What goes up must come down: the emerging spectrum of inhibitory receptors

Comprehensive Phenotyping of Dendritic Cells in Cancer Patients by Flow Cytometry

Dendritic cells (DCs) play a crucial role in the complex interplay between tumor cells and the immune system. During the elimination phase of cancer immunoediting, immunostimulatory DCs are critical for the control of tumor growth. During the escape phase, regulatory DCs sustain tumor tolerance and contribute to the development of the immunosuppressive tumor microenvironment that characterizes this phase. Moreover, increasing evidence indicates that DCs are also critical for the success of cancer immunotherapy. Hence, there is increasing need to fully characterize DC subsets and their activatory/inhibitory profile in cancer patients. In this review, we describe the role played by different DC subsets in the different phases of cancer immunoediting, the function exerted by different activatory and inhibitory molecules expressed on DC surface, and the cytokines produced by distinct DC subsets, in order to provide an overview on the DC features that may be useful to be assessed when dealing with the flow cytometric characterization of DCs in cancer patients. © 2020 International Society for Advancement of Cytometry.

Dendritic cell tolerogenicity: a key mechanism in immunomodulation by vitamin D receptor agonists

Dendritic cells (DC) induce or tolerize T cells, and tolerogenic DCs can promote the development of regulatory T cells (Treg) with suppressive activity. Thus, the possibility of manipulating DCs and enhancing their tolerogenic properties using different pharmacologic or biologic agents could be exploited to control a variety of chronic immuno-mediated inflammatory conditions. Among agents able to promote induction of tolerogenic DCs, vitamin D receptor (VDR) agonists have attracted considerable attention, also because of their potential in clinical translation. DCs are key targets for the immunomodulatory effects of VDR agonists, which shape DC phenotype and function, enhancing their tolerogenicity in adaptive immune responses. Tolerogenic DCs induced by a short treatment with VDR agonists promote CD4+CD25+Foxp3+ Treg cells that are able to mediate transplantation tolerance and to arrest the development of autoimmune diseases. VDR agonists not only favor induction of CD4+CD25+ Treg cells, but can also enhance their recruitment at inflammatory sites. The tolerogenic properties induced by VDR agonists in DCs, leading to enhanced Treg cell development, likely contribute to the beneficial activity of these hormone-like molecules in autoimmune disease and graft rejection models, highlighting their applicability to the treatment of chronic inflammatory conditions sustained by autoreactive or alloreactive immune responses.

Induction of tolerogenic dendritic cells by vitamin D receptor agonists

Dendritic cells induce and regulate T cell responses, and tolerogenic dendritic cells (DCs) can promote the development of regulatory T cells with suppressive activity. Thus, the possibility to manipulate DCs using different pharmacological or biological agents enables them to exert tolerogenic activities, could be exploited to better control a variety of chronic inflammatory conditions, from autoimmune diseases to allograft rejection. A variety of both biological and pharmacological agents can induce tolerogenic DCs, and several in vitro studies have demonstrated that human regulatory T cells can be induced by DCs manipulated to acquire and/or enhance tolerogenic properties, with in vivo data also accumulating. Within this context, we have explored the immunoregulatory activities of vitamin D receptor (VDR) agonists, secosteroid hormones able to induce tolerogenic DCs and regulatory T cells. Tolerogenic DCs induced by a short treatment with VDR agonists promote CD4(+) CD25(+) Foxp3(+) suppressor T cells that are able to mediate transplantation tolerance and to arrest the development of autoimmune diseases. VDR agonists not only favour the induction of CD4(+) CD25(+) regulatory T cells, but can also enhance their recruitment to inflammatory sites. VDR agonists have been proven effective and safe drugs in a variety of autoimmune disease and graft rejection models, highlighting their potential applicability to chronic inflammatory conditions sustained by autoreactive or alloreactive immune responses. In addition to the topical treatment of psoriasis, a Th1-mediated autoimmune disease of the skin where VDR agonists are the most used topical drugs; these agents might eventually find a broader application in the treatment of inflammatory conditions, where their modulatory effects on DCs enhancing T cells with regulatory functions could turn out to be quite beneficial.

Analysis of individual natural killer cell responses

Typical assays for natural killer (NK) cell function assess the responses of entire NK cell populations. It is now possible to determine the responses of individual NK cells. Herein, two representative assays are described along with examples of how they have helped clarify current understanding of NK cell biology.

Interactions between natural killer cells, cortisol and prolactin in malaria during pregnancy

Natural killer cells derived from pluripotent hematopoietic stem cells are important cells of the immune system that have two main functions: a cytolytic activity and a cytokine-producing capacity. These functions are tightly regulated by numerous activating and inhibitory receptors, including newly discovered receptors that selectively trigger the cytolytic activity in a major histocompatibility complex independent manner. Based on their defining function of spontaneous cytotoxicity without prior immunization, natural killer (NK) cells have been thought to play a critical role in immune surveillance and cancer therapy. New insights into NK cell biology have suggested their major roles in the control of infections, particularly in Plasmodium falciparum infection and in fetal implantation. P. falciparum is the main protozoan parasite responsible for malaria causing 200-300 million clinical cases and killing over 3 million people each year. This review provides an update on NK cell function, ontogeny and biology in order to better understand the role of NK cells in pregnancy in regions where malaria is endemic. Understanding mechanisms of NK cell functions may lead to novel therapeutic strategies for the treatment of human disease, in general, and particularly in the fight against malaria.

Expression of the inhibitory receptor ILT3 on dendritic cells is dispensable for induction of CD4+Foxp3+ regulatory T cells by 1,25-dihydroxyvitamin D3

1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) is a secosteroid hormone that renders dendritic cells (DCs) tolerogenic, favoring the induction of regulatory T cells. Induction of DCs with tolerogenic properties by 1,25(OH)(2)D(3) is associated with increased selective expression of immunoglobulin-like transcript 3 (ILT3), suggesting its involvement in the immunoregulatory properties of this hormone. Here we show an in vivo correlate of the increased ILT3 expression on DCs in healing psoriatic lesions following topical treatment with the 1,25(OH)(2)D(3) analog calcipotriol. Analysis of DC subsets reveals a differential regulation of ILT3 expression by 1,25(OH)(2)D(3), with a marked up-regulation in myeloid DCs but no effect on its expression by plasmacytoid DCs. A regulatory role for ILT3 expressed on DCs is indicated by the increased interferon-gamma (IFN-gamma) secretion promoted by anti-ILT3 addition to cultures of DCs and T cells, but this effect is blunted in 1,25(OH)(2)D(3)-treated DCs, suggesting ILT3-independent mechanisms able to regulate T-cell activation. Although ILT3 expression by DCs is required for induction of regulatory T cells, DC pretreatment with 1,25(OH)(2)D(3) leads to induction of CD4(+)Foxp3(+) cells with suppressive activity irrespective of the presence of neutralizing anti-ILT3 monoclonal antibody (mAb), indicating that ILT3 expression is dispensable for the capacity of 1,25(OH)(2)D(3)-treated DCs to induce regulatory T cells.

