CD84 expression on human hematopoietic progenitor cells

Terminal deoxynucleotidyl transferase and CD84 identify human multi-potent lymphoid progenitors

Lymphoid specification in human hematopoietic progenitors is not fully understood. To better associate lymphoid identity with protein-level cell features, we conduct a highly multiplexed single-cell proteomic screen on human bone marrow progenitors. This screen identifies terminal deoxynucleotidyl transferase (TdT), a specialized DNA polymerase intrinsic to VDJ recombination, broadly expressed within CD34+ progenitors prior to B/T cell emergence. While these TdT+ cells coincide with granulocyte-monocyte progenitor (GMP) immunophenotype, their accessible chromatin regions show enrichment for lymphoid-associated transcription factor (TF) motifs. TdT expression on GMPs is inversely related to the SLAM family member CD84. Prospective isolation of CD84lo GMPs demonstrates robust lymphoid potentials ex vivo, while still retaining significant myeloid differentiation capacity, akin to LMPPs. This multi-omic study identifies human bone marrow lymphoid-primed progenitors, further defining the lympho-myeloid axis in human hematopoiesis.

Role of signaling lymphocytic activation molecule family of receptors in the pathogenesis of rheumatoid arthritis: insights and application

Rheumatoid arthritis (RA) is an autoimmune disease characterized by chronic inflammation and joint damage. The signaling lymphocytic activation molecule (SLAMF) family of receptors are expressed on various hematopoietic and non-hematopoietic cells and can regulate both immune cell activation and cytokine production. Altered expression of certain SLAMF receptors contributes to aberrant immune responses in RA. In RA, SLAMF1 is upregulated on T cells and may promote inflammation by participating in immune cell-mediated responses. SLAMF2 and SLAMF4 are involved in regulating monocyte tumor necrosis factor production and promoting inflammation. SLAMF7 activates multiple inflammatory pathways in macrophages to drive inflammatory gene expression. SLAMF8 inhibition can reduce inflammation in RA by blocking ERK/MMPs signaling. Of note, there are differences in SLAMF receptor (SFR) expression between normal and arthritic joint tissues, suggesting a role as potential diagnostic biomarkers. This review summarizes recent advances on the roles of SLAMF receptors 1, 2, 4, 7, and 8 in RA pathogenesis. However, further research is needed to elucidate the mechanisms of SLAMF regulation of immune cells in RA. Understanding interactions between SLAMF receptors and immune cells will help identify selective strategies for targeting SLAMF signaling without compromising normal immunity. Overall, the SLAMF gene family holds promise as a target for precision medicine in RA, but additional investigation of the underlying immunological mechanisms is needed. Targeting SLAMF receptors presents opportunities for new diagnostic and therapeutic approaches to dampen damaging immune-mediated inflammation in RA.

Unravelling human hematopoietic progenitor cell diversity through association with intrinsic regulatory factors

Hematopoietic stem and progenitor cell (HSPC) transplantation is an essential therapy for hematological conditions, but finer definitions of human HSPC subsets with associated function could enable better tuning of grafts and more routine, lower-risk application. To deeply phenotype HSPCs, following a screen of 328 antigens, we quantified 41 surface proteins and functional regulators on millions of CD34+ and CD34- cells, spanning four primary human hematopoietic tissues: bone marrow, mobilized peripheral blood, cord blood, and fetal liver. We propose more granular definitions of HSPC subsets and provide new, detailed differentiation trajectories of erythroid and myeloid lineages. These aspects of our revised human hematopoietic model were validated with corresponding epigenetic analysis and in vitro clonal differentiation assays. Overall, we demonstrate the utility of using molecular regulators as surrogates for cellular identity and functional potential, providing a framework for description, prospective isolation, and cross-tissue comparison of HSPCs in humans.

