Face off--the interplay between activating and inhibitory immune receptors

Persistence of a Skewed Repertoire of NK Cells in People with HIV-1 on Long-Term Antiretroviral Therapy

HIV-1 infection greatly alters the NK cell phenotypic and functional repertoire. This is highlighted by the expansion of a rare population of FcRγ- NK cells exhibiting characteristics of traditional immunologic memory in people with HIV (PWH). Although current antiretroviral therapy (ART) effectively controls HIV-1 viremia and disease progression, its impact on HIV-1-associated NK cell abnormalities remains unclear. To address this, we performed a longitudinal analysis detailing conventional and memory-like NK cell characteristics in n = 60 PWH during the first 4 y of ART. Throughout this regimen, a skewed repertoire of cytokine unresponsive FcRγ- memory-like NK cells persisted and accompanied an overall increase in NK surface expression of CD57 and KLRG1, suggestive of progression toward immune senescence. These traits were linked to elevated serum inflammatory biomarkers and increasing Ab titers to human CMV, with human CMV viremia detected in approximately one-third of PWH at years 1-4 of ART. Interestingly, 40% of PWH displayed atypical NK cell subsets, representing intermediate stages of NK-poiesis based on single-cell multiomic trajectory analysis. Our findings indicate that NK cell irregularities persist in PWH despite long-term ART, underscoring the need to better understand the causative mechanisms that prevent full restoration of immune health in PWH.

Mapping the interplay between NK cells and HIV: therapeutic implications

Although highly effective at durably suppressing plasma HIV-1 viremia, combination antiretroviral therapy (ART) treatment regimens do not eradicate the virus, which persists in long-lived CD4+ T cells. This latent viral reservoir serves as a source of plasma viral rebound following treatment interruption, thus requiring lifelong adherence to ART. Additionally, challenges remain related not only to access to therapy but also to a higher prevalence of comorbidities with an inflammatory etiology in treated HIV-1+ individuals, underscoring the need to explore therapeutic alternatives that achieve sustained virologic remission in the absence of ART. Natural killer (NK) cells are uniquely positioned to positively impact antiviral immunity, in part due to the pleiotropic nature of their effector functions, including the acquisition of memory-like features, and, therefore, hold great promise for transforming HIV-1 therapeutic modalities. In addition to defining the ability of NK cells to contribute to HIV-1 control, this review provides a basic immunologic understanding of the impact of HIV-1 infection and ART on the phenotypic and functional character of NK cells. We further delineate the qualities of "memory" NK cell populations, as well as the impact of HCMV on their induction and subsequent expansion in HIV-1 infection. We conclude by highlighting promising avenues for optimizing NK cell responses to improve HIV-1 control and effect a functional cure, including blockade of inhibitory NK receptors, TLR agonists to promote latency reversal and NK cell activation, CAR NK cells, BiKEs/TriKEs, and the role of HIV-1-specific bNAbs in NK cell-mediated ADCC activity against HIV-1-infected cells.

CD200R1L is a functional evolutionary conserved activating receptor in human neutrophils

Inhibitory and activating immune receptors play a key role in modulating the amplitude and duration of immune responses during infection and in maintaining immune balance in homeostatic conditions. The CD200 Receptor (CD200R) gene family in humans encodes one inhibitory receptor, CD200R1, and one putative activating member, CD200R1 Like (CD200R1L). It is demonstrated that CD200R1L is endogenously expressed by human neutrophils and activates cellular functions such as reactive oxygen species (ROS) production via Syk, PI3Kβ, PI3Kδ, and Rac GTPase signaling. Phylogenetic analysis shows that CD200R1L is present in many species among vertebrates, ranging from birds to primates, suggesting that evolutionary conservation of this receptor is critical for protection against co‐evolving pathogens. The duplication event that generated CD200R1L from CD200R occurred several times throughout evolution, supporting convergent evolution of CD200R1L. In our phylogenetic trees, CD200R1L has longer branch lengths than CD200R1 in most species, suggesting that CD200R1L is evolving faster than CD200R1. It is proposed that CD200R1L represents a hitherto uncharacterized activating receptor on human neutrophils.

TARM1 contributes to development of arthritis by activating dendritic cells through recognition of collagens

TARM1 is a member of the leukocyte immunoglobulin-like receptor family and stimulates macrophages and neutrophils in vitro by associating with FcRγ. However, the function of this molecule in the regulation of the immune system is unclear. Here, we show that Tarm1 expression is elevated in the joints of rheumatoid arthritis mouse models, and the development of collagen-induced arthritis (CIA) is suppressed in Tarm1-/- mice. T cell priming against type 2 collagen is suppressed in Tarm1-/- mice and antigen-presenting ability of GM-CSF-induced dendritic cells (GM-DCs) from Tarm1-/- mouse bone marrow cells is impaired. We show that type 2 collagen is a functional ligand for TARM1 on GM-DCs and promotes DC maturation. Furthermore, soluble TARM1-Fc and TARM1-Flag inhibit DC maturation and administration of TARM1-Fc blocks the progression of CIA in mice. These results indicate that TARM1 is an important stimulating factor of dendritic cell maturation and could be a good target for the treatment of autoimmune diseases.

Co-inhibitory Receptor Signaling in T-Cell-Mediated Autoimmune Glomerulonephritis

Autoimmune glomerulonephritis occurs as a consequence of autoantibodies and T-cell effector functions that target autoantigens. Co-signaling through cell surface receptors profoundly influences the optimal activation of T cells. The scope of this review is signaling mechanisms and the functional roles of representative T-cell co-inhibitory receptors in the regulation of autoimmune glomerulonephritis, along with current therapeutic challenges mainly on preclinical trials. Co-inhibitory receptors utilize both shared and unique signaling pathway, suggesting specialized functions that provide the rationale behind therapies for autoimmune glomerulonephritis by targeting these inhibitory receptors. These receptors largely suppress Th1 immunity, modify Th17 and Th2 immune response, and enhance Treg function. Anti-cytotoxic T-lymphocyte-associated protein 4 (CTLA4) immunoglobulin (Ig), which is able to block both activating CD28 and inhibitory CTLA4 signaling, has been shown in preclinical and clinical investigations to have effects on glomerular disease. Other inhibitory receptors for treating glomerulonephritis have not been clinically tested, and efficacy of manipulating these pathways requires further preclinical investigation. While immune checkpoint inhibition using anti-CTLA4 antibodies and anti-programmed cell death 1 (PD-1)/PD-L1 antibodies has been approved for the treatment of several cancers, blockade of CTLA4 and PD-1/PD-L1 is associated with adverse effects that resemble autoimmune disorders, including systemic vasculitis. A renal autoimmune vasculitis model features an initial Th17 dominancy followed later by a Th1-dominant outcome and Treg cells that attenuate autoreactive T-cell function. Toward the development of effective therapies for T-cell-mediated autoimmune glomerulonephritis, it would be preferable to pay attention to the impact of the inhibitory pathways in immunological renal disease settings.

