While there was no difference in the proportion or number of TFH cells, defined as CD4+ICOS+CXCR5+, at day 35 post-infection, or at any other time point examined earlier in the infection (data not shown), the percentage and number of these cells in the spleen decreased in Il21−/− mice as the infection progressed to day 56 post-infection (Fig. 4C). This evidence concerns the gene ICOS and infection.