SLAM-family receptors come of age as a potential molecular target in cancer immunotherapy

The signaling lymphocytic activation molecule (SLAM) family receptors were discovered in immune cells for the first time. The SLAM-family receptors are a significant player in cytotoxicity, humoral immune responses, autoimmune diseases, lymphocyte development, cell survival, and cell adhesion. There is growing evidence that SLAM-family receptors have been involved in cancer progression and heralded as a novel immune checkpoint on T cells. Previous studies have reported the role of SLAMs in tumor immunity in various cancers, including chronic lymphocytic leukemia, lymphoma, multiple myeloma, acute myeloid leukemia, hepatocellular carcinoma, head and neck squamous cell carcinoma, pancreas, lung, and melanoma. Evidence has deciphered that the SLAM-family receptors may be targeted for cancer immunotherapy. However, our understanding in this regard is not complete. This review will discuss the role of SLAM-family receptors in cancer immunotherapy. It will also provide an update on recent advances in SLAM-based targeted immunotherapies.

Molecular Mechanisms in Murine Syngeneic Leukemia Stem Cells

Acute Myeloid Leukemia (AML) is a severe disease with a very high relapse rate. AML relapse may be attributable to leukemic stem cells (LSC). Notably, the "cancer stem cell" theory, which relates to LSCs, is controversial and criticized due to the technical peculiarities of the xenotransplant of human cells into mice. In this study, we searched for possible LSCs in an immunocompetent synergetic mice model. First, we found phenotypic heterogeneity in the ML23 leukemia line. We prospectively isolated a sub-population using the surface markers cKit⁺CD9^-CD48⁺Mac1^-/low, which have the potency to relapse the disease. Importantly, this sub-population can pass in syngeneic hosts and retrieve the heterogeneity of the parental ML23 leukemia line. The LSC sub-population resides in various organs. We present a unique gene expression signature of the LSC in the ML23 model compared to the other sub-populations. Interestingly, the ML23 LSC sub-population expresses therapeutic targeted genes such as CD47 and CD93. Taken together, we present the identification and molecular characterization of LSCs in a syngeneic murine model.

A Bioinformatics View on Acute Myeloid Leukemia Surface Molecules by Combined Bayesian and ABC Analysis

“Big omics data” provoke the challenge of extracting meaningful information with clinical benefit. Here, we propose a two-step approach, an initial unsupervised inspection of the structure of the high dimensional data followed by supervised analysis of gene expression levels, to reconstruct the surface patterns on different subtypes of acute myeloid leukemia (AML). First, Bayesian methodology was used, focusing on surface molecules encoded by cluster of differentiation (CD) genes to assess whether AML is a homogeneous group or segregates into clusters. Gene expressions of 390 patient samples measured using microarray technology and 150 samples measured via RNA-Seq were compared. Beyond acute promyelocytic leukemia (APL), a well-known AML subentity, the remaining AML samples were separated into two distinct subgroups. Next, we investigated which CD molecules would best distinguish each AML subgroup against APL, and validated discriminative molecules of both datasets by searching the scientific literature. Surprisingly, a comparison of both omics analyses revealed that CD339 was the only overlapping gene differentially regulated in APL and other AML subtypes. In summary, our two-step approach for gene expression analysis revealed two previously unknown subgroup distinctions in AML based on surface molecule expression, which may guide the differentiation of subentities in a given clinical–diagnostic context.

Markers for human haematopoietic stem cells: The disconnect between an identification marker and its function

The haematopoietic system is a classical stem cell hierarchy that maintains all the blood cells in the body. Haematopoietic stem cells (HSCs) are rare, highly potent cells that reside at the apex of this hierarchy and are historically some of the most well studied stem cells in humans and laboratory models, with haematopoiesis being the original system to define functional cell types by cell surface markers. Whilst it is possible to isolate HSCs to near purity, we know very little about the functional activity of markers to purify HSCs. This review will focus on the historical efforts to purify HSCs in humans based on cell surface markers, their putative functions and recent advances in finding functional markers on HSCs.

Integrating transcription-factor abundance with chromatin accessibility in human erythroid lineage commitment

Master transcription factors (TFs) directly regulate present and future cell states by binding DNA regulatory elements and driving gene-expression programs. Their abundance influences epigenetic priming to different cell fates at the chromatin level, especially in the context of differentiation. In order to link TF protein abundance to changes in TF motif accessibility and open chromatin, we developed InTAC-seq, a method for simultaneous quantification of genome-wide chromatin accessibility and intracellular protein abundance in fixed cells. Our method produces high-quality data and is a cost-effective alternative to single-cell techniques. We showcase our method by purifying bone marrow (BM) progenitor cells based on GATA-1 protein levels and establish high GATA-1-expressing BM cells as both epigenetically and functionally similar to erythroid-committed progenitors.