NK Cell Adoptive Immunotherapy of Cancer: Evaluating Recognition Strategies and Overcoming Limitations

Natural killer (NK) cells, the primary effector cells of the innate immune system, utilize multiple strategies to recognize tumor cells by (1) detecting the presence of activating receptor ligands, which are often upregulated in cancer; (2) targeting cells that have a loss of major histocompatibility complex (MHC); and (3) binding to antibodies that bind to tumor-specific antigens on the tumor cell surface. All these strategies have been successfully harnessed in adoptive NK cell immunotherapies targeting cancer. In this review, we review the applications of NK cell therapies across different tumor types. Similar to other forms of immunotherapy, tumor-induced immune escape and immune suppression can limit NK cell therapies' efficacy. Therefore, we also discuss how these limitations can be overcome by conferring NK cells with the ability to redirect their tumor-targeting capabilities and survive the immune-suppressive tumor microenvironment. Finally, we also discuss how future iterations can benefit from combination therapies with other immunotherapeutic agents.

Chimeric Antigen Receptor Expressing Natural Killer Cells for the Immunotherapy of Cancer

Adoptive cell therapy has emerged as a powerful treatment for advanced cancers resistant to conventional agents. Most notable are the remarkable responses seen in patients receiving autologous CD19-redirected chimeric antigen receptor (CAR) T cells for the treatment of B lymphoid malignancies; however, the generation of autologous products for each patient is logistically cumbersome and has restricted widespread clinical use. A banked allogeneic product has the potential to overcome these limitations, yet allogeneic T-cells (even if human leukocyte antigen-matched) carry a major risk of graft-versus-host disease (GVHD). Natural killer (NK) cells are bone marrow-derived innate lymphocytes that can eliminate tumors directly, with their activity governed by the integration of signals from activating and inhibitory receptors and from cytokines including IL-15, IL-12, and IL-18. NK cells do not cause GVHD or other alloimmune or autoimmune toxicities and thus, can provide a potential source of allogeneic “off-the-shelf” cellular therapy, mediating major anti-tumor effects without inducing potentially lethal alloreactivity such as GVHD. Given the multiple unique advantages of NK cells, researchers are now exploring the use of CAR-engineered NK cells for the treatment of various hematological and non-hematological malignancies. Herein, we review preclinical data on the development of CAR-NK cells, advantages, disadvantages, and current obstacles to their clinical use.

Coevolution of Siglec-11 and Siglec-16 via gene conversion in primates

Background

Siglecs-11 and -16 are members of the sialic acid recognizing Ig-like lectin family, and expressed in same cells. Siglec-11 functions as an inhibitory receptor, whereas Siglec-16 exhibits activating properties. In humans, SIGLEC11 and SIGLEC16 gene sequences are extremely similar in the region encoding the extracellular domain due to gene conversions. Human SIGLEC11 was converted by the nonfunctional SIGLEC16P allele, and the converted SIGLEC11 allele became fixed in humans, possibly because it provides novel neuroprotective functions in brain microglia. However, the detailed evolutionary history of SIGLEC11 and SIGLEC16 in other primates remains unclear.

Results

We analyzed SIGLEC11 and SIGLEC16 gene sequences of multiple primate species, and examined glycan binding profiles of these Siglecs. The phylogenetic tree demonstrated that gene conversions between SIGLEC11 and SIGLEC16 occurred in the region including the exon encoding the sialic acid binding domain in every primate examined. Functional assays showed that glycan binding preference is similar between Siglec-11 and Siglec-16 in all analyzed hominid species. Taken together with the fact that Siglec-11 and Siglec-16 are expressed in the same cells, Siglec-11 and Siglec-16 are regarded as paired receptors that have maintained similar ligand binding preferences via gene conversions. Relaxed functional constraints were detected on the SIGLEC11 and SIGLEC16 exons that underwent gene conversions, possibly contributing to the evolutionary acceptance of repeated gene conversions. The frequency of nonfunctional SIGLEC16P alleles is much higher than that of SIGLEC16 alleles in every human population.

Conclusions

Our findings indicate that Siglec-11 and Siglec-16 have been maintained as paired receptors by repeated gene conversions under relaxed functional constraints in the primate lineage. The high prevalence of the nonfunctional SIGLEC16P allele and the fixation of the converted SIGLEC11 imply that the loss of Siglec-16 and the gain of Siglec-11 in microglia might have been favored during the evolution of human lineage.

Electronic supplementary material

The online version of this article (10.1186/s12862-017-1075-z) contains supplementary material, which is available to authorized users.

NKG2C+NKG2A- Natural Killer Cells are Associated with a Lower Viral Set Point and may Predict Disease Progression in Individuals with Primary HIV Infection

Natural killer (NK) cells are the first line of defense against pathogens of the immune system and also play an important role in resistance against HIV. The activating receptor NKG2C and the inhibitory receptor NKG2A co-modulate the function of NK cells by recognizing the same ligand, HLA-E. However, the role of NKG2A and NKG2C on viral set point and the prediction of HIV disease progression have been rarely reported. In this study, we determined the expression of NKG2C or NKG2A on the surface of NK cells from 22 individuals with primary HIV infection (PHI) stage and 23 HIV-negative normal control (NC) subjects. The CD4⁺ T cell count and plasma level of HIV RNA in the infected individuals were longitudinally followed-up for about 720 days. The proportion of NKG2C⁺NKG2A^- NK cells was higher in subjects from the low set point group and was negatively correlated with the viral load. In addition, strong anti-HIV activities were observed in NKG2C⁺ NK cells from the HIV-positive donors. Furthermore, a proportion of NKG2C⁺NKG2A^- NK cells >35.45%, and a ratio of NKG2C/NKG2A >1.7 were predictive for higher CD4⁺ T cell counts 720 days after infection. Collectively, the experimental results allow us to draw the conclusion that NKG2C⁺ NK cells might exert an antiviral effect and that the proportion of NKG2C⁺NKG2A^- NK cells, and the ratio of NKG2C/NKG2A, are potential biomarkers for predicting HIV disease progression.

Evolutionary and polymorphism analyses reveal the central role of BTN3A2 in the concerted evolution of the BTN3 gene family

The butyrophilin 3 (BTN3) receptors are implicated in the T lymphocytes regulation and present a wide plasticity in mammals. In order to understand how these genes have been diversified, we studied their evolution and show that the three human BTN3 are the result of two successive duplications in Primates and that the three genes are present in Hominoids and the Old World Monkey groups. A thorough phylogenetic analysis reveals a concerted evolution of BTN3 characterized by a strong and recurrent homogenization of the region encoding the signal peptide and the immunoglobulin variable (IgV) domain in Hominoids, where the sequences of BTN3A1 or BTN3A3 are replaced by BTN3A2 sequence. In human, the analysis of the diversity of these genes in 1683 individuals representing 26 worldwide populations shows that the three genes are polymorphic, with more than 46 alleles for each gene, and marked by extreme homogenization of the IgV sequences. The same analysis performed for the BTN2 genes shows also a concerted evolution; however, it is not as strong and recurrent as for BTN3. This study shows that BTN3 receptors are marked by extreme concerted evolution at the IgV domain and that BTN3A2 plays a central role in this evolution.

