Activation of the Lectin Pathway by Natural IgM in a Model of Ischemia/Reperfusion Injury

Prognostic role of mesangial IgM deposition in IgA nephropathy: a long-term cohort study

BACKGROUND

The clinical significance of mesangial immunoglobulin (Ig) M deposition in IgA nephropathy (IgAN) has been less explored and remains a topic of debate. Therefore, our study aimed to investigate the prognostic value of mesangial IgM deposition in a long-term follow-up cohort of IgAN patients.

METHODS

A unicentric retrospective study was conducted on 93 consecutive IgAN patients (median age 41 years, 68% male, eGFR 48.7 mL/min, proteinuria 1.1 g/g) from 2010 to 2015. They were followed until end-stage kidney disease (ESKD), death, or until the end of the study in January 2021, with a median follow-up of 7 years. An independent pathologist evaluated the IgM immunofluorescence pattern, Oxford MEST-C score, and transmission electron microscopy (TEM) lesions following a comprehensive protocol.

RESULTS

In our cohort, 70% had mesangial IgM-positive deposits, while 30% were IgM-negative. Both groups were similar in age, sex, prevalence of arterial hypertension, Charlson comorbidity scores, kidney function (eGFR and proteinuria), pathology findings (Oxford MEST-C score, IgG and C3 immune deposition), and TEM analysis. Treatment with RASI and immunosuppression, and death rates were also comparable. However, 37% of IgM-positive patients progressed to ESKD, significantly higher than the 11% in the IgM-negative group. Univariate and multivariate Cox proportional hazards regression analyses identified lower eGFR, higher Oxford MEST-C score, and mesangial IgM deposits as independent factors associated with shorter kidney survival.

CONCLUSIONS

Our study highlights mesangial IgM deposition as a potential risk factor for ESKD in patients with advanced IgAN, laying a foundation for further research in this area.

Host glycosylation of immunoglobulins impairs the immune response to acute Lyme disease

BACKGROUND

Lyme disease is caused by the bacteria Borreliella burgdorferi sensu lato (Bb) transmitted to humans from the bite of an infected Ixodes tick. Current diagnostics for Lyme disease are insensitive at the early disease stage and they cannot differentiate between active infections and people with a recent history of antibiotic-treated Lyme disease.

METHODS

Machine learning technology was utilized to improve the prediction of acute Lyme disease and identify sialic acid and galactose sugar structures (N-glycans) on immunoglobulins associated specifically at time points during acute Lyme disease time. A plate-based approach was developed to analyze sialylated N-glycans associated with anti-Bb immunoglobulins. This multiplexed approach quantitates the abundance of Bb-specific IgG and the associated sialic acid, yielding an accuracy of 90% in a powered study.

FINDINGS

It was demonstrated that immunoglobulin sialic acid levels increase during acute Lyme disease and following antibiotic therapy and a 3-month convalescence, the sialic acid level returned to that found in healthy control subjects (p < 0.001). Furthermore, the abundance of sialic acid on Bb-specific IgG during acute Lyme disease impaired the host's ability to combat Lyme disease via lymphocytic receptor FcγRIIIa signaling. After enzymatically removing the sialic acid present on Bb-specific antibodies, the induction of cytotoxicity from acute Lyme disease patient antigen-specific IgG was significantly improved.

INTERPRETATION

Taken together, Bb-specific immunoglobulins contain increased sialylation which impairs the host immune response during acute Lyme disease. Furthermore, this Bb-specific immunoglobulin sialyation found in acute Lyme disease begins to resolve following antibiotic therapy and convalescence.

FUNDING

Funding for this study was provided by the Coulter-Drexel Translational Research Partnership Program as well as from a Faculty Development Award from the Drexel University College of Medicine Institute for Molecular Medicine and Infectious Disease and the Department of Microbiology and Immunology.

IgM N-glycosylation correlates with COVID-19 severity and rate of complement deposition

The glycosylation of IgG plays a critical role during human severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, activating immune cells and inducing cytokine production. However, the role of IgM N-glycosylation has not been studied during human acute viral infection. The analysis of IgM N-glycosylation from healthy controls and hospitalized coronavirus disease 2019 (COVID-19) patients reveals increased high-mannose and sialylation that correlates with COVID-19 severity. These trends are confirmed within SARS-CoV-2-specific immunoglobulin N-glycan profiles. Moreover, the degree of total IgM mannosylation and sialylation correlate significantly with markers of disease severity. We link the changes of IgM N-glycosylation with the expression of Golgi glycosyltransferases. Lastly, we observe antigen-specific IgM antibody-dependent complement deposition is elevated in severe COVID-19 patients and modulated by exoglycosidase digestion. Taken together, this work links the IgM N-glycosylation with COVID-19 severity and highlights the need to understand IgM glycosylation and downstream immune function during human disease.

A nanobody-based complement inhibitor targeting complement component 2 reduces hemolysis in a complement humanized mouse model of autoimmune hemolytic anemia

C2 is an attractive therapeutic target for many complement-mediated diseases. We developed Nab1B10, a new anti-C2 nanobody that potently and selectively inhibits both the classical and lectin pathways of complement activation. Mechanistically, Nab1B10 binds to the C2a portion of C2 and inhibits the assembly of C3 convertase C4b2a. Nab1B10 cross-reacts with monkey but not rodent C2 and inhibits classical pathway-mediated hemolysis. Using a new complement humanized mouse model of autoimmune hemolytic anemia (AIHA), we demonstrated that Nab1B10 abolished classical pathway complement activation-mediated hemolysis in vivo. We also developed C2-neutralizing bi- and tetra-valent antibodies based on Nab1B10 and found these antibodies significantly more potent than the other anti-C2 monoclonal antibody that is already in clinical trials. These data suggest that these novel C2-neutralizing nanobodies could be further developed as new therapeutics for many complement-mediated diseases, in which pathogenesis is dependent on the classical and/or lectin pathway of complement activation.

IgM N-glycosylation correlates with COVID-19 severity and rate of complement deposition

The glycosylation of IgG plays a critical role during human SARS-CoV-2, activating immune cells and inducing cytokine production. However, the role of IgM N-glycosylation has not been studied during acute viral infection in humans. In vitro evidence suggests that the glycosylation of IgM inhibits T cell proliferation and alters complement activation rates. The analysis of IgM N-glycosylation from healthy controls and hospitalized COVID-19 patients reveals that mannosylation and sialyation levels associate with COVID-19 severity. Specifically, we find increased di- and tri-sialylated glycans and altered mannose glycans in total serum IgM in severe COVID-19 patients when compared to moderate COVID-19 patients. This is in direct contrast with the decrease of sialic acid found on the serum IgG from the same cohorts. Moreover, the degree of mannosylation and sialylation correlated significantly with markers of disease severity: D-dimer, BUN, creatinine, potassium, and early anti-COVID-19 amounts of IgG, IgA, and IgM. Further, IL-16 and IL-18 cytokines showed similar trends with the amount of mannose and sialic acid present on IgM, implicating these cytokines' potential to impact glycosyltransferase expression during IgM production. When examining PBMC mRNA transcripts, we observe a decrease in the expression of Golgi mannosidases that correlates with the overall reduction in mannose processing we detect in the IgM N-glycosylation profile. Importantly, we found that IgM contains alpha-2,3 linked sialic acids in addition to the previously reported alpha-2,6 linkage. We also report that antigen-specific IgM antibody-dependent complement deposition is elevated in severe COVID-19 patients. Taken together, this work links the immunoglobulin M N-glycosylation with COVID-19 severity and highlights the need to understand the connection between IgM glycosylation and downstream immune function during human disease.