Blood protein profiles related to preterm birth and retinopathy of prematurity

BACKGROUND

Nearly one in ten children is born preterm. The degree of immaturity is a determinant of the infant's health. Extremely preterm infants have higher morbidity and mortality than term infants. One disease affecting extremely preterm infants is retinopathy of prematurity (ROP), a multifactorial neurovascular disease that can lead to retinal detachment and blindness. The advances in omics technology have opened up possibilities to study protein expressions thoroughly with clinical accuracy, here used to increase the understanding of protein expression in relation to immaturity and ROP.

METHODS

Longitudinal serum protein profiles the first months after birth in 14 extremely preterm infants were integrated with perinatal and ROP data. In total, 448 unique protein targets were analyzed using Proximity Extension Assays.

RESULTS

We found 20 serum proteins associated with gestational age and/or ROP functioning within mainly angiogenesis, hematopoiesis, bone regulation, immune function, and lipid metabolism. Infants with severe ROP had persistent lower levels of several identified proteins during the first postnatal months.

CONCLUSIONS

The study contributes to the understanding of the relationship between longitudinal serum protein levels and immaturity and abnormal retinal neurovascular development. This is essential for understanding pathophysiological mechanisms and to optimize diagnosis, treatment and prevention for ROP.

IMPACT

Longitudinal protein profiles of 14 extremely preterm infants were analyzed using a novel multiplex protein analysis platform combined with perinatal data. Proteins associated with gestational age at birth and the neurovascular disease ROP were identified. Among infants with ROP, longitudinal levels of the identified proteins remained largely unchanged during the first postnatal months. The main functions of the proteins identified were angiogenesis, hematopoiesis, immune function, bone regulation, lipid metabolism, and central nervous system development. The study contributes to the understanding of longitudinal serum protein patterns related to gestational age and their association with abnormal retinal neuro-vascular development.

High MHC-II expression in Epstein-Barr virus-associated gastric cancers suggests that tumor cells serve an important role in antigen presentation

EBV-associated gastric adenocarcinomas (EBVaGCs) often exhibit better clinical outcomes than EBV negative gastric cancers (GCs), which could be related to their consistent expression of foreign viral antigens. Antigen-presenting cells (APCs) present peptide antigens in the context of the class-II major histocompatibility complex (MHC-II). During inflammatory conditions, epithelial cells express MHC-II and function as accessory APCs. Utilizing RNA-seq data from nearly 400 GC patients, we determined the impact of EBV-status on expression of MHC-II components, genes involved in their regulation, and T-cell co-stimulation. Virtually all MHC-II genes were significantly upregulated in EBVaGCs compared to normal tissues, or other GC subtypes. Genes involved in antigen presentation were also significantly upregulated in EBVaGCs, as were the key MHC-II transcriptional regulators CIITA and RFX5. This was unexpected as the EBV encoded BZLF1 protein can repress CIITA transcription and is expressed in many EBVaGCs. Furthermore, MHC-II upregulation was strongly correlated with elevated intratumoral levels of interferon-gamma. In addition, expression of co-stimulatory molecules involved in T-cell activation and survival was also significantly increased in EBVaGCs. Thus, gastric adenocarcinoma cells may functionally contribute to the highly immunogenic tumor microenvironment observed in EBVaGCs via a previously unappreciated role in interferon-induced antigen presentation.