The TGF-β-induced up-regulation of NKG2DLs requires AKT/GSK-3β-mediated stabilization of SP1

Natural killer (NK) cells play an important role in preventing cancer development. NK group 2 member D (NKG2D) is an activating receptor expressed in the membrane of NK cells. Tumour cells expressing NKG2DL become susceptible to an immune‐dependent rejection mainly mediated by NK cells. The paradoxical roles of transforming growth factor beta (TGF‐β) in regulation of NKG2DL are presented in many studies, but the mechanism is unclear. In this study, we showed that TGF‐β up‐regulated the expression of NKG2DLs in both PC3 and HepG2 cells. The up‐regulation of NKG2DLs was characterized by increasing the expression of UL16‐binding proteins (ULBPs) 1 and 2. TGF‐β treatment also increased the expression of transcription factor SP1. Knockdown of SP1 significantly attenuated TGF‐β‐induced up‐regulation of NKG2DLs in PC3 and HepG2 cells, suggesting that SP1 plays a key role in TGF‐β‐induced up‐regulation of NKG2DLs. TGF‐β treatment rapidly increased SP1 protein expression while not mRNA level. It might be due to that TGF‐β can elevate SP1 stability by activating PI3K/AKT signalling pathway, subsequently inhibiting GSK‐3β activity and decreasing the association between SP1 and GSK‐3β. Knockdown of GSK‐3β further verified our findings. Taken together, these results revealed that AKT/GSK‐3β‐mediated stabilization of SP1 is required for TGF‐β induced up‐regulation of NKG2DLs. Our study provided valuable evidence for exploring the tumour immune modulation function of TGF‐β.

Paired Siglec receptors generate opposite inflammatory responses to a human-specific pathogen

Paired immune receptors display near-identical extracellular ligand-binding regions but have intracellular sequences with opposing signaling functions. While inhibitory receptors dampen cellular activation by recognizing self-associated molecules, the functions of activating counterparts are less clear. Here, we studied the inhibitory receptor Siglec-11 that shows uniquely human expression in brain microglia and engages endogenous polysialic acid to suppress inflammation. We demonstrated that the human-specific pathogen Escherichia coli K1 uses its polysialic acid capsule as a molecular mimic to engage Siglec-11 and escape killing. In contrast, engagement of the activating counterpart Siglec-16 increases elimination of bacteria. Since mice do not have paired Siglec receptors, we generated a model by replacing the inhibitory domain of mouse Siglec-E with the activating module of Siglec-16. Siglec-E16 enhanced proinflammatory cytokine expression and bacterial killing in macrophages and boosted protection against intravenous bacterial challenge. These data elucidate uniquely human interactions of a pathogen with Siglecs and support the long-standing hypothesis that activating counterparts of paired immune receptors evolved as a response to pathogen molecular mimicry of host ligands for inhibitory receptors.

Cord Blood as a Source of Natural Killer Cells

Cord blood (CB) offers several unique advantages as a graft source for hematopoietic stem cell transplantation (HSCT). The risk of relapse and graft vs. host disease after cord blood transplantation (CBT) is lower than what is typically observed after other graft sources with a similar degree of human leukocyte antigen mismatch. Natural killer (NK) cells have a well-defined role in both innate and adaptive immunity and as the first lymphocytes to reconstitute after HSCT and CBT, and they play a significant role in protection against early relapse. In this article, we highlight the uses of CB NK cells in transplantation and adoptive immunotherapy. First, we will describe differences in the phenotype and functional characteristics of NK cells in CB as compared with peripheral blood. Then, we will review some of the obstacles we face in using resting CB NK cells for adoptive immunotherapy, and discuss methods to overcome them. We will review the current literature on killer-cell immunoglobulin-like receptors ligand mismatch and outcomes after CBT. Finally, we will touch on current strategies for the use of CB NK cells in cellular immunotherapy.

Structural basis for simultaneous recognition of an O-glycan and its attached peptide of mucin family by immune receptor PILRα

Paired Ig-like type 2 receptor α (PILRα) recognizes a wide range of O-glycosylated mucin and related proteins to regulate broad immune responses. However, the molecular characteristics of these recognitions are largely unknown. Here we show that sialylated O-linked sugar T antigen (sTn) and its attached peptide region are both required for ligand recognition by PILRα. Furthermore, we determined the crystal structures of PILRα and its complex with an sTn and its attached peptide region. The structures show that PILRα exhibits large conformational change to recognize simultaneously both the sTn O-glycan and the compact peptide structure constrained by proline residues. Binding and functional assays support this binding mode. These findings provide significant insight into the binding motif and molecular mechanism (which is distinct from sugar-recognition receptors) by which O-glycosylated mucin proteins with sTn modifications are recognized in the immune system as well as during viral entry.

Apoptotic cells suppress mast cell inflammatory responses via the CD300a immunoreceptor

After cecal ligation and puncture, mice lacking the phosphatidylserine receptor CD300a on mast cells show more neutrophil recruitment to the peritoneal cavity, improved bacterial clearance, and extended survival.

When a cell undergoes apoptosis, phosphatidylserine (PS) is exposed on the outer leaflet of the plasma membrane. PS acts as an “eat-me” signal to direct phagocytes expressing PS receptors to engulf the apoptotic cell. We recently reported that the immunoreceptor CD300a, which is expressed on myeloid cells, is a PS receptor. We show that CD300a does not facilitate macrophage phagocytosis of apoptotic cells. Instead, CD300a delivers an inhibitory signal in mast cells to suppress production of LPS-induced inflammatory cytokines and chemokines. After cecal ligation and puncture (CLP), when a large number of cells undergo apoptosis in the peritoneal cavity, CD300a-deficient peritoneal mast cells produced more chemoattractant and recruited more neutrophils than did wild-type (WT) mast cells. As a result, CD300a-deficient mice showed increased neutrophil recruitment and improved bacterial clearance in the peritoneal cavity, and survived longer than WT mice. Antibody blockade of CD300a–PS interactions improved bacterial clearance and extended survival of WT mice subjected to CLP. These results indicated that CD300a is a nonphagocytic PS receptor that regulates mast cell inflammatory responses to microbial infections.

Paired opposing leukocyte receptors recognizing rapidly evolving ligands are subject to homogenization of their ligand binding domains

Some leukocyte receptors come in groups of two or more where the partners share ligand(s) but transmit opposite signals. Some of the ligands, such as MHC class I, are fast evolving, raising the problem of how paired opposing receptors manage to change in step with respect to ligand binding properties and at the same time conserve opposite signaling functions. An example is the KLRC (NKG2) family, where opposing variants have been conserved in both rodents and primates. Phylogenetic analyses of the KLRC receptors within and between the two orders show that the opposing partners have been subject to post-speciation gene homogenization restricted mainly to the parts of the genes that encode the ligand binding domains. Concerted evolution similarly restricted is demonstrated also for the KLRI, KLRB (NKR-P1), KLRA (Ly49), and PIR receptor families. We propose the term merohomogenization for this phenomenon and discuss its significance for the evolution of immune receptors.