Functional and structural characterization of two populations of human monocyte-derived dendritic cells

BACKGROUND

The characterization of human monocyte-derived dendritic cells (HM-DC) subsets have been a very difficult and elusive task because of the lack of appropriate reagents. We, therefore utilized several diverse approaches to evaluate two populations of HM-DC including flow cytometry, ultra-structural evaluation by electron microscopy, and functional assays. In addition, we studied the kinetics of the expression of antigens on HM-DC at diverse intervals of time and identify surface markers and functional differences of these two HM-DC subsets.

RESULTS

This study identified that a phenotype of HM-DC as defined by CD11c+, CD86+, and CD40+ could be separated in the presence or absence of TGF-beta1 into two different subsets of DC: (i) HM-DC without Birbeck granuli (Mo-DC) and (ii) HM-DC with Birbeck granuli (Mo-LC). Furthermore, the functional studies showed that the HM-DC treated with TGF-beta1 (Mo-LC) exhibited the presence of Birbeck granuli and could actively divide. In addition, after undergoing more than four cell divisions, these cells split into at least two additional subsets of Mo-LC: (iia) Mo-LC with high forward scatter (FSC) and (iib) Mo-LC with normal FSC. In contrast, the Mo-DC cultured in absence of TGF-beta1 did not exhibit Birbeck granuli, showed reduced ability to divide, and kept the normal FSC when analyzed.

CONCLUSIONS

This study enabled us to determine in HM-DC: (i) the existence of antigenic and functional differences between various subpopulations of Mo-DC and Mo-LC; (ii) the existence of differences in the kinetics of antigens expression among the subsets of Mo-DC and Mo-LC; (iii) the existence of specific markers for each of the subpopulations of HM-DC.

Immunoglobulin-like transcripts ILT2, ILT3 and ILT7 are expressed by human dendritic cells and down-regulated following activation

Immunoglobulin-like transcripts (ILT) represent novel immunoglobulin superfamily receptors that are expressed in myeloid, lymphoid and dendritic cells (DC). Here, we studied by gene expression profiling with DNA microarrays ILT expression in different DC subsets, including plasmacytoid DC (PDC), monocyte-derived DC (Mo-DC) and DC obtained by in vitro differentiation from CD34(+) progenitor cells, and DC activated in the presence of different activating agents. ILT2 and ILT3 were expressed in PDC, Mo-DC and DC obtained from CD34(+) cells. ILT7 mRNA was present in PDC, but absent in Mo-DC and DC obtained from CD34(+) cells, indicating that ILT7 mRNA expression seems to be a marker for PDC. CpG-DNA and inflammatory stimuli, such as TNF alpha, prostaglandin E2 (PGE2) and soluble CD40 ligand (sCD40L), and different combinations thereof are frequently employed for DC activation. Here, we demonstrate that ILT2 and ILT3 expression is down-regulated following DC activation by CpG-DNA and inflammatory stimuli at both mRNA and protein levels. Thus, activation of human DC with such stimuli involves down-regulation of inhibitory ILT2 and ILT3 receptors, and this could represent a novel mechanism contributing to DC activation.

Leukocyte immunoglobulin-like receptors: novel innate receptors for human basophil activation and inhibition

Basophils, recruited from the blood to tissues, have been implicated by their presence in diverse allergic disorders including bronchial asthma, allergic rhinitis, and cutaneous contact hypersensitivity. We hypothesized that like other leukocytes involved in inflammatory responses, basophils would express members of the leukocyte immunoglobulin-like receptor (LIR) family of immuno-regulatory molecules on their cell surface. We identified LIR7, an activating member coupled to the common Fc receptor gamma chain, and LIR3, an inhibitory member containing cytoplasmic immunoreceptor tyrosine-based inhibitory motifs, on these cells from human peripheral blood. Cross-linking of LIR7 resulted in the concentration-dependent net release of histamine (29.8 +/- 10.8%) and cysteinyl leukotrienes (cysLTs) (31.4 +/- 8.7 ng/10(6) basophils) that were maximal at 30 minutes, and of interleukin-4 (IL-4) (410.2 +/- 61.6 pg/10(6) basophils) that was maximal at 4 hours and comparable with the response initiated by cross-linking of the high-affinity receptor for immunoglobulin E (FcepsilonRI). Coligation of LIR3 to LIR7 or to FcepsilonRI by means of a second monoclonal antibody significantly inhibited net histamine release, cysLT production, and IL-4 generation. That LIR3 is profoundly counter-regulatory for both adaptive and innate receptors suggests a broad role in containment of the inflammatory response.

Src homology region 2-containing protein tyrosine phosphatase-2 (SHP-2) can play a direct role in the inhibitory function of killer cell Ig-like receptors in human NK cells

The inhibitory forms of killer cell Ig-like receptors (KIR) are MHC class I-binding receptors that are expressed by human NK cells and prevent their attack of normal cells. Substantial evidence indicates that the mechanism of KIR-mediated inhibition involves recruitment of the protein tyrosine phosphatase, Src homology region 2-containing protein tyrosine phosphatase (SHP)-1, to phosphorylated immunoreceptor tyrosine-based inhibitory motifs (ITIMs). However, the functional significance of parallel recruitment of a SHP-1-related phosphatase, SHP-2, to KIR ITIMs has not been addressed. In the present study, our results with mutant forms of a classical KIR, KIR3DL1, show a direct correlation between SHP-2 recruitment and functional inhibition of target cell conjugation and cytotoxicity. In addition, KIR3DL1 inhibition of target cell cytotoxicity is blocked by overexpression of a dominant-negative form of SHP-2. Finally, KIR3DL1 fused directly with the catalytic domain of SHP-2 inhibits both target cell conjugation and cytotoxicity responses. These results strongly indicate that SHP-2 catalytic activity plays a direct role in inhibitory KIR functions, and SHP-2 inhibits NK cell activation in concert with SHP-1.

