Control of T Lymphocyte Signaling by Ly108, a Signaling Lymphocytic Activation Molecule Family Receptor Implicated in Autoimmunity

Abstract

The signaling lymphocytic activation molecule family of receptors has been implicated in the pathophysiology of autoimmunity in humans and mice. One member of the family, Ly108, was strongly linked to lupus susceptibility in mice. High expression of a Ly108 isoform, Ly108-1, was observed in lymphocytes of lupus-prone mice. Herein, we examined the molecular basis for the influence of Ly108 on lupus susceptibility by studying Ly108 signal transduction in T cells. We observed that Ly108 was able to mediate a tyrosine phosphorylation signal implicating Ly108, Vav-1, and c-Cbl in a manner strictly dependent on engagement of the extracellular domain of Ly108 and co-expression of the Src homology 2 (SH2) domain-containing adaptor signaling lymphocytic activation molecule (SLAM)-associated protein (SAP). Evaluation of T cells from mice carrying mutations in the SAP-FynT pathway indicated that Ly108-triggered protein tyrosine phosphorylation was due to the capacity of SAP to recruit FynT. Importantly, Ly108-1 was more apt at triggering tyrosine phosphorylation signals in T cells when compared with the predominant Ly108 isoform found in non-lupus-prone mice, Ly108-2. This difference was due in part to the presence in Ly108-1 of a unique intra-cytoplasmic tyrosine-based motif that promoted Ly108 signal transduction. Together these data provided a molecular explanation for the involvement of Ly108 in lupus susceptibility in mice.

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

“…FYN is considered the key Src-family kinase required for SLAM-R signaling, based on its SAP-dependent association with several other SLAM-Rs (13)(14)(15)(16)(17)(18)(19)(20). Although LCK was previously implicated in phosphorylation of both SLAM and CD84 (17,24), no direct association between LCK and a SLAM-R had been detected to date.…”

“…Previous studies either relied on overexpression constructs for biochemical analysis of NTB-A signaling (19) or focused on thymocytes, NK cells, or NK cell lines (13)(14)(15)(16)(17)(18)(19)(20). In the latter case, it is possible that cell type-based differences in the expression of FYN, LCK, SAP, or the SLAM family receptor examined could account for differences in the associations detected.…”

“…Two Srcfamily kinases are primarily involved in initiating TCR signaling: LCK and FYN (12). FYN is the major Src-family kinase shown to mediate signal transduction of SLAM-Rs (including NTB-A) in thymocytes and NK cells (13)(14)(15)(16)(17)(18)(19)(20). To examine the role of both FYN and LCK in RICD, we first used highly selective inhibitors for FYN (SU6656) (21) or LCK [LCK inhibitor I (LCKi)] (22).…”

“…The cytoplasmic tail of NTB-A contains three tyrosinebased motifs that regulate signal transduction: Y273 is part of a putative ITIM, whereas Y284 and Y308 are each part of an ITSM (8). Because SAP binding to NTB-A is reported to be dependent on phosphorylation of one or more of these tyrosines (19,20), we next asked which tyrosines in the NTB-A cytoplasmic tail are important for increased SAP-dependent LCK recruitment and phosphorylation during restimulation. To address this question, we stably expressed wild-type (WT) NTB-A or single tyrosine-tophenylalanine (Y→F) NTB-A cytoplasmic tail mutants (Y273F, Y284F, or Y308F) in the NTB-A-negative T cell line PEER (10).…”

Altered expression of signalling lymphocyte activation molecule (SLAM) family receptors CS1 (CD319) and 2B4 (CD244) in patients with systemic lupus erythematosus

“…Analysis of expression of CS1 isoforms indicates differential expression of CS1-L and CS1-S isoform in SLE PBMCs, reminiscent of Ly108 expression in lupus-prone mice [59,60]. The CS1-S isoform does not contain two ITSMs and does not mediate signalling [38].…”