C1q as a target molecule to treat human disease: What do mouse studies teach us?

The complement system is a field of growing interest for pharmacological intervention. Complement protein C1q, the pattern recognition molecule at the start of the classical pathway of the complement cascade, is a versatile molecule with additional non-canonical actions affecting numerous cellular processes. Based on observations made in patients with hereditary C1q deficiency, C1q is protective against systemic autoimmunity and bacterial infections. Accordingly, C1q deficient mice reproduce this phenotype with susceptibility to autoimmunity and infections. At the same time, beneficial effects of C1q deficiency on disease entities such as neurodegenerative diseases have also been described in murine disease models. This systematic review provides an overview of all currently available literature on the C1q knockout mouse in disease models to identify potential target diseases for treatment strategies focusing on C1q, and discusses potential side-effects when depleting and/or inhibiting C1q.

Tranexamic acid is associated with reduced complement activation in trauma patients with hemorrhagic shock and hyperfibrinolysis on thromboelastography

: Trauma with hemorrhagic shock causes massive tissue plasminogen activator release, plasmin generation, and hyperfibrinolysis. Tranexamic acid (TXA) has recently been used to treat bleeding in trauma by preventing plasmin generation to limit fibrinolysis. Trauma patients also have increased complement activation that correlates with mortality and organ failure, but the source of activation is not clear, and plasmin has recently been shown to efficiently cleave C3 and C5 to their activated fragments. We hypothesized that trauma patients in hemorrhagic shock with hyperfibrinolysis on thromboelastography (TEG) LY30 would have increased complement activation at early time points, as measured by soluble C5b-9 complex, and TXA would prevent this. Plasma samples were obtained from an unrelated, previously performed IRB-approved prospective randomized study of trauma patients. Three groups were studied with n = 5 patients in each group: patients without hyperfibrinolysis (TEG LY30 < 3%) (who therefore did not get TXA), patients with hyperfibrinolysis (TEG LY30 > 3%) who did not get TXA, and patients with hyperfibrinolysis who were then treated with TXA. We found that patients who did not receive TXA, regardless of fibrinolytic phenotype, had elevated soluble C5b-9 levels at 6 h relative to emergency department levels. In contrast, all five patients with initial TEG LY30 more than 3% and were then treated with TXA had reduced soluble C5b-9 levels at 6 h relative to emergency department levels. There were no differences in PF1 + 2, Bb, or C4d levels between groups, suggesting that coagulation and complement activation pathways may not be primarily responsible for the observed differences.

Complement Initiation Varies by Sex in Intestinal Ischemia Reperfusion Injury

Intestinal ischemia reperfusion (IR)-induced tissue injury represents an acute inflammatory response with significant morbidity and mortality. The mechanism of IR-induced injury is not fully elucidated, but recent studies suggest a critical role for complement activation and for differences between sexes. To test the hypothesis that complement initiation differs by sex in intestinal IR, we performed intestinal IR on male and female WT C57B6L/, C1q-/-, MBL-/-, or properdin (P)-/- mice. Intestinal injury, C3b and C5a production and ex vivo secretions were analyzed. Initial studies demonstrated a difference in complement mRNA and protein in male and female WT mice. In response to IR, male C1q-, MBL- and P-deficient mice sustained less injury than male WT mice. In contrast, only female MBL-/- mice sustained significantly less injury than female wildtype mice. Importantly, wildtype, C1q-/- and P-/- female mice sustained significant less injury than the corresponding male mice. In addition, both C1q and MBL expression and deposition increased in WT male mice, while only elevated MBL expression and deposition occurred in WT female mice. These data suggested that males use both C1q and MBL pathways, while females tend to depend on lectin pathway during intestinal IR. Females produced significantly less serum C5a in MBL-/- and P-/- mice. Our findings suggested that complement activation plays a critical role in intestinal IR in a sex-dependent manner.

Therapeutic Targeting of the Complement System: From Rare Diseases to Pandemics

The complement system was discovered at the end of the 19th century as a heat-labile plasma component that "complemented" the antibodies in killing microbes, hence the name "complement." Complement is also part of the innate immune system, protecting the host by recognition of pathogen-associated molecular patterns. However, complement is multifunctional far beyond infectious defense. It contributes to organ development, such as sculpting neuron synapses, promoting tissue regeneration and repair, and rapidly engaging and synergizing with a number of processes, including hemostasis leading to thromboinflammation. Complement is a double-edged sword. Although it usually protects the host, it may cause tissue damage when dysregulated or overactivated, such as in the systemic inflammatory reaction seen in trauma and sepsis and severe coronavirus disease 2019 (COVID-19). Damage-associated molecular patterns generated during ischemia-reperfusion injuries (myocardial infarction, stroke, and transplant dysfunction) and in chronic neurologic and rheumatic disease activate complement, thereby increasing damaging inflammation. Despite the long list of diseases with potential for ameliorating complement modulation, only a few rare diseases are approved for clinical treatment targeting complement. Those currently being efficiently treated include paroxysmal nocturnal hemoglobinuria, atypical hemolytic-uremic syndrome, myasthenia gravis, and neuromyelitis optica spectrum disorders. Rare diseases, unfortunately, preclude robust clinical trials. The increasing evidence for complement as a pathogenetic driver in many more common diseases suggests an opportunity for future complement therapy, which, however, requires robust clinical trials; one ongoing example is COVID-19 disease. The current review aims to discuss complement in disease pathogenesis and discuss future pharmacological strategies to treat these diseases with complement-targeted therapies. SIGNIFICANCE STATEMENT: The complement system is the host's defense friend by protecting it from invading pathogens, promoting tissue repair, and maintaining homeostasis. Complement is a double-edged sword, since when dysregulated or overactivated it becomes the host's enemy, leading to tissue damage, organ failure, and, in worst case, death. A number of acute and chronic diseases are candidates for pharmacological treatment to avoid complement-dependent damage, ranging from the well established treatment for rare diseases to possible future treatment of large patient groups like the pandemic coronavirus disease 2019.

Meta-heterogeneity: Evaluating and Describing the Diversity in Glycosylation Between Sites on the Same Glycoprotein

Mass spectrometry-based glycoproteomics has gone through some incredible developments over the last few years. Technological advances in glycopeptide enrichment, fragmentation methods, and data analysis workflows have enabled the transition of glycoproteomics from a niche application, mainly focused on the characterization of isolated glycoproteins, to a mature technology capable of profiling thousands of intact glycopeptides at once. In addition to numerous biological discoveries catalyzed by the technology, we are also observing an increase in studies focusing on global protein glycosylation and the relationship between multiple glycosylation sites on the same protein. It has become apparent that just describing protein glycosylation in terms of micro- and macro-heterogeneity, respectively, the variation and occupancy of glycans at a given site, is not sufficient to describe the observed interactions between sites. In this perspective we propose a new term, meta-heterogeneity, to describe a higher level of glycan regulation: the variation in glycosylation across multiple sites of a given protein. We provide literature examples of extensive meta-heterogeneity on relevant proteins such as antibodies, erythropoietin, myeloperoxidase, and a number of serum and plasma proteins. Furthermore, we postulate on the possible biological reasons and causes behind the intriguing meta-heterogeneity observed in glycoproteins.

Activation of final complement components after kidney transplantation as a marker of delayed graft function severity

Background

Ischaemia-reperfusion (I/R) damage is a relevant cause of delayed graft function (DGF). Complement activation is involved in experimental I/R injury, but few data are available from kidney transplant (KT) patients. We studied the dynamics of membrane attack complex (C5b-9) as a soluble fraction (SC5b-9) and the histological deposit pattern of C3b, complement Factor H (FH) and C5b-9 in DGF patients.