Novel mesenchymal and haematopoietic cell isoforms of the SHP-2 docking receptor, PZR: identification, molecular cloning and effects on cell migration

SHP-2 (Src homology phosphatase type-2) is essential for haematopoietic skeletal and vascular development. Thus the identification of its binding partners is critically important. In the present study, we describe a unique monoclonal antibody, WM78, which interacts with PZR, a SHP-2 binding partner. Furthermore, we identify two novel isoforms of PZR, PZRa and PZRb, derived by differential splicing from a single gene transcription unit on human chromosome 1q24. All are type 1 transmembrane glycoproteins with identical extracellular and transmembrane domains, but differ in their cytoplasmic tails. The PZR intracellular domain contains two SHP-2 binding immunoreceptor tyrosine-based inhibitory motifs (VIY(246)AQL and VVY(263)ADI) which are not present in PZRa and PZRb. Using the WM78 monoclonal antibody, which recognizes the common extracellular domain of the PZR isoforms, we demonstrate that the PZR molecules are expressed on mesenchymal and haematopoietic cells, being present on the majority of CD34(+)CD38(+) and early clonogenic progenitors, and at lower levels on CD34(+)CD38(-) cells and the hierarchically more primitive pre-colony forming units. Interestingly, we show by reverse transcriptase-PCR that the PZR isoforms are differentially expressed in haematopoietic, endothelial and mesenchymal cells. Both PZR and PZRb are present in CD133(+) precursors and endothelial cells, PZRb predominates in mesenchymal and committed myelomonocytic progenitor cells, and all three isoforms occur in erythroid precursor cell lines. Importantly, using SHP-2 mutant (Delta 46-110) and SHP-2 rescue of embryonic fibroblasts stably expressing the PZR isoforms, we demonstrate for the first time that PZR, but not PZRa or PZRb, facilitates fibronectin- dependent migration of cells expressing a competent SHP-2 molecule. These observations will be instrumental in determining the mechanisms whereby PZR isoforms regulate cell motility.

Natural killer cell activation receptors in innate immunity to infection

Natural killer (NK) cells are best known for their capacity to kill tumors but they are also critical in early innate responses to infection, especially herpesviruses. Recent studies indicate that NK cell receptors involved in tumor target specificity are also involved in responses to viral infections.

In vitro correlates of Ld-restricted resistance to toxoplasmic encephalitis and their critical dependence on parasite strain

Resistance to murine toxoplasmic encephalitis has been precisely and definitively mapped to the L(d) class I gene. Consistent with this, CD8(+) T cells can adoptively transfer resistance to toxoplasmic encephalitis. However, cytotoxic CD8(+) T cells, capable of killing class I-matched, infected target cells, are generated during the course of Toxoplasma gondii infection even in mice lacking the L(d) gene. L(d)-restricted killing could not be demonstrated, and the functional correlate of the L(d) gene has therefore remained elusive. Herein, L(d)-restricted killing of T. gondii-infected target cells is demonstrated for the first time. L(d)-restricted killing is critically dependent on the strain of T. gondii and is observed with all the derivatives of type II strains tested, but not with a type I strain. These results have important implications for vaccine development.

Biology of natural killer cells in cancer and infection

NK cells are the important cells of the immune system derived from stem cells in the marrow. Their physiology is tightly regulated to control proliferation, cytotoxicity and cytokine production. In cancer, NK cells may be abnormal due to the cancer itself or possibly related to its therapy. The finding of class I recognizing inhibitory receptors may play a role in stem cell transplant rejection, immune surveillance and cancer immunotherapy. NK cells should no longer be thought of as direct cytotoxic killers alone, as they clearly play a critical role in cytokine production which may be important to control cancer and infection. Understanding NK cell function and homing may lead to novel therapeutic strategies for the treatment of human disease.

SHP-1- and phosphotyrosine-independent inhibitory signaling by a killer cell Ig-like receptor cytoplasmic domain in human NK cells

Killer cell Ig-like receptors (KIR) are MHC class I-binding immunoreceptors that can suppress activation of human NK cells through recruitment of the Src homology 2-containing protein tyrosine phosphatase-1 (SHP-1) to two immunoreceptor tyrosine-based inhibitory motifs (ITIMs) in their cytoplasmic domains. KIR2DL4 (2DL4; CD158d) is a structurally distinct member of the KIR family, which is expressed on most, if not all, human NK cells. 2DL4 contains only one ITIM in its cytoplasmic domain and an arginine in its transmembrane region, suggesting both inhibitory and activating functions. While 2DL4 can activate IFN-gamma production, dependent upon the transmembrane arginine, the function of the single ITIM of 2DL4 remains unknown. In this study, tandem ITIMs of KIR3DL1 (3DL1) and the single ITIM of 2DL4 were directly compared in functional and biochemical assays. Using a retroviral transduction method, we show in human NK cell lines that 1) the single ITIM of 2DL4 efficiently inhibits natural cytotoxicity responses; 2) the phosphorylated single ITIM recruits SHP-2 protein tyrosine phosphatase, but not SHP-1 in NK cells; 3) expression of dominant-negative SHP-1 does not block the ability of 2DL4 to inhibit natural cytotoxicity; 4) surprisingly, mutation of the tyrosine within the single ITIM does not completely abolish inhibitory function; and 5) this correlates with weak SHP-2 binding to the mutant ITIM of 2DL4 in NK cells and a corresponding nonphosphorylated ITIM peptide in vitro. These results reveal new aspects of the KIR-inhibitory pathway in human NK cells, which are SHP-1 and phosphotyrosine independent.

The biology of natural killer cells in cancer, infection, and pregnancy

OBJECTIVE

NK cells are important cells of the immune system. They are ultimately derived from pluripotent hematopoietic stem cells. NK cell cytotoxicity and other functions are tightly regulated by numerous activating and inhibitory receptors including newly discovered receptors that selectively recognize major histocompatibility complex class I alleles. Based on their defining function of spontaneous cytotoxicity without prior immunization, NK cells have been thought to play a critical role in immune surveillance and cancer therapy. However, new insights into NK cell biology have suggested major roles for NK cells in infection control and uterine function. The purpose of this review is to provide an update on NK cell function, ontogeny, and biology in order to better understand the role of NK cells in health and disease.

DATA SOURCES

In the Medline database, the major subject heading "Natural Killer Cells" was introduced in 1983, identifying 16,848 citations as of December 31, 2000. Since 1986, there have been approximately 1000 citations per year under this subject heading. In this database, 68% of manuscripts are limited to human NK cells; 40% of citations cross with the major sub-heading of cytotoxicity, 40% with cytokines, 36% with neoplasm, 5% with antibody-dependent cellular cytotoxicity, 2.8% with pregnancy, and 1.3% with infection. Of references from the year 2000-2001, 46 were selected to combine with contributions from earlier literature.

CONCLUSIONS

NK cells should no longer be thought of as direct cytotoxic killers alone as they clearly serve a critical role in cytokine production which may be important to control cancer, infection, and fetal implantation. Understanding mechanisms of NK cell functions may lead to novel therapeutic strategies for the treatment of human disease.

