PDCD1 and infection: In accordance with this idea, both Il21-/- and Il21r-/- mice showed higher frequencies of CD44high and PD-1+ CD4+ T cells in the spleen at day 120 post-infection when compared to WT C57BL/6 mice (frequencies CD4+CD44high of 82±5% and 62±14% for Il21-/- and Il21r-/- vs 14±2% for WT C57BL/6; frequencies CD4+PD-1+ of 74±5 and 61±7 for Il21-/- and Il21r-/- vs 8±2 for WT C57BL/6; P<0.01, Kruskal-Wallis test).