The covalent interaction of C3 with IgG immune complexes

Natural killer cells in cancer immunotherapy

Natural killer (NK) cells, as innate lymphocytes, possess cytotoxic capabilities and engage target cells through a repertoire of activating and inhibitory receptors. Particularly, natural killer group 2, member D (NKG2D) receptor on NK cells recognizes stress-induced ligands-the MHC class I chain-related molecules A and B (MICA/B) presented on tumor cells and is key to trigger the cytolytic response of NK cells. However, tumors have developed sophisticated strategies to evade NK cell surveillance, which lead to failure of tumor immunotherapy. In this paper, we summarized these immune escaping strategies, including the downregulation of ligands for activating receptors, upregulation of ligands for inhibitory receptors, secretion of immunosuppressive compounds, and the development of apoptosis resistance. Then, we focus on recent advancements in NK cell immune therapies, which include engaging activating NK cell receptors, upregulating NKG2D ligand MICA/B expression, blocking inhibitory NK cell receptors, adoptive NK cell therapy, chimeric antigen receptor (CAR)-engineered NK cells (CAR-NK), and NKG2D CAR-T cells, especially several vaccines targeting MICA/B. This review will inspire the research in NK cell biology in tumor and provide significant hope for improving cancer treatment outcomes by harnessing the potent cytotoxic activity of NK cells.

A review of hypersensitivity methods to detect immune responses to SARS-CoV-2

The investigation of the immune response after SARS-CoV-2 infection has been the goal of many researchers worldwide. The study of humoral immune responses and in vitro T cell production after infection requires the obtaining of individualized blood samples to test the presence of antibodies or activated T cells specific for the virus. In vitro T cell studies are especially troublesome due to the need for more specialized resources often outside the daily routine of clinical laboratories. For this reason the development of a simple and objective method to achieve these T cell studies is needed. In this manuscript we reviewed the hypersensitivity reactions, the theoretical basis and the historical background of delayed type hypersensitivity (DTH) which uses the principles of use of this test in the clinical setting for the past century. In the second part of the review, we focus on COVID adaptive immune responses, to understand the differences and challenges offered by this new application of DTH to investigate immune responses elicited after infection. In the last part of the review a vision provided for the use of this test to investigate the immunogenicity elicited by the vaccines. In our opinion, the clinical guidelines of immune assessment of SARS-CoV-2-infected or vaccinated individuals should include this simple and low-cost test to measure T-cell immunity. Rationale and improved vaccination schemes could be obtained after its implementation in the routine assessment of immunity in this pandemic situation.

Serological Evidence for the Association Between Epstein-Barr Virus Infection and Sjögren's Syndrome

Background

Exposure to Epstein-Barr virus (EBV) infection has been hypothesized to be an important risk factor for multiple rheumatic diseases, but the serological evidence so far for its role in Sjögren’s syndrome (SjS) is not clearly established yet. This study aimed to assess the seroepidemiological associations of antibodies to EBV with SjS.

Methods

A seroepidemiological study containing 119 patients with SjS and 65 healthy controls was first performed, in which the associations of SjS with four commonly studied EBV antibodies including IgM-anti-viral capsid antigen (anti-VCA) antibody, IgG-anti-VCA antibody, IgG-anti-early antigen (anti-EA) antibody, and IgG-anti-EBV nuclear antigen 1 (anti-EBNA1) antibody were evaluated. A systematic review and meta-analysis of eligible seroepidemiological studies was also carried out, and data syntheses were performed using random-effect meta-analysis.

Results

In the case-control study, the patients with SjS had both a significantly higher prevalence of IgG-anti-EA antibody positivity (31.9% vs. 3.1%, P < 0.001) and high titers of IgG-anti-EA antibody (P < 0.001) than healthy controls. The titer of IgG-anti-VCA antibody was significantly increased in the patients with SjS compared with healthy controls (P < 0.001). IgG-anti-EA antibody seropositive patients with SjS had lower levels of both C3 (P = 0.002) and C4 (P = 0.02), and the titer of IgG-anti-EA antibody was inversely related to the levels of both C3 (r = -0.31, P < 0.001) and C4 (r = -0.20, P = 0.03). A total of 14 eligible studies on the serological associations between EBV infection and SjS were finally included into the meta-analysis, which suggested obvious associations of SjS with IgM-anti-VCA antibody [Odds ratio (OR) = 5.77, 95%CI 1.73–19.25, P = 0.004] and IgG-anti-EA antibody (OR = 9.97, 95%CI 4.58-21.67, P < 0.00001).

