Functional and biochemical characterization of murine effector and memory CD4 T cells

Naive CD4 T cells that encounter pathogen are activated to form effector T cells that clear the infection by orchestrating the anamnestic immune response. During this primary response, long-lived memory CD4 T cells are generated which, during re-infection, mediate a faster and more efficient secondary response. We demonstrate here, distinct phenotype, function, activation requirements and intracellular signaling in naive, memory and effector CD4 T cell subsets. We show that effectors exhibit faster kinetics and overall hyper-responsiveness to cognate as well as non-cognate stimuli while memory CD4 T cells are hypo-responsive to noncognate stimuli. We establish that effector and memory subsets can be distinguished by a novel activation profile to antigen versus anti-CD3 stimuli. Since signaling events controlling these functional responses of effector and memory T cells are not known, we demonstrate here that relative to naive CD4 T cells, memory CD4 T cells exhibit a profound and specific decrease in expression of the linker/adapter molecule SLP-76, with concomitant decreases in total cellular tyrosine phosphorylation following TCR/CD3 ligation. Effector CD4 T cells, by contrast, express normal to elevated levels of SLP-76 protein relative to naive cells and exhibit hyper-phosphorylation of LAT, SLP-76 and Erkl/2 kinases. Our results suggest that the enhanced effector cell response derives from amplification of TCR-mediated signaling through the hyper-phosphorylation of linker/adapters and MAP kinases, whereas the memory cell responses may derive from unusually efficient coupling of low levels of SLP-76 directly to downstream mediators, resulting in a shorter biochemical pathway to memory T cell activation and differentiation.