Identification of a specific self-reactive IgM antibody that initiates intestinal ischemia/reperfusion injury

A single-chain antibody construct with specificity of a natural IgM antibody reduces hepatic ischemia reperfusion injury in mice

Natural immunoglobulin M (IgM) antibodies have been shown to recognize post-ischemic neoepitopes following reperfusion of tissues and to activate complement. Specifically, IgM antibodies and complement have been shown to drive hepatic ischemia reperfusion injury (IRI). Herein, we investigate the therapeutic effect of C2 scFv (single-chain antibody construct with specificity of a natural IgM antibody) on hepatic IRI in C57BL/6 mice. Compared with PBS-treated mice, C2 scFv-treated mice displayed almost no necrotic areas, significant reduction in serum ALT, AST and LDH levels, and significantly reduced in the number of TUNEL positive cells. Moreover, C2 scFv-treated mice exhibited a notable reduction in inflammatory cells after hepatic IRI than PBS-treated mice. The serum IL-6, IL-1β, TNF-α and MPC-1 levels were also severely suppressed by C2 scFv. Interestingly, C2 scFv reconstituted hepatic inflammation and IRI in Rag1-/- mice. We found that C2 scFv promoted hepatic cell death and increased inflammatory cytokines and infiltration of inflammatory cells after hepatic IRI in Rag1-/- mice. In addition, IgM and complement 3d (C3d) were deposited in WT mice and in Rag1-/- mice reconstituted with C2 scFv, indicating that C2 scFv can affect IgM binding and complement activation and reconstitute hepatic IRI. C3d expression was significantly lower in C57BL/6 mice treated with C2 scFv compared to PBS, indicating that excessive exogenous C2 scFv inhibited complement activation. These data suggest that C2 scFv alleviates hepatic IRI by blocking complement activation, and treatment with C2 scFv may be a promising therapy for hepatic IRI.

B-cells in pulmonary arterial hypertension: friend, foe or bystander?

There is an unmet need for new therapeutic strategies that target alternative pathways to improve the prognosis of patients with pulmonary arterial hypertension (PAH). As immunity has been involved in the development and progression of vascular lesions in PAH, we review the potential contribution of B-cells in its pathogenesis and evaluate the relevance of B-cell-targeted therapies. Circulating B-cell homeostasis is altered in PAH patients, with total B-cell lymphopenia, abnormal subset distribution (expansion of naïve and antibody-secreting cells, reduction of memory B-cells) and chronic activation. B-cells are recruited to the lungs through local chemokine secretion, and activated by several mechanisms: 1) interaction with lung vascular autoantigens through cognate B-cell receptors; 2) costimulatory signals provided by T follicular helper cells (interleukin (IL)-21), type 2 T helper cells and mast cells (IL-4, IL-6 and IL-13); and 3) increased survival signals provided by B-cell activating factor pathways. This activity results in the formation of germinal centres within perivascular tertiary lymphoid organs and in the local production of pathogenic autoantibodies that target the pulmonary vasculature and vascular stabilisation factors (including angiotensin-II/endothelin-1 receptors and bone morphogenetic protein receptors). B-cells also mediate their effects through enhanced production of pro-inflammatory cytokines, reduced anti-inflammatory properties by regulatory B-cells, immunoglobulin (Ig)G-induced complement activation, and IgE-induced mast cell activation. Precision-medicine approaches targeting B-cell immunity are a promising direction for select PAH conditions, as suggested by the efficacy of anti-CD20 therapy in experimental models and a trial of rituximab in systemic sclerosis-associated PAH.

Secreted IgM modulates IL-10 expression in B cells

IL-10+ B cells are critical for immune homeostasis and restraining immune responses in infection, cancer, and inflammation; however, the signals that govern IL-10+ B cell differentiation are ill-defined. Here we find that IL-10+ B cells expand in mice lacking secreted IgM ((s)IgM-/-) up to 10-fold relative to wildtype (WT) among all major B cell and regulatory B cell subsets. The IL-10+ B cell increase is polyclonal and presents within 24 hours of birth. In WT mice, sIgM is produced prenatally and limits the expansion of IL-10+ B cells. Lack of the high affinity receptor for sIgM, FcμR, in B cells translates into an intermediate IL-10+ B cell phenotype relative to WT or sIgM-/- mice. Our study thus shows that sIgM regulates IL-10 programming in B cells in part via B cell-expressed FcμR, thereby revealing a function of sIgM in regulating immune homeostasis.

The role of complement C3 in the outcome of regional myocardial infarction

Coronary heart disease leading to myocardial ischemia is a major cause of heart failure. A hallmark of heart failure is myocardial fibrosis. Using a murine model of myocardial ischemia/reperfusion injury (IRI), we showed that, following IRI, in mice genetically deficient in the central factor of complement system, C3, myocardial necrosis was reduced compared with WT mice. Four weeks after the ischemic period, the C3^-/- mice had significantly less cardiac fibrosis and better cardiac function than the WT controls. Overall, our results suggest that innate immune response through complement C3 plays an important role in necrotic cell death, which contributes to the cardiac fibrosis that underlies post-infarction heart failure.

Membrane attack complexes, endothelial cell activation, and direct allorecognition

Endothelial cells (ECs) form a critical immune interface regulating both the activation and trafficking of alloreactive T cells. In the setting of solid organ transplantation, donor-derived ECs represent sites where alloreactive T cells encounter major and minor tissue-derived alloantigens. During this initial encounter, ECs may formatively modulate effector responses of these T cells through expression of inflammatory mediators. Direct allorecognition is a process whereby recipient T cells recognize alloantigen in the context of donor EC-derived HLA molecules. Direct alloresponses are strongly modulated by human ECs and are galvanized by EC-derived inflammatory mediators.

Complement are immune proteins that mark damaged or foreign surfaces for immune cell activation. Following labeling by natural IgM during ischemia reperfusion injury (IRI) or IgG during antibody-mediated rejection (ABMR), the complement cascade is terminally activated in the vicinity of donor-derived ECs to locally generate the solid-phase inflammatory mediator, the membrane attack complex (MAC). Via upregulation of leukocyte adhesion molecules, costimulatory molecules, and cytokine trans-presentation, MAC strengthen EC:T cell direct alloresponses and qualitatively shape the alloimmune T cell response. These processes together promote T cell-mediated inflammation during solid organ transplant rejection.

In this review we describe molecular pathways downstream of IgM- and IgG-mediated MAC assembly on ECs in the setting of IRI and ABMR of tissue allografts, respectively. We describe work demonstrating that MAC deposition on ECs generates ‘signaling endosomes’ that sequester and post-translationally enhance the stability of inflammatory signaling molecules to promote EC activation, a process potentiating EC-mediated direct allorecognition. Additionally, with consideration to first-in-human xenotransplantation procedures, we describe clinical therapeutics based on inhibition of the complement pathway. The complement cascade critically mediates EC activation and improved understanding of relevant effector pathways will uncover druggable targets to obviate dysregulated alloimmune T cell infiltration into tissue allografts.

Flow Cytometry-Based Quantification and Analysis of Myocardial B-cells

A growing body of evidence shows that B-lymphocytes play an important role in the context of myocardial physiology and myocardial adaptation to injury. However, the literature reports contrasting data on the prevalence of myocardial B-cells. B-cells have been reported to be both among the most prevalent immune cells in the rodent heart or to be present, but at a markedly lower prevalence than myeloid cells, or to be quite rare. Similarly, several groups have described that the number of myocardial B-cells increases after acute ischemic myocardial injury, but one group reported no changes in the number of B-cells of the injured myocardium. Implementation of a shared, reproducible method to assess the prevalence of myocardial B-cells is critical to harmonize observations from different research groups and thus promote the advancement of the study of B-cell myocardial interactions. Based on our experience, the seemingly contrasting observations reported in the literature likely stem from the fact that murine myocardial B-cells are mostly intravascular and connected to the microvascular endothelium. Therefore, the number of B-cells recovered from a murine heart is exquisitely sensitive to the perfusion conditions used to clean the organ and to the method of digestion used. Here we report an optimized protocol that accounts for these two critical variables in a specific way. This protocol empowers reproducible, flow cytometry-based analysis of the number of murine myocardial B-cells and allows researchers to distinguish extravascular vs. intravascular myocardial B-cells.

