Inhibition of the Lectin Pathway of Complement Activation Reduces Acute Respiratory Distress Syndrome Severity in a Mouse Model of SARS-CoV-2 Infection

Most patients with COVID-19 in the intensive care unit develop an acute respiratory distress syndrome characterized by severe hypoxemia, decreased lung compliance, and high vascular permeability. Activation of the complement system is a hallmark of moderate and severe COVID-19, with abundant deposition of complement proteins in inflamed tissue and on the endothelium during COVID-19. Using a transgenic mouse model of SARS-CoV-2 infection, we assessed the therapeutic utility of an inhibitory antibody (HG4) targeting MASP-2, a key enzyme in the lectin pathway. Treatment of infected mice with HG4 reduced the disease severity score and improved survival vs mice that received an isotype control antibody. Administration of HG4 significantly reduced the lung injury score, including alveolar inflammatory cell infiltration, alveolar edema, and alveolar hemorrhage. The ameliorating effect of MASP-2 inhibition on the severity of COVID-19 pathology is reflected by a significant reduction in the proinflammatory activation of brain microglia in HG4-treated mice.

Multiorgan MRI findings after hospitalisation with COVID-19 in the UK (C-MORE): a prospective, multicentre, observational cohort study

INTRODUCTION

The multiorgan impact of moderate to severe coronavirus infections in the post-acute phase is still poorly understood. We aimed to evaluate the excess burden of multiorgan abnormalities after hospitalisation with COVID-19, evaluate their determinants, and explore associations with patient-related outcome measures.

METHODS

In a prospective, UK-wide, multicentre MRI follow-up study (C-MORE), adults (aged ≥18 years) discharged from hospital following COVID-19 who were included in Tier 2 of the Post-hospitalisation COVID-19 study (PHOSP-COVID) and contemporary controls with no evidence of previous COVID-19 (SARS-CoV-2 nucleocapsid antibody negative) underwent multiorgan MRI (lungs, heart, brain, liver, and kidneys) with quantitative and qualitative assessment of images and clinical adjudication when relevant. Individuals with end-stage renal failure or contraindications to MRI were excluded. Participants also underwent detailed recording of symptoms, and physiological and biochemical tests. The primary outcome was the excess burden of multiorgan abnormalities (two or more organs) relative to controls, with further adjustments for potential confounders. The C-MORE study is ongoing and is registered with ClinicalTrials.gov, NCT04510025.

FINDINGS

Of 2710 participants in Tier 2 of PHOSP-COVID, 531 were recruited across 13 UK-wide C-MORE sites. After exclusions, 259 C-MORE patients (mean age 57 years [SD 12]; 158 [61%] male and 101 [39%] female) who were discharged from hospital with PCR-confirmed or clinically diagnosed COVID-19 between March 1, 2020, and Nov 1, 2021, and 52 non-COVID-19 controls from the community (mean age 49 years [SD 14]; 30 [58%] male and 22 [42%] female) were included in the analysis. Patients were assessed at a median of 5·0 months (IQR 4·2-6·3) after hospital discharge. Compared with non-COVID-19 controls, patients were older, living with more obesity, and had more comorbidities. Multiorgan abnormalities on MRI were more frequent in patients than in controls (157 [61%] of 259 vs 14 [27%] of 52; p<0·0001) and independently associated with COVID-19 status (odds ratio [OR] 2·9 [95% CI 1·5-5·8]; padjusted=0·0023) after adjusting for relevant confounders. Compared with controls, patients were more likely to have MRI evidence of lung abnormalities (p=0·0001; parenchymal abnormalities), brain abnormalities (p<0·0001; more white matter hyperintensities and regional brain volume reduction), and kidney abnormalities (p=0·014; lower medullary T1 and loss of corticomedullary differentiation), whereas cardiac and liver MRI abnormalities were similar between patients and controls. Patients with multiorgan abnormalities were older (difference in mean age 7 years [95% CI 4-10]; mean age of 59·8 years [SD 11·7] with multiorgan abnormalities vs mean age of 52·8 years [11·9] without multiorgan abnormalities; p<0·0001), more likely to have three or more comorbidities (OR 2·47 [1·32-4·82]; padjusted=0·0059), and more likely to have a more severe acute infection (acute CRP >5mg/L, OR 3·55 [1·23-11·88]; padjusted=0·025) than those without multiorgan abnormalities. Presence of lung MRI abnormalities was associated with a two-fold higher risk of chest tightness, and multiorgan MRI abnormalities were associated with severe and very severe persistent physical and mental health impairment (PHOSP-COVID symptom clusters) after hospitalisation.

INTERPRETATION

After hospitalisation for COVID-19, people are at risk of multiorgan abnormalities in the medium term. Our findings emphasise the need for proactive multidisciplinary care pathways, with the potential for imaging to guide surveillance frequency and therapeutic stratification.

FUNDING

UK Research and Innovation and National Institute for Health Research.

Clinical characteristics with inflammation profiling of long COVID and association with 1-year recovery following hospitalisation in the UK: a prospective observational study

BACKGROUND

No effective pharmacological or non-pharmacological interventions exist for patients with long COVID. We aimed to describe recovery 1 year after hospital discharge for COVID-19, identify factors associated with patient-perceived recovery, and identify potential therapeutic targets by describing the underlying inflammatory profiles of the previously described recovery clusters at 5 months after hospital discharge.

METHODS

The Post-hospitalisation COVID-19 study (PHOSP-COVID) is a prospective, longitudinal cohort study recruiting adults (aged ≥18 years) discharged from hospital with COVID-19 across the UK. Recovery was assessed using patient-reported outcome measures, physical performance, and organ function at 5 months and 1 year after hospital discharge, and stratified by both patient-perceived recovery and recovery cluster. Hierarchical logistic regression modelling was performed for patient-perceived recovery at 1 year. Cluster analysis was done using the clustering large applications k-medoids approach using clinical outcomes at 5 months. Inflammatory protein profiling was analysed from plasma at the 5-month visit. This study is registered on the ISRCTN Registry, ISRCTN10980107, and recruitment is ongoing.

