C5a anaphylatoxin is a major regulator of activating versus inhibitory FcgammaRs in immune complex-induced lung disease

Lipopolysaccharide Impedes Bone Repair in FcγRIIB-Deficient Mice

Chronic inflammation contributes to the development of skeletal disorders in patients with systemic lupus erythematosus (SLE). Activation of the host immune response stimulates osteoclast activity, which in turn leads to bone loss. Regenerating bone in the inflammatory microenvironments of SLE patients with critical bone defects remains a great challenge. In this study, we utilized lipopolysaccharide (LPS) to imitate locally and systemically pathogenic bacterial infection and examined the bone regeneration performance of LPS-associated mandibular and tibial bone regeneration impairment in FcγRIIB-/- mice. Our results indicated that a loss of FcγRIIB alleviates bone regeneration in both mandibles and tibiae. After LPS induction, FcγRIIB-/- mice were susceptible to impaired fracture healing in tibial and mandibular bones. LPS decreased the mineralization to collagen ratio in FcγRIIB-/- mice, indicating a mineralization defect during bone repair. An osteoblast-associated gene (Col1a1) was attenuated in FcγRIIB-deficient mice, whereas Bglap, Hhip, and Creb5 were further downregulated with LPS treatment in FcγRIIB-/- mice compared to FcγRIIB-/- mice. Alpl and Bglap expression was dcreased in osteoblasts derived from bone chips. An osteoclast-associated gene, Tnfsf11/Tnfrsf11 ratio, ewas increased in LPS-induced FcγRIIB-/- mice and in vitro. Furthermore, systemic LPS was relatively potent in stimulating production of pro-inflammatory cytokines including TNF-α, IL-6, and MCP-1 in FcγRIIB-/- mice compared to FcγRIIB-/- mice. The levels of TNF-α, IFN-β, IL-1α, and IL-17A were increased, whereas IL-10 and IL-23 were decreased in FcγRIIB-/- mice treated locally with LPS. These findings suggest that both local and systemic LPS burden can exacerbate bone regeneration impairment, delay mineralization and skeletal repair, and induce inflammation in SLE patients.

Nomenclature of allergic diseases and hypersensitivity reactions: Adapted to modern needs: An EAACI position paper

The exponential growth of precision diagnostic tools, including omic technologies, molecular diagnostics, sophisticated genetic and epigenetic editing, imaging and nano-technologies and patient access to extensive health care, has resulted in vast amounts of unbiased data enabling in-depth disease characterization. New disease endotypes have been identified for various allergic diseases and triggered the gradual transition from a disease description focused on symptoms to identifying biomarkers and intricate pathogenetic and metabolic pathways. Consequently, the current disease taxonomy has to be revised for better categorization. This European Academy of Allergy and Clinical Immunology Position Paper responds to this challenge and provides a modern nomenclature for allergic diseases, which respects the earlier classifications back to the early 20th century. Hypersensitivity reactions originally described by Gell and Coombs have been extended into nine different types comprising antibody- (I-III), cell-mediated (IVa-c), tissue-driven mechanisms (V-VI) and direct response to chemicals (VII). Types I-III are linked to classical and newly described clinical conditions. Type IVa-c are specified and detailed according to the current understanding of T1, T2 and T3 responses. Types V-VI involve epithelial barrier defects and metabolic-induced immune dysregulation, while direct cellular and inflammatory responses to chemicals are covered in type VII. It is notable that several combinations of mixed types may appear in the clinical setting. The clinical relevance of the current approach for allergy practice will be conferred in another article that will follow this year, aiming at showing the relevance in clinical practice where various endotypes can overlap and evolve over the lifetime.

Subversion of the Complement System by Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic pathogen heavily implicated in chronic diseases. Immunocompromised patients that become infected with P. aeruginosa usually are afflicted with a lifelong chronic infection, leading to worsened patient outcomes. The complement system is an integral piece of the first line of defense against invading microorganisms. Gram-negative bacteria are thought to be generally susceptible to attack from complement; however, P. aeruginosa can be an exception, with certain strains being serum resistant. Various molecular mechanisms have been described that confer P. aeruginosa unique resistance to numerous aspects of the complement response. In this review, we summarize the current published literature regarding the interactions of P. aeruginosa and complement, as well as the mechanisms used by P. aeruginosa to exploit various complement deficiencies and the strategies used to disrupt or hijack normal complement activities.

C5a-licensed phagocytes drive sterilizing immunity during systemic fungal infection

Systemic candidiasis is a common, high-mortality, nosocomial fungal infection. Unexpectedly, it has emerged as a complication of anti-complement C5-targeted monoclonal antibody treatment, indicating a critical niche for C5 in antifungal immunity. We identified transcription of complement system genes as the top biological pathway induced in candidemic patients and as predictive of candidemia. Mechanistically, C5a-C5aR1 promoted fungal clearance and host survival in a mouse model of systemic candidiasis by stimulating phagocyte effector function and ERK- and AKT-dependent survival in infected tissues. C5ar1 ablation rewired macrophage metabolism downstream of mTOR, promoting their apoptosis and enhancing mortality through kidney injury. Besides hepatocyte-derived C5, local C5 produced intrinsically by phagocytes provided a key substrate for antifungal protection. Lower serum C5a concentrations or a C5 polymorphism that decreases leukocyte C5 expression correlated independently with poor patient outcomes. Thus, local, phagocyte-derived C5 production licenses phagocyte antimicrobial function and confers innate protection during systemic fungal infection.

The complement receptor C5aR2 regulates neutrophil activation and function contributing to neutrophil-driven epidermolysis bullosa acquisita

Introduction

The function of the second receptor for the complement cleavage product C5a, C5aR2, is poorly understood and often neglected in the immunological context. Using mice with a global deficiency of C5aR2, we have previously reported an important role of this receptor in the pathogenesis of the neutrophil-driven autoimmune disease epidermolysis bullosa acquisita (EBA). Based on in vitro analyses, we hypothesized that the absence of C5aR2 specifically on neutrophils is the cause of the observed differences. Here, we report the generation of a new mouse line with a LysM-specific deficiency of C5aR2.

Methods

LysM-specific deletion of C5aR2 was achieved by crossing LysM^cre mice with tdTomato-C5ar2^fl/fl mice in which the tdTomato-C5ar2 gene is flanked by loxP sites. Passive EBA was induced by subcutaneous injection of rabbit anti-mouse collagen type VII IgG. The effects of targeted deletion of C5ar2 on C5a-induced effector functions of neutrophils were examined in in vitro assays.

Results

We confirm the successful deletion of C5aR2 at both the genetic and protein levels in neutrophils. The mice appeared healthy and the expression of C5aR1 in bone marrow and blood neutrophils was not negatively affected by LysM-specific deletion of C5aR2. Using the antibody transfer mouse model of EBA, we found that the absence of C5aR2 in LysM-positive cells resulted in an overall amelioration of disease progression, similar to what we had previously found in mice with global deficiency of C5aR2. Neutrophils lacking C5aR2 showed decreased activation after C5a stimulation and increased expression of the inhibitory Fcγ receptor FcγRIIb.

Discussion

Overall, with the data presented here, we confirm and extend our previous findings and show that C5aR2 in neutrophils regulates their activation and function in response to C5a by potentially affecting the expression of Fcγ receptors and CD11b. Thus, C5aR2 regulates the finely tuned interaction network between immune complexes, Fcγ receptors, CD11b, and C5aR1 that is important for neutrophil recruitment and sustained activation. This underscores the importance of C5aR2 in the pathogenesis of neutrophil-mediated autoimmune diseases.

