IL-21-producing Tfh cells are activated during acute <i>P</i>. <i>chabaudi</i> infection.

<p>(A) Flow cytometric analysis of representative naïve (top row) and infected mice (8 days post-infection, bottom row). Gates show frequency of CD3⁺CD4⁺CD44^high cells expressing CXCR5 and PD-1. (B) Frequency and (C) total numbers of Tfh cells, defined as CD3⁺CD4⁺CD44^highCXCR5⁺PD-1⁺, in WT C57BL/6, <i>Il21</i>^<i>-/-</i>, <i>Il21r</i>^<i>-/-</i> and <i>Ighm</i> mice. (D) Flow cytometric analysis representative of infected WT C57BL/6 mice (8 days post-infection) corresponding to IL-21 intracellular staining on CD4⁺ T cells (red), overlaid on side scatter light vs CD44 (left) and CXCR5 vs PD-1 (right) from CD3⁺CD4⁺ T cells. Numbers show frequency of IL-21-producing CD4⁺ T cells with high expression of CD44 (left), and their differential expression of CXCR5 and PD-1 (right). (E) Differential combination of expression (+) or absence of expression (–) of CD44, CXCR5 and PD-1 (bottom left) on IL-21-producing CD3⁺CD4⁺ T cells at different days post-infection in the spleen of WT C57BL/6 mice. (F) Flow cytometric analysis of IFN-γ (green line) on CD3⁺CD4⁺CD44^highCXCR5⁺PD-1⁺IL-21⁺ T cells from the spleen of WT C57BL/6 mice, 8 days post-<i>P</i>. <i>chabaudi</i> infection (representative of 4 mice). (G) Serum IL-6 at day 6 post-<i>P</i>. <i>chabaudi</i> infection. Statistical significance was obtained using the Kruskal-Wallis test comparing each time point with its respective basal level (day 0 post-infection) (*, P<0.05; **, P<0.01; ***, P<0.001), or comparing with the data obtained from the WT C57BL/6 group (#, P<0.05; # #, P<0.01). Bars represent median values. Data are representative of at least two independent experiments and were obtained in groups of 4–7 mice per time point.