TGFB1 and viral infectious disease: Because CD4 T helper subsets begin to diverge within the first few days of viral infection (Choi et al., 2013b) and we found fewer Tfh and more Th1 precursor cells in the absence of TGF-β signals during LCMV infection (Figure 2—figure supplement 1A), we questioned when TGF-β was required for Tfh differentiation during influenza virus infection.