Inhibition of Immune Responses by ITAM-Bearing Receptors

An immunomodulatory role of Fc receptor γ chain independent of FcγR ligation by IgG in acute neuroinflammation triggered by MPTP intoxication

Aberrant microglial activation is a prominent feature of neuroinflammation, which is implicated in the pathogenesis of neurological disorders. Fc receptor common γ-chain (FcRγ), one of the two immunoreceptor tyrosine-based activation motif-bearing adaptor proteins, is abundantly expressed in microglia. It couples with different receptors, such as receptors for the Fc portion of IgG. In this study, we observed increased FcRγ expression along with increased IgG-binding during acute neuroinflammation triggered by MPTP intoxication, where adaptive immune responses should not be involved. Notably, FcRγ was expressed not only in the cell membrane but also in the cytoplasm in the activated microglia. FcRγ deficiency exacerbated microglial activation, pro-inflammatory factor upregulation, nigral dopaminergic neuronal loss and motor deficits, implicating a beneficial role of FcRγ in this model. Blockade of Fcγ receptor ligation by IgG in mice by Endoglycosidase S treatment, a bacterial endo-β-N-acetylglucosaminidase cleaving specifically the Asn297-linked glycan of IgG, or by using the mice deficient in mature B cells (muMT) with IgG production defects, did not show similar phenotypes to those observed in FcRγ-deficient mice, indicating that the beneficial effect mediated by FcRγ did not depend on FcγR ligation by IgG. Further, FcRγ knockout aggravated the expression and activation of STAT1 in microglia, suggesting FcRγ modulated neuroinflammation by dampening STAT1 signaling. Collectively, these results revealed that FcRγ-associated receptors could function as negative regulators of neuroinflammation and dopaminergic neurodegeneration.

CD3ζ ITAMs enable ligand discrimination and antagonism by inhibiting TCR signaling in response to low-affinity peptides

The T cell antigen receptor (TCR) contains ten immunoreceptor tyrosine-based activation motif (ITAM) signaling sequences distributed within six CD3 subunits; however, the reason for such structural complexity and multiplicity is unclear. Here we evaluated the effect of inactivating the three CD3ζ chain ITAMs on TCR signaling and T cell effector responses using a conditional 'switch' mouse model. Unexpectedly, we found that T cells expressing TCRs containing inactivated (non-signaling) CD3ζ ITAMs (6F-CD3ζ) exhibited reduced ability to discriminate between low- and high-affinity ligands, resulting in enhanced signaling and cytokine responses to low-affinity ligands because of a previously undetected inhibitory function of CD3ζ ITAMs. Also, 6F-CD3ζ TCRs were refractory to antagonism, as predicted by a new in silico adaptive kinetic proofreading model that revises the role of ITAM multiplicity in TCR signaling. Finally, T cells expressing 6F-CD3ζ displayed enhanced cytolytic activity against solid tumors expressing low-affinity ligands, identifying a new counterintuitive approach to TCR-mediated cancer immunotherapy.

LFA-1 Activation in T-Cell Migration and Immunological Synapse Formation

Integrin LFA-1 plays a critical role in T-cell migration and in the formation of immunological synapses. LFA-1 functions through interacting with its ligands with differing affinities: low, intermediate, and high. Most prior research has studied how LFA-1 in the high-affinity state regulates the trafficking and functions of T cells. LFA-1 is also presented in the intermediate-affinity state on T cells, however, the signaling to activate LFA-1 to the intermediate-affinity state and the role of LFA-1 in this affinity state both remain largely elusive. This review briefly summarizes the activation and roles of LFA-1 with varied ligand-binding affinities in the regulation of T-cell migration and immunological synapse formation.

LMAN1 is a receptor for house dust mite allergens

Development of therapies with the potential to change the allergic asthmatic disease course will require the discovery of targets that play a central role during the initiation of an allergic response, such as those involved in the process of allergen recognition. We use a receptor glycocapture technique to screen for house dust mite (HDM) receptors and identify LMAN1 as a candidate. We verify the ability of LMAN1 to directly bind HDM allergens and demonstrate that LMAN1 is expressed on the surface of dendritic cells (DCs) and airway epithelial cells (AECs) in vivo. Overexpression of LMAN1 downregulates NF-κB signaling in response to inflammatory cytokines or HDM. HDM promotes binding of LMAN1 to the FcRγ and recruitment of SHP1. Last, peripheral DCs of asthmatic individuals show a significant reduction in the expression of LMAN1 compared with healthy controls. These findings have potential implications for the development of therapeutic interventions for atopic disease.

Engineering Surfaces with Immune Modulating Properties of Mucin Hydrogels

Hydrogels of cross-linked mucin glycoproteins (Muc-gel) have shown strong immune-modulating properties toward macrophages in vitro, which are translated in vivo by the dampening of the foreign body response to implantation in mice. Beyond mucin hydrogels, other biomaterials such as sensors, electrodes, and other long-term implants would also benefit from such immune-modulating properties. In this work, we aimed to transfer the bioactivity observed for three-dimensional Muc-gels to the surface of two model materials by immobilizing mucin into thin films (Muc-film) using covalent layer-by-layer assembly. We tested how the surface immobilization of mucins affects macrophage responses compared to Muc-gels. We showed that Muc-films on soft polyacrylamide gels mimic Muc-gel in their modulation of macrophage responses with activated gene expression of inflammatory cytokines on day 1 and then dampening them on day 3. Also, the markers of polarized macrophages, M1 and M2, were expressed at the same level for macrophages on Muc-film-coated soft polyacrylamide gels and Muc-gel. In contrast, Muc-film-coated hard polystyrene led to a different macrophage response compared to Muc-gel, having no activated expression of inflammatory cytokines and a different M1 marker expression. This suggested that the substrate mechanical properties and mucin molecular configuration determined by substrate-mucin interactions affect mucin immune-modulating properties. We conclude that mucin immune-modulating properties can be transferred to materials by mucin surface immobilization but will be dependent on the substrate chemical and mechanical properties.

Spatial organization of FcγR and TLR2/1 on phagosome membranes differentially regulates their synergistic and inhibitory receptor crosstalk

Many innate immune receptors function collaboratively to detect and elicit immune responses to pathogens, but the physical mechanisms that govern the interaction and signaling crosstalk between the receptors are unclear. In this study, we report that the signaling crosstalk between Fc gamma receptor (FcγR) and Toll-like receptor (TLR)2/1 can be overall synergistic or inhibitory depending on the spatial proximity between the receptor pair on phagosome membranes. Using a geometric manipulation strategy, we physically altered the spatial distribution of FcγR and TLR2 on single phagosomes. We demonstrate that the signaling synergy between FcγR and TLR2/1 depends on the proximity of the receptors and decreases as spatial separation between them increases. However, the inhibitory effect from FcγRIIb on TLR2-dependent signaling is always present and independent of receptor proximity. The overall cell responses are an integration from these two mechanisms. This study presents quantitative evidence that the nanoscale proximity between FcγR and TLR2 functions as a key regulatory mechanism in their signaling crosstalk.

Toll receptors remodel epithelia by directing planar-polarized Src and PI3K activity

Toll-like receptors are essential for animal development and survival, with conserved roles in innate immunity, tissue patterning, and cell behavior. The mechanisms by which Toll receptors signal to the nucleus are well characterized, but how Toll receptors generate rapid, localized signals at the cell membrane to produce acute changes in cell polarity and behavior is not known. We show that Drosophila Toll receptors direct epithelial convergent extension by inducing planar-polarized patterns of Src and PI3-kinase (PI3K) activity. Toll receptors target Src activity to specific sites at the membrane, and Src recruits PI3K to the Toll-2 complex through tyrosine phosphorylation of the Toll-2 cytoplasmic domain. Reducing Src or PI3K activity disrupts planar-polarized myosin assembly, cell intercalation, and convergent extension, whereas constitutive Src activity promotes ectopic PI3K and myosin cortical localization. These results demonstrate that Toll receptors direct cell polarity and behavior by locally mobilizing Src and PI3K activity.

