IgM from BALB/c mice enhances germinal center responses.

<p>On day 0, BALB/c mice were immunized i.v. with WT or Cµ13 IgM specific for KLH (50 µg/mouse) or SRBC (0.2 ml of a solution with HA titer 1:32) 30 min before 10 µg KLH (A), 5×10⁵ (B) or 5×10⁶ SRBC (C) were administered via the same route; controls received antigens or specific IgM alone. Spleens were harvested on day 10. Splenocytes from half of each spleen were analyzed by flow cytometry; germinal center B cells were gated as GL7⁺PNA⁺ amongst B220⁺ cells (Figure S1) and the percentages of germinal center B cells were quantified (A-C, upper left panels). The other halves of the spleens were sectioned, stained with anti-B220 (blue), anti-MOMA (green) and PNA (red), and analyzed for number of PNA⁺ germinal centers in B cell follicles by confocal microscopy (A-C, upper right panels); each image is a representative area (1725 µm × 1295 µm) for 2-3 whole sections with original magnification ×10 (A-C, lower panels). Germinal center responses of mice immunized with specific IgM alone were always lower than the responses of mice immunized with antigens alone (not shown). Data are representative of two experiments with each antigen dose. ns = not significant; * = p < 0.05; ** = p < 0.01.