Natural antibodies, autoantibodies and complement activation in tissue injury

Highly pathogenic natural monoclonal antibody B4-IgM recognizes a post-translational modification comprised of acetylated N-terminal methionine followed by aspartic or glutamic acid

The natural monoclonal antibody B4-IgM recognizes murine annexin 4 (mAn4) and exacerbates ischemia-reperfusion injury in many mouse models. During apoptosis, the intracellular mAn4 protein translocates to the membrane surface, remaining attached to the outer membrane leaflet where it is recognized by the anti-mAn4 B4-IgM antibody. B4-IgM does not recognize human annexin 4 (hAn4). However, the B4-IgM antibody epitope was detected by Western blot of unknown human proteins and by flow cytometry on all studied human cell lines undergoing apoptosis and on a minor subset of healthy cells. The B4-IgM antibody also recognizes the epitope on necrotic cells in cytoplasmic proteins, apparently entering through pores large enough to allow natural antibodies to penetrate the cells and bind to the epitope expressed on self-proteins. Using proteomics and site-directed mutagenesis, we found that B4-IgM binds to an epitope with post-translationally modified acetylated N-terminal methionine, followed by either glutamic or aspartic acid. The epitope is not induced by apoptosis or injury because this modification can also occur during protein translation. This finding reveals an additional novel mechanism whereby injured cells are detected by natural antibodies that initiate pathogenic complement activation through the recognition of epitopes that are shared across multiple proteins found in variable cell lines.

The utility of immunoglobulin A/complement 3 and immunoglobulin G/immunoglobulin M ratios in the assessment of disease activation in patients with Behçet disease

OBJECTIVE

Pathogenesis of Behçet disease (BD) has not yet been clearly revealed and there is no ideal test for the estimation of disease activation at present. This study aimed to assess the efficiencies of IgG/IgM and IgA/C3 ratios in determining activation of BD.

METHOD

This retrospective cohort study consisted of 140 patients with BD. Patients were divided into two groups: (1) active BD (n = 89) and (2) inactive BD (n = 51) and were compared in terms of  demographic features, clinical characteristics and laboratory test results. IgA/C3 and IgG/IgM ratios were compared according to organ system involvement; receiver operating characteristic (ROC) curve analysis was performed in order to assess the performance of IgA/C3 and IgG/IgM ratios in determining patient disease status.

RESULTS

Significantly higher levels of erythrocyte sedimentation rate, C-reactive protein, IgA, G, C4, IgA/C3, IgG/IgM ratios (p = .007 for IgA and p < .001 for others) and significantly lower levels of IgM and C3 were observed in patients with active BD (p < .001). The IgG/IgM ratio was significantly higher in patients with vascular involvement (p = .017) and the IgA/C3 ratio was significantly higher in patients with arthritis (p = .007). Cut-off values of 0.019 (70.8% sensitivity, 62% specificity) and 7.08 (84.3% sensitivity, 80% specificity) were determined for IgA/C3 and IgG/IgM ratios, respectively.

CONCLUSION

IgA/C3 and IgG/IgM ratios may be used as additional parameters for the assessment of BD status.

C2 IgM Natural Antibody Enhances Inflammation and Its Use in the Recombinant Single Chain Antibody-Fused Complement Inhibitor C2-Crry to Target Therapeutics to Joints Attenuates Arthritis in Mice

Natural IgM antibodies (NAbs) have been shown to recognize injury-associated neoepitopes and to initiate pathogenic complement activation. The NAb termed C2 binds to a subset of phospholipids displayed on injured cells, and its role(s) in arthritis, as well as the potential therapeutic benefit of a C2 NAb-derived ScFv-containing protein fused to a complement inhibitor, complement receptor-related y (Crry), on joint inflammation are unknown. Our first objective was to functionally test mAb C2 binding to apoptotic cells from the joint and also evaluate its inflammation enhancing capacity in collagen antibody-induced arthritis (CAIA). The second objective was to generate and test the complement inhibitory capacity of C2-Crry fusion protein in the collagen-induced arthritis (CIA) model. The third objective was to demonstrate in vivo targeting of C2-Crry to damaged joints in mice with arthritis. The effect of C2-NAb on CAIA in C57BL/6 mice was examined by inducing a suboptimal disease. The inhibitory effect of C2-Crry in DBA/1J mice with CIA was determined by injecting 2x per week with a single dose of 0.250 mg/mouse. Clinical disease activity (CDA) was examined, and knee joints were fixed for analysis of histopathology, C3 deposition, and macrophage infiltration. In mice with suboptimal CAIA, at day 10 there was a significant (p < 0.017) 74% increase in the CDA in mice treated with C2 NAb, compared to mice treated with F632 control NAb. In mice with CIA, at day 35 there was a significant 39% (p < 0.042) decrease in the CDA in mice treated with C2-Crry. Total scores for histopathology were also 50% decreased (p < 0.0005) in CIA mice treated with C2-Crry. C3 deposition was significantly decreased in the synovium (44%; p < 0.026) and on the surface of cartilage (42%; p < 0.008) in mice treated with C2-Crry compared with PBS treated CIA mice. Furthermore, C2-Crry specifically bound to apoptotic fibroblast-like synoviocytes in vitro, and also localized in the knee joints of arthritic mice as analyzed by in vivo imaging. In summary, NAb C2 enhanced arthritis-related injury, and targeted delivery of C2-Crry to inflamed joints demonstrated disease modifying activity in a mouse model of human inflammatory arthritis.