Adoptive T cell therapy: Boosting the immune system to fight cancer

Cellular therapies have shown increasing promise as a cancer treatment. Encouraging results against hematologic malignancies are paving the way to move into solid tumors. In this review, we will focus on T-cell therapies starting from tumor infiltrating lymphocytes (TILs) to optimized T-cell receptor-modified (TCR) cells and chimeric antigen receptor-modified T cells (CAR-Ts). We will discuss the positive preclinical and clinical findings of these approaches, along with some of the persisting barriers that need to be overcome to improve outcomes.

A Fish Leukocyte Immune-Type Receptor Uses a Novel Intracytoplasmic Tail Networking Mechanism to Cross-Inhibit the Phagocytic Response

Channel catfish (Ictalurus punctatus) leukocyte immune-type receptors (IpLITRs) are a family of immunoregulatory proteins shown to regulate several innate immune cell effector responses, including phagocytosis. The precise mechanisms of IpLITR-mediated regulation of the phagocytic process are not entirely understood, but we have previously shown that different IpLITR-types use classical as well as novel pathways for controlling immune cell-mediated target engulfment. To date, all functional assessments of IpLITR-mediated regulatory actions have focused on the independent characterization of select IpLITR-types in transfected cells. As members of the immunoglobulin superfamily, many IpLITRs share similar extracellular Ig-like domains, thus it is possible that various IpLITR actions are influenced by cross-talk mechanisms between different IpLITR-types; analogous to the paired innate receptor paradigm in mammals. Here, we describe in detail the co-expression of different IpLITR-types in the human embryonic AD293 cell line and examination of their receptor cross-talk mechanisms during the regulation of the phagocytic response using imaging flow cytometry, confocal microscopy, and immunoprecipitation protocols. Overall, our data provides interesting new insights into the integrated control of phagocytosis via the antagonistic networking of independent IpLITR-types that requires the selective recruitment of inhibitory signaling molecules for the initiation and sustained cross-inhibition of phagocytosis.

Immunoregulatory Property of C-Type Lectin-Like Receptors in Fibrosing Interstitial Lung Diseases

The innate immune system identifies exogenous threats or endogenous stress through germline-encoded receptors called pattern recognition receptors (PRRs) that initiate consecutive downstream signaling pathways to control immune responses. However, the contribution of the immune system and inflammation to fibrosing interstitial lung diseases (ILD) remains poorly understood. Immunoreceptor tyrosine-based motif-bearing C-type lectin-like receptors (CTLRs) may interact with various immune cells during tissue injury and wound repair processes. Dectin-1 is a CTLR with dominant mechanisms manifested through its intracellular signaling cascades, which regulate fibrosis-promoting properties through gene transcription and cytokine activation. Additionally, immune impairment in ILD facilitates microbiome colonization; hence, Dectin-1 is the master protector in host pulmonary defense against fungal invasion. Recent progress in determining the signaling pathways that control the balance of fibrosis has implicated immunoreceptor tyrosine-based motif-bearing CTLRs as being involved, either directly or indirectly, in the pathogenesis of fibrosing ILD.

Targeting Cancer Stem Cells by Genetically Engineered Chimeric Antigen Receptor T Cells

The term cancer stem cell (CSC) starts 25 years ago with the evidence that CSC is a subpopulation of tumor cells that have renewal ability and can differentiate into several distinct linages. Therefore, CSCs play crucial role in the initiation and the maintenance of cancer. Moreover, it has been proposed throughout several studies that CSCs are behind the failure of the conventional chemo-/radiotherapy as well as cancer recurrence due to their ability to resist the therapy and their ability to re-regenerate. Thus, the need for targeted therapy to eliminate CSCs is crucial; for that reason, chimeric antigen receptor (CAR) T cells has currently been in use with high rate of success in leukemia and, to some degree, in patients with solid tumors. This review outlines the most common CSC populations and their common markers, in particular CD133, CD90, EpCAM, CD44, ALDH, and EGFRVIII, the interaction between CSCs and the immune system, CAR T cell genetic engineering and signaling, CAR T cells in targeting CSCs, and the barriers in using CAR T cells as immunotherapy to treat solid cancers.

Siglecs that Associate with DAP12

Siglecs are a family of transmembrane receptor-like glycan-recognition proteins expressed primarily on leukocytes. Majority of Siglecs have an intracellular sequence motif called immunoreceptor tyrosine-based inhibitory motif (ITIM) and associate with Src homology region 2 domain-containing tyrosine phosphatase-1 (SHP-1), and negatively regulate tyrosine phosphorylation-mediated intracellular signaling events. On the other hand, some Siglecs have a positively charged amino acid residue in the transmembrane domain and associate with DNAX activation protein of 12 kDa (DAP12), which in turn recruits spleen tyrosine kinase (Syk). These DAP12-associated Siglecs play diverse functions. For example, Siglec-15 is conserved throughout vertebrate evolution and plays a role in bone homeostasis by regulating osteoclast development and function. Human Siglec-14 and -16 have inhibitory counterparts (Siglec-5 and -11, respectively), which show extremely high sequence similarity with them at the extracellular domain but interact with SHP-1. The DAP12-associated Siglec in such "paired receptor" configuration counteracts the pathogens that exploit the inhibitory counterpart. Polymorphisms (mutations) that render DAP12-associated inactive Siglecs are found in humans, and some of these appear to be associated with sensitivity or resistance of human hosts to bacterially induced conditions. Studies of mouse Siglec-H have revealed complex and intriguing functions it plays in regulating adaptive immunity. Many questions remain unanswered, and further molecular and genetic studies of DAP12-associated Siglecs will yield valuable insights with translational relevance.

Chronic Ly49H Receptor Engagement in vivo Decreases NK Cell Response to Stimulation Through ITAM-Dependent and Independent Pathways Both in vitro and in vivo

Natural killer (NK) cells play an important role in the innate immune response. The summation of activation and inhibitory signals delivered through cell surface membrane receptors determines NK cell function. However, the continuous engagement of an activating receptor on NK cells appears to render the cells hyporesponsive to stimulation through other unrelated activating receptors. The mechanism by which this takes place remains unclear. Herein we demonstrate that continuous in vivo engagement of the Ly49H receptor with its ligand, m157, results in Ly49H⁺ NK cells that are hyporesponsive to further stimulation by other ITAM-dependent and independent receptors, while Ly49H⁻ NK cells remain unaffected. The hyporesponsiveness of the NK cell correlates with the degree of Ly49H receptor downmodulation on its cell surface. We observe defects in calcium flux in the hyporesponsive NK cells following stimulation through the NK1.1 receptor. In addition, we observe differences in signaling molecules that play a role in calcium flux, including spleen tyrosine kinase (Syk) at baseline and phosphorylated phospholipase C gamma 2 (p-PLCγ2) at both baseline and following stimulation through NK1.1. We also demonstrate that various ITAM associated activation receptors, including Ly49H, remain associated with their respective adaptor molecules. With regard to in vivo NK cell function, we did not find differences in the formation of metastatic lung lesions following IV injection of B16 melanoma cells. However, we did observe defects in rejection of missing-self targets in vivo. The data suggest that continuous engagement of the Ly49H activating receptor on NK cells results in hyporesponsiveness of the NK cells to all of the ITAM-dependent and independent receptors we analyzed due to altered signaling pathways downstream of the receptor and adaptor molecule.