FINDINGS

2320 participants discharged from hospital between March 7, 2020, and April 18, 2021, were assessed at 5 months after discharge and 807 (32·7%) participants completed both the 5-month and 1-year visits. 279 (35·6%) of these 807 patients were women and 505 (64·4%) were men, with a mean age of 58·7 (SD 12·5) years, and 224 (27·8%) had received invasive mechanical ventilation (WHO class 7-9). The proportion of patients reporting full recovery was unchanged between 5 months (501 [25·5%] of 1965) and 1 year (232 [28·9%] of 804). Factors associated with being less likely to report full recovery at 1 year were female sex (odds ratio 0·68 [95% CI 0·46-0·99]), obesity (0·50 [0·34-0·74]) and invasive mechanical ventilation (0·42 [0·23-0·76]). Cluster analysis (n=1636) corroborated the previously reported four clusters: very severe, severe, moderate with cognitive impairment, and mild, relating to the severity of physical health, mental health, and cognitive impairment at 5 months. We found increased inflammatory mediators of tissue damage and repair in both the very severe and the moderate with cognitive impairment clusters compared with the mild cluster, including IL-6 concentration, which was increased in both comparisons (n=626 participants). We found a substantial deficit in median EQ-5D-5L utility index from before COVID-19 (retrospective assessment; 0·88 [IQR 0·74-1·00]), at 5 months (0·74 [0·64-0·88]) to 1 year (0·75 [0·62-0·88]), with minimal improvements across all outcome measures at 1 year after discharge in the whole cohort and within each of the four clusters.

INTERPRETATION

The sequelae of a hospital admission with COVID-19 were substantial 1 year after discharge across a range of health domains, with the minority in our cohort feeling fully recovered. Patient-perceived health-related quality of life was reduced at 1 year compared with before hospital admission. Systematic inflammation and obesity are potential treatable traits that warrant further investigation in clinical trials.

FUNDING

UK Research and Innovation and National Institute for Health Research.

Role of the lectin pathway of complement in hematopoietic stem cell transplantation-associated endothelial injury and thrombotic microangiopathy

Hematopoietic stem cell transplantation-associated thrombotic microangiopathy (HSCT-TMA) is a life-threatening syndrome that occurs in adult and pediatric patients after hematopoietic stem cell transplantation. Nonspecific symptoms, heterogeneity within study populations, and variability among current diagnostic criteria contribute to misdiagnosis and underdiagnosis of this syndrome. Hematopoietic stem cell transplantation and associated risk factors precipitate endothelial injury, leading to HSCT-TMA and other endothelial injury syndromes such as hepatic veno-occlusive disease/sinusoidal obstruction syndrome, idiopathic pneumonia syndrome, diffuse alveolar hemorrhage, capillary leak syndrome, and graft-versus-host disease. Endothelial injury can trigger activation of the complement system, promoting inflammation and the development of endothelial injury syndromes, ultimately leading to organ damage and failure. In particular, the lectin pathway of complement is activated by damage-associated molecular patterns (DAMPs) on the surface of injured endothelial cells. Pattern-recognition molecules such as mannose-binding lectin (MBL), collectins, and ficolins—collectively termed lectins—bind to DAMPs on injured host cells, forming activation complexes with MBL-associated serine proteases 1, 2, and 3 (MASP-1, MASP-2, and MASP-3). Activation of the lectin pathway may also trigger the coagulation cascade via MASP-2 cleavage of prothrombin to thrombin. Together, activation of complement and the coagulation cascade lead to a procoagulant state that may result in development of HSCT-TMA. Several complement inhibitors targeting various complement pathways are in clinical trials for the treatment of HSCT-TMA. In this article, we review the role of the complement system in HSCT-TMA pathogenesis, with a focus on the lectin pathway.

Analysis of Serological Biomarkers of SARS-CoV-2 Infection in Convalescent Samples From Severe, Moderate and Mild COVID-19 Cases

Precision monitoring of antibody responses during the COVID-19 pandemic is increasingly important during large scale vaccine rollout and rise in prevalence of Severe Acute Respiratory Syndrome-related Coronavirus-2 (SARS-CoV-2) variants of concern (VOC). Equally important is defining Correlates of Protection (CoP) for SARS-CoV-2 infection and COVID-19 disease. Data from epidemiological studies and vaccine trials identified virus neutralising antibodies (Nab) and SARS-CoV-2 antigen-specific (notably RBD and S) binding antibodies as candidate CoP. In this study, we used the World Health Organisation (WHO) international standard to benchmark neutralising antibody responses and a large panel of binding antibody assays to compare convalescent sera obtained from: a) COVID-19 patients; b) SARS-CoV-2 seropositive healthcare workers (HCW) and c) seronegative HCW. The ultimate aim of this study is to identify biomarkers of humoral immunity that could be used to differentiate severe from mild or asymptomatic SARS-CoV-2 infections. Some of these biomarkers could be used to define CoP in further serological studies using samples from vaccination breakthrough and/or re-infection cases. Whenever suitable, the antibody levels of the samples studied were expressed in International Units (IU) for virus neutralisation assays or in Binding Antibody Units (BAU) for ELISA tests. In this work we used commercial and non-commercial antibody binding assays; a lateral flow test for detection of SARS-CoV-2-specific IgG/IgM; a high throughput multiplexed particle flow cytometry assay for SARS-CoV-2 Spike (S), Nucleocapsid (N) and Receptor Binding Domain (RBD) proteins); a multiplex antigen semi-automated immuno-blotting assay measuring IgM, IgA and IgG; a pseudotyped microneutralisation test (pMN) and an electroporation-dependent neutralisation assay (EDNA). Our results indicate that overall, severe COVID-19 patients showed statistically significantly higher levels of SARS-CoV-2-specific neutralising antibodies (average 1029 IU/ml) than those observed in seropositive HCW with mild or asymptomatic infections (379 IU/ml) and that clinical severity scoring, based on WHO guidelines was tightly correlated with neutralisation and RBD/S antibodies. In addition, there was a positive correlation between severity, N-antibody assays and intracellular virus neutralisation.

The Contribution of Complement to the Pathogenesis of IgA Nephropathy: Are Complement-Targeted Therapies Moving from Rare Disorders to More Common Diseases?

