Restimulation-Induced Cell Death of LCK at NTB-A Receptors during SAP Facilitates Recruitment and Activation

Upon TCR restimulation, activated, cycling T cells can undergo a self-regulatory form of apoptosis known as restimulation-induced cell death (RICD). We previously demonstrated that RICD is impaired in T cells from patients with X-linked lymphoproliferative disease, which lack SLAM-associated protein (SAP) expression. Both SAP and the specific SLAM receptor NK, T, and B cell Ag (NTB-A) are required for RICD, but the mechanism by which these molecules promote a strong, proapoptotic signal through the TCR remains unclear. In this article, we show that the Src-family kinase LCK, but not FYN, associates with NTB-A in activated human T cells. This association increased after TCR restimulation in a SAP-dependent manner, requiring both immunoreceptor tyrosine-based switch motifs in the NTB-A cytoplasmic tail. Both NTB-A–associated LCK phosphorylation and kinase activity were enhanced in restimulated T cells, amplifying proximal TCR signaling. In contrast, TCR-induced LCK association with NTB-A, as well as phosphorylation and kinase activity, was reduced in T cells from patients with X-linked lymphoproliferative disease or normal T cells transfected with SAP-specific small interfering RNA, consistent with RICD resistance. Collectively, our data reveal how SAP nucleates a previously unknown signaling complex involving NTB-A and LCK to potentiate RICD of activated human T cells. D uring an immune response, Ag-specific T cells undergo extensive clonal expansion that gives rise to effector and memory cells (1). This proliferative phase must be tightly regulated to prevent excessive T cell accumulation and immunopathological damage to self-tissues (2). One mechanism that maintains T cell homeostasis in this context is restimulation-induced cell death (RICD). RICD is an autoregulatory form of apoptosis that constrains the size of the effector T cell pool at the peak of the immune response (3). Following TCR re-engagement in the presence of IL-2, activated cycling T cells upregulate certain proapoptotic molecules, such as FAS ligand (FASL) and BIM, which participate in apoptosis induction. However, the biochemical mechanism that converts TCR signaling into a proapoptotic signal for activated T cells remains poorly understood. Recently, the clinical significance of this process was underscored in patients with X-linked lymphoproliferative disease (XLP) (4–6). Most cases of XLP are linked to mutations or deletions in SH2D1A, which encodes the signaling lymphocyte activation molecule (SLAM)–associated protein (SAP) (7). XLP is characterized by reduced Ab responses, dysregulated cytokine production , and impaired T cell and NK cell cytotoxicity (7). Despite these deficiencies, XLP patients often present with lymphadenopathy and transient bouts of excess …