Complement as a vital nexus of the pathobiological connectome for acute respiratory distress syndrome: An emerging therapeutic target

The hallmark of acute respiratory distress syndrome (ARDS) pathobiology is unchecked inflammation-driven diffuse alveolar damage and alveolar-capillary barrier dysfunction. Currently, therapeutic interventions for ARDS remain largely limited to pulmonary-supportive strategies, and there is an unmet demand for pharmacologic therapies targeting the underlying pathology of ARDS in patients suffering from the illness. The complement cascade (ComC) plays an integral role in the regulation of both innate and adaptive immune responses. ComC activation can prime an overzealous cytokine storm and tissue/organ damage. The ARDS and acute lung injury (ALI) have an established relationship with early maladaptive ComC activation. In this review, we have collected evidence from the current studies linking ALI/ARDS with ComC dysregulation, focusing on elucidating the new emerging roles of the extracellular (canonical) and intracellular (non-canonical or complosome), ComC (complementome) in ALI/ARDS pathobiology, and highlighting complementome as a vital nexus of the pathobiological connectome for ALI/ARDS via its crosstalking with other systems of the immunome, DAMPome, PAMPome, coagulome, metabolome, and microbiome. We have also discussed the diagnostic/therapeutic potential and future direction of ALI/ARDS care with the ultimate goal of better defining mechanistic subtypes (endotypes and theratypes) through new methodologies in order to facilitate a more precise and effective complement-targeted therapy for treating these comorbidities. This information leads to support for a therapeutic anti-inflammatory strategy by targeting the ComC, where the arsenal of clinical-stage complement-specific drugs is available, especially for patients with ALI/ARDS due to COVID-19.

Impaired B cell Expression of the Inhibitory Fcγ Receptor IIB in Myasthenia Gravis

Myasthenia gravis (MG) is an autoimmune disease in which pathogenic immunoglobulin G (IgG) antibodies (Abs) bind to acetylcholine receptors (AChR) or to functionally related molecules at the neuromuscular junction. B cell expression of the inhibitory IgG receptor, FcγRIIB, maintains peripheral immune tolerance and its absence renders B cells hyperresponsive to autoantigen. Here, we report that FcγRIIB expression levels are substantially reduced in B lineage cells derived from immunotherapy-naïve patients with AChR-Ab⁺ early-onset MG (EOMG). In contrast, genetic variants associated with impaired FcγRIIB expression are not enriched in MG, indicating post-transcriptional dysregulation. FcγR-targeted therapies could have therapeutic benefits in MG. This article is protected by copyright. All rights reserved.

Analysis of Complement Gene Expression, Clinical Associations, and Biodistribution of Complement Proteins in the Synovium of Early Rheumatoid Arthritis Patients Reveals Unique Pathophysiologic Features

Rheumatoid arthritis (RA) is an autoimmune disease characterized by synovial hyperplasia and inflammation. The finding of autoantibodies in seropositive RA suggests that complement system activation might play a pathophysiologic role due to the local presence of immune complexes in the joints. Our first objective was to explore the Pathobiology of Early Arthritis Cohort (PEAC) mRNA sequencing data for correlations between clinical disease severity as measured by DAS28-ESR (disease activity score in 28 joints for erythrocyte sedimentation rate) and complement system gene expression, both in the synovium and in blood. Our second objective was to determine the biodistribution using multiplex immunohistochemical staining of specific complement activation proteins and inhibitors from subjects in the Accelerating Medicines Partnership (AMP) RA/SLE study. In the PEAC study, there were significant positive correlations between specific complement gene mRNA expression levels in the synovium and DAS28-ESR for the following complement genes: C2, FCN1, FCN3, CFB, CFP, C3AR1, C5AR1, and CR1 Additionally, there were significant negative correlations between DAS28-ESR and Colec12, C5, C6, MASP-1, CFH, and MCP In the synovium there were also significant positive correlations between DAS28-ESR and FcγR1A, FcγR1B, FcγR2A, and FcγR3A Notably, CFHR4 synovial expression was positively correlated following treatment with the DAS28-ESR at 6 mo, suggesting a role in worse therapeutic responses. The inverse correlation of C5 RNA expression in the synovium may underlie the failure of significant benefit from C5/C5aR inhibitors in clinical trials performed in patients with RA. Multiplex immunohistochemical analyses of early RA synovium reveal significant evidence of regional alterations of activation and inhibitory factors that likely promote local complement activation.

Trastuzumab/Pertuzumab combination therapy stimulates anti-tumor responses through complement-dependent cytotoxicity and phagocytosis

Standard-of-care treatment for advanced HER2+ breast cancers (BC) is comprised of two HER2-specific monoclonal antibodies (mAb), Trastuzumab (T) and Pertuzumab (P) with chemotherapy. While this combination (T+P) is highly effective, its synergistic mechanism of action (MOA) is not completely known. Initial studies had demonstrated that Pertuzumab suppressed HER2 hetero-dimerization as the potential therapeutic MOA, thus the improved outcome associated with the T+P combination MOA compared to Trastuzumab alone has been widely reported as being due to Pertuzumab-mediated suppression of HER2 signaling in combination with Trastuzumab-mediated induction of anti-tumor immunity. Unraveling this MOA may be critical to extend this combination strategy to other antigens or other cancers, as well as improving this current treatment modality. Using novel murine and human versions of Pertuzumab, we found it induced both Antibody-Dependent-Cellular-Phagocytosis (ADCP) by tumor-associated macrophages and suppression of HER2 oncogenic signaling. Most significantly, we identified that only T+P combination therapy, but not when either antibody used in isolation, allows for the activation of the classical complement pathway, resulting in both direct complement-dependent cytotoxicity (CDC) as well as complement-dependent cellular phagocytosis (CDCP) of HER2+ BC cells. Notably, we show that tumor expression of C1q was positively associated with survival outcome in HER2+ BC patients, whereas expression of complement regulators CD55 and CD59 were inversely correlated, suggesting the importance of complement activity in clinical outcomes. Accordingly, inhibition of C1 activity in mice abolished the synergistic therapeutic activity of T+P therapy, whereas knockdown of CD55 and CD59 expression enhanced T+P efficacy. In summary, our study identifies classical complement activation as a significant anti-tumor MOA for T+P therapy that may be functionally enhanced to augment therapeutic efficacy in the clinic.

C5aR2 deficiency ameliorates inflammation in murine epidermolysis bullosa acquisita by regulating FcγRIIb expression on neutrophils

Epidermolysis bullosa acquisita (EBA) is a rare blistering skin disease induced by autoantibodies directed against type VII collagen (COL7). Transfer of antibodies against murine COL7 (mCOL7) into mice mimics the effector phase of EBA and results in a subepidermal blistering phenotype. Activation of the complement system, and especially the C5a/C5aR1 axis driving neutrophil activation, are critical for EBA pathogenesis. However, the role of the alternative C5a receptor, C5aR2, which is commonly thought to be more immunosuppressive, in the pathogenesis of EBA is still elusive. Therefore, we sought to delineate the functional relevance of C5aR2 during the effector phase of EBA. Unexpectedly, C5aR2-deficient (C5ar2^-/-) mice showed an attenuated disease phenotype, suggesting a pathogenic contribution of C5aR2 to disease progression. In vitro, C5ar2^-/- neutrophils exhibited significantly reduced (Ca²⁺)_i flux, reactive oxygen species release, and migratory capacity when activated with immune complexes or exposed to C5a. These functions were completely absent when C5ar1^-/- neutrophils were activated. Moreover, C5aR2 deficiency more than tripled FcγRIIb expression on neutrophils thus lowering the A/I ratio of FcγRs and impeding the sustainment of inflammation. Collectively, we demonstrate here a pro-inflammatory contribution of C5aR2 to the pathogenesis of antibody-induced tissue damage in experimental EBA.

There Is Strength in Numbers: Quantitation of Fc Gamma Receptors on Murine Tissue-Resident Macrophages

Many of the effector functions of antibodies rely on the binding of antibodies/immune complexes to cellular Fcγ receptors (FcγRs). Since the majority of innate immune effector cells express both activating and inhibitory Fc receptors, the outcome of the binding of immune complexes to cells of a given population is influenced by the relative affinities of the respective IgG subclasses to these receptors, as well as by the numbers of activating and inhibitory FcγRs on the cell surface. A group of immune cells that has come into focus more recently is the various subsets of tissue-resident macrophages. The central functions of FcγRs on tissue macrophages include the clearance of opsonized pathogens, the removal of small immune complexes from the circulation and the depletion of antibody-opsonized cells in the therapy of autoimmunity and cancer. Despite these essential functions of FcγRs on tissue-resident macrophages, an in-depth quantification of FcγRs is lacking. Thus, the aim of our current study was to quantify the various Fcγ receptors on macrophages in murine liver, lung, kidney, brain, skin and spleen. Our study identified a pronounced heterogeneity between FcγR expression patterns of the different tissue macrophages, which may reflect their specialized functions within their unique niches in different organ environments.