Primary IgA nephropathy (IgAN) is a leading cause of chronic kidney disease and kidney failure for which there is no disease-specific treatment. However, this could change, since novel therapeutic approaches are currently being assessed in clinical trials, including complement-targeting therapies. An improved understanding of the role of the lectin and the alternative pathway of complement in the pathophysiology of IgAN has led to the development of these treatment strategies. Recently, in a phase 2 trial, treatment with a blocking antibody against mannose-binding protein-associated serine protease 2 (MASP-2, a crucial enzyme of the lectin pathway) was suggested to have a potential benefit for IgAN. Now in a phase 3 study, this MASP-2 inhibitor for the treatment of IgAN could mark the start of a new era of complement therapeutics where common diseases can be treated with these drugs. The clinical development of complement inhibitors requires a better understanding by physicians of the biology of complement, the pathogenic role of complement in IgAN, and complement-targeted therapies. The purpose of this review is to provide an overview of the role of complement in IgAN, including the recent discovery of new mechanisms of complement activation and opportunities for complement inhibitors as the treatment of IgAN.

Enterococcus faecalis Escapes Complement-Mediated Killing via Recruitment of Complement Factor H

BACKGROUND

Enterococcus faecalis is considered to be the most important species of enterococci responsible for blood stream infections in critically ill patients. In blood, the complement system is activated via the classical pathway (CP), the lectin pathway (LP), or the alternative pathway (AP), and it plays a critical role in opsonophagocytosis of bacteria including E faecalis.

METHODS

In a mouse model of enterococcus peritonitis, BALB-C mice were challenged with a high dose of E faecalis 12 hours after intraperitoneal administration of anti-Factor H (FH) antibodies or isotype control. Four hours later, control mice developed higher bacterial burden in blood and organs compared with mice treated with anti-FH antibodies.

RESULTS

We demonstrate that complement recognition molecules C1q, CL-11, and murine ficolin-A bind the enterococcus and drive the CP and the LP in human and mouse. We further describe that E faecalis evades the AP by recruitment of FH on its surface. Our results show a strong C3b deposition on E faecalis via both the CP and the LP but not through the AP.

CONCLUSIONS

These findings indicate that E faecalis avoids the complement phagocytosis by the AP via sequestering complement FH from the host blood.

Cardioprotection by an anti-MASP-2 antibody in a murine model of myocardial infarction

Background

Myocardial ischaemia-reperfusion injury is a major cause of mortality and morbidity in the developed world. Many approaches have been investigated to counteract the pathological consequences associated with acute myocardial infarction (AMI) and cardiac remodelling. It is accepted that inflammation, and therefore activation of the complement pathway, is a crucial step in the pathogenesis of this injury, and many attempts have been made to ameliorate the infarction and consequent dysfunction using anticomplement therapy, with mixed success. Recently, the lectin complement activation pathway involving the mannose-binding lectin-associated serine protease 2 (MASP-2) has been shown to be an important mediator of the inflammatory response in ischaemia/reperfusion injury in the heart. In this study, therefore, we aimed to investigate the feasibility of using monoclonal antibodies raised against MASP-2 in a murine model of AMI.

Methods

Mice were injected with anti-MASP-2 antibody or control 18 hours prior to experimental infarction by ligation of the left anterior descending coronary artery for 30 min followed by 120 min reperfusion. The developed infarct was measured, and blood was collected for analysis of lectin pathway functional activity.

Results and conclusions

We found that mice treated with anti-MASP-2 antibody had smaller infarcts than those treated with control antibody. We believe this may represent a valuable step forward in the protection of the myocardium against ischaemia-reperfusion injury.

Mannan binding lectin-associated serine protease-2 (MASP-2) critically contributes to post-ischemic brain injury independent of MASP-1

BACKGROUND

Complement activation via the lectin activation pathway (LP) has been identified as the key mechanism behind post-ischemic tissue inflammation causing ischemia-reperfusion injury (IRI) which can significantly impact the clinical outcome of ischemic disease. This work defines the contributions of each of the three LP-associated enzymes-mannan-binding lectin-associated serine protease (MASP)-1, MASP-2, and MASP-3-to ischemic brain injury in experimental mouse models of stroke.

METHODS

Focal cerebral ischemia was induced in wild-type (WT) mice or mice deficient for defined complement components by transient middle cerebral artery occlusion (tMCAO) or three-vessel occlusion (3VO). The inhibitory MASP-2 antibody was administered systemically 7 and 3.5 days before and at reperfusion in WT mice in order to assure an effective MASP-2 inhibition throughout the study. Forty-eight hours after ischemia, neurological deficits and infarct volumes were assessed. C3 deposition and microglia/macrophage morphology were detected by immunohistochemical, immunofluorescence, and confocal analyses.

RESULTS

MASP-2-deficient mice (MASP-2(-/-)) and WT mice treated with an antibody that blocks MASP-2 activity had significantly reduced neurological deficits and histopathological damage after transient ischemia and reperfusion compared to WT or control-treated mice. Surprisingly, MASP-1/3(-/-) mice were not protected, while mice deficient in factor B (fB(-/-)) showed reduced neurological deficits compared to WT mice. Consistent with behavioral and histological data, MASP-2(-/-) had attenuated C3 deposition and presented with a significantly higher proportion of ramified, surveying microglia in contrast to the hypertrophic pro-inflammatory microglia/macrophage phenotype seen in the ischemic brain tissue of WT mice.

CONCLUSIONS

This work demonstrates the essential role of the low-abundant MASP-2 in the mediation of cerebral ischemia-reperfusion injury and demonstrates that targeting MASP-2 by an inhibitory therapeutic antibody markedly improved the neurological and histopathological outcome after focal cerebral ischemia. These results contribute to identifying the key lectin pathway component driving brain tissue injury following cerebral ischemia and call for a revision of the presently widely accepted view that MASP-1 is an essential activator of the lectin pathway effector component MASP-2.

