T cells bearing the CD44hi "memory" phenotype display characteristics of activated cells in G1 stage of cell cycle.
T cells capable of anamnestic proliferative responses to antigen in vitro (i.e., "memory" cells) have been shown to display the CD44hi CD45RBlo surface phenotype. To assess the state of activation of these T cells, CD4+ T cells expressing the CD44hi or CD45RBlo phenotype were compared to CD4+ T cells expressing the CD44lo or CD45RBhi phenotype in the context of expression of the "activated" (asialo-GM1hi) vs "resting" (asialo-GM1lo) phenotype and in the context of cell size, total protein content, and total RNA content. Dual fluorescence analysis demonstrated that all CD4+ T cells expressing the CD44hi phenotype also expressed the asialo-GM1hi phenotype associated with cell activation. In vitro proliferative assays confirmed that the CD4+ asialo-GM1hi, the CD4+ CD45RBlo, and the CD4+ CD44hi FACS-sorted populations displayed stronger in vitro responsiveness to stimulation with immobilized anti-CD3 mAB than the CD4+ asialo-GM1lo, CD45RBhi, or CD44lo populations. Acridine orange analysis of sorted CD44hi/lo fractions revealed that the diploid (G1) population of the CD44hi T cells displayed a higher mean RNA content than the CD44lo T cells. Similarly, the CD44hi T cells displayed a higher mean cell size and a higher mean total protein content than the CD44lo CD4+ T cells. Similar results were obtained with asialo-GM1 and CD45RB subsets of CD4+ T cells. The basal rate of protein synthesis, as determined by [3H]leucine incorporation, was approximately 50% higher in the CD44hi small CD4+ T cells than in the CD44lo CD4+ T cells. Based on the knowledge that cell size, total protein and RNA content, and responsiveness to signals inducing proliferation are lowest in G0 stage of cycle and increase through G1 stage of cycle, it appears that the CD44hi CD45RBlo T cells exist in a higher activation state than CD44lo CD45RBhi T cells. The previously demonstrated association of CD44hi CD45RBlo phenotype with memory T cells suggests that the CD44hi memory T cells are maintained in G1 (not necessarily cycling) rather than resting "out of cycle" in G0.