Methods

We evaluated SC5b-9 levels in 59 recipients: 38 with immediate graft function and 21 with DGF. The SC5b-9 was measured at admission for KT and 7 days after KT. DGF-kidney biopsies (n = 12) and a control group of 1-year protocol biopsies without tissue damage (n = 4) were stained for C5b-9, C3b and FH.

Results

SC5b-9 increased significantly in DGF patients (Day 0: 6621 ± 2202 mAU/L versus Day 7: 9626 ± 4142  mAU/L; P = 0.006), while it remained stable in non-DGF patients. Days 0-7 increase >5% was the better cut-off associated with DGF versus non-DGF patient discrimination (sensitivity = 81%). In addition, SC5b-9 increase was related to DGF duration and worse graft function, and independently associated with DGF occurrence. SC5b-9, C3b and FH stains were observed in tubular epithelial cells basal membrane. DGF-kidney biopsies showed a more frequently high-intensity stain, a higher number of tubules with positive stain and larger perimeter of tubules with positive stains for SC5b-9, C3b and FH than control patients.

Conclusions

Both SC5b-9 levels and SC5b-9, C3b and FH deposits in tubular epithelial cells basal membrane are highly expressed in patients experiencing DGF. SC5b-9 levels increase could be useful as a marker of DGF severity.

Complement: Bridging the innate and adaptive immune systems in sterile inflammation

The complement system is a collection of soluble and membrane-bound proteins that together act as a powerful amplifier of the innate and adaptive immune systems. Although its role in infection is well established, complement is becoming increasingly recognized as a key contributor to sterile inflammation, a chronic inflammatory process often associated with noncommunicable diseases. In this context, damaged tissues release danger signals and trigger complement, which acts on a range of leukocytes to augment and bridge the innate and adaptive immune systems. Given the detrimental effect of chronic inflammation, the complement system is therefore well placed as an anti-inflammatory drug target. In this review, we provide a general outline of the sterile activators, effectors, and targets of the complement system and a series of examples (i.e., hypertension, cancer, allograft transplant rejection, and neuroinflammation) that highlight complement's ability to bridge the 2 arms of the immune system.

Mannose Binding Lectin, S100 B Protein, and Brain Injuries in Neonates With Perinatal Asphyxia

Perinatal asphyxia triggers an acute inflammatory response in the injured brain. Complement activation and neuroinflammation worsen brain damage after a systemic ischemia/reperfusion insult. The increase of mannose binding lectin (MBL) during asphyxia may contribute to the brain damage, via activation of the complement lectin pathway. The possible role of MBL2 gene variants in influencing the severity of post-asphyxia brain injuries is still unexplored. This retrospective study included 53 asphyxiated neonates: 42 underwent therapeutic hypothermia (TH) and 11 did not because they were admitted to the NICU later than 6 h after the hypoxic insult. Blood samples from TH-treated and untreated patients were genotyped for MBL2 gene variants, and biomarker plasma levels (MBL and S100 B protein) were measured at different time points: during hypothermia, during rewarming, and at 7-10 days of life. The timing of blood sampling, except for the T1 sample, was the same in untreated infants. Highest (peak) levels of MBL and MBL2 genotypes were correlated to neuroimaging brain damage or death and long-term neurodevelopmental delay. MBL2 wild-type genotype was associated with the highest MBL levels and worst brain damage on MRI (p = 0.046) at 7-10 days after hypoxia. MBL increased in both groups and S100B decreased, slightly more in treated than in untreated neonates. The progressive increase of MBL (p = 0.08) and to be untreated with TH (p = 0.08) increased the risk of brain damage or death at 7-10 days of life, without affecting neurodevelopmental outcomes at 1 year. The effect of TH on MBL plasma profiles is uncertain.

Complement activation by autoantigen recognition in the growth process of benign prostatic hyperplasia

The pathophysiology of benign prostatic hyperplasia (BPH) remained unclear. Here, we concentrated on the complement activation in the growth of BPH using a rat model. BPH tissues were harvested from rats after rat urogenital sinus implantation. The local expression and deposition levels of C1q, C3, mannose-binding lectin (MBL), factor B (FB), and C5b-9 in the rat and human BPH tissues were analyzed by real-time RT-PCR, western blotting and immunohistochemistry (IHC). Serum IgG levels in the rat BPH model were analyzed by ELISA, and IHC was used to assess tissue localization. Proteins binding serum IgG autoantibody in the BPH rats were isolated by immunoprecipitation. C1q, C3, MBL, FB and C5b-9 were highly localized in rat BPH tissues compared to normal tissues. In contrast, C3, FB and C5b-9, but not C1q and MBL, were abundantly detected in human BPH tissues compared to normal tissues. Diffuse localization of IgG in rat BPH tissues was found. Heat shock protein 90, annexin, α-smooth muscle actin, and β-actin were identified as targets for IgG autoantibodies in the BPH model. Our results strongly suggested the role for complement activation in the growth process of BPH, likely triggered by classical pathway activation with autoantibodies.

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.

Role of Complement Activation in Allograft Inflammation

PURPOSE

Novel paradigms have broadened our understanding of mechanisms through which complement mediates allograft inflammation/injury. Herein we review advances in the field and highlight therapeutic implications.

RECENT FINDINGS

Pre-clinical and translational human trials have elucidated complement-dependent mechanisms of post-transplant ischemia-reperfusion (I/R) injury. Immune cell-derived, and intracellular, complement activation are newly linked to proinflammatory T cell immunity relevant to allograft rejection. Complement-induced immune regulation, including C5a ligation of C5a receptor 2 on T cells, C5a/C5a receptor 1 interactions on regulatory myeloid cells, and C1q binding to CD8+ T cells can inhibit proinflammatory T cells and/or prolong murine allograft survival. Pilot trials of complement inhibition to treat/prevent human I/R- or antibody-initiated allograft injury show promise.

SUMMARY

The complement system participates in allograft injury through multiple context- dependent mechanisms involving various components and receptors. These new insights along with development and implementation of individualized complement inhibitory strategies have potential to improve transplant outcomes.

Clinical significance of mesangial IgM deposition in patients with IgA nephropathy

BACKGROUND

Glomerular immunoglobulin G deposition in patients with immunoglobulin A nephropathy (IgAN) has been shown to be associated with adverse renal outcomes. Clinical significance of mesangial immunoglobulin M (IgM) deposition in these patients remains to be proven.

METHODS

One hundred patients who had a diagnosis of IgAN between 2001 and 2017 were enrolled. Patients were divided into two groups based on mesangial IgM deposition status. Groups were compared for demographic, clinical, and pathologic variables at baseline and in follow-up. Cox regression analysis was performed to evaluate the effect of mesangial IgM positivity on renal survival.

RESULTS

IgM-positive group included 51% of participants. Baseline demographic and clinical parameters were not significantly different between groups. Mesangial IgM deposition was significantly associated with a higher segmental sclerosis score (p = 0.008). At last visit, median serum creatinine was higher (p = 0.021) and eGFR was lower (p = 0.006) in IgM-positive group. Nineteen (19%) of all patients reached the combined primary outcome which includes doubling in serum creatinine or evolution to ESRD. Cumulative renal survival was lower (p = 0.001) and resistant disease was more frequent in IgM-positive group (p = 0.026). Renal survival at 15 years was 94.2% and 59.7% in IgM-negative and IgM-positive groups, respectively (p = 0.006). Time-averaged proteinuria (HR 2.9; 95% CI 1.9-4.5; p < 0.001) and mesangial IgM deposition (HR, 13.2; 95% CI 1.9-93.1; p = 0.01) were found to be independent predictors of unfavorable renal outcomes.