Collectin-11 detects stress-induced L-fucose pattern to trigger renal epithelial injury

Physiochemical stress induces tissue injury as a result of the detection of abnormal molecular patterns by sensory molecules of the innate immune system. Here, we have described how the recently discovered C-type lectin collectin-11 (CL-11, also known as CL-K1 and encoded by COLEC11) recognizes an abnormal pattern of L-fucose on postischemic renal tubule cells and activates a destructive inflammatory response. We found that intrarenal expression of CL-11 rapidly increases in the postischemic period and colocalizes with complement deposited along the basolateral surface of the proximal renal tubule in association with L-fucose, the potential binding ligand for CL-11. Mice with either generalized or kidney-specific deficiency of CL-11 were strongly protected against loss of renal function and tubule injury due to reduced complement deposition. Ex vivo renal tubule cells showed a marked capacity for CL-11 binding that was induced by cell stress under hypoxic or hypothermic conditions and prevented by specific removal of L-fucose. Further analysis revealed that cell-bound CL-11 required the lectin complement pathway-associated protease MASP-2 to trigger complement deposition. Given these results, we conclude that lectin complement pathway activation triggered by ligand-CL-11 interaction in postischemic tissue is a potent source of acute kidney injury and is amenable to sugar-specific blockade.

Complement factor H in its alternative identity as adrenomedullin-binding protein 1

Complement factor H has been extensively studied since its discovery 50 years ago, and its role in the complement system is quite well established. It has another role, however, as a binding protein for the regulatory peptide adrenomedullin. Part of this role appears to be protection of adrenomedullin from proteolytic degradation. The binding interaction is unusual and merits further investigation. Adrenomedullin has potential therapeutic uses in diseases affecting the vasculature, and factor H has been administered with adrenomedullin in some animal models of disease.

Further studies of the down-regulation by Factor I of the C3b feedback cycle using endotoxin as a soluble activator and red cells as a source of CR1 on sera of different complotype

In this paper we have extended our earlier studies of the action of increasing Factor I concentration on complement activation by using a soluble activator, lipopolysaccharide (LPS) endotoxin, and using human erythrocytes as a source of CR1 - the co-factor needed for the final clip of iC3b to C3dg by Factor I. Using this more physiological system, the results show that we can predict that a quite modest increase in Factor I concentration - 22 µg/ml of extra Factor I - will convert the activity of the highest risk sera to those of the lowest risk. Preliminary experiments have been performed with erythrocytes allotyped for CR1 number. While we have not been able to perform an adequate study of their co-factor activities in our assays, preliminary experiments suggest that when Factor I levels are increased the difference produced by different allotypes of red cells is largely overcome. This suggests that in patients with paroxysmal nocturnal haemoglobinuria (PNH) treated with eculizumab, additional treatment with Factor I may be very useful in reducing the need for blood transfusion. We have also explored the age-related allele frequency for the two polymorphisms of Factor H and the polymorphism of C3. In our population, unlike the 1975 study, we found no age variation in the allele frequency in these polymorphisms. This may, however, reflect that the Cambridge BioResource volunteers do not include many very young or very elderly patients, and in general comprise a population not greatly at risk of death from infectious disease.

Complotype affects the extent of down-regulation by Factor I of the C3b feedback cycle in vitro

Sera from a large panel of normal subjects were typed for three common polymorphisms, one in C3 (R102G) and two in Factor H (V62I and Y402H), that influence predisposition to age-related macular degeneration and to some forms of kidney disease. Three groups of sera were tested; those that were homozygous for the three risk alleles; those that were heterozygous for all three; and those homozygous for the low-risk alleles. These groups vary in their response to the addition of exogenous Factor I when the alternative complement pathway is activated by zymosan. Both the reduction in the maximum amount of iC3b formed and the rate at which the iC3b is converted to C3dg are affected. For both reactions the at-risk complotype requires higher doses of Factor I to produce similar down-regulation. Because iC3b reacting with the complement receptor CR3 is a major mechanism by which complement activation gives rise to inflammation, the breakdown of iC3b to C3dg can be seen to have major significance for reducing complement-induced inflammation. These findings demonstrate for the first time that sera from subjects with different complement alleles behave as predicted in an in-vitro assay of the down-regulation of the alternative complement pathway by increasing the concentration of Factor I. These results support the hypothesis that exogenous Factor I may be a valuable therapeutic aid for down-regulating hyperactivity of the C3b feedback cycle, thereby providing a treatment for age-related macular degeneration and other inflammatory diseases of later life.

Trypanosoma cruzi calreticulin inhibits the complement lectin pathway activation by direct interaction with L-Ficolin

Trypanosoma cruzi, the agent of Chagas' disease, the sixth neglected tropical disease worldwide, infects 10-12 million people in Latin America. Differently from T. cruzi epimastigotes, trypomastigotes are complement-resistant and infective. CRPs, T-DAF, sialic acid and lipases explain at least part of this resistance. In vitro, T. cruzi calreticulin (TcCRT), a chaperone molecule that translocates from the ER to the parasite surface: (a) Inhibits the human classical complement activation, by interacting with C1, (b) As a consequence, an increase in infectivity is evident and, (c) It inhibits angiogenesis and tumor growth. We report here that TcCRT also binds to the L-Ficolin collagenous portion, thus inhibiting approximately between 35 and 64% of the human complement lectin pathway activation, initiated by L-Ficolin, a property not shared by H-Ficolin. While L-Ficolin binds to 60% of trypomastigotes and to 24% of epimastigotes, 50% of the former and 4% of the latter display TcCRT on their surfaces. Altogether, these data indicate that TcCRT is a parasite inhibitory receptor for Ficolins. The resulting evasive activities, together with the TcCRT capacity to inhibit C1, with a concomitant increase in infectivity, may represent T. cruzi strategies to inhibit important arms of the innate immune response.

Septicaemia models using Streptococcus pneumoniae and Listeria monocytogenes: understanding the role of complement properdin

Streptococcus pneumoniae and Listeria monocytogenes, pathogens which can cause severe infectious disease in human, were used to infect properdin-deficient and wildtype mice. The aim was to deduce a role for properdin, positive regulator of the alternative pathway of complement activation, by comparing and contrasting the immune response of the two genotypes in vivo. We show that properdin-deficient and wildtype mice mounted antipneumococcal serotype-specific IgM antibodies, which were protective. Properdin-deficient mice, however, had increased survival in the model of streptococcal pneumonia and sepsis. Low activity of the classical pathway of complement and modulation of FcγR2b expression appear to be pathogenically involved. In listeriosis, however, properdin-deficient mice had reduced survival and a dendritic cell population that was impaired in maturation and activity. In vitro analyses of splenocytes and bone marrow-derived myeloid cells support the view that the opposing outcomes of properdin-deficient and wildtype mice in these two infection models is likely to be due to a skewing of macrophage activity to an M2 phenotype in the properdin-deficient mice. The phenotypes observed thus appear to reflect the extent to which M2- or M1-polarised macrophages are involved in the immune responses to S. pneumoniae and L. monocytogenes. We conclude that properdin controls the strength of immune responses by affecting humoral as well as cellular phenotypes during acute bacterial infection and ensuing inflammation.

