Soluble TNF produced by B cells is essential for organization of secondary lymphoid organs and for efficient humoral immune responses. (A) A breeding strategy to generate mice expressing only membrane form of TNF specifically in B or T cells (Tm-B-TNF KO and Tm-T-TNF KO mice). (B-E) Mice were immunized i.p. with 10⁸ SRBCs, and frozen section analyzed on day 8. Representative images from 1 of 2 independent experiments (n = 5 mice per group) are shown. (B) Frozen spleen sections were stained with antibodies anti-B220 (red)/anti-CR1 (blue), PNA (red), anti-CD3(blue)/IgD (red). Arrows indicate residual FDCs and GCs in spleen of Tm-B-TNF KO mice. Original magnification is ×200. Right panels show quantification of FDC and GC areas. Data represent means ± SEM. (C) Frozen sections of PP were stained with anti-CR1 (blue)/anti-B220 (red) antibodies. Original magnification is ×100. (D) Frozen sections of MLNs were stained with anti-B220 and anti-CR1 antibody to visualize B-cell follicles and FDCs, respectively. Arrow indicates residual FDCs staining in MLNs of Tm-B-TNF KO mice. Original magnification is ×100. Right panel shows quantification of FDC areas. Data represent means ± SEM. (E) Soluble TNF from B cells is required for generation of IgG response to T-cell–dependent antigen. Mice were immunized i.f.p. with 10⁸ SRBCs and specific IgG response was measured on day 21. One of 2 independent experiments with similar results is shown. Data represent means ± SD; n = 5 mice per group, *P < .05, **P < .01, ***P < .001.