How ITAMs inhibit signaling

The effect of Fc region affinity of protein-based antibody-recruiting molecules on antibody-dependent cellular cytotoxicity

Previously, we reported anticancer molecules, Fc-binding antibody-recruiting molecules (Fc-ARMs), which crosslink proteins on cancer cells with endogenous immunoglobulin Gs (IgGs) via their Fc region. The mobilized IgGs on cancer cells can accommodate natural killer cells to induce antibody-dependent cellular cytotoxicity (ADCC). Because previous Fc-ARMs utilized Fc-binding peptides, their affinity to IgGs is weak, which resulted in the limited induction capability of ADCC. Previous Fc-ARMs also unitized small molecular ligands to cancer cells, which limited their universal applicability to any cancer cells. A recent study reported that protein-based Fc-ARMs might overcome the issues associated with non-proteinous Fc-ARMs. Here, we examined the universality of a protein-based Fc-ARM by replacing its tumor-binding domain with a human epidermal growth factor receptor 2 (HER2)-specific affibody (ZHER2:342). We also examined the requirement of its Fc-binding domain affinity. We found that the Fc-ARMs accepted an affibody as a tumor-binding domain to induce ADCC. Furthermore, the required residence time of the complex between Fc-ARM and IgG was ∼102 min, which was comparable to that when monoclonal antibodies bind to their specific antigens. However, we found that the extent of ADCC induced by Fc-ARM was lower than that of conventional IgG-mediated ADCC, indicating that further enhancement of the affinity of the antibody-binding terminus and tumor-binding terminus of Fc-ARM may be needed to achieve ADCC equivalent to that of conventional IgG-mediated ADCC.

Ablation of SYK Kinase from Expanded Primary Human NK Cells via CRISPR/Cas9 Enhances Cytotoxicity and Cytokine Production

CMV infection alters NK cell phenotype and function toward a more memory-like immune state. These cells, termed adaptive NK cells, typically express CD57 and NKG2C but lack expression of the FcRγ-chain (gene: FCER1G, FcRγ), PLZF, and SYK. Functionally, adaptive NK cells display enhanced Ab-dependent cellular cytotoxicity (ADCC) and cytokine production. However, the mechanism behind this enhanced function is unknown. To understand what drives enhanced ADCC and cytokine production in adaptive NK cells, we optimized a CRISPR/Cas9 system to ablate genes from primary human NK cells. We ablated genes that encode molecules in the ADCC pathway, such as FcRγ, CD3ζ, SYK, SHP-1, ZAP70, and the transcription factor PLZF, and tested subsequent ADCC and cytokine production. We found that ablating the FcRγ-chain caused a modest increase in TNF-α production. Ablation of PLZF did not enhance ADCC or cytokine production. Importantly, SYK kinase ablation significantly enhanced cytotoxicity, cytokine production, and target cell conjugation, whereas ZAP70 kinase ablation diminished function. Ablating the phosphatase SHP-1 enhanced cytotoxicity but reduced cytokine production. These results indicate that the enhanced cytotoxicity and cytokine production of CMV-induced adaptive NK cells is more likely due to the loss of SYK than the lack of FcRγ or PLZF. We found the lack of SYK expression could improve target cell conjugation through enhanced CD2 expression or limit SHP-1-mediated inhibition of CD16A signaling, leading to enhanced cytotoxicity and cytokine production.

Features, modulation and analysis of glycosylation patterns of therapeutic recombinant immunoglobulin A

Increasing the production of recombinant antibodies while ensuring high and stable protein quality remains a challenge in mammalian cell culture. This review is devoted to advances in the field of obtaining stable and optimal glycosylation of therapeutic antibodies based on IgA, as well as the subsequent issues of glycosylation control of glycoproteins during their production. Current studies also demonstrate a general need for a more fundamental understanding of the use of CHO cell-based producer cell lines, through which the glycoprofile of therapeutic IgA antibodies is produced and the dependence of glycosylation on culture conditions could be controlled. Optimization of glycosylation improves the therapeutic efficacy and can expand the possibilities for the creation of highly effective glycoprotein therapeutic drugs. Current status and trends in glycan analysis of therapeutic IgA, dominantly based on mass spectrometry and lectin microarrays are herein summarised as well.