Conclusions

The findings from this study provide strong serological evidence for the association between EBV infection and SjS. SjS has obvious associations with IgM-anti-VCA antibody and IgG-anti-EA antibody. IgG-anti-EA antibody is linked to low levels of C3 and C4 in the patients with SjS, the significance of which needs to be addressed in further studies.

Complement activation by IgG containing immune complexes regulates the interaction of C1q with its ligands

Classical pathway activation of the compl ement system is initiated by the binding of the globular head domains of glycoprotein C1q to its corresponding ligand leading to both C1 activation and C3 convertase formation. However, the whereabouts and function of C1q after complement activation have only been marginally investigated. This report presents two mechanisms of action that remove bound C1q from a complement activating IgG immune complex in concentrated serum. The first mechanism details that sequential activation of the classical and alternative pathways releases bound C1q from an immune complex and that the dissociated C1q is subsequently found in complex with complement fragment C3c. The second mechanism is the displacement of C1q from an immune complex by the addition of near physiologic concentrations of purified or serum C1q. This activity can also be demonstrated using serum depleted of C3, normal serum chelated in EDTA, or purified C1. Fresh C1q in C3-depleted serum was found to replace dissociated C1q on the immune complex. C1q dissociated from immune complexes by the mechanism of C1q displacement is able to bind B and T lymphoblastoid cells that express receptors and ligands for both the collagen like region and the globular head domains of C1q. C1q dissociated from immune complexes by the mechanism of C3 activation do not bind these cells. This result suggests that C3 bound to C1q during complement activation and dissociation interferes with the ability of released C1q to access C1q receptors and ligands, particularly receptors for the globular head domains. These underlying mechanisms that regulate the interaction of C1q with its ligands reveal a novel function for complement activation during the immune response.

Antigen-specific airway IL-33 production depends on FcγR-mediated incorporation of the antigen by alveolar macrophages in sensitized mice

Although interleukin (IL)-33 is a candidate for the aggravation of asthma, the mechanisms underlying antigen-specific IL-33 production in the lung are unclear. Therefore, we analysed the mechanisms in mice. Intra-tracheal administration of ovalbumin (OVA) evoked increases in IL-33 and IL-33 mRNA in the lungs of both non-sensitized and OVA-sensitized mice, and the increases in the sensitized mice were significantly higher than in the non-sensitized mice. However, intra-tracheal administration of bovine serum albumin did not increase the IL-33 level in the OVA-sensitized mice. Depletion of neither mast cells/basophils nor CD4⁺ cells abolished the OVA-induced IL-33 production in sensitized mice, suggesting that the antigen recognition leading to the IL-33 production was not related with either antigen-specific IgE-bearing mast cells/basophils or memory CD4⁺ Th2 cells. When a fluorogenic substrate-labelled OVA (DQ-OVA) was intra-tracheally administered, the lung cells of sensitized mice incorporated more DQ-OVA than those of non-sensitized mice. The lung cells incorporating DQ-OVA included B-cells and alveolar macrophages. The allergic IL-33 production was significantly reduced by treatment with anti-FcγRII/III mAb. Depletion of alveolar macrophages by clodronate liposomes significantly suppressed the allergic IL-33 production, whereas depletion of B-cells by anti-CD20 mAb did not. These results suggest that the administered OVA in the lung bound antigen-specific IgG Ab, and then alveolar macrophages incorporated the immune complex through FcγRII/III on the cell surface, resulting in IL-33 production in sensitized mice. The mechanisms underlying the antigen-specific IL-33 production may aid in development of new pharmacotherapies.

