Fc gamma receptor IIa suppresses type I and III interferon production by human myeloid immune cells

IgA2 immune complexes selectively promote inflammation by human CD103+ dendritic cells

While immunoglobulin A (IgA) is well known for its neutralizing and anti-inflammatory function, it is becoming increasingly clear that IgA can also induce human inflammatory responses by various different immune cells. Yet, little is known about the relative role of induction of inflammation by the two IgA subclasses i.e. IgA1, most prominent subclass in circulation, and IgA2, most prominent subclass in the lower intestine. Here, we set out to study the inflammatory function of IgA subclasses on different human myeloid immune cell subsets, including monocytes, and in vitro differentiated macrophages and intestinal CD103+ dendritic cells (DCs). While individual stimulation with IgA immune complexes only induced limited inflammatory responses by human immune cells, both IgA subclasses strongly amplified pro-inflammatory cytokine production upon co-stimulation with Toll-like receptor (TLR) ligands such as Pam3CSK4, PGN, and LPS. Strikingly, while IgA1 induced slightly higher or similar levels of pro-inflammatory cytokines by monocytes and macrophages, respectively, IgA2 induced substantially more inflammation than IgA1 by CD103+ DCs. In addition to pro-inflammatory cytokine proteins, IgA2 also induced higher mRNA expression levels, indicating that amplification of pro-inflammatory cytokine production is at least partially regulated at the level of gene transcription. Interestingly, cytokine amplification by IgA1 was almost completely dependent on Fc alpha receptor I (FcαRI), whilst blocking this receptor only partially reduced cytokine induction by IgA2. In addition, IgA2-induced amplification of pro-inflammatory cytokines was less dependent on signaling through the kinases Syk, PI3K, and TBK1/IKKϵ. Combined, these findings indicate that IgA2 immune complexes, which are most abundantly expressed in the lower intestine, particularly promote inflammation by human CD103+ intestinal DCs. This may serve an important physiological function upon infection, by enabling inflammatory responses by this otherwise tolerogenic DC subset. Since various inflammatory disorders are characterized by disturbances in IgA subclass balance, this may also play a role in the induction or exacerbation of chronic intestinal inflammation.

Conformational state of C-reactive protein is critical for reducing immune complex-triggered type I interferon response: Implications for pathogenic mechanisms in autoimmune diseases imprinted by type I interferon gene dysregulation

Presence of autoantibodies targeting nuclear constituents, i.e., double-stranded DNA and small nuclear ribonucleoproteins (snRNPs), remain a cornerstone in systemic lupus erythematosus (SLE). Fcγ receptor IIa (FcγRIIa) dependent uptake of nucleic acid containing immune complexes (ICs) by plasmacytoid dendritic cells (PDCs) can activate toll-like receptors (TLRs) such as TLR7 and TLR9 resulting in type I interferon (IFN) production. Previously, the classical liver-derived acute-phase reactant C-reactive protein (CRP) has been suggested to reduce IC-induced type I IFN production, whereas monomeric (mCRP) vs. pentameric (pCRP) mediated effects have not yet been unraveled. Herein, peripheral blood mononuclear cells (PBMCs) or enriched blood DCs from healthy volunteers were stimulated with SLE sera, snRNP-IgG (ICs), or TLR ligands with or without pCRP, mCRP, or anti-FcγRIIa antibody. Type I IFNs and cytokine responses were investigated using quantitative PCR, ELISA, and flow cytometry. pCRP inhibited IFN gene expression in PBMCs and enriched DCs after incubation with ICs, compared to ICs alone, whereas mCRP had significantly less inhibitory effect. The effect was independent on the order in which IC or CRP was added to the cells. In addition, pCRP inhibited IFN induced by other TLR stimulators, implicating broader inhibitory effects induced by pCRP. We demonstrate pronounced immunoregulatory functions of CRP whereas the inhibitory properties were evidently dependent on CRP's intact conformational state. The inhibition of type I IFNs was not due to competition of FcγRs, or binding of CRP to the ICs. Our findings have implications for autoimmune IC-mediated conditions imprinted by type I IFN gene dysregulation.

