IL21 and infection: CXCR5intPD-1int TFH cells were the primary source of IL-21 expression early during the course of infection, whereas TFH cells with high expression of CXCR5 and PD-1 (CXCR5highPD-1high) also expressed IL-21 during the resolution phase of the infection (Ryg-Cornejo et al., 2016).