CONCLUSIONS

In conclusion, we demonstrated that mesangial IgM deposition independently associated with worse renal outcomes in patients with IgA nephropathy.

Acute heart failure following myocardial infarction: complement activation correlates with the severity of heart failure in patients developing cardiogenic shock

AIMS

Heart failure (HF) is an impending complication to myocardial infarction. We hypothesized that the degree of complement activation reflects severity of HF following acute myocardial infarction.

METHODS AND RESULTS

The LEAF trial (LEvosimendan in Acute heart Failure following myocardial infarction) evaluating 61 patients developing HF within 48 h after percutaneous coronary intervention-treated ST-elevation myocardial infarction herein underwent a post hoc analysis. Blood samples were drawn from inclusion to Day 5 and at 42 day follow-up, and biomarkers were measured with enzyme immunoassays. Regional myocardial contractility was measured by echocardiography as wall motion score index (WMSI). The cardiogenic shock group (n = 9) was compared with the non-shock group (n = 52). Controls (n = 44) were age-matched and sex-matched healthy individuals. C4bc, C3bc, C3bBbP, and sC5b-9 were elevated in patients at inclusion compared with controls (P < 0.01). The shock group had higher levels compared with the non-shock group for all activation products except C3bBbP (P < 0.05). At Day 42, all products were higher in the shock group (P < 0.05). In the shock group, sC5b-9 correlated significantly with WMSI at baseline (r = 0.68; P = 0.045) and at Day 42 (r = 0.84; P = 0.036). Peak sC5b-9 level correlated strongly with WMSI at Day 42 (r = 0.98; P = 0.005). Circulating endothelial cell activation markers sICAM-1 and sVCAM-1 were higher in the shock group during the acute phase (P < 0.01), and their peak levels correlated with sC5b-9 peak level in the whole HF population (r = 0.32; P = 0.014 and r = 0.30; P = 0.022, respectively).

CONCLUSIONS

Complement activation discriminated cardiogenic shock from non-shock in acute ST-elevation myocardial infarction complicated by HF and correlated with regional contractility and endothelial cell activation, suggesting a pathogenic role of complement in this condition.

Ischemia augments alloimmune injury through IL-6-driven CD4+ alloreactivity

Ischemia reperfusion injuries (IRI) are unavoidable in solid organ transplantation. IRI augments alloimmunity but the mechanisms involved are poorly understood. Herein, we examined the effect of IRI on antigen specific alloimmunity. We demonstrate that ischemia promotes alloimmune activation, leading to more severe histological features of rejection, and increased CD4+ and CD8+ T cell graft infiltration, with a predominantly CD8+ IFNγ+ infiltrate. This process is dependent on the presence of alloreactive CD4+ T cells, where depletion prevented infiltration of ischemic grafts by CD8+ IFNγ+ T cells. IL-6 is a known driver of ischemia-induced rejection. Herein, depletion of donor antigen-presenting cells reduced ischemia-induced CD8+ IFNγ+ allograft infiltration, and improved allograft outcomes. Following prolonged ischemia, accelerated rejection was observed despite treatment with CTLA4Ig, indicating that T cell costimulatory blockade failed to overcome the immune activating effect of IRI. However, despite severe ischemic injury, treatment with anti-IL-6 and CTLA4Ig blocked IRI-induced alloimmune injury and markedly improved allograft survival. We describe a novel pathway where IRI activates innate immunity, leading to upregulation of antigen specific alloimmunity, resulting in chronic allograft injury. Based on these findings, we describe a clinically relevant treatment strategy to overcome the deleterious effect of IRI, and provide superior long-term allograft outcomes.

Complement Dependence of Murine Costimulatory Blockade-Resistant Cellular Cardiac Allograft Rejection

Building on studies showing that ischemia-reperfusion-(I/R)-injury is complement dependent, we tested links among complement activation, transplantation-associated I/R injury, and murine cardiac allograft rejection. We transplanted BALB/c hearts subjected to 8-h cold ischemic storage (CIS) into cytotoxic T-lymphocyte associated protein 4 (CTLA4)Ig-treated wild-type (WT) or c3-/- B6 recipients. Whereas allografts subjected to 8-h CIS rejected in WT recipients with a median survival time (MST) of 37 days, identically treated hearts survived >60 days in c3-/- mice (p < 0.05, n = 4-6/group). Mechanistic studies showed recipient C3 deficiency prevented induction of intragraft and serum chemokines/cytokines and blunted the priming, expansion, and graft infiltration of interferon-γ-producing, donor-reactive T cells. MST of hearts subjected to 8-h CIS was >60 days in mannose binding lectin (mbl1-/- mbl2-/- ) recipients and 42 days in factor B (cfb-/- ) recipients (n = 4-6/group, p < 0.05, mbl1-/- mbl2-/- vs. cfb-/- ), implicating the MBL (not alternative) pathway. To pharmacologically target MBL-initiated complement activation, we transplanted BALB/c hearts subjected to 8-h CIS into CTLA4Ig-treated WT B6 recipients with or without C1 inhibitor (C1-INH). Remarkably, peritransplantation administration of C1-INH prolonged graft survival (MST >60 days, p < 0.05 vs. controls, n = 6) and prevented CI-induced increases in donor-reactive, IFNγ-producing spleen cells (p < 0.05). These new findings link donor I/R injury to T cell-mediated rejection through MBL-initiated, complement activation and support testing C1-INH administration to prevent CTLA4Ig-resistant rejection of deceased donor allografts in human transplant patients.

Deconstructing the Lectin Pathway in the Pathogenesis of Experimental Inflammatory Arthritis: Essential Role of the Lectin Ficolin B and Mannose-Binding Protein-Associated Serine Protease 2

Complement plays an important role in the pathogenesis of rheumatoid arthritis. Although the alternative pathway (AP) is known to play a key pathogenic role in models of rheumatoid arthritis, the importance of the lectin pathway (LP) pattern recognition molecules such as ficolin (FCN) A, FCN B, and collectin (CL)-11, as well as the activating enzyme mannose-binding lectin-associated serine protease-2 (MASP-2), are less well understood. We show in this article that FCN A-/- and CL-11-/- mice are fully susceptible to collagen Ab-induced arthritis (CAIA). In contrast, FCN B-/- and MASP-2-/-/sMAp-/- mice are substantially protected, with clinical disease activity decreased significantly (p < 0.05) by 47 and 70%, respectively. Histopathology scores, C3, factor D, FCN B deposition, and infiltration of synovial macrophages and neutrophils were similarly decreased in FCN B-/- and MASP-2-/-/sMAp-/- mice. Our data support that FCN B plays an important role in the development of CAIA, likely through ligand recognition in the joint and MASP activation, and that MASP-2 also contributes to the development of CAIA, likely in a C4-independent manner. Decreased AP activity in the sera from FCN B-/- and MASP-2-/-/sMAp-/- mice with arthritis on adherent anti-collagen Abs also support the hypothesis that pathogenic Abs, as well as additional inflammation-related ligands, are recognized by the LP and operate in vivo to activate complement. Finally, we also speculate that the residual disease seen in our studies is driven by the AP and/or the C2/C4 bypass pathway via the direct cleavage of C3 through an LP-dependent mechanism.