Novel γ-sarcoglycan interactors in murine muscle membranes

BACKGROUND

The sarcoglycan complex (SC) is part of a network that links the striated muscle cytoskeleton to the basal lamina across the sarcolemma. The SC coordinates changes in phosphorylation and Ca⁺⁺-flux during mechanical deformation, and these processes are disrupted with loss-of-function mutations in gamma-sarcoglycan (Sgcg) that cause Limb girdle muscular dystrophy 2C/R5.

METHODS

To gain insight into how the SC mediates mechano-signaling in muscle, we utilized LC-MS/MS proteomics of SC-associated proteins in immunoprecipitates from enriched sarcolemmal fractions. Criteria for inclusion were co-immunoprecipitation with anti-Sgcg from C57BL/6 control muscle and under-representation in parallel experiments with Sgcg-null muscle and with non-specific IgG. Validation of interaction was performed in co-expression experiments in human RH30 rhabdomyosarcoma cells.

RESULTS

We identified 19 candidates as direct or indirect interactors for Sgcg, including the other 3 SC proteins. Novel potential interactors included protein-phosphatase-1-catalytic-subunit-beta (Ppp1cb, PP1b) and Na⁺-K⁺-Cl^--co-transporter NKCC1 (SLC12A2). NKCC1 co-localized with Sgcg after co-expression in human RH30 rhabdomyosarcoma cells, and its cytosolic domains depleted Sgcg from cell lysates upon immunoprecipitation and co-localized with Sgcg after detergent permeabilization. NKCC1 localized in proximity to the dystrophin complex at costameres in vivo. Bumetanide inhibition of NKCC1 cotransporter activity in isolated muscles reduced SC-dependent, strain-induced increases in phosphorylation of extracellular signal-regulated kinases 1 and 2 (ERK1/2). In silico analysis suggests that candidate SC interactors may cross-talk with survival signaling pathways, including p53, estrogen receptor, and TRIM25.

CONCLUSIONS

Results support that NKCC1 is a new SC-associated signaling protein. Moreover, the identities of other candidate SC interactors suggest ways by which the SC and NKCC1, along with other Sgcg interactors such as the membrane-cytoskeleton linker archvillin, may regulate kinase- and Ca⁺⁺-mediated survival signaling in skeletal muscle.

Extending traditional antibody therapies: Novel discoveries in immunotherapy and clinical applications

Immunotherapy has been well regarded as one of the safer and antigen-specific anti-cancer treatments compared to first-generation chemotherapy. Since Coley's discovery, researchers focused on engineering novel antibody-based therapies. Including artificial and modified antibodies, such as antibody fragments, antibody-drug conjugates, and synthetic mimetics, the variety of immunotherapy has been rapidly expanding in the last few decades. Genetic and chemical modifications to monoclonal antibody have been brought into academia, in vivo trials, and clinical applications. Here, we have looked around antibodies overall. First, we elucidate the antibody structure and its cytotoxicity mechanisms. Second, types of therapeutic antibodies are presented. Additionally, there is a summarized list of US Food and Drug Administration (FDA)-approved therapeutic antibodies and recent clinical trials. This review provides a comprehensive overview of both the general function of therapeutic antibodies and a few main variations in development, including recent advent with the proposed mechanism of actions, and we introduce types of therapeutic antibodies, clinical trials, and approved commercial immunotherapeutic drugs.