Unique-region phosphorylation targets LynA for rapid degradation, tuning its expression and signaling in myeloid cells

The activity of Src-family kinases (SFKs), which phosphorylate immunoreceptor tyrosine-based activation motifs (ITAMs), is a critical factor regulating myeloid-cell activation. We reported previously that the SFK LynA is uniquely susceptible to rapid ubiquitin-mediated degradation in macrophages, functioning as a rheostat regulating signaling (Freedman et al., 2015). We now report the mechanism by which LynA is preferentially targeted for degradation and how cell specificity is built into the LynA rheostat. Using genetic, biochemical, and quantitative phosphopeptide analyses, we found that the E3 ubiquitin ligase c-Cbl preferentially targets LynA via a phosphorylated tyrosine (Y32) in its unique region. This distinct mode of c-Cbl recognition depresses steady-state expression of LynA in macrophages derived from mice. Mast cells, however, express little c-Cbl and have correspondingly high LynA. Upon activation, mast-cell LynA is not rapidly degraded, and SFK-mediated signaling is amplified relative to macrophages. Cell-specific c-Cbl expression thus builds cell specificity into the LynA checkpoint.

Bacterial Clearance Is Enhanced by α2,3- and α2,6-Sialyllactose via Receptor-Mediated Endocytosis and Phagocytosis

Sialyllactose (SL) is a representative human milk oligosaccharide (HMO) of human breast milk. The roles of SL in infant brain development and immunity have been reported in previous studies.

Sialyllactose (SL) is a representative human milk oligosaccharide (HMO) of human breast milk. The roles of SL in infant brain development and immunity have been reported in previous studies. In this study, we identified the impact of SL on innate immunity. Our results showed that the administration of SL had significant efficacy on bacterial clearance in Pseudomonas aeruginosa K-infected mice. We also examined the role of SL in the human THP-1 macrophage-like cell line. SL effectively promoted receptor-mediated endocytosis and phagocytosis. Furthermore, SL accelerated the recruitment of Rac1 to the cell membrane, leading to the generation of reactive oxygen species for the elimination of phagocytosed bacteria. Our findings provide a new perspective on the role of SL in breast milk and suggest its application as a therapeutic agent to treat bacterial and viral infections.

NK cell receptor NKG2D sets activation threshold for the NCR1 receptor early in NK cell development

The activation of natural killer (NK) cells depends on a change in the balance of signals from inhibitory and activating receptors. The activation threshold values of NK cells are thought to be set by engagement of inhibitory receptors during development. Here, we found that the activating receptor NKG2D specifically set the activation threshold for the activating receptor NCR1 through a process that required the adaptor DAP12. As a result, NKGD2-deficient (Klrk1^-/-) mice controlled tumors and cytomegalovirus infection better than wild-type controls through the NCR1-induced production of the cytokine IFN-γ. Expression of NKG2D before the immature NK cell stage increased expression of the adaptor CD3ζ. Reduced expression of CD3ζ in Klrk1^-/- mice was associated with enhanced signal transduction through NCR1 and CD3ζ-deficiency resulted in hyper-responsiveness to stimulation via NCR1. Thus, an activating receptor developmentally set the activity of another activating receptor on NK cells and determined NK cell-reactivity to cellular threats.

Src family kinase tyrosine phosphorylates Toll-like receptor 4 to dissociate MyD88 and Mal/Tirap, suppressing LPS-induced inflammatory responses

Src family kinases (SFKs) are a family of protein tyrosine kinases containing nine members: Src, Lyn, Fgr, Hck, Lck, Fyn, Blk, Yes, and Ylk. Although SFK activation is a major immediate signaling event in LPS/Toll-like receptor 4 (TLR4) signaling, its precise role has remained elusive due to various contradictory results obtained from a certain SFK member-deficient mice or cells. The observed inconsistencies may be due to the compensation or redundancy by other SFKs upon a SFK deficiency. The chemical rescuing approach was suggested to induce temporal and precise SFK activation in living cells, thereby limiting the chance of cellular adaption to a SFK-deficient condition. Using the rescuing approach, we demonstrate that restoring SFK activity not only induces tyrosine phosphorylation of TLR4, but also inhibits LPS-induced NFκB and JNK1/2 activation and consequently suppresses LPS-induced cytokine production. TLR4 normally recruits TIR domain-containing adaptors in response to LPS, however, temporally restored SFK activation disrupts the LPS-induced association of MyD88 and Mal/Tirap with TLR4. Additionally, using kinase-dead SFK-Lyn (Y397/508F) and constitutively active SFK-Lyn (Y508F), we found that the kinase-dead SFK inhibits TLR4 tyrosine phosphorylation with reduced binding affinity to TLR4, while the kinase-active SFK strongly binds to TLR4 and promotes TLR4 tyrosine phosphorylation, suggesting that SFK kinase activity is required for TLR4 tyrosine phosphorylation and TLR4-SFK interaction. Together, our results demonstrate that SFK activation induces TLR4 tyrosine phosphorylation, consequently dissociating MyD88 and Mal/Tirap from TLR4 and inhibiting LPS-induced inflammatory responses, suggesting a negative feedback loop regulated by SFK-induced tyrosine phosphorylation in TLR4.

Lyn and Fyn function as molecular switches that control immunoreceptors to direct homeostasis or inflammation

Immunoreceptors can transduce either inhibitory or activatory signals depending on ligand avidity and phosphorylation status, which is modulated by the protein kinases Lyn and Fyn. Here we show that Lyn and Fyn control immune receptor signaling status. SHP-1 tyrosine 536 phosphorylation by Lyn activates the phosphatase promoting inhibitory signaling through the immunoreceptor. By contrast, Fyn-dependent phosphorylation of SHP-1 serine 591 inactivates the phosphatase, enabling activatory immunoreceptor signaling. These SHP-1 signatures are relevant in vivo, as Lyn deficiency exacerbates nephritis and arthritis in mice, whereas Fyn deficiency is protective. Similarly, Fyn-activating signature is detected in patients with lupus nephritis, underlining the importance of this Lyn-Fyn balance. These data show how receptors discriminate negative from positive signals that respectively result in homeostatic or inflammatory conditions.Src-family kinases Fyn and Lyn are signaling components downstream of ITAM-bearing antigen receptors. Here the authors show that by phosphorylating SHP-1 at different residues, Lyn and Fyn can have opposing regulatory effects on ITAM receptors.

TREM2-Ligand Interactions in Health and Disease

The protein triggering receptor expressed on myeloid cells-2 (TREM2) is an immunomodulatory receptor with a central role in myeloid cell activation and survival. In recent years, the importance of TREM2 has been highlighted by the identification of coding variants that increase risk for Alzheimer's disease and other neurodegenerative diseases. Animal studies have further shown the importance of TREM2 in neurodegenerative and other inflammatory disease models including chronic obstructive pulmonary disease, multiple sclerosis, and stroke. A mechanistic understanding of TREM2 function remains elusive, however, due in part to the absence of conclusive information regarding the identity of endogenous TREM2 ligands. While many TREM2 ligands have been proposed, their physiological role and mechanism of engagement remain to be determined. In this review, we highlight the suggested roles of TREM2 in these diseases and the recent advances in our understanding of TREM2 and discuss putative TREM2-ligand interactions and their potential roles in signaling during health and disease. We develop a model based on the TREM2 structure to explain how different TREM2 ligands might interact with the receptor and how disease risk variants may alter ligand interactions. Finally, we propose future experimental directions to establish the role and importance of these different interactions on TREM2 function.

