IL-21 is produced during <i>P</i>. <i>chabaudi</i> infection and required to control chronic infection.

<p>(A) IL-21 mRNA in spleen cells of <i>P</i>. <i>chabaudi</i>-infected mice measured by real-time quantitative RT-PCR. Parasitemia (B) and total rbc counts (C) were determined in WT C57BL/6 (closed circles), <i>Il21</i>^<i>-/-</i> (open circles) and <i>Il21r</i>^<i>-/-</i> (open squares) mice. (D) Individual examples of spleens from <i>Il21r</i>^<i>-/-</i> (a) <i>Il21</i>^<i>-/-</i> (b) and WT C57BL/6 (c) mice at day 120 post-infection, and a spleen from an age-matched WT C57BL/6 naïve mouse (d). Bar, 1 cm. (E) Total number of nucleated live splenocytes were determined with a hemocytometer in WT C57BL/6 (black bars), <i>Il21</i>^<i>-/-</i> (open bars) and <i>Il21r</i>^<i>-/-</i> (stripped bars) mice. (F) Numbers of Ter119⁺ and Ter119^– cells in the spleen of WT C57BL/6 (black bars) and <i>Il21r</i>^<i>-/-</i> (striped bars) at day 32 post-infection. Data are representative of two or more independent experiments and are obtained in groups of 5–10 mice per time point. Statistical significance was obtained using Mann Whitney U test or Kruskal-Wallis test. *, P<0.05; **, P<0.01; ***, P<0.001; ****, P<0.0001. Error bars correspond to mean ± SEM.