Statistical analysis

Statistical analysis comparing two or three different groups at a single time point was performed in Prism software (v. 7.0a) using a Student’s t test or one-way ANOVA, respectively. When data from multiple experiments were combined, a randomized block ANOVA was used in R v.3.4.4 statistical software to account for natural variability between experimental dates by modeling the genotype (WT vs. ICOS KO vs. ICOS YF) as a fixed effect and the experimental date as a random effect. The particular test used for each individual panel shown, as well as the group size, is specified in each figure legend. P values are displayed as follows: ns = not significant, * p<0.05, ** p<0.01, *** p<0.001. All data were graphed using Prism software and are presented as the mean ± standard deviation unless otherwise noted in the figure legend.