CD4 and viral infectious disease: Although its role in the CD8+ T cell was initially thought to be non-essential [5], studies have recently shown that T-bet is involved in the differentiation of short-lived effector CD8+ T cells [6] at the expense of central memory cells [7], and to account for the defect in the development of central memory CD8+ T cells that occurs in CD4+ T-cell-deficient mice with viral infections [7].