CD11b Expression Identifies CD8+CD28+ T Lymphocytes with Phenotype and Function of Both Naive/Memory and Effector Cells

A previously unreported CD8+CD28+CD11b+ T cell subset occurs in healthy individuals and expands in patients suffering from primary viral infections. In functional terms, these cells share the features of naive/memory CD8+CD28+CD11b− and terminally differentiated effector CD8+CD28−CD11b+ subpopulations. Like CD28− cells, CD28+CD11b+ lymphocytes have the ability to produce IFN-γ, to express perforin granules in vivo, and to exert a potent cytolytic activity. Moreover, these cells can respond to chemotactic stimuli and can efficiently cross the endothelial barrier. In contrast, like their CD11b− counterpart, they still produce IL-2 and retain the ability to proliferate following mitogenic stimuli. The same CD28+CD11b+ subpopulation detected in vivo could be generated by culturing naive CD28+CD11b− cells in the presence of mitogenic stimuli following the acquisition of a CD45RO+ memory phenotype. Considering both phenotypic and functional properties, we argue that this subset may therefore constitute an intermediate phenotype in the process of CD8+ T cell differentiation and that the CD11b marker expression can distinguish between memory- and effector-type T cells in the human CD8+CD28+ T cell subset.