Approaches to Enhance Natural Killer Cell-Based Immunotherapy for Pediatric Solid Tumors

Treatment of metastatic pediatric solid tumors remain a significant challenge, particularly in relapsed and refractory settings. Standard treatment has included surgical resection, radiation, chemotherapy, and, in the case of neuroblastoma, immunotherapy. Despite such intensive therapy, cancer recurrence is common, and most tumors become refractory to prior therapy, leaving patients with few conventional treatment options. Natural killer (NK) cells are non-major histocompatibility complex (MHC)-restricted lymphocytes that boast several complex killing mechanisms but at an added advantage of not causing graft-versus-host disease, making use of allogeneic NK cells a potential therapeutic option. On top of their killing capacity, NK cells also produce several cytokines and growth factors that act as key regulators of the adaptive immune system, positioning themselves as ideal effector cells for stimulating heavily pretreated immune systems. Despite this promise, clinical efficacy of adoptive NK cell therapy to date has been inconsistent, prompting a detailed understanding of the biological pathways within NK cells that can be leveraged to develop "next generation" NK cell therapies. Here, we review advances in current approaches to optimizing the NK cell antitumor response including combination with other immunotherapies, cytokines, checkpoint inhibition, and engineering NK cells with chimeric antigen receptors (CARs) for the treatment of pediatric solid tumors.

Siglec-G Deficiency Ameliorates Hyper-Inflammation and Immune Collapse in Sepsis via Regulating Src Activation

Hyper-inflammation during acute phase and sequential hypo-inflammation during immunosuppressive phase in macrophages/monocytes lead to multiorgan failure syndrome and immune collapse of sepsis, in which toll-like receptor (TLR)-triggered inflammatory responses play a major role. Here, we reported that Siglecg deficiency attenuated TLR4-triggered pro-inflammatory cytokine production and increased anti-inflammatory cytokine [interleukin-10 [IL-10]] production in vivo and in vitro at both acute and immunosuppressive phases. Siglecg deficiency also protected mice from lipopolysaccharide (LPS)-induced sepsis with less inflammation in the lung and less tissue destruction in the spleen. Siglec-G inhibited proto-oncogene tyrosine-protein kinase Src (Src) activation via recruiting and activating tyrosine phosphatase Src homology region 2 domain-containing phosphatase-1 (SHP1) through immunoreceptor tyrosine-based inhibitory motif (ITIM) domain. Src could inhibit TLR4-induced inflammatory cytokines and promote anti-inflammatory cytokine IL-10. Mechanical investigation showed that Src could interact with and phosphorylate STAT3. Src could also promote HIF1α degradation through activating GSK3β. Our study reveals that Siglec-G orchestrates TLR-induced inflammation, which outlines that blocking Siglec-G or activating Src may be a promising strategy for both acute and chronic inflammatory diseases.

FcεRI γ-Chain Negatively Modulates Dectin-1 Responses in Dendritic Cells

The inhibitory effect of immunoreceptor tyrosine-based activation motif (ITAM)-containing adapters DAP12 and FcεRI γ-chain (FcRγ) has been found in many immune functions. Herein, we have further explored the role of these adapters in C-type lectin receptors response. We identified that FcRγ, but not DAP12, could negatively regulate the Dectin-1 responses in dendritic cells (DCs). Loss of FcRγ or both DAP12 and FcRγ enhanced the maturation and cytokine production in DCs upon Dectin-1 activation compared to normal cells, whereas DCs lacking only DAP12 showed little changes. In addition, increments of T cell activation and T helper 17 polarization induced by FcRγ-deficient DCs were observed both in vitro and in vivo. Examining the Dectin-1 signaling, we revealed that the activations of several signaling molecules were augmented in FcRγ-deficient DCs stimulated with Dectin-1 ligands. Furthermore, we demonstrated that the association of phosphatases SHP-1 and PTEN with FcRγ may contribute to the negative regulation of FcRγ in Dectin-1 activation in DCs. These results extend the inhibitory effect of ITAM-containing adapters to Dectin-1 response in immune functions, even though Dectin-1 contains an ITAM-like intracellular domain. According to the role of Dectin-1 in responding to microbes and tumor cells, our finding may have applications in the development of vaccine and cancer therapy.

