Pillai, ShivVilla, Sophia Moriah2026-07-0720262026-06-032026Villa, Sophia Moriah. 2026. Characterizing CD4 T Cells Involved in the Extrafollicular Response. Masters Thesis, Harvard Medical School.32696850https://p2p8-sa-zuvru-a9vusux.re-cotta.com/handle/1/42744160The importance of extrafollicular adaptive immune responses is evident across diverse disease states. However, the precise identity of the T and B lymphocyte subsets that drive dysregulation and pathogenic extrafollicular autoantibody production remains unclear in conditions such as lupus, chronic graft-versus-host disease, and IgG4-related disease. Prior studies in COVID-19 and HIV have shown that class-switched antibodies can arise in the absence of germinal center activity, supporting the idea that class switch recombination may occur outside the germinal center and could depend on alternative forms of CD4 T cell help. This project investigated CD4 T cells interacting with B cells outside the conventional germinal center response. T-B conjugates increased substantially following sheep red blood cell immunization, indicating that these interactions expand during an active immune response. Importantly, the B cells bound within these conjugates were IgD+, indicating that the interacting B cell population was unswitched and consistent with an early stage of the humoral response. In LCMV infection, analysis of interacting CD4 T cells showed a heterogeneous population that included T-bet+ and Foxp3+ cells, with relatively few Bcl6+ cells, suggesting that B cell-interacting CD4 T cells outside the germinal center are more diverse than previously appreciated. Further phenotypic analysis identified distinct ICOS+PSGL1- and ICOS+PSGL1+ CD4 T cell populations also present in the sorted T-B pairs. The ICOS+PSGL1- population aligned more closely with follicular helper features, whereas the ICOS+PSGL1+ population represented a broader Th1-focused helper population. Bulk RNA sequencing supported this distinction, showing that ICOS+PSGL1+ cells exhibit a mixed transcriptional program rather than a purely follicular helper identity. Together, these findings support the existence of a heterogeneous CD4 T cell population that engages unswitched B cells outside the classical germinal center pathway and may contribute to early extrafollicular B cell help.application/pdfenImmunologyThesis or Dissertation2026-07-070009-0002-7417-329X