Activation of complement factor B contributes to murine and human myocardial ischemia/reperfusion injury

The pathophysiology of myocardial injury that results from cardiac ischemia and reperfusion (I/R) is incompletely understood. Experimental evidence from murine models indicates that innate immune mechanisms including complement activation via the classical and lectin pathways are crucial. Whether factor B (fB), a component of the alternative complement pathway required for amplification of complement cascade activation, participates in the pathophysiology of myocardial I/R injury has not been addressed. We induced regional myocardial I/R injury by transient coronary ligation in WT C57BL/6 mice, a manipulation that resulted in marked myocardial necrosis associated with activation of fB protein and myocardial deposition of C3 activation products. In contrast, in fB-/- mice, the same procedure resulted in significantly reduced myocardial necrosis (% ventricular tissue necrotic; fB-/- mice, 20 ± 4%; WT mice, 45 ± 3%; P < 0.05) and diminished deposition of C3 activation products in the myocardial tissue (fB-/- mice, 0 ± 0%; WT mice, 31 ± 6%; P<0.05). Reconstitution of fB-/- mice with WT serum followed by cardiac I/R restored the myocardial necrosis and activated C3 deposition in the myocardium. In translational human studies we measured levels of activated fB (Bb) in intracoronary blood samples obtained during cardio-pulmonary bypass surgery before and after aortic cross clamping (AXCL), during which global heart ischemia was induced. Intracoronary Bb increased immediately after AXCL, and the levels were directly correlated with peripheral blood levels of cardiac troponin I, an established biomarker of myocardial necrosis (Spearman coefficient = 0.465, P < 0.01). Taken together, our results support the conclusion that circulating fB is a crucial pathophysiological amplifier of I/R-induced, complement-dependent myocardial necrosis and identify fB as a potential therapeutic target for prevention of human myocardial I/R injury.

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.

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.

Mannose-Binding Lectin: Biologic Characteristics and Role in the Susceptibility to Infections and Ischemia-Reperfusion Related Injury in Critically Ill Neonates

The mannose-binding lectin (MBL) is a member of the collectin family, belonging to the innate immunity system. Genetic, biologic, and clinical properties of MBL have been widely investigated throughout the last decades, although some interesting aspects of its potential clinical relevance are still poorly understood. Low circulating concentrations of MBL have been associated with increased risk of infection and poor neurologic outcome in neonates. On the other hand, an excessive and uncontrolled inflammatory response by the neonatal intestine after the exposure to luminal bacteria, leading to an increased production of MBL, may be involved in the onset of necrotizing enterocolitis. The purpose of the present review is to summarize the current knowledge about genetic and biologic characteristics of MBL and its role in the susceptibility to infections and to ischemia-reperfusion related tissue injuries to better explore its clinical relevance during the perinatal period and the possible future therapeutic applications.

Complement activation, regulation, and molecular basis for complement-related diseases

The complement system is an essential element of the innate immune response that becomes activated upon recognition of molecular patterns associated with microorganisms, abnormal host cells, and modified molecules in the extracellular environment. The resulting proteolytic cascade tags the complement activator for elimination and elicits a pro-inflammatory response leading to recruitment and activation of immune cells from both the innate and adaptive branches of the immune system. Through these activities, complement functions in the first line of defense against pathogens but also contributes significantly to the maintenance of homeostasis and prevention of autoimmunity. Activation of complement and the subsequent biological responses occur primarily in the extracellular environment. However, recent studies have demonstrated autocrine signaling by complement activation in intracellular vesicles, while the presence of a cytoplasmic receptor serves to detect complement-opsonized intracellular pathogens. Furthermore, breakthroughs in both functional and structural studies now make it possible to describe many of the intricate molecular mechanisms underlying complement activation and the subsequent downstream events, as well as its cross talk with, for example, signaling pathways, the coagulation system, and adaptive immunity. We present an integrated and updated view of complement based on structural and functional data and describe the new roles attributed to complement. Finally, we discuss how the structural and mechanistic understanding of the complement system rationalizes the genetic defects conferring uncontrolled activation or other undesirable effects of complement.

Acute Liver Injury Is Independent of B Cells or Immunoglobulin M

Background & Aims

Acute liver injury is a clinically important pathology and results in the release of Danger Associated Molecular Patterns, which initiate an immune response. Withdrawal of the injurious agent and curtailing any pathogenic secondary immune response may allow spontaneous resolution of injury. The role B cells and Immunoglobulin M (IgM) play in acute liver injury is largely unknown and it was proposed that B cells and/or IgM would play a significant role in its pathogenesis.

Methods

Tissue from 3 models of experimental liver injury (ischemia-reperfusion injury, concanavalin A hepatitis and paracetamol-induced liver injury) and patients transplanted following paracetamol overdose were stained for evidence of IgM deposition. Mice deficient in B cells (and IgM) were used to dissect out the role B cells and/or IgM played in the development or resolution of injury. Serum transfer into mice lacking IgM was used to establish the role IgM plays in injury.

Results

Significant deposition of IgM was seen in the explanted livers of patients transplanted following paracetamol overdose as well as in 3 experimental models of acute liver injury (ischemia-reperfusion injury, concanavalin A hepatitis and paracetamol-induced liver injury). Serum transfer into IgM-deficient mice failed to reconstitute injury (p = 0.66), despite successful engraftment of IgM. Mice deficient in both T and B cells (RAG1-/-) mice (p<0.001), but not B cell deficient (μMT) mice (p = 0.93), were significantly protected from injury. Further interrogation with T cell deficient (CD3εKO) mice confirmed that the T cell component is a key mediator of sterile liver injury. Mice deficient in B cells and IgM mice did not have a significant delay in resolution following acute liver injury.

Discussion

IgM deposition appears to be common feature of both human and murine sterile liver injury. However, neither IgM nor B cells, play a significant role in the development of or resolution from acute liver injury. T cells appear to be key mediators of injury. In conclusion, the therapeutic targeting of IgM or B cells (e.g. with Rituximab) would have limited benefit in protecting patients from acute liver injury.

Inhalation anesthesia of rats: influence of the fraction of inspired oxygen on limb ischemia/reperfusion injury

Inhalation anesthesia with isoflurane is a well-established and safe method used in small laboratory animals. In most cases oxygen is used as a carrier gas for isoflurane, but room air or mixtures of oxygen with air or nitrous oxide are also being used. Anesthesia is therefore administered using different fractions of inspired oxygen (FiO2), and this may have consequences for the outcome of experiments. The aim of the present study was to investigate the influence of FiO2 on rat hind limb ischemia/reperfusion injury and to refine the used inhalation anesthesia. Male Wistar rats were subjected to 3.5 h of ischemia and 2 h of reperfusion, and divided into three groups according to FiO2 in the O2/air/isoflurane anesthesia gas mixture: 40%, 60%, and 100% O2 Normal, healthy rats were used as controls. Muscle edema and creatine kinase MM, a marker for myocyte necrosis, were significantly increased with 40% FiO2 as compared with 100% FiO2 (P < 0.05). Partial pressure of oxygen, oxygen saturation, and oxyhemoglobin were significantly higher in the 100% O2 group as compared with 40% O2 No significant differences were detected for other parameters, such as the oxidative stress markers malondialdehyde and superoxide dismutase. We conclude that a refined inhalation anesthesia setting using 40% FiO2, reflecting more or less the clinical situation, leads to a more severe and more physiologically relevant reperfusion injury than higher FiO2. Oxidative stress did not correlate with FiO2 and seemed to have no influence on reperfusion injury.

