Identification of novel markers for mouse CD4⁺ T follicular helper cells

CD4⁺ T follicular helper (T_FH) cells are central for generation of long-term B-cell immunity. A defining phenotypic attribute of T_FH cells is the expression of the chemokine R CXCR5, and T_FH cells are typically identified by co-expression of CXCR5 together with other markers such as PD-1, ICOS, and Bcl-6. Herein, we report high-level expression of the nutrient transporter folate R 4 (FR4) on T_FH cells in acute viral infection. Distinct from the expression profile of conventional T_FH markers, FR4 was highly expressed by naive CD4⁺ T cells, was downregulated after activation and subsequently re-expressed on T_FH cells. Furthermore, FR4 expression was maintained, albeit at lower levels, on memory T_FH cells. Comparative gene expression profiling of FR4^hi versus FR4^lo Ag-specific CD4⁺ effector T cells revealed a molecular signature consistent with T_FH and T_H1 subsets, respectively. Interestingly, genes involved in the purine metabolic pathway, including the ecto-enzyme CD73, were enriched in T_FH cells compared with T_H1 cells, and phenotypic analysis confirmed expression of CD73 on T_FH cells. As there is now considerable interest in developing vaccines that would induce optimal T_FH cell responses, the identification of two novel cell surface markers should be useful in characterization and identification of T_FH cells following vaccination and infection.