Characterization of Ly108 in the thymus: evidence for distinct properties of a novel form of Ly108

Characterization of Ly108-H1 Signaling Reveals Ly108-3 Expression and Additional Strain-Specific Differences in Lupus Prone Mice

Ly108 (SLAMF6) is a homophilic cell surface molecule that binds SLAM-associated protein (SAP), an intracellular adapter protein that modulates humoral immune responses. Furthermore, Ly108 is crucial for the development of natural killer T (NKT) cells and CTL cytotoxicity. Significant attention has been paid towards expression and function of Ly108 since multiple isoforms were identified, i.e., Ly108-1, Ly108-2, Ly108-3, and Ly108-H1, some of which are differentially expressed in several mouse strains. Surprisingly, Ly108-H1 appeared to protect against disease in a congenic mouse model of Lupus. Here, we use cell lines to further define Ly108-H1 function in comparison with other isoforms. We show that Ly108-H1 inhibits IL-2 production while having little effect upon cell death. With a refined method, we could detect phosphorylation of Ly108-H1 and show that SAP binding is retained. We propose that Ly108-H1 may regulate signaling at two levels by retaining the capability to bind its extracellular as well as intracellular ligands, possibly inhibiting downstream pathways. In addition, we detected Ly108-3 in primary cells and show that this isoform is also differentially expressed between mouse strains. The presence of additional binding motifs and a non-synonymous SNP in Ly108-3 further extends the diversity between murine strains. This work highlights the importance of isoform awareness, as inherent homology can present a challenge when interpreting mRNA and protein expression data, especially as alternatively splicing potentially affects function.

SLAM Associated Protein Signaling in T Cells: Tilting the Balance Toward Autoimmunity

T cell activation is the result of the integration of signals across the T cell receptor and adjacent co-receptors. The signaling lymphocyte activation molecules (SLAM) family are transmembrane co-receptors that modulate antigen driven T cell responses. Signal transduction downstream of the SLAM receptor is mediated by the adaptor protein SLAM Associated Protein (SAP), a small intracellular protein with a single SH2 binding domain that can recruit tyrosine kinases as well as shield phosphorylated sites from dephosphorylation. Balanced SLAM-SAP signaling within T cells is required for healthy immunity, with deficiency or overexpression prompting autoimmune diseases. Better understanding of the molecular pathways involved in the intracellular signaling downstream of SLAM could provide treatment targets for these autoimmune diseases.

T-B Lymphocyte Interactions Promote Type 1 Diabetes Independently of SLAM-Associated Protein

Signaling lymphocytic activation molecule-associated protein (SAP), a critical intracellular signaling molecule for T-B lymphocyte interactions, drives T follicular helper (Tfh) cell development in germinal centers (GCs). High-affinity islet autoantibodies predict type 1 diabetes (T1D) but do not cause β cell destruction. This paradox intimates Tfh cells as key pathologic effectors, consistent with an observed Tfh signature in T1D. To understand how fully developed Tfh (GC Tfh) contribute to different autoimmune processes, we investigated the role of SAP in T1D and autoantibody-mediated arthritis. Whereas spontaneous arthritis depended on SAP in the autoantibody-mediated K/BxN model, organized insulitis and diabetes onset were unabated, despite a blocked anti-insulin vaccine response in SAP-deficient NOD mice. GC Tfh and GC B cell development were blocked by loss of SAP in K/BxN mice. In contrast, although GC B cell formation was markedly reduced in SAP-deficient NOD mice, T cells with a GC Tfh phenotype were found at disease sites. CXCR3⁺ CCR6^- (Tfh1) subset bias was observed among GC Tfh cells infiltrating the pancreas of NOD mice, which was enhanced by loss of SAP NOD T cells override SAP requirement to undergo activation and proliferation in response to Ag presentation, demonstrating the potential for productive cognate T-B lymphocyte interactions in T1D-prone mice. We find that SAP is essential when autoantibody-driven immune complexes promote inflammation but is not required for effective organ-specific autoimmune attack. Thus, Tfh induced in classic GC reactions are dispensable for T1D, but the autoimmune process in the NOD model retains pathogenic Tfh without SAP.

SLAMF6 clustering is required to augment T cell activation

The signaling lymphocytic activation molecule (SLAM) family is comprised of nine distinct receptors that are expressed exclusively on hematopoietic cells. Most of these transmembrane receptors are homotypic by nature and downstream signaling occurs when cells that express the same SLAM receptor interact. Previous studies have determined that anti-SLAMF6 antibodies can have a therapeutic effect in autoimmunity and cancer. However, little is known about the role of SLAMF6 in the adaptive immune responses and in order to utilize SLAMF6 interventional approaches, a better understanding of the biology of this receptor in T cell is warranted. Accordingly, the objective of our study was to investigate both functionally and structurally the role of SLAMF6 in T cell receptor (TCR) mediated responses. Biochemical and genetic experiments revealed that SLAMF6 was required for productive TCR downstream signaling. Interestingly, SLAMF6 ectodomain was required for its function, but not for its recruitment to the immunological synapse. Flow-cytometry analysis demonstrated that tyrosine 308 of the tail of SLAMF6 was crucial for its ability to enhance T cell function. Imaging studies revealed that SLAMF6 clustering, specifically with the TCR, resulted in dramatic increase in downstream signaling. Mechanistically, we showed that SLAMF6 enhanced T cell function by increasing T cell adhesiveness through activation of the small GTPase Rap1. Taken together SLAMF6 is an important regulator of T cell activation where both its ectodomain and its endodomain contribute differentially to T cell functions. Additional studies are underway to better evaluate the role of anti-SLAMF6 approaches in specific human diseases.

