SAP signaling: a dual mechanism of action

Case Report: Meningoencephalitis With Thrombotic Occlusive Vasculopathy in a Young EBV-Naïve Boy Is Associated With a Novel SH2D1A Mutation

X-linked lymphoproliferative disease (XLP1) is a combined immunodeficiency characterized by severe immune dysregulation caused by mutations in the SH2D1A/SAP gene. Loss or dysfunction of SH2D1A is associated with the inability in clearing Epstein-Barr-Virus (EBV) infections. Clinical manifestation is diverse and ranges from life-threatening hemophagocytic lymphohistiocytosis (HLH) and fulminant infectious mononucleosis (FIM) to lymphoma and antibody deficiency. Rare manifestations include aplastic anemia, chronic gastritis and vasculitis. Herein, we describe the case of a previously healthy eight-year old boy diagnosed with XLP1 presenting with acute non-EBV acute meningoencephalitis with thrombotic occlusive vasculopathy. The patient developed multiple cerebral aneurysms leading to repeated intracerebral hemorrhage and severe cerebral damage. Immunological examination was initiated after development of a susceptibility to infections with recurrent bronchitis and one episode of severe pneumonia and showed antibody deficiency with pronounced IgG1-3-4 subclass deficiency. We could identify a novel hemizygous SH2D1A point mutation affecting the start codon. Basal levels of SAP protein seemed to be detectable in CD8⁺ and CD4⁺ T- and CD56⁺ NK-cells of the patient what indicated an incomplete absence of SAP. In conclusion, we could demonstrate a novel SH2D1A mutation leading to deficient SAP protein expression and a rare clinical phenotype of non-EBV associated acute meningoencephalitis with thrombotic occlusive vasculopathy.

Immune Functions of Signaling Lymphocytic Activation Molecule Family Molecules in Multiple Myeloma

Simple Summary

Multiple myeloma (MM) is an incurable hematological malignancy characterized by an increase in abnormal plasma cells. Disease progression, drug resistance, and immunosuppression in MM are associated with immune-related molecules, such as immune checkpoint and co-stimulatory molecules, present in the tumor microenvironment. Novel agents targeting these cell-surface molecules are currently under development, including monoclonal antibodies, bispecific monoclonal antibodies, and chimera antigen receptor T-cell therapies. In this review, we focus on the signaling lymphocytic activation molecule family receptors and provide an overview of their biological functions and novel therapies in MM.

Abstract

The signaling lymphocytic activation molecule (SLAM) family receptors are expressed on various immune cells and malignant plasma cells in multiple myeloma (MM) patients. In immune cells, most SLAM family molecules bind to themselves to transmit co-stimulatory signals through the recruiting adaptor proteins SLAM-associated protein (SAP) or Ewing’s sarcoma-associated transcript 2 (EAT-2), which target immunoreceptor tyrosine-based switch motifs in the cytoplasmic regions of the receptors. Notably, SLAMF2, SLAMF3, SLAMF6, and SLAMF7 are strongly and constitutively expressed on MM cells that do not express the adaptor proteins SAP and EAT-2. This review summarizes recent studies on the expression and biological functions of SLAM family receptors during the malignant progression of MM and the resulting preclinical and clinical research involving four SLAM family receptors. A better understanding of the relationship between SLAM family receptors and MM disease progression may lead to the development of novel immunotherapies for relapse prevention.

The SLAMF3 rs509749 polymorphism correlates with malignant potential in multiple myeloma

The signaling lymphocytic activation molecule family 3 (SLAMF3) is highly expressed on plasma cells from patients with multiple myeloma (MM) and induces high malignant potential by ERK signaling mediated via the interaction with adaptor proteins SHP2 and GRB2. This study focused on the single-nucleotide polymorphism (SNP) of the SLAMF3 gene (rs509749, 1804A>G, M602V) in MM. The SNP G allele was a major type, and the frequencies of the GG, GA, and AA genotypes were 61.8%, 29.4%, and 8.8%, respectively, in patients with MM, which was almost the same as in healthy the control group in the Japanese population. Interestingly, patients with GG genotypes had significantly shorter overall survival times than patients with GA/AA genotypes. Consistent with those results, SLAMF3-overexpressing KMS-34 cells with the G allele (V₆₀₂) had higher cell proliferation potential and were more resistant to anti-MM agents than those with the A allele (M₆₀₂). When those cells were subcutaneously inoculated into NOG mice, tumor sizes in mice receiving V₆₀₂ cells rapidly increased, and survival was significantly shorter than in mice injected with M₆₀₂ cells. Furthermore, SLAMF3 V₆₀₂ molecules bound more tightly to SHP2 and GRB2, with increased SHP2 and ERK phosphorylation compared with M₆₀₂ cells. The mRNA expression of cell cycle-related genes (CCND1 and CCNE1) and anti-apoptotic genes (BCL2L and p21) was increased in V₆₀₂ cells compared with M₆₀₂ cells. The results thus suggested that the G allele of SLAMF3 SNP rs509749 may be associated with MM disease progression.