The biology of natural killer cells and implications for therapy of human disease

Natural killer (NK) cells are unique lymphocytes capable of lysing target cells without prior immunization. NK cells activated with cytokines, like interleukin-2 (IL-2), have been used since the 1980s as adoptive immunotherapy against metastatic solid tumors, but their effectiveness has been limited. The mechanisms by which NK cells recognize their targets are complex, including newly identified receptors that recognize class I MHC molecules. Understanding these mechanisms may support the use of NK cells as clinical therapy against infectious diseases and cancer. We have been interested in the use of NK cells clinically for their potential to eradicate minimal residual disease and prevent relapses after autologous stem cell transplantation. Several strategies are discussed to increase the specificity and efficacy of NK cell therapy. One method is to increase the targeting of NK cells by the use of monoclonal antibodies. Another approach uses allogeneic NK cells to overcome the inhibitory receptor mechanisms that may block target cell lysis by recognition of class I molecules. These and other novel strategies may prove to be attractive and effective immunotherapeutic tools to manipulate NK cells to fight human disease.

Flow cytometric assessment of the reactivity of a panel of monoclonal antibodies (mAb) against two populations of human dendritic cells (DC)

BACKGROUND

The identification of antigens on human DC has been a very difficult and elusive task because of the lack of appropriate reagents. Therefore, we evaluated by flow cytometry a panel of mAb that recognize antigens on human DC, aiming to determine the kinetics of DC antigen expression at 7, 14, 21 and 28 days in (i) Dermal DC like cells (Mo-DC) and (ii) Langerhans cell like DC (Mo-LC). In addition we aimed to identify markers for DC subpopulations.

RESULTS

It was found at day 7, that mAb BG6, HP-F1, BU10, RFD-1, CMRF-44 recognized <20% of Mo-DC. In contrast, 7H5, ZM3.8, CDlb/c, 55K-2, MMR1.16, MMR190.BB3 and L25 reacted with >50% of Mo-DC. Moreover, 7H5, ZM3.8, CMRF-56, CDlb/c, 55K-2, MMR1.16, MMR190.BB3 and L25 showed increased MFI reactivity against Mo-DC. mAb BG6, BU10 and CMRF-44 recognized <20% Mo-LC while RFD-1 reacted with 21% of Mo-LC. In contrast, HP-F1 showed 87% of Mo-LC positive. Also, 7H5, ZM3.8, RFD-7, MR15-2, CDlb/c, 55K-2, MMR1.16, MMR190.BB3 and L25 reacted with >50% of Mo-LC. The increase in % of positive cells was paralleled by MFI increases. At day 14, fourteen mAb recognized >50% of the Mo-DC, while five recognized 20-50% of Mo-DC. BG6 reacted with 7% of the Mo-DC. Nineteen mAb recognized >48% of Mo-LC while BG6 had negative reactivity. At day 21 and 28, all mAb reacted with >20% of Mo-DC and yielded a significant MFI with Mo-DC. Also nineteen mAb yielded significant MFI with Mo-LC while RFD-7 did not.

CONCLUSIONS

The immunophenotyping assays demonstrated differences between the two DC populations as well as variations in the reactivity of the mAb at diverse time points, suggesting the existence of subpopulations within the Mo-DC and Mo-LC.

Murine cytomegalovirus is regulated by a discrete subset of natural killer cells reactive with monoclonal antibody to Ly49H

Antiviral roles of natural killer (NK) cell subsets were examined in C57BL/6 mice infected with murine cytomegalovirus (MCMV) and other viruses, including lymphocytic choriomeningitis virus (LCMV), vaccinia virus (VV), and mouse hepatitis virus (MHV). Each virus vigorously induced an NK cell infiltrate into the peritoneal cavity and liver, causing some redistributions of NK cell subsets defined by monoclonal antibody (mAb) directed against Ly49A, C/I, D, and G2. Striking results were seen with a mAb (1F8) reactive with the positively signaling molecule Ly49H, present in MCMV-resistant C57BL/6 mice. mAb 1F8 also stains Ly49 C and I, but exclusion of those reactivities with mAb 5E6, which recognizes Ly49 C and I, indicated that Ly49H(+) cells infiltrated the peritoneal cavity and liver and were particularly effective at synthesizing interferon gamma. Depletion of 1F8(+) but not 5E6(+) cells in vivo by mAb injections enhanced MCMV titers by 20-1,000-fold in the spleen and approximately fivefold in the liver. Titers of LCMV or VV were not enhanced. These anti-MCMV effects were attributed to prototypical NK1.1(+)CD3(-) NK cells and not to NK1.1(+)CD3(+) "NK/T" cells. This is the first evidence that control of a virus infection in vivo is mediated by a distinct NK cell subset.

Stage-specific modulation of IFN-regulatory factor 4 function by Krüppel-type zinc finger proteins

Optimal humoral responses depend on the activation of Ag-specific B cells, followed by their progression toward a fully differentiated phenotype. Acquisition of stage-appropriate patterns of gene expression is crucial to this differentiation program. However, the molecular mechanisms used by B cells to modulate gene expression as they complete their maturation program are poorly understood. IFN-regulatory factor 4 (IRF-4) plays a critical role in mature B cell function. Using the transcriptional regulation of the human B cell activation marker CD23 as a model system, we have previously demonstrated that IRF-4 is induced in response to B cell-activating stimuli and that it acts as a transactivator of CD23 gene expression. We have furthermore found that IRF-4 function can be blocked by B cell lymphomas 6 (BCL-6) protein, a Krüppel-type zinc finger repressor normally expressed in germinal center B cells. However, CD23 expression is known to be down-regulated in plasma cells despite high level expression of IRF-4 and the lack of BCL-6, suggesting that in plasma cells the IRF-4-mediated induction of CD23 is prevented by its interaction with a distinct repressor. In this set of studies, we demonstrate that IRF-4 interacts with B lymphocyte-induced maturation protein/positive regulatory domain I-binding factor 1 (Blimp1/PRD1-BF1), a Krüppel-type zinc finger protein whose expression correlates with terminal B cell differentiation. Functional studies indicate that Blimp1, like BCL-6, can block IRF-4-transactivating ability. These findings thus support a model whereby IRF-4 function is modulated in a stage-specific manner by its interaction with developmentally restricted sets of Krüppel-type zinc finger proteins.

Uterine macrophages express the gp49B inhibitory receptor in midgestation

Mouse gp49B is an immunoreceptor tyrosine-based inhibitory motif-bearing receptor identified on mast cells and NK cells. In this report, however, we show that this receptor is expressed on macrophages accumulating in the uterine metrial gland in midgestation, along with gp49A that has a very homologous extracellular domain with gp49B but has a short cytoplasmic domain without ITIM. Culture of bone marrow cells in the conditioned medium of the metrial gland resulted in the selective proliferation of macrophages expressing both Fcgamma-activating receptors and gp49B inhibitory receptor. Stimulation of macrophages with immobilized IgG, but not with anti-FcgammaRII/III, induced a considerable amount of TNF-alpha and IL-10 production, suggesting that the high-affinity receptor for IgG (FcgammaRI) can transmit activating signals in cytokine production of macrophages. Furthermore, coligation of gp49B with FcgammaRI resulted in the inhibition of TNF-alpha production. Thus, our data provide evidence that gp49B is an endogenous negative regulator of macrophage activation and may regulate the function of macrophages during pregnancy.

