CD4 and influenza: CD4+ T cells were then stimulated with IL-2 and influenza-specific peptide (nucleoprotein NP311–325), and NP311–325 tetramer–positive CD4+ T cell populations including T follicular helper (TFH), T follicular regulatory (TFR), TEFF, and conventional Treg (cTreg) cells were analyzed (Fig. 2A).