Deposition of HEL-PE ICs on FDCs and CXCR5-dependent encounter by cognate B cells. (A) LN section showing distribution of HEL-PE and staining for CD35 (green) and B220 (blue), 24 h after immunization. Bars: (left) 200 µm; (right) 50 µm. Data are representative of follicles from six inguinal LNs in three experiments. (B and C) Flow cytometric analysis of B cells 12 h after transfer to mice that had received anti-PE antibody and been immunized with HEL-PE or SA-PE, or mice receiving HEL-PE alone. Plots were pregated for CD19⁺CD45.1⁺ or CD45.2⁺ B cells. (B) HEL-PE capture by MD4 B cells (CD45.1⁻CD45.2⁺) compared with nontransgenic (noncognate) CD45.1⁺CD45.2⁺ control cells. (C) CD86 and CD83 expression on noncognate wild-type (WT) MD4 or Cxcr5^−/− MD4 B cells in passively immunized mice that had received HEL-PE or SA-PE as indicated. (D) Summary of flow cytometric data of the type in B and C, pooled from 18 experiments. Each point indicates an individual LN (at least six mice per group). Bars indicate means. (E and F) Flow cytometric analysis of wild-type MD4 B cells transferred into Cr2^−/− hosts (E) and summary of HEL-PE binding, CD86 and CD83 expression for MD4 B cells transferred to anti-PE–treated wild-type, and Cr2^−/− recipients and wild-type recipients that did not receive ant-PE (F). Bars indicate means. (G and H) Flow cytometry of wild-type MD4 B cells transferred into mice that had received anti-PE antibody and been immunized with DEL-PE (G) and summary of antigen capture and activation marker expression (H). Plots were pregated for CD19⁺CD45.2⁺ B cells. Data are from two (E and F) or three (G and H) experiments. Each point (F and H) indicates an individual LN (at least four mice per group). Bars indicate means. Numbers in quadrants (B, C, E, and G) indicate percentage of gated cells.