STING Targeting in Lung Diseases

The cGAS-STING pathway displays important functions in the regulation of innate and adaptive immunity following the detection of microbial and host-derived DNA. Here, we briefly summarize biological functions of STING and review recent literature highlighting its important contribution in the context of respiratory diseases. Over the last years, tremendous progress has been made in our understanding of STING activation, which has favored the development of STING agonists or antagonists with potential therapeutic benefits. Antagonists might alleviate STING-associated chronic inflammation and autoimmunity. Furthermore, pharmacological activation of STING displays strong antiviral properties, as recently shown in the context of SARS-CoV-2 infection. STING agonists also elicit potent stimulatory activities when used as an adjuvant promoting antitumor responses and vaccines efficacy.

C-Reactive Protein Controls IL-23 Production by Human Monocytes

C-reactive protein (CRP) is an acute-phase protein in humans that is produced in high quantities by the liver upon infection and under inflammatory conditions. Although CRP is commonly used as a marker of inflammation, CRP can also directly contribute to inflammation by eliciting pro-inflammatory cytokine production by immune cells. Since CRP is highly elevated in serum under inflammatory conditions, we have studied the CRP-induced cytokine profile of human monocytes, one of the main innate immune cell populations in blood. We identified that CRP is relatively unique in its capacity to induce production of the pro-inflammatory cytokine IL-23, which was in stark contrast to a wide panel of pattern recognition receptor (PRR) ligands. We show that CRP-induced IL-23 production was mediated at the level of gene transcription, since CRP particularly promoted gene transcription of IL23A (encoding IL-23p19) instead of IL12A (encoding IL-12p35), while PRR ligands induce the opposite response. Interestingly, when CRP stimulation was combined with PRR ligand stimulation, as for example, occurs in the context of sepsis, IL-23 production by monocytes was strongly reduced. Combined, these data identify CRP as a unique individual ligand to induce IL-23 production by monocytes, which may contribute to shaping systemic immune responses under inflammatory conditions.

Monocytes Exposed to Immune Complexes Reduce pDC Type 1 Interferon Response to Vidutolimod

Vidutolimod, also known as CMP-001, is a virus-like particle composed of the Qβ bacteriophage coat protein encasing a TLR9 agonist. Vidutolimod injected intratumorally is showing promise in early phase clinical trials based on its ability to alter the tumor microenvironment and induce an anti-tumor immune response. We previously demonstrated that the in vivo efficacy of vidutolimod is dependent on the presence of anti-Qβ antibodies that enhance opsonization and uptake of vidutolimod by TLR9-expressing plasmacytoid dendritic cells (pDCs). Here, we evaluated the effect of immune complexes, including anti-Qβ-coated vidutolimod, on induction of Type 1 Interferon production by peripheral blood mononuclear cells in response to vidutolimod and soluble TLR9 agonists. Immune complexes, including but not limited to anti-Qβ-coated vidutolimod, indirectly suppressed TLR9-mediated Type 1 Interferon production by pDCs in a monocyte-dependent manner. These findings indicate that anti-Qβ-coated vidutolimod has effects in addition to those mediated by TLR9 that could have important clinical implications for understanding the mechanism of action of this exciting new approach to in situ immunization and cancer immunotherapy.

Physiological and Pathological Inflammation Induced by Antibodies and Pentraxins

Macrophages play a key role in induction of inflammatory responses. These inflammatory responses are mostly considered to be instigated by activation of pattern recognition receptors (PRRs) or cytokine receptors. However, recently it has become clear that also antibodies and pentraxins, which can both activate Fc receptors (FcRs), induce very powerful inflammatory responses by macrophages that can even be an order of magnitude greater than PRRs. While the physiological function of this antibody-dependent inflammation (ADI) is to counteract infections, undesired activation or over-activation of this mechanism will lead to pathology, as observed in a variety of disorders, including viral infections such as COVID-19, chronic inflammatory disorders such as Crohn's disease, and autoimmune diseases such as rheumatoid arthritis. In this review we discuss how physiological ADI provides host defense by inducing pathogen-specific immunity, and how erroneous activation of this mechanism leads to pathology. Moreover, we will provide an overview of the currently known signaling and metabolic pathways that underlie ADI, and how these can be targeted to counteract pathological inflammation.