Bibliometric and visual analysis of intestinal ischemia reperfusion from 2004 to 2022

Background

Intestinal ischemia/reperfusion (I/R) injury is a common tissue-organ damage occurring in surgical practice. This study aims to comprehensively review the collaboration and impact of countries, institutions, authors, subject areas, journals, keywords, and critical literature on intestinal I/R injury from a bibliometric perspective, and to assess the evolution of clustering of knowledge structures and identify hot trends and emerging topics.

Methods

Articles and reviews related to intestinal I/R were retrieved through subject search from Web of Science Core Collection. Bibliometric analyses were conducted on Excel 365, CiteSpace, VOSviewer, and Bibliometrix (R-Tool of R-Studio).

Results

A total of 1069 articles and reviews were included from 2004 to 2022. The number of articles on intestinal I/R injury gradually plateaued, but the number of citations increased. These publications were mainly from 985 institutions in 46 countries, led by China and the United States. Liu Kx published the most papers, while Chiu Cj had the largest number of co-citations. Analysis of the journals with the most outputs showed that most journals focused on surgical sciences, cell biology, and immunology. Macroscopic sketch and microscopic characterization of the entire knowledge domain were achieved through co-citation analysis. The roles of cell death, exosomes, intestinal flora, and anesthetics in intestinal I/R injury are the current and developing research focuses. The keywords "dexmedetomidine", "proliferation", and "ferroptosis" may also become new trends and focus of future research.

Conclusion

This study comprehensively reviews the research on intestinal I/R injury using bibliometric and visualization methods, and will help scholars better understand the dynamic evolution of intestinal I/R injury and provide directions for future research.

The Role of B Cells in Cardiomyopathy and Heart Failure

PURPOSE OF REVIEW

To summarize the current knowledge on the role that B lymphocytes play in heart failure.

RECENT FINDINGS

Several studies from murine models have shown that B cells modulate cardiac adaptation to injury and ultimately affect the degree of cardiac dysfunction after acute ischemic damage. In addition, a B cell-modulating small molecule was recently shown to have beneficial effects in humans with heart failure with preserved ejection fraction. B lymphocytes are specialized immune cells present in all jawed vertebrates. They are characteristically known for their ability to produce antibodies, but they have other functions and are important players in virtually all forms of immune responses. A growing body of evidence indicates that B cells are intimately connected with the heart and that B cell dysregulation might play a role in the pathogenesis and progression of both heart failure with reduced ejection fraction and heart failure with preserved ejection fraction. B cells are therefore gathering attention as potential targets for the development of novel immunomodulatory-based treatments for heart failure.

FcᵧRIIb protects from reperfusion injury by controlling antibody and type I IFN-mediated tissue injury and death

Intestinal ischemia and reperfusion (I/R) is accompanied by an exacerbated inflammatory response characterized by deposition of IgG, release of inflammatory mediators, and intense neutrophil influx in the small intestine, resulting in severe tissue injury and death. We hypothesized that FcᵧRIIb activation by deposited IgG could inhibit tissue damage during I/R. Our results showed that I/R induction led to the deposition of IgG in intestinal tissue during the reperfusion phase. Death upon I/R occurred earlier and was more frequent in FcᵧRIIb^-/- than WT mice. The higher lethality rate was associated with greater tissue injury and bacterial translocation to other organs. FcᵧRIIb^-/- mice presented changes in the amount and repertoire of circulating IgG, leading to increased IgG deposition in intestinal tissue upon reperfusion in these mice. Depletion of intestinal microbiota prevented antibody deposition and tissue damage in FcᵧRIIb^-/- mice submitted to I/R. We also observed increased production of ROS on neutrophils harvested from the intestines of FcᵧRIIb^-/- mice submitted to I/R. In contrast, FcᵧRIII^-/- mice presented reduced tissue damage and neutrophil influx after reperfusion injury, a phenotype reversed by FcᵧRIIb blockade. In addition, we observed reduced IFN-β expression in the intestines of FcᵧRIII^-/- mice after I/R, a phenotype that was also reverted by blocking FcᵧRIIb. IFNAR^-/- mice submitted to I/R presented reduced lethality and TNF release. Altogether our results demonstrate that antibody deposition triggers FcᵧRIIb to control IFN-β and IFNAR activation and subsequent TNF release, tailoring tissue damage, and death induced by reperfusion injury.

Immunomodulatory Treatment Strategies Targeting B Cells for Heart Failure

Cardio-oncology, a nascent specialty, has evolved as a concerted strategy to address the cardiovascular complications of cancer therapies. On the other hand, emerging evidence has shown that some anti-tumor drugs, such as CD20-targeted rotuximab, also have markedly cardioprotective effects in addition to treating cancers. Rituximab is a CD20-targeted monoclonal antibody and kill tumor B-cells through antibody-mediated and antibody-independent pathways, indicating that B cells participate and promote the progression of cardiovascular diseases. In this review, we mainly present the evidence that B cells contribute to the development of hypertrophy, inflammation, and maladaptive tissue remodeling, with the aim of proposing novel immunomodulatory therapeutic strategies targeting B cells and their products for the treatment of heart failure.

Natural antibodies and CRP drive anaphylatoxin production by urate crystals

In gout, crystallization of uric acid in the form of monosodium urate (MSU) leads to a painful inflammatory response. MSU crystals induce inflammation by activating the complement system and various immune cell types, and by inducing necrotic cell death. We previously found that the soluble pattern recognition molecule C-reactive protein (CRP) recognizes MSU crystals, while enhancing complement activation. In the absence of CRP, MSU crystals still induced complement activation, suggesting additional CRP-independent mechanisms of complement activation. In the present study, we searched for additional MSU crystal-binding complement activators. We found that all healthy individuals, even unborn children, have MSU crystal-specific immunoglobulin M (IgM) in their blood. This indicates that innate IgM, also known as natural IgM, recognizes these crystals. In serum lacking IgM and CRP, MSU crystals showed negligible complement activation as assessed by the production of the anaphylatoxins C4a, C3a, and C5a (listed in order of production via the classical complement pathway). We show that IgM and CRP both activate the classical complement pathway on MSU crystals. CRP was more efficient at fixating active C1 on the crystals and inducing release of the most inflammatory anaphylatoxin C5a, indicating non-redundant functions of CRP. Notably, while CRP recognizes MSU crystals but not the related calcium pyrophosphate dihydrate (CPPD) crystals, natural IgM bound to both, suggesting common and distinct mechanisms of recognition of individual crystal types by complement activators.

Loss of Id3 (Inhibitor of Differentiation 3) Increases the Number of IgM-Producing B-1b Cells in Ischemic Skeletal Muscle Impairing Blood Flow Recovery During Hindlimb Ischemia

OBJECTIVE

Neovascularization can maintain and even improve tissue perfusion in the setting of limb ischemia during peripheral artery disease. The molecular and cellular mechanisms mediating this process are incompletely understood. We investigate the potential role(s) for Id3 (inhibitor of differentiation 3) in regulating blood flow in a murine model of hindlimb ischemia (HLI). Approach and Results: HLI was modeled through femoral artery ligation and resection and blood flow recovery was quantified by laser Doppler perfusion imaging. Mice with global Id3 deletion had significantly impaired perfusion recovery at 14 and 21 days of HLI. Endothelial- or myeloid cell-specific deletion of Id3 revealed no effect on perfusion recovery while B-cell-specific knockout of Id3 (Id3BKO) revealed a significant attenuation of perfusion recovery. Flow cytometry revealed no differences in ischemia-induced T cells or myeloid cell numbers at 7 days of HLI, yet there was a significant increase in B-1b cells in Id3BKO. Consistent with these findings, ELISA (enzyme-linked immunoassay) demonstrated increases in skeletal muscle and plasma IgM. In vitro experiments demonstrated reduced proliferation and increased cell death when endothelial cells were treated with conditioned media from IgM-producing B-1b cells and tibialis anterior muscles in Id3BKO mice showed reduced density of total CD31+ and αSMA+CD31+ vessels.