Mapping the ligand of the NK inhibitory receptor Ly49A on living cells

We have used a recombinant, biotinylated form of the mouse NK cell inhibitory receptor, Ly49A, to visualize the expression of MHC class I (MHC-I) ligands on living lymphoid cells. A panel of murine strains, including MHC congenic lines, was examined. We detected binding of Ly49A to cells expressing H-2D(d), H-2D(k), and H-2D(p) but not to those expressing other MHC molecules. Cells of the MHC-recombinant strain B10.PL (H-2(u)) not only bound Ly49A but also inhibited cytolysis by Ly49A(+) effector cells, consistent with the correlation of in vitro binding and NK cell function. Binding of Ly49A to H-2D(d)-bearing cells of different lymphoid tissues was proportional to the level of H-2D(d) expression and was not related to the lineage of the cells examined. These binding results, interpreted in the context of amino acid sequence comparisons and the recently determined three-dimensional structure of the Ly49A/H-2D(d) complex, suggest a role for amino acid residues at the amino-terminal end of the alpha1 helix of the MHC-I molecule for Ly49A interaction. This view is supported by a marked decrease in affinity of an H-2D(d) mutant, I52 M, for Ly49A. Thus, allelic variation of MHC-I molecules controls measurable affinity for the NK inhibitory receptor Ly49A and explains differences in functional recognition in different mouse strains.

Negative regulation of growth hormone receptor/JAK2 signaling by signal regulatory protein alpha

Signal regulatory proteins (SIRPs) are receptor-like transmembrane proteins, the majority of which contain a cytoplasmic proline-rich region and four cytoplasmic tyrosines that, when phosphorylated, bind SH2 domain-containing protein tyrosine phosphatases (SHP). We demonstrated previously that growth hormone (GH) induces tyrosyl phosphorylation of SIRPalpha and association of SIRPalpha with SHP-2. The GH-activated tyrosine kinase JAK2 associates with and tyrosyl-phosphorylates SIRPalpha1. Here we show that JAK2-SIRPalpha1 association does not require phosphotyrosines in SIRPalpha1 or JAK2 or the proline-rich region of SIRPalpha1. However, when the C-terminal 30 amino acids of SIRPalpha1 containing the proline-rich region and tyrosine 495 are deleted, tyrosyl phosphorylation of SIRPalpha1 by JAK2 and association of SHP-2 with SIRPalpha1 are reduced. GH-dependent tyrosyl phosphorylation of JAK2 is reduced when wild-type SIRPalpha1 compared with SIRPalpha1 lacking the four cytoplasmic tyrosines (SIRP 4YF) is expressed in cells, suggesting that SIRPalpha1 negatively regulates GHR/JAK2 signaling. Consistent with reduced JAK2 activity, overexpression of wild-type SIRPalpha1 but not SIRP 4YF reduces GH-induced phosphorylation of ERKs 1 and 2, STAT3, and STAT5B. These results suggest that SIRPalpha1 is a negative regulator of GH signaling and that the ability of SIRPalpha1 mutants to negatively regulate GHR-JAK2 signaling correlates with their ability to bind SHP-2.

LIRs/ILTs/MIRs, inhibitory and stimulatory Ig-superfamily receptors expressed in myeloid and lymphoid cells

Cells exhibit a complex network of inhibitory and stimulatory signaling pathways, which interact with each other to maintain an homeostatic balance and modulate cellular responses to external stimuli. During most of the 1980s, a great effort was put into the characterization of stimulatory cell surface receptors for cytokines and growth factors. In the last decade, a large number of inhibitory receptors have been identified and it has become apparent that inhibitory signaling pathways are subject to intricate regulatory mechanisms. Inhibitory and stimulatory signaling pathways work in concert with each other to establish activation thresholds and provide sensitive tuning mechanisms that help control cellular responses. LIRs/ILTs/MIRs are a novel family of inhibitory and stimulatory receptors expressed both in myeloid and lymphoid cells. They contain two or four immunoglobulin-like domains in the extracellular region and their cytoplasmic domains are either very short and without any signaling motifs or are long and contain a variable number of immunoreceptor tyrosine-based inhibition motifs (ITIMs). LIRs within the first group send stimulatory signals by association with the FcR common gamma chain and LIRs within the second group deliver inhibitory signals by association with the protein tyrosine phosphatase SHP-1. This review summarizes our current knowledge on the LIRs, their ligands, and biological functions.

FDF03, a novel inhibitory receptor of the immunoglobulin superfamily, is expressed by human dendritic and myeloid cells

In this study, we describe human FDF03, a novel member of the Ig superfamily expressed as a monomeric 44-kDa transmembrane glycoprotein and containing a single extracellular V-set Ig-like domain. Two potential secreted isoforms were also identified. The gene encoding FDF03 mapped to chromosome 7q22. FDF03 was mostly detected in hemopoietic tissues and was expressed by monocytes, macrophages, and granulocytes, but not by lymphocytes (B, T, and NK cells), indicating an expression restricted to cells of the myelomonocytic lineage. FDF03 was also strongly expressed by monocyte-derived dendritic cells (DC) and preferentially by CD14+/CD1a- DC derived from CD34+ progenitors. Moreover, flow cytometric analysis showed FDF03 expression by CD11c+ blood and tonsil DC, but not by CD11c- DC precursors. The FDF03 cytoplasmic tail contained two immunoreceptor tyrosine-based inhibitory motif (ITIM)-like sequences. When overexpressed in pervanadate-treated U937 cells, FDF03 was tyrosine-phosphorylated and recruited Src homology-2 (SH2) domain-containing protein tyrosine phosphatase (SHP)-2 and to a lesser extent SHP-1. Like engagement of the ITIM-bearing receptor LAIR-1/p40, cross-linking of FDF03 inhibited calcium mobilization in response to CD32/FcgammaRII aggregation in transfected U937 cells, thus demonstrating that FDF03 can function as an inhibitory receptor. However, in contrast to LAIR-1/p40, cross-linking of FDF03 did not inhibit GM-CSF-induced monocyte differentiation into DC. Thus, FDF03 is a novel ITIM-bearing receptor selectively expressed by cells of myeloid origin, including DC, that may regulate functions other than that of the broadly distributed LAIR-1/p40 molecule.