High Level Expression of MHC-II in HPV+ Head and Neck Cancers Suggests that Tumor Epithelial Cells Serve an Important Role as Accessory Antigen Presenting Cells

High-risk human papillomaviruses (HPVs) are responsible for a subset of head and neck squamous cell carcinomas (HNSCC). Expression of class II major histocompatibility complex (MHC-II) is associated with antigen presenting cells (APCs). During inflammation, epithelial cells can be induced to express MHC-II and function as accessory APCs. Utilizing RNA-seq data from over 500 HNSCC patients from The Cancer Genome Atlas, we determined the impact of HPV-status on the expression of MHC-II genes and related genes involved in their regulation, antigen presentation, and T-cell co-stimulation. Expression of virtually all MHC-II genes was significantly upregulated in HPV+ carcinomas compared to HPV- or normal control tissue. Similarly, genes that encode products involved in antigen presentation were also significantly upregulated in the HPV+ cohort. In addition, the expression of CIITA and RFX5-regulators of MHC-II-were significantly upregulated in HPV+ tumors. This coordinated upregulation of MHC-II genes was correlated with higher intratumoral levels of interferon-gamma in HPV+ carcinomas. Furthermore, genes that encode various co-stimulatory molecules involved in T-cell activation and survival were also significantly upregulated in HPV+ tumors. Collectively, these results suggest a previously unappreciated role for epithelial cells in antigen presentation that functionally contributes to the highly immunogenic tumor microenvironment observed in HPV+ HNSCC.

Spatial Analysis of Single Fiber Cells of the Developing Ocular Lens Reveals Regulated Heterogeneity of Gene Expression

The developing eye lens presents an exceptional paradigm for spatial transcriptomics. It is composed of highly organized long, slender transparent fiber cells, which differentiate from the edges of the anterior epithelium of the lens (equator), attended by high expression of crystallins, which generates transparency. Every fiber cell, therefore, is an optical unit whose refractive properties derive from its gene activity. Here, we probe this tangible relationship between the gene activity and the phenotype by studying the expression of all known 17 crystallins and 77 other non-crystallin genes in single fiber cells isolated from three states/regions of differentiation, allowing us to follow molecular progression at the single-cell level. The data demonstrate highly variable gene activity in cortical fibers, interposed between the nascent and the terminally differentiated fiber cell transcription. These data suggest that the so-called stochastic, highly heterogeneous gene activity is a regulated intermediate in the realization of a functional phenotype.

CD84 cell surface signaling molecule: An emerging biomarker and target for cancer and autoimmune disorders

CD84 (SLAMF5) is a member of the SLAM family of cell-surface immunoreceptors. Broadly expressed on most immune cell subsets, CD84 functions as a homophilic adhesion molecule, whose signaling can activate or inhibit leukocyte function depending on the cell type and its stage of activation or differentiation. CD84-mediated signaling regulates diverse immunological processes, including T cell cytokine secretion, natural killer cell cytotoxicity, monocyte activation, autophagy, cognate T:B interactions, and B cell tolerance at the germinal center checkpoint. Recently, alterations in CD84 have been related to autoimmune and lymphoproliferative disorders. Specific allelic variations in CD84 are associated with autoimmune diseases such as systemic lupus erythematosus and rheumatoid arthritis. In chronic lymphocytic leukemia, CD84 mediates intrinsic and stroma-induced survival of malignant cells. In this review, we describe our current understanding of the structure and function of CD84 and its potential role as a therapeutic target and biomarker in inflammatory autoimmune disorders and cancer.

Elucidating the Role of CD84 and AHR in Modulation of LPS-Induced Cytokines Production by Cruciferous Vegetable-Derived Compounds Indole-3-Carbinol and 3,3'-Diindolylmethane

Modulation of the immune system by cancer protective food bioactives has preventive and therapeutic importance in prostate cancer, but the mechanisms remain largely unclear. The current study tests the hypothesis that the diet-derived cancer protective compounds, indole-3-carbinol (I3C) and 3,3′-diindolylmethane (DIM), affect the tumor microenvironment by regulation of inflammatory responses in monocytes and macrophages. We also ask whether I3C and DIM act through the aryl hydrocarbon (AHR)-dependent pathway or the signaling lymphocyte activation molecule (SLAM) family protein CD84-mediated pathway. The effect of I3C and DIM was examined using the human THP-1 monocytic cell in its un-differentiated (monocyte) and differentiated (macrophage) state. We observed that I3C and DIM inhibited lipopolysaccharide (LPS) induction of IL-1β mRNA and protein in the monocyte form but not the macrophage form of THP-1. Interestingly, CD84 mRNA but not protein was inhibited by I3C and DIM. AHR siRNA knockdown experiments confirmed that the inhibitory effects of I3C and DIM on IL-1β as well as CD84 mRNA are regulated through AHR-mediated pathways. Additionally, the AHR ligand appeared to differentially regulate other LPS-induced cytokines expression. Hence, cross-talk between AHR and inflammation-mediated pathways, but not CD84-mediated pathways, in monocytes but not macrophages may contribute to the modulation of tumor environments by I3C and DIM in prostate cancer.