The Role of Adaptor Proteins in the Biology of Natural Killer T (NKT) Cells

Adaptor proteins contribute to the selection, differentiation and activation of natural killer T (NKT) cells, an innate(-like) lymphocyte population endowed with powerful immunomodulatory properties. Distinct from conventional T lymphocytes NKT cells preferentially home to the liver, undergo a thymic maturation and differentiation process and recognize glycolipid antigens presented by the MHC class I-like molecule CD1d on antigen presenting cells. NKT cells express a semi-invariant T cell receptor (TCR), which combines the Vα14-Jα18 chain with a Vβ2, Vβ7, or Vβ8 chain in mice and the Vα24 chain with the Vβ11 chain in humans. The avidity of interactions between their TCR, the presented glycolipid antigen and CD1d govern the selection and differentiation of NKT cells. Compared to TCR ligation on conventional T cells engagement of the NKT cell TCR delivers substantially stronger signals, which trigger the unique NKT cell developmental program. Furthermore, NKT cells express a panoply of primarily inhibitory NK cell receptors (NKRs) that control their self-reactivity and avoid autoimmune activation. Adaptor proteins influence NKT cell biology through the integration of TCR, NKR and/or SLAM (signaling lymphocyte-activation molecule) receptor signals or the variation of CD1d-restricted antigen presentation. TCR and NKR ligation engage the SH2 domain-containing leukocyte protein of 76kDa slp-76 whereas the SLAM associated protein SAP serves as adaptor for the SLAM receptor family. Indeed, the selection and differentiation of NKT cells selectively requires co-stimulation via SLAM receptors. Furthermore, SAP deficiency causes X-linked lymphoproliferative disease with multiple immune defects including a lack of circulating NKT cells. While a deletion of slp-76 leads to a complete loss of all peripheral T cell populations, mutations in the SH2 domain of slp-76 selectively affect NKT cell biology. Furthermore, adaptor proteins influence the expression and trafficking of CD1d in antigen presenting cells and subsequently selection and activation of NKT cells. Adaptor protein complex 3 (AP-3), for example, is required for the efficient presentation of glycolipid antigens which require internalization and processing. Thus, our review will focus on the complex contribution of adaptor proteins to the delivery of TCR, NKR and SLAM receptor signals in the unique biology of NKT cells and CD1d-restricted antigen presentation.

2B4 Mediates Inhibition of CD8+ T Cell Responses via Attenuation of Glycolysis and Cell Division

We recently showed that 2B4 expression on memory T cells in human renal transplant recipients was associated with reduced rates of rejection. To investigate whether 2B4 functionally underlies graft acceptance during transplantation, we established an experimental model in which 2B4 was retrogenically expressed on donor-reactive murine CD8+ T cells (2B4rg), which were then transferred into naive recipients prior to skin transplantation. We found that constitutive 2B4 expression resulted in significantly reduced accumulation of donor-reactive CD8+ T cells following transplantation and significantly prolonged graft survival following transplantation. This marked reduction in alloreactivity was due to reduced proliferation of CD8+ Thy1.1+ 2B4rg cells as compared with control cells, underpinned by extracellular flux analyses demonstrating that 2B4-deficient (2B4KO) CD8+ cells activated in vitro exhibited increased glycolytic capacity and upregulation of gene expression profiles consistent with enhanced glycolytic machinery as compared with wild type controls. Furthermore, 2B4KO CD8+ T cells primed in vivo exhibited significantly enhanced ex vivo uptake of a fluorescent glucose analogue. Finally, the proliferative advantage associated with 2B4 deficiency was only observed in the setting of glucose sufficiency; in glucose-poor conditions, 2B4KO CD8+ T cells lost their proliferative advantage. Together, these data indicate that 2B4 signals function to alter T cell glucose metabolism, thereby limiting the proliferation and accumulation of CD8+ T cells. Targeting 2B4 may therefore represent a novel therapeutic strategy to attenuate unwanted CD8+ T cell responses.

Dissection of SAP-dependent and SAP-independent SLAM family signaling in NKT cell development and humoral immunity

Chen et al. dissect SAP-dependent and SAP-independent SLAM family signaling in the regulation of NKT cell development and follicular T helper cell differentiation using a novel mouse model lacking all seven SLAM family receptors.

