Constitutive Lck Activity Drives Sensitivity Differences between CD8+ Memory T Cell Subsets

CD8(+) T cells develop increased sensitivity following Ag experience, and differences in sensitivity exist between T cell memory subsets. How differential TCR signaling between memory subsets contributes to sensitivity differences is unclear. We show in mouse effector memory T cells (T-EM) that >50% of lymphocyte-specific protein tyrosine kinase (Lck) exists in a constitutively active conformation, compared with < 20% in central memory T cells (T-CM). Immediately proximal to Lck signaling, we observed enhanced Zap-70 phosphorylation in T-EM following TCR ligation compared with T-CM. Furthermore, we observed superior cytotoxic effector function in T-EM compared with T-CM, and we provide evidence that this results from a lower probability of T-CM reaching threshold signaling owing to the decreased magnitude of TCR-proximal signaling. We provide evidence that the differences in Lck constitutive activity between CD8(+) T-CM and T-EM are due to differential regulation by SH2 domain-containing phosphatase-1 (Shp-1) and C-terminal Src kinase, and we use modeling of early TCR signaling to reveal the significance of these differences. We show that inhibition of Shp-1 results in increased constitutive Lck activity in T-CM to levels similar to T-EM, as well as increased cytotoxic effector function in T-CM. Collectively, this work demonstrates a role for constitutive Lck activity in controlling Ag sensitivity, and it suggests that differential activities of TCR-proximal signaling components may contribute to establishing the divergent effector properties of T-CM and T-EM. This work also identifies Shp-1 as a potential target to improve the cytotoxic effector functions of T-CM for adoptive cell therapy applications.