Novel anti-myeloma immunotherapies targeting the SLAM family of receptors

Treatment for multiple myeloma (MM) has significantly advanced in the last decade with the introduction of proteasome inhibitors and immunomodulatory therapies. Unfortunately, MM continues to cause significant morbidity and most patients eventually succumb to the disease. As in other areas of cancer, immunotherapy in MM has also evolved and holds promise to deliver long-lasting remissions or even cure. The signaling lymphocyte activation molecules (SLAM) family of surface proteins represents a group of potential targets for immunotherapy in MM as some of the family members are expressed consistently on plasma cells and also on myeloma propagating pre-plasma cells. Here, we review the SLAM family members in detail, describe their tissue distribution, biologic pathways, as well as relevant pre-clinical studies and clinical trials in MM. Our review demonstrates the value of SLAM family receptors as potential targets for anti-myeloma immunotherapies and outlines how immunotherapeutic approaches can be developed.

The SLAM family member CD84 is regulated by ADAM10 and calpain in platelets

BACKGROUND AND OBJECTIVE

Ectodomain shedding is a major mechanism to modulate platelet receptor signaling and to downregulate platelet reactivity. Proteins of the a disintegrin and metalloproteinase (ADAM) family are implicated in the shedding of various platelet receptors. The signaling lymphocyte activation molecule (SLAM) family receptor CD84 is highly expressed in platelets and immune cells, but its role in platelet physiology is not well explored. Because of its ability to form homodimers, CD84 has been suggested to mediate contact-dependent signaling and contribute to thrombus stability. However, nothing is known about the cellular regulation of CD84.

METHODS

We studied the regulation of CD84 in murine platelets by biochemical approaches and use of three different genetically modified mouse lines. Regulation of CD84 in human platelets was studied using inhibitors and biochemical approaches.

RESULTS

We show that CD84 is cleaved from the surface of human and murine platelets in response to different shedding inducing agents and platelet receptor agonists. CD84 downregulation occurs through ectodomain-shedding and intracellular cleavage. Studies in transgenic mice identified ADAM10 as the principal sheddase responsible for CD84 cleavage, whereas ADAM17 was dispensable. Western blot analyses revealed calpain-mediated intracellular cleavage of the CD84 C-terminus, occurring simultaneously with, but independently of, ectodomain shedding. Furthermore, analysis of plasma and serum samples from transgenic mice demonstrated that CD84 is constitutively shed from the platelet surface by ADAM10 in vivo.

CONCLUSIONS

These results reveal a dual regulation mechanism for platelet CD84 by simultaneous extra- and intracellular cleavage that may modulate platelet-platelet and platelet-immune cell interactions.

Very small embryonic-like stem cells purified from umbilical cord blood lack stem cell characteristics

Very small embryonic-like (VSEL) cells have been described as putatively pluripotent stem cells present in murine bone marrow and human umbilical cord blood (hUCB) and as such are of high potential interest for regenerative medicine. However, there remain some questions concerning the precise identity and properties of VSEL cells, particularly those derived from hUCB. For this reason, we have carried out an extensive characterisation of purified populations of VSEL cells from a large number of UCB samples. Consistent with a previous report, we find that VSEL cells are CXCR4⁺, have a high density, are indeed significantly smaller than HSC and have an extremely high nuclear/cytoplasmic ratio. Their nucleoplasm is unstructured and stains strongly with Hoechst 33342. A comprehensive FACS screen for surface markers characteristic of embryonic, mesenchymal, neuronal or hematopoietic stem cells revealed negligible expression on VSEL cells. These cells failed to expand in vitro under a wide range of culture conditions known to support embryonic or adult stem cell types and a microarray analysis revealed the transcriptional profile of VSEL cells to be clearly distinct both from well-defined populations of pluripotent and adult stem cells and from the mature hematopoietic lineages. Finally, we detected an aneuploid karyotype in the majority of purified VSEL cells by fluorescence in situ hybridisation. These data support neither an embryonic nor an adult stem cell like phenotype, suggesting rather that hUCB VSEL cells are an aberrant and inactive population that is not comparable to murine VSEL cells.