A Review on the Function and Regulation of ARHGDIB/RhoGDI2 Expression Including the Hypothetical Role of ARHGDIB/RhoGDI2 Autoantibodies in Kidney Transplantation

Challenging and still unsolved problems in kidney transplantation are risk stratification and the treatment of humoral rejection. Antibody-mediated rejection is an important cause of early and chronic rejection. The impact of donor-specific HLA antibodies on antibody-mediated rejection–causing graft damage is well known, but the clinical relevance of non-HLA antibodies remains unclear. Recently, in 2 independent studies, a new correlation was found between the presence of non-HLA anti-Rho guanosine diphosphate dissociation inhibitor 2 (ARHGDIB) antibodies and increased graft failure. RhoGDI2, another name for ARHGDIB, is a negative regulator of the Rho guanosine triphosphate (RhoGTP)ases RhoA, Rac1m, and Cdc42, whose main function is regulating the actin network in a variety of cells. RhoGDI2 is mainly expressed intracellularly, and some expression is observed on the cell surface. Currently, there is no mechanism known to explain this correlation. Additionally, the reason why the antibodies are produced is unknown. In this review, we will address these questions, provide an overview of other diseases in which these antibodies are prevalent, and describe the physiological role of RhoGDI2 itself. If the mechanism and impact of RhoGDI2 antibodies in kidney graft failure are known, improved risk stratification can be provided to decrease the rate of donor kidney graft failure.

Pathophysiology of Acute Illness and Injury

The pathophysiology of acute illness and injury recognizes three main effectors: infection, trauma, and ischemia-reperfusion injury. Each of them can act by itself or in combination with the other two in developing a systemic inflammatory reaction syndrome (SIRS) that is a generalized reaction to the morbid event. The time course of SIRS is variable and influenced by the number and severity of subsequent insults (e.g., reparative surgery, acquired hospital infections). It occurs simultaneously with a complex of counter-regulatory mechanisms (compensatory anti-inflammatory response syndrome, CARS) that limit the aggressive effects of SIRS. In adjunct, a progressive dysfunction of the acquired (lymphocytes) immune system develops with increased risk for immunoparalysis and associated infectious complications. Both humoral and cellular effectors participate to the development of SIRS and CARS. The most important humoral mediators are pro-inflammatory (IL-1β, IL-6, IL-8, IL-12) and anti-inflammatory (IL-4, IL-10) cytokines and chemokines, complement, leukotrienes, and PAF. Effector cells include neutrophils, monocytes, macrophages, lymphocytes, and endothelial cells. The endothelium is a key factor for production of remote organ damage as it exerts potent chemo-attracting effects on inflammatory cells, allows for leukocyte trafficking into tissues and organs, and promotes further inflammation by cytokines release. Moreover, the loss of vasoregulatory properties and the increased permeability contribute to the development of hypotension and tissue edema. Finally, the disseminated activation of the coagulation cascade causes the widespread deposition of microthrombi with resulting maldistribution of capillary blood flow and ultimately hypoxic cellular damage. This mechanism together with increased vascular permeability and vasodilation is responsible for the development of the multiple organ dysfunction syndrome (MODS).

Gain of function mutant of complement factor B K323E mimics pathogenic C3NeF autoantibodies in convertase assays

Complement convertases are enzymatic complexes, which play a critical role in propagation and amplification of the complement cascade. Under physiological conditions, convertases decay shortly after being formed in either spontaneous or inhibitor-driven process. Prolongation of their half-life by C3NeF autoantibodies that prevent convertase dissociation results in pathogenic condition often manifested by renal diseases. However, the diagnosis of convertase abnormalities is difficult due to the labile nature of these enzymes and low credibility of existing methods. Only recently, two-step functional assays employing C5-depleted serum or C5 inhibitors were introduced. Their advantage is convertase formation in the physiological milieu of whole serum and the drawback is inter-assay variability due to variations in rabbit erythrocytes used for the haemolysis-based readout. Abovementioned problems demand the application of the internal standard in each experiment. Obtaining a defined preparation of autoantibodies is complicated due to ethical and practical considerations. We found that recombinant, his-tagged factor B (fB) variant K323E retains full hemolytic activity and possess the ability to form convertases with prolonged half-life either in fB-depleted serum or when mixed with normal human serum. Such dominant character of K323E mutation allows using recombinant protein as a reference in functional convertase assays, not limited to these using rabbit erythrocytes. Additionally, our results demonstrate that gain of function mutations in complement components mimic the phenotype of C3NeF. Hence, patients with such "genetic C3NeF" would not benefit from B-cell depletion (e.g. by rituximab) and therefore should be properly diagnosed in order to choose suitable therapeutic intervention.