Complement in the Homeostatic and Ischemic Brain

The complement system is a component of the immune system involved in both recognition and response to pathogens, and it is implicated in an increasing number of homeostatic and disease processes. It is well documented that reperfusion of ischemic tissue results in complement activation and an inflammatory response that causes post-reperfusion injury. This occurs following cerebral ischemia and reperfusion and triggers secondary damage that extends beyond the initial infarcted area, an outcome that has rationalized the use of complement inhibitors as candidate therapeutics after stroke. In the central nervous system, however, recent studies have revealed that complement also has essential roles in synaptic pruning, neurogenesis, and neuronal migration. In the context of recovery after stroke, these apparent divergent functions of complement may account for findings that the protective effect of complement inhibition in the acute phase after stroke is not always maintained in the subacute and chronic phases. The development of effective stroke therapies based on modulation of the complement system will require a detailed understanding of complement-dependent processes in both early neurodegenerative events and delayed neuro-reparatory processes. Here, we review the role of complement in normal brain physiology, the events initiating complement activation after cerebral ischemia-reperfusion injury, and the contribution of complement to both injury and recovery. We also discuss how the design of future experiments may better characterize the dual role of complement in recovery after ischemic stroke.

Complement activation contributes to the anti-methicillin-resistant Staphylococcus aureus effect of natural anti-keratin antibody

Methicillin-resistant Staphylococcus aureus (MRSA) remains a major public health problem worldwide because of its strong resistance to a variety of antibiotics. Natural immunoglobulin (Ig) M antibodies have been reported to protect against microbial infections. In the present study, the function of a monoclonal natural anti-keratin antibody IgM (named 3B4) in MRSA infection was evaluated. The binding of 3B4 to MRSA was studied using immunofluorescence assay and flow cytometry (FCM). The binding of 3B4 to mannose-binding lectin (MBL) and complement activation were detected by ELISA. For the in vivo study, transgenic mice for the VH gene from 3B4 (TgVH 3B4) were used. After infection, the bacterial burden was examined in the kidney, spleen and enterocelia. Inflammatory cytokine levels and the neutrophil ratio in peritoneal lavage fluid (PLF) were assessed by ELISA and FCM, respectively. Additionally, the total serum hemolytic activity (CH50) in the early stage of infection was detected by ELISA. The results showed that 3B4 bound directly to MRSA and MBL, and the interaction between 3B4 and MRSA/MBL led to the activation of the classic and the MBL pathway in vitro. After 48 h of MRSA infection, the bacterial load in the kidney, spleen and enterocelia was significantly decreased in TgVH 3B4 mice (P < 0.05) compared with wild-type mice. Levels of IL-6, TNF-α, and IFN-γ were increased after MRSA infection. The levels of IL-6 and TNF-α in TgVH 3B4 mice were decreased by 49.1% and 59.4% compared to wild-type mice. Additionally, the neutrophil ratio in the PLF of TgVH 3B4 mice was decreased by 65.9%. The CH50 value was significantly higher in TgVH 3B4 mice than in wild-type mice, indicating that 3B4 promoted the activation of the complement system in MRSA infected mice. The results reveal an important role of 3B4 in the anti-MRSA immune response, and the complement activation contributes to this effect.

Complement: an overview for the clinician

The complement system is an essential component of the immune system. It is a highly integrative system and has a number of functions, including host defense, removal of injured cells and debris, modulation of metabolic and regenerative processes, and regulation of adaptive immunity. Complement is activated via different pathways and it is regulated tightly by several mechanisms to prevent host injury. Imbalance between complement activation and regulation can manifest in disease and injury to self. This article provides an outline of complement activation pathways, regulatory mechanisms, and normal physiologic functions of the system.

The endothelial deprotection hypothesis for lupus pathogenesis: the dual role of C1q as a mediator of clearance and regulator of endothelial permeability

Systemic lupus erythematosus (SLE) is a heterogeneous multifactorial systemic autoimmune disease affecting several organs. SLE can start relatively early in life and results in impaired quality of life and shortened life expectancy because of a gradual disease progression leading to cardiovascular, renal and neoplastic disease. The basic mechanisms of the pathogenesis of the disease still remain to be clarified. It is clear that complement proteins play a key and complex role in the development of SLE. Complement component C1q has been known to be a fundamental component of lupus development, but most explanations focus on its role in apoptotic debris removal. Importantly, C1q was recently found to play a key role in the maintenance of vascular endothelial integrity.

We suggest that apoptotic products, endothelial cells and extracellular matrix components, which display negatively charged moieties, compete for binding to molecules of the innate humoral immune response, like C1q. Genetic or acquired factors leading to an increased load of apoptotic cell debris and decrease or absence of C1q therefore interfere with the regulation of endothelial permeability and integrity. Furthermore, we suggest that lupus is the net result of an imbalance between the two functions of immune clearance and vascular endothelial integrity maintenance, an imbalance triggered and sustained by autoimmunity, which skews C1q consumption by IgG-mediated complement classical pathway activation on autoantigens. In this triangle of innate clearance, autoimmunity and endothelial integrity, C1q plays a central role.

Hence, we interpret the pathogenesis of lupus by identifying three key components, namely innate immune clearance, autoimmunity and endothelial integrity and we establish a link between these components based on the protective role that innate clearance molecules play in endothelial renewal. By including the vasoprotective role of C1q in the interpretation of SLE development we attempt to provide novel explanations for the symptoms, organ damage, diagnostic and therapeutic difficulties of the disease.

MBL2 gene polymorphisms increase the risk of adverse neurological outcome in preterm infants: a preliminary prospective study

BACKGROUND

As described in animal models, the lectin-complement pathway is central to the propagation of ischemia-reperfusion injuries in many tissues, including the brain. Similarly, it might affect the genesis of brain damage in preterm infants. MBL2 gene single-nucleotide polymorphisms (SNPs), regulating mannose-binding lectin (MBL) serum levels, could predict the risk of adverse neurological outcome in these infants.

METHODS

To evaluate the association between SNPs of the MBL2 gene and long-term neurological outcomes in preterm infants, 75 infants (gestational age (GA) ≤ 32 wk) were observed in a prospective longitudinal study and assessed by clinical and instrumental exams at 12 and 24 mo of corrected age (CA). They were genotyped for the promoter polymorphism -221 and for the exon-1 variant alleles (at codons 52, 54, and 57) of the MBL2 gene.

RESULTS

The MBL2 exon-1 OO genotype was more frequent in children with an adverse neurological outcome (5/35; 7%) than in controls (0/40; 0%), P = 0.045. The risk of intraventricular hemorrhage in carriers of the genotype OO was marked, without reaching statistical significance (odds ratio: 8.67; 95% confidence interval: 0.87-86.06; P = 0.07).

CONCLUSION

Preterm infants who are carriers of MBL2 exon-1 OO genotype are exposed to an increased risk of adverse neurological outcomes.

Lessons learned from mice deficient in lectin complement pathway molecules

The lectin pathway of the complement system is initiated when the pattern-recognition molecules, mannose-binding lectin (MBL), ficolins or collectin-11, bind to invading pathogens or damaged host cells. This leads to activation of MBL/ficolin/collectin-11 associated serine proteases (MASPs), which in turn activate downstream complement components, ultimately leading to elimination of the pathogen. Mice deficient in the key molecules of lectin pathway of complement have been generated in order to build knowledge of the molecular mechanisms of the lectin pathway in health and disease. Despite differences in the genetic arrangements of murine and human orthologues of lectin pathway molecules, the knockout mice have proven to be valuable models to explore the effect of deficiency states in humans. In addition, new insight and unexpected findings on the diverse roles of lectin pathway molecules in complement activation, pathogen infection, coagulation, host tissue injury and developmental biology have been revealed by in vivo investigations. This review provides an overview of the mice deficient in lectin pathway molecules and highlights some of the most important findings that have resulted from studies of these.

