Successive annual influenza vaccination induces a recurrent oligoclonotypic memory response in circulating T follicular helper cells

T follicular helper (Tfh) CD4 cells are crucial providers of B cell help during adaptive immune responses. A circulating population of CD4 T cells, termed cTfh, have similarity to lymphoid Tfh, can provide B cell help, and responded to influenza vaccination. However, it is unclear whether human vaccination-induced cTfh respond in an antigen-specific manner and whether they form long-lasting memory. Here, we identified a cTfh population that expressed multiple T cell activation markers and could be readily identified by coexpression of ICOS and CD38. This subset expressed more Bcl-6, c-Maf, and IL-21 than other blood CD4 subsets. Influenza vaccination induced a strong response in the ICOS+CD38+ cTfh at day 7, and this population included hemagglutinin-specific cells by tetramer staining and antigen-stimulated Activation Induced Marker (AIM) expression. Moreover, TCRB sequencing identified a clonal response in ICOS+CD38+ cTfh that correlated strongly with the increased circulating ICOS+CD38+ cTfh frequency and the circulating plasmablast response. In subjects who received successive annual vaccinations, a recurrent oligoclonal response was identified in the ICOS+CD38+ cTfh subset at 7 days after every vaccination. These oligoclonal responses in ICOS+CD38+ cTfh after vaccination persisted in the ICOS-CD38- cTfh repertoire in subsequent years, suggesting clonal maintenance in a memory reservoir in the more-stable ICOS-CD38- cTfh subset. These data highlight the antigen-specificity, lineage relationships and memory properties of human cTfh responses to vaccination, providing new avenues for tracking and monitoring cTfh responses during infection and vaccination in humans.