FIG 3.

MHC-II-dependent vaccine protection is partially dependent on CD4⁺ T cells. CD4-deficient (CD4 KO), MHC-II KO, and WT C57BL/6 mice were vaccinated s.c. with 10 μg of PIV and challenged i.p. with 1 × 10⁷ genomic copies of C. burnetii NMI 28 dpv. Mice receiving Alhydrogel adjuvant alone served as unvaccinated controls. (A) The relative body weight was determined throughout the infection. Splenomegaly (B) and splenic bacterial burden (C) were assessed 14 dpi to compare protection. (D and E) Spleen sections from WT, CD4 KO, and MHC-II KO mice were evaluated 14 dpi for histiocytic inflammation in red pulp based on the following scale: 0, no accumulations of macrophages; 1, small accumulations of macrophages; 2, small to moderate accumulations of macrophages; and 3, moderate to large accumulations of macrophages. Differences in spleen volume between groups were largely the result of extramedullary hematopoiesis. The results are expressed as the percent splenomegaly, i.e., (spleen weight/body weight) × 100. The bacterial burden was determined by real-time quantitative PCR (qPCR) and is expressed as log₁₀ C. burnetii com1 gene copy numbers. Each experimental group includes five mice, with error bars representing the standard deviations from the mean. *, P < 0.05; **, P < 0.01; ***, P < 0.001; ****, P < 0.0001 (as determined by two-way ANOVA with Dunnett’s multiple-comparison test [A], one-way ANOVA with Tukey’s multiple-comparison test [B and C], or Kruskal-Wallis with Dunn’s multiple-comparison test [E]).