CONCLUSIONS

This study is the first to demonstrate a role for B-cell-specific Id3 in maintaining blood flow recovery during HLI. Results suggest a role for Id3 in promoting blood flow during HLI and limiting IgM-expressing B-1b cell expansion. These findings present new mechanisms to investigate in peripheral artery disease pathogenesis.

Set Up for Failure: Pre-Existing Autoantibodies in Lung Transplant

Lung transplant patients have the lowest long-term survival rates compared to other solid organ transplants. The complications after lung transplantation such as primary graft dysfunction (PGD) and ultimately chronic lung allograft dysfunction (CLAD) are the main reasons for this limited survival. In recent years, lung-specific autoantibodies that recognize non-HLA antigens have been hypothesized to contribute to graft injury and have been correlated with PGD, CLAD, and survival. Mounting evidence suggests that autoantibodies can develop during pulmonary disease progression before lung transplant, termed pre-existing autoantibodies, and may participate in allograft injury after transplantation. In this review, we summarize what is known about pulmonary disease autoantibodies, the relationship between pre-existing autoantibodies and lung transplantation, and potential mechanisms through which pre-existing autoantibodies contribute to graft injury and rejection.

Therapeutic Potential of B-1a Cells in Intestinal Ischemia-reperfusion Injury

BACKGROUND

Acute mesenteric ischemia is a common surgical emergency. Restoration of blood flow is a critical objective of treating this pathology. However, many patients suffer from ischemia-reperfusion (I/R) injuries at the time of revascularization, requiring prolonged hospitalizations. B-1a cells are a subtype of B lymphocytes with roles in regulating inflammation and tissue injury by spontaneous release of natural IgM and IL-10. We hypothesized that treatment with B-1a cells protects mice from intestinal I/R.

METHODS

Mesenteric ischemia was induced in mice by placing a vascular clip on the superior mesenteric artery for 60 minutes. At the time of reperfusion, B-1a cells or PBS control were instilled into the peritoneal cavity (PerC) of mice. PerC lavage, blood, intestine, and lungs were collected 4 h after reperfusion. Serum organ injury and inflammatory markers such as ALT, AST, LDH, lactate, IL-6, as well as lung and gut histology and myeloperoxidase (MPO) were assessed.

RESULTS

In intestinal I/R, B-1a cell frequency and number in the PerC were significantly decreased compared to sham-operated mice. There was an increase in the serum levels of ALT, AST, LDH, lactate, and IL-6 when comparing the vehicle group with the sham group. These increases were significantly reduced in the B-1a cell treated group. B-1a cell treatment significantly decreased the intestine and lung injury scores as well as MPO content, compared to vehicle treated mice. B-1a cell treatment resulted in a reduction of apoptotic cells in these tissues. Serum IgM levels were decreased in intestinal I/R, while treatment with B-1a cells significantly increased their levels towards normal levels.

CONCLUSIONS

B-1a cell treatment at the time of mesenteric reperfusion ameliorates end organ damage and reduces systemic inflammation through the improvement of serum IgM levels. Preserving B-1a cells pool could serve as a novel therapeutic avenue in intestinal I/R injury.

The Emerging Role of B Lymphocytes in Cardiovascular Disease

B cells are traditionally known for their ability to produce antibodies in the context of adaptive immune responses. However, over the last decade B cells have been increasingly recognized as modulators of both adaptive and innate immune responses, as well as players in an important role in the pathogenesis of a variety of human diseases. Here, after briefly summarizing our current understanding of B cell biology, we present a systematic review of the literature from both animal models and human studies that highlight the important role that B lymphocytes play in cardiac and vascular disease. While many aspects of B cell biology in the vasculature and, to an even greater extent, in the heart remain unclear, B cells are emerging as key regulators of cardiovascular adaptation to injury.

A novel injury site-natural antibody targeted complement inhibitor protects against lung transplant injury

Complement is known to play a role in ischemia and reperfusion injury (IRI). A general paradigm is that complement is activated by self-reactive natural IgM antibodies (nAbs), after they engage postischemic neoepitopes. However, a role for nAbs in lung transplantation (LTx) has not been explored. Using mouse models of LTx, we investigated the role of two postischemic neoepitopes, modified annexin IV (B4) and a subset of phospholipids (C2), in LTx. Antibody deficient Rag1-/- recipient mice were protected from LTx IRI. Reconstitution with either B4 or C2nAb restored IRI, with C2 significantly more effective than B4 nAb. Based on these information, we developed/characterized a novel complement inhibitor composed of single-chain antibody (scFv) derived from the C2 nAb linked to Crry (C2scFv-Crry), a murine inhibitor of C3 activation. Using an allogeneic LTx, in which recipients contain a full nAb repertoire, C2scFv-Crry targeted to the LTx, inhibited IRI, and delayed acute rejection. Finally, we demonstrate the expression of the C2 neoepitope in human donor lungs, highlighting the translational potential of this approach.

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.

Mannose-binding lectin (MBL) and the lectin complement pathway play a role in cutaneous ischemia and reperfusion injury

Objectives

Cutaneous ischemia/reperfusion (CI/R) injury has shown to play a significant role in chronic wounds such as decubitus ulcers, diabetic foot ulcers, atherosclerotic lesions, and venous stasis wounds. CI/R also plays a role in free tissue transfer in reconstructive microsurgery and has been linked to clinical burn-depth progression after thermal injury. While the role of the complement system has been elucidated in multiple organ systems, evidence is lacking with respect to its role in the skin. Therefore, we evaluated the role of the complement system in CI/R injury.

Methods

Using a single pedicle skin flap mouse model of acute CI/R, we performed CI/R in wild-type (WT) mice and complement knock out (KO) mice, deficient in either C1q (C1q KO; classical pathway inhibition), mannose-binding lectin (MBL null; lectin pathway inhibition) or factor B (H2Bf KO; alternative pathway inhibition). Following 10 h ischemia and 7 days reperfusion, mice were sacrificed, flaps harvested and flap viability assessed via Image J software. The flap necrotic area was expressed as % total flap area. In another group, mice were sacrificed following CI/R with 10 h ischemia and 48 h reperfusion. Two cranial skin flap samples were taken for gene expression analysis of IL1b, IL6, TNFα, ICAM1, VCAM1, IL10, IL13 using real-time polymerase chain reaction (RT-PCR).

Results

Following CI/R, MBL null mice had a statistically significant smaller %necrotic flap area compared to WT mice (10.6 vs. 43.1%; p<0.05) suggesting protection from CI/R. A significantly reduced mean %necrotic flap area was not seen in either C1q KO or H2Bf KO mice relative to WT (22.9 and 31.3 vs. 43.1%; p=0.08 and p=0.244, respectively). There were no statistically significant differences between groups for markers of inflammation (TNFα, ICAM1, VCAM1, IL1b, IL6). In contrast, mRNA levels of IL10, a regulator of inflammation, were significantly increased in the MBL null group (p=0.047).

Conclusions

We demonstrated for the first time a significant role of MBL and the lectin complement pathway in ischemia/reperfusion injury of the skin and a potential role for IL10 in attenuating CI/R injury, as IL10 levels were significantly increased in the tissue from the CI/R-protected MBL null group.

