Differential T cell receptor‐mediated signaling in naive and memory CD4 T cells

Naive and memory CD4 T cells differ in cell surface phenotype, function, activation requirements, and modes of regulation. To investigate the molecular bases for the dichotomies between naive and memory CD4 T cells and to understand how the T cell receptor (TCR) directs diverse functional outcomes, we investigated proximal signaling events triggered through the TCR/CD3 complex in naive and memory CD4 T cell subsets isolated on the basis of CD45 isoform expression. Naive CD4 T cells signal through TCR/CD3 similar to unseparated CD4 T cells, producing multiple tyrosine‐phosphorylated protein species overall and phosphorylating the T cell‐specific ZAP‐70 tyrosine kinase which is recruited to the CD3ζ subunit of the TCR. Memory CD4 T cells, however, exhibit a unique pattern of signaling through TCR/CD3. Following stimulation through TCR/CD3, memory CD4 T cells produce fewer species of tyrosine‐phosphorylated substrates and fail to phosphorylate ZAP‐70, yet unphosphorylated ZAP‐70 can associate with the TCR/CD3 complex. Moreover, a 26/28‐kDa phosphorylated doublet is associated with CD3ζ in resting and activated memory but not in naive CD4 T cells. Despite these differences in the phosphorylation of ZAP‐70 and CD3‐associated proteins, the ZAP‐70‐related kinase, p72syk, exhibits similar phosphorylation in naive and memory T cell subsets, suggesting that this kinase could function in place of ZAP‐70 in memory CD4T cells. These results indicate that proximal signals are differentially coupled to the TCR in naive versus memory CD4 T cells, potentially leading to distinct downstream signaling events and ultimately to the diverse functions elicited by these two CD4 T cell subsets.