Human Cell Line-Derived Monoclonal IgA Antibodies for Cancer Immunotherapy

IgA antibodies have great potential to improve the functional diversity of current IgG antibody-based cancer immunotherapy options. However, IgA production and purification is not well established, which can at least in part be attributed to the more complex glycosylation as compared to IgG antibodies. IgA antibodies possess up to five N-glycosylation sites within their constant region of the heavy chain as compared to one site for IgG antibodies. The human GlycoExpress expression system was developed to produce biotherapeutics with optimized glycosylation and used here to generate a panel of IgA isotype antibodies directed against targets for solid (TA-mucin 1, Her2, EGFR, Thomsen–Friedenreich) and hematological (CD20) cancer indications. The feasibility of good manufacturing practice was shown by the production of 11 g IgA within 35 days in a one liter perfusion bioreactor, and IgA antibodies in high purity were obtained after purification. The monoclonal IgA antibodies possessed a high sialylation degree, and no non-human glycan structures were detected. Kinetic analysis revealed increased avidity antigen binding for IgA dimers as compared to monomeric antibodies. The IgA antibodies exhibited potent Fab- and Fc-mediated functionalities against cancer cell lines, whereby especially granulocytes are recruited. Therefore, for patients who do not sufficiently benefit from therapeutic IgG antibodies, IgA antibodies may complement current regiment options and represent a promising strategy for cancer immunotherapy. In conclusion, a panel of novel biofunctional IgA antibodies with human glycosylation was successfully generated.

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.

CD11b activation suppresses TLR-dependent inflammation and autoimmunity in systemic lupus erythematosus

Genetic variations in the ITGAM gene (encoding CD11b) strongly associate with risk for systemic lupus erythematosus (SLE). Here we have shown that 3 nonsynonymous ITGAM variants that produce defective CD11b associate with elevated levels of type I interferon (IFN-I) in lupus, suggesting a direct link between reduced CD11b activity and the chronically increased inflammatory status in patients. Treatment with the small-molecule CD11b agonist LA1 led to partial integrin activation, reduced IFN-I responses in WT but not CD11b-deficient mice, and protected lupus-prone MRL/Lpr mice from end-organ injury. CD11b activation reduced TLR-dependent proinflammatory signaling in leukocytes and suppressed IFN-I signaling via an AKT/FOXO3/IFN regulatory factor 3/7 pathway. TLR-stimulated macrophages from CD11B SNP carriers showed increased basal expression of IFN regulatory factor 7 (IRF7) and IFN-β, as well as increased nuclear exclusion of FOXO3, which was suppressed by LA1-dependent activation of CD11b. This suggests that pharmacologic activation of CD11b could be a potential mechanism for developing SLE therapeutics.

The many faces of Mac-1 in autoimmune disease

Mac-1 (CD11b/CD18) is a β2 integrin classically regarded as a pro-inflammatory molecule because of its ability to promote phagocyte cytotoxic functions and enhance the function of several effector molecules such as FcγR, uPAR, and CD14. Nevertheless, recent reports have revealed that Mac-1 also plays significant immunoregulatory roles, and genetic variants in ITGAM, the gene that encodes CD11b, confer risk for the autoimmune disease systemic lupus erythematosus (SLE). This has renewed interest in the physiological roles of this integrin and raised new questions on how its seemingly opposing biological functions may be regulated. Here, we provide an overview of the CD18 integrins and how their activation may be regulated as this may shed light on how the opposing roles of Mac-1 may be elicited. We then discuss studies that exemplify Mac-1's pro-inflammatory versus regulatory roles particularly in the context of IgG immune complex-mediated inflammation. This includes a detailed examination of molecular mechanisms that could explain the risk-conferring effect of rs1143679, a single nucleotide non-synonymous Mac-1 polymorphism associated with SLE.