Allorecognition in a basal chordate consists of independent activating and inhibitory pathways

Histocompatibility in the basal chordate Botryllus schlosseri is controlled by the polymorphisms of a single gene: the fuhc. A polymorphic candidate receptor (fester) appeared to play roles in both initiating the reaction and discriminating between fuhc alleles. Here we report the characterization of a related protein, uncle fester. uncle fester is not polymorphic, and although coexpressed with fester, has different functional properties. Loss-of-function studies demonstrate that uncle fester was required for incompatible reactions but has no role in interactions between compatible individuals. Furthermore, stimulation with monoclonal antibodies could initiate a rejection phenotype on a single colony, and in both assays the severity of the rejection could be manipulated. These findings suggest that allorecognition in Botryllus consists of independent pathways that control compatible and incompatible outcomes that are integrated within the interacting cells, and may provide insight into basal processes conserved in allorecognition responses throughout the metazoa.

Immune inhibitory receptors: essential regulators of phagocyte function

Phagocytes, including neutrophils, monocytes, and macrophages, play a crucial role in host defense by recognition and elimination of invading pathogens. Phagocytic cells produce reactive oxygen species (ROS), inflammatory cytokines, and chemokines, leading to bacterial killing and to recruitment and activation of additional immune cells. However, inflammatory mediators are potentially harmful for the host and their production is therefore tightly controlled by multiple regulatory mechanisms. One such mechanism is immune suppression by immune inhibitory receptors, which are increasingly acknowledged as potent regulators of the immune response. So far, research has focused on the role of these receptors in the regulation of NK cells, B cells, and T cells. Importantly, an accumulating number of inhibitory receptors have been identified on phagocytes. Here, we review the role of inhibitory receptors in the regulation of phagocyte cytokine production, migration, apoptosis, ROS production, and phagocytosis. Furthermore, we discuss the intracellular mechanisms utilized by distinct inhibitory receptors to regulate specific phagocyte functions. We demonstrate that inhibitory receptors are important regulators of the immune response, which bacteria can use to their advantage.

Uterine natural killer (uNK) cells and their missions during pregnancy: a review

Natural killer (NK) cells are lymphocytes of the innate immune system. The aim of this review is to describe the properties and roles of NK cells in the human uterus during pregnancy. Uterine natural killer cells (uNK) constitute a major lymphocyte population during early gestation in the uterus. The uterine natural killer cells are recognized owing to their CD56(bright), CD16(-), CD3(-) phenotype. Their number increases in the first trimester with a subsequent decline as pregnancy progresses. They have been shown to be closely associated with cells of the extravillous trophoblast (EVT) and spiral arteries. They play important roles in remodeling of the spiral arteries, control of trophoblast invasion and in the development of the placenta. Some studies have shown the number and repertoire of receptors of uNK differ between women with healthy pregnancies and those with pathologic pregnancies, such as pre-eclampsia or intrauterine growth retardation. During pregnancy, the cytotoxic characteristics of the uterine killer cells are not directed towards the fetus, and scientists continue to question and explore this phenomenon with increasing evidence that these cells may perform differing beneficial roles during pregnancy. Contrary to their previously suspected "hostile" characteristics, the uterine killer cells are considered to be "friendly" and appear to be essential and very important regulators of successful implantation and pregnancy.

Differential recruitment of activating and inhibitory Fc gamma RII during phagocytosis

Human myeloid cells express both activating and inhibitory receptors of the FcgammaRII family. FcgammaRIIA mediates processes associated with cell activation, including phagocytosis of IgG-opsonized particles, whereas coengagement of the inhibitory FcgammaRIIB downregulates such signaling. We analyzed the relative recruitment of these two receptors during phagocytosis of IgG-coated particles by ts20 Chinese hamster fibroblast cells cotransfected with both receptors carrying distinguishable fluorescent protein tags. We found that FcgammaRIIA is substantially enriched at sites of particle binding relative to its inhibitory counterpart, with a greater than 2-fold increase in the local ratio of activating to inhibitory receptor compared with that for the plasma membrane as a whole. Experiments with chimeric receptors revealed that the preferential enrichment of FcgammaRIIA results from differences between the extracellular domains of the receptors, and indicated that the lesser recruitment of FcgammaRIIB limits its ability to effectively inhibit FcgammaRIIA-mediated phagocytosis. Mutagenesis studies indicated that FcgammaRIIA residues leucine 132 and phenylalanine 160, which lie in IgG-binding regions of FcgammaRIIA and which differ in FcgammaRIIB, both contribute to the local relative enrichment of FcgammaRIIA by increasing its affinity for IgG1 relative to that of FcgammaRIIB. In human monocytes, engagement of approximately equal amounts of FcgammaRIIB was required to substantially inhibit FcgammaRIIA-mediated phagocytosis. These results demonstrate that differences in affinity for IgG between activating and inhibitory FcgammaR can result in substantial local changes in their relative concentrations during phagocytosis, with important functional consequences.

Modulation of paired immunoglobulin-like type 2 receptor signaling alters the host response to Staphylococcus aureus-induced pneumonia

Paired immunoglobulin-like type 2 receptors (PILRs) inhibitory PILRalpha and activating PILRbeta are predominantly expressed on myeloid cells. Their functions in host defense and inflammation are largely unknown, and in this study, we evaluated their roles in an acute Staphylococcus aureus pneumonia model. Compared to their respective controls, Pilrb(-/-) mice or mice in which PILRalpha was activated with an agonistic antibody showed improved clearance of pulmonary staphylococci and improved survival. These mice had reduced serum or bronchoalveolar lavage fluid levels of interleukin-1beta (IL-1beta), tumor necrosis factor alpha (TNF-alpha), and IL-6 and elevated levels of gamma interferon (IFN-gamma), IL-12, and IL-10. In contrast, mice in which PILRbeta was activated had increased lung bacterial burdens and higher mortality coupled with an intense proinflammatory response with highly elevated levels of IL-1beta, TNF-alpha, and IL-6. Treatment groups with reduced bacterial burdens had higher levels of Keratinocyte-derived chemokine (KC), macrophage inflammatory protein 2 (MIP-2), and MIP-1alpha in bronchoalveolar lavage fluid and an increased influx of neutrophils and macrophages to the lungs. Consistent with our in vivo findings, bone marrow-derived macrophages from Pilrb(-/-) mice released significantly less IL-1beta and TNF-alpha and more IFN-gamma and IL-12 than did the wild-type macrophages when directly stimulated with heat-killed S. aureus. To our knowledge, this is the first evidence that S. aureus directly interacts with PILRbeta. It provides a mechanism by which manipulating the balance in favor of an inhibitory PILR signal, by activation of PILRalpha or deletion of PILRbeta, helps to control acute S. aureus-mediated pneumonia and attenuate the inflammatory response. These results highlight the importance of PILRs in innate immunity and the control of inflammation.