Complement Activation in the Treatment of B-Cell Malignancies

Unconjugated monoclonal antibodies (mAb) have revolutionized the treatment of B-cell malignancies. These targeted drugs can activate innate immune cytotoxicity for therapeutic benefit. mAb activation of the complement cascade results in complement-dependent cytotoxicity (CDC) and complement receptor-mediated antibody-dependent cellular phagocytosis (cADCP). Clinical and laboratory studies have showed that CDC is therapeutically important. In contrast, the biological role and clinical effects of cADCP are less well understood. This review summarizes the available data on the role of complement activation in the treatment of mature B-cell malignancies and proposes future research directions that could be useful in optimizing the efficacy of this important class of drugs.

Complement System: a Neglected Pathway in Immunotherapy

Approved for the treatment of autoimmune diseases, hematological malignancies, and solid cancers, several monoclonal antibodies (mAb) make use of complement in their mechanism of action. Such an assessment is based on comprehensive investigations that used mouse models, in vitro studies, and analyses from patients at initiation (basal level to highlight deficiencies) and after treatment initiation (mAb impact on complement), which have further provided key insights into the importance of the complement activation and/or complement deficiencies in mAb activity. Accordingly, new approaches can now be developed with the final objective of increasing the clinical efficacy of mAb. These improvements include (i) the concurrent administration of fresh frozen plasma during mAb therapy; (ii) mAb modifications such as immunoglobulin G subclass switching, Fc mutation, or IgG hexamerization to improve the fixation and activation of C1q; (iii) optimization of the target recognition to induce a higher complement-dependent cytotoxicity (CDC) and/or complement-dependant cellular cytotoxicity (CDCC); and (iv) the control of soluble and cellular complement inhibitors.

Glycosylation-dependent opsonophagocytic activity of staphylococcal protein A antibodies

All currently licensed antibodies against bacteria target exotoxins. For most pathogens, neutralization of toxin(s) is not sufficient to prevent bacterial replication. Antibodies against surface determinants represent better candidates to enhance opsonophagocytic killing, but the mechanisms of action of such antibodies have not been systematically studied. Staphylococcal protein A is a conserved surface protein of Staphylococcus aureus and a crucial virulence determinant that manipulates B-cell responses and blocks deposition of opsonin. Monoclonal antibodies directed against SpA represent potential therapeutic agents as well as a formidable tool to identify and optimize effector functions of antibodies that can promote bacterial clearance.

Antibodies may bind to bacterial pathogens or their toxins to control infections, and their effector activity is mediated through the recruitment of complement component C1q or the engagement with Fcγ receptors (FcγRs). For bacterial pathogens that rely on a single toxin to cause disease, immunity correlates with toxin neutralization. Most other bacterial pathogens, including Staphylococcus aureus, secrete numerous toxins and evolved multiple mechanisms to escape opsonization and complement killing. Several vaccine candidates targeting defined surface antigens of S. aureus have failed to meet clinical endpoints. It is unclear that such failures can be solely attributed to the poor selection of antibody targets. Thus far, studies to delineate antibody-mediated uptake and killing of Gram-positive pathogens remain extremely limited. Here, we exploit 3F6-hIgG1, a human monoclonal antibody that binds and neutralizes the abundant surface-exposed Staphylococcal protein A (SpA). We find that galactosylation of 3F6-hIgG1 that favors C1q recruitment is indispensable for opsonophagocytic killing of staphylococci and for protection against bloodstream infection in animals. However, the simple removal of fucosyl residues, which results in reduced C1q binding and increased engagement with FcγR, maintains the opsonophagocytic killing and protective attributes of the antibody. We confirm these results by engineering 3F6-hIgG1 variants with biased binding toward C1q or FcγRs. While the therapeutic benefit of monoclonal antibodies against infectious disease agents may be debatable, the functional characterization of such antibodies represents a powerful tool for the development of correlates of protection that may guide future vaccine trials.

Mechanistic inferences from clinical reports of SARS-CoV-2

SARS-CoV-2 was identified as the causative pathogen in an outbreak of viral pneumonia cases originating in Wuhan, China, with an ensuing rapid global spread that led it to be declared a pandemic by the WHO on March 11, 2020. Given the threat to public health posed by sequelae of SARS-CoV-2 infection, the literature surrounding patient presentation in severe and non-severe cases, transmission rates and routes, management strategies, and initial clinical trial results have become available at an unprecedented pace. In this review we collate current clinical and immunologic reports, comparing these to reports of previous coronaviruses to identify mechanisms driving progression to severe disease in some patients. In brief, we propose a model wherein dysregulated type I interferon signalling leads to aberrant recruitment and accumulation of innate immune lineages in the lung, impairing establishment of productive adaptive responses, and permitting a pathologic pro-inflammatory state. Finally, we extend these findings to suggest possible treatment options that may merit investigation in randomized clinical trials.

Fc Gamma Receptors and Complement Component 3 Facilitate Anti-fVIII Antibody Formation

Anti-factor VIII (fVIII) alloantibodies, which can develop in patients with hemophilia A, limit the therapeutic options and increase morbidity and mortality of these patients. However, the factors that influence anti-fVIII antibody development remain incompletely understood. Recent studies suggest that Fc gamma receptors (FcγRs) may facilitate recognition and uptake of fVIII by recently developed or pre-existing naturally occurring anti-fVIII antibodies, providing a mechanism whereby the immune system may recognize fVIII following infusion. However, the role of FcγRs in anti-fVIII antibody formation remains unknown. In order to define the influence of FcγRs on the development of anti-fVIII antibodies, fVIII was injected into WT or FcγR knockout recipients, followed by evaluation of anti-fVIII antibodies. Anti-fVIII antibodies were readily observed following fVIII injection into FcγR knockouts, with similar anti-fVIII antibody levels occurring in FcγR knockouts as detected in WT mice injected in parallel. As antibodies can also fix complement, providing a potential mechanism whereby anti-fVIII antibodies may influence anti-fVIII antibody formation independent of FcγRs, fVIII was also injected into complement component 3 (C3) knockout recipients in parallel. Similar to FcγR knockouts, C3 knockout recipients developed a robust response to fVIII, which was likewise similar to that observed in WT recipients. As FcγRs or C3 may compensate for each other in recipients only deficient in FcγRs or C3 alone, we generated mice deficient in both FcγRs and C3 to test for potential antibody effector redundancy in anti-fVIII antibody formation. Infusion of fVIII into FcγRs and C3 (FcγR × C3) double knockouts likewise induced anti-fVIII antibodies. However, unlike individual knockouts, anti-fVIII antibodies in FcγRs × C3 knockouts were initially lower than WT recipients, although anti-fVIII antibodies increased to WT levels following additional fVIII exposure. In contrast, infusion of RBCs expressing distinct alloantigens into FcγRs, C3 or FcγR × C3 knockout recipients either failed to change anti-RBC levels when compared to WT recipients or actually increased antibody responses, depending on the target antigen. Taken together, these results suggest FcγRs and C3 can differentially impact antibody formation following exposure to distinct alloantigens and that FcγRs and C3 work in concert to facilitate early anti-fVIII antibody formation.

Anti-Citrullinated Protein Antibodies in Patients with Rheumatoid Arthritis: Biological Effects and Mechanisms of Immunopathogenesis

Individuals with high anti-citrullinated protein antibody (ACPA) titers have an increased risk of developing rheumatoid arthritis (RA). Although our knowledge of the generation and production of ACPAs has continuously advanced during the past decade, our understanding on the pathogenic mechanisms of how ACPAs interact with immune cells to trigger articular inflammation is relatively limited. Citrullination disorders drive the generation and maintenance of ACPAs, with profound clinical significance in patients with RA. The loss of tolerance to citrullinated proteins, however, is essential for ACPAs to exert their pathogenicity. N-linked glycosylation, cross-reactivity and the structural interactions of ACPAs with their citrullinated antigens further direct their biological functions. Although questions remain in the pathogenicity of ACPAs acting as agonists for a receptor-mediated response, immune complex (IC) formation, complement system activation, crystallizable fragment gamma receptor (FcγR) activation, cross-reactivity to joint cartilage and neutrophil extracellular trap (NET)-related mechanisms have all been suggested recently. This paper presents a critical review of the characteristics and possible biological effects and mechanisms of the immunopathogenesis of ACPAs in patients with RA.