Antibodies to receptors are associated with biomarkers of inflammation and myocardial damage in heart failure

INTRODUCTION

Naturally occurring antibodies are linked to inflammation, tissue injury and apoptosis, processes also linked to heart failure. Associations between antibodies, inflammation and myocardial damage, have not been elucidated in heart failure.

OBJECTIVE

We investigated if 25 antibodies to receptors expressed in the cardiovascular system were associated with troponin-T, biomarkers of inflammation and clinical measures of disease severity, in patients with heart failure.

METHODS

Antibodies in sera from patients (n=191) with ischemic (n=155) or non-ischemic (n=36) heart failure were measured with full-receptor sandwich enzyme-linked immunosorbent assays. All patients underwent coronary angiography with determination of left ventricular ejection fraction (LVEF) and left ventricular end-diastolic pressure (LVEDP). Measured biomarkers included troponin-T, C-reactive protein, erythrocyte sedimentation rate, fibrinogen and neopterin.

RESULTS

Stabilin-1-antibodies correlated with troponin-T (β 0.23 p=0.008), soluble endoglin-antibodies with erythrocyte sedimentation rate (β 0.19, p=0.007) and fibrinogen (β 0.28, p<0.001). Platelet-derived growth factor subunit β-antibodies were associated with neopterin (β 0.17, p=0.002). All antibodies were correlated (R 0.26 to 0.91) and formed 4 principal components (PCs). Patients with high CRP and high PC2 had higher NYHA class and patients with high troponin-T and high PC1 had lower LVEDP (interactions, all p<0.05).

CONCLUSION

Antibodies to receptors are correlated and are associated with biomarkers of inflammation and myocardial damage, which further modifies their association with disease severity in heart failure. Their functional activity and immunological function, remain undecided.

Immunoglobulin M: Restrainer of Inflammation and Mediator of Immune Evasion by Plasmodium falciparum Malaria

Immunoglobulin M (IgM) is an ancient antibody class that is found in all vertebrates, with the exception of coelacanths, and is indispensable in both innate and adaptive immunity. The equally ancient human malaria parasite, Plasmodium falciparum, formed an intimate relationship with IgM with which it co-evolved. In this article, we discuss the association between IgM and human malaria parasites, building on several recent publications that implicate IgM as a crucial molecule that determines both host and parasite survival. Consequently, a better understanding of this association may lead to the development of improved intervention strategies.

TLR2 modulates antibodies required for intestinal ischemia/reperfusion-induced damage and inflammation

In multiple clinical conditions, including trauma and hemorrhage, reperfusion magnifies ischemic tissue damage. Ischemia induces expression of multiple neoantigens, including lipid alterations that are recognized by the serum protein, β2-glycoprotein I (β2-GPI). During reperfusion, binding of β2-GPI by naturally occurring Abs results in an excessive inflammatory response that may lead to death. As β2-GPI is critical for intestinal ischemia/reperfusion (IR)-induced tissue damage and TLR2 is one of the proposed receptors for β2-GPI, we hypothesized that IR-induced intestinal damage and inflammation require TLR2. Using TLR2(-/-) mice, we demonstrate that TLR2 is required for IR-induced mucosal damage, as well as complement activation and proinflammatory cytokine production. In response to IR, TLR2(-/-) mice have increased serum β2-GPI compared with wild-type mice, but β2-GPI is not deposited on ischemic intestinal tissue. In addition, TLR2(-/-) mice also did not express other novel Ags, suggesting a sequential response. Unlike other TLRs, TLR2(-/-) mice lacked the appropriate Ab repertoire to induce intestinal IR tissue damage or inflammation. Together, these data suggest that, in addition to the inflammatory response, IR-induced injury requires TLR2 for naturally occurring Ab production.

Deficiency of complement receptors CR2/CR1 in Cr2⁻/⁻ mice reduces the extent of secondary brain damage after closed head injury

Complement activation at the C3 convertase level has been associated with acute neuroinflammation and secondary brain injury after severe head trauma. The present study was designed to test the hypothesis that Cr2^-/- mice, which lack the receptors CR2/CD21 and CR1/CD35 for complement C3-derived activation fragments, are protected from adverse sequelae of experimental closed head injury. Adult wild-type mice and Cr2^-/- mice on a C57BL/6 genetic background were subjected to focal closed head injury using a standardized weight-drop device. Head-injured Cr2^-/- mice showed significantly improved neurological outcomes for up to 72 hours after trauma and a significantly decreased post-injury mortality when compared to wild-type mice. In addition, the Cr2^-/- genotype was associated with a decreased extent of neuronal cell death at seven days post-injury. Western blot analysis revealed that complement C3 levels were reduced in the injured brain hemispheres of Cr2^-/- mice, whereas plasma C3 levels remained unchanged, compared to wild-type mice. Finally, head-injured Cr2^-/- had an attenuated extent of post-injury C3 tissue deposition, decreased astrocytosis and microglial activation, and attenuated immunoglobulin M deposition in injured brains compared to wild-type mice. Targeting of these receptors for complement C3 fragments (CR2/CR1) may represent a promising future approach for therapeutic immunomodulation after traumatic brain injury.