Heterogeneity in the CD200R paired receptor family

Paired receptors are groups of closely related membrane proteins that have the potential to either inhibit or activate. The CD200R family consists of one inhibitory member, CD200R and various numbers of activating genes according to species with three defined in C57BL/6 mice. A genomic PCR strategy was used to examine the repertoire of genes in both laboratory and wild-derived mice. Most mouse strains tested (18/22) had three activating genes, and 16 of these had either the combination of CD200RLa, Lb, and Lc or CD200RLa, Lb, and Le. The Lc and Le genes were mutually exclusive and were equally common (10 strains). Wild-derived mice varied more with one example of strains with one, two, and four activating genes. An inhibitory CD200R gene was detected in each mouse strain, although two slightly different sequences were found in both laboratory and wild-derived mice. This diversity is probably being driven by pathogens but is less extensive than for many NK paired receptors such as KIR and Ly49. It is possible that myeloid paired receptors are involved in immune regulation of responses against pathogens rather than directly killing infected cells as for NK cells and, hence, under less intense evolutionary pressure.

Regulation of Immune Responses by the Activating and Inhibitory Myeloid-Associate Immunoglobuline-Like Receptors (MAIR) (CD300)

Activating and inhibitory cell surface receptors play important roles in regulation of immune responses. Recent progress has demonstrated that many inhibitory receptors pair with activating, as well as inhibitory, isoforms, both of whose genes are located in small clusters on a chromosome. We and others identified paired activating and inhibitory immunoglobulin-like receptors, designated myeloid-associated immunoglobulin-like receptors (MAIR) (CD300). MAIR is a multigene family consisting of nine genes on a small segment of mouse chromosome 11. MAIR family receptors are preferentially expressed on myeloid cells, including macrophages, dendritic cells, granulocytes, and bone-marrow-derived cultured mast cells, and a subset of B cells and regulate activation of these cells. Thus, MAIR plays an important role in innate immunity mediated by myeloid cells.

Natural killer cell receptors and their ligands in liver diseases

The liver is a distinctive immune organ with predominant innate immunity, being rich in innate immune cells such as natural killer (NK) cells. In humans, NK cells comprise about 30%-50% of intrahepatic lymphocytes, whereas peripheral blood lymphocytes contain about 5%-20% NK cells. Accumulating evidence suggests that NK cells play an important role not only in host defense against invading microorganisms and tumor transformation in the liver but also in liver injury and repair. In recent years, significant progress has been made in terms of understanding how NK cells recognize their target cells and carry out their effector functions. It is now clear that NK cells are strictly regulated by numerous activating and inhibitory NK cell receptors that recognize various classes of cell surface ligands, some of which are expressed by normal healthy cells. Therefore, to further elucidate the involvement of NK cells in the pathogenesis of liver diseases, an understanding of recent advances in NK cell biology is crucial. This review provides an overview of recent advances in our knowledge of human NK cell receptors and their ligands in the context of liver diseases.

The human CD94 gene encodes multiple, expressible transcripts including a new partner of NKG2A/B

CD94/NKG2A is an inhibitory receptor expressed by natural killer (NK) cells and a subset of CD8+ T cells. Ligation of CD94/NKG2A by its ligand HLA-E results in tyrosine phosphorylation of the NKG2A immunoreceptor tyrosine-based inhibitory motifs, and recruitment and activation of the SH2 domain-bearing tyrosine phosphatase-1, which in turn suppresses activation signals. The nkg2a gene encodes two isoforms, NKG2A and NKG2B, with the latter lacking the stem region. We identified three new alternative transcripts of the cd94 gene in addition to the originally described canonical CD94Full. One of the transcripts, termed CD94-T4, lacks the portion that encodes the stem region. CD94-T4 associates with both NKG2A and NKG2B, but preferentially associates with the latter. This is probably due to the absence of a stem region in both CD94-T4 and NKG2B. CD94-T4/NKG2B is capable of binding HLA-E and, when expressed in E6-1 Jurkat T cells, inhibits TCR mediated signals, demonstrating that this heterodimer is functional. Coevolution of stemless isoforms of CD94 and NKG2A that preferentially pair with each other to produce a functional heterodimer indicates that this may be more than a serendipitous event. CD94-T4/NKG2B may contribute to the plasticity of the NK immunological synapse by insuring an adequate inhibitory signal.

PIR-B-deficient mice are susceptible to Salmonella infection

Paired Ig-like receptors of activating (PIR-A) and inhibitory (PIR-B) isoforms are expressed by many hematopoietic cells, including B lymphocytes and myeloid cells. To determine the functional roles of PIR-A and PIR-B in primary bacterial infection, PIR-B-deficient (PIR-B(-/-)) and wild-type (WT) control mice were injected i.v. with an attenuated strain of Salmonella enterica Typhimurium (WB335). PIR-B(-/-) mice were found to be more susceptible to Salmonella infection than WT mice, as evidenced by high mortality rate, high bacterial loads in the liver and spleen, and a failure to clear bacteria from the circulation. Although blood levels of major cytokines and Salmonella-specific Abs were mostly comparable in the two groups of mice, distinct patterns of inflammatory lesions were found in their livers at 7-14 days postinfection: diffuse spreading along the sinusoids in PIR-B(-/-) mice vs nodular restricted localization in WT mice. PIR-B(-/-) mice have more inflammatory cells in the liver but fewer B cells and CD8(+) T cells in the spleen than WT mice at 14 days postinfection. PIR-B(-/-) bone marrow-derived macrophages (BMMphi) failed to control intracellular replication of Salmonella in vitro, in part due to inefficient phagosomal oxidant production, when compared with WT BMMphi. PIR-B(-/-) BMMphi also produced more nitrite and TNF-alpha upon exposure to Salmonella than WT BMMphi did. These findings suggest that the disruption of PIR-A and PIR-B balance affects their regulatory roles in host defense to bacterial infection.

The counterbalance theory for evolution and function of paired receptors

Paired receptors are families of membrane proteins containing similar extracellular regions but differing in their potential for signaling with one type able to give inhibitory signals and the other activating. Inhibitory receptors could be good targets for pathogens to restrict immune responses against them. Here we suggest that activating members may have evolved to counterbalance pathogens utilizing the inhibitory pathway. Thus, if a pathogen utilizes any part of the inhibitory receptor to downregulate responses against itself, it may, because of similarities in structure, also bind the activating receptor and give an opposing signal. We evaluate recent structural data on SIRPalpha (signal regulatory protein) and LILRB1 (leukocyte immunoglobulin-like receptor subfamily B member 1) showing evidence of pathogen pressure in nonligand-binding regions of these receptors together with data on pathogen binding to PIRs (paired Ig-like receptors) to provide support for this theory.

