Fig. 4

L. reuteri promotes B cell IgA-responses in a Tfh-PD-1-dependent manner. a The area of IgA⁺ plasma cells (anti-CD138) quantified in ileal and colonic tissues, n = 5 mice per group (duplicate slides per mouse, scale bars equal 100 μm). b Free IgA (μg/ml, ELISA) in the lumen of ileum and colon (n = 10–11 mice per group) and c Serum IgA and IgG concentrations (μg/ml, ELISA, n = 5-6 mice per group). d Flow cytometry of IgA⁺ bacteria in ileum (n = 16-17 mice per group). e, f Ileal microbiome assessed by 16S rRNA gene amplicon sequencing. Microbial community diversity was calculated as α-diversity in mice treated with L. reuteri and/or DSS compared to controls, n = 4–9 mice per group. Data are presented as median values. g Localization of PD-1⁺ T cells in PPs stained with anti-PD-1 (red), anti-CD3 (green), and Hoechst (blue, scale bar equals 100 μm) from three independent experiments. h Flow cytometry of Tfh cells and PD-1 expression (n = 13-14 mice per group). A histogram of PPs and spleen indicating a uniquely high expression of PD-1 in PPs T cells. i Design of PD-1 monoclonal antibodies blocking (blue arrows) and CD4⁺ T cell depletion (black arrows) experiments and effects on the number of B cell subsets (n = 6–13 mice per group). j, k Free IgA production and quantification of IgA⁺ bacteria in the ileum from PD-1 (mAb) blocking experiments (n = 6 mice per group). Data are presented as mean ± SEM. *p < 0.05, **p < 0.01, ***p < 0.001, ****p < 0.0001 using two-tailed Student’s t test or ANOVA with Tukey’s post hoc test