Sustained reduction of alloantibody secreting plasma cells and donor specific antibody with proteasome inhibition in mice

The long-lived plasma cells, which develop after alloantigen sensitization, produce donor specific alloantibodies (DSAs) that generate a positive serum cross-match and preclude transplantation. Bortezomib, a proteasome inhibitor, is being investigated in clinical desensitization protocols, however preclinical studies in a transplant model are nonexistent. We hypothesized that sustained treatment with only a proteasome inhibitor would eliminate plasma cells and reduce DSA over time. Cardiac allografts were transplanted into murine recipients. Eight weeks after allograft rejection the proteasome inhibitor, bortezomib, was injected intravenously twice weekly for 60 days. Serum alloantibody responses were assayed using flow cross-match. Total and alloreactive plasma cell numbers were enumerated using flow cytometry and ELISPOT. All recipients of cardiac allografts rejected their graft promptly within 16 days and demonstrated alloantibody by flow cross-match. DSA was sustained in the control mice while mice treated with bortezomib had sustained elimination of DSA and a marked reduction in plasma cell population. Also, bortezomib was associated with an increased level of BLyS. Within a murine model, proteasome inhibition can eliminate alloantibody secreting plasma cells, and reduce alloantibody. Cessation of bortezomib is not associated with return of DSA.

R-spondin3 prevents mesenteric ischemia/reperfusion-induced tissue damage by tightening endothelium and preventing vascular leakage

Inflammation and vascular injury triggered by ischemia/reperfusion (I/R) represent a leading cause of morbidity and mortality in a number of clinical settings. Wnt and its homolog partners R-spondins, in addition to regulating embryonic development have recently been demonstrated to serve as wound-healing agents in inflammation-associated conditions. Here we ask whether R-spondins could prevent inflammation-associated tissue damage in ischemic disorders and thus investigate the role of R-spondin3 (R-spo3) in a mouse model of mesenteric I/R. We demonstrate that R-spo3 ameliorates mesenteric I/R-induced local intestinal as well as remote lung damage by suppressing local and systemic cytokine response and deposition of IgM and complement in intestinal tissues. We also show that decreased inflammatory response is accompanied by tightening of endothelial cell junctions and reduction in vascular leakage. We conclude that R-spo3 stabilizes endothelial junctions and inhibits vascular leakage during I/R and thereby mitigates the inflammatory events and associated tissue damage. Our findings uniquely demonstrate a protective effect of R-spo3 in I/R-related tissue injury and suggest a mechanism by which it may have these effects.

Challenging the role of adaptive immunity in neurotrauma: Rag1(-/-) mice lacking mature B and T cells do not show neuroprotection after closed head injury

The role of adaptive immunity in contributing to post-traumatic neuroinflammation and neuropathology after head injury remains largely unexplored. The present study was designed to investigate the pathophysiological sequelae of closed head injury in Rag1(-/-) mice devoid of mature B and T lymphocytes. C57BL/6 wild-type and Rag1(-/-) mice were subjected to experimental closed head injury, using a standardized weight-drop device. Outcome parameters consisted of neurological scoring, quantification of blood-brain barrier (BBB) function, measurement of inflammatory markers and mediators of apoptosis in serum and brain tissue, and assessment of neuronal cell death, astrogliosis, and tissue destruction. There was no difference between wild-type and Rag1(-/-) mice with regard to injury severity and neurological impairment for up to 7 days after head injury. The extent of BBB dysfunction was in a similar range for both groups. Quantification of complement activation fragments in serum revealed significantly attenuated C3a levels in Rag1(-/-) mice compared to wild-type animals. In contrast, the levels of pro- and anti-inflammatory cytokines and pro-apoptotic and anti-apoptotic mediators remained in a similar range for both groups, and the histological analysis of brain sections did not reveal a difference in reactive astrogliosis, tissue destruction, and neuronal cell death in Rag1(-/-) compared to wild-type mice. These findings suggest that adaptive immunity is not of crucial importance for initiating and sustaining the inflammatory neuropathology after closed head injury. The attenuated extent of post-traumatic complement activation seen in Rag1(-/-) mice implies a cross-talk between innate and adaptive immune responses, which requires further investigation in future studies.

Autoimmunity against cardiac troponin I in ischaemia reperfusion injury

AIMS

Autoimmunity against cardiac troponin I (cTnI) has deleterious effects on the infarcted myocardium early after onset of ischaemia. Here, we explored the impact of cTnI-autoimmunity in the long term. Furthermore, we studied the effects of cTnI-autoimmunity on the infarcted myocardium following revascularization measures in terms of ischaemia reperfusion injury (IRI), which resembles clinical reality more closely.