Are Fc Gamma Receptor Polymorphisms Important in HIV-1 Infection Outcomes and Latent Reservoir Size?

Fc gamma receptors (FcγR) are cell surface glycoproteins which trigger specific effector-cell responses when cross-linked with the Fc portions of immunoglobulin (IgG) antibodies. During HIV-1 infection, the course of disease progression, ART response, and viral reservoir size vary in different individuals. Several factors may account for these differences; however, Fc gamma receptor gene polymorphisms, which influence receptor binding to IgG antibodies, are likely to play a key role. FcγRIIa (CD32) was recently reported as a potential marker for latent HIV reservoir, however, this assertion is still inconclusive. Whether FcγR polymorphisms influence the size of the viral reservoir, remains an important question in HIV cure studies. In addition, potential cure or viral suppression methods such as broadly neutralizing antibody (bNAbs) may depend on FcγRs to control the virus. Here, we discuss the current evidence on the potential role played by FcγR polymorphisms in HIV-1 infection, treatment and vaccine trial outcomes. Importantly, we highlight contrasting findings that may be due to multiple factors and the relatively limited data from African populations. We recommend further studies especially in sub-Saharan Africa to confirm the role of FcγRIIa in the establishment of latent reservoir and to determine their influence in therapies involving bNAbs.

Kawasaki-like diseases and thrombotic coagulopathy in COVID-19: delayed over-activation of the STING pathway?

We previously made the hypothesis that STING contributes to COVID-19. The present review detail new arguments for over-activation of STING pathways in COVID-19, following the description of hyper-coagulability and Kawasaki-like diseases in children. Indeed, Kawasaki disease is induced by overreaction of innate cells following exposition to various viruses, including herpes viruses which trigger STING. It predisposes to diffuse vasculitis and aneurysms, whereas STING is over-expressed in arterial aneurisms. The redness at the inoculation site of bacillus Calmette-Guérin, a specific feature of Kawasaki disease, is reproduced by activation of the STING pathway, which is inhibited upstream by aspirin, intravenous immunoglobulins, and Vitamin-D. SARS-CoV2 binding to ACE2 can lead to excessive angiotensin II signaling, which activates the STING pathway in mice. Over-activation of the STING-pathway promotes hyper-coagulability through release of interferon-β and tissue factor by monocytes-macrophages. Aspirin and dipyridamole, besides their anti-platelet activity, also reduce tissue factor procoagulant activity, and aspirin inhibits the STING pathway upstream of STING. Aspirin and dipyridamole may be used, in combination with drugs blocking downstream the activation of the STING pathway, like inhibitors of IL-6R and JAK/STAT pathways. The risk of bleeding should be low as bleeding has not been reported in severe COVID-19 patients.

IgG Subclasses Shape Cytokine Responses by Human Myeloid Immune Cells through Differential Metabolic Reprogramming

IgG Abs are crucial for various immune functions, including neutralization, phagocytosis, and Ab-dependent cellular cytotoxicity. In this study, we identified another function of IgG by showing that IgG immune complexes elicit distinct cytokine profiles by human myeloid immune cells, which are dependent on FcγR activation by the different IgG subclasses. Using monoclonal IgG subclasses with identical Ag specificity, our data demonstrate that the production of Th17-inducing cytokines, such as TNF, IL-1β, and IL-23, is particularly dependent on IgG2, whereas type I IFN responses are controlled by IgG3, and IgG1 is able to regulate both. In addition, we identified that subclass-specific cytokine production is orchestrated at the posttranscriptional level through distinct glycolytic reprogramming of human myeloid immune cells. Combined, these data identify that IgG subclasses provide pathogen- and cell type-specific immunity through differential metabolic reprogramming by FcγRs. These findings may be relevant for future design of Ab-related therapies in the context of infectious diseases, chronic inflammation, and cancer.