Nonstochastic coexpression of activation receptors on murine natural killer cells

Murine natural killer cells (NK) express lectin-like activation and inhibitory receptors, including the CD94/NKG2 family of receptors that bind Qa-1, and the Ly-49 family that recognizes major histocompatibility complex class I molecules. Here, we demonstrate that cross-linking of NK cells with a new specific anti-Ly-49H mAb induced NK cell cytotoxicity and cytokine production. Ly-49H is expressed on a subset of NK cells and can be coexpressed with Ly-49 inhibitory receptors. However, unlike Ly-49 inhibitory receptors, Ly-49H is not detectable on naive splenic CD3(+) T cells, indicating that Ly-49H may be an NK cell-specific activation receptor. In further contrast to the stochastically expressed Ly-49 inhibitory receptors, Ly-49H is preferentially expressed with the Ly-49D activation receptor, and expression of both Ly-49H and Ly-49D is augmented on NK cells that lack receptors for Qa-1 tetramers. On developing splenic NK1.1(+) cells, Ly-49D and Ly-49H are expressed later than the inhibitory receptors. These results directly demonstrate that Ly-49H activates primary NK cells, and suggest that expression of Ly-49 activation receptors by NK cells may be specifically regulated on NK cell subsets. The simultaneous expression of multiple activation receptors by individual NK cells contrasts with that of T cell antigen receptors and is relevant to the role of NK cells in innate immunity.

CD72 negatively regulates signaling through the antigen receptor of B cells

The immunoreceptor tyrosine-based inhibition motif (ITIM) is found in various membrane molecules such as CD22 and the low-affinity Fc receptor for IgG in B cells and the killer cell-inhibitory receptor and Ly-49 in NK cells. Upon tyrosine phosphorylation at the ITIMs, these molecules recruit SH2 domain-containing phosphatases such as SH2-containing tyrosine phosphatase-1 and negatively regulate cell activity. The B cell surface molecule CD72 carries an ITIM and an ITIM-like sequence. We have previously shown that CD72 is phosphorylated and recruits SH2-containing tyrosine phosphatase-1 upon cross-linking of the Ag receptor of B cells (BCR). However, whether CD72 modulates BCR signaling has not yet been elucidated. In this paper we demonstrate that expression of CD72 down-modulates both extracellular signal-related kinase (ERK) activation and Ca2+ mobilization induced by BCR ligation in the mouse B lymphoma line K46micromlambda, whereas BCR-mediated ERK activation was not reduced by the ITIM-mutated form of CD72. Moreover, coligation with CD72 with BCR reduces BCR-mediated ERK activation in spleen B cells of normal mice. These results indicate that CD72 negatively regulates BCR signaling. CD72 may play a regulatory role in B cell activation, probably by setting a threshold for BCR signaling.

SHP-1 regulation of p62(DOK) tyrosine phosphorylation in macrophages

SHP-1 plays key roles in the modulation of hematopoietic cell signaling. To ascertain the impact of SHP-1 on colony-stimulating factor-1 (CSF-1)-mediated survival and proliferative signaling, we compared the CSF-1 responses of primary bone marrow macrophages (BMM) from wild-type and SHP-1-deficient motheaten (me/me) mice. CSF-1-induced protein tyrosine phosphorylation levels were similar in wild-type and me/me BMM, except for the constitutive hyperphosphorylation of a 62-kDa phosphoprotein (pp62) in me/me macrophages. pp62 was identified as the RASGAP-associated p62(DOK) and was shown to be the major CSF-1R-associated tyrosine-phosphorylated protein in CSF-1-treated BMM. p62(DOK) was found to be constitutively associated with SHP-1 in BMM and in 293T cells, co-expressing p62(dok) and either wild-type or catalytically inert SHP-1 (SHP-1 C453S). In both cell types, the interaction of SHP-1 with p62(DOK) occurred independently of p62(DOK) tyrosine phosphorylation, but only the tyrosine-phosphorylated p62(DOK) was bound by SHP-1 C453S in a far Western analysis. These findings suggest a constitutive association of SHP-1 and p62(DOK) that is either conformation-dependent or indirect as well as a direct, inducible association of the SHP-1 catalytic domain with tyrosine-phosphorylated p62(DOK). p62(DOK) hyperphosphorylation is not associated with altered CSF-1-induced RAS signaling or proliferation. However, whereas wild-type macrophages undergo cell death following CSF-1 removal, me/me macrophages exhibit prolonged survival in the absence of growth factor. Thus, p62(DOK) is a major SHP-1 substrate whose tyrosine phosphorylation correlates with growth factor-independent survival in macrophages.

Interaction of the NK cell inhibitory receptor Ly49A with H-2Dd: identification of a site distinct from the TCR site

Natural killer cell function is controlled by interaction of NK receptors with MHC I molecules expressed on target cells. We describe the binding of bacterially expressed Ly49A, the prototype murine NK inhibitory receptor, to similarly engineered H-2Dd. Despite its homology to C-type lectins, Ly49A binds independently of carbohydrate and Ca2+ and shows specificity for MHC I but not bound peptide. The affinity of the Ly49A/H-2Dd interaction as determined by surface plasmon resonance is from 6 to 26 microM at 25 degrees C and is greater by ultracentrifugation at 4 degrees C. Biotinylated Ly49A stains H-2Dd-expressing cells. Competition experiments indicate that the Ly49A and T cell receptor (TCR) binding sites on MHC I are distinct, suggesting complex regulation of cells that bear both TCR and NK cell receptors.

Cutting Edge: Expression of Functional CD94/NKG2A Inhibitory Receptors on Fetal NK1.1+Ly-49− Cells: A Possible Mechanism of Tolerance During NK Cell Development

Fetal liver- and thymus-derived NK1.1+ cells do not express known Ly-49 receptors. Despite the absence of Ly-49 inhibitory receptors, fetal and neonatal NK1.1+Ly-49− cells can distinguish between class Ihigh and class Ilow target cells, suggesting the existence of other class I-specific inhibitory receptors. We demonstrate that fetal NK1.1+Ly-49− cell lysates contain CD94 protein and that a significant proportion of fetal NK cells are bound by Qa1b tetramers. Fetal and adult NK cells efficiently lyse lymphoblasts from Kb−/−Db−/− mice. Qa1b-specific peptides Qdm and HLA-CW4 leader peptide specifically inhibited the lysis of these blasts by adult and fetal NK cells. Qdm peptide also inhibited the lysis of Qa1b-transfected human 721.221 cells by fetal NK cells. Taken together, these results suggest that the CD94/NKG2A receptor complex is the major known inhibitory receptor for class I (Qa1b) molecules on developing fetal NK cells.

The NKR-P1B Gene Product Is an Inhibitory Receptor on SJL/J NK Cells

The mouse NKR-P1 family includes at least three genes: NKR-P1A, -B, -C. Neither surface expression nor function of the NKR-P1B gene product has previously been shown. Here, we demonstrate that the SJL/J allele of the NKR-P1B gene product is expressed on SJL/J NK cells, and is recognized by PK136 mAb. Interestingly, the same mAb does not recognize the NKR-P1B gene product of C57BL/6. We have also generated a novel mAb, 1C10, that recognizes an activation receptor on SJL/J NK cells. Activation of the NKR-P1B receptor-inhibited 1C10 mAb induced redirected lysis and recruited SHP-1, indicating that NKR-P1B is an inhibitory receptor. Therefore, the mouse NKR-P1 gene family, like the Ly49 family, includes both activation and inhibitory receptors.