Abrogated RANKL expression in properdin-deficient mice is associated with better outcome from collagen-antibody-induced arthritis

Introduction

Properdin amplifies the alternative pathway of complement activation. In the present study, we evaluated its role in the development of collagen antibody-induced arthritis (CAIA).

Methods

Arthritis was induced by intraperitoneal injection of a collagen antibody cocktail into properdin-deficient (KO) and wild-type (WT) C57BL/6 mice. Symptoms of disease were evaluated daily. The degree of joint damage was assessed histologically and with immunostaining for bone-resorption markers. Phenotypes of cell populations, their receptor expression, and intracellular cytokine production were determined with flow cytometry. Osteoclast differentiation of bone marrow (BM) precursors was evaluated by staining for tartrate-resistant acid phosphatase (TRAP).

Results

Properdin-deficient mice developed less severe CAIA than did WT mice. They showed significantly improved clinical scores and downregulated expression of bone-resorption markers in the joints at day 10 of disease. The frequencies of Ly6G⁺CD11b⁺ cells were fewer in BM, blood, and synovial fluid (SF) of KO than of WT CAIA mice. The receptor activator of nuclear factor κB ligand (RANKL) was downregulated on arthritic KO neutrophils from BM and the periphery. Decreased C5a amounts in KO SF contributed to lower frequencies of CD5aR⁺-bearing neutrophils. In blood, surface C5aR was detected on KO Ly6G⁺ cells as a result of low receptor engagement. Circulating CD4⁺ T cells had an altered ability to produce interleukin (IL)-17 and interferon (IFN)-γ and to express RANKL. In KO CAIA mice, decreased frequencies of CD4⁺ T cells in the spleen were related to low CD86 expression on Ly6G^highCD11b⁺ cells. Arthritic KO T cells spontaneously secreted IFN-γ but not IL-17 and IL-6, and responded to restimulation with less-vigorous cytokine production in comparison to WT cells. Fewer TRAP-positive mature osteoclasts were found in KO BM cell cultures.

Conclusions

Our data show that the active involvement of properdin in arthritis is related to an increased proinflammatory cytokine production and RANKL expression on immune cells and to a stimulation of the RANKL-dependent osteoclast differentiation.

Complement Factor MASP-2 is activated after acute myocardial ischemia in human (116.28)

Acute blockage of coronary artery results in myocardial injury manifested in clinical situations such as myocardial infarction (MI). Following ischemia, inflammation is provoked and may be related to the clinical outcomes. We studied the acute inflammatory response immediately following myocardial ischemia in human. Our results showed that in acute MI patients circulation levels of complement factor MASP-2 were significantly reduced comparing with those of healthy individuals or of coronary artery disease (CAD) patients without acute MI. This reduction was not due to genetic polymorphism of MASP-2 in acute MI patients. We hypothesized that MASP-2 was activated after acute myocardial ischemia, and tested this in cardiac patients undergoing surgically induced transient global heart ischemia. Our results showed that MASP-2 was significantly reduced in coronary circulation after global heart ischemia and was independently correlated with the post-operative increase of the myocardial injury marker, cardiac Troponin I. These data suggested that MASP-2 is activated in human acute heart ischemia and associated with myocardial injury.

Growth temperature-dependent expression of structural variants of Listeria monocytogenes lipoteichoic acid

Investigating the expression of lipoteichoic acid (LTA) from Listeria monocytogenes, we found two distinct structural variants of LTA (LTA1 and LTA2) using NMR and MS technology. While both LTA consisted of a poly-glycerophosphate backbone (differing in length) bound via a disaccharide to a diacyl-glycerol moiety, one LTA type (LTA2) possessed a second diacyl-glycerol moiety linked to the disaccharide via a phosphodiester. As examined in vitro, LTA2 in contrast to LTA1 failed to activate the L-ficolin dependent pathway of complement. Most interestingly, growth temperature had a strong influence on the expression levels of LTA1 and LTA2 in the cell wall: while the amount of LTA1 was comparable, the expression of LTA2 was low when Listeria had grown at room temperature (ratio of LTA1 to LTA2 was 1:0.06), but increased when Listeria had been cultivated at 37°C (ratio of LTA1 to LTA2 was 1:0.68). The observed shift in LTA expression, probably accompanying the switch from the saprophytic to the virulent entity, indicates an important adaptation to the different structural requirements inside the host cells.

The role of properdin in murine zymosan-induced arthritis

Using properdin-deficient and wild-type mice, we have investigated the role of properdin in development and progression of zymosan-induced arthritis. At the initial phase of local, zymosan-induced inflammation, properdin-deficient and wild-type mice showed bone erosion, proteoglycan loss and cell infiltration. Compared to wild-type, properdin-deficient mice had reduced C5a and IL-6 but similar synovial TNF-alpha and sRANKL levels. Both groups showed a systemic immune response. Elevated IFN-gamma production and STAT1 signaling in splenocytes and a shift to Th1 response in popliteal lymph nodes were observed in properdin-deficient mice. Properdin-deficient mice had significantly less circulating zymosan-specific IgG antibodies than wild-type. In the chronic phase, the lack of properdin resulted in significant proteoglycan loss in the joints and lower cartilageneous STAT1 expression. The level of synovial C5a on day 30 was comparable in both groups, but C5aR staining was more apparent in the joints of properdin-deficient mice. Our data show that properdin is an important player in processes involved in inflammatory joint degradation.