METHODS AND RESULTS

After immunization with either cTnI (n= 10) or a control buffer (n= 14), A/J mice underwent chronic coronary artery ligation. Another group of mice immunized with cTnI (n= 13) underwent temporary coronary artery occlusion and were compared with non-immunized controls (n= 17). Left ventricular function was evaluated by echocardiography. Hearts were obtained for histological evaluation. Immunological responses were quantified by analysis of cytokine and chemokine patterns as well as anti-cTnI antibody titres. Myocardial inflammation and cardiac dysfunction were detectable as late as 180 days after myocardial infarction (MI). Previous cTnI-immunization enhanced myocardial inflammation and dysfunction. Mice subjected to cTnI-immunization before IRI exhibited a higher inflammation score, an upregulated expression of pro-inflammatory chemokines (IP-10, MIP-1, Ltn, RANTES, TCA-3) and chemokine receptors (CCR2, CCR5), increased interleukin (IL)-2, interferon (IFN)-g, and decreased IL-10 production along with a markedly reduced fractional shortening after IRI compared with the controls.

CONCLUSION

Our results demonstrate for the first time that cTnI-induced autoimmune response not only leads to increased myocardial inflammation and impaired cardiac function 180 days after chronic coronary artery ligation, but also exacerbates ischaemia/reperfusion injury compared with non-immunized controls. Hence, the presence of cTnI-autoimmunity could render subjects more vulnerable to prospective myocardial injury, be it MI, or secondary revascularization measures.

CR2+ marginal zone B cell production of pathogenic natural antibodies is C3 independent

Intestinal ischemia-reperfusion (IR)-induced damage requires complement receptor 2 (CR2) for generation of the appropriate natural Ab repertoire. Pathogenic Abs recognize neoantigens on the ischemic tissue, activate complement, and induce intestinal damage. Because C3 cleavage products act as ligands for CR2, we hypothesized that CR2(hi) marginal zone B cells (MZBs) require C3 for generation of the pathogenic Abs. To explore the ability of splenic CR2(+) B cells to generate the damaging Ab repertoire, we adoptively transferred either MZBs or follicular B cells (FOBs) from C57BL/6 or Cr2(-/-) mice into Rag-1(-/-) mice. Adoptive transfer of wild type CR2(hi) MZBs but not CR2(lo) FOBs induced significant damage, C3 deposition, and inflammation in response to IR. In contrast, similarly treated Rag-1(-/-) mice reconstituted with either Cr2(-/-) MZB/B1 B cells (B1Bs) or FOBs lacked significant intestinal damage and displayed limited complement activation. To determine whether C3 cleavage products are critical in CR2-dependent Ab production, we evaluated the ability of the natural Ab repertoire of C3(-/-) mice to induce damage in response to IR. Infusion of C3(-/-) serum into Cr2(-/-) mice restored IR-induced tissue damage. Furthermore, Rag-1(-/-) mice sustained significant damage after infusion of Abs from C3(-/-) but not Cr2(-/-) mice. Finally, adoptive transfer of MZBs from C3(-/-) mice into Rag-1(-/-) mice resulted in significant tissue damage and inflammation. These data indicate that CR2 expression on MZBs is sufficient to induce the appropriate Abs required for IR-induced tissue damage and that C3 is not critical for generation of the pathogenic Abs.

Helicobacter infection alters MyD88 and Trif signalling in response to intestinal ischaemia-reperfusion

Ischaemia-reperfusion-induced intestinal injury requires both Toll-like receptor 4 (TLR4) signalling through myeloid differentiation primary response gene (88) (MyD88) and complement activation. As a common Gram-negative intestinal pathogen, Helicobacter hepaticus signals through TLR4 and upregulates the complement inhibitor, decay accelerating factor (DAF; CD55). Since ischaemia-reperfusion (IR) injury is complement dependent, we hypothesized that Helicobacter infection may alter IR-induced intestinal damage. Infection increased DAF transcription and subsequently decreased complement activation in response to IR without altering intestinal damage in wild-type mice. Ischaemia-reperfusion induced similar levels of DAF mRNA expression in uninfected wild-type, MyD88(-/-) or TIR-domain-containing adaptor-inducing interferon-β (Trif)-deficient mice. However, during infection, IR-induced DAF transcription was significantly attenuated in Trif-deficient mice. Likewise, IR-induced intestinal damage, complement component 3 deposition and prostaglandin E(2) production were attenuated in Helicobacter-infected, Trif-deficient but not MyD88(-/-) mice. While infection attenuated IR-induced cytokine production in wild-type and MyD88(-/-) mice, there was no further decrease in Trif-deficient mice. These data indicate distinct roles for MyD88 and Trif in IR-induced inflammation and suggest that chronic, undetected infections, such as Helicobacter, alter the use of the adaptor proteins to induce damage.

