CD28 and lymphopenia: This study examined several parameters as indicators of premature T cell aging in NOD mice compared to age-matched control Balb/c mice, including the number of total, naïve, and memory CD4+ T cells in the spleen; the percentage of naïve and memory cells in CD4+ T cells; the ratio of naïve to memory CD4+ T cells; IL-2 production and the percentage of CD28+ cells in CD4+ T cells after Con A stimulation; the percentage of FAS+CD44+ cells in CD4+ T cells; and lymphopenia-induced proliferation [1].