Leukocyte-Associated Ig-Like Receptor-1 Functions as an Inhibitory Receptor on Cytotoxic T Cells

Leukocyte associated Ig-like receptor-1 (LAIR-1) is a surface molecule expressed on human mononuclear leukocytes that functions as an inhibitory receptor on human NK cells. In addition to NK cells, LAIR-1 is expressed on T cells, B cells, macrophages, and dendritic cells. Most cells express two biochemically distinct forms of LAIR-1, which we now show are likely alternative splice variants of the same gene. Cross-linking of LAIR-1 on human T cell clones results in inhibition of cytotoxicity only in T cell clones that lack CD28 and are able to spontaneously lyse certain targets in vitro. Moreover, the cytolytic activity of freshly isolated T cells, which is thought to be mainly due to “effector” T cells, can be inhibited by anti-LAIR-1 mAb. Thus, LAIR-1 functions as an inhibitory receptor not only on NK cells, but also on human T cells. This indicates that LAIR-1 provides a mechanism of regulation of effector T cells and may play a role in the inhibition of unwanted bystander responses mediated by Ag-specific T cells.

Immunoreceptor tyrosine-based inhibition motif-bearing receptors regulate the immunoreceptor tyrosine-based activation motif-induced activation of immune competent cells

ITIM-bearing receptors, a family which only recently has been recognized, play a key role in the regulation of the ITAM-induced activation of immune competent cells. The mechanism of ITM-mediated regulation in various cells was recently clarified. The present review focuses on ITIM bearing membrane proteins that negatively regulate the activation of cells when co-crosslinked with ITAM containing receptors, illustrates the inhibitory processes by the negative regulation of B-, NK-, T-cells and mast cells and summarizes current views on the mechanism of ITIM-mediated inhibition.

Specificity of the SH2 Domains of SHP-1 in the Interaction with the Immunoreceptor Tyrosine-Based Inhibitory Motif-Bearing Receptor gp49B

Inhibitory receptors on hemopoietic cells critically regulate cellular function. Despite their expression on a variety of cell types, these inhibitory receptors signal through a common mechanism involving tyrosine phosphorylation of the immunoreceptor tyrosine-based inhibitory motif (ITIM), which engages Src homology 2 (SH2) domain-containing cytoplasmic tyrosine or inositol phosphatases. In this study, we have investigated the proximal signal-transduction pathway of an ITIM-bearing receptor, gp49B, a member of a newly described family of murine NK and mast cell receptors. We demonstrate that the tyrosine residues within the ITIMs are phosphorylated and serve for the association and activation of the cytoplasmic tyrosine phosphatase SHP-1. Furthermore, we demonstrate a physiologic association between gp49B and SHP-1 by coimmunoprecipitation studies from NK cells. To address the mechanism of binding between gp49B and SHP-1, binding studies involving glutathione S-transferase SHP-1 mutants were performed. Utilizing the tandem SH2 domains of SHP-1, we show that either SH2 domain can interact with phosphorylated gp49B. Full-length SHP-1, with an inactivated amino SH2 domain, also retained gp49B binding. However, binding to gp49B was disrupted by inactivation of the carboxyl SH2 domain of full-length SHP-1, suggesting that in the presence of the phosphatase domain, the carboxyl SH2 domain is required for the recruitment of phosphorylated gp49B. Thus, gp49B signaling involves SHP-1, and this association is dependent on tyrosine phosphorylation of the gp49B ITIMs, and an intact SHP-1 carboxyl SH2 domain.

Cutting Edge: Human Myeloid Cells Express an Activating ILT Receptor (ILT1) That Associates with Fc Receptor γ-Chain

Ig-like transcripts (ILTs) encode cell surface receptors expressed on myeloid and lymphoid cells that are structurally and functionally related to killer cell inhibitory receptors. One ILT, designated ILT1, contains a short cytoplasmic domain that lacks sequence motifs implicated in signal transduction. Its function is unknown. Similar short cytoplasmic domains have been observed in activating NK cell receptors and FcαR, which transduce stimulatory signals via associated DAP12 and FcεRIγ proteins, respectively. Here we show that ILT1 receptor is selectively expressed on myeloid cells, functions as an activating receptor, and associates with FcεRIγ rather than DAP12.

Type 1 diabetes: the facts fit a deficient inhibitory signal given by MHC class II

This paper presents a hypothesis regarding the aetiology of Type 1 (autoimmune) diabetes, which suggests that autoimmunity is normally prevented by an inhibitory or negative signal delivered by MHC molecules, and that in Type 1 diabetes it is the inability of beta cells to deliver sufficient negative signal from MHC Class II that drives the underlying autoimmune process. Based on a broad survey of the diabetes literature, a list of clinical, pathological, experimental and epidemiological 'facts' about Type 1 diabetes is presented which are considered to be widely accepted as proven. The new theory is then compared to other recent theories on the aetiology of diabetes with regard to its ability to explain these accepted 'facts'.

MHC Class I Mosaic Mice Reveal Insights into Control of Ly49C Inhibitory Receptor Expression in NK Cells

We have analyzed lymphocyte development in natural MHC class I chimeric mice, generated through a transgenic approach in β2-microglobulin (β2m)−/− mice. In these mice, MHC class I+ cells coexist with an equal proportion of MHC class I-deficient cells. These MHC class I mosaic mice had normal numbers of CD8+ T cells, which had a target cell specificity similar to that of wild-type mice. Consequently, the mice did not develop any signs of autoimmunity. They also had normal numbers of NK cells. This allowed an examination of the MHC class I influence on the expression of the Ly49C inhibitory receptor on NK cells. This receptor binds to H-2Kb. It is expressed at low levels on NK cells in wild-type mice of the H-2b haplotype, but at markedly higher levels on NK cells in β2m−/− mice and other strains of mice lacking expression of H-2Kb. Relatively little is known about how MHC class I molecules affect expression of the Ly49 receptors. Through the analysis of the present MHC class I mosaic mice, we demonstrate that the expression levels of Ly49C on NK cells is a consequence not only of MHC class I expression in the environment, but also of the expression of MHC class I molecules by the NK cells themselves. These findings are discussed in relation to the biological role of the calibration of the Ly49 inhibitory receptor expression in relation to self-MHC class I.