Human natural killer cells

Natural killer (NK) cells were discovered more than 30 years ago. NK cells are large granular lymphocytes that belong to the innate immune system because unlike T or B lymphocytes of the adaptive or antigen-specific immune system, NK cells do not rearrange T-cell receptor or immunoglobulin genes from their germline configuration. During the past 2 decades there has been a substantial gain in our understanding of what and how NK-cells "see," lending important insights into their functions and purpose in normal immune surveillance. The most recent discoveries in NK-cell receptor biology have fueled translational research that has led to remarkable results in treating human malignancy.

Hepatitis C virus-induced secretion of inflammatory chemokines preferentially recruits NKG2A+CD8+ T cells

In patients with hepatitis C, a loss-of-function mutation of chemokine receptor CCR5 (CCR5Delta32) has been shown to be associated with spontaneous viral clearance and lower levels of hepatic inflammation. In the present study, we show that CCR5 is coexpressed with the inhibitory NKG2A receptor on CD8(+) T cells. Consequently, CCR5(+) T cells were highly susceptible to NKG2A-mediated inhibition of cytotoxic activity and NKG2A(+) lymphocytes were preferentially attracted by CCR5 ligands induced by hepatitis C virus E2 antigen. Thus, CCR5 is likely to exert immunoregulatory effects in hepatitis C virus infection by preferentially recruiting CD8(+) T cells bearing the inhibitory NKG2A receptor to the liver.

Itch-/- alphabeta and gammadelta T cells independently contribute to autoimmunity in Itchy mice

E3 ubiquitin ligases determine which intracellular proteins are targets of the ubiquitin conjugation pathway and thus play a key role in determining the half-life, subcellular localization and/or activation status of their target proteins. Itchy mice lack the E3 ligase, Itch, and show dysregulation of T lymphocytes and the induction of a lethal autoimmune inflammatory condition. Itch is widely expressed in hematopoietic and nonhematopoietic cells, and we demonstrate that disease is transferred exclusively by hematopoietic cells. Moreover, distinct manifestations of the autoimmune inflammatory phenotype are contributed by discrete populations of lymphocytes. The presence of Itch-deficient alphabeta T cells drives expansion of peritoneal B1b cells and elevated IgM levels, which correlate with itching and pathology. In contrast, Itch(-/-) interleukin-4-producing gammadelta T cells, even in the absence of alphabeta T cells, are associated with elevated levels of IgE and an inflammatory condition. These data indicate that disruption of an E3 ubiquitin ligase in alphabeta T cells can subvert a B-cell subpopulation, which normally functions to control particular microbial pathogens in a T-independent manner, to contribute to autoimmunity. In addition, disruption of Itch in innate gammadelta T cells can influence autoimmune pathology and might therefore require distinct therapeutic intervention.

Increased NKG2A found in cytotoxic natural killer subset in HIV-1 patients with advanced clinical status

OBJECTIVES

To investigate the expression of NKG2A on cytotoxic natural killer (NK) cells in HIV-1-infected patients.

DESIGN

A cross-sectional study was conducted to investigate the NKG2A expression on NK cell subsets among HIV-infected individuals at different clinical stages and HIV negative controls.

METHODS

113 HIV-1 infected and 43 uninfected individuals were enrolled in this study. The HIV-1 infected patients were further categorized into three groups, CD4 cell count > 500 cells/microl (n = 44), CD4 cell count of 200-500 cells/microl (n = 49) and CD4 cell count < 200 cells/microl (n = 20). Flow cytometry was used to determine the expression of NKG2A on NK cell subsets. A flow-based assay was employed to quantify the NK cytotoxicity.

RESULTS

Fewer cytotoxic NK cells and more dysfunctional NK cells were observed in patients with advanced clinical conditions. Higher NKG2A expression level in cytotoxic NK subset were found in later stages HIV infection, 25.6% in groups with CD4 cell count > 500 cells/microl, 40.9% in groups with CD4 cell count 200-500 cells/microl and 48.3% in groups with CD4 cell count < 200 cells/microl. Lower NK mediated cytotoxicity, which was associated with the decrease in cytotoxic NK cell number and higher NKG2A expression on cytotoxic NK subsets, was found in AIDS patients compared with patients at early stage of infection. A reverse association between the percentage of NKG2A positive cells in cytotoxic NK subset and CD4 cell count was observed in all HIV-1 infected groups.

CONCLUSION

Fewer cytotoxic NK cells and higher NKG2A expression in cytotoxic NK subset was found in patients in late stage HIV infection. Such a phenomenon may relate to the escape of HIV-1-infected CD4+ T cells from being attacked by NK cells.

Species-specific evolution of immune receptor tyrosine based activation motif-containing CEACAM1-related immune receptors in the dog

BACKGROUND

Although the impact of pathogens on the evolution of the mammalian immune system is still under debate, proteins, which both regulate immune responses and serve as cellular receptors for pathogens should be at the forefront of pathogen-driven host evolution. The CEA (carcinoembryonic antigen) gene family codes for such proteins and indeed shows tremendous species-specific variation between human and rodents. Since little is known about the CEA gene family in other lineages of placental mammals, we expected to gain new insights into the evolution of the rapidly diverging CEA family by analyzing the CEA family of the dog.

RESULTS

Here we describe the complete CEA gene family in the dog. We found that the gene coding for the ITIM-bearing immunoregulatory molecule CEACAM1 gave rise to a recent expansion of the canine CEA gene family by gene duplication, similar to that previously found in humans and mice. However, while the murine and human CEACAMs (carcinoembryonic antigen-related cell adhesion molecules) are predominantly secreted and GPI-anchored, respectively, in the dog, most of the CEACAMs represent ITAM-bearing transmembrane proteins. One of these proteins, CEACAM28, exhibits nearly complete sequence identity with the ligand-binding N domain of CEACAM1, but antagonizing signaling motifs in the cytoplasmic tail. Comparison of nonsynonymous and synonymous substitutions indicates that the CEACAM28 N domain is under the strongest purifying selection of all canine CEACAM1-related CEACAMs. In addition, CEACAM28 shows a similar expression pattern in resting immune cells and tissues as CEACAM1. However, upon activation CEACAM28 mRNA and CEACAM1 mRNA are differentially regulated.

CONCLUSION

Thus, CEACAM1 and CEACAM28 are the first paired immune receptors identified within the CEA gene family, which are expressed on T cells and are most likely involved in the fine-tuning of T cell responses. The direction of gene conversion accompanied by purifying selection and expression in immune cells suggests the possibility that CEACAM28 evolved in response to selective pressure imposed by species-specific pathogens.

