Combined loss of IL-6 and IL-21 compromises GC formation in influenza infection. Analysis of GC B cells in C57BL/6 (WT), IL-6, IL-21, and IL-6/IL-21 double-deficient mice (DKO). Mice were analyzed on day 10 after influenza infection. Results shown are from three to six independent experiments, totaling 4 naive WT and 21 WT, 8 Il6^−/−, 8 Il21^−/−, and 16 DKO-infected mice, respectively. (A) Cells from the draining mLNs and from the spleen were stained for GC B cells with α-B220, α-Fas, and PNA, and the percentage of B220⁺ cells that were also PNA⁺/Fas⁺ is shown. (B and C) Frequency distribution of GC B cells in spleens and mLNs from WT and mutant mice analyzed on day 10 of infection. (D) Ratio of GC area to B cell follicle area in spleens of WT and DKO animals on day 10, as measured from histological sections. Each symbol represents an individual animal. (E and F) Frequency distribution of GC B cells from WT and mutant mice analyzed on day 21 of infection. Each symbol represents an individual animal. Statistical analyses used Tukey’s multiple comparison tests. ***, P < 0.001; **, P = 0.001–0.001; *, P = 0.01–0.05. Bars and numbers show mean percentage with ± SEM. Results are from three to six independent experiments. (G) Representative histological staining to detect GCs in spleens from control or mutant mice 10 d after influenza infection. Paraffin sections were stained with α-GL7, α-B220, and α-CD3. Bars, 50 µm.