Phosphoinositide 3-kinase p110 delta regulates natural antibody production, marginal zone and B-1 B cell function, and autoantibody responses

B-1 and marginal zone (MZ) B cells produce natural Abs, make Ab responses to microbial pathogens, and contribute to autoimmunity. Although the delta isoform of the PI3K p110 catalytic subunit is essential for development of these innate-like B cells, its role in the localization, activation, and function of normal B-1 and MZ B cells is not known. Using IC87114, a highly selective inhibitor of p110delta enzymatic activity, we show that p110delta is important for murine B-1 and MZ B cells to respond to BCR clustering, the TLR ligands LPS and CpG DNA, and the chemoattractants CXCL13 and sphingosine 1-phosphate. In these innate-like B cells, p110delta activity mediates BCR-, TLR- and chemoattractant-induced activation of the Akt prosurvival kinase, chemoattractant-induced migration, and TLR-induced proliferation. Moreover, we found that TLR-stimulated Ab responses by B-1 and MZ B cells, as well as the localization of MZ B cells in the spleen, depend on p110delta activity. Finally, we show that the in vivo production of natural Abs requires p110delta and that p110delta inhibitors can reduce in vivo autoantibody responses. Thus, targeting p110delta may be a novel approach for regulating innate-like B cells and for treating Ab-mediated autoimmune diseases.

Early complement factors in the local tissue immunocomplex generated during intestinal ischemia/reperfusion injury

Recent work reveals that the innate immune system is able to recognize self-targets and initiate an inflammatory response similar to that of pathogens. One novel example of this innate autoimmunity is ischemia/reperfusion (I/R) injury, in which reperfusion of the ischemic tissues elicits an acute inflammatory response activated by natural IgM (nIgM) binding to ischemia-specific self-antigens, which are non-muscle myosin heavy chains type II (NMHC-II) subtype A and C. Subsequently, the complement lectin pathway is activated and eventually tissue injury occurs. Although earlier studies in the intestinal model showed that the classical complement pathway did not initiate I/R injury, C1q deposition was still observed in the local injured tissues by imaging analysis. Moreover, the involvement of the alternative complement pathway became unclear due to conflicting reports using different knockout mice. To explore the immediate downstream pathway following nIgM-ischemic antigen interaction, we isolated the nIgM-ischemic antigen immunocomplexes from the local tissue of animals treated in the intestinal I/R injury model, and examined the presence of initial molecules of three complement pathways. Our results showed that mannan-binding lectin (MBL), the early molecule of the lectin pathway, was present in the nIgM-ischemic Ag immunocomplex. In addition, C1q, the initial molecule of the classical pathway was also detected on the immunocomplex. However, Factor B, the early molecule in the alternative pathway, was not detected in the immunocomplex. To further examine the role of the alternative pathway in I/R injury, we utilized Factor B knockout mice in the intestinal model. Our results showed that Factor B knockout mice were not protected from local tissue injury, and their complement system was activated in the local tissues by nIgM during I/R. These results indicated that the lectin complement pathway operates immediately downstream of the nIgM-ischemic antigen interaction during intestinal I/R. Furthermore, the classical complement pathway also appears to interact with the of nIgM-ischemic antigen immunocomplex. Finally, the alternative complement pathway is not involved in I/R injury induction in the current intestinal model.

The human IgM pentamer is a mushroom-shaped molecule with a flexural bias

The textbook planar model of pentameric IgM, a potent activator of complement C1q, is based upon the crystallographic structure of IgG. Although widely accepted, key predictions of this model have not yet been directly confirmed, which is particularly important since IgG lacks a major Ig fold domain in its Fc region that is present in IgM. Here, we construct a homology-based structural model of the IgM pentamer using the recently obtained crystallographic structure of IgE Fc, which has this additional Ig domain, under the constraint that all of the cysteine residues known to form disulfide bridges both within each monomer and between monomers are bonded together. In contrast to the planar model, this model predicts a non-planar, mushroom-shaped complex, with the central portion formed by the C-terminal domains protruding out of the plane formed by the Fab domains. This unexpected conformation of IgM is, however, directly confirmed by cryo-atomic force microscopy of individual human IgM molecules. Further analysis of this model with free energy calculations of out-of-plane Fab domain rotations reveals a pronounced asymmetry favoring flexions toward the central protrusion. This bias, together with polyvalent attachment to cell surface antigen, would ensure that the IgM pentamer is oriented on the cell membrane with its C1q binding sites fully exposed to the solution, and thus provides a mechanistic explanation for the first steps of C1q activation by IgM.