Immunotherapy in Multiple Myeloma

Multiple myeloma is a complex disease and immune dysfunction has been known to play an important role in the disease pathogenesis, progression, and drug resistance. Recent efforts in drug development have been focused on immunotherapies to modify the MM disease process. Here, we summarize the emerging immunotherapies in the MM treatment landscape.

Mechanisms of Resistance to Anti-CD38 Daratumumab in Multiple Myeloma

Daratumumab (Dara) is the first-in-class human-specific anti-CD38 mAb approved for the treatment of multiple myeloma (MM). Although recent data have demonstrated very promising results in clinical practice and trials, some patients do not achieve a partial response, and ultimately all patients undergo progression. Dara exerts anti-MM activity via antibody-dependent cell-mediated cytotoxicity (ADCC), antibody-dependent cellular phagocytosis (ADCP), complement-dependent cytotoxicity (CDC), and immunomodulatory effects. Deregulation of these pleiotropic mechanisms may cause development of Dara resistance. Knowledge of this resistance may improve the therapeutic management of MM patients.

An intranasally administered monoclonal antibody cocktail abrogates ricin toxin-induced pulmonary tissue damage and inflammation

Ricin toxin, a plant-derived, mannosylated glycoprotein, elicits an incapacitating and potentially lethal inflammatory response in the airways following inhalation. Uptake of ricin by alveolar macrophages (AM) and other pulmonary cell types occurs via two parallel pathways: one mediated by ricin's B subunit (RTB), a galactose-specific lectin, and one mediated by the mannose receptor (MR;CD206). Ricin's A subunit (RTA) is a ribosome-inactivating protein that triggers apoptosis in mammalian cells. It was recently reported that a single monoclonal antibody (MAb), PB10, directed against an immunodominant epitope on RTA and administered intravenously, was able to rescue Rhesus macaques from lethal aerosol dose of ricin. In this study, we now demonstrate in mice that the effectiveness PB10 is significantly improved when combined with a second MAb, SylH3, against RTB. Mice treated with PB10 alone survived lethal-dose intranasal ricin challenge, but experienced significant weight loss, moderate pulmonary inflammation (e.g., elevated IL-1 and IL-6 levels, PMN influx), and apoptosis of lung macrophages. In contrast, mice treated with the PB10/SylH3 cocktail were essentially impervious to pulmonary ricin toxin exposure, as evidenced by no weight loss, no change in local IL-1 and IL-6 levels, retention of lung macrophages, and a significant dampening of PMN recruitment into the bronchoalveolar lavage (BAL) fluids. The PB10/SylH3 cocktail only marginally reduced ricin binding to target cells in the BAL, suggesting that the antibody mixture neutralizes ricin by interfering with one or more steps in the RTB- and MR-dependent uptake pathways.

The Complex Association of FcγRIIb With Autoimmune Susceptibility

FcγRIIb is the only inhibitory Fc receptor and controls many aspects of immune and inflammatory responses. The observation 19 years ago that Fc γ RIIb ^-/- mice generated by gene targeting in 129 derived ES cells developed severe lupus like disease when backcrossed more than 7 generations into C57BL/6 background initiated extensive research on the functional understanding of this strong autoimmune phenotype. The genomic region in the distal part of Chr1 both in human and mice in which the Fc γ R gene cluster is located shows a high level of complexity in relation to the susceptibility to SLE. Specific haplotypes of closely linked genes including the Fc γ RIIb and Slamf genes are associated with increased susceptibility to SLE both in mice and human. Using forward and reverse genetic approaches including in human GWAS and in mice congenic strains, KO mice (germline and cell type specific, on different genetic background), knockin mice, overexpressing transgenic mice combined with immunological models such as adoptive transfer of B cells from Ig transgenic mice the involved genes and the causal mutations and their associated functional alterations were analyzed. In this review the results of this 19 years extensive research are discussed with a focus on (genetically modified) mouse models.

Complement activation on neutrophils initiates endothelial adhesion and extravasation

Neutrophils are essential to the pathogenesis of many inflammatory diseases. In the autoantibody-mediated K/BxN model of inflammatory arthritis, the alternative pathway (AP) of complement and Fc gamma receptors (FcγRs) are required for disease development while the classical pathway is dispensable. The reason for this differential requirement is unknown. We show that within minutes of K/BxN serum injection complement activation (CA) is detected on circulating neutrophils, as evidenced by cell surface C3 fragment deposition. CA requires the AP factor B and FcγRs but not C4, implying that engagement of FcγRs by autoantibody or immune complexes directly triggers AP C3 convertase assembly. The absence of C5 does not prevent CA on neutrophils but diminishes the upregulation of adhesion molecules. In vivo two-photon microscopy reveals that CA on neutrophils is critical for neutrophil extravasation and generation of C5a at the site of inflammation. C5a stimulates the release of neutrophil proteases, which contribute to the degradation of VE-cadherin, an adherens junction protein that regulates endothelial barrier integrity. C5a receptor antagonism blocks the extracellular release of neutrophil proteases, suppressing VE-cadherin degradation and neutrophil transendothelial migration in vivo. These results elucidate the AP-dependent intravascular neutrophil-endothelial interactions that initiate the inflammatory cascade in this disease model but may be generalizable to neutrophil extravasation in other inflammatory processes.

C5a receptor 1-/- mice are protected from the development of IgE-mediated experimental food allergy

BACKGROUND

Food-induced anaphylaxis is a serious allergic reaction caused by Fcε-receptor activation on mast cells (MCs). The exact mechanisms breaking oral tolerance and the effector pathways driving food allergy remain elusive. As complement is activated in food-induced anaphylaxis, we aimed to assess the role of C5a in disease pathogenesis.

METHODS

Oral antigen-induced food-induced anaphylaxis was induced in BALB/c wild-type (wt) and C5ar1-/- mice. Readouts included diarrhea development, changes in rectal temperature, hematocrit, antigen-specific serum IgE, MCPT-1, and intestinal MC numbers, as well as FcεR1-mediated MC functions including C5a receptor 1 (C5aR1) regulation. Further, histamine-mediated hypothermia and regulation of endothelial tight junctions were determined.

RESULTS

Repeated oral OVA challenge resulted in diarrhea, hypothermia, increased hematocrit, high OVA-specific serum IgE, and MCPT-1 levels in wt mice. Male C5ar1-/- mice were completely whereas female C5ar1-/- mice were partially protected from anaphylaxis development. Serum MCPT-1 levels were reduced gender-independent, whereas IgE levels were reduced in male but not in female C5ar1-/- mice. Mechanistically, IgE-mediated degranulation and IL-6 production from C5ar1-/- BMMCs of both sexes were significantly reduced. Importantly, FcεR1 cross-linking strongly upregulated C5aR1 MC expression in vitro and in vivo. Finally, C5ar1-/- male mice were largely protected from histamine-induced hypovolemic shock, which was associated with protection from histamine-induced barrier dysfunction in vitro following C5aR targeting.

CONCLUSIONS

Our findings identify C5aR1 activation as an important driver of IgE-mediated food allergy through regulation of allergen-specific IgE production, FcεR1-mediated MC degranulation, and histamine-driven effector functions preferentially in male mice.

A histopathologic study of the controlling role of T cells on experimental periodontitis in rats

Background/purpose

The onset and progression of periodontitis involve bacterial infection and the immune response. T cells function in the immune response and reportedly induce bone resorption in inflammatory bone loss. However, the exact role of T cells in periodontal destruction remains unclear. Using our experimental model of periodontitis, we aimed to investigate the influence of T cells on periodontal destruction.

