CTLA4 and infection: In this study, our results showed that, compared to WT mice, influenza A infection resulted in decreased expression of checkpoint molecules GITR, BTLA, TIM-3, PD-1, CTLA-4, and LAIR-1 in lung tissues of KI or KO mice on day 7 after infection, and expression levels of these molecules presented a positive correlation with viral TICD50 in lungs of WT mice but negative correlation with TCID50 in lungs of KI or KO mice although the significant correlation was not observed in all them (Figure 5 and Supplemental Figure 1).