A novel inhibitor of the alternative pathway of complement attenuates intestinal ischemia/reperfusion-induced injury

Complement activation has been demonstrated to contribute significantly to the expression of IR-induced tissue damage. Each of the three complement pathways, classic, alternative, and lectin, has been implicated in the instigation of tissue pathology. In this study, we used a selective inhibitor of the alternative pathway, that is, a soluble form of complement receptor of the immunoglobulin superfamily (CRIg-Fc) to determine whether it can prevent IR tissue injury. We demonstrate that treatment of C57B1/6 mice prior to mesenteric IR prevents local (intestinal) and remote (lung) injury by limiting deposition of complement and entry of polymorphonuclear cells to the sites of injury. Our results show that CRIg-Fc represents a candidate to limit IR injury as it occurs in various clinical conditions.

The regulation of liver cell survival by complement

Complement effectors are known to contribute to host cell injury in several inflammatory diseases. Contrary to this paradigm, in this study utilizing surgical liver resection (partial hepatectomy) in various complement-deficient mice as a model, we have demonstrated that complement anaphylatoxins C3a and C5a are required for the survival of liver cells during regeneration. The mechanisms of these cytoprotective functions of complement were related to the regulation of IL-6 and TNF production or release after liver resection. Disturbances in the cytokine milieu, induced by a loss of complement activity, were found to alter prosurvival signaling, including the IL-6/STAT3 and PI3K/Akt/mammalian target of rapamycin pathways. In conclusion, this study documents functions of complement proteins as prosurvival factors that, through their interactions with cytokines, inhibit apoptotic signaling in proliferating cells of epithelial origin.

Evidence for allergen-specific IgE of maternal origin in human placenta

BACKGROUND

Immunoglobulin E (IgE) has been identified on macrophage-like cells in the villi of human placenta, irrespective of the serum IgE levels or allergy status of the mother. The origin of placental IgE is debated and it is not known if it is spontaneously produced, so-called 'natural IgE', or if it has any specificity for certain allergens. The aim of this study was to investigate if placental IgE originates from mother or child and to analyse its specificity.

METHODS

Immunoglobulin E was eluted from placenta by lowering the pH. Total and allergen-specific IgEs were measured in placenta eluate, maternal and cord blood plasma by means of ImmunoCAP (Phadia AB). The levels of natural antibodies were determined with an anti-phosphorylcholine (PC) enzyme-linked immunosorbent assay, as natural IgE has been shown in one previous publication with this assay.

RESULTS

Detectable amounts of IgE were eluted from 11/12 full-term placentas. Natural (anti-PC) IgE antibodies were detected in low amounts in maternal plasma but not in the placental eluate or in cord blood plasma. There was a significant correlation between the amount of total IgE eluted from placenta and the levels of total IgE in maternal plasma; however, not between maternal and cord blood plasma. Allergen-specific IgE was only found in placental eluates from mothers with specific IgE towards these allergens. Furthermore, there was a significant correlation between the amount of allergen-specific IgE eluted from placenta and the levels of allergen-specific IgE in maternal plasma. Allergen-specific IgE could not be detected in cord blood.

CONCLUSION

These results suggest a maternal origin of placental IgE, which can be allergen-specific.

A reevaluation of the role of IgM Non-HLA antibodies in cardiac transplantation

BACKGROUND

Preexisting IgG antibodies to donor human leukocyte antigens (HLA) are a risk factor for rapid allograft rejection. However, non-HLA antibodies, of the IgM class, also called autoreactive antibodies, are not believed to affect graft survival. The aim of this study was to determine the incidence and clinical relevance of pretransplant lymphocytotoxic non-HLA IgM antibodies on long-term cardiac allograft survival.

METHODS

A retrospective study of 616 adult recipients of cardiac allografts, transplanted at this center between 1991 and 2003, has been performed. Antibodies in pretransplant sera were initially defined using complement-dependent cytotoxicity assays, and subsequently analyzed for HLA specificities using solid phase assays.

RESULTS

HLA antibodies were present in 69 of 616 heart recipients (58 IgG, 11 IgM); in 22 of these, the antibodies were donor-specific. Non-HLA IgM antibodies were detected in 59 of 616 recipients who did not have HLA-specific antibodies; these patients had a 1, 2, 5, and 10 year survival of 55.9%, 54.2%, 49.9%, and 43.3% compared with 75.8%, 73.7%, 66.6%, and 52.8% for those without antibodies (P=0.0085 log-rank test). Multivariate analysis demonstrated pretransplant non-HLA IgM antibodies to be an independent risk factor for mortality (P=0.0001). Myocardial histology of postmortem heart and cardiac biopsies suggested an association with ischemic damage and "primary" allograft failure.

CONCLUSIONS

We propose the hypothesis that the presence of cytotoxic IgM antibodies to non-HLAs before heart transplantation maybe a risk factor for early allograft failure.

Complement component C5a mediates hemorrhage-induced intestinal damage

BACKGROUND

Complement has been implicated in the pathogenesis of intestinal damage and inflammation in multiple animal models. Although the exact mechanism is unknown, inhibition of complement prevents hemodynamic alterations in hemorrhage.