Duality of B Cell-CXCL13 Axis in Tumor Immunology

Tumor immunity is a rapidly evolving area of research consisting of many possible permutations of immune cell tumor interactions that are dependent upon cell type, tumor type, and stage in tumor progression. At the same time, the majority of cancer immunotherapies have been focused on modulating the T cell-mediated antitumor immune response and have largely ignored the potential utility that B cells possess with respect to tumor immunity. Therefore, this motivated an exploration into the role that B cells and their accompanying chemokine, CXCL13, play in tumor immunity across multiple tumor types. Both B cells and CXCL13 possess dualistic impacts on tumor progression and tumor immunity which is furthered detail in this review. Specifically, various B cells subtypes are able to suppress or enhance several important immunological functions. Paradoxically, CXCL13 has been shown to drive several pro-growth and invasive signaling pathways across multiple tumor types, while also, correlating with improved survival and immune cell tumor localization in other tumor types. Potential tools for better elucidating the mechanisms by which B cells and CXCL13 impact the antitumor immune response are also discussed. In addition, multiples strategies are proposed for modulating the B cell-CXCL13 axis for cancer immunotherapies.

IgG Immune Complexes Break Immune Tolerance of Human Microglia

Microglia are phagocytic cells involved in homeostasis of the brain and are key players in the pathogenesis of multiple sclerosis (MS). A hallmark of MS diagnosis is the presence of IgG Abs, which appear as oligoclonal bands in the cerebrospinal fluid. In this study, we demonstrate that myelin obtained post mortem from 8 out of 11 MS brain donors is bound by IgG Abs. Importantly, we show that IgG immune complexes strongly potentiate activation of primary human microglia by breaking their tolerance for microbial stimuli, such as LPS and Poly I:C, resulting in increased production of key proinflammatory cytokines, such as TNF and IL-1β. We identified FcγRI and FcγRIIa as the two main responsible IgG receptors for the breaking of immune tolerance of microglia. Combined, these data indicate that IgG immune complexes potentiate inflammation by human microglia, which may play an important role in MS-associated inflammation and the formation of demyelinating lesions.

Activation of activating Fc gamma receptors down-regulates the levels of interferon β, interferon γ and interferon λ1 in porcine alveolar macrophages during PRRSV infection

Porcine activating Fc gamma receptors (FcγRI and FcγRIII) have been cloned and characterized for many years. However, their roles in interferon (IFN) antiviral immune response to porcine reproductive and respiratory syndrome virus (PRRSV) infection have not yet been investigated extensively. In this study, PRRSV infection assay showed that PRRSV increased significantly the transcription of IFN-β, IFN-γ and IFN-λ1 in porcine alveolar macrophages (PAMs) in early infection and decreased significantly the transcription of IFN-β, IFN-γ and IFN-λ1 in PAMs in late infection. Activation assay showed that specific activation of FcγRI or FcγRIII in PAMs decreased significantly the transcription of IFN-β, IFN-γ and IFN-λ1 and increased significantly the transcription of transforming growth factor β1 (TGF-β1). PRRSV infection assay mediated by FcγRI and FcγRIII showed that specific activation of FcγRI or FcγRIII in PAMs during PRRSV infection decreased significantly the transcription of IFN-β, IFN-γ and IFN-λ1, but increased significantly the transcription of TGF-β1 and enhanced significantly viral replication. In conclusion, our studies suggested that activating FcγR signaling inhibited the transcriptional levels of IFN-β, IFN-γ and IFN-λ1 in PAMs in response to PRRSV infection.

Antibody-Mediated Porcine Reproductive and Respiratory Syndrome Virus Infection Downregulates the Production of Interferon-α and Tumor Necrosis Factor-α in Porcine Alveolar Macrophages via Fc Gamma Receptor I and III