Mouse CD84 is a pan-leukocyte cell-surface molecule that modulates LPS-induced cytokine secretion by macrophages

CD84 is 1 of the 9 SLAM family cell-surface receptors involved in leukocyte activation. The CD84 ectodomain is highly glycosylated, and its cytoplasmic tail contains 2 copies of an ITSM, which can be phosphorylated. Here, we report that although mouse CD84 was present on all BM HSCs, its expression declined in developing thymic and BM lymphocytes. However, CD84 expression levels did increase significantly during the later maturation stages and were expressed abundantly on mature B and T cells. Among lymphocyte subsets, the highest expression was found on innate-like lymphocytes; specifically, on NKT and marginal zone B cells. Splenic CD4+ T(FH) cells exhibited higher levels of CD84 compared with the other CD4+ T cell subsets. CD84 was expressed abundantly on monocytes, macrophages, granulocytes, and DCs. Moreover, as the function of CD84 in myeloid cells remains unknown, we focused on the role this receptor plays in mouse macrophage activation. Transfection of CD84 in RAW-264.7 macrophages led to an increase in MAPK phosphorylation and NF-κB activation upon LPS stimulation. Concomitantly, the presence of CD84 increased the LPS-induced secretion of TNF-α and MCP-1 but lowered IL-10 and IL-6 production significantly. This modulatory effect was mediated by Y(300) within the second ITSM of CD84. Additionally, CD84 knock-down decreased TNF-α and IL-6 production in LPS-activated BMDMs. Taken together, these results show that mouse CD84 is a pan-leukocyte receptor, able to modulate signaling pathways downstream of TLR4, and regulates macrophage cell-fate decisions and effector functions.

The SLAM and SAP gene families control innate and adaptive immune responses

The nine SLAM-family genes, SLAMF1-9, a subfamily of the immunoglobulin superfamily, encode differentially expressed cell-surface receptors of hematopoietic cells. Engagement with their ligands, which are predominantly homotypic, leads to distinct signal transduction events, for instance those that occur in the T or NK cell immune synapse. Upon phosphorylation of one or more copies of a unique tyrosine-based signaling motif in their cytoplasmic tails, six of the SLAM receptors recruit the highly specific single SH2-domain adapters SLAM-associated protein (SAP), EAT-2A, and/or EAT-2B. These adapters in turn bind to the tyrosine kinase Fyn and/or other protein tyrosine kinases connecting the receptors to signal transduction networks. Individuals deficient in the SAP gene, SH2D1A, develop an immunodeficiency syndrome: X-linked lympho-proliferative disease. In addition to operating in the immune synapse, SLAM receptors initiate or partake in multiple effector functions of hematopoietic cells, for example, neutrophil and macrophage killing and platelet aggregation. Here we discuss the current understanding of the structure and function of these recently discovered receptors and adapter molecules in the regulation of adaptive and innate immune responses.

Differential expression of CD150 (SLAM) family receptors by human hematopoietic stem and progenitor cells

OBJECTIVES

Human hematopoietic stem cell (HSC)-containing grafts are most commonly used to treat various blood diseases, including leukemias and autoimmune disorders. CD150 (SLAM) family receptors have recently been shown to be differentially expressed by mouse HSC and progenitor cells. Members of the CD150 family are key regulators of leukocyte activation and differentiation. The goal of the present study is to analyze the expression patterns of the CD150 receptors CD48, CD84, CD150 (SLAM), CD229 (Ly9), and CD244 (2B4) on the different sources of human hematopoietic stem and progenitor cells.

MATERIALS AND METHODS

Expression of CD150 receptors was analyzed on human mobilized peripheral blood CD133(+)-isolated cells and CD34(+) bone marrow (BM) and umbilical cord blood (CB) cells using multicolor flow cytometry.