NF-kappa B p50/p65 affects the frequency of Ly49 gene expression by NK cells

In mice, acquisition of Ly49 receptors characterizes one of the developmental stages of NK cells. We previously described a novel Ly49 promoter, Pro1, involved in Ly49 gene regulation in immature NK cells. Pro1 transcriptional activity requires a NF-kappaB binding site; however, only NF-kappaB/p50 binding to this element was observed. Cotransfection of NF-kappaB/p65 with Ly49g Pro1 in LNK cells induced a decrease in the transcriptional activity of the core promoter. Moreover, decreasing NF-kappaB/p65 protein expression by RNA interference increases Pro1 transcriptional activity. A high rate of NF-kappaB/p65 degradation in LNK cells correlates with Pro1 activity, since treatment with the proteasome inhibitor MG132 increased levels of NF-kappaB/p65 protein and decreased Pro1 activity. In addition, analysis of the Ly49 repertoire in NF-kappaB/p50 null mice reveals a decrease in the proportion of NK cells expressing a given Ly49 molecule. The defect in Ly49 expression is observed in the bone marrow and the spleen with a similar altered pattern of developmental stages in each tissue. The frequency of Ly49 expression in NF-kappaB/p52 null mice is slightly increased, indicating the specific role of NF-kappaB/p50 in Ly49 gene activation. These results suggest that NF-kappaB p50/p65 plays a major role in the initiation of Ly49 gene expression in NK cells.

The microglial networks of the brain and their role in neuronal network plasticity after lesion

Microglia are the resident inflammatory cells of the central nervous system (CNS) extending a network of processes in the CNS parenchyma. Following axon lesion to neurons, most extensively studied in motoneurons, there is a typical retrograde response at the cell body level, including the removal or 'stripping' of synapses from the perikaryon and dendrites of affected cells. Microglia have been attributed a main and active role in this process, although also an involvement of activated astrocytes has been suggested. The signaling pathways for this 'synaptic stripping' have so far been unknown, but recently some classical immune recognition molecules, the MHC class I molecules, have been shown to have a strong influence on the strength and pattern of the synapse elimination response. Since there is an expression of MHC class I in both neurons and glia, in particular microglia, as well as MHC class I related receptors in axons and microglia, there are good reasons to believe that classical immune recognition signaling between neurons and glia underlies part of the 'stripping' response. A role for microglia in an interplay with synapses based on this type of signaling is further exemplified by the fact that, in the absence of some MHC class I related receptors normally found on microglia during development, profound effects on synaptic function and biochemistry have been demonstrated. Such effects may be linked to the development of various disorders of the CNS, such as degenerative disease.

MHC class I expression and synaptic plasticity after nerve lesion

An axon lesion to a bulbar or spinal motoneuron is followed by a typical retrograde response at the cell body level, including the removal or 'stripping' of synapses from the perikaryon and dendrites of affected cells. Both activated microglia and astrocytes have been attributed roles in this process. The signalling pathways for this 'synaptic stripping' have so far been unknown, but recently a classical set of immune recognition molecules, the MHC class I molecules, have been shown to have a strong influence on the strength and pattern of the synapse elimination response. Thus, when MHC class I signalling is severely impaired in mice lacking the MHC class I subunit beta2-microglobulin (beta2m) and transporter associated with antigen processing 1 (TAP 1) genes, both of which are necessary for surface expression of MHC class I, there is a stronger elimination of synapses from injured neurons, with the surplus elimination directed towards clusters of putatively inhibitory synapses. Moreover, the regenerative capacity of motoneurons in such mice is lower than in wild-type animals. The expression of MHC class I, as well as MHC class I-related receptors in both neurons and glia, lend support to a hypothesis that classical immune recognition signalling between neurons and glia underlie part of the 'stripping' response.

Identification of CLEC12B, an inhibitory receptor on myeloid cells

Activation of immune cells has to be tightly controlled to prevent detrimental hyperactivation. In this regulatory process molecules of the C-type lectin-like family play a central role. Here we describe a new member of this family, CLEC12B. The extracellular domain of CLEC12B shows considerable homology to the activating natural killer cell receptor NKG2D, but unlike NKG2D, CLEC12B contains an immunoreceptor tyrosine-based inhibition motif in its intracellular domain. Despite the homology, CLEC12B does not appear to bind NKG2D ligands and therefore does not represent the inhibitory counterpart of NKG2D. However, CLEC12B has the ability to counteract NKG2D-mediated signaling, and we show that this function is dependent on the immunoreceptor tyrosine-based inhibition motif and the recruitment of the phosphatases SHP-1 and SHP-2. Using monoclonal anti-CLEC12B antibodies we found de novo expression of this receptor on in vitro generated human macrophages and on the human myelo-monocytic cell line U937 upon phorbol 12-myristate 13-acetate treatment, suggesting that this receptor plays a role in myeloid cell function.

Siglec-15: an immune system Siglec conserved throughout vertebrate evolution

Siglecs are vertebrate cell-surface receptors that recognize sialylated glycans. Here we have identified and characterized a novel Siglec, named Siglec-15. Siglec-15 is a type-I transmembrane protein consisting of: (i) two immunoglobulin (Ig)-like domains, (ii) a transmembrane domain containing a lysine residue, and (iii) a short cytoplasmic tail. Siglec-15 is expressed on macrophages and/or dendritic cells of human spleen and lymph nodes. We show that the extracellular domain of Siglec-15 preferentially recognizes the Neu5Acalpha2-6GalNAcalpha- structure. Siglec-15 associates with the activating adaptor proteins DNAX activation protein (DAP)12 and DAP10 via its lysine residue in the transmembrane domain, implying that it functions as an activating signaling molecule. Siglec-15 is the second human Siglec identified to have an activating signaling potential; unlike Siglec-14, however, it does not have an inhibitory counterpart. Orthologs of Siglec-15 are present not only in mammals but also in other branches of vertebrates; in contrast, no other known Siglec expressed in the immune system has been conserved throughout vertebrate evolution. Thus, Siglec-15 probably plays a conserved, regulatory role in the immune system of vertebrates.

Induction of CD16+ CD56bright NK cells with antitumour cytotoxicity not only from CD16- CD56bright NK Cells but also from CD16- CD56dim NK cells

The aim of this study was to examine the effect of cytokines on different subsets of NK cells, while especially focusing on CD16(-) CD56(dim) cells and CD16(-) CD56(bright) cells. When human peripheral blood mononuclear cells (PBMC) were cultured with a combination of IL-2, IL-12 and IL-15 for several days, a minor population of CD56(bright) NK cells expanded up to 15%, and also showed potent cytotoxicities against various cancer cells. Sorting experiments revealed that unconventional CD16(-) CD56(+) NK cells (CD16(-) CD56(dim) NK cells and CD16(-) CD56(bright) NK cells, both of which are less than 1% in PBMC) much more vigorously proliferated after cytokine stimulation, whereas predominant CD16(+) CD56(dim) NK cells proliferated poorly. In addition, many of the resting CD16(-) CD56(bright) NK cells developed into CD16(+) CD56(bright) NK cells, and CD16(-) CD56(dim) NK cells developed into CD16(-) CD56(bright) NK cells and also further into CD16(+) CD56(bright) NK cells by the cytokines. CSFE label experiments further substantiated the proliferation capacity of each subset and the developmental process of CD16(+) CD56(bright) NK cells. Both CD16(-) CD56(dim) NK cells and CD16(-) CD56(bright) NK cells produced large amounts of IFN-gamma and Fas-ligands. The CD16(+) CD56(bright) NK cells showed strong cytotoxicities against not only MHC class I (-) but also MHC class I (+) tumours regardless of their expression of CD94/NKG2A presumably because they expressed NKG2D as well as natural cytotoxicity receptors. The proliferation of CD16(+) CD56(bright) NK cells was also induced when PBMC were stimulated with penicillin-treated Streptococcus pyogenes, thus suggesting their role in tumour immunity and bacterial infections.