Biliary glycoprotein (CD66a), a cell adhesion molecule of the immunoglobulin superfamily, on human lymphocytes: structure, expression and involvement in T cell activation

The biliary glycoproteins (BGP or CD66a), a group of different splice variants of a single gene, are members of the carcinoembryonic antigen family and the immunoglobulin superfamily. Recently, we detected CD66a on IL-2 activated lymphocytes. In this study we characterized the structure and the expression pattern of BGP on human lymphocytes and investigated its role in T cell activation. Lymphocytes express 2 of the 13 known splice variants, i.e. BGPa and BGPb, which are glycosylated in a lymphocyte-specific manner. Both BGPa and BGPb have the long cytoplasmic tail, which contains two immunoreceptor tyrosine-based inhibitory motif (ITIM)-like motifs, but differ in their extracellular region containing 4 and 3 immunoglobulin-like domains, respectively. On PBL BGP is expressed in small amounts only on B cells and Th cells. Stimulation with IL-2 leads to a strong up-regulation of BGP by these cells, and induces de novo BGP expression on gammabeta T cells, CD8+ and CD56+ cells, but not on CD16+ lymphocytes. This up-regulation of BGP seems to be part of the physiological process of T cell activation, since stimulation with anti-CD3 mAb is sufficient to induce BGP up-regulation. Based on the presence of the two ITIM-like motifs, one may expect that BGP inhibits T cell activation, but surprisingly, engagement of BGP enhances the proliferation of anti-CD3-stimulated T cells.

The MHC class I binding proteins LIR-1 and LIR-2 inhibit Fc receptor-mediated signaling in monocytes

The MHC class I binding proteins leukocyte immunoglobulin-like receptor (LIR)-1 and -2 recognize a similar broad spectrum of HLA-A, -B and -C alleles but are differentially expressed in lymphocytes, monocytes, and dendritic cells. In monocytes, phosphorylation of LIR-1 and LIR-2 results in the binding of the tyrosine phosphatase SHP-1. Coligation of either LIR with Fcgamma receptor I (CD64) inhibits tyrosine phosphorylation of the associated Fc receptor gamma chain and Syk molecules, as well as intracellular calcium mobilization. These findings suggest that LIR-1 and LIR-2 function as unique MHC class I receptors involved in the inhibition or down-modulation of monocyte activation signals, particularly those mediated through the receptors for IgG, IgE and IgA.

Paired immunoglobulin-like receptor (PIR)-A is involved in activating mast cells through its association with Fc receptor gamma chain

Paired immunoglobulin-like receptor (PIR)-A and PIR-B possess similar ectodomains with six immunoglobulin-like loops, but have distinct transmembrane and cytoplasmic domains. PIR-B bears immunoreceptor tyrosine-based inhibitory motif (ITIM) sequences in its cytoplasmic domain that recruit Src homology (SH)2 domain-containing tyrosine phosphatases SHP-1 and SHP-2, leading to inhibition of B and mast cell activation. In contrast, the PIR-A protein has a charged Arg residue in its transmembrane region and a short cytoplasmic domain that lacks ITIM sequences. Here we show that Fc receptor gamma chain, containing an immunoreceptor tyrosine-based activation motif (ITAM), associates with PIR-A. Cross-linking of this PIR-A complex results in mast cell activation such as calcium mobilization in an ITAM-dependent manner. Thus, our data provide evidence for the existence of two opposite signaling pathways upon PIR aggregation. PIR-A induces the stimulatory signal by using ITAM in the associated gamma chain, whereas PIR-B mediates the inhibitory signal through its ITIMs.

Cloning of a novel C-type lectin expressed by murine macrophages

We report the cloning of a novel macrophage-restricted C-type lectin by differential display polymerase chain reaction. This molecule, named mouse macrophage C-type lectin, is a 219-amino acid, type II transmembrane protein with a single extracellular C-type lectin domain. Northern blot analysis indicates that it is expressed in cell lines and normal mouse tissues in a macrophage-restricted manner. The cDNA and genomic sequences of mouse macrophage C-type lectin indicate that it is related to the Group II animal C-type lectins. The mcl gene locus has been mapped between the genes for the interleukin-17 receptor and CD4 on mouse chromosome 6, the same chromosome as the mouse natural killer cell gene complex.

Natural killer cell receptors

In killing of cellular targets, natural killer cells employ receptors that activate them and receptors specific for MHC class I that inhibit their activation. Progress in understanding the inhibitory receptors has been rapid, and indications are that they fall into two distinct structural types that appear to utilize the same inhibitory signaling cascade; meanwhile, components of the activation cascade are being elucidated, permitting us to integrate the pathways involved.

Cutting Edge: The B Cell Surface Protein CD72 Recruits the Tyrosine Phosphatase SHP-1 upon Tyrosine Phosphorylation

Activation signals of lymphocytes are negatively regulated by the membrane molecules carrying the immunoreceptor tyrosine-based inhibition motif (ITIM). Upon tyrosine phosphorylation, ITIMs recruit SH2-containing phosphatases such as SHP-1, resulting in down-modulation of cell activation. We showed that the cytoplasmic domain of the CD72 molecule carries an ITIM and is associated in vitro with SHP-1 upon tyrosine phosphorylation. Moreover, cross-linking of B cell Ag receptor (BCR) enhances both tyrosine phosphorylation of CD72 and association of CD72 with SHP-1 in B cell line WEHI-231. These results indicate that CD72 recruits SHP-1 upon tyrosine phosphorylation induced by BCR signaling, suggesting that CD72 is a negative regulator of BCR signaling.

HLA-E is a major ligand for the natural killer inhibitory receptor CD94/NKG2A

We previously showed that the availability of a nonamer peptide derived from certain HLA class I signal sequences is a necessary requirement for the stabilization of endogenous HLA-E expression on the surface of 721.221 cells. This led us to examine the ability of HLA-E to protect HLA class I transfectants from natural killer (NK) cell-mediated lysis. It was possible to implicate the CD94/NKG2A complex as an inhibitory receptor recognizing this class Ib molecule by using as target a .221 transfectant selectively expressing surface HLA-E. HLA-E had no apparent inhibitory effect mediated through the identified Ig superfamily (Ig-SF) human killer cell inhibitory receptors or ILT2/LIR1. Further studies of CD94/NKG2+ NK cell-mediated recognition of .221 cells transfected with different HLA class I allotypes (i.e., -Cw4, -Cw3, -B7) confirmed that the inhibitory interaction was mediated by CD94/NKG2A recognizing the surface HLA-E molecule, because only antibodies directed against either HLA-E, CD94, or CD94/NKG2A specifically restored lysis. Surface stabilization of HLA-E in cold-treated .221 cells loaded with appropriate peptides was sufficient to confer protection, resulting from recognition of the HLA class Ib molecule by the CD94/NKG2A inhibitory receptor. Consistent with the prediction that the ligand for CD94/NKG2A is expressed ubiquitously, our examination of HLA-E antigen distribution indicated that it is detectable on the surface of a wide variety of cell types.