Materials and methods

Male athymic nude (Nu) and euthymic wild-type (WT) rats were divided into the immunized (I-Nu and I-WT), non-immunized (nI-Nu and nI-WT). The immunized groups were immunized intraperitoneally with lipopolysaccharide (LPS). The non-immunized groups received phosphate-buffered saline (PBS). Nothing was administered to the non-treated groups. LPS was applied to the right palatal gingival sulcus in the immunized and non-immunized groups daily for 20 days. Loss of attachment, numbers of inflammatory cells and osteoclasts, and levels of alveolar bone were investigated histopathologically and histometrically. Osteoclasts were stained with tartrate-resistant acid phosphatase. The numbers of IL-4-positive cells were evaluated immunohistologically.

Results

Loss of attachment, numbers of inflammatory cells, levels of alveolar bone, and the number of osteoclasts were significantly increased in the nI-WT group compared with the nI-Nu group. However, the parameters were significantly increased in the I-Nu group compared with the I-WT group. The number of IL-4-positive cells was greater in the I-WT group than in the I-Nu group.

Conclusion

T cells promote inflammation in non-immunized animals; however, they regulate these processes in immunized animals.

The Human FcγRII (CD32) Family of Leukocyte FcR in Health and Disease

FcγRs have been the focus of extensive research due to their key role linking innate and humoral immunity and their implication in both inflammatory and infectious disease. Within the human FcγR family FcγRII (activatory FcγRIIa and FcγRIIc, and inhibitory FcγRIIb) are unique in their ability to signal independent of the common γ chain. Through improved understanding of the structure of these receptors and how this affects their function we may be able to better understand how to target FcγR specific immune activation or inhibition, which will facilitate in the development of therapeutic monoclonal antibodies in patients where FcγRII activity may be desirable for efficacy. This review is focused on roles of the human FcγRII family members and their link to immunoregulation in healthy individuals and infection, autoimmunity and cancer.

Antibody-dependent and -independent mechanisms of inflammatory arthritis

Inflammatory arthritis encompasses a set of common diseases characterized by immune-mediated attack on joint tissues. Most but not all affected patients manifest circulating autoantibodies. Decades of study in human and animal arthritis have identified key roles for autoantibodies in immune complexes and through direct modulation of articular biology. However, joint inflammation can arise because of pathogenic T cells and other pathways that are antibody-independent. Here we review the evidence for these parallel tracks, in animal models and in humans, to explore the range of mechanisms engaged in the pathophysiology of arthritis and to highlight opportunities for targeted therapeutic intervention.

Ferumoxytol Is Not Retained in Kidney Allografts in Patients Undergoing Acute Rejection

PURPOSE

To evaluate whether ultrasmall superparamagnetic iron oxide nanoparticle (USPIO)-enhanced magnetic resonance imaging (MRI) can detect allograft rejection in pediatric kidney transplant patients.

PROCEDURES

The USPIO ferumoxytol has a long blood half-life and is phagocytosed by macrophages. In an IRB-approved single-center prospective clinical trial, 26 pediatric patients and adolescents (age 10-26 years) with acute allograft rejection (n = 5), non-rejecting allografts (n = 13), and normal native kidneys (n = 8) underwent multi-echo T2* fast spoiled gradient-echo (FSPGR) MRI after intravenous injection (p.i.) of 5 mg Fe/kg ferumoxytol. T2* relaxation times at 4 h p.i. (perfusion phase) and more than 20 h p.i. (macrophage phase) were compared with biopsy results. The presence of rejection was assessed using the Banff criteria, and the prevalence of macrophages on CD163 immunostains was determined based on a semi-quantitative scoring system. MRI and histology data were compared among patient groups using t tests, analysis of variance, and regression analyses with a significance threshold of p < 0.05.

RESULTS

At 4 h p.i., mean T2* values were 6.6 ± 1.5 ms for native kidneys and 3.9 ms for one allograft undergoing acute immune rejection. Surprisingly, at 20-24 h p.i., one rejecting allograft showed significantly prolonged T2* relaxation times (37.0 ms) compared to native kidneys (6.3 ± 1.7 ms) and non-rejecting allografts (7.6 ± 0.1 ms). Likewise, three additional rejecting allografts showed significantly prolonged T2* relaxation times compared to non-rejecting allografts at later post-contrast time points, 25-97 h p.i. (p = 0.008). Histological analysis revealed edema and compressed microvessels in biopsies of rejecting allografts. Allografts with and without rejection showed insignificant differences in macrophage content on histopathology (p = 0.44).

CONCLUSION

After ferumoxytol administration, renal allografts undergoing acute rejection show prolonged T2* values compared to non-rejecting allografts. Since histology revealed no significant differences in macrophage content, the increasing T2* value is likely due to the combined effect of reduced perfusion and increased edema in rejecting allografts.

Effects of complement and serum IgG on rituximab-dependent natural killer cell-mediated cytotoxicity against Raji cells

Accumulating evidence indicates that the anti-CD20 monoclonal antibody rituximab significantly improves the clinical prognosis of patients with non-Hodgkin lymphoma and chronic lymphocytic leukemia. However, a number of patients relapse or fail to respond to rituximab. To further understand the cause of this, polymorphisms of FcγRIIIa were initially detected in healthy volunteers. Subsequently, the rituximab-dependent natural killer (NK) cell-mediated cytotoxicity of different FcγRIIIa genotypes was assessed by a cytotoxicity assay in vitro. Ultimately, the effect of human serum immunoglobulin (Ig) G and complement on rituximab-dependent NK cell-mediated cytotoxicity was evaluated in vitro. It was revealed that FcγRIIIa polymorphisms were associated with the antibody-dependent cell-mediated cytotoxicity (ADCC) of NK cells. In addition, the ADCC of NK cells with FcγRIIIa-158 V/V was increased compared with that of FcγRIIIa-158 V/F. The serum IgG and rituximab Fc segment was able to bind competitively with NK cell FcγRIIIa. It was observed that serum IgG inhibited, whereas complement enhanced rituximab-induced NK-cell mediated ADCC. Therefore, various agents administered synchronously with rituximab may modulate the efficacy of this agent and ultimately its toxicity against tumor cells.

The Pyrazole Derivative BTP2 Attenuates IgG Immune Complex-induced Inflammation

Store-operated calcium entry (SOCE) is the most common mode of calcium influx in non-excitable cells, including immune cells. The two STIM isoforms mediate SOCE as well as Fc receptor (FcR)-downstream activation of macrophages and mast cells-which appears to be relevant in vivo, in models of antibody-dependent tissue injury and allergy. Hence, the pathway of SOCE may be a therapeutic target for treatment of immune complex (IC)-mediated autoimmunity and allergic asthma. The pyrazole derivative, BTP2 is an efficient inhibitor of SOCE, which has already been shown to attenuate allergic inflammation. However, its effect on Fc gamma receptor (FcγR) signaling and IC-induced tissue injury had not yet been studied. Here, we show that BTP2 is a potent inhibitor of SOCE in primary macrophages, blocking FcγR-mediated responses. To investigate the effect of inhibition of SOCE in IC-mediated tissue injury, we induced reverse passive Arthus reaction to IgG immune complexes in the skin and lungs of BTP2- or control-treated mice. Treatment with BTP2 resulted in markedly attenuated inflammation in both the skin and the lungs. Our findings indicate the involvement of SOCE in FcγR-mediated responses in vitro and in vivo and suggest that BTP2-mediated inhibition of SOCE may have a therapeutic potential on IC-mediated autoimmunity.

