The Importance of LAT in the Activation, Homeostasis, and Regulatory Function of T Cells

Abstract

LAT (linker for activation of T cells) is a transmembrane adaptor protein that plays an essential role in TCR-mediated signaling and thymocyte development. Because LAT-deficient mice have an early block in thymocyte development, we utilized an inducible system to delete LAT in primary T cells to study LAT function in T cell activation, homeostasis, and survival. Deletion of LAT caused primary T cells to become unresponsive to stimulation from the TCR and impaired T cell homeostatic proliferation and long term survival. Furthermore, deletion of LAT led to reduced expression of Foxp3, CTLA-4, and CD25 in T reg cells and impaired their function. Consequently, mice with LAT deleted developed a lymphoproliferative syndrome similar to that in LATY136F mice, although less severe. Our data implicate that LAT has positive and negative roles in the regulation of mature T cells.Upon engagement of the TCR (T cell receptor), 3 LAT (linker for activation of T cells) is phosphorylated at multiple tyrosine residues by ZAP-70 kinase (1) and functions as a protein scaffold to assemble a large membrane-tethered signalosome (1-3). LAT binding of Gads leads to the membrane recruitment of SLP-76, which in turn interacts with Itk, as well as Vav1 and PLC-␥1. Through its interaction with LAT and SLP-76, PLC-␥1 is also brought to the plasma membrane. Upon activation by ZAP-70 and Itk, PLC-␥1 hydrolyzes phosphatidylinositol 4,5-bisphosphate into the secondary messengers inositol trisphosphate and diacylglycerol (4, 5). Inositol trisphosphate induces Ca 2ϩ mobilization, whereas diacylglycerol binds to RasGRP1 to activate the Ras-MAPK pathway (6, 7). LAT also contributes to Ras-MAPK activation through recruitment of the Grb2-Sos complex to the plasma membrane to activate Ras (1, 8). Initiation of these signaling cascades eventually leads to activation of transcription factors that regulate the genes critical for T cell proliferation and effector functions.The essential role of LAT in T cell activation was initially demonstrated in LAT-deficient Jurkat T cells. Although the proximal signals upstream of LAT phosphorylation remain intact in these cells, TCR-mediated calcium mobilization, Ras-MAPK activation, and NFAT activation are all abolished (9, 10). LAT function during T cell development has also been well characterized. Thymocyte development in LAT Ϫ/Ϫ mice is completely blocked at the CD25 ϩ CD44 Ϫ DN3 stage, indicating an absolute requirement for LAT in pre-TCR signaling (11). Our recent studies using LAT conditional knock-out mice show that LAT is also required during thymocyte development at the DP to SP transition, as positive selection in LAT-deficient DP thymocytes is markedly compromised in these mice. Consequently, the maturation of SP thymocytes is severely blocked (12). In addition, a point mutation at LAT tyrosine 136 (Y136F), which specifically abolishes the LAT-PLC-␥1 interaction, renders both positive and negative thymic selection processes defective in the LATY136F knock-in mice (13), highlighting the importance of LAT-m...

Targeting Galectin-1 Overcomes Breast Cancer-Associated Immunosuppression and Prevents Metastatic Disease

“…We used a qRT-PCR array for screening genes functionally associated with immune tolerance and anergy (data not shown). Notably, among other discrete changes, we observed a consistent altered expression of Lat, a gene that encodes a tyrosine phosphorylated transmembrane adaptor protein whose expression is associated with the suppressive activity of T reg cells (37,38). Recent findings showed that inhibition of LAT in mice leads to impaired immune tolerance due to diminished suppressive activity of T reg cells (37).…”

A THEMIS : SHP 1 complex promotes T‐cell survival

“…Because TCR ligation induces recruitment of THEMIS, via GRB2, onto LAT (Paster et al , 2013), we infer that THEMIS allows transport of SHP1 and/or SHP2 close to ligand-activated TCRs, thus dampening TCR-ITAM phosphorylation and downstream signalling cascades. LAT deficiency also leads to augmented TCR-ITAM and ZAP-70 phosphorylation (Salek et al , 2013), further supporting the role of LAT as a hub for signal regulation (Acuto et al , 2008; Mingueneau et al , 2009; Shen et al , 2010). However, the in vivo consequences of LAT and THEMIS deficiency are substantially different (Acuto et al , 2008; Mingueneau et al , 2009; Fu et al , 2013; Shen et al , 2010), reflecting perhaps the capacity of the TCR to utilize additional signalling conduits that cannot tightly control the TCR signal as LAT does (Roncagalli et al , 2014).…”

Enhanced Function of Redirected Human T Cells Expressing Linker for Activation of T Cells That Is Resistant to Ubiquitylation

“…Phosphorylated LAT recruits a number of signaling molecules resulting in calcium influx, nuclear factor of activated T cells (NFAT) activation, and ultimately expression of cytokine genes (Malissen et al, 2005). Studies in mice with knockout or modified knock-in LAT molecules have revealed that LAT has a function in both the differentiation of T cells along the Th1 and Th2 pathways, as well as in the control of lymphocyte proliferation (Mingueneau et al, 2009;Roncagalli et al, 2010;Shen et al, 2010). The function of LAT in postthymic mature T cells is relatively unknown, and the function in human T cells is not well studied, as most studies to date have been done in mouse T cells.…”