Non-T Cell Activation Linker (NTAL) Negatively Regulates TREM-1/DAP12-Induced Inflammatory Cytokine Production in Myeloid Cells

The engagement of triggering receptor expressed on myeloid cells 1 (TREM-1) on macrophages and neutrophils leads to TNF-alpha and IL-8 production and enhances inflammatory responses to microbial products. For signal transduction, TREM-1 couples to the ITAM-containing adapter DNAX activation protein of 12 kDa (DAP12). In general, ITAM-mediated signals lead to cell activation, although DAP12 was recently implicated in inhibitory signaling in mouse macrophages and dendritic cells. To date, signals downstream of the TREM-1 and DAP12 complex in myeloid cells are poorly defined. By analyzing receptor-induced tyrosine phosphorylation patterns, we discovered that the ligation of TREM-1 leads to tyrosine phosphorylation of the non-T cell activation linker (NTAL; also called linker of activation in B cells or LAB) in a myelomonocytic cell line and primary human granulocytes. Using RNA interference to decrease the expression levels of NTAL, we demonstrate that in NTAL knockdown cell lines the phosphorylation of ERK1/2 is enhanced. In addition, low levels of NTAL are correlated with decreased and delayed mobilization of Ca(2+) after TREM-1 triggering. Most importantly, we demonstrate that NTAL acts as a negative regulator of TNF-alpha and IL-8 production after stimulation via TREM-1. Our results show that activation signals delivered via DAP12 can be counterbalanced by the adaptor NTAL, identifying NTAL as gatekeeper of TREM-1/DAP12-induced signaling in myeloid cells.

Allogeneic hematopoietic transplantation and natural killer cell recognition of missing self

Although the optimal donor for allogeneic hematopoietic stem cell transplantation (HSCT) is a human leukocyte antigen-matched sibling, 75% of patients do not have a match, and alternatives are matched unrelated volunteers, unrelated umbilical cord blood units, and full-haplotype-mismatched family members. To cure leukemia, allogeneic HSCT relies on donor T cells in the allograft, which promote engraftment, eradicate malignant cells, and reconstitute immunity. Here, we focus on the open issues of rejection, graft-versus-host disease (GVHD), and infections and the benefits of natural killer (NK) cell alloreactivity and its underlying mechanisms. Donor-versus-recipient NK cell alloreactivity derives from a mismatch between inhibitory receptors for self-major histocompatibility complex (MHC) class I molecules on donor NK clones and the MHC class I ligands on recipient cells. These NK clones sense the missing expression of the self-MHC class I allele on the allogeneic targets and mediate alloreactions. HSCT from 'NK alloreactive' donors controls acute myeloid relapse without causing GVHD. We review the translation of NK cell recognition of missing self into the clinical practice of allogeneic hematopoietic transplantation and discuss how it has opened innovative perspectives in the cure of leukemia.

Involvement of the fractalkine pathway in the pathogenesis of childhood hemolytic uremic syndrome

Thrombotic microangiopathy and acute renal failure are cardinal features of postdiarrheal hemolytic uremic syndrome (HUS). These conditions are related to endothelial and epithelial cell damage induced by Shiga toxin (Stx) through the interaction with its globotriaosyl ceramide receptor. However, inflammatory processes contribute to the pathogenesis of HUS by sensitizing cells to Stx fractalkine (FKN), a CX(3)C transmembrane chemokine expressed on epithelial and endothelial cells upon activation, is involved in the selective migration and adhesion of specific leukocyte subsets to tissues. Here, we demonstrated a selective depletion of circulating mononuclear leukocytes expressing the receptor for FKN (CX(3)CR1) in patients with HUS. We found a unique phenotype in children with HUS distinct from that seen in healthy, uremic, or infected controls, in which monocytes lost CX(3)CR1, down-modulated CD62L, and increased CD16. In addition, the CD56(dim) natural killer (NK) subpopulation was decreased, leading to an altered peripheral CD56(dim)/CD56(bright) ratio from 10.0 to 4.5. It is noteworthy that a negative correlation existed between the percentage of circulating CX(3)CR1(+) leukocytes and the severity of renal failure. Finally, CX(3)CR1(+) leukocytes were observed in renal biopsies from patients with HUS. We suggest that the interaction of CX(3)CR1(+) cells with FKN present on activated endothelial cells may contribute to renal injury in HUS.

The role of cystatins in cells of the immune system

The cystatins constitute a large group of evolutionary related proteins with diverse biological activities. Initially, they were characterized as inhibitors of lysosomal cysteine proteases - cathepsins. Cathepsins are involved in processing and presentation of antigens, as well as several pathological conditions such as inflammation and cancer. Recently, alternative functions of cystatins have been proposed: they also induce tumour necrosis factor and interleukin 10 synthesis and stimulate nitric oxide production. The aim of the present review was the analysis of data on cystatins from NCBI GEO database and the literature, and obtained in microarray and serial analysis of gene expression (SAGE) experiments. The expression of cystatins A, B, C, and F in macrophages, dendritic cells and natural killer cells of the immune system, during differentiation and activation is discussed.

Discovery of Siglec-14, a novel sialic acid receptor undergoing concerted evolution with Siglec-5 in primates

Immune receptors that show high mutual sequence similarity and have antagonizing signaling properties are called paired receptors, and are believed to fine-tune immune responses. Siglecs are sialic acid-recognizing receptors of the immunoglobulin (Ig) superfamily expressed on immune cells. Human Siglec-5, encoded by SIGLEC5 gene, has four extracellular Ig-like domains and a cytosolic inhibitory motif. We discovered human Siglec-14 with three Ig-like domains, encoded by the SIGLEC14 gene, adjacent to SIGLEC5. Human Siglec-14 has almost complete sequence identity with human Siglec-5 at the first two Ig-like domains, shows a glycan binding preference similar to that of human Siglec-5, and associates with the activating adapter protein DAP12. Thus, Siglec-14 and Siglec-5 appear to be the first glycan binding paired receptors. Near-complete sequence identity of the amino-terminal part of human Siglec-14 and Siglec-5 indicates partial gene conversion between SIGLEC14 and SIGLEC5. Remarkably, SIGLEC14 and SIGLEC5 in other primates also show evidence of gene conversions within each lineage. Evidently, balancing the interactions between Siglec-14, Siglec-5 and their common ligand(s) had selective advantage during the course of evolution. The "essential arginine" critical for sialic acid recognition in both Siglec-14 and Siglec-5 is present in humans but mutated in almost all great ape alleles.