CD38 Antibodies in Multiple Myeloma: Mechanisms of Action and Modes of Resistance

MM cells express high levels of CD38, while CD38 is expressed at relatively low levels on normal lymphoid and myeloid cells, and in some non-hematopoietic tissues. This expression profile, together with the role of CD38 in adhesion and as ectoenzyme, resulted in the development of CD38 antibodies for the treatment of multiple myeloma (MM). At this moment several CD38 antibodies are at different phases of clinical testing, with daratumumab already approved for various indications both as monotherapy and in combination with standards of care in MM. CD38 antibodies have Fc-dependent immune effector mechanisms, such as complement-dependent cytotoxicity (CDC), antibody-dependent cellular cytotoxicity (ADCC), and antibody-dependent cellular phagocytosis (ADCP). Inhibition of ectoenzymatic function and direct apoptosis induction may also contribute to the efficacy of the antibodies to kill MM cells. The CD38 antibodies also improve host-anti-tumor immunity by the elimination of regulatory T cells, regulatory B cells, and myeloid-derived suppressor cells. Mechanisms of primary and/or acquired resistance include tumor-related factors, such as reduced cell surface expression levels of the target antigen and high levels of complement inhibitors (CD55 and CD59). Differences in frequency or activity of effector cells may also contribute to differences in outcome. Furthermore, the microenvironment protects MM cells to CD38 antibody-induced ADCC by upregulation of anti-apoptotic molecules, such as survivin. Improved understanding of modes of action and mechanisms of resistance has resulted in rationally designed CD38-based combination therapies, which will contribute to further improvement in outcome of MM patients.

Complement in the Initiation and Evolution of Rheumatoid Arthritis

The complement system is a major component of the immune system and plays a central role in many protective immune processes, including circulating immune complex processing and clearance, recognition of foreign antigens, modulation of humoral and cellular immunity, removal of apoptotic and dead cells, and engagement of injury resolving and tissue regeneration processes. In stark contrast to these beneficial roles, however, inadequately controlled complement activation underlies the pathogenesis of human inflammatory and autoimmune diseases, including rheumatoid arthritis (RA) where the cartilage, bone, and synovium are targeted. Recent studies of this disease have demonstrated that the autoimmune response evolves over time in an asymptomatic preclinical phase that is associated with mucosal inflammation. Notably, experimental models of this disease have demonstrated that each of the three major complement activation pathways plays an important role in recognition of injured joint tissue, although the lectin and amplification pathways exhibit particularly impactful roles in the initiation and amplification of damage. Herein, we review the complement system and focus on its multi-factorial role in human patients with RA and experimental murine models. This understanding will be important to the successful integration of the emerging complement therapeutics pipeline into clinical care for patients with RA.

The critical role of C5a as an initiator of neutrophil-mediated autoimmune inflammation of the joint and skin

The deposition of IgG autoantibodies in peripheral tissues and the subsequent activation of the complement system, which leads to the accumulation of the anaphylatoxin C5a in these tissues, is a common hallmark of diverse autoimmune diseases, including rheumatoid arthritis (RA) and pemphigoid diseases (PDs). C5a is a potent chemoattractant for granulocytes and mice deficient in its precursor C5 or its receptor C5aR1 are resistant to granulocyte recruitment and, consequently, to tissue inflammation in several models of autoimmune diseases. However, the mechanism whereby C5a/C5aR regulates granulocyte recruitment in these diseases has remained elusive. Mechanistic studies over the past five years into the role of C5a/C5aR1 in the K/BxN serum arthritis mouse model have provided novel insights into the mechanisms C5a/C5aR1 engages to initiate granulocyte recruitment into the joint. It is now established that the critical actions of C5a/C5aR1 do not proceed in the joint itself, but on the luminal endothelial surface of the joint vasculature, where C5a/C5aR1 mediate the arrest of neutrophils on the endothelium by activating β₂ integrin. Then, C5a/C5aR1 induces the release of leukotriene B₄ (LTB₄) from the arrested neutrophils. The latter, subsequently, initiates by autocrine/paracrine actions via its receptor BLT1 the egress of neutrophils from the blood vessel lumen into the interstitial. Compelling evidence suggests that this C5a/C5aR1-LTB₄/BLT1 axis driving granulocyte recruitment in arthritis may represent a more generalizable biological principle critically regulating effector cell recruitment in other IgG autoantibody-induced diseases, such as in pemphigoid diseases. Thus, dual inhibition of C5a and LTB₄, as implemented in nature by the lipocalin coversin in the soft-tick Ornithodoros moubata, may constitute a most effective therapeutic principle for the treatment of IgG autoantibody-driven diseases.

Targeting IgG in Arthritis: Disease Pathways and Therapeutic Avenues

Rheumatoid arthritis (RA) is a polygenic and multifactorial syndrome. Many complex immunological and genetic interactions are involved in the final outcome of the clinical disease. Autoantibodies (rheumatoid factors, anti-citrullinated peptide/protein antibodies) are present in RA patients' sera for a long time before the onset of clinical disease. Prior to arthritis onset, in the autoantibody response, epitope spreading, avidity maturation, and changes towards a pro-inflammatory Fc glycosylation phenotype occurs. Genetic association of epitope specific autoantibody responses and the induction of inflammation dependent and independent changes in the cartilage by pathogenic autoantibodies emphasize the crucial contribution of antibody-initiated inflammation in RA development. Targeting IgG by glyco-engineering, bacterial enzymes to specifically cleave IgG/alter N-linked Fc-glycans at Asn 297 or blocking the downstream effector pathways offers new avenues to develop novel therapeutics for arthritis treatment.

Progressive changes in composition of lymphocytes in lung tissues from patients with non-small-cell lung cancer

Immune cell infiltration is a common feature of many human solid tumors. Innate and adaptative immune systems contribute to tumor immunosurveillance. We investigated whether tumors evade immune surveillance by inducing states of tolerance and/or through the inability of some immune subpopulations to effectively penetrate tumor nests. Immunohistochemistry and flow cytometry analysis were used to study the composition and distribution of immune subpopulations in samples of peripheral blood, tumor tissue (TT), adjacent tumor tissue (ATT), distant non-tumor tissue (DNTT), cancer nests, cancer stroma, and invasive margin in 61 non-small-cell lung cancer (NSCLC) patients. A significantly higher percentage of T and B cells and significantly lower percentage of NK cells were detected in TT than in DNTT. Memory T cells (CD4+CD45RO+, CD8+CD45RO+) and activated T cells (CD8+DR+) were more prevalent in TT. Alongside this immune activation, the percentage of T cells with immunosuppressive activity was higher in TT than in DNTT. B- cells were practically non-existent in tumor nests and were preferentially located in the invasive margin. The dominant NK cell phenotype in peripheral blood and DNTT was the cytotoxic phenotype (CD56+ CD16+), while the presence of these cells was significantly decreased in ATT and further decreased in TT. Finally, the immunologic response differed between adenocarcinoma and squamous cell carcinoma and according to the tumor differentiation grade. These findings on the infiltration of innate and adaptative immune cells into tumors contribute to a more complete picture of the immune reaction in NSCLC.

C5aR activation in the absence of C5a: A new disease mechanism of autoimmune hemolytic anemia in mice

IgG Fc receptors (FcγRs) and the C5a anaphylatoxin receptor (C5aR) were identified as key regulators of type II autoimmune injury in mice. However, and with respect to C5aR, the relative importance of C5a for IgG autoantibody-induced cellular destruction remained unclear. Using an experimental model of autoimmune hemolytic anemia (AIHA), we here report marked differences in the development of AIHA between mice lacking C5aR and C5-deficient (Hc⁰ ) strain, indicating a limited role of C5 in this type of C5aR-regulated disease. Ex-vivo-analyses of liver homogenates from anemic Hc⁰ mice demonstrate C5a-independent C5aR activation, upregulation of FcγR expression and amplification of erythrophagocytosis by macrophages. As assessed by pharmacological inhibition studies, targeting of C5aR, but not of C5, is effective in treating experimental AIHA. Collectively, these results define a previously unrecognized disease mechanism of C5aR activation in AIHA that does not necessarily involve C5 and C5a.

