Memory T cells. Memory CD4 T cells derived from monkey peripheral blood mononuclear cells (PBMC) were studied before and after SIV challenge. Naive CD4 T cells (Naive), effector memory CD4 T cells (EM) and central memory CD4 T cells (CM) were identified as CD3+CD4+CD95–CD28+, CD3+CD4+CD95+CD28– and CD3+CD4+CD95+CD28+, respectively. Vac, vaccinated monkey (n = 5); Con, control monkey (n = 4).