The many faces of FcγRI: implications for therapeutic antibody function

Fcγ receptor I (FcγRI or CD64) is the sole human Fc receptor with high affinity for monovalent IgG. While it contains an immunoreceptor tyrosine-based activation motif in its cytoplasmic domain, binding of FcγRI can result in a complex array of activating and inhibitory outcomes. For instance, binding of monomeric IgG provides a low-intensity tonic signal through FcγRI that is necessary for full interferon γ receptor signaling in the same cell. Interaction of FcγRI with larger high-avidity complexes can result in phagocytosis, the generation of reactive oxygen species, as well as the synthesis and release of inflammatory cytokines. However, numerous reports also document potent anti-inflammatory effects brought about by FcγRI engagement with immune complexes such as the inhibition of IFNγ and TLR4 signaling, and secretion of interleukin-10. This has led to conflicting hypotheses regarding the function of FcγRI, especially with regard to its role in the efficacy of several therapeutic monoclonal antibodies. While many of these issues are still unclear, continued characterization of the regulation and context dependence of FcγRI function, as well as the molecular mechanisms responsible for these various outcomes, will improve our understanding of FcγRI biology as well as the therapeutic strategies designed to harness or constrain its actions.

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.

Biochemical and Functional Insights into the Integrated Regulation of Innate Immune Cell Responses by Teleost Leukocyte Immune-Type Receptors

Across vertebrates, innate immunity consists of a complex assortment of highly specialized cells capable of unleashing potent effector responses designed to destroy or mitigate foreign pathogens. The execution of various innate cellular behaviors such as phagocytosis, degranulation, or cell-mediated cytotoxicity are functionally indistinguishable when being performed by immune cells isolated from humans or teleost fishes; vertebrates that diverged from one another more than 450 million years ago. This suggests that vital components of the vertebrate innate defense machinery are conserved and investigating such processes in a range of model systems provides an important opportunity to identify fundamental features of vertebrate immunity. One characteristic that is highly conserved across vertebrate systems is that cellular immune responses are dependent on specialized immunoregulatory receptors that sense environmental stimuli and initiate intracellular cascades that can elicit appropriate effector responses. A wide variety of immunoregulatory receptor families have been extensively studied in mammals, and many have been identified as cell- and function-specific regulators of a range of innate responses. Although much less is known in fish, the growing database of genomic information has recently allowed for the identification of several immunoregulatory receptor gene families in teleosts. Many of these putative immunoregulatory receptors have yet to be assigned any specific role(s), and much of what is known has been based solely on structural and/or phylogenetic relationships with mammalian receptor families. As an attempt to address some of these shortcomings, this review will focus on our growing understanding of the functional roles played by specific members of the channel catfish (Ictalurus punctatus) leukocyte immune-type receptors (IpLITRs), which appear to be important regulators of several innate cellular responses via classical as well as unique biochemical signaling networks.

Enhancing the safety of antibody-based immunomodulatory cancer therapy without compromising therapeutic benefit: Can we have our cake and eat it too?

INTRODUCTION

Monoclonal antibodies (mAbs) targeting checkpoint inhibitors have demonstrated clinical benefit in treating patients with cancer and have paved the way for additional immune-modulating mAbs such as those targeting costimulatory receptors. The full clinical utility of these agents, however, is hampered by immune-related adverse events (irAEs) that can occur during therapy.

AREAS COVERED

We first provide a general overview of tumor immunity, followed by a review of the two major classes of immunomodulatory mAbs being developed as cancer therapeutics: checkpoint inhibitors and costimulatory receptor agonists. We then discuss therapy-associated adverse events. Finally, we describe in detail the mechanisms driving their therapeutic activity, with an emphasis on interactions between antibody fragment crystallizable (Fc) domains and Fc receptors (FcR).

EXPERT OPINION

Given that Fc-FcR interactions appear critical in facilitating the ability of immunomodulatory mAbs to elicit both therapeutically useful as well as adverse effects, the engineering of mAbs that can effectively engage their targets while limiting interaction with FcRs might represent a promising future avenue for developing the next generation of immune-enhancing tumoricidal agents with increased safety and retention of efficacy.

