ILC3 suppress TfH-dependent B cell class switching in an MHCII and IL-4–dependent manner. B cells were sort-purified and cultured in vitro alone (n = 12), with TfH (n = 12) or with TfH in combination with either (CD3⁺CD4⁺/CXCR5⁺PD1⁺)CD25⁺GITR⁺ TfR (n = 11), (CD3⁺CD4⁺/CXCR5⁻PD1⁻)CD25⁺GITR⁺ T reg cells (n = 6), wild-type ILC3 (n = 8), or ILC3 from MHCII^ΔILC3 mice (n = 6). Statistical comparisons performed by one-way ANOVA; data pooled from three independent experiments. (A and B) Representative flow cytometry plots (A) and frequencies (B) of (CD3⁻MHCII⁺/B220⁺CD19⁺)GL7⁺IgG1⁺ class-switched B cells. (C–F) Expression of Cd40l (C), Tgfb, Ifng, and Il17a (D), Il21 (E), and Il4 (F) in sort-purified B cells, T cells, or TfH assessed by qPCR (n = 3 per group), representative of two independent experiments. Statistical comparisons performed by Student’s t test. (G) Concentration of IL-4 protein in supernatants from cocultures containing B cells and TfH cultured with wild-type of MHCII-deficient ILC3, as in A and B. Statistical comparisons performed by Student’s t test. (H and I) Cell numbers of mLN (CD3⁻MHCII⁺/B220⁺ CD19⁺)GL7⁺Fas⁺ GC B cells (H) and (CD3⁻MHCII⁺)B220⁻IgA⁺ plasma cells in the cLPL of anti–IL-4 (n = 5) or isotype (n = 5) control–treated MHCII^ΔILC3 and H2-Ab1^fl/fl (n = 6) mice (I). (J and K) Relative abundance of Helicobacter spp. (J) and H. typhlonius (K) in total colonic mucosal bacteria preparations from anti–IL-4 or isotype control–treated MHCII^ΔILC3 and H2-Ab1^fl/fl mice. (H–K) Statistical comparisons performed by one-way ANOVA; data representative of two independent experiments. All data shown as mean ± SEM; * P ≤ 0.05, **P ≤ 0.01; ***P ≤ 0.001.