Identification of murine antigen-specific T follicular helper cells using an activation-induced marker assay.

Protective antibody (Ab) responses induced by natural infection or vaccination play a central role in defense against invasive pathogens. Germinal centers (GCs) are the sites of Ab affinity maturation and T follicular helper (Tfh) cells are a critical factor for driving GC formation and B cell selection. Therefore characterization of antigen (Ag)-specific Tfh cells is increasingly essential to define the mechanistic basis of protective antibody responses. However, since Tfh are weak producers of cytokines it is difficult to detect Ag-specific Tfh cells using conventional intracellular cytokine staining (ICS). Here, we report an assay identifying mouse Ag-specific Tfh cells by assessing the upregulation of surface activation-induced markers (AIM). Murine lymph node (LN)-derived Tfh cells largely retained CXCR5 and PD-1 expression following 18-hour cell culture. After influenza infection or influenza hemagglutinin (HA) protein vaccination of mice, stimulation of lymph node cell suspensions with peptide pools or whole protein drove upregulation of CD25, OX40 (CD134), ICOS (CD278) and CD154 on Tfh cells. Upregulation of either CD154 or CD25/OX40 proved a sensitive method for delineating HA-specific Tfh cells. This assay provides the opportunity to quantify antigen-specific Tfh cells in mice without the need for transgenic models or MHC-II tetramer reagents restricted to specific epitopes.