MATERIALS AND METHODS

C57Bl/6, complement 5 deficient (C5-/-) and sufficient (C5+/+) mice were subjected to 25% blood loss. In some cases, C57Bl/6 mice were treated with C5a receptor antagonist (C5aRa) post-hemorrhage. Intestinal injury, leukotriene B4, and myeloperoxidase production were assessed for each treatment group of mice.

RESULTS

Mice subjected to significant blood loss without major trauma develop intestinal inflammation and tissue damage within 2 hours. We report here that complement 5 (C5) deficient mice are protected from intestinal tissue damage when subjected to hemorrhage (injury score = 0.36 compared with wildtype hemorrhaged animal injury score = 2.89; P < 0.05). We present evidence that C5a represents the effector molecule because C57Bl/6 mice treated with a C5a receptor antagonist displayed limited intestinal injury (injury score = 0.88), leukotriene B4 (13.16 pg/mg tissue), and myeloperoxidase (115.6 pg/mg tissue) production compared with hemorrhaged C57Bl/6 mice (P < 0.05).

CONCLUSIONS

Complement activation is important in the development of hemorrhage-induced tissue injury and C5a generation is critical for tissue inflammation and damage. Thus, therapeutics targeting C5a may be useful therapeutics for hemorrhage-associated injury.

Nature and functions of autoantibodies

Antibodies that react with self-molecules occur in healthy individuals and are referred to as natural antibodies or autoantibodies. Natural autoantibodies are mainly IgM, are encoded by unmutated V(D)J genes and display a moderate affinity for self-antigens. They provide a first line of defense against infections, probably serve housekeeping functions and contribute to the homeostasis of the immune system. By contrast, high-affinity, somatically mutated IgG autoantibodies reflect a pathologic process whereby homeostatic pathways related to cell clearance, antigen-receptor signaling or cell effector functions are disturbed. In some autoimmune disorders, autoantibodies might be present before disease onset, show remarkable specificity and serve as biomarkers providing an opportunity for diagnosis and therapeutic intervention. In organ-specific autoimmune diseases, such as myasthenia gravis or pemphigus, autoantibodies directly bind to and injure target organs. In systemic autoimmune diseases, autoantibodies react with free molecules, such as phospholipids, as well as cell surface and nucleoprotein antigens, forming pathogenic antigen-antibody (immune) complexes. These autoantibodies injure tissues and organs through engagement of Fc gammaR activation of complement as well as internalization and activation of Toll-like receptors. Activation of intracellular Toll-like receptors in plasmacytoid dendritic cells leads to the production of type I interferon, whereas engagement of intracellular Toll-like receptors on antigen-presenting cells stimulates cell activation and the production of other inflammatory cytokines. Thus, immune complexes might perpetuate a positive feedback loop amplifying inflammatory responses.

Role of biological modifiers regulating the immune response after trauma

Trauma induces a profound immunological dysfunction. This is characterised by an early state of hyperinflammation, followed by a phase of immunosuppression with increased susceptibility to infection and multiple organ failure. Therapeutic strategies directed at restoring immune homeostasis after traumatic injuries have largely failed in translation from "bench to bedside". The present review illustrates the role of biological modifiers of the posttraumatic immune response by portraying different modalities of therapeutic immune modulation. The emphasis is placed on anti-inflammatory (steroids) and immune-stimulatory (interferon) pharmacological strategies and modified resuscitative strategies, as well as more unconventional immunomodulatory approaches, such as immunonutrition.

A novel role of complement in mobilization: immunodeficient mice are poor granulocyte-colony stimulating factor mobilizers because they lack complement-activating immunoglobulins

Complement (C) and innate immunity emerge as important and underappreciated modulators of mobilization of hematopoietic stem/progenitor cells (HSPC). We reported that (a) C becomes activated in bone marrow (BM) during granulocyte-colony-stimulating factor (G-CSF)-induced mobilization by the classic immunoglobulin (Ig)-dependent pathway and that (b) C3 cleavage fragments increase the responsiveness of HSPC to a stromal derived factor-1 gradient. Since patients suffering from severe combined immunodeficiency (SCID) mobilize poorly, we hypothesized that this could be directly linked to the lack of C activating Ig in these patients. In the current study to better elucidate the role of C activation in HSPC mobilization, we mobilized mice that lack Ig (RAG2, SCID, and Jh) by G-CSF or zymosan, compounds that activate C by the classic Ig-dependent and the alternative Ig-independent pathways, respectively. In addition, we evaluated mobilization in C5-deficient animals. Mobilization was evaluated by measuring the number of colony-forming unit-granulocyte macrophage and leukocytes circulating in peripheral blood. We found that (a) G-CSF- but not zymosan-induced mobilization was severely reduced in RAG2, SCID, and Jh mice; (b) impaired G-CSF-induced mobilization was restored after infusion of purified wild-type Ig; and (c) mobilization was severely reduced in C5-deficient mice. These data provide strong evidence that the C system plays a pivotal role in mobilization of HSPC and that egress of HSPC from BM occurs as part of an immune response. Disclosure of potential conflicts of interest is found at the end of this article.