MZB cells and NK1.1⁺-cell help are only partially responsible for the TNP-Ficoll IgM response (A) On day 7 after TNP-Ficoll immunization of naive or splenectomized mice, serum was obtained for flow cytometric analysis of TNP-Ficoll–specific IgM and IgG2a levels. Bars represent the mean (± SE) MFI values of groups of 6 animals each. *P < .05 for comparison between groups. (B) TNP-Ficoll immunization was performed in naive or 2-Gy–irradiated C57BL/6 nude mice. Serum TNP-Ficoll IgM levels were measured on day 7 after immunization. Bars represent means (± SE) MFI of 4 naive nonimmunized, 5 naive immunized, and 6 irradiated immunized mice. *P < .05 for comparison between groups; §P < .05 for comparison with all other groups. (C) At 1 week before TNP-Ficoll immunization, 6 × 10⁶ NK1.1⁺ or NK1.1⁻ cells (n = 4 per group) from C57BL/6 nude naive mice were transferred to BALB/c nude naive recipients. Serum TNP-Ficoll IgM levels were measured on day 7 after immunization and compared with those of immunized and control BALB/c nude mice (n = 3 per group). Bars represent means (± SE) MFI of each group. *P < .05 for comparison between groups; §P < .05 for comparison with all other groups.