Multiple tolerance defects contribute to the breach of B cell tolerance in New Zealand Black chromosome 1 congenic mice

Lupus is characterized by a loss of B cell tolerance leading to autoantibody production. In this study, we explored the mechanisms underlying this loss of tolerance using B6 congenic mice with an interval from New Zealand Black chromosome 1 (denoted c1(96–100)) sufficient for anti-nuclear antibody production. Transgenes for soluble hen egg white lysozyme (sHEL) and anti-HEL immunoglobulin were crossed onto this background and various tolerance mechanisms examined. We found that c1(96–100) mice produced increased levels of IgM and IgG anti-HEL antibodies compared to B6 mice and had higher proportions of germinal center B cells and long-lived plasma cells, suggesting a germinal center-dependent breach of B cell anergy. Consistent with impaired anergy induction, c1(96–100) double transgenic B cells showed enhanced survival and CD86 upregulation. Hematopoietic chimeric sHEL mice with a mixture of B6 and c1(96–100) HEL transgenic B cells recapitulated these results, suggesting the presence of a B cell autonomous defect. Surprisingly, however, there was equivalent recruitment of B6 and c1(96–100) B cells into germinal centers and differentiation to splenic plasmablasts in these mice. In contrast, there were increased proportions of c1(96–100) T follicular helper cells and long-lived plasma cells as compared to their B6 counterparts, suggesting that both B and T cell defects are required to breach germinal center tolerance in this model. This possibility was further supported by experiments showing an enhanced breach of anergy in double transgenic mice with a longer chromosome 1 interval with additional T cell defects.

Invariant NKT Cell Activation Is Potentiated by Homotypic trans-Ly108 Interactions

Invariant NKT (iNKT) cells are innate lymphocytes that respond to glycolipids presented by the MHC class Ib molecule CD1d and are rapidly activated to produce large quantities of cytokines and chemokines. iNKT cell development uniquely depends on interactions between double-positive thymocytes that provide key homotypic interactions between signaling lymphocyte activation molecule (SLAM) family members. However, the role of SLAM receptors in the differentiation of iNKT cell effector subsets and activation has not been explored. In this article, we show that C57BL/6 mice containing the New Zealand Black Slam locus have profound alterations in Ly108, CD150, and Ly9 expression that is associated with iNKT cell hyporesponsiveness. This loss of function was only apparent when dendritic cells and iNKT cells had a loss of SLAM receptor expression. Using small interfering RNA knockdowns and peptide-blocking strategies, we demonstrated that trans-Ly108 interactions between dendritic cells and iNKT cells are critical for robust activation. LY108 costimulation similarly increased human iNKT cell activation. Thus, in addition to its established role in iNKT cell ontogeny, Ly108 regulates iNKT cell function in mice and humans.

CRISPR-Mediated Triple Knockout of SLAMF1, SLAMF5 and SLAMF6 Supports Positive Signaling Roles in NKT Cell Development

The SLAM family receptors contribute to diverse aspects of lymphocyte biology and signal via the small adaptor molecule SAP. Mutations affecting SAP lead to X-linked lymphoproliferative syndrome Type 1, a severe immunodysregulation characterized by fulminant mononucleosis, dysgammaglobulinemia, and lymphoproliferation/lymphomas. Patients and mice having mutations affecting SAP also lack germinal centers due to a defect in T:B cell interactions and are devoid of invariant NKT (iNKT) cells. However, which and how SLAM family members contribute to these phenotypes remains uncertain. Three SLAM family members: SLAMF1, SLAMF5 and SLAMF6, are highly expressed on T follicular helper cells and germinal center B cells. SLAMF1 and SLAMF6 are also implicated in iNKT development. Although individual receptor knockout mice have limited iNKT and germinal center phenotypes compared to SAP knockout mice, the generation of multi-receptor knockout mice has been challenging, due to the genomic linkage of the genes encoding SLAM family members. Here, we used Cas9/CRISPR-based mutagenesis to generate mutations simultaneously in Slamf1, Slamf5 and Slamf6. Genetic disruption of all three receptors in triple-knockout mice (TKO) did not grossly affect conventional T or B cell development and led to mild defects in germinal center formation post-immunization. However, the TKO worsened defects in iNKT cells development seen in SLAMF6 single gene-targeted mice, supporting data on positive signaling and potential redundancy between these receptors.