Cross Talk between Inhibitory Immunoreceptor Tyrosine-Based Activation Motif-Signaling and Toll-Like Receptor Pathways in Macrophages and Dendritic Cells

The innate immune cells sense microbial infection and self-ligands by pathogen recognition receptors (PRRs), such as toll-like receptors (TLRs) and regulatory receptors (RRs), associated with immunoreceptor tyrosine-based activation motif (ITAM). Rapid activation and concerted action of PRRs signaling and feedback inhibitory mechanisms must be engaged to ensure the host defense functions and to prevent cytotoxicity associated with excessive activation. ITAM-associated RRs can generate stimulatory or, paradoxically, inhibitory signals. The network of ITAM-associated RR, together with TLR-signaling pathways, are responsible for immunogenic or tolerogenic responses of macrophages and dendritic cells to their microenvironment. In macrophages, TLR4 signaling is inhibited by low-avidity ligation of ITAM-associated receptors, while high-avidity ligation of ITAM-associated receptors results in potentiation of TLR4 signaling together with resistance to extracellular cytokine microenvironment signals. In contrast to macrophages, TLR7/9 signaling in plasmacytoid DCs (pDCs) is inhibited by high-avidity ligation of ITAM-associated RR, while low-avidity ligation does not show any effect. Surprisingly, interference of ITAM-associated receptor signaling with TLR pathways has not been reported in conventional dendritic cells. Here, we present an overview of molecular mechanisms acting at the crossroads of TLR and ITAM-signaling pathways and address the question of how the high-avidity engagement of the ITAM-associated receptors in pDCs inhibits TLR7/9 signaling. Cellular context and spatiotemporal engagement of ITAM- and TLR-signaling pathways are responsible for different outcomes of macrophage versus pDC activation. While the cross-regulation of cytokine and TLR signaling, together with antigen presentation, are the principal functions of ITAM-associated RR in macrophages, the major role of these receptors in pDCs seems to be related to inhibition of cytokine production and reestablishment of a tolerogenic state following pDC activation. Pharmacologic targeting of TLR and ITAM signaling could be an attractive new therapeutic approach for treatment of chronic infections, cancer, and autoimmune and inflammatory diseases related to pDCs.

Host Immune Responses in HIV-1 Infection: The Emerging Pathogenic Role of Siglecs and Their Clinical Correlates

A better understanding of the mechanisms employed by HIV-1 to escape immune responses still represents one of the major tasks required for the development of novel therapeutic approaches targeting a disease still lacking a definitive cure. Host innate immune responses against HIV-1 are key in the early phases of the infection as they could prevent the development and the establishment of two hallmarks of the infection: chronic inflammation and viral reservoirs. Sialic acid-binding immunoglobulin-like lectins (Siglecs) belong to a family of transmembrane proteins able to dampen host immune responses and set appropriate immune activation thresholds upon ligation with their natural ligands, the sialylated carbohydrates. This immune-modulatory function is also targeted by many pathogens that have evolved to express sialic acids on their surface in order to escape host immune responses. HIV-1 envelope glycoprotein 120 (gp120) is extensively covered by carbohydrates playing active roles in life cycle of the virus. Indeed, besides forming a protecting shield from antibody recognition, this coat of N-linked glycans interferes with the folding of viral glycoproteins and enhances virus infectivity. In particular, the sialic acid residues present on gp120 can bind Siglec-7 on natural killer and monocytes/macrophages and Siglec-1 on monocytes/macrophages and dendritic cells. The interactions between these two members of the Siglec family and the sialylated glycans present on HIV-1 envelope either induce or increase HIV-1 entry in conventional and unconventional target cells, thus contributing to viral dissemination and disease progression. In this review, we address the impact of Siglecs in the pathogenesis of HIV-1 infection and discuss how they could be employed as clinic and therapeutic targets.

Cross-talk between pathogen recognizing Toll-like receptors and immunoglobulin Fc receptors in immunity

The individual role of pathogen-binding Toll-like receptors (TLRs) and antibody-binding Fc receptors (FcRs) during pathogenic infections has been studied extensively. However, combined activation of these different receptor classes has received little attention, even though they are triggered simultaneously when immune cells bind antibody-opsonized pathogens. In the last few years, it has become evident that joined activation of TLRs and FcRs substantially tailors inflammatory immune responses, which is an efficient and controlled mechanism of the host to act upon invading pathogens. In this review, we discuss the mechanisms of cross-talk between different TLRs and FcRs and the resulting inflammatory immune responses. Furthermore, we propose how chronic activation via this cross-talk might be detrimental in inflammatory (auto) immune diseases. We conclude with the potential exploitation of the interplay between TLRs and FcRs for monoclonal antibody therapy to target tumors. Future interests in this field of research include establishing a more detailed and mechanistic understanding of the mode of action of TLR and FcR cross-talk and exploration of its physiological importance in health and disease. This may furthermore open up novel therapeutic options for intervention in inflammatory diseases or cancer.

Siglec-H protects from virus-triggered severe systemic autoimmunity

Siglec-H is a key negative regulator of the type I interferon pathway, reducing the incidence of autoimmunity after viral infection.

It is controversial whether virus infections can contribute to the development of autoimmune diseases. Type I interferons (IFNs) are critical antiviral cytokines during virus infections and have also been implicated in the pathogenesis of systemic lupus erythematosus. Type I IFN is mainly produced by plasmacytoid dendritic cells (pDCs). The secretion of type I IFN of pDCs is modulated by Siglec-H, a DAP12-associated receptor on pDCs. In this study, we show that Siglec-H–deficient pDCs produce more of the type I IFN, IFN-α, in vitro and that Siglec-H knockout (KO) mice produce more IFN-α after murine cytomegalovirus (mCMV) infection in vivo. This did not impact control of viral replication. Remarkably, several weeks after a single mCMV infection, Siglec-H KO mice developed a severe form of systemic lupus–like autoimmune disease with strong kidney nephritis. In contrast, uninfected aging Siglec-H KO mice developed a mild form of systemic autoimmunity. The induction of systemic autoimmune disease after virus infection in Siglec-H KO mice was accompanied by a type I IFN signature and fully dependent on type I IFN signaling. These results show that Siglec-H normally serves as a modulator of type I IFN responses after infection with a persistent virus and thereby prevents induction of autoimmune disease.

DAP12 Inhibits Pulmonary Immune Responses to Cryptococcus neoformans

Cryptococcus neoformans is an opportunistic fungal pathogen that is inhaled into the lungs and can lead to life-threatening meningoencephalitis in immunocompromised patients. Currently, the molecular mechanisms that regulate the mammalian immune response to respiratory cryptococcal challenge remain poorly defined. DAP12, a signaling adapter for multiple pattern recognition receptors in myeloid and natural killer (NK) cells, has been shown to play both activating and inhibitory roles during lung infections by different bacteria and fungi. In this study, we demonstrate that DAP12 plays an important inhibitory role in the immune response to C. neoformans Infectious outcomes in DAP12(-/-) mice, including survival and lung fungal burden, are significantly improved compared to those in C57BL/6 wild-type (WT) mice. We find that eosinophils and macrophages are decreased while NK cells are increased in the lungs of infected DAP12(-/-) mice. In contrast to WT NK cells, DAP12(-/-) NK cells are able to repress C. neoformans growth in vitro Additionally, DAP12(-/-) macrophages are more highly activated than WT macrophages, with increased production of tumor necrosis factor (TNF) and CCL5/RANTES and more efficient uptake and killing of C. neoformans These findings suggest that DAP12 acts as a brake on the pulmonary immune response to C. neoformans by promoting pulmonary eosinophilia and by inhibiting the activation and antifungal activities of effector cells, including NK cells and macrophages.