RESULTS

CD244 was present on most CD133(+)Lin(-)-mobilized cells and CD34(+)Lin(-) BM and CB cells, including virtually all CD38(-)Lin(-) primitive progenitor cells. CD48 had a restricted expression pattern on CD133(+)Lin(-)CD38(-) cells, while its levels were significantly higher in CD34(+)Lin(-) BM and CB cells. In addition, CD84 was present on a significant number of CD133(+)Lin(-) cells, but only on a small fraction of CD133(+)Lin(-)CD38(-) peripheral blood mobilized cells. In contrast, CD84 was expressed on practically all CD34(+)Lin(-) BM cells. No CD150 expression was observed in mobilized peripheral blood CD133(+)Lin(-) or CD34(+)Lin(-) BM and CB cells. Furthermore, only a small fraction of CD34(+)Lin(-) BM and CB cells expressed CD229.

CONCLUSIONS

Our results show that CD150 family molecules are present on human hematopoietic stem and progenitor cells and that their expression patterns differ between humans and mice.

Regulation of cellular and humoral immune responses by the SLAM and SAP families of molecules

SAP (SLAM-associated protein) was identified in 1998 as an adaptor molecule involved in the intracellular signaling pathways elicited through the cell surface receptor SLAM and as the protein defective in the human immunodeficiency X-linked lymphoproliferative disease (XLP). During the past eight years, it has been established that the SLAM family of cell surface receptors (SLAM, 2B4, NTB-A, Ly9, CD84) and the SAP family of adaptors (SAP, EAT-2, ERT) play critical roles in lymphocyte development, differentiation, and acquisition of effector functions. Studies of these proteins have shown unexpected roles in cytokine production by T cells and myeloid cells, T cell-dependent humoral immune responses, NK cell-mediated cytotoxicity, and NKT cell development. This review highlights recent findings that have improved our understanding of the roles of the SLAM and SAP families of molecules in immune regulation and discusses how perturbations in the signaling pathways involving these proteins can result in different disease states.

The role of SLAM family receptors in immune cell signaling

The signaling lymphocyte-activating molecule (SLAM) family immunoreceptors are expressed in a wide array of immune cells, including both T and B lymphocytes. By virtue of their ability to transduce tyrosine phosphorylation signals through the so-called ITSM (immunoreceptor tyrosine-based switch motif) sequences, they play an important part in regulating both innate and adaptive immune responses. The critical role of the SLAM immunoreceptors in mediating normal immune reactions was highlighted in recent findings that SAP, a SLAM-associated protein, modulates the activities of various immune cells through interactions with different members of the SLAM family expressed in these cells. Importantly, mutations or deletions of the sap gene in humans result in the X-linked lymphoproliferative syndrome. In this review, we summarize current knowledge and survey the latest developments in signal transduction events triggered by the activation of SLAM family receptors in different cell types.

The CD2 family of natural killer cell receptors

The CD2 family of receptors is evolutionarily conserved and widely expressed on cells within the hematopoietic compartment. In recent years several new members have been identified with important roles in the immune system. CD2 family members regulate natural killer (NK) cell lytic activity and inflammatory cytokine production when engaged by ligands on tumor cells. Furthermore, a subfamily of CD2 receptors, the CD 150-like molecules, has been implicated in the pathogenesis of X-linked lymphoproliferative disease (XLP). Many of these receptors have now been shown to bind homophilically or heterophilically to other molecules within the family. With these discoveries a novel mechanism for lymphocyte regulation has emerged: CD2 family members on NK cells engage ligands on neighboring NK cells, leading to NK cell stimulation. Moreover, heterotypic stimulatory interactions between NK cells and other leukocytes also occur. In this manner, CD2 family members may provide interlymphocyte communication that maintains organization within the hematopoietic compartment and amplifies immune responses. This review discusses these multiple roles for CD2 family members, focusing specifically on the regulation of NK cells.

