CXCR5 and infection: Studies using Il21−/− and Il21r−/− mice show that the in vivo generation of CXCR5+PD-1+ TFH cells in these mice is as robust as in wt mice following NP-KLH (152) or NP-CGG (147) immunization, or infection with LCMV (148) or Influenza (149).