A Comprehensive Immunoreceptor Phosphotyrosine-based Signaling Network Revealed by Reciprocal Protein-Peptide Array Screening

Cells of the immune system communicate with their environment through immunoreceptors. These receptors often harbor intracellular tyrosine residues, which, when phosphorylated upon receptor activation, serve as docking sites to recruit downstream signaling proteins containing the Src Homology 2 (SH2) domain. A systematic investigation of interactions between the SH2 domain and the immunoreceptor tyrosine-based regulatory motifs (ITRM), including inhibitory (ITIM), activating (ITAM), or switching (ITSM) motifs, is critical for understanding cellular signal transduction and immune function. Using the B cell inhibitory receptor CD22 as an example, we developed an approach that combines reciprocal or bidirectional phosphopeptide and SH2 domain array screens with in-solution binding assays to identify a comprehensive SH2-CD22 interaction network. Extending this approach to 194 human ITRM sequences and 78 SH2 domains led to the identification of a high-confidence immunoreceptor interactome containing 1137 binary interactions. Besides recapitulating many previously reported interactions, our study uncovered numerous novel interactions. The resulting ITRM-SH2 interactome not only helped to fill many gaps in the immune signaling network, it also allowed us to associate different SH2 domains to distinct immune functions. Detailed analysis of the NK cell ITRM-mediated interactions led to the identification of a network nucleated by the Vav3 and Fyn SH2 domains. We showed further that these SH2 domains have distinct functions in cytotoxicity. The bidirectional protein-peptide array approach described herein may be applied to the numerous other peptide-binding modules to identify potential protein-protein interactions in a systematic and reliable manner.

Regulation of human mesenchymal stem cell differentiation by TREM-2

Activation of the triggering receptor expressed on myeloid cells 2 (TREM-2) regulates myeloid cell function in vitro. However, the failure to detect TREM-2 protein expression in vivo has hampered studies on immunological and other physiological TREM-2 functions. This study demonstrates that TREM-2 is expressed by human mesenchymal stem cells (h-MSCs) and responds to the toll-like receptor (TLR) ligand lipopolysaccharide (LPS). Knockdown of TREM-2 in h-MSCs using a small interfering RNA (siRNA) reduced the expression levels of TLR2, TLR4, and TLR6, inhibited osteogenic, chondrogenic, and adipogenic differentiation under specific induction conditions, and enhanced LPS-evoked inflammatory cytokine production. Thus, activation of TREM-2 may restrain h-MSC immune activation and promote differentiation for tissue repair.

Dendritic cell-bound IgE functions to restrain allergic inflammation at mucosal sites

Antigen-mediated cross-linking of Immunoglobulin E (IgE) bound to mast cells/basophils via FcɛRI, the high affinity IgE Fc-receptor, is a well-known trigger of allergy. In humans, but not mice, dendritic cells (DCs) also express FcɛRI that is constitutively occupied with IgE. In contrast to mast cells/basophils, the consequences of IgE/FcɛRI signals for DC function remain poorly understood. We show that humanized mice that express FcɛRI on DCs carry IgE like non-allergic humans and do not develop spontaneous allergies. Antigen-specific IgE/FcɛRI cross-linking fails to induce maturation or production of inflammatory mediators in human DCs and FcɛRI-humanized DCs. Furthermore, conferring expression of FcɛRI to DCs decreases the severity of food allergy and asthma in disease-relevant models suggesting anti-inflammatory IgE/FcɛRI signals. Consistent with the improved clinical parameters in vivo, antigen-specific IgE/FcɛRI cross-linking on papain or lipopolysaccharide-stimulated DCs inhibits the production of pro-inflammatory cytokines and chemokines. Migration assays confirm that the IgE-dependent decrease in cytokine production results in diminished recruitment of mast cell progenitors; providing a mechanistic explanation for the reduced mast cell-dependent allergic phenotype observed in FcɛRI-humanized mice. Our study demonstrates a novel immune regulatory function of IgE and proposes that DC-intrinsic IgE signals serve as a feedback mechanism to restrain allergic tissue inflammation.

Intrathecal heat shock protein 60 mediates neurodegeneration and demyelination in the CNS through a TLR4- and MyD88-dependent pathway

Background

Toll-like receptors (TLR) constitute a highly conserved class of receptors through which the innate immune system responds to both pathogen- and host-derived factors. Although TLRs are involved in a wide range of central nervous system (CNS) disorders including neurodegenerative diseases, the molecular events leading from CNS injury to activation of these innate immune receptors remain elusive. The stress protein heat shock protein 60 (HSP60) released from injured cells is considered an endogenous danger signal of the immune system. In this context, the main objective of the present study was to investigate the impact of extracellular HSP60 on the brain in vivo.

Results

We show here that HSP60 injected intrathecally causes neuronal and oligodendrocyte injury in the CNS in vivo through TLR4-dependent signaling. Intrathecal HSP60 results in neuronal cell death, axonal injury, loss of oligodendrocytes, and demyelination in the cerebral cortex of wild-type mice. In contrast both mice lacking TLR4 and the TLR adaptor molecule MyD88 are protected against deleterious effects induced by HSP60. In contrast to the exogenous TLR4 ligand, lipopolysaccharide, intrathecal HSP60 does not induce such a considerable inflammatory response in the brain. In the CNS, endogenous HSP60 is predominantly expressed in neurons and released during brain injury, since the cerebrospinal fluid (CSF) from animals of a mouse stroke model contains elevated levels of this stress protein compared to the CSF of sham-operated mice.

Conclusions

Our data show a direct toxic effect of HSP60 towards neurons and oligodendrocytes in the CNS. The fact that these harmful effects involve TLR4 and MyD88 confirms a molecular pathway mediated by the release of endogenous TLR ligands from injured CNS cells common to many forms of brain diseases that bi-directionally links CNS injury and activation of the innate immune system to neurodegeneration and demyelination in vivo.

Electronic supplementary material

The online version of this article (doi:10.1186/s13024-015-0003-1) contains supplementary material, which is available to authorized users.

Triggering receptor expressed on myeloid cells-2 fine-tunes inflammatory responses in murine Gram-negative sepsis

During infections, TLR-mediated responses require tight regulation to allow for pathogen removal, while preventing overwhelming inflammation and immunopathology. The triggering receptor expressed on myeloid cells (TREM)-2 negatively regulates inflammation by macrophages and impacts on phagocytosis, but the function of endogenous TREM-2 during infections is poorly understood. We investigated TREM-2's role in regulating TLR4-mediated inflammation by studying wild-type and TREM-2(-/-) mice challenged with LPS and found TREM-2 to dampen early inflammation. Augmented early inflammation in TREM-2(-/-) animals was followed by an accelerated resolution and ultimately improved survival, associated with the induction of the negative regulator A20. Upon infection with Escherichia coli, the otherwise beneficial effect of an exaggerated early immune response in TREM-2(-/-) animals was counteracted by a 50% reduction in bacterial phagocytosis. In line with this, TREM-2(-/-) peritoneal macrophages (PMs) exhibited augmented inflammation following TLR4 stimulation, demonstrating the presence and negative regulatory functionality of TREM-2 on primary PMs. Significantly, we identified a high turnover rate because TREM-2 RNA is 25-fold down-regulated and the protein proteasomally degraded upon LPS encounter, thus ensuring a tightly regulated and versatile system that modulates inflammation. Our results illustrate TREM-2's effects on infection-triggered inflammation and identify TREM-2 as a potential target to prevent overwhelming inflammation while preserving antibacterial-effector functions.

Proteomic analysis of lymphoblastoid cells from Nasu-Hakola patients: a step forward in our understanding of this neurodegenerative disorder

Nasu-Hakola disease (NHD) is a recessively inherited rare disorder characterized by a combination of neuropsychiatric and bone symptoms which, while being unique to this disease, do not provide a rationale for the unambiguous identification of patients. These individuals, in fact, are likely to go unrecognized either because they are considered to be affected by other kinds of dementia or by fibrous dysplasia of bone. Given that dementia in NHD has much in common with Alzheimer’s disease and other neurodegenerative disorders, it cannot be expected to achieve the differential diagnosis of this disease without performing a genetic analysis. Under this scenario, the availability of protein biomarkers would indeed provide a novel context to facilitate interpretation of symptoms and to make the precise identification of this disease possible. The work here reported was designed to generate, for the first time, protein profiles of lymphoblastoid cells from NHD patients. Two-dimensional electrophoresis (2-DE) and nano liquid chromatography-tandem mass spectrometry (nLC-MS/MS) have been applied to all components of an Italian family (seven subjects) and to five healthy subjects included as controls. Comparative analyses revealed differences in the expression profile of 21 proteins involved in glucose metabolism and information pathways as well as in stress responses.