T/B-cell interactions are more transient in response to weak stimuli in SLE-prone mice

Changes in immune function during the course of systemic lupus erythematosus (SLE) are well characterized. Class-switched antinuclear antibodies are the hallmark of SLE, and T/B-cell interactions are thus critical. However, changes in immune function contributing to disease susceptibility are unknown. Here, we have analyzed primary T and B cells from a mouse model of SLE prior to the onset of disease. To allow cognate T-cell activation with low affinity, we have developed a lower potency peptide ligand for the OTII TCR. T- and B-cell couples formed less frequently and retained their polarity less efficiently preferentially in response to low-affinity stimulation in SLE-prone mice. This matched decreased recruitment of actin and Vav1 and an enhanced PKCΘ recruitment to the cellular interface in T cells. The induction of the GC B-cell marker GL7 was increased in T/B cell couples from SLE-prone mice when the T-cell numbers were limited. However, the overall gene expression changes were marginal. Taken together, the enhanced cell-couple transience may allow a more efficient sampling of a large number of T/B cell couples, preferentially in response to limiting stimuli, therefore enhancing the immune reactivity in the development of SLE.

SAP facilitates recruitment and activation of LCK at NTB-A receptors during restimulation-induced cell death

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.

Expansion of an osteopontin-expressing T follicular helper cell subset correlates with autoimmunity in B6.Sle1b mice and is suppressed by the H1-isoform of the Slamf6 receptor

The costimulatory receptor Slamf6 partially controls lupus-related autoimmunity in congenic Sle1b mice; for instance, the presence of the protein isoform Slamf6-H1 in Sle1b.Slamf6-H1 mice mitigates disease. Here, we report that young Sle1b mice, but not Sle1b.Slamf6-H1 or B6 mice, contain a memory T-helper cell subset identified by ]mt]2-fold increase in expression of 17 genes, chief among which is Spp1, encoding the cytokine osteopontin (OPN). These T follicular helper (TFH) cells, including OPN(+) TFH cells, expand concomitantly with severity of the disease. By contrast, Sle1b.Slamf6-H1 or Sle1b.SAP(-)/(-) mice do not develop autoantibodies and the number of T(FH) cells is 5 times lower than in age-matched Sle1b mice. By comparing Sle1b and Sle1b.OPN(-)/(-) mice, we find that the lack of OPN expression impedes early autoantibody production. Furthermore, on the adoptive transfer of Sle1b.OPN(-)/(-) CD4(+) T cells into bm12 recipients autoantibody production and germinal center formation is reduced compared to recipients of Sle1b.OPN(+/+) CD4(+) T cells. We propose a model in which OPN provides a survival signal for a precursor T(FH) cell subset, which is a key factor in autoimmunity.

A role for Ly108 in the induction of promyelocytic zinc finger transcription factor in developing thymocytes

The promyelocytic zinc finger transcription factor (PLZF) is required for the development of activated phenotypes in NKT and other innate T lymphocytes. Although strong TCR stimulation has been implicated in the induction of PLZF, the factors regulating PLZF expression are incompletely understood. We show in this study that costimulation of preselection double-positive thymocytes through the signaling lymphocyte activation molecule family receptor Ly108 markedly enhanced PLZF expression compared with that induced by TCR stimulation alone. Costimulation with Ly108 increased expression of early growth response protein (Egr)-2 and binding of Egr-2 to the promoter of Zbtb16, which encodes PLZF, and resulted in PLZF levels similar to those seen in NKT cells. In contrast, costimulation with anti-CD28 failed to enhance Egr-2 binding and Zbtb16 expression. Moreover, mice lacking Ly108 showed decreased numbers of PLZF-expressing CD4(+) T cells. Together, these results support a potential role for Ly108 in the induction of PLZF.

The receptor Ly108 functions as a SAP adaptor-dependent on-off switch for T cell help to B cells and NKT cell development

Humans and mice deficient in the adaptor protein SAP (Sh2d1a) have a major defect in humoral immunity, resulting from a lack of T cell help for B cells. The role of SAP in this process is incompletely understood. We found that deletion of receptor Ly108 (Slamf6) in CD4(+) T cells reversed the Sh2d1a(-/-) phenotype, eliminating the SAP requirement for germinal centers. This potent negative signaling by Ly108 required immunotyrosine switch motifs (ITSMs) and SHP-1 recruitment, resulting in high amounts of SHP-1 at the T cell:B cell synapse, limiting T cell:B cell adhesion. Ly108-negative signaling was important not only in CD4(+) T cells; we found that NKT cell differentiation was substantially restored in Slamf6(-/-)Sh2d1a(-/-) mice. The ability of SAP to regulate both positive and negative signals in T cells can explain the severity of SAP deficiency and highlights the importance of SAP and SHP-1 competition for Ly108 ITSM binding as a rheostat for the magnitude of T cell help to B cells.