C1q, the recognition subcomponent of the classical pathway of complement, drives microglial activation

Microglia, central nervous system (CNS) resident phagocytic cells, persistently police the integrity of CNS tissue and respond to any kind of damage or pathophysiological changes. These cells sense and rapidly respond to danger and inflammatory signals by changing their cell morphology; by release of cytokines, chemokines, or nitric oxide; and by changing their MHC expression profile. We have shown previously that microglial biosynthesis of the complement subcomponent C1q may serve as a reliable marker of microglial activation ranging from undetectable levels of C1q biosynthesis in resting microglia to abundant C1q expression in activated, nonramified microglia. In this study, we demonstrate that cultured microglial cells respond to extrinsic C1q with a marked intracellular Ca(2+) increase. A shift toward proinflammatory microglial activation is indicated by the release of interleukin-6, tumor necrosis factor-alpha, and nitric oxide and the oxidative burst in rat primary microglial cells, an activation and differentiation process similar to the proinflammatory response of microglia to exposure to lipopolysaccharide. Our findings indicate 1) that extrinsic plasma C1q is involved in the initiation of microglial activation in the course of CNS diseases with blood-brain barrier impairment and 2) that C1q synthesized and released by activated microglia is likely to contribute in an autocrine/paracrine way to maintain and balance microglial activation in the diseased CNS tissue.

Trypanosoma cruzi calreticulin: a possible role in Chagas' disease autoimmunity

Trypanosoma cruzi (T. cruzi) is the causative agent of Chagas' disease, an endemic and chronic illness that affects 18 million people in Latin America. The mechanisms underlying its pathogenesis are controversial. There is a growing body of evidence supporting the view that T. cruzi infection elicits severe autoimmune responses in the host, which significantly contribute to the pathogenesis of Chagas' disease, and several recent studies have reported the presence of autoantibodies and effector T lymphocytes against parasite and self antigens in infected patients and experimentally infected animals. T. cruzi calreticulin (TcCRT) is a 45kDa protein, immunogenic in humans, rabbits and mice. It has a high degree of homology with human (HuCRT) and mouse calreticulin (MoCRT), which would explain why an immune response to TcCRT could contribute to autoimmune reactions in Chagas' disease. Anti-TcCRT antibodies generated in A/J mice immunized with recombinant TcCRT (rTcCRT) reacted with rHuCRT and bound to neonatal and adult isogenic cardiomyocytes cultured in vitro. Interestingly, histological alterations, such as edema formation and cell infiltrates, which include CD3(+) cells, were detected in heart sections from immunized animals. Therefore, in rTcCRT-immunized mice, an autoimmune reaction against host CRT, paralleled by histological cardiac alterations, suggests a role of the parasite molecule in the induction of immunologically mediated heart tissue damage. The data presented here propose that TcCRT participates in the induction of cardiac autoimmunity in Chagas' disease.

Expression of M-Ficolin in human monocytes and macrophages

M-Ficolin is a member of the ficolin family of proteins, which is expressed by monocytes. We have determined the expression of this gene in various populations of this lineage in man and found lower levels of M-Ficolin mRNA in the more mature CD14(+)CD16(+) monocytes as compared to the classical CD14(++) monocytes. Monocyte-derived macrophages generated by in vitro culture for 5 days strongly reduced M-Ficolin mRNA and protein. Mature tissue macrophages from the lung and from breast milk also showed a very low level of M-Ficolin transcripts. When cells of the monocytic cell line Mono Mac 6 cell were treated with TLR2 and TLR4 ligands for 24 h then there was an average of 6- and 9-fold induction of the M-Ficolin mRNA, respectively. After 72 h induction was in average 30- and 80-fold for TLR2 and TLR4 stimulation, respectively. Treatment of monocyte-derived macrophages for 3 days with TLR4 ligand gave an average 4-fold induction and alveolar macrophages treated with TLR4 ligand showed a 12-fold induction. These data show that M-Ficolin expression is silenced in macrophages but can be re-activated after prolonged activation via TLRs.

Ficolins: novel pattern recognition molecules of the innate immune response

Ficolins are members of the collectin family of proteins which are able to recognize pathogen-associated molecular pattern (PAMP) on microbial surfaces. Upon binding to their specific PAMP, ficolins may trigger activation of the immune system by either binding to cellular receptors for collectins or by initiating activation of complement via the lectin pathway. For the latter, the human ficolins (i.e. L-, H- and M-ficolin) and murine ficolin-A were shown to associate with the lectin pathway-specific serine protease MBL-associated serine protease-2 (MASP-2) and catalyse its activation which in turn activates C4 and C4b-bound C2 to generate the C3 convertase C4b2a. There is mounting evidence underlining the lectin nature of ficolins with a wide range of carbohydrate moieties recognized on microbial surfaces. However, not all members of the ficolin family appear to act as lectin pathway recognition components. For example, murine ficolin-B does not associate with MASP-2 and appears to be absent in plasma and other humoral fluids. Its stringent cellular localization points to other functions within the immune response, possibly acting as an intracellular scavenger to target and facilitate clearance of PAMP-bearing debris. When comparing ficolin orthologues from different species, it appears evident that human, murine, and porcine ficolins differ in many aspects, a specific point that we aim to address in this review.

Stat3 is involved in control of MASP2 gene expression

Little is known about determinants regulating expression of Mannan-binding lectin associated serine protease-2 (MASP-2), the effector component of the lectin pathway of complement activation. Comparative bioinformatic analysis of the MASP2 promoter regions in human, mouse, and rat, revealed conservation of two putative Stat binding sites, termed StatA and StatB. Site directed mutagenesis specific for these sites was performed. Transcription activity was decreased 5-fold when StatB site was mutated in the wildtype reporter gene construct. Gel retardation and competition assays demonstrated that proteins contained in the nuclear extract prepared from HepG2 specifically bound double-stranded StatB oligonucleotides. Supershift analysis revealed Stat3 to be the major specific binding protein. We conclude that Stat3 binding is important for MASP2 promoter activity.

Functional MASP2 single nucleotide polymorphism plays no role in psoriasis

BACKGROUND

Psoriasis is a heritable disease and genome-wide scans have implicated several loci of susceptibility. The gene for MASP-2, a protease involved in complement activation, is located within one of these loci on chromosome 1p.

OBJECTIVES

To assess whether partial or total MASP-2 deficiency is a risk factor for developing psoriasis.

METHODS

We screened a cohort of patients affected by plaque psoriasis and their parents by restriction fragment length polymorphism analyses.

RESULTS

We detected a single nucleotide polymorphism that leads to an amino acid exchange, which results in dissociation of MASP-2 from a carbohydrate recognition complex.

CONCLUSIONS

We show that this mutant allele is not associated with psoriasis. There was no favoured transmission from parents to affected offspring. The calculated allele frequency in this psoriasis group (Scottish and English) was 0.0326, and in the unaffected group 0.0379.