Molecular mechanisms of action of anti-TNF-α agents - Comparison among therapeutic TNF-α antagonists

Tumor necrosis factor (TNF)-α is a potent pro-inflammatory and pathological cytokines in inflammatory diseases such as rheumatoid arthritis and inflammatory bowel diseases. Anti-TNF-α therapy has been established as an efficacious therapeutic strategy in these diseases. In clinical settings, three monoclonal anti-TNF-α full IgG1 antibodies infliximab, adalimumab, and golimumab, PEGylated Fab' fragment of anti-TNF-α antibody certolizumab pegol, extracellular domain of TNF receptor 2/IgG1-Fc fusion protein etanercept, are almost equally effective for rheumatoid arthritis. Although monoclonal full IgG1 antibodies are able to induce clinical and endoscopic remission in inflammatory bowel diseases, certolizumab pegol without Fc portion has been shown to be less effective for inflammatory bowel diseases compared to full IgG1 antibodies. In addition, there are no evidences that etanercept leads clinical remission in inflammatory bowel diseases. Besides the common effect of anti-TNF-α agents on neutralization of soluble TNF-α, each anti-TNF-α agent has its own distinctive pharmacological properties which cause the difference in clinical efficacies. Here we focus on the distinctions of action of anti-TNF-α agents especially in following points; (1) blocking ability against ligands, transmembrane TNF-α and lymphotoxin, (2) effects toward transmembrane TNF-α-expressing cells, (3) effects toward Fcγ receptor-expressing cells, (4) degradation and distribution in inflamed tissue. Accumulating evidence will give us the idea how to modify anti-TNF-α agents to enhance the clinical efficacy in inflammatory diseases.

Biological evaluation of liposome-encapsulated hemoglobin surface-modified with a novel PEGylated nonphospholipid amphiphile

Traumatic injury is often associated with hemorrhagic shock. Liposome-encapsulated hemoglobin (LEH) is being developed as an artificial oxygen carrier to address post-hemorrhage oxygen and volume deficit. Here, we report a new composition of LEH based on the use of polyethylene glycol (PEG2K ) conjugated with nonphospholipid hexadecylcarbamoylmethylhexadecanoate (HDAS) to modify the surface of LEH particles. LEH was manufactured by the high-pressure homogenization method using dipalmitoylphosphatidylcholine (∼38 mol%), cholesterol (∼38 mol%), HDAS (∼20 mol%), and highly purified stroma-free human hemoglobin. HDAS-PEG2K was postinserted into the resultant LEH to generate HDAS-PEG2K -LEH. We investigated the potential immune response to HDAS-PEG2K -LEH in a mice model. At the same time, the preparation was tested in a rat model to study the effect of repeated HDAS-PEG2K -LEH injection over 4 weeks. We found that HDAS-PEG2K modification substantially reduced the circulating levels of anaphylatoxins C3a and C5a, as well as plasma levels of thromboxane B2, in mice. Repeated injections of HDAS-PEG2K -LEH in rats did not appear to alter its clearance profile after 4 weeks of treatment. No antibody response against human hemoglobin or PEG was detected in rat plasma. Histological observations of lung, liver, spleen, and kidney were not significantly different between saline-treated rats and HDAS-PEG2K -LEH-treated rats. Immunohistochemical staining for rat heme oxygenase-1 (HO-1) did not show induced expression of HO-1 in these organs. These results suggest that the new surface modification of LEH is immune-neutral and does not adversely affect histology even after repeated administration.

Neuroprotection in stroke by complement inhibition and immunoglobulin therapy

Activation of the complement system occurs in a variety of neuroinflammatory diseases and neurodegenerative processes of the CNS. Studies in the last decade have demonstrated that essentially all of the activation components and receptors of the complement system are produced by astrocytes, microglia, and neurons. There is also rapidly growing evidence to indicate an active role of the complement system in cerebral ischemic injury. In addition to direct cell damage, regional cerebral ischemia and reperfusion (I/R) induces an inflammatory response involving complement activation and generation of active fragments, such as C3a and C5a anaphylatoxins, C3b, C4b, and iC3b. The use of specific inhibitors to block complement activation or their mediators such as C5a, can reduce local tissue injury after I/R. Consistent with therapeutic approaches that have been successful in models of autoimmune disorders, many of the same complement inhibition strategies are proving effective in animal models of cerebral I/R injury. One new form of therapy, which is less specific in its targeting of complement than monodrug administration, is the use of immunoglobulins. Intravenous immunoglobulin (IVIG) has the potential to inhibit multiple components of inflammation, including complement fragments, pro-inflammatory cytokine production and leukocyte cell adhesion. Thus, IVIG may directly protect neurons, reduce activation of intrinsic inflammatory cells (microglia) and inhibit transendothelial infiltration of leukocytes into the brain parenchyma following an ischemic stroke. The striking neuroprotective actions of IVIG in animal models of ischemic stroke suggest a potential therapeutic potential that merits consideration for clinical trials in stroke patients.

