STX11 expression is not required for functional B cells but for functional CD4 T cells. (A) Mixed BM chimera: 80% JHT (no B cells) and 20% Stx11^−/− (n = 7) or WT CD45.1 C57BL/6J (n = 5) BM cells were mixed and injected into irradiated JHT recipients and infected with 200 PFU LCMV-WE 10 wk later. d14 p.i. from n = 2 independent experiments. (B) GC B cells (Fas⁺GL7⁺), isotype-switched (IgM⁻IgD⁻) B cells, and plasmablasts (CD138⁺) as percentages of CD19⁺B220⁺ B cells in spleens. (C) Viral titers in spleen. Dotted line indicates the detection limit. (D) LCMV NP-specific IgG levels (left) and IgG avidity in serum (right). (E) BM chimera: 80% Tcra^−/− and 20% Stx11^−/− (n = 10) or Lyst^−/− BM cells (n = 7) or WT CD45.1 C57BL/6J (n = 11) in n = 2 independent experiments were mixed and injected into irradiated Tcra^−/− recipients and infected with 200 PFU LCMV 8 wk later. d14 p.i. (F) Isotype-switched, GC B cells, and plasmablasts as percentages of CD19⁺B220⁺ B cells. (G) Viral titers in spleen. (H) LCMV NP-specific IgG levels (left) and IgG avidity in serum (right). For avidity measurements, antigen-captured serum IgG was treated with PBS or 8 M urea. (A–H) Mean and ± SEMs of Mann–Whitney U test are shown; depending on normal distribution *P < 0.05, *** P < 0.001, **** < 0.0001, ns indicates not significant.