Mannan-binding lectin-associated serine protease 2 is critical for the development of renal ischemia reperfusion injury and mediates tissue injury in the absence of complement C4

Mannan-binding lectin-associated serine protease 2 (MASP-2) has been described as the essential enzyme for the lectin pathway (LP) of complement activation. Since there is strong published evidence indicating that complement activation via the LP critically contributes to ischemia reperfusion (IR) injury, we assessed the effect of MASP-2 deficiency in an isogenic mouse model of renal transplantation. The experimental transplantation model used included nephrectomy of the remaining native kidney at d 5 post-transplantation. While wild-type (WT) kidneys grafted into WT recipients (n=7) developed acute renal failure (control group), WT grafts transplanted into MASP-2-deficient recipients (n=7) showed significantly better kidney function, less C3 deposition, and less IR injury. In the absence of donor or recipient complement C4 (n=7), the WT to WT phenotype was preserved, indicating that the MASP-2-mediated damage was independent of C4 activation. This C4-bypass MASP-2 activity was confirmed in mice deficient for both MASP-2 and C4 (n=7), where the protection from postoperative acute renal failure was no greater than in mice with MASP-2 deficiency alone. Our study highlights the role of LP activation in renal IR injury and indicates that injury occurs through MASP-2-dependent activation events independent of C4.

Membrane lipid interactions in intestinal ischemia/reperfusion-induced Injury

Ischemia, lack of blood flow, and reperfusion, return of blood flow, are a common phenomenon affecting millions of Americans each year. Roughly 30,000 Americans per year experience intestinal ischemia-reperfusion (IR), which is associated with a high mortality rate. Previous studies of the intestine established a role for neutrophils, eicosanoids, the complement system and naturally occurring antibodies in IR-induced pathology. Furthermore, data indicate involvement of a lipid or lipid-like moiety in mediating IR-induced damage. It has been proposed that antibodies recognize exposure of neo-antigens, triggering action of the complement cascade. While it is evident that the pathophysiology of IR-induced injury is complex and multi-factorial, we focus this review on the involvement of eicosanoids, phospholipids and neo-antigens in the early pathogenesis. Lipid changes occurring in response to IR, neo-antigens exposed and the role of a phospholipid transporter, phospholipid scramblase 1 will be discussed.

Mannan-binding lectin in cardiovascular disease

Cardiovascular disease remains the leading cause of mortality and morbidity worldwide so research continues into underlying mechanisms. Since innate immunity and its potent component mannan-binding lectin have been proven to play an important role in the inflammatory response during infection and ischaemia-reperfusion injury, attention has been paid to its role in the development of cardiovascular complications as well. This review provides a general outline of the structure and genetic polymorphism of MBL and its role in inflammation/tissue injury with emphasis on associations with cardiovascular disease. MBL appears to be involved in the pathogenesis of atherosclerosis and, in consequence, coronary artery disease and also inflammation and tissue injury after myocardial infarction and heart transplantation. The relationship between MBL and disease is rather complex and depends on different genetic and environmental factors. That could be why the data obtained from animal and clinical studies are sometimes contradictory proving not for the first time that innate immunity is a "double-edge sword," sometimes beneficial and, at other times disastrous for the host.

RGMb is a novel binding partner for PD-L2 and its engagement with PD-L2 promotes respiratory tolerance

We report that programmed death ligand 2 (PD-L2), a known ligand of PD-1, also binds to repulsive guidance molecule b (RGMb), which was originally identified in the nervous system as a co-receptor for bone morphogenetic proteins (BMPs). PD-L2 and BMP-2/4 bind to distinct sites on RGMb. Normal resting lung interstitial macrophages and alveolar epithelial cells express high levels of RGMb mRNA, whereas lung dendritic cells express PD-L2. Blockade of the RGMb-PD-L2 interaction markedly impaired the development of respiratory tolerance by interfering with the initial T cell expansion required for respiratory tolerance. Experiments with PD-L2-deficient mice showed that PD-L2 expression on non-T cells was critical for respiratory tolerance, but expression on T cells was not required. Because PD-L2 binds to both PD-1, which inhibits antitumor immunity, and to RGMb, which regulates respiratory immunity, targeting the PD-L2 pathway has therapeutic potential for asthma, cancer, and other immune-mediated disorders. Understanding this pathway may provide insights into how to optimally modulate the PD-1 pathway in cancer immunotherapy while minimizing adverse events.

Immunoglobulin M, C-reactive protein and complement activation in rat hepatic ischemia-reperfusion injury

BACKGROUND

Ischemia-reperfusion (I/R) models have shown that C-reactive protein (CRP) and immunoglobulin M (IgM) are involved in complement activation. Binding of CRP and IgM to damaged cell membranes initiates complement activation and aggravates I/R injury in various organs. However, the time course of CRP- and IgM-mediated complement activation and the relation to hepatocellular injury and inflammation in liver I/R are unknown.

AIM

To evaluate the time course of IgM- and CRP-related complement activation and the relation to hepatocellular injury and inflammation in a hepatic I/R rat model.

METHODS

Male Wistar rats were allocated to (1) five groups of animals exposed to 60 min of partial ischemia (70%) induced via clamping of the left segmental portal triad, followed by 0, 3, 6, 12 or 24 h of reperfusion (n = 6 in each group); (2) five groups of sham-operated animals with corresponding reperfusion times (n = 5), and (3) a control group sacrificed before ischemia (n = 5). Hepatocellular injury, inflammatory response, rat plasma CRP and IgM levels and immunohistochemical depositions of CRP, IgM and C3 were assessed for each group.

RESULTS

Histopathological injury scores of hematoxylin and eosin sections of ischemic liver lobes demonstrated increasing values throughout the reperfusion time with a peak at 12 h. Plasma aminotransferases (alanine aminotransferase and aspartate aminotransferase) significantly increased after 3 h of reperfusion, peaking at 6 h (3,100 ± 800 U/l; p < 0.05). Hepatic neutrophil influx significantly increased from 3 to 6 h of reperfusion (p < 0.05) and demonstrated the highest value at 12 h (1.1 ± 0.2 U/mg of protein). Plasma IL-6 levels in the ischemia groups showed peak values after 6 h of reperfusion, decreasing significantly thereafter (p < 0.05). Plasma CRP values reached highest levels after 3 h of reperfusion (mean 91 ± 5% of control pool), decreasing significantly thereafter. Rat IgM concentrations in plasma did not significantly change throughout the reperfusion time. Immunohistochemical depositions of IgM, CRP and C3 in ischemic lobes demonstrated a similar pattern in time, reaching maximum values at 12 h of reperfusion. The percentages of depositions of CRP and IgM were significantly correlated [r(S) = 0.569; p < 0.001; Spearman test]. The time course of C3 and CRP depositions throughout reperfusion and C3 and IgM staining were significantly similar [r(S) = 0.797 and r(S) = 0.656, respectively; p < 0.0001; ANOVA].

CONCLUSIONS

CRP and IgM depositions demonstrate a parallel time course throughout the reperfusion to hepatocellular damage, inflammatory response and activated complement deposition in this rat hepatic I/R model. Furthermore, the time course of CRP and IgM depositions was significantly similar to that of activated complement depositions.

Molecular determinants of sterile inflammation

Necrotic cell death alerts the acquired immune system to activate naïve T cells even in the absence of non-self derived molecules (e.g. pathogens). In addition, sterile necrosis leads to innate immune-mediated acute inflammation. The dying cells still represent a threat to the body that should be eliminated by the host immune response. Although the inflammatory response plays important roles in protecting the host and repairing tissues, it can also cause the collateral damage to normal tissues that underlies disease pathogenesis. Tissue resident macrophages recognize the danger signals released from necrotic cells via the pattern recognition receptors and secrete IL-1 that results in acute neutrophilic inflammation. This article will review our current knowledge especially focusing on the role of IL-1 in the sterile necrotic cell death induced inflammation.