More than just antibodies: Protective mechanisms of a mucosal vaccine against fish pathogen Flavobacterium columnare

A recently developed attenuated vaccine for Flavobacterium columnare has been demonstrated to provide superior protection for channel catfish, Ictalurus punctatus, against genetically diverse columnaris isolates. We were interested in examining the mechanisms of this protection by comparing transcriptional responses to F. columnare challenge in vaccinated and unvaccinated juvenile catfish. Accordingly, 58 day old fingerling catfish (28 days post-vaccination or unvaccinated control) were challenged with a highly virulent F. columnare isolate (BGSF-27) and gill tissues collected pre-challenge (0 h), and 1 h and 2 h post infection, time points previously demonstrated to be critical in early host-pathogen interactions. Following RNA-sequencing and transcriptome assembly, differential expression (DE) analysis within and between treatments revealed several patterns and pathways potentially underlying improved survival of vaccinated fish. Most striking was a pattern of dramatically higher basal expression of an array of neuropeptides (e.g. somatostatin), hormones, complement factors, and proteases at 0 h in vaccinated fish. Previous studies indicate these are likely the preformed mediators of neuroendocrine cells and/or eosinophilic granular (mast-like) cells within the fish gill. Following challenge, these elements fell to almost undetectable levels (>100-fold downregulated) by 1 h in vaccinated fish, suggesting their rapid release and/or cessation of synthesis following degranulation. Concomitantly, levels of pro-inflammatory cytokines (IL-1b, IL-8, IL-17) were induced in unvaccinated fish. In contrast, in vaccinated catfish, we observed widespread induction of genes needed for collagen deposition and tissue remodeling. Taken together, our results indicate an important component of vaccine protection in fish mucosal tissues may be the sensitization, proliferation and arming of resident secretory cells in the period between primary and secondary challenge.

Regulation and function of anaphylatoxins and their receptors in allergic asthma

Allergic asthma is a disease of the airways driven by maladaptive T helper 2 (Th2) and Th17 immune response against harmless, airborne substances. The hallmarks of this disease are airway hyperresponsiveness (AHR), eosinophilic and neutrophilic airway inflammation and mucus overproduction. Distinct dendric cell (DC) subsets together with airway epithelial and pulmonary vascular endothelial cells play critical roles in allergen sensing and in driving T cell differentiation towards Th2 and Th17 effector or regulatory T cells (Treg). Previous studies suggested already a pivotal role for the anaphylatoxins (C5a, C3a) in the pathogenesis of allergic asthma. During sensitization for example it is described, that C3a promotes, whereas C5a protects from the development of maladaptive immunity during allergen sensitization. Here we will discuss the role of the anaphylatoxins (C3a, C5a) and their receptors during the pathogenesis of allergic asthma, and specifically in lung DC biology. We will also have a look on canonical and non-canonical complement activation and we will discuss novel concepts on how the adaptive immune system can regulate the function of ATRs also in the context of allergic asthma.

Modulating Cytotoxic Effector Functions by Fc Engineering to Improve Cancer Therapy

In the last two decades, monoclonal antibodies have revolutionized the therapy of cancer patients. Although antibody therapy has continuously been improved, still a significant number of patients do not benefit from antibody therapy. Therefore, rational optimization of the antibody molecule by Fc engineering represents a major area of translational research to further improve this potent therapeutic option. Monoclonal antibodies are able to trigger a variety of effector mechanisms. Especially Fc-mediated effector functions such as antibody-dependent cell-mediated cytotoxicity (ADCC), antibody-dependent cellular phagocytosis (ADCP), and complement- dependent cytotoxicity (CDC) are considered important in antibody therapy of cancer. Novel mechanistic insights into the action of monoclonal antibodies allowed the development of various Fc engineering approaches to modulate antibodies' effector functions. Strategies in modifying the Fc glycosylation profile (Fc glyco-engineering) or approaches in engineering the protein backbone (Fc protein engineering) have been intensively evaluated. In the current review, Fc engineering strategies resulting in improved ADCC, ADCP and CDC activity are summarized and discussed.

Yersinia enterocolitica Exploits Signal Crosstalk between Complement 5a Receptor and Toll-like Receptor 1/2 and 4 to Avoid the Bacterial Clearance in M cells

In the intestinal mucosal surface, microfold cells (M cells) are the representative gateway for the uptake of luminal antigens. At the same time, M cells are the primary infection site for pathogens invading mucosal surface for their infection. Although it is well recognized that many mucosal pathogens exploit the M cells for their infection, the mechanism to infect M cells utilized by pathogens is not clearly understood yet. In this study, we found that M cells expressing complement 5a (C5a) receptor (C5aR) also express Toll-like receptor (TLR) 1/2 and TLR4. Infection of Yersinia enterocolitica, an M cell-invading pathogen, synergistically regulated cyclic adenosine monophosphate-dependent protein kinase A (cAMP-PKA) signaling which are involved in signal crosstalk between C5aR and TLRs. In addition, Y. enterocolitica infection into M cells was enhanced by C5a treatment and this enhancement was abrogated by C5a antagonist treatment. Finally, Y. enterocolitica infection into M cells was unsuccessful in C5aR knock-out mice. Collectively, we suggest that exploit the crosstalk between C5aR and TLR signaling is one of infection mechanisms utilized by mucosal pathogens to infect M cells.

Innate immune mediators in cancer: between defense and resistance

Chronic inflammation in the tumor microenvironment and evasion of the antitumor effector immune response are two of the emerging hallmarks required for oncogenesis and cancer progression. The innate immune system not only plays a critical role in perpetuating these tumor-promoting hallmarks but also in developing antitumor adaptive immune responses. Thus, understanding the dual role of the innate system in cancer immunology is required for the design of combined immunotherapy strategies able to tackle established tumors. Here, we review recent advances in the understanding of the role of cell populations and soluble components of the innate immune system in cancer, with a focus on complement, the adapter molecule Stimulator of Interferon Genes, natural killer cells, myeloid cells, and B cells.

Old dogs-new tricks: immunoregulatory properties of C3 and C5 cleavage fragments

The activation of the complement system by canonical and non-canonical mechanisms results in the generation of multiple C3 and C5 cleavage fragments including anaphylatoxins C3a and C5a as well as opsonizing C3b/iC3b. It is now well appreciated that anaphylatoxins not only act as pro-inflammatory mediators but as immunoregulatory molecules that control the activation status of cells and tissue at several levels. Likewise, C3b/iC3b is more than the opsonizing fragment that facilitates engulfment and destruction of targets by phagocytes. In the circulation, it also facilitates the transport and delivery of bacteria and immune complexes to phagocytes, through a process known as immune adherence, with consequences for adaptive immunity. Here, we will discuss non-classical immunoregulatory properties of C3 and C5 cleavage fragments. We highlight the influence of anaphylatoxins on Th2 and Th17 cell development during allergic asthma with a particular emphasis on their role in the modulation of CD11b⁺ conventional dendritic cells and monocyte-derived dendritic cells. Furthermore, we discuss the control of anaphylatoxin-mediated activation of dendritic cells and allergic effector cells by adaptive immune mechanisms that involve allergen-specific IgG1 antibodies and plasma or regulatory T cell-derived IL-10 production. Finally, we take a fresh look at immune adherence with a particular focus on the development of antibacterial cytotoxic T-cell responses.

Drug-induced pyoderma gangrenosum: a model to understand the pathogenesis of pyoderma gangrenosum

Pyoderma gangrenosum (PG) is a rare autoinflammatory condition in which the alteration of neutrophil function and the innate immune response play key roles in its pathogenesis. Cases of PG have been reported in patients being treated with certain medications, which may help us to understand some of the possible pathways involved in the aetiology of PG. The aim of this review is to review the cases of PG triggered by certain drugs and try to thoroughly understand the pathogenesis of the disease. To accomplish this, a PubMed search was completed using the following words: pyoderma gangrenosum, neutrophilic dermatosis, pathophysiology, drug-induced pyoderma gangrenosum. In total, we found 43 cases of drug-induced PG. Most of them were caused by colony-stimulating factors and small-molecule tyrosine kinase inhibitors. We propose that drugs induce PG through various mechanisms such as dysfunctional neutrophil migration and function, dysregulated inflammatory response, promotion of keratinocyte apoptosis and alteration of epigenetic mechanisms. PG is a rare condition with complex pathophysiology and drug-induced cases are even more scarce; this is the main limitation of this review. Understanding the possible mechanisms of drug-induced PG, via abnormal neutrophil migration and function, abnormal inflammation, keratinocyte apoptosis and alteration of epigenetic mechanisms would help to better understand the pathogenesis of PG and ultimately to optimize targeted therapy.