Antibody-dependent enhancement (ADE) contributes to the pathogenesis of porcine reproductive and respiratory syndrome virus (PRRSV)-persistent infection. However, the mechanisms of PRRSV-ADE infection are still confusing. A clear understanding of the event upon virus infection by the ADE pathway has become crucial for developing efficient intervention of the PRRSV infection. In this study, an ADE assay showed that PRRSV-ADE infection in porcine alveolar macrophages (AMs) significantly decreased the production of interferon-α (IFN-α) and tumor necrosis factor-α (TNF-α), and significantly increased the production of interleukine-10 (IL-10). A gene knockdown assay based on small interfering RNA (siRNA) showed that both Fc gamma receptor I (FcγRI) and FcγRIII in porcine AMs were involved in PRRSV-ADE infection. An activation assay showed that specific activation of FcγRI or FcγRIII in porcine AMs during PRRSV infection not only significantly decreased the production of IFN-α and TNF-α, but also significantly increased the production of IL-10 and significantly facilitated PRRSV replication. In conclusion, our studies suggested that ADE downregulated the production of IFN-α and TNF-α in porcine AMs maybe via FcγRI and FcγRIII, thereby leading to enhanced PRRSV infection.

Dysregulated Fcγ receptor IIa-induced cytokine production in dendritic cells of lupus nephritis patients

Systemic lupus erythematosus (SLE) is an autoimmune disease of unknown etiology. One of the key factors associated with SLE pathogenesis is excessive production of type I interferons (IFNs). This could result from increased activation of type I IFN-stimulating pathways, but also from decreased activation of type I IFN-inhibitory pathways. Recently, we have identified that immunoglobulin (Ig)G immune complexes strongly inhibit type I IFN production in healthy individuals by inhibitory signaling through Fcγ receptor IIa (FcγRIIa) on dendritic cells (DCs). Because, in SLE patients, immune complexes are characteristically present, we assessed whether FcγR-induced suppression of type I IFN is functional in DCs of SLE patients. We divided the SLE patients into one group without, and one group with, previous major organ involvement, for which we chose nephritis as a prototypical example. We show that DCs of lupus nephritis patients displayed impaired FcγR-mediated type I IFN inhibition compared to SLE patients without major organ involvement or healthy controls. We verified that this impaired type I IFN inhibition was not related to differences in disease activity, medication, FcγRIIa expression or expression of IFN regulatory transcription factors (IRF)1 and IRF5. In addition, we identified that DCs of lupus nephritis patients show increased FcγR-induced interleukin (IL)-1β production, which is another important cytokine that promotes kidney inflammation. Taken together, these data indicate that DCs of lupus nephritis patients display altered FcγR-mediated regulation of cytokine production, resulting in elevated levels of type I IFN and IL-1β. This dysregulation may contribute to the development of nephritis in SLE patients.

Fc Gamma Receptor Polymorphisms Modulated the Vaccine Effect on HIV-1 Risk in the HVTN 505 HIV Vaccine Trial

HIV Vaccine Trials Network (HVTN) 505 was a phase 2b efficacy trial of a DNA/recombinant adenovirus 5 (rAd5) HIV vaccine regimen. Although the trial was stopped early for lack of overall efficacy, later correlates of risk and sieve analyses generated the hypothesis that the DNA/rAd5 vaccine regimen protected some vaccinees from HIV infection yet enhanced HIV infection risk for others. Here, we assessed whether and how host Fc gamma receptor (FcγR) genetic variations influenced the DNA/rAd5 vaccine regimen's effect on HIV infection risk. We found that vaccine receipt significantly increased HIV acquisition compared with placebo receipt among participants carrying the FCGR2C-TATA haplotype (comprising minor alleles of four FCGR2C single-nucleotide polymorphism [SNP] sites) (hazard ratio [HR] = 9.79, P = 0.035) but not among participants without the haplotype (HR = 0.86, P = 0.67); the interaction of vaccine and haplotype effect was significant (P = 0.034). Similarly, vaccine receipt increased HIV acquisition compared with placebo receipt among participants carrying the FCGR3B-AGA haplotype (comprising minor alleles of the 3 FCGR3B SNPs) (HR = 2.78, P = 0.058) but not among participants without the haplotype (HR = 0.73, P = 0.44); again, the interaction of vaccine and haplotype was significant (P = 0.047). The FCGR3B-AGA haplotype also influenced whether a combined Env-specific CD8+ T-cell polyfunctionality score and IgG response correlated significantly with HIV risk; an FCGR2A SNP and two FCGR2B SNPs influenced whether anti-gp140 antibody-dependent cellular phagocytosis correlated significantly with HIV risk. These results provide further evidence that Fc gamma receptor genetic variations may modulate HIV vaccine effects and immune function after HIV vaccination.IMPORTANCE By analyzing data from the HVTN 505 efficacy trial of a DNA/recombinant adenovirus 5 (rAd5) vaccine regimen, we found that host genetics, specifically Fc gamma receptor genetic variations, influenced whether receiving the DNA/rAd5 regimen was beneficial, neutral, or detrimental to an individual with respect to HIV-1 acquisition risk. Moreover, Fc gamma receptor genetic variations influenced immune responses to the DNA/rAd5 vaccine regimen. Thus, Fc gamma receptor genetic variations should be considered in the analysis of future HIV vaccine trials and the development of HIV vaccines.