An in vivo role for Trypanosoma cruzi calreticulin in antiangiogenesis

Angiogenesis leads to neovascularization from existing blood vessels. It is associated with tumor growth and metastasis and is regulated by pro- and antiangiogenic molecules, some of them currently under clinical trials for cancer treatment. During the last few years we have cloned, sequenced and expressed a Trypanosoma cruzi calreticulin gene (TcCRT). Its product, TcCRT, a 45 kDa protein, is more than 50% identical to human CRT (HuCRT). TcCRT, present on the surface of trypomastigotes, binds both C1q and mannan binding lectin and inhibits the classical activation pathway of human complement. Since TcCRT is highly homologous to a functional antiangiogenic fragment from HuCRT (aa 120-180), recombinant (r) and native (n) TcCRT were tested in their antiangiogenic effects, in the chick embryonic chorioallantoid membrane (CAM) assay. Both proteins mediated highly significant antiangiogenic effects in the in vivo CAM assay. This effect was further substantiated in experiments showing that the plasmid construct pSecTag/TcCRT also displayed significant antiangiogenic properties, as compared to the empty vector. Most likely, the fact that antiangiogenic substances act preferentially on growing neoplasic tissues, but not on already established tumors, is due to their effects on emerging blood vessels. The results shown here indicate that TcCRT, like its human counterpart, has antiangiogenic properties. These properties may explain, at least partly, the reported antineoplasic effect of experimental T. cruzi infection.

Does Trypanosoma cruzi calreticulin modulate the complement system and angiogenesis?

Calreticulin, a calcium-binding protein that is highly conserved in its multiple functions, is present in a wide spectrum of subcellular compartments in virtually every cell of higher organisms. In this article, we propose a dual role for parasite calreticulin, with emphasis on the Trypanosoma cruzi model. By modulating the vertebrate complement system, calreticulin might provide the parasite with an effective immune-escape mechanism. Alternatively, by inhibiting angiogenesis, the parasite molecule might protect the host from ongoing neoplasic aggressions. Many questions are still unanswered, particularly those regarding the consequences that these interactions could have in vivo for both the parasite and the host.

F(ab')2 antibody fragments against Trypanosoma cruzi calreticulin inhibit its interaction with the first component of human complement

Trypanosoma cruzi calreticulin (TcCRT), described in our laboratory, retains several important functional features from its vertebrate homologues. We have shown that recombinant TcCRT inhibits the human complement system when it binds to the collagenous portion of C1q. The generation of classical pathway convertases and membrane attack complexes is thus strongly inhibited. In most T. cruzi-infected individuals, TcCRT is immunogenic and mediates the generation of specific antibodies. By reverting the C1q / TcCRT interaction, a parasite immune evasion strategy, these antibodies contribute to the host/parasite equilibrium. In an in vitro correlate of this situation, we show that the Clq/TcCRT interaction is inhibited by F(ab')2 polyclonal anti-TcCRT IgG fragments. It is therefore feasible that in infected humans anti-TcCRT antibodies participate in reverting an important parasite strategy aimed at inhibiting the classical complement pathway. Thus, membrane-bound TcCRT interacts with the collagenous portion Clq, and this Clq is recognized by the CD91-bound host cell CRT, thus facilitating parasite internalization. Based on our in vitro results, it could be proposed that the in vivo interaction between TcCRT and vertebrate Clq could be inhibited by F(ab')2 fragments anti-rTcCRT or against its S functional domain, thus interfering with the internalization process.

Involvement of the lectin pathway of complement activation in antimicrobial immune defense during experimental septic peritonitis

A critical first line of defense against infection is constituted by the binding of natural antibodies to microbial surfaces, activating the complement system via the classical complement activation pathway. In this function, the classical activation pathway is supported and amplified by two antibody-independent complement activation routes, i.e., the lectin pathway and the alternative pathway. We studied the contribution of the different complement activation pathways in the host defense against experimental polymicrobial peritonitis induced by cecal ligation and puncture by using mice deficient in either C1q or factors B and C2. The C1q-deficient mice lack the classical complement activation pathway. While infection-induced mortality of wild-type mice was 27%, mortality of C1q-deficient mice was increased to 60%. Mice with a deficiency of both factors B and C2 lack complement activation via the classical, the alternative, and the lectin pathways and exhibit a mortality of 92%, indicating a significant contribution of the lectin and alternative pathways of complement activation to survival. For 14 days after infection, mannan-binding lectin (MBL)-dependent activation of C4 was compromised. Serum MBL-A and MBL-C levels were significantly reduced for 1 week, possibly due to consumption. mRNA expression profiles did not lend support for either of the two MBL genes to respond as typical acute-phase genes. Our results demonstrate a long-lasting depletion of MBL-A and MBL-C from serum during microbial infection and underline the importance of both the lectin and the alternative pathways for antimicrobial immune defense.

The classical activation pathway of the human complement system is specifically inhibited by calreticulin from Trypanosoma cruzi

The high resistance of Trypanosoma cruzi trypomastigotes, the causal agent of Chagas' disease, to complement involves several parasite strategies. In these in vitro studies, we show that T. cruzi calreticulin (TcCRT) and two subfragments thereof (TcCRT S and TcCRT R domains) bind specifically to recognition subcomponents of the classical and lectin activation pathways (i.e., to collagenous tails of C1q and to mannan-binding lectin) of the human complement system. As a consequence of this binding, specific functional inhibition of the classical pathway and impaired mannan-binding lectin to mannose were observed. By flow cytometry, TcCRT was detected on the surface of viable trypomastigotes and, by confocal microscopy, colocalization of human C1q with surface TcCRT of infective trypomastigotes was visualized. Taken together, these findings imply that TcCRT may be a critical factor contributing to the ability of trypomastigotes to interfere at the earliest stages of complement activation.

