We compared the phosphorylation of both Lck Con394 and Lck Y505 in resting JR209 TCM and TEM by Western blot analysis, which showed that relative to TCM, TEM had greater levels of pY394 and lower levels of pY505 (Fig. results in increased constitutive Lck activity in TCM to levels similar to TEM, as well as increased cytotoxic effector function in TCM. Together, this work demonstrates a role for constitutive Lck activity in controlling antigen sensitivity, and suggests that differential activities of TCR-proximal signaling components may contribute to establishing the divergent effector properties of TCM and TEM. This work also identifies Shp-1 as a potential target to improve the cytotoxic effector functions of TCM for adoptive cell therapy applications. Introduction T cell effector functions are initiated by ligation of the TCR Mupirocin with a MHC presenting antigen peptide (pMHC) on the surface of an APC . T cell sensitivity is substantially increased following antigen experience and maturation and can vary between antigen-experienced memory subsets , which has been attributed in part to enhanced TCR-proximal signaling . TCM and TEM have unique gene expression and cytokine signaling signatures , which result in distinct effector capacities . As a result, TCM have an enhanced ability to confer host protection against viral and bacterial challenge  as well as enhanced therapeutic antitumor CDC25B responses compared with TEM . However, TEM possess greater cytotoxic properties , which suggests that this superior properties of TCM result from greater proliferation upon antigen re-encounter and preferential homing to secondary lymphoid tissues [7, 9] despite a deficiency in cytotoxic properties compared with TEM . The contributions of TCR signaling components that confer differences in activation sensitivity and functional outcomes between CD8+ TCM and TEM remain unclear. Initiation of T cell signaling by TCR ligation leads to a sequence of well-characterized signaling events, including Lck phosphorylation of CD3 ITAMs  and Zap-70 . Active Lck is present in T cells prior to TCR stimulation , and exists in equilibrium between four says, based on phosphorylation of activating Y394 and inhibitory Y505 . It Mupirocin is unclear if the level of constitutively active Lck differs significantly between T cell subsets, and whether any such differences in Lck activity would contribute to establishing differential antigen sensitivities. This premise is usually supported by recent work by Manz and colleagues , which showed that increasing Lck activity through inhibition of Csk leads to enhanced downstream signaling following T cell stimulation. Here, we show that TCM and TEM possess differential constitutive Lck activities, driven in part by differential regulation by Shp-1 and Csk. In response to the moderate affinity (9.3M) self-antigen gp100209-217,2M, differences in proximal T cell signaling resulted in significantly different probabilities of TCM and TEM achieving full cytotoxic effector function. Comparatively higher constitutive Lck activity can explain the more robust proximal antigen-dependent signaling and cytotoxic effector function of TEM. Given the importance of both TCR dwell time and Lck in driving TCR signaling , our results suggest that T cells sensitivity may be influenced by constitutive Lck activity, which varies sufficiently between TCM and TEM to establish differential antigen sensitivities. Materials and Methods Reagents and materials All cells were cultured in RPMI 1640 l-glutamine-supplemented media (Life Technologies) with the inclusion of 10% FBS (ThermoScientific), sodium pyruvate, MEM non-essential amino acids, and penicillin/streptomycin (Life Technologies). Anti-Lck (SPM413 and 2102), anti-pY394-Lck (Tyr 394), anti-pTyr505-Lck (pY505.4), anti-GST (K-18), anti-Zap-70 (1E7.2), anti-Shp-1 (C19), anti-Csk (C-20), anti-Cbp (PAG-C1), anti-rabbit IgG F(ab)2-APC, anti-mouse IgG F(ab)2-FITC, and Mupirocin normal rabbit IgG isotype control were from Santa Cruz Biotechnology. FITC-conjugated anti-CD8a (53-6.7), PerCp-Cy5.5-conjugated anti-CD44 (IM7) and PE-conjugated CD62L (MEL-14) were from eBiosciences. Anti-pShp-1 (S591) and annti-pShp-1 (Y536) were from ECM Biosciences. Anti-pZap-70 (Y319) and anti-pShp-1 (Y564) were from Cell Signaling Technology. Anti-pY142 (K25-407.69), and anti-Themis (Q1-1103) were from BD Pharmingen. Anti-rabbit IRDye 680LT, anti-goat IRDye 680 LT and anti-mouse 800CW IRDye were from LI-COR Biosciences. APC-anti-mouse IgG (poly4053) was from Biolegend. Anti-phosphotyrosine (4G10) was from Millipore. Anti-actin was from Acris Antibodies. Anti-Cbp (EPR9705) was from Abcam. rCD3- with N-terminal GST tag was from Novus Biologicals. Prolong Gold antifade with DAPI, Fura 2-AM, Lystotracker Red, and Calcein Green AM was from Life Technologies. gp100209-217(2M) peptide (IMDQVPFSV) was from Biosynthesis Inc. Animals and cell culture Mupirocin JR209 TCM and TEM cells were obtained, as previously described . Major lymph nodes.