Lack of Fc Gamma Receptor IIIA Promotes Rather than Suppresses Humoral and Cellular Immune Responses after Mucosal or Parenteral Immunization with Antigen and Adjuvants

The Fcγ receptor IIIA (FcγRIIIA) has traditionally been known as a positive regulator of immune responses. Consistent with this, mice deficient in FcγRIIIA are protected from various inflammation-associated pathologies including several autoimmune diseases. In contrast to this accepted dogma, we show here that mice lacking FcγRIIIA developed increased rather than reduced both humoral and cellular immune responses to mucosal (sublingual) immunization with ovalbumin (OVA) given together with the strong mucosal adjuvant cholera toxin as well as to parenteral (subcutaneous) immunization with OVA in complete Freund's adjuvant. After either route of immunization, in comparison with concomitantly immunized wild-type mice, FcγRIIIA-/- mice had increased serum anti-OVA IgG (IgG1 but not IgG2) antibody responses as well as augmented cellular responses that included memory B cells and effector T cells. The increments in immune responses in FcγRIIIA-/- mice were similar to those seen in FcγRIIB-/- mice. Furthermore, OVA-pulsed FcγRIIIA-/- DCs, similar to OVA-specific FcγRIIB-/- DCs, had enhanced capacity to activate OVA-specific OT-II T cells, which was even further pronounced when DCs were pulsed with IgG1-complexed OVA. Our data support an inhibitory-regulatory role of FcγRIIIA on vaccine/adjuvant-induced immune responses and demonstrate that lack of FcγRIIIA can promote rather than suppress both humoral and cellular immune responses.

C5a Regulates IL-1β Production and Leukocyte Recruitment in a Murine Model of Monosodium Urate Crystal-Induced Peritonitis

Gouty arthritis results from the generation of monosodium urate (MSU) crystals within joints. These MSU crystals elicit acute inflammation characterized by massive infiltration of neutrophils and monocytes that are mobilized by the pro-inflammatory cytokine IL-1β. MSU crystals also activate the complement system, which regulates the inflammatory response; however, it is unclear whether or how MSU-mediated complement activation is linked to IL-1β release in vivo, and the various roles that might be played by individual components of the complement cascade. Here we show that exposure to MSU crystals in vivo triggers the complement cascade, leading to the generation of the biologically active complement proteins C3a and C5a. C5a, but not C3a, potentiated IL-1β and IL-1α release from LPS–primed MSU-exposed peritoneal macrophages and human monocytic cells in vitro; while in vivo MSU–induced C5a mediated murine neutrophil recruitment as well as IL-1β production at the site of inflammation. These effects were significantly ameliorated by treatment of mice with a C5a receptor antagonist. Mechanistic studies revealed that C5a most likely increased NLRP3 inflammasome activation via production of reactive oxygen species (ROS), and not through increased transcription of inflammasome components. Therefore we conclude that C5a generated upon MSU-induced complement activation increases neutrophil recruitment in vivo by promoting IL-1 production via the generation of ROS, which activate the NLRP3 inflammasome. Identification of the C5a receptor as a key determinant of IL-1-mediated recruitment of inflammatory cells provides a novel potential target for therapeutic intervention to mitigate gouty arthritis.

Complement C5a Receptor is the Key Initiator of Neutrophil Adhesion Igniting Immune Complex-induced Arthritis

The deposition of immune complexes (IC) in tissues induces a "type III hypersensitivity" that results in tissue damage and underlies the pathogenesis of many autoimmune diseases. The neutrophil is the first immune cell recruited into sites of IC deposition and plays a critical role in shaping the overall tissue response. However, the mechanism by which IC initiate and propagate neutrophil infiltration into tissue is not known. Here, using intravital multiphoton joint imaging of IC-induced arthritis in live mice, we found that the complement C5a receptor (C5aR) was the key initiator of neutrophil adhesion on joint endothelium. C5a presented on joint endothelium induced β2 integrin-dependent neutrophil arrest, facilitating neutrophil spreading and transition to crawling, and subsequent leukotriene B₄ receptor (BLT1)-mediated extravasation of the first neutrophils. The chemokine receptor CCR1 promoted neutrophil crawling on the joint endothelium while CXCR2 amplified late neutrophil recruitment and survival once in the joint. Thus, imaging arthritis has defined a new paradigm for type III hypersensitivity where C5a directly initiates neutrophil adhesion on the joint endothelium igniting inflammation.

Macrophages as Effectors of Acute and Chronic Allograft Injury

Organ transplants give a second chance of life to patients with end-stage organ failure. However, the immuno-logical barriers prove to be very challenging to overcome and graft rejection remains a major hurdle to long-term transplant survival. For decades, adaptive immunity has been the focus of studies, primarily based on the belief that T cells are necessary and sufficient for rejection. With better-developed immunosuppressive drugs and protocols that effectively control adaptive cells, innate immune cells have emerged as key effector cells in triggering graft injury and have therefore attracted much recent attention. In this review, we discuss current understanding of macrophages and their role in transplant rejection, their dynamics, distinct phenotypes, locations, and functions. We also discuss novel therapeutic approaches under development to target macrophages in transplant recipients.

C5a Induces the Synthesis of IL-6 and TNF-α in Rat Glomerular Mesangial Cells through MAPK Signaling Pathways

Inflammatory response has been reported to contribute to the renal lesions in rat Thy-1 nephritis (Thy-1N) as an animal model of human mesangioproliferative glomerulonephritis (MsPGN). Besides C5b-9 complex, C5a is also a potent pro-inflammatory mediator and correlated to severity of various nephritic diseases. However, the role of C5a in mediating pro-inflammatory cytokine production in rats with Thy-1N is poorly defined. In the present studies, the levels of C5a, interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α) were first determined in the renal tissues of rats with Thy-1N. Then, the expression of IL-6 and TNF-α was detected in rat glomerular mesangial cells (GMC) stimulated with our recombinant rat C5a in vitro. Subsequently, the activation of mitogen-activated protein kinase (MAPK) signaling pathways (p38 MAPK, ERK1/2 and JNK) and their roles in the regulation of IL-6 and TNF-α production were examined in the GMC induced by C5a. The results showed that the levels of C5a, IL-6 and TNF-α were markedly increased in the renal tissues of Thy-1N rats. Rat C5a stimulation in vitro could up-regulate the expression of IL-6 and TNF-α in rat GMC, and the activation of MAPK signaling pathways was involved in the induction of IL-6 and TNF-α. Mechanically, p38 MAPK activation promoted IL-6 production, while either ERK1/2 or JNK activation promoted TNF-α production in the GMC with exposure to C5a. Taken together, these data implicate that C5a induces the synthesis of IL-6 and TNF-α in rat GMC through the activation of MAPK signaling pathways.

Pathogenic autoantibodies in lupus nephritis

Lupus nephritis is a major complication of systemic lupus erythematosus (SLE) and is associated with a high rate of morbidity and mortality. While many different immunologic and nonimmunologic factors contribute to disease expression in lupus nephritis, a large body of evidence suggests that the production of anti-DNA antibodies and the formation of glomerular immune deposits are important initial events in the pathogenesis of the disease. This review will summarize our current understanding of the differences between pathogenic and nonpathogenic autoantibodies, the mechanisms by which these autoantibodies induce renal injury and the effector mechanisms which are subsequently activated by the deposited autoantibodies that ultimately lead to the expression of the different lupus lesions.

C5a of Cynoglossus semilaevis has anaphylatoxin-like properties and promotes antibacterial and antiviral defense

Activation of the complement system leads to the cleavage of component factor C5 into C5a and C5b. C5a can induce chemotaxis and inflammatory responses in mammals. The function of C5a in fish is poorly understood. In this study, we report the identification and analysis of a C5 homologue, CsC5, from tongue sole (Cynoglossus semilaevis). CsC5 is composed of 1683 amino acid residues that include an anaphylatoxin homologous domain. Expression of CsC5 could be detected in a variety of tissues and was up-regulated by bacterial or viral pathogen infection. Purified recombinant CsC5a (rCsC5a) could bind to peripheral blood leukocytes (PBL) and stimulate PBL chemotaxis, proliferation, respiratory burst, acid phosphatase activity, and phagocytosis. Tongue sole administered rCsC5a exhibited enhanced resistance against bacterial and viral infections. These results indicate that CsC5a is an anaphylatoxin with a role in innate immune defense against bacterial and viral infections.