Post-rituximab immunoglobulin M (IgM) hypogammaglobulinemia

Rituximab is a B cell depleting monoclonal antibody that targets the B cell-specific cell surface antigen CD20 and is currently used to treat several autoimmune diseases. The elimination of mature CD20-positive B lymphocytes committed to differentiate into autoantibody-producing plasma cells is considered to be the major effect of rituximab, that makes it a beneficial biological agent in treating autoimmune diseases. Hypogammaglobulinemia has been reported after rituximab therapy in patients with lymphoma and rheumatoid arthritis. Similar data are scarce for other autoimmune diseases. Low immunoglobulin G (IgG) or hypogammaglobulinemia has attracted the most attention because of its significant role in protective immunity. However, the incidence and clinical implications of low immunoglobulin M (IgM) or hypogammaglobulinemia have not been studied in detail. This review will focus on the frequency and the clinical concerns of low IgM levels that result as a consequence of the administration of rituximab. The etiopathogenic mechanisms underlying post-rituximab IgM hypogammaglobulinemia and its implications are presented. The long-term consequences, if any, are not known or documented. Multiple factors may be involved in whether IgG or IgM decreases secondary to rituximab therapy. It is possible that the autoimmune disease itself may be one of the important factors. The dose, frequency and number of infusions appear to be important variables. Post-rituximab therapy immunoglobulin levels return to normal. During this process. IgM levels take a longer time to return to normal levels when compared to IgG or other immunoglobulins. IgM deficiency persists after B cell repopulation to normal levels has occurred. Laboratory animals and humans deficient in IgM can have multiple infections. Specific pharmacologic agents or biologic therapy that address and resolve IgM deficiency are currently unavailable. If the clinical situation so warrants, then prophylactic antibiotics may be indicated and perhaps helpful. Research in this iatrogenic phenomenon will provide a better understanding of not only the biology of IgM, but also the factor(s) that control its production and regulation, besides its influence if any, on rituximab therapy.

Secreted IgM: New tricks for an old molecule

Secreted IgM (sIgM) is a multifunctional evolutionary conserved antibody that is critical for the maintenance of tissue homeostasis as well as the development of fully protective humoral responses to pathogens. Constitutive secretion of self- and polyreactive natural IgM, produced mainly by B-1 cells, provides a circulating antibody that engages with autoantigens as well as invading pathogens, removing apoptotic and other cell debris and initiating strong immune responses. Pathogen-induced IgM production by B-1 and conventional B-2 cells strengthens this early, passive layer of IgM-mediated immune defense and regulates subsequent IgG production. The varied effects of secreted IgM on immune homeostasis and immune defense are facilitated through its binding to numerous different cell types via different receptors. Recent studies identified a novel function for pentameric IgM, namely as a transporter for the effector protein ″apoptosis-inhibitor of macrophages″ (AIM/CD5L). This review aims to provide a summary of the known functions and effects of sIgM on immune homeostasis and immune defense, and its interaction with its various receptors, and to highlight the many critical immune regulatory functions of this ancient and fascinating immunoglobulin.

The five dimensions of B cell tolerance

B cell tolerance has been generally understood to be an acquired property of the immune system that governs antibody specificity in ways that avoid auto-toxicity. As useful as this understanding has proved, it fails to fully explain the existence of auto-reactive specificities in healthy individuals and contribution these may have to health. Mechanisms underlying B cell tolerance are considered to select a clonal repertoire that generates a collection of antibodies that do not bind self, ie tolerance operates more or less in three dimensions that largely spare autologous cells and antigens. Yet, most B lymphocytes in humans and probably in other vertebrates are auto-reactive and absence of these auto-reactive B cells is associated with disease. We suggest that auto-reactivity can be embodied by extending the concept of tolerance by two further dimensions, one of time and circumstance and one that allows healthy cells to actively resist injury. In this novel concept, macromolecular recognition by the B cell receptor leading to deletion, anergy, receptor editing or B cell activation is extended by taking account of the time of development of normal immune responses (4th dimension) and the accommodation (or tolerance) of normal cells to bound antibody, activation of complement, and interaction with inflammatory cells (fifth dimension). We discuss how these dimensions contribute to understanding B cell biology in health or disease.

Targeted complement inhibition salvages stressed neurons and inhibits neuroinflammation after stroke in mice

Ischemic stroke results from the interruption of blood flow to the brain resulting in long-term motor and cognitive neurological deficits, and it is a leading cause of death and disability. Current interventions focus on the restoration of blood flow to limit neuronal death, but these treatments have a therapeutic window of only a few hours and do not address post-stroke cerebral inflammation. The complement system, a component of the innate immune system, is activated by natural immunoglobulin M (IgM) antibodies that recognize neoepitopes expressed in the brain after ischemic stroke. We took advantage of this recognition system to inhibit complement activation locally in the ischemic area in mice. A single chain antibody recognizing a post-ischemic neoepitope linked to a complement inhibitor (termed B4Crry) was administered systemically as a single dose after stroke and shown to specifically target the ischemic hemisphere and improve long-term motor and cognitive recovery. We show that complement opsonins guide microglial phagocytosis of stressed but salvageable neurons, and that by locally and transiently inhibiting complement deposition, B4Crry prevented phagocytosis of penumbral neurons and inhibited pathologic complement and microglial activation that otherwise persisted for several weeks after stroke. B4Crry was protective in adult, aged, male and female mice and had a therapeutic window of at least 24 hours after stroke. Furthermore, the epitope recognized by B4Crry in mice is overexpressed in the ischemic penumbra of acute stroke patients, but not in the contralateral tissue, highlighting the translational potential of this approach.

Early complementopathy predicts the outcomes of patients with trauma

Background

Complementopathy (rapid complement activation and consumption after trauma) has been reported in trauma patients, but the underlying mechanism of these phenomena and their clinical significance remain unclear. This study aimed to determine the complement/complement pathway activation and identify the association of complement activation with clinical outcomes in trauma patients.

Methods

We studied 33 trauma patients with mean Injury Severity Score of 25, and 25 healthy volunteers. Sera were collected on patients’ arrival at the emergency department, as well as 1, 2, 3, 5, and 7 days after trauma, to measure the levels of terminal complement activation product soluble C5b-9 (sC5b-9) by ELISA. In addition, the functional complement activation pathway was evaluated using a commercial complement system screening kit.

Results

Serum concentrations of sC5b-9 (complement terminal pathway activity) were significantly increased in trauma patients throughout the entire observation period except on day 1. Complement terminal activities were significantly higher in 27 of 33 patients with systemic inflammatory response syndrome (SIRS) than non-SIRS patients on day 2, day 5, and day 7. Increased serum levels of sC5b-9 positively correlated with SIRS. Functional complement analysis revealed that the classical pathway was the predominant pathway responsible for complement activation. Burn patients tended to have a greater and prolonged classical pathway activation than non-burn patients, and burn injury and blunt injury were associated with higher blood levels of sC5b-9 than penetrating injury.

Discussion

Early complement activation through the classical pathway after trauma is observed and positively correlated with the development of SIRS. Thus, monitoring of the complement system might be beneficial in the care of critically injured patients.

Level of evidence

III.

Study type

Prognostic.

Adaptive Immune Responses Contribute to Post-ischemic Cardiac Remodeling

Myocardial infarction (MI) is a common condition responsible for mortality and morbidity related to ischemic heart failure. Accumulating experimental and translational evidence support a crucial role for innate immunity in heart failure and adverse heart remodeling following MI. More recently, the role of adaptive immunity in myocardial ischemia has been identified, mainly in rodents models of both transient and permanent heart ischemia. The present review summarizes the experimental evidence regarding the role of lymphocytes and dendritic cells in myocardial remodeling following coronary artery occlusion. Th1 and potentially Th17 CD4⁺ T cell responses promote adverse heart remodeling, whereas regulatory T cells appear to be protective, modulating macrophage activity, cardiomyocyte survival, and fibroblast phenotype. The role of CD8⁺ T cells in this setting remains unknown. B cells contribute to adverse cardiac remodeling through the modulation of monocyte trafficking, and potentially the production of tissue-specific antibodies. Yet, further substantial efforts are still required to confirm experimental data in human MI before developing new therapeutic strategies targeting the adaptive immune system in ischemic cardiac diseases.