Molecular and cellular pathogenesis of X-linked lymphoproliferative disease

X-linked lymphoproliferative disease (XLP) is an inherited immune defect caused by mutations in the Src homology 2 domain-containing gene 1A, which encodes the adapter protein, signaling lymphocytic activation molecule (SLAM)-associated protein (SAP). SAP is expressed in T cells, natural killer (NK) cells, and NKT cells, where it binds to the cytoplasmic domain of the surface receptor SLAM (CD150) and the related receptors, 2B4 (CD244), CD84, Ly9 (CD229), NK-T-B-antigen, and CD2-like receptor-activating cytotoxic T cells. SAP also binds to the Src family tyrosine kinase Fyn and recruits it to SLAM, which leads to the generation of downstream phosphotyrosine signals. While the roles of the SLAM family receptors are only beginning to be understood, experiments suggest that these molecules regulate important aspects of lymphocyte function, such as proliferation, cytokine secretion, cytotoxicity, and antibody production. Thus, in XLP patients who lack functional SAP, the SLAM family receptors may not signal properly. This property likely contributes to the phenotypes of XLP, including fulminant infectious mononucleosis, lymphoma, and hypogammaglobulinemia. Further studies of SAP and the SLAM family receptors will provide insights into XLP and elucidate the signaling events regulating lymphocyte ontogeny and function.

A specific heptapeptide from a phage display peptide library homes to bone marrow and binds to primitive hematopoietic stem cells

Phage display peptide libraries have enabled the discovery of peptides that selectively target specific organs. Selection of organ-specific peptides is mediated through binding of peptides displayed on phage coat protein to adhesion molecules expressed within targeted organs. Hematopoietic stem cells selectively home to bone marrow, and certain adhesion receptors critical to this function have been demonstrated. Using a phage display library, we identified a specific peptide that trafficked to murine bone marrow in vivo. We independently isolated exactly the same heptapeptide from the entire library by in vitro biopanning on primitive lineage-depleted, Hoechst 33342(dull)/rhodamine 123(dull) murine bone marrow stem cells and confirmed peptide binding to these cells by immunofluorescence studies. We demonstrated bone marrow-specific homing of the peptide by an in vivo assay in which the animals were injected with the phage displaying peptide sequence, and immunofluorescence analysis of multiple organs was performed. We also showed that the peptide significantly decreased the homing of stem cells to the bone marrow but not to the spleen 3 hours after transplantation using fluorescently labeled Lin(-)Sca(+) hematopoietic cells in an in vivo homing assay. The peptide sequence has a partial (5/7) amino acid sequence homology with a region of CD84. This discovery represents the first application of the phage display methodology to the bone marrow and stem cells and led to the identification of a specific heptapeptide that homes to bone marrow, binds to primitive stem cells, and plays a role in stem cell homing.

Differential expression of SAP and EAT-2-binding leukocyte cell-surface molecules CD84, CD150 (SLAM), CD229 (Ly9) and CD244 (2B4)

The CD150 (SLAM) family consists of nine leukocyte cell-surface proteins involved in lymphocyte activation that belong to the immunoglobulin (Ig) superfamily. Six members of this family--CD84, CD150 (SLAM), CD229 (Ly9), CD244 (2B4), NTB-A, and CS1--associate with adapter proteins--SLAM-associated protein (SAP) and EAT-2. SAP is a short intracellular molecule that is mutated in humans with X-linked lymphoproliferative disease. Flow cytometric analysis of the expression of CD84, CD150, CD229, and CD244 cell-surface receptors on several leukocyte and lymphocyte subsets was performed. CD84 and CD150 were present on thymocytes, mature T cells and antigen-presenting cells. The expression of CD84 and CD150 was high on memory T cells. CD150 expression was strongly up-regulated after cell activation. In contrast to CD84, CD150 was absent on resting monocytes and immature dendritic cells (DCs). CD229 presented a pattern of expression restricted to lymphocytes. CD244 was preferentially expressed on natural killer cells, CD8(+) effector cells, resting monocytes, basophils, and eosinophils. We describe a broader distribution of CD84, CD150, CD229, and CD244 than previously reported and show that they are differentially expressed on hematopoietic cells. The heterogeneous expression of these receptors indicates that these molecules may play non-redundant functions in the regulation of both innate and adaptive immune responses.