Initiation of the alternative pathway of murine complement by immune complexes is dependent on N-glycans in IgG antibodies

OBJECTIVE

Collagen antibody-induced arthritis in mice exhibits a requirement for amplification by the alternative pathway of complement. Although the alternative pathway is activated by spontaneous hydrolysis, it is not known whether this pathway can also be initiated directly by IgG antibodies in immune complexes (ICs). IgG lacking terminal sialic acid and galactose (G0 IgG) can activate the lectin pathway of complement, but it is not known if G0 IgG can also activate the classical or alternative pathway. The purpose of this study was to examine the mechanism of initiation of the alternative pathway of complement by ICs.

METHODS

We used adherent ICs containing bovine type II collagen (CII) and 4 monoclonal antibodies (mAb) to CII (adCII-IC). C3 activation was measured in the presence of sera from wild-type C57BL/6 mice or from mice deficient in informative complement components. The mAb were used intact or after enzyme digestion to create G0 IgG or to completely remove the N-glycan.

RESULTS

Both the classical and alternative pathways, but not the lectin pathway, mediated C3 activation induced by the adCII-IC. Mannose inhibited the alternative pathway-mediated C3 activation but had no effect on the classical pathway, and N-glycans in IgG were required by the alternative pathway but not the classical pathway. Both the classical and alternative pathways mediated C3 activation induced by G0 IgG. Mannose-binding lectin bound avidly to G0 IgG, but lectin pathway-mediated C3 activation was only slightly increased by G0 IgG.

CONCLUSION

The alternative pathway of complement is capable of initiating C3 activation induced by adCII-IC and requires the presence of N-glycans on the IgG. G0 IgG activates both the classical and alternative pathways more strongly than the lectin pathway.

Coupling of C3bi to IgG inhibits the tyrosine phosphorylation signaling cascade downstream Syk and reduces cytokine induction in monocytes

The effect of coupling C3bi to immunoglobulin G (IgG) immune complexes (IC) on their ability to produce protein tyrosine phosphorylation and activation of the mitogen-activated protein kinase (MAPK) and the Akt/protein kinase B (PKB) routes was assessed in human monocytes. Cross-linking Fc receptors for IgG activated the protein tyrosine kinase Syk, phospholipases Cgamma1 and Cgamma2, the MAPK cascade, and the Akt/PKB route. Linkage of C3bi to the gamma-chain of IgG produced a decrease of the protein bands displaying tyrosine phosphorylation, whereas the MAPK cascades and the Akt/PKB route remained almost unaffected. Zymosan particles, which because of their beta-glucan content mimic the effect of fungi, produced a limited increase of tyrosine-phosphorylated protein bands, whereas treatment of zymosan under conditions adequate for C3bi coating increased its ability to induce protein tyrosine phosphorylation. Noteworthy, this was also observed under conditions where other components of serum might be bound by zymosan particles, for instance, serum IgG, thereby suggesting their potential involvement in Syk activation. The induction of cytokines showed a changing pattern consistent with the changes observed in the signaling pathways. IC induced monocyte chemoattractant protein-1 (MCP-1)/CC chemokine ligand 2 (CCL2), interleukin (IL)-1beta, and eotaxin-2/CCL24, which were not observed with C3bi-coated IC. Zymosan induced the expression of tumor necrosis factor alpha (TNF-alpha), TNF-beta, IL-10, IL-6, and MCP-2/CCL8, whereas the cytokine signature of C3bi-coated zymosan also included interferon-inducible protein 10/CXC chemokine ligand 10, platelet-derived growth factor-BB, and I-309/CCL1. Taken together, these findings indicate that C3bi targets the phagocytic cargo, and engagement or diversion of the Syk route determines the phagocyte response.