FcγR-TLR Cross-Talk Enhances TNF Production by Human Monocyte-Derived DCs via IRF5-Dependent Gene Transcription and Glycolytic Reprogramming

Antigen-presenting cells (APCs) such as dendritic cells (DCs) are crucial for initiation of adequate inflammatory responses, which critically depends on the cooperated engagement of different receptors. In addition to pattern recognition receptors (PRRs), Fc gamma receptors (FcγRs) have recently been identified to be important in induction of inflammation by DCs. FcγRs that recognize IgG immune complexes, which are formed upon opsonization of pathogens, induce pro-inflammatory cytokine production through cross-talk with PRRs such as Toll-like receptors (TLRs). While the physiological function of FcγR-TLR cross-talk is to provide protective immunity against invading pathogens, undesired activation of FcγR-TLR cross-talk, e.g., by autoantibodies, also plays a major role in the development of chronic inflammatory disorders such as rheumatoid arthritis (RA). Yet, the molecular mechanisms of FcγR-TLR cross-talk are still largely unknown. Here, we identified that FcγR-TLR cross-talk-induced cytokine production critically depends on activation of the transcription factor interferon regulatory factor 5 (IRF5), which results from induction of two different pathways that converge on IRF5 activation. First, TLR stimulation induced phosphorylation of TBK1/IKKε, which is required for IRF5 phosphorylation and subsequent activation. Second, FcγR stimulation induced nuclear translocation of IRF5, which is essential for gene transcription by IRF5. We identified that IRF5 activation by FcγR-TLR cross-talk amplifies pro-inflammatory cytokine production by increasing cytokine gene transcription, but also by synergistically inducing glycolytic reprogramming, which is another essential process for induction of inflammatory responses by DCs. Combined, here we identified IRF5 as a pivotal component of FcγR-TLR cross-talk in human APCs. These data may provide new potential targets to suppress chronic inflammation in autoantibody-associated diseases that are characterized by undesired or excessive FcγR-TLR cross-talk, such as RA, systemic sclerosis, and systemic lupus erythematous.

FcγRIII stimulation breaks the tolerance of human nasal epithelial cells to bacteria through cross-talk with TLR4

The nasal cavity displays immune tolerance to commensal bacteria under homeostatic conditions, which is rapidly converted to a pro-inflammatory response upon infection. Yet, the factors that control this conversion are still largely unknown. Here, we provide evidence that Fc gamma receptor III (FcγRIII) stimulation breaks immune tolerance to bacteria in the human nasal cavity through activation of nasal epithelial cells, which are the first line of defense against invading microbes. While under steady-state conditions human nasal epithelial cells were completely non-responsive to Gram-negative bacteria P. aeruginosa or TLR4 ligand LPS, IgG opsonization of bacteria, as occurs upon infection, strongly induced production of pro-inflammatory agents such as IL-6 and IL-8. This breaking of tolerance to bacteria was completely dependent on FcγRIII, which amplified cytokine gene transcription through cross-talk with TLR4. In addition, we identified that epithelial cells from patients suffering from chronic rhinosinusitis with nasal polyps do not display LPS tolerance, thereby providing an explanation for the disturbed host defense responses of these patients. Taken together, these data are the first to identify FcγR expression on nasal epithelial cells, as well as to identify its important role in controlling the balance between tolerance and inflammation in the nasal cavity.