Development and evaluation of the first immunochromatographic test that can detect specific antibodies against Cryptosporidium parvum

Cryptosporidium parvum is a major cause of diarrhea among human and calves, resulting in severe health hazards and drastic economic losses, respectively. Although C. parvum infection leads to high morbidity and mortality in immunocompromised patients and bovine calves, this infection remains a neglected disease. Currently available diagnostic tests for C. parvum are primarily based on detection of oocysts, DNA, or secreted antigens in fecal specimens. Demonstration of specific antibodies with a rapid immunochromatographic test (ICT) will be advantageous not only in providing a simple, rapid, accurate, and affordable tool but also in surveillance because of the ability to recognize recent and past infections. Herein, we developed two ICTs using the diagnostic antigen CpP23 and immunodominant antigen CpGP15 to detect C. parvum-specific antibodies in cattle sera. Because of unavailability of a reference test for antibody detection, evaluation and validation of our developed ICTs were conducted using reference cattle samples and unknown field cattle sera. Serum samples were simultaneously tested by a previously validated enzyme-linked immunosorbent assay (ELISA) using the same antigens (CpGP15 and CpP23). ICTs showed substantial ability to discriminate between positive and negative control cattle sera for both CpGP15 and CpP23. Even against field sera, high sensitivity, specificity, and agreement rates were recorded for ICTs compared with the previously validated ELISA with the same antigens (CpGP15 = 78.78%, 100%, and 85.11%; CpP23 = 80%, 100%, and 80.56%, respectively). Moreover, a high correlation was observed between the test band intensity of ICTs and optical density of ELISA, particularly in the case of CpP23-specific IgM. To our knowledge, this study represents the first development of ICTs that can detect C. parvum-specific antibodies. Our tests will contribute greatly to C. parvum infection control in cattle by providing a method for on-site diagnosis of early and latent infections.

Antibodies aggravate the development of ischemic heart failure

Heart-specific antibodies have been widely associated with myocardial infarction (MI). However, it remains unclear whether autoantibodies mediate disease progression or are a byproduct of cardiac injury. To disambiguate the role of immunoglobulins in MI, we characterized the development of ischemic heart failure in agammaglobulinemic mice (AID^-/-μS^-/-). Although these animals can produce functional B cells, they cannot synthesize secretory IgM (μS^-/-) or perform Ig class switching (AID^-/-), leading to complete antibody deficiency. Agammaglobulinemia did not affect overall post-MI survival but resulted in a significant reduction in infarct size. Echocardiographic analyses showed that, compared with wild-type infarcted control mice, AID^-/-μS^-/- mice exhibited improved cardiac function and reduced remodeling on day 56 post-MI. These differences remained significant even after animals with matched infarct sizes were compared. Infarcted AID^-/-μS^-/- mice also showed reduced myocardial expression levels of transcripts known to promote adverse remodeling, such as matrix metalloproteinase-9, collagen type I a1, collagen type III a1, and IL-6. An unbiased screening of the heart reactivity potential in the plasma of wild-type MI animals revealed the presence of antibodies that target the myocardial scar and collagenase-sensitive epitopes. Moreover, we found that IgG accumulated within the scar tissues of infarcted mice and remained in close proximity with cells expressing Fcγ receptors (CD16/32), suggesting the existence of an in situ IgG-Fcγ receptor axis. Collectively, our study results confirm that antibodies contribute to ischemic heart failure progression and provide novel insights into the mechanisms underlying this phenomenon. NEW & NOTEWORTHY Our study sheds some light on the long-standing debate over the relevance of autoantibodies in heart failure and might stimulate future research in the field. The observation of extracellular matrix-specific antibodies and the detection of Fcγ receptor-expressing cells within the scar provide novel insights into the mechanisms by which antibodies may contribute to adverse remodeling.

Anti-HIV IgM protects against mucosal SHIV transmission

OBJECTIVE

Worldwide, most new HIV infections occur through mucosal exposure. Immunoglobulin M (IgM) is the first antibody class generated in response to infectious agents; IgM is present in the systemic circulation and in mucosal fluids as secretory IgM. We sought to investigate for the first time the role of IgM in preventing AIDS virus acquisition in vivo.

DESIGN

Recombinant polymeric monoclonal IgM was generated from the neutralizing monoclonal IgG1 antibody 33C6-IgG1, tested in vitro, and given by passive intrarectal immunization to rhesus macaques 30 min before intrarectal challenge with simian-human immunodeficiency virus (SHIV) that carries an HIV-1 envelope gene.

RESULTS

In vitro, 33C6-IgM captured virions more efficiently and neutralized the challenge SHIV with a 50% inhibitory molar concentration (IC50) that was 1 log lower than that for 33C6-IgG1. The IgM form also exhibited significantly higher affinity and avidity compared with 33C6-IgG1. After intrarectal administration, 33C6-IgM prevented viremia in four out of six rhesus macaques after high-dose intrarectal SHIV challenge. Five out of six rhesus macaques given 33C6-IgG1 were protected at a five times higher molar concentration compared with the IgM form; all untreated controls became highly viremic. Rhesus macaques passively immunized with 33C6-IgM with breakthrough infection had notably early development of autologous neutralizing antibody responses.

CONCLUSION

Our primate model data provide the first proof-of-concept that mucosal IgM can prevent mucosal HIV transmission and have implications for HIV prevention and vaccine development.

Complement Activation in Liver Transplantation: Role of Donor Macrosteatosis and Implications in Delayed Graft Function

The complement system anchors the innate inflammatory response by triggering both cell-mediated and antibody-mediated immune responses against pathogens. The complement system also plays a critical role in sterile tissue injury by responding to damage-associated molecular patterns. The degree and duration of complement activation may be a critical variable controlling the balance between regenerative and destructive inflammation following sterile injury. Recent studies in kidney transplantation suggest that aberrant complement activation may play a significant role in delayed graft function following transplantation, confirming results obtained from rodent models of renal ischemia/reperfusion (I/R) injury. Deactivating the complement cascade through targeting anaphylatoxins (C3a/C5a) might be an effective clinical strategy to dampen reperfusion injury and reduce delayed graft function in liver transplantation. Targeting the complement cascade may be critical in donor livers with mild to moderate steatosis, where elevated lipid burden amplifies stress responses and increases hepatocyte turnover. Steatosis-driven complement activation in the donor liver may also have implications in rejection and thrombolytic complications following transplantation. This review focuses on the roles of complement activation in liver I/R injury, strategies to target complement activation in liver I/R, and potential opportunities to translate these strategies to transplanting donor livers with mild to moderate steatosis.

Endothelial Dysfunction in Kidney Transplantation

Kidney transplantation entails a high likelihood of endothelial injury. The endothelium is a target of choice for injury by ischemia-reperfusion, alloantibodies, and autoantibodies. A certain degree of ischemia-reperfusion injury inevitably occurs in the immediate posttransplant setting and can manifest as delayed graft function. Acute rejection episodes, whether T-cell or antibody-mediated, can involve the graft micro- and macrovasculature, leading to endothelial injury and adverse long-term consequences on graft function and survival. In turn, caspase-3 activation in injured and dying endothelial cells favors the release of extracellular vesicles (apoptotic bodies and apoptotic exosome-like vesicles) that further enhance autoantibody production, complement deposition, and microvascular rarefaction. In this review, we present the evidence for endothelial injury, its causes and long-term consequences on graft outcomes in the field of kidney transplantation.

