CD4 and infection: Given that in vivo blocking of IFNγ reduces the expression of MHCII genes in intestinal crypt cells and at the same time lowers the number of cytotoxic CD4 T cells in lamina propria of aged mice, it is conceivable that, as previously shown in infections and disease models, expression of MHCII genes by epithelial cells during aging is important for expansion of this T cell population in intestinal lamina propria13,27.