Immunoglobulin treatment reduces atherosclerosis in apolipoprotein E-/- low-density lipoprotein receptor-/- mice via the complement system

Atherosclerosis is associated with activation of the immune system. Intravenously applied normal polyclonal immunoglobulins (IVIg) have broad therapeutic applications in the treatment of autoimmune and systemic inflammatory diseases. Recently, IVIg have been shown to inhibit atherogenesis in experimental animal models. To investigate the role of the complement system in this process, we used third complement component-deficient (C3(-/-)) and control atherosclerosis-prone apolipoprotein E (ApoE) and low-density lipoprotein receptor (LDLR) double knock-out mice fed a normal diet. IVIg treatment reduced lesion fraction area in the aortic root of complement-sufficient mice whereas the lesion fraction area of C3(-/-) mice was not affected. Thus, complement activation plays a role in the anti-atherosclerotic effects of IVIg, possibly by C3-derived fragments generated through Fc-dependent complement activation.

Rituximab infusion promotes rapid complement depletion and acute CD20 loss in chronic lymphocytic leukemia

Complement plays an important role in the immunotherapeutic action of the anti-CD20 mAb rituximab, and therefore we investigated whether complement might be the limiting factor in rituximab therapy. Our in vitro studies indicate that at high cell densities, binding of rituximab to human CD20(+) cells leads to loss of complement activity and consumption of component C2. Infusion of rituximab in chronic lymphocytic leukemia patients also depletes complement; sera of treated patients have reduced capacity to C3b opsonize and kill CD20(+) cells unless supplemented with normal serum or component C2. Initiation of rituximab infusion in chronic lymphocytic leukemia patients leads to rapid clearance of CD20(+) cells. However, substantial numbers of B cells, with significantly reduced levels of CD20, return to the bloodstream immediately after rituximab infusion. In addition, a mAb specific for the Fc region of rituximab does not bind to these recirculating cells, suggesting that the rituximab-opsonized cells were temporarily sequestered by the mononuclear phagocytic system, and then released back into the circulation after the rituximab-CD20 complexes were removed by phagocytic cells. Western blots provide additional evidence for this escape mechanism that appears to occur as a consequence of CD20 loss. Treatment paradigms to prevent this escape, such as use of engineered or alternative anti-CD20 mAbs, may allow for more effective immunotherapy of chronic lymphocytic leukemia.

Release of arachidonic acid by stimulation of opsonic receptors in human monocytes: the FcgammaR and the complement receptor 3 pathways

The role of the opsonic receptors FcgammaR and CR3 on the release of arachidonic acid (AA) by human monocytes was studied using IgG-ovalbumin (OVA) equivalence immune complexes (IC), anti-OVA IgG bound to OVA-coupled latex beads, and C3bi-bound IC. Release of AA was produced by IC and latex-OVA beads bound to IgG, whereas binding of C3bi to IC inhibited the ability of IC to release AA. In contrast, coating of zymosan particles with C3bi enhanced AA release as compared with that produced by non-coated particles. Masking of C3bi on C3bi-bound IC by incubation with anti-C3 IgG resulted in the recovery of their ability to release AA, thereby suggesting that binding of C3b by IC reduces their flogogenic effects, whereas opsonization of microbial walls by complement may enhance their proinflammatory potential. The binding/uptake of opsonized zymosan particles was inhibited by anti-CR3 Ab and C3bi-bound IC, but not by beta-glucan, mannan, and anti-Toll-like receptor 2 Ab. These findings show that cooperative engagement of CR3 on both the lectin-like site involved in beta-glucan binding and the I-domain involved in C3bi binding, as it can be observed in the innate immune response, produces AA release, whereas the unique interaction of C3bi-bound IC with the I-domain of CR3, as it may occur in the adaptive immune response, diverts the IC lattice from a productive interaction with FcgammaR linked to AA release.

Flow cytometry based detection of HLA alloantibody mediated classical complement activation

