CD28 and infection: Interestingly, naive untreated CD28−/loxCre+/− mice and naive CD57BL/6 mice showed similar numbers of CD4+ T cell subsets except for an increased number of CD4+ T cells and central memory cells in littermate control mice, demonstrating that untreated CD28−/loxCre+/− littermate control mice are phenotypically similar to wild-type mouse until cd28 gene deletion is induced with tamoxifen prior to re-infection.