Figure 6.

B cells and virus-specific IgG level are significantly reduced in Irf2^−/− mouse brains. (A) Brain slides from five Irf2^−/− moribund mice and six SVN-infected WT mice with high CNS viral titers from Fig. 2 D were stained with antibodies specific for CD3 (T cell), B220 (B cell), granzyme (cytotoxic T and NK cells), and Mac2 (macrophage). Immune cell infiltrates were semiquantitatively scored using a scale of zero to four as follows: 0, absent; 1, minimal; 2, mild; 3, moderate; and 4, marked. (B) Images that consistently sample eight different regions in the brains (Fig. S2) of WT and Irf2^−/− mice were taken, and the B cell staining present in those images was quantified by ImageJ (measurement of number of positive pixels) or by eye (number of B cells). The p-values were determined by the unpaired, two-tailed Student’s t test (*, P < 0.05). pos, positive. (C) SINV-specific IgG and IgM antibodies in serum and brain homogenate samples of infected mice were measured by ELISA, as detailed in the ELISA section of Materials and methods. Mean OD value represents the level of virus-specific IgG or IgM antibody. Serum samples from three to six mice (except for n = 2 for Irf2^−/− on day 7 p.i.) and brain homogenates from three mice per genotype per time point were tested for antibody titers. Error bars represent SD. Bonferroni posttests: *, P < 0.05. (D) PRNT was performed on serum samples of uninfected (n = 1 per genotype) and day 5 SVN-challenged WT and Irf2^−/− mice (n = 4–5 per genotype). Infection with a constant concentration of virus incubated with serum samples from uninfected mice resulted in ∼200 plaques per well. The point at which the dilution curve crosses the dotted line represents the concentration of serum from SVN-infected mice required to reduce the number of plaques by 50%.