FIG. 4.

Early and enhanced IFN-γ production in ICOS^−/− mice in response to reinfection with C. trachomatis. T cells containing IFN-γ were enumerated in frozen cross sections of the uteruses of infected mice. Results are given as average percentages ± standard errors of the means of total CD4⁺ T cells that double stained for IFN-γ⁺ at the indicated time points postreinfection. Cells were counted in at least three fields (1,250 μm² each at ×20 magnification), from left and right uterine horns from three independent experiments with at least three mice per group. (A) Frozen sections were colabeled with anti-IFN-γ (red, Texas Red) and anti-CD4⁺(green, FITC) and double-labeled of IFN-γ⁺ CD4⁺ T cells (yellow) were counted (B). Negative controls include the uteruses from Chlamydia-infected IFN-γ^−/− mice (C) or isotype controls (not shown) labeled and scored identically to tissues from the WT, ICOS^−/−, or CD28^−/− mice. Photographs shown at a magnification of ×40. * (P < 0.05) and ** (P < 0.01) denote statistically significant differences between the WT and either CD28^−/− or ICOS^−/− groups, while # (P < 0.05) and ## (P < 0.01) denote statistically significant differences between CD28^−/− and ICOS^−/− groups.