IgG4 can induce an M2-like phenotype in human monocyte-derived macrophages through FcγRI

Antibodies evoke cellular responses through the binding of their Fc region to Fc receptors, most of which contain immunoreceptor tyrosine-based activation motif domains and are thus considered "activating." However, there is a growing appreciation of these receptors for their ability to deliver an inhibitory signal as well. We previously described one such phenomenon whereby interferon (IFN)γ signaling is inhibited by immune complex signaling through FcγRI. To understand the implications of this in the context of therapeutic antibodies, we assessed individual IgG subclasses to determine their ability to deliver this anti-inflammatory signal in monocyte-derived macrophages. Like IgG1, we found that IgG4 is fully capable of inhibiting IFNγ-mediated events. In addition, F(ab')2 fragments that interfere with FcγRI signaling reversed this effect. For mAbs developed with either an IgG1 or an IgG4 constant region for indications where inflammation is undesirable, further examination of a potential Fc-dependent contribution to their mechanism of action is warranted.

A Lupus-Associated Mac-1 Variant Has Defects in Integrin Allostery and Interaction with Ligands under Force

Leukocyte CD18 integrins increase their affinity for ligand by transmitting allosteric signals to and from their ligand-binding αI domain. Mechanical forces induce allosteric changes that paradoxically slow dissociation by increasing the integrin/ligand bond lifetimes, referred to as catch bonds. Mac-1 formed catch bonds with its ligands. However, a Mac-1 gene (ITGAM) coding variant (rs1143679, R77H), which is located in the β-propeller domain and is significantly associated with systemic lupus erythematosus risk, exhibits a marked impairment in 2D ligand affinity and affinity maturation under mechanical force. Targeted mutations and activating antibodies reveal that the failure in Mac-1 R77H allostery is rescued by induction of cytoplasmic tail separation and full integrin extension. These findings demonstrate roles for R77, and the β-propeller in which it resides, in force-induced allostery relay and integrin bond stabilization. Defects in these processes may have pathological consequences, as the Mac-1 R77H variant is associated with increased susceptibility to lupus.

Control of cytokine production by human fc gamma receptors: implications for pathogen defense and autoimmunity

Control of cytokine production by immune cells is pivotal for counteracting infections via orchestration of local and systemic inflammation. Although their contribution has long been underexposed, it has recently become clear that human Fc gamma receptors (FcγRs), which are receptors for the Fc region of immunoglobulin G (IgG) antibodies, play a critical role in this process by controlling tissue- and pathogen-specific cytokine production. Whereas individual stimulation of FcγRs does not evoke cytokine production, FcγRs cell-type specifically interact with various other receptors for selective amplification or inhibition of particular cytokines, thereby tailoring cytokine responses to the immunological context. The physiological function of FcγR-mediated control of cytokine production is to counteract infections with various classes of pathogens. Upon IgG opsonization, pathogens are simultaneously recognized by FcγRs as well as by various pathogen-sensing receptors, leading to the induction of pathogen class-specific immune responses. However, when erroneously activated, the same mechanism also contributes to the development of autoimmune diseases such as rheumatoid arthritis and systemic lupus erythematosus. In this review, we discuss control of cytokine production as a novel function of FcγRs in human innate immune cells in the context of homeostasis, infection, and autoimmunity and address the possibilities for future therapeutic exploitation.

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.

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.

Motif switches: decision-making in cell regulation

Tight regulation of gene products from transcription to protein degradation is required for reliable and robust control of eukaryotic cell physiology. Many of the mechanisms directing cell regulation rely on proteins detecting the state of the cell through context-dependent, tuneable interactions. These interactions underlie the ability of proteins to make decisions by combining regulatory information encoded in a protein's expression level, localisation and modification state. This raises the question, how do proteins integrate available information to correctly make decisions? Over the past decade pioneering work on the nature and function of intrinsically disordered protein regions has revealed many elegant switching mechanisms that underlie cell signalling and regulation, prompting a reevaluation of their role in cooperative decision-making.