Complement-dependent cytotoxicity (CDC) panel reactive antibody (PRA) testing is used to assess recipient presensitization and post-transplant alloantibody formation in transplant recipients. However, CDC test results can be affected by false-positive reactions brought about by autoantibodies or antilymphocyte reagents. As an alternative to the CDC-PRA assay, detection of HLA alloantibodies using HLA antigen-coated microbeads (FlowPRA test) was recently established. FlowPRA testing, however, does not distinguish between (presumably more harmful) complement-fixing and noncomplement-fixing alloantibodies. In this study, we established a novel assay allowing flow cytometric detection of HLA alloantibody dependent classical complement activation using the FlowPRA test. For the detection of complement activation, FlowPRA beads were incubated with sera from highly sensitized dialysis patients (CDC-PRA reactivity >60%) and then stained for C4 (C4d, C4c) and C3 (C3d, C3c) fragments, as well as C1q deposition using indirect immunofluorescence. We demonstrate alloantibody induced induction of C4 fragment, and in parallel C1q deposition to HLA class I or class II beads. As shown by immunoblotting, C4 staining was not due to the presence of preformed C4 fragment-IgG/M complexes. Indeed, C4 fragment deposition in our in vitro system was demonstrated to result from de novo complement activation. First, inactivation of C4 by treatment of sera with methylamine, which inhibits cleavage of the internal thioester, completely abolished C4 fragment deposition. Second, C4 fragment deposition was not observed in the evaluation of C4-free immunoadsorption eluates obtained from highly sensitized dialysis patients. After supplementation with complement, however, eluates induced C4 deposition. Deposition of C4 split products and C1q was temperature-dependent with maximum binding after incubation at 4 degrees C for 60 min. In contrast, maximum C3 fragment deposition was found at 37 degrees C. At this temperature, C3 deposition occurred in an alloantibody and C4-independent fashion, presumably as a result of alternative complement activation. In summary, we describe a novel cell-independent and easy-to-perform PRA test that permits flow cytometry based detection of alloantibody induced classical complement activation. Future studies will have to evaluate its possible relevance as an alternative to CDC-PRA testing in clinical transplantation.

Role of membrane cofactor protein (CD46) in regulation of C4b and C3b deposited on cells

C4b and C3b deposited on host cells undergo limited proteolytic cleavage by regulatory proteins. Membrane cofactor protein (MCP; CD46), factor H, and C4b binding protein mediate this reaction, known as cofactor activity, that also requires the plasma serine protease factor I. To explore the roles of the fluid phase regulators vs those expressed on host cells, a model system was used examining complement fragments deposited on cells transfected with human MCP as assessed by FACS and Western blotting. Following incubation with Ab and complement on MCP(+) cells, C4b was progressively cleaved over the first hour to C4d and C4c. There was no detectable cleavage of C4b on MCP(-) cells, indicating that MCP (and not C4BP in the serum) primarily mediates this cofactor activity. C3b deposition was not blocked on MCP(+) cells because classical pathway activation occurred before substantial C4b cleavage. Cleavage, though, of deposited C3b was rapid (<5 min) and iC3b was the dominant fragment on MCP(-) and MCP(+) cells. Studies using a function-blocking mAb further established factor H as the responsible cofactor. If the level of Ab sensitization was reduced 8-fold or if Mg(2+)-EGTA was used to block the classical pathway, MCP efficiently inhibited C3b deposition mediated by the alternative pathway. Thus, for the classical pathway, MCP is the cofactor for C4b cleavage and factor H for C3b cleavage. However, if the alternative pathway mediates C3b deposition, then MCP's cofactor activity is sufficient to restrict complement activation.

How antibodies to a ubiquitous cytoplasmic enzyme may provoke joint-specific autoimmune disease

Arthritis in the K/BxN mouse model results from pathogenic immunoglobulins (Igs) that recognize the ubiquitous cytoplasmic enzyme glucose-6-phosphate isomerase (GPI). But how is a joint-specific disease of autoimmune and inflammatory nature induced by systemic self-reactivity? No unusual amounts or sequence, splice or modification variants of GPI expression were found in joints. Instead, immunohistological examination revealed the accumulation of extracellular GPI on the lining of the normal articular cavity, most visibly along the cartilage surface. In arthritic mice, these GPI deposits were amplified and localized with IgG and C3 complement. Similar deposits were found in human arthritic joints. We propose that GPI-anti-GPI complexes on articular surfaces initiate an inflammatory cascade via the alternative complement pathway, which is unbridled because the cartilage surface lacks the usual cellular inhibitors. This may constitute a generic scenario of arthritogenesis, in which extra-articular proteins coat the cartilage or joint extracellular matrix.