CMV-encoded Fcγ receptors: modulators at the interface of innate and adaptive immunity

The constant region of IgG antibodies mediates antiviral activities upon engaging host Fcγ receptors (FcγRs) expressed by a variety of immune cells, such as antibody-dependent cellullar cytotoxcity (ADCC) executed by natural killer (NK)cells. Human cytomegalovirus (HCMV) is unique among viruses by encoding also an array of several Fcγ-binding glycoproteins with cell surface disposition and concomitant incorporation into the virion. Evidence is increasing that the virus-encoded Fcγ receptors differ in their Fcγ binding mode but effectively operate as adversaries of host FcγRs since they are able to prevent IgG-mediated triggering of activating host FcγRs, i.e., FcγRI, FcγRIIA, and FcγRIIIA. Here we discuss virus-encoded FcγRs as the first known HCMV inhibitors of IgG-mediated immunity which could account for the limited efficacy of HCMV hyperimmune globulin in clinical settings. A better understanding of their molecular mode of action opens up new perspectives for improving IgG therapies against HCMV disease.

CEACAM2 negatively regulates hemi (ITAM-bearing) GPVI and CLEC-2 pathways and thrombus growth in vitro and in vivo

Carcinoembryonic antigen-related cell adhesion molecule-2 (CEACAM2) is a cell-surface glycoprotein expressed on blood, epithelial, and vascular cells. CEACAM2 possesses adhesive and signaling properties mediated by immunoreceptor tyrosine-based inhibitory motifs. In this study, we demonstrate that CEACAM2 is expressed on the surface and in intracellular pools of platelets. Functional studies of platelets from Ceacam2(-/-)-deficient mice (Cc2(-/-)) revealed that CEACAM2 serves to negatively regulate collagen glycoprotein VI (platelet) (GPVI)-FcRγ-chain and the C-type lectinlike receptor 2 (CLEC-2) signaling. Cc2(-/-) platelets displayed enhanced GPVI and CLEC-2-selective ligands, collagen-related peptide (CRP), collagen, and rhodocytin (Rhod)-mediated platelet aggregation. They also exhibited increased adhesion on type I collagen, and hyperresponsive CRP and CLEC-2-induced α and dense granule release compared with wild-type platelets. Furthermore, using intravital microscopy to ferric chloride (FeCl3)-injured mesenteric arterioles and laser-induced injury of cremaster muscle arterioles, we herein show that thrombi formed in Cc2(-/-) mice were larger and more stable than wild-type controls in vivo. Thus, CEACAM2 is a novel platelet immunoreceptor that acts as a negative regulator of platelet GPVI-collagen interactions and of ITAM receptor CLEC-2 pathways.

Shifting FcγRIIA-ITAM from activation to inhibitory configuration ameliorates arthritis

Rheumatoid arthritis-associated (RA-associated) inflammation is mediated through the interaction between RA IgG immune complexes and IgG Fc receptors on immune cells. Polymorphisms within the gene encoding the human IgG Fc receptor IIA (hFcγRIIA) are associated with an increased risk of developing RA. Within the hFcγRIIA intracytoplasmic domain, there are 2 conserved tyrosine residues arranged in a noncanonical immunoreceptor tyrosine-based activation motif (ITAM). Here, we reveal that inhibitory engagement of the hFcγRIIA ITAM either with anti-hFcγRII F(ab')2 fragments or intravenous hIgG (IVIg) ameliorates RA-associated inflammation, and this effect was characteristic of previously described inhibitory ITAM (ITAMi) signaling for hFcαRI and hFcγRIIIA, but only involves a single tyrosine. In hFcγRIIA-expressing mice, arthritis induction was inhibited following hFcγRIIA engagement. Moreover, hFcγRIIA ITAMi-signaling reduced ROS and inflammatory cytokine production through inhibition of guanine nucleotide exchange factor VAV-1 and IL-1 receptor-associated kinase 1 (IRAK-1), respectively. ITAMi signaling was mediated by tyrosine 304 (Y304) within the hFcγRIIA ITAM, which was required for recruitment of tyrosine kinase SYK and tyrosine phosphatase SHP-1. Anti-hFcγRII F(ab')2 treatment of inflammatory synovial cells from RA patients inhibited ROS production through induction of ITAMi signaling. These data suggest that shifting constitutive hFcγRIIA-mediated activation to ITAMi signaling could ameliorate RA-associated inflammation.

Plasmacytoid dendritic cells, interferon signaling, and FcγR contribute to pathogenesis and therapeutic response in childhood immune thrombocytopenia

Immune thrombocytopenia (ITP) is an autoimmune disorder of childhood characterized by immune-mediated destruction of platelets. The mechanisms underlying the pathogenesis of ITP and the therapeutic efficacy of intravenous immunoglobulins (IVIG) in this disorder remain unclear. We show that monocytes from patients with ITP have a distinct gene expression profile, with increased expression of type I interferon response (IR) genes. Plasma from ITP patients had increased levels of several cytokines indicative of immune activation, including an increase in interferon-α. ITP patients also had an increase in plasmacytoid dendritic cells (pDCs) compared to healthy donors. Therapy-induced remission of ITP was associated with abrogation of the IR gene signature in monocytes without reduction in the levels of circulating interferon-α itself. IVIG altered the ratio of activating/inhibitory Fcγ receptors (FcγRs) in vivo primarily by reducing FcγRIII (CD16). The engagement of activating FcγRs was required for IVIG-mediated abrogation of monocyte response to exogenous interferon-α in culture. Moreover, plasma from ITP patients led to activation of monocytes and myeloid DCs in culture with an increase in T cell stimulatory capacity; this activation depended on the engagement of activating FcγRs and interferon-α receptor (IFNAR) and was inhibited by antibody-mediated blockade of these pathways. These data point to a central role of type I interferon in the pathogenesis of ITP and suggest targeting pDCs and blockade of IR as potential therapeutic approaches in this disorder. They also provide evidence for the capacity of IVIG to extinguish IR in vivo, which may contribute to its effects in autoimmunity.

Expression of the B-cell receptor component CD79a on immature myeloid cells contributes to their tumor promoting effects

The role of myeloid derived suppressor cells (MDSCs) in promoting tumorigenesis is well-established, and significant effort is being made to further characterize surface markers on MDSCs both for better diagnosis and as potential targets for therapy. Here we show that the B cell receptor adaptor molecule CD79a is unexpectedly expressed on immature bone marrow myeloid cells, and is upregulated on MDSCs generated in multiple different mouse models of metastatic but not non-metastatic cancer. CD79a on MDSCs is upregulated and activated in response to soluble factors secreted by tumor cells. Activation of CD79a on mouse MDSCs, by crosslinking with a specific antibody, maintained their immature phenotype (CD11b+Gr1+), enhanced their migration, increased their suppressive effect on T cell proliferation, and increased secretion of pro-tumorigenic cytokines such as IL-6 and CCL22. Furthermore, crosslinking CD79a on myeloid cells activated signaling through Syk, BLNK, ERK and STAT3 phosphorylation. In vivo, CD79+ myeloid cells showed enhanced ability to promote primary tumor growth and metastasis. Finally we demonstrate that CD79a is upregulated on circulating myeloid cells from lung cancer patients, and that CD79a+ myeloid cells infiltrate human breast tumors. We propose that CD79a plays a functional role in the tumor promoting effects of myeloid cells, and may represent a novel target for cancer therapy.

