CD11b expression as a marker to distinguish between recently activated effector CD8(+) T cells and memory cells

CD8(+) T cells in different activation states have been difficult to identi fy phenotypically. In this study we have investigated whether Mac-1 (CD11b) expression can be used as a criterion to distinguish between recently acti vated effector cells and memory cells belonging to the CD8(+) T cell subset . Polyclonal virus-specific effector and memory CD8(+) T cells from lymphoc ytic choriomeningitis- and vesicular stomatitis virus-infected mice were vi sualized through staining for intracellular IFN-gamma or binding of MHC-pep tide tetramers, and Mac-1 expression was evaluated, Naive T cells and most virus-specific memory CD8(+) T cells express little or no Mac-1 independent of the virus model employed, In contrast, the majority of CD8(+) T cells p resent during acute infection express a significant level of Mac-1 and, sim ilarly, Mac-1 expression is found on secondary effecters generated in respo nse to viral re-exposure. We therefore suggest that high Mac-1 expression d efines a subset of circulating effector cells and that the presence of this marker on antigen-specific CD8(+) T cells signifies recent activation.