Signaling lymphocytic activation molecule (SLAM)–associated protein (SAP) mutations in X-linked lymphoproliferative disease (XLP) lead to defective NKT cell development and impaired humoral immunity. Because of the redundancy of SLAM family receptors (SFRs) and the complexity of SAP actions, how SFRs and SAP mediate these processes remains elusive. Here, we examined NKT cell development and humoral immunity in mice completely deficient in SFR. We found that SFR deficiency severely impaired NKT cell development. In contrast to SAP deficiency, SFR deficiency caused no apparent defect in follicular helper T (T_FH) cell differentiation. Intriguingly, the deletion of SFRs completely rescued the severe defect in T_FH cell generation caused by SAP deficiency, whereas SFR deletion had a minimal effect on the defective NKT cell development in SAP-deficient mice. These findings suggest that SAP-dependent activating SFR signaling is essential for NKT cell selection; however, SFR signaling is inhibitory in SAP-deficient T_FH cells. Thus, our current study revises our understanding of the mechanisms underlying T cell defects in patients with XLP.

Primary immunodeficiencies and the control of Epstein-Barr virus infection

Human primary immunodeficiency (PID) states, where mutations in single immune system genes predispose individuals to certain infectious agents and not others, are experiments of nature that hold important lessons for the immunologist. The number of genetically defined PIDs is rising rapidly, as is the opportunity to learn from them. Epstein-Barr virus (EBV), a human herpesvirus, has long been of interest because of its complex interaction with the immune system. Thus, it causes both infectious mononucleosis (IM), an immunopathologic disease associated with exaggerated host responses, and at least one malignancy, EBV-positive lymphoproliferative disease, when those responses are impaired. Here, we describe the full range of PIDs currently linked with an increased risk of EBV-associated disease. These provide examples where IM-like immunopathology is fatally exaggerated, and others where responses impaired at the stage of induction, expansion, or effector function predispose to malignancy. Current evidence from this rapidly moving field supports the view that lesions in both natural killer cell and T cell function can lead to EBV pathology.

Simultaneous TCR and CD244 signals induce dynamic downmodulation of CD244 on human antiviral T cells

Various cosignaling molecules on T cells can contribute to activation, inhibition, or exhaustion, depending on context. The surface receptor signaling lymphocytic activation molecule (SLAM) family receptor CD244 (2B4/SLAMf4) has been shown to be capable of either inhibitory or enhancing effects upon engagement of its ligand CD48 (SLAMf2). We examined phenotypes of CD8 T cells from HIV(+) and HIV(neg) human donors, specific for HIV and/or respiratory syncytial virus. Cultured and ex vivo CD8 T cells expressed PD-1, CD244, and TIM-3. We found that ex vivo CD8 T cells downregulated CD244 in response to superantigen. Furthermore, cognate peptide induced rapid downregulation of both CD244 and TIM-3, but not PD-1, on CD8 T cell clones. CD244 downmodulation required simultaneous signaling via both TCR and CD244 itself. Using a pH-sensitive fluorophore conjugated to avidin-Ab tetramers, we found that CD244 crosslinking in the presence of TCR signaling resulted in rapid transport of CD244 to an acidic intracellular compartment. Downregulation was not induced by PMA-ionomycin, or prevented by PI3K inhibition, implicating a TCR-proximal signaling mechanism. CD244 internalization occurred within hours of TCR stimulation and required less peptide than was required to induce IFN-γ production. The degree of CD244 internalization varied among cultured CD8 T cell lines of different specificities, and correlated with the enhancement of IFN-γ production in response to CD48 blockade in HIV(+), but not HIV(neg), subjects. Our results indicate that rapid CD244 internalization is induced by a two-signal mechanism and plays a role in modulation of antiviral CD8 T cell responses by CD48-CD244 signaling.

Non-catalytic tyrosine-phosphorylated receptors

Leukocytes play a critical role in recognizing and responding to infection and cancer. Central to this function is an array of cell-surface receptors that lack sequence homology. Many of these receptors have in common the fact that their signaling involves phosphorylation of cytoplasmic domains by extrinsic tyrosine kinases. These non-catalytic tyrosine-phosphorylated receptors (NTRs) share a number of other features, including small size and optimal stimulation by surface-associated ligands. We argue here that NTRs are also likely to share the same kinetic-segregation triggering mechanism, which involves segregation of the engaged NTR from receptor tyrosine phosphatases with large ectodomains such as CD45 and CD148. NTRs signal through tyrosine-containing cytoplasmic motifs, which recruit distinct cytoplasmic signaling proteins when phosphorylated, transducing activatory or inhibitory signals. They have two features that make them uniquely well suited to their role in immune recognition of infection and cancer. Their modular structure enables the coupling of many rapidly evolving receptors with diverse ligand specificities to the same conserved signaling machinery. Their similarity in size and shared signaling machinery enables them to colocalize at cell-cell interfaces when they engage ligands, facilitating the integration of activatory and inhibitory signals from multiple receptors at the cell surface.