The role of complement in tumor growth

Complement is a central part of the immune system that has developed as a first defense against non-self cells. Neoplastic transformation is accompanied by an increased capacity of the malignant cells to activate complement. In fact, clinical data demonstrate complement activation in cancer patients. On the basis of the use of protective mechanisms by malignant cells, complement activation has traditionally been considered part of the body's immunosurveillance against cancer. Inhibitory mechanisms of complement activation allow cancer cells to escape from complement-mediated elimination and hamper the clinical efficacy of monoclonal antibody-based cancer immunotherapies. To overcome this limitation, many strategies have been developed with the goal of improving complement-mediated effector mechanisms. However, significant work in recent years has identified new and surprising roles for complement activation within the tumor microenvironment. Recent reports suggest that complement elements can promote tumor growth in the context of chronic inflammation. This chapter reviews the data describing the role of complement activation in cancer immunity, which offers insights that may aid the development of more effective therapeutic approaches to control cancer.

Activation of the lectin pathway of complement in pig-to-human xenotransplantation models

BACKGROUND

Natural IgM containing anti-Gal antibodies initiates classic pathway complement activation in xenotransplantation. However, in ischemia-reperfusion injury, IgM also induces lectin pathway activation. The present study was therefore focused on lectin pathway as well as interaction of IgM and mannose-binding lectin (MBL) in pig-to-human xenotransplantation models.

METHODS

Activation of the different complement pathways was assessed by cell enzyme-linked immunosorbent assay using human serum on wild-type (WT) and α-galactosyl transferase knockout (GalTKO)/hCD46-transgenic porcine aortic endothelial cells (PAEC). Colocalization of MBL/MASP2 with IgM, C3b/c, C4b/c, and C6 was investigated by immunofluorescence in vitro on PAEC and ex vivo in pig leg xenoperfusion with human blood. Influence of IgM on MBL binding to PAEC was tested using IgM depleted/repleted and anti-Gal immunoabsorbed serum.

RESULTS

Activation of all the three complement pathways was observed in vitro as indicated by IgM, C1q, MBL, and factor Bb deposition on WT PAEC. MBL deposition colocalized with MASP2 (Manders' coefficient [3D] r=0.93), C3b/c (r=0.84), C4b/c (r=0.86), and C6 (r=0.80). IgM colocalized with MBL (r=0.87) and MASP2 (r=0.83). Human IgM led to dose-dependently increased deposition of MBL, C3b/c, and C6 on WT PAEC. Colocalization of MBL with IgM (Pearson's coefficient [2D] rp=0.88), C3b/c (rp=0.82), C4b/c (rp=0.63), and C6 (rp=0.81) was also seen in ex vivo xenoperfusion. Significantly reduced MBL deposition and complement activation was observed on GalTKO/hCD46-PAEC.

CONCLUSION

Colocalization of MBL/MASP2 with IgM and complement suggests that the lectin pathway is activated by human anti-Gal IgM and may play a pathophysiologic role in pig-to-human xenotransplantation.

Impact of mannose-binding lectin deficiency on radiocontrast-induced renal dysfunction

Contrast-induced nephropathy (CIN) is the third leading cause of acute renal failure in hospitalized patients. Endothelial dysfunction, renal medullary ischemia, and tubular toxicity are regarded as the most important factors in the pathogenesis of CIN. Mannose-binding lectin (MBL), a pattern recognition protein of the lectin pathway of complement, has been found to aggravate and mediate tissue damage during experimental renal ischemia/reperfusion (I/R) injury which was alleviated by inhibition with C1 inhibitor, a potent MBL, and lectin pathway inhibitor. In this paper, we highlight the potential role of MBL in the pathogenesis of human CIN. In experimental I/R models, MBL was previously found to induce tubular cell death independent of the complement system. In addition, after binding to vascular endothelial cells, MBL and its associated serine proteases were able to trigger a proinflammatory reaction and contribute to endothelial dysfunction. In humans, urinary MBL was increased after administration of contrast media and in individuals with CIN. Moreover, individuals with normal/high MBL levels were at increased risk to develop radiocontrast-induced renal dysfunction. Hence, MBL and the lectin pathway seem to be a promising target given that a licensed, powerful, human recombinant inhibitor exits to be added to the scarce armamentarium currently available for prophylaxis of CIN.

Application of a hemolysis assay for analysis of complement activation by perfluorocarbon nanoparticles

Nanoparticles offer new options for medical diagnosis and therapeutics with their capacity to specifically target cells and tissues with imaging agents and/or drug payloads. The unique physical aspects of nanoparticles present new challenges for this promising technology. Studies indicate that nanoparticles often elicit moderate to severe complement activation. Using human in vitro assays that corroborated the mouse in vivo results we previously presented mechanistic studies that define the pathway and key components involved in modulating complement interactions with several gadolinium-functionalized perfluorocarbon nanoparticles (PFOB). Here we employ a modified in vitro hemolysis-based assay developed in conjunction with the mouse in vivo model to broaden our analysis to include PFOBs of varying size, charge and surface chemistry and examine the variations in nanoparticle-mediated complement activity between individuals. This approach may provide the tools for an in-depth structure-activity relationship study that will guide the eventual development of biocompatible nanoparticles.

FROM THE CLINICAL EDITOR

Unique physical aspects of nanoparticles may lead to moderate to severe complement activation in vivo, which represents a challenge to clinical applicability. In order to guide the eventual development of biocompatible nanoparticles, this team of authors report a modified in vitro hemolysis-based assay developed in conjunction with their previously presented mouse model to enable in-depth structure-activity relationship studies.

B lymphocytes trigger monocyte mobilization and impair heart function after acute myocardial infarction

Acute myocardial infarction is a severe ischemic disease responsible for heart failure and sudden death. Here, we show that after acute myocardial infarction in mice, mature B lymphocytes selectively produce Ccl7 and induce Ly6C(hi) monocyte mobilization and recruitment to the heart, leading to enhanced tissue injury and deterioration of myocardial function. Genetic (Baff receptor deficiency) or antibody-mediated (CD20- or Baff-specific antibody) depletion of mature B lymphocytes impeded Ccl7 production and monocyte mobilization, limited myocardial injury and improved heart function. These effects were recapitulated in mice with B cell-selective Ccl7 deficiency. We also show that high circulating concentrations of CCL7 and BAFF in patients with acute myocardial infarction predict increased risk of death or recurrent myocardial infarction. This work identifies a crucial interaction between mature B lymphocytes and monocytes after acute myocardial ischemia and identifies new therapeutic targets for acute myocardial infarction.

Emerging role of the mannose-binding lectin-dependent pathway of complement activation in clinical organ transplantation

PURPOSE OF REVIEW

Over the past decade, the role of the complement system in solid organ transplantation has received increased attention. A number of experimental and epidemiological studies have suggested that the lectin pathway plays a role in infectious complications, rejection and long-term outcome after transplantation. This review discusses recent data on the role of the lectin pathway in solid organ transplantation.

RECENT FINDINGS

Studies on the role of mannose-binding lectin (MBL) in organ transplantation have shown an association of MBL-deficient states with an increased risk of infection after liver and simultaneous pancreas-kidney transplantation. On the contrary, a high MBL status in the recipient has been associated with poorer organ survival and increased rejection associated damage in various transplant settings. Experimental data points towards a role for MBL in ischemia-reperfusion damage in various organs. Several lines of evidence suggest that MBL may contribute to immunoglobulin-mediated complement activation in both ischemia-reperfusion and rejection. The interaction of MBL with IgM may be of particular importance in this setting.

SUMMARY

We review recent epidemiological data on the role of MBL in solid organ transplantation. We relate these findings to the emerging experimental data and attempt to explain some of the conflicting results on beneficial and harmful effects of the lectin pathway.