The tyrosine kinase Syk differentially regulates Toll-like receptor signaling downstream of the adaptor molecules TRAF6 and TRAF3

Toll-like receptors (TLRs) are a major family of pattern recognition receptors, and they play a crucial role in innate immune responses. Activation of TLR4 signaling at the plasma membrane by its ligand lipopolysaccharide (LPS) stimulates a proinflammatory pathway dependent on the E3 ubiquitin ligase TRAF6 (tumor necrosis factor receptor-associated factor 6) and the kinase TAK1 (transforming growth factor β-activated kinase 1), whereas TLR4 signaling at endosomes stimulates the production of type I interferons (IFNs) through a pathway that depends on TRAF3 and the kinase TBK1 (TANK-binding kinase-1). We found that the nonreceptor tyrosine kinase Syk partially mediated the endocytosis of TLR4, but it also played a dual role in TLR4-mediated signaling. LPS-dependent stimulation of TLR4 in Syk-deficient macrophages led to enhanced activation of TAK1 and increased production of proinflammatory cytokines compared to that in wild-type macrophages. In contrast, Syk-deficient macrophages exhibited decreased TLR4-dependent activation of TBK1 signaling and production of type I IFNs. We found that Syk was present in both TRAF6- and TRAF3-containing signaling complexes; however, the LPS-dependent, lysine 63-linked ubiquitination of TRAF6 and TRAF3 was oppositely regulated by Syk. We identified the domains of Syk that interacted with TRAF3, TRAF6, TAK1, and TBK1, factors activated by multiple TLRs, which suggests that Syk may act as a common regulator of various TLR responses. Together, our results demonstrate the opposing regulatory roles of Syk in TLR-mediated TRAF6 and TRAF3 signaling pathways, which suggests that Syk may fine-tune the innate immune response to lessen inflammation.

β(2) integrins inhibit TLR responses by regulating NF-κB pathway and p38 MAPK activation

Outside-in signals from β(2) integrins require immunoreceptor tyrosine-based activation motif adapters in myeloid cells that are known to dampen TLR responses. However, the relationship between β(2) integrins and TLR regulation is unclear. Here we show that deficiency in β(2) integrins (Itgb2(-/-) ) causes hyperresponsiveness to TLR stimulation, demonstrating that β(2) integrins inhibit signals downstream of TLR ligation. Itgb2(-/-) macrophages and dendritic cells produced more IL-12 and IL-6 than WT cells when stimulated with TLR agonists and Itgb2(-/-) mice produced more inflammatory cytokines than WT mice when injected with LPS. TLR hypersensitivity was not the result of insufficient ABIN-3, A20, Hes-1, or IRAK-M expression, nor to changes in IL-10 production or sensitivity, though Itgb2(-/-) macrophages had reduced p38 MAPK phosphorylation after LPS treatment. Furthermore, a Cbl-b-MyD88 regulatory axis is not required for TLR inhibition in macrophages. Instead, Itgb2(-/-) macrophages presented with enhanced IκBα degradation, leading to changes in NF-κB recruitment to target promoters and elevated cytokine, chemokine, and anti-apoptotic gene transcription. Thus, β(2) integrins limit TLR signaling by inhibiting NF-κB pathway activation and promoting p38 MAPK activation, thereby fine-tuning TLR-induced inflammatory responses.

Negative regulation of osteoclast precursor differentiation by CD11b and β2 integrin-B-cell lymphoma 6 signaling

Negative regulation of osteoclastogenesis is important for bone homeostasis and prevention of excessive bone resorption in inflammatory and other diseases. Mechanisms that directly suppress osteoclastogenesis are not well understood. In this study we investigated regulation of osteoclast differentiation by the β2 integrin CD11b/CD18 that is expressed on myeloid lineage osteoclast precursors. CD11b-deficient mice exhibited decreased bone mass that was associated with increased osteoclast numbers and decreased bone formation. Accordingly, CD11b and β2 integrin signaling suppressed osteoclast differentiation by preventing receptor activator of NF-κB ligand (RANKL)-induced induction of the master regulator of osteoclastogenesis nuclear factor of activated T cells, cytoplasmic 1 (NFATc1) and of downstream osteoclast-related NFATc1 target genes. CD11b suppressed induction of NFATc1 by the complementary mechanisms of downregulation of RANK expression and induction of recruitment of the transcriptional repressor B-cell lymphoma 6 (BCL6) to the NFATC1 gene. These findings identify CD11b as a negative regulator of the earliest stages of osteoclast differentiation, and provide an inducible mechanism by which environmental cues suppress osteoclastogenesis by activating a transcriptional repressor that makes genes refractory to osteoclastogenic signaling.

Regulation of TLR7/9 signaling in plasmacytoid dendritic cells

Plasmacytoid dendritic cells (pDCs), also known as type I interferon (IFN)-producing cells, are specialized immune cells characterized by their extraordinary capabilities of mounting rapid and massive type I IFN response to nucleic acids derived from virus, bacteria or dead cells. PDCs selectively express endosomal Toll-like receptor (TLR) 7 and TLR9, which sense viral RNA and DNA respectively. Following type I IFN and cytokine responses, pDCs differentiate into antigen presenting cells and acquire the ability to regulate T cell-mediated adaptive immunity. The functions of pDCs have been implicated not only in antiviral innate immunity but also in immune tolerance, inflammation and tumor microenvironments. In this review, we will focus on TLR7/9 signaling and their regulation by pDC-specific receptors.

A novel phage element of Salmonella enterica serovar Enteritidis P125109 contributes to accelerated type III secretion system 2-dependent early inflammation kinetics in a mouse colitis model

Salmonella enterica subsp. I serovar Enteritidis exhibits type III secretion system 2 (TTSS2)-dependent early colonization and inflammation kinetics faster than those of closely related S. enterica serovar Typhimurium. To investigate the accelerated TTSS-2-dependent pathogenic potential of S. Enteritidis, we focused on its genome. Results of a previously published comparative genomic study revealed the presence of mutually exclusive genes in both serovars. In this study, we investigated the roles of six S. Enteritidis-specific genes in vivo by using differential fluorescence induction (DFI) through putative gene-specific promoters. The promoter construct associated with the gene locus SEN1140 induced green fluorescent protein (GFP) expression in the gut lumen, lamina propria, mesenteric lymph nodes, and related systemic organs. To further investigate the potential role of SEN1140, we compared a SEN1140 deletion mutant with S. Typhimurium in a TTSS1-deficient background. Interestingly, the S. Enteritidis mutant lacking SEN1140 did not show the unique TTSS-2-dependent early colonization and inflammation kinetic phenotype of S. Typhimurium. Consistent with this result, complementation of SEN1140 restored the TTSS-2-dependent accelerated inflammatory potential of S. Enteritidis. This report presents a suitable screening strategy that uses a combination of DFI, fluorescence-activated cell sorting, quantitative PCR, and wild-type isogenic tagged-strain techniques to explore the unique roles of S. Enteritidis-specific genes in bacterial pathogenesis.

Natural killer cells and their role in rheumatoid arthritis: friend or foe?

Rheumatoid arthritis (RA) is a long-term disease that leads to inflammation of the joints and surrounding tissues. Natural killer (NK) cells are an important part of the innate immune system and are responsible for the first line of defense against pathogens during the initial immune challenge before the adaptive immune system eventually eliminates the infectious burden. NK cells have the capacity to damage normal cells or through interaction with other cells such as dendritic cells, macrophages, and T cells cause autoimmune diseases, such as RA. NK cells isolated from the joints of patients with RA suggest that they may play a role in this disease. However, the involvement of NK cells in RA pathology is not fully elucidated. Both protective and detrimental roles of NK cells in RA have recently been reported. A better understanding of NK cells' role in RA might help to develop new therapeutic strategies for treatment of the RA or other autoimmune diseases. We have decided in this paper to focus on the NK cell biology, and attempt to bring the interested readership of this Journal up to date on the NK cell, specifically its possible relation to RA.

