to antigen. In the naïve state, an in vivo limiting dilution assay revealed hundreds more precursor T cells than defined by pMHCII tetramers, finding high-affinity T cells comprise only 5–30% of the total naïve repertoire. Upon immunization, low-affinity T cells expand similarly to higher-affinity counterparts, demonstrating no affinity maturation or preferential expansion of the highest-affinity T cells as the immune response progressed. These findings demonstrate affinity diversity of CD4 T cells is maintained without a selective shift favoring the highest-affinity cells.