Blockade of IgM-Mediated Inflammation Alters Wound Progression in a Swine Model of Partial-Thickness Burn

In a mouse model, a second-degree burn elicits a severe inflammatory response that is mediated by circulating autoantibody specific for a neoantigen (nonmuscle myosin). Nonmuscle myosin is expressed by injured tissue, leading to amplified ulceration and scarring. We hypothesize that a synthetic peptide (N2) can mimic the neoantigen and competitively inhibit the autoantibody, decreasing inflammation, and reducing the extent of burn injury in a preclinical swine model of burn. Second-degree burns were created on young swine using brass cylinders, warmed to varying temperatures before skin contact. Animals were treated in double-blind fashion with normal saline, control peptide, or blocking peptide. Biopsies were taken at 2 hours, 1, 4, 7, and 14 days after burn injury. Burn wound healing parameters were assessed. Immunohistochemical staining for Ki-67, immunoglobulin (Ig)M, and interleukin (IL)-8 were also performed. N2 blocking peptide administration decreased dermal injury at 4 days with increased reepithelization, indicating more rapid healing. N2 normalized skin histology by 14 days and showed improved epidermal healing. Granulation tissue thickness was decreased, and there was an accompanying decrease in neutrophil infiltration. The basal layer of epidermis in N2-treated animals displayed more cells positive for Ki-67, suggesting a prompter regenerative capacity. Immunohistochemical staining demonstrated decreased deposition of immunoglobulin M and interleukin-8 after thermal injury in animals treated with N2 peptide, in comparison to controls. The findings of this study identify N2 blocking a specific inflammatory pathway, as a novel therapeutic approach, preventing the evolution of cutaneous burn injuries in a preclinical animal model.

Natural immunoglobulin M initiates an inflammatory response important for both hepatic ischemia reperfusion injury and regeneration in mice

Complement plays a role in both hepatic ischemia reperfusion (IR) injury (IRI) and liver regeneration, but it is not clear how complement is activated in either process. We investigated the role of self-reactive immunoglobulin M (IgM) antibodies in activating complement after hepatic IR and liver resection. Natural IgM antibodies that recognize danger-associated molecular patterns (neoepitopes) activate complement following both hepatic IR and liver resection. Antibody-deficient Rag1^-/- mice were protected from hepatic IRI, but had increased hepatic injury and an impaired regenerative response after 70% partial hepatectomy (PHx). We identified two IgM monoclonal antibodies (mAbs) that specifically reversed the effect of Rag1 deficiency in both models; B4 (recognizes Annexin IV) and C2 (recognizes subset of phospholipids). Focusing on the B4-specific response, we demonstrated sinusoidal colocalization of IgM and C3d in Rag1^-/- mice that were reconstituted with B4 mAb, and furthermore that the Annexin IV neoepitope is specifically and similarly expressed after both hepatic IR and PHx in wild-type (WT) mice. A single-chain antibody construct (scFv) derived from B4 mAb blocked IgM binding and reduced injury post-IR in WT mice, although, interestingly, B4scFv did not alter regeneration post-PHx, indicating that anti-Annexin IV antibodies are sufficient, but not necessary, for the regenerative response in the context of an entire natural antibody repertoire. We also demonstrated expression of the B4 neoepitope in postischemic human liver samples obtained posttransplantation and a corollary depletion in IgM recognizing the B4 and C2 neoepitopes in patient sera following liver transplantation. Conclusion: These data indicate an important role for IgM in hepatic IRI and regeneration, with a similar cross-species injury-specific recognition system that has implications for the design of neoepitope targeted therapeutics. (Hepatology 2018;67:721-735).

Tissue conservation for transplantation

Pathophysiological changes that occur during ischemia and subsequent reperfusion cause damage to tissues procured for transplantation and also affect long-term allograft function and survival. The proper preservation of organs before transplantation is a must to limit these injuries as much as possible. For decades, static cold storage has been the gold standard for organ preservation, with mechanical perfusion developing as a promising alternative only recently. The current literature points to the need of developing dedicated preservation protocols for every organ, which in combination with other interventions such as ischemic preconditioning and therapeutic additives offer the possibility of improving organ preservation and extending it to multiple times its current duration. This review strives to present an overview of the current body of knowledge with regard to the preservation of organs and tissues destined for transplantation.

IL-17A Produced by Innate Lymphoid Cells Is Essential for Intestinal Ischemia-Reperfusion Injury

Ischemia-reperfusion (IR) injury to the small intestine following clamping of the superior mesenteric artery results in an intense local inflammatory response that is characterized by villous damage and neutrophil infiltration. IL-17A, a cytokine produced by a variety of cells in response to inflammatory cytokines released following tissue injury, has been implicated in IR injury. Using Il17a^-/- , Il23r^-/- , and Rorc^-/- mice and administration of anti-IL-17A and anti-IL-23 neutralizing Abs to wild-type mice, we demonstrate that intestinal IR injury depends on IL-17A and that IL-17A is downstream of the binding of autoantibody to ischemia-conditioned tissues and subsequent complement activation. Using bone marrow chimeras, we demonstrate that the IL-17A required for intestinal IR injury is derived from hematopoietic cells. Finally, by transferring autoantibody-rich sera into Rag2γc^-/- and Rag2^-/- mice, we demonstrate that innate lymphoid cells are the main producers of IL-17A in intestinal IR injury. We propose that local production of IL-17A by innate lymphoid cells is crucial for the development of intestinal IR injury and may provide a therapeutic target for clinical exploitation.

A Novel Mechanism for Generating the Interferon Signature in Lupus: Opsonization of Dead Cells by Complement and IgM

OBJECTIVE

In vitro studies suggest that the type I interferon (IFN) signature seen in most lupus patients results from Fcγ receptor-mediated uptake of nucleic acid-containing immune complexes by plasmacytoid dendritic cells and engagement of endosomal Toll-like receptors. The aim of this study was to reexamine the pathogenesis of the IFN signature in vivo.

METHODS

Lupus was induced in mice by injecting pristane. Some mice were treated with normal immunoglobulin or with cobra venom factor to deplete complement. The IFN signature was evaluated by polymerase chain reaction. The IFN signature also was determined in C4-deficient patients and control subjects.

RESULTS

Wild-type C57BL/6 mice with pristane-induced lupus developed a strong IFN signature, which was absent in immunoglobulin-deficient (μMT), C3^-/- , and CD18^-/- mice. Intravenous infusion of normal IgM, but not IgG, restored the IFN signature in μMT mice, and the IFN signature in wild-type mice was inhibited by depleting complement, suggesting that opsonization by IgM and complement is involved in IFN production. Consistent with that possibility, the levels of "natural" IgM antibodies reactive with dead cells were increased in pristane-treated wild-type mice compared with untreated controls, and in vivo phagocytosis of dead cells was impaired in C3-deficient mice. To examine the clinical relevance of these findings, we identified 10 C4-deficient patients with lupus-like disease and compared them with 152 C4-intact patients and 21 healthy controls. In comparison with C4-intact patients, C4-deficient patients had a different clinical/serologic phenotype and lacked the IFN signature.

CONCLUSION

These studies define previously unrecognized roles of natural IgM, complement, and complement receptors in generating the IFN signature in lupus.

Complement Recognition Pathways in Renal Transplantation

The complement system, consisting of soluble and cell membrane-bound components of the innate immune system, has defined roles in the pathophysiology of renal allograft rejection. Notably, the unavoidable ischemia-reperfusion injury inherent to transplantation is mediated through the terminal complement activation products C5a and C5b-9. Furthermore, biologically active fragments C3a and C5a, produced during complement activation, can modulate both antigen presentation and T cell priming, ultimately leading to allograft rejection. Earlier work identified renal tubule cell synthesis of C3, rather than hepatic synthesis of C3, as the primary source of C3 driving these effects. Recent efforts have focused on identifying the local triggers of complement activation. Collectin-11, a soluble C-type lectin expressed in renal tissue, has been implicated as an important trigger of complement activation in renal tissue. In particular, collectin-11 has been shown to engage L-fucose at sites of ischemic stress, activating the lectin complement pathway and directing the innate immune response to the distressed renal tubule. The interface between collectin-11 and L-fucose, in both the recipient and the allograft, is an attractive target for therapies intended to curtail renal inflammation in the acute phase.

Natural IgM antibodies that bind neoepitopes exposed as a result of spinal cord injury , drive secondary injury by activating complement

Background

Natural IgM antibodies (Abs) function as innate immune sensors of injury via recognition of neoepitopes expressed on damaged cells, although how this recognition systems function following spinal cord injury (SCI) exposes various neoepitopes and their precise nature remains largely unknown. Here, we investigated the role of two natural IgM monoclonal Abs (mAbs), B4 and C2, that recognize post-ischemic neoepitopes following ischemia and reperfusion in other tissues.