Arthritis critically dependent on innate immune system players

K/BxN T cell receptor transgenic mice are a model of inflammatory arthritis, similar to rheumatoid arthritis. Disease in these animals is focused specifically on the joints but stems from autoreactivity to a ubiquitously expressed antigen, glucose-6-phosphate isomerase (GPI). T and B cells are both required for disease initiation, but anti-GPI immunoglobulins (Igs), alone, can induce arthritis in lymphocyte-deficient recipients. Here, we show that the arthritogenic Igs act through both Fc receptors (in particular, FcgammaRIII) and the complement network (C5a). Surprisingly, the alternative pathway of complement activation is critical, while classical pathway components are entirely dispensable. We suggest that autoimmune disease, even one that is organ specific, can occur when mobilization of an adaptive immune response results in runaway activation of the innate response.

Serine 132 is the C3 covalent attachment point on the CH1 domain of human IgG1

The covalent binding of C3 (complement component C3) to antigen-antibody complexes (Ag.Ab; immune complexes (ICs)) is a key event in the uptake, transport, presentation, and elimination of Ag in the form of Ag.Ab.C3b (IC.C3b). Upon interaction of C3 with IgG.IC, C3b.C3b.IgG covalent complexes are formed that are detected on SDS-polyacrylamide gel electrophoresis by two bands corresponding to C3b.C3b (band A) and C3b.IgG (band B) covalent complexes. This allows one to evaluate the covalent binding of C3b to IgG antibodies. It has been described that C3b can attach to both the Fab (on the CH1 domain) and the Fc regions of IgG. Here the covalent interaction of C3b to the CH1 domain, a region previously described spanning residues 125-147, has been studied. This region of the CH1 domain is exposed to solvent and contains a cluster of six potential acceptor sites for ester bond formation with C3b (four Ser and two Thr). A set of 10 mutant Abs were generated with the putative acceptor residues substituted by Ala, and we studied their covalent interaction with C3b. Single (Ser-131, Ser-132, Ser-134, Thr-135, Ser-136, and Thr-139), double (positions 131-132), and multiple (positions 134-135-136, 131-132-134-135-136, and 131-132-134-135-136-139) mutants were produced. None of the mutants (single, double, or multiple) abolished completely the ability of IgG to bind C3b, indicating the presence of C3b binding regions other than in the CH1 domain. However, all mutant Abs, in which serine at position 132 was replaced by Ala, showed a significant decrease in the ability to form C3b.IgG covalent complexes, whereas the remaining mutants had normal activity. In addition we examined ICs using the F(ab')2 fragment of the mutant Abs, and only those containing Ala at position 132 (instead of Ser) failed to bind C3b. Thus Ser-132 is the binding site for C3b on the CH1 domain of the heavy chain, in the Fab region of human IgG.

Administration of IgG Fc fragments prevents glomerular injury in experimental immune complex nephritis

Most human nephritis is due to glomerular deposition and/or formation of immune complexes (IC). In cultured mesangial cells, Fc receptor stimulation induces proliferation, matrix synthesis, and release of several mediators implicated in the initiation and progression of glomerular injury. Since Ig Fc fragments in vitro modified these phenomena, we studied the effects of systemic administration of IgG Fc fragments on the evolution of experimental IC nephritis. Fc fragment injection (1 mg/day i.p.) to rats with ongoing nephritis (proteinuria 20-50 mg/24 h vs 9 +/- 0.2 mg/24 h in controls) markedly ameliorates proteinuria, renal function, and morphological renal lesions. This was accompanied by a reduction in the renal synthesis of chemokines (monocyte chemoattractant protein-1, IFN-inducible protein-10, and cytokine-induced neutrophil chemoattractant-1), matrix proteins, and growth factors (platelet-derived growth factor, and TGF-beta), and in the activity of transcription factors. The treatment did not affect the glomerular deposition of IgG IC and complement C1q. In contrast, a decrease in the renal expression and production of C3 was observed without changes in serum complement levels. In vitro, very low complement consumption and no C3b covalent interaction were observed with Fc fragments, confirming that they did not modify systemic complement activity. These results indicate that the administration of Fc fragments prevents the development of glomerular damage in an aggressive model of proliferative glomerulonephritis through mechanisms involving a reduced local generation of complement, chemokines and growth factors. Modulation of IC-mesangial cell interaction by Fc fragment administration could represent a new approach to the treatment of severe immune nephritis.