Profiling the CD8low phenotype, an alternative career choice for CD8 T cells during primary differentiation.

A CD8+ T cell of naive phenotype has multiple career choices during its primary differentiation into an effector cell population. One of these career options is becoming a CD8low T cell. We have previously shown by in vitro studies that CD8low T cells have lost expression of CD8 surface protein and mRNA and are poorly cytolytic. In line with poor cytolytic function, CD8low T cells express low levels of perforin and granzyme B and C, mediators of the granule-exocytosis machinery. However, CD8low T cells express IFN-gamma and substantial amounts of IL-4, the signature cytokines of type 1 and type 2 T-cell polarization, respectively. Here, we argue that the CD8low phenotype is an alternative career choice for any naive CD8+ T cell during primary activation but that the probability of choosing this option is greatly enhanced by both IL-4 and strong activation conditions. CD8low T cells have downregulated CD8 alpha/beta heterodimers and no preferential CD8 alpha/alpha homodimer expression. As shown by anti-CD8 Ab blocking experiments, surface CD8 substantially contributes to the CD8 T cell's effector function (i.e. cytokine expression and cytolytic activity). The distinct effector profile of CD8low T cells gives an example of the complexity of different CD8 T cell careers during primary effector differentiation.