Methods

Identification of post-SCI expressed neoepitopes was examined using previously characterized monoclonal Abs (B4 and C2 mAbs). The role of post-SCI neoepitopes and their recognition by natural IgM Abs in propagating secondary injury was examined in Ab-deficient Rag1−/− or wild type C57BL/6 mice using Ab reconstitution experiments and neoepitope-targeted therapeutic studies, respectively.

Results

Administration of B4 or C2 mAb following murine SCI increased lesion size and worsened functional outcome in otherwise protected Ab-deficient Rag1−/− mice. Injury correlated with colocalized deposition of IgM and C3d in injured spinal cords from both mAb reconstituted Rag1−/− mice and untreated wild-type mice. Depletion of peritoneal B1 B cells, a source of natural Abs, reduced circulating levels of IgM with B4 (annexin-IV) and C2 (subset of phospholipids) reactivity, reduced IgM and complement deposition in the spinal cord, and protected against SCI. We therefore investigated whether the B4 neoepitope represents a therapeutic target for complement inhibition. B4-Crry, a fusion protein consisting of a single-chain Ab derived from B4 mAb, linked to the complement inhibitor Crry, significantly protected against SCI. B4-Crry exhibited a dual function in that it inhibited both the binding of pathogenic IgM and blocked complement activation in the spinal cord.

Conclusions

This study identifies important neoepitopes expressed within the spinal cord after injury. These neoepitopes are recognized by clonally specific natural IgM Abs that activate complement and drive pathology. We demonstrate that these neoepitopes represent novel targets for the therapeutic delivery of a complement inhibitor, and possibly other payload, to the injured spinal cord.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-017-0894-6) contains supplementary material, which is available to authorized users.

Complement in Kidney Transplantation

The complement system is considered to be an important part of innate immune system with a significant role in inflammation processes. The activation can occur through classical, alternative, or lectin pathway, resulting in the creation of anaphylatoxins C3a and C5a, possessing a vast spectrum of immune functions, and the assembly of terminal complement cascade, capable of direct cell lysis. The activation processes are tightly regulated; inappropriate activation of the complement cascade plays a significant role in many renal diseases including organ transplantation. Moreover, complement cascade is activated during ischemia/reperfusion injury processes and influences delayed graft function of kidney allografts. Interestingly, complement system has been found to play a role in both acute cellular and antibody-mediated rejections and thrombotic microangiopathy. Therefore, complement system may represent an interesting therapeutical target in kidney transplant pathologies.

Pathogenesis of Diffuse Alveolar Hemorrhage in Murine Lupus

OBJECTIVE

Diffuse alveolar hemorrhage (DAH) in lupus patients confers >50% mortality, and the cause is unknown. We undertook this study to examine the pathogenesis of DAH in C57BL/6 mice with pristane-induced lupus, a model of human lupus-associated DAH.

METHODS

Clinical/pathologic and immunologic manifestations of DAH in pristane-induced lupus were compared with those of DAH in humans. Tissue distribution of pristane was examined by mass spectrometry. Cell types responsible for disease were determined by in vivo depletion using clodronate liposomes and antineutrophil monoclonal antibodies (anti-Ly-6G). The effect of complement depletion with cobra venom factor (CVF) was examined.

RESULTS

After intraperitoneal injection, pristane migrated to the lung, causing cell death, small vessel vasculitis, and alveolar hemorrhage similar to that seen in DAH in humans. B cell-deficient mice were resistant to induction of DAH, but susceptibility was restored by infusing IgM. C3^-/- and CD18^-/- mice were also resistant, and DAH was prevented in wild-type mice by CVF. Induction of DAH was independent of Toll-like receptors, inflammasomes, and inducible nitric oxide. Mortality was increased in interleukin-10 (IL-10)-deficient mice, and pristane treatment decreased IL-10 receptor expression in monocytes and STAT-3 phosphorylation in lung macrophages. In vivo neutrophil depletion was not protective, while treatment with clodronate liposomes prevented DAH, which suggests that macrophage activation is central to DAH pathogenesis.

CONCLUSION

The pathogenesis of DAH involves opsonization of dead cells by natural IgM and complement followed by complement receptor-mediated lung inflammation. The disease is macrophage dependent, and IL-10 is protective. Complement inhibition and/or macrophage-targeted therapies may reduce mortality in lupus-associated DAH.

Microbiota-Induced Antibodies Are Essential for Host Inflammatory Responsiveness to Sterile and Infectious Stimuli

The indigenous intestinal microbiota is frequently considered an additional major organ of the human body and exerts profound immunomodulating activities. Germ-free (GF) mice display a significantly different inflammatory responsiveness pattern compared with conventional (CV) mice, and this was dubbed a "hyporesponsive phenotype." Taking into account that the deposition of immune complexes is a major event in acute inflammation and that GF mice have a distinct Ig repertoire and B cell activity, we aimed to evaluate whether this altered Ig repertoire interferes with the inflammatory responsiveness of GF mice. We found that serum transfer from CV naive mice was capable of reversing the inflammatory hyporesponsiveness of GF mice in sterile inflammatory injury induced by intestinal ischemia and reperfusion, as well as in a model of lung infection by Klebsiella pneumoniae Transferring serum from Ig-deficient mice to GF animals did not alter their response to inflammatory insult; however, injecting purified Abs from CV animals restored inflammatory responsiveness in GF mice, suggesting that natural Abs present in serum were responsible for altering GF responsiveness. Mechanistically, injection of serum and Ig from CV mice into GF animals restored IgG deposition, leukocyte influx, NF-κB activation, and proinflammatory gene expression in inflamed tissues and concomitantly downregulated annexin-1 and IL-10 production. Thus, our data show that microbiota-induced natural Abs are pivotal for host inflammatory responsiveness to sterile and infectious insults.

Therapeutic hypothermia modulates complement factor C3a and C5a levels in a rat model of hypoxic ischemic encephalopathy

BACKGROUND

Therapeutic hypothermia (HT) is the only intervention that improves outcomes in neonatal hypoxic-ischemic encephalopathy (HIE). However, the multifactorial mechanisms by which HT impacts HIE are incompletely understood. The complement system plays a major role in the pathogenesis of ischemia-reperfusion injuries such as HIE. We have previously demonstrated that HT modulates complement activity in vitro.

METHODS

Term equivalent rat pups were subjected to unilateral carotid ligation followed by hypoxia (8% O2) for 45 min to simulate HIE. A subset of animals was subjected to HT (31-32°C for 6 h). Plasma and brain levels of C3a and C5a were measured. Receptors for C3a (C3aR) and C5a (C5aR) along with C1q, C3, and C9 were characterized in neurons, astrocytes, and microglia.

RESULTS

We found that HT increased systemic expression of C3a and decreased expression of C5a after HIE. In the brain, C3aR and C5aR are predominantly expressed on microglia after HIE. HT increased local expression of C3aR and decreased expression on C5aR after HIE. Furthermore, HT decreased local expression of C1q, C3-products, and C9 in the brain.

CONCLUSION

HT is associated with significant alteration of complement effectors and their cognate receptors. Complement modulation may improve outcomes in neonatal HIE.

Resolvin D1 Alleviates the Lung Ischemia Reperfusion Injury via Complement, Immunoglobulin, TLR4, and Inflammatory Factors in Rats

Lung ischemia-reperfusion injury (LIRI) is still an unsolved medical issue, which negatively affects the prognosis of many lung diseases. The aim of this study is to determine the effects of RvD1 on LIRI and the potential mechanisms involved. The results revealed that the levels of complement, immunoglobulin, cytokines, sICAM-1, MPO, MDA, CINC-1, MCP-1, ANXA-1, TLR4, NF-κBp65, apoptosis index, and pulmonary permeability index were increased, whereas the levels of SOD, GSH-PX activity, and oxygenation index were decreased in rats with LIRI. Except for ANXA-1, these responses induced by LIRI were significantly inhibited by RvD1 treatment. In addition, LIRI-induced structure damages of lung tissues were also alleviated by RvD1 as shown by H&E staining and transmission electron microscopy. The results suggest that RvD1 may play an important role in protection of LIRI via inhibition of complement, immunoglobulin, and neutrophil activation; down-regulation of TLR4/NF-κB; and the expression of a variety of inflammatory factors.

