CXCR5 and infection: Intriguingly, we found that direct TGF-β promoted the differentiation of Tfh precursor cells at day 3 post infection (p.i.), such that there were about 1/3 fewer CD25lo CXCR5+ Tfh precursor cells in the absence of direct TGF-β signals (WT = 60.25% ± 4, KO = 42% ± 3.3) (Figure 2—figure supplement 1A).