Fc receptor-targeted therapies for the treatment of inflammation, cancer and beyond

The direct or indirect targeting of antibody Fc receptors (FcRs) presents unique opportunities and interesting challenges for the treatment of inflammatory diseases, cancer and infection. Biological responses induced via the Fc portions of antibodies are powerful, complex and unusual, and comprise both activating and inhibitory effects. These properties can be exploited in the engineering of therapeutic monoclonal antibodies to improve their activity in vivo. FcRs have also emerged as key participants in the pathogenesis of several important autoimmune diseases, including systemic lupus erythematosus and rheumatoid arthritis. Therapeutic approaches based on antagonizing FcR function with small molecules or biological drugs such as monoclonal antibodies and recombinant soluble FcR ectodomains have gained momentum. This Review addresses various strategies to manipulate FcR function to overcome immune complex-mediated inflammatory diseases, and considers approaches to improve antibody-based anticancer therapies.

Tim-3 marks human natural killer cell maturation and suppresses cell-mediated cytotoxicity

Natural killer (NK) cells are innate lymphocytes that play an important role against viral infections and cancer. This effect is achieved through a complex mosaic of inhibitory and activating receptors expressed by NK cells that ultimately determine the magnitude of the NK-cell response. The T-cell immunoglobulin- and mucin domain-containing (Tim)-3 receptor was initially identified as a T-helper 1-specific type I membrane protein involved in regulating T-cell responses. Human NK cells transcribe the highest amounts of Tim-3 among lymphocytes. Tim-3 protein is expressed on essentially all mature CD56(dim)CD16(+) NK cells and is expressed heterogeneously in the immature CD56(bright)CD16(-) NK-cell subset in blood from healthy adults and in cord blood. Tim-3 expression was induced on CD56(bright)CD16(-) NK cells after stimulation with IL-15 or IL-12 and IL-18 in vitro, suggesting that Tim-3 is a maturation marker on NK cells. Whereas Tim-3 has been used to identify dysfunctional T cells, NK cells expressing high amounts of Tim-3 are fully responsive with respect to cytokine production and cytotoxicity. However, when Tim-3 was cross-linked with antibodies it suppressed NK cell-mediated cytotoxicity. These findings suggest that NK-cell responses may be negatively regulated when NK cells encounter target cells expressing cognate ligands of Tim-3.

ITAM-coupled receptors inhibit IFNAR signaling and alter macrophage responses to TLR4 and Listeria monocytogenes

ITAM-coupled receptors play an essential role in regulating macrophage activation and function by cross-regulating signaling from heterologous receptors. We investigated mechanisms by which ITAM-associated receptors inhibit type I IFN (IFN-α/β) signaling in primary human macrophages and tested the effects of simultaneous ligation of ITAM-associated receptors and TLR4 on TLR4-induced Jak-STAT signaling that is mediated by autocrine IFN-β. Preligation of ITAM-coupled β2 integrins and FcγRs inhibited proximal signaling by the type I IFN receptor IFNAR. Cross-inhibition of IFNAR signaling by β2 integrins resulted in decreased Jak1 activation and was mediated by partial downregulation of the IFNAR1 subunit and MAPK-dependent induction of USP18, which blocks the association of Jak1 with IFNAR2. Simultaneous engagement of ITAM-coupled β2 integrins or Dectin-1 with TLR4 did not affect TLR4-induced direct activation of inflammatory target genes such as TNF or IL6 but abrogated subsequent induction of IFN response genes that is mediated by autocrine IFN-β signaling. Type I IFNs promote macrophage death postinfection by Listeria monocytogenes. Consequently, attenuation of IFN responses by β2 integrins protected primary human macrophages from L. monocytogenes-induced apoptosis. These results provide a mechanism for cross-inhibition of type I IFN signaling by ITAM-coupled β2 integrins and demonstrate that ITAM signaling qualitatively modulates macrophage responses to pathogen-associated molecular patterns and pathogens by selectively suppressing IFN responses.

IgG1 and IVIg induce inhibitory ITAM signaling through FcγRIII controlling inflammatory responses

Intravenous immunoglobulin (IVIg) has been used in the treatment of several autoimmune and inflammatory diseases. However, its mechanism of action remains incompletely understood. Here, we investigated the possibility that IVIg induces its anti-inflammatory effects through activating Fcγ receptors bearing an immunoreceptor tyrosine-based activation motif (ITAM) in the FcRγ signaling adaptor. Recently, the concept of inhibitory ITAM (ITAMi) has emerged as a new means to negatively control the immune response. We found that interaction of FcRγ-associated mouse or human FcγRIII with uncomplexed IgG1 or IVIg, or with bivalent anti-FcγRIII F(ab')(2) reduced calcium responses, reactive oxygen species production, endocytosis, and phagocytosis, induced by heterologous activating receptors on monocyte/macrophages and FcγRIII(+) transfectants. Inhibition required the ITAMi configuration of the FcγRIII-associated FcRγ subunit and SHP-1 recruitment involving formation of intracellular "inhibisome" clusters containing FcγRIII, and the targeted heterologous activating receptor. IVIg as well as anti-FcγRIII treatments controlled the development of nonimmune mediated inflammation in vivo independently of FcγRIIB. These results demonstrate that circulating immunoglobulins (Ig)Gs are not functionally inert but act through continuous interaction with FcγRIII-inducing ITAMi signaling to maintain immune homeostasis. These data support a new mechanism of action for IVIg and demonstrate the therapeutic potential of FcγRIIIA targeting in inflammation.

Role of ITAM signaling module in signal integration

Diverse cell types use a small number of evolutionarily conserved signaling modules to integrate external cues and elicit distinct functions. A question thus arises as to how does a receptor, which contains a single signaling module, produce distinct outcomes to diverse signals, particularly if such module is shared amongst a family of receptors? Emerging data suggest that many immunoreceptors, all of which use a conserved ITAM-module for their signaling, can couple with members of additional classes of membrane receptors to deliver unique signal(s) to the cell. We discuss the possible biological purposes and mechanisms behind these interactions at the plasma membrane. We offer a conceptual framework to understand information processing within the immune system and discuss the new biology of old receptors involving their structural and functional collaborations that evolved to deliver unique signal(s) to the cell using a limited set of conserved signaling modules.

TREM-2, triggering receptor expressed on myeloid cell-2, negatively regulates TLR responses in dendritic cells

DCs play a key role in defense against infections and also in preventing inflammatory and autoimmune diseases. The response of DCs to pathogens is tightly regulated by many mechanisms to allow for appropriate, but not pathogenic, responses. We previously showed that DCs with deficiencies for two ITAM-bearing signaling adapters, DAP12 and FcRγ, produce more inflammatory cytokines upon treatment with Toll-like receptor (TLR) agonists than WT DCs. Here, we investigated whether the TREM-2 receptor pairs with DAP12 to inhibit TLR responses in DCs. TREM-2-deficient BMDCs showed increased inflammatory cytokine and type I IFN production in response to TLR ligation. Additionally, TREM-2-deficient BMDCs had increased TLR-induced maturation and were more efficient at inducing antigen-specific T-cell proliferation upon CpG DNA stimulation compared with WT BMDCs. Finally, we showed that a TREM-2 ligand is expressed on the surface of BMDCs, suggesting that the TREM-2 receptor transduces inhibitory signals due to recognition of an endogenous ligand.