Role of Natural IgM Autoantibodies (IgM-NAA) and IgM Anti-Leukocyte Antibodies (IgM-ALA) in Regulating Inflammation

Natural IgM autoantibodies (IgM-NAA) are rapidly produced to inhibit pathogens and abrogate inflammation mediated by invading microorganisms and host neoantigens. IgM-NAA achieve this difficult task by being polyreactive with low binding affinity but with high avidity, characteristics that allow these antibodies to bind antigenic determinants shared by pathogens and neoantigens. Hence the same clones of natural IgM can bind and mask host neoantigens as well as inhibit microorganisms. In addition, IgM-NAA regulate the inflammatory response via mechanisms involving binding of IgM to apoptotic cells to enhance their removal and binding of IgM to live leukocytes to regulate their function. Secondly, we review how natural IgM prevents autoimmune disorders arising from pathogenic IgG autoantibodies as well as by autoreactive B and T cells that have escaped tolerance mechanisms. Thirdly, using IgM knockout mice, we show that regulatory B and T cells require IgM to effectively regulate inflammation mediated by innate, adaptive and autoimmune mechanisms. It is therefore not surprising why the host positively selects such autoreactive B1 cells that generate protective IgM-NAA, which are also evolutionarily conserved. Fourthly, we show that IgM anti-leukocyte autoantibodies (IgM-ALA) levels and their repertoire can vary in normal humans and disease states and this variation may partly explain the observed differences in the inflammatory response after infection, ischemic injury or after a transplant. Finally we also show how protective IgM-NAA can be rendered pathogenic under non-physiological conditions. IgM-NAA have therapeutic potential. Polyclonal IgM infusions can be used to abrogate ongoing inflammation. Additionally, inflammation arising after ischemic kidney injury, e.g., during high-risk elective cardiac surgery or after allograft transplantation, can be prevented by pre-emptively infusing polyclonal IgM, or DC pretreated ex vivo with IgM, or by increasing in vivo IgM with a vaccine approach. Cell therapy with IgM pretreated cells, is appealing as less IgM will be required.

Ischemia/Reperfusion

Ischemic disorders, such as myocardial infarction, stroke, and peripheral vascular disease, are the most common causes of debilitating disease and death in westernized cultures. The extent of tissue injury relates directly to the extent of blood flow reduction and to the length of the ischemic period, which influence the levels to which cellular ATP and intracellular pH are reduced. By impairing ATPase-dependent ion transport, ischemia causes intracellular and mitochondrial calcium levels to increase (calcium overload). Cell volume regulatory mechanisms are also disrupted by the lack of ATP, which can induce lysis of organelle and plasma membranes. Reperfusion, although required to salvage oxygen-starved tissues, produces paradoxical tissue responses that fuel the production of reactive oxygen species (oxygen paradox), sequestration of proinflammatory immunocytes in ischemic tissues, endoplasmic reticulum stress, and development of postischemic capillary no-reflow, which amplify tissue injury. These pathologic events culminate in opening of mitochondrial permeability transition pores as a common end-effector of ischemia/reperfusion (I/R)-induced cell lysis and death. Emerging concepts include the influence of the intestinal microbiome, fetal programming, epigenetic changes, and microparticles in the pathogenesis of I/R. The overall goal of this review is to describe these and other mechanisms that contribute to I/R injury. Because so many different deleterious events participate in I/R, it is clear that therapeutic approaches will be effective only when multiple pathologic processes are targeted. In addition, the translational significance of I/R research will be enhanced by much wider use of animal models that incorporate the complicating effects of risk factors for cardiovascular disease. © 2017 American Physiological Society. Compr Physiol 7:113-170, 2017.

CD6 Receptor Regulates Intestinal Ischemia/Reperfusion-induced Injury by Modulating Natural IgM-producing B1a Cell Self-renewal

Intestinal ischemia/reperfusion (I/R) injury is a relatively common pathological condition that can lead to multi-organ failure and mortality. Regulatory mechanism for this disease is poorly understood, although it is established that circulating pathogenic natural IgM, which is primarily produced by B1a cells outside of the peritoneal cavity, are integrally involved. CD6 was originally identified as a marker for T cells and was later found to be present on some subsets of B cells in humans; however, whether CD6 plays any role in intestinal I/R-induced injury and, if so, the underlying mechanisms, remain unknown. Here we report that CD6^-/- mice were significantly protected from intestinal inflammation and mucosal damage compared with WT mice in a model of intestinal I/R-induced injury. Mechanistically, we found that CD6 was selectively expressed on B1 cells outside of the bone marrow and peritoneal cavity and that pathogenic natural IgM titers were reduced in the CD6^-/- mice in association with significantly decreased B1a cell population. Our results reveal an unexpected role of CD6 in the pathogenesis of intestinal IR-induced injury by regulating the self-renewal of B1a cells.

Anti-LG3 Antibodies Aggravate Renal Ischemia-Reperfusion Injury and Long-Term Renal Allograft Dysfunction

Pretransplant autoantibodies to LG3 and angiotensin II type 1 receptors (AT1R) are associated with acute rejection in kidney transplant recipients, whereas antivimentin autoantibodies participate in heart transplant rejection. Ischemia-reperfusion injury (IRI) can modify self-antigenic targets. We hypothesized that ischemia-reperfusion creates permissive conditions for autoantibodies to interact with their antigenic targets and leads to enhanced renal damage and dysfunction. In 172 kidney transplant recipients, we found that pretransplant anti-LG3 antibodies were associated with an increased risk of delayed graft function (DGF). Pretransplant anti-LG3 antibodies are inversely associated with graft function at 1 year after transplantation in patients who experienced DGF, independent of rejection. Pretransplant anti-AT1R and antivimentin were not associated with DGF or its functional outcome. In a model of renal IRI in mice, passive transfer of anti-LG3 IgG led to enhanced dysfunction and microvascular injury compared with passive transfer with control IgG. Passive transfer of anti-LG3 antibodies also favored intrarenal microvascular complement activation, microvascular rarefaction and fibrosis after IRI. Our results suggest that anti-LG3 antibodies are novel aggravating factors for renal IRI. These results provide novel insights into the pathways that modulate the severity of renal injury at the time of transplantation and their impact on long-term outcomes.

Manipulation of the glycan-specific natural antibody repertoire for immunotherapy

Natural immunoglobulin derived from innate-like B lymphocytes plays important roles in the suppression of inflammatory responses and represents a promising therapeutic target in a growing number of allergic and autoimmune diseases. These antibodies are commonly autoreactive and incorporate evolutionarily conserved specificities, including certain glycan-specific antibodies. Despite this conservation, exposure to bacterial polysaccharides during innate-like B lymphocyte development, through either natural exposure or immunization, induces significant changes in clonal representation within the glycan-reactive B cell pool. Glycan-reactive natural antibodies (NAbs) have been reported to play protective and pathogenic roles in autoimmune and inflammatory diseases. An understanding of the composition and functions of a healthy glycan-reactive NAb repertoire is therefore paramount. A more thorough understanding of NAb repertoire development holds promise for the design of both biological diagnostics and therapies. In this article, we review the development and functions of NAbs and examine three glycan specificities, represented in the innate-like B cell pool, to illustrate the complex roles environmental antigens play in NAb repertoire development. We also discuss the implications of increased clonal plasticity of the innate-like B cell repertoire during neonatal and perinatal periods, and the prospect of targeting B cell development with interventional therapies and correct defects in this important arm of the adaptive immune system.