Role of calreticulin from parasites in its interaction with vertebrate hosts

Although parasites range from protozoan to complex, evolutionary advanced arthropods, in general, a hallmark of parasite life cycles is their ability to adapt to changes in temperature, pH and host defense strategies. Calreticulin, a calcium-binding protein, highly conserved and multifunctional, is present in every cell of higher organisms, except erythrocytes. The surprising array of calreticulin-associated functions include lectin-like chaperoning, calcium storage and signaling, modulation of gene expression, cell adhesion, enhancement of phagocytosis of C1q or collectin opsonized apoptotic cells, inhibition of angiogenesis and tumoral growth, inhibition of perforin pore formation in T and NK cells, and inhibition of C1q-dependent complement activation. Likewise, calreticulin is present in a wide spectrum of sub cellular compartments. Parasite calreticulin shows a surprisingly high degree of conservation within the framework of its functional domains. Its role within the parasite/host relationship needs to be assessed further, in particular with regard to its impact on parasite infectivity, by helping to evade from its hosts' immune response. With special emphasis on calreticulin from Trypanosoma cruzi, the intracellular protozoan agent of American trypanosomiasis (Chagas' disease), we wish to exemplify and highlight the various implications of parasite calreticulin, within the pathophysiology of parasite-mediated human and animal disease.

Molecular comparison of apocrine released and cytoplasmic resident carbonic anhydrase II

We have previously shown that carbonic anhydrase II usually described as a cytoplasmic resident isoform (cCAH II) is secreted by the rat coagulating gland (sCAH II) via the apocrine secretion mode. To get more detailed information why CAH II is cytoplasmic resident in some organs and secreted in others we cloned and sequenced the cDNA of rat coagulating gland sCAH II. The sequence of the secretory form was found to be completely identical with the cCAH II. Therefore, a signal peptide targeting sCAH II for apocrine secretion can be excluded. Considering the fact that other apocrine secreted proteins are glycosylated, cCAH II and sCAH II were analyzed for carbohydrate substitutions. As expected for a cytoplasmic protein, no glycan modification could be identified in cCAH II. In contrast, sCAH II carried exclusively Gal, GlcNAc and Fuc residues in a molar ratio of 1:0.8:0.5. Carbohydrate linkage analyses demonstrated the presence of terminal Fuc, terminal, 3-substituted and 3,6-disubstituted Gal as well as 4-substituted and 3,4-disubstituted GlcNAc. The composition of the glycan constituents as well as deglycosylation experiments clearly proved that sCAH II carries neither conventional mammalian-type N-glycans nor mucin-type O-linked sugar chains. Lacking a signal peptide for ER translocation, glycosylation of sCAH II must occur within the cytoplasmic compartment. Further studies have to elucidate whether or not glycosylation of sCAH II is essential for the apocrine release of the protein.

Natural substrates and inhibitors of mannan-binding lectin-associated serine protease-1 and -2: a study on recombinant catalytic fragments

Mannan-binding lectin-associated serine protease (SP) (MASP)-1 and MASP-2 are modular SP and form complexes with mannan-binding lectin, the recognition molecule of the lectin pathway of the complement system. To characterize the enzymatic properties of these proteases we expressed their catalytic region, the C-terminal three domains, in Escherichia coli. Both enzymes autoactivated and cleaved synthetic oligopeptide substrates. In a competing oligopeptide substrate library assay, MASP-1 showed extreme Arg selectivity, whereas MASP-2 exhibited a less restricted, trypsin-like specificity. The enzymatic assays with complement components showed that cleavage of intact C3 by MASP-1 and MASP-2 was detectable, but was only approximately 0.1% of the previously reported efficiency of C3bBb, the alternative pathway C3-convertase. Both enzymes cleaved C3i 10- to 20-fold faster, but still at only approximately 1% of the efficiency of MASP-2 cleavage of C2. We believe that C3 is not the natural substrate of either enzyme. MASP-2 cleaved C2 and C4 at high rates. To determine the role of the individual domains in the catalytic region of MASP-2, the second complement control protein module together with the SP module and the SP module were also expressed and characterized. We demonstrated that the SP domain alone can autoactivate and cleave C2 as efficiently as the entire catalytic region, while the second complement control protein module is necessary for efficient C4 cleavage. This behavior strongly resembles C1s. Each MASP-1 and MASP-2 fragment reacted with C1-inhibitor, which completely blocked the enzymatic action of the enzymes. Nevertheless, relative rates of reaction with alpha-2-macroglobulin and C1-inhibitor suggest that alpha-2-macroglobulin may be a significant physiological inhibitor of MASP-1.

In vivo biosynthesis of endogenous and of human C1 inhibitor in transgenic mice: tissue distribution and colocalization of their expression

We have produced transgenic mice expressing human C1 inhibitor mRNA and protein under the control of the human promoter and regulatory elements. The transgene was generated using a minigene construct in which most of the human C1 inhibitor gene (C1NH) was replaced by C1 inhibitor cDNA. The construct retained the promoter region extending 1.18 kb upstream of the transcription start site, introns 1 and 2 as well as a stretch of 2.5 kb downstream of the polyadenylation site, and therefore carried all known elements involved in transcriptional regulation of the C1NH gene. Mice with high serum levels of human C1 inhibitor, resulting from multiple tandem integrations of the C1 inhibitor transgene, were selected. Immunohistochemistry in combination with in situ hybridization was applied to localize the sites of C1 inhibitor biosynthesis and to demonstrate its local production in brain, spleen, liver, heart, kidney, and lung. The distribution of human C1 inhibitor-expressing cells was qualitatively indistinguishable from that of its mouse counterpart, but expression levels of the transgene were significantly higher. In the spleen, production of C1 inhibitor was colocalized with that of a specific marker for white pulp follicular dendritic cells. This study demonstrates a stringently regulated expression of both the endogenous and the transgenic human C1 inhibitor gene and reveals local biosynthesis of C1 inhibitor at multiple sites in which the components of the macromolecular C1 complex are also produced.

The mannan-binding lectin-associated serine proteases (MASPs) and MAp19: four components of the lectin pathway activation complex encoded by two genes

Mannan-binding lectin (MBL) and ficolins (L-ficolin and H-ficolin) initiate the lectin pathway of complement activation upon binding to microbial carbohydrates. The activation is mediated by associated serine proteases, termed MASPs, since they were discovered as MBL-associated serine proteases. The MASP family comprises three serine proteases, MASP-1, MASP-2 and MASP-3 and a non-enzymatic protein, MAp19. The MASPs show identical domain structure, shared also with C1r and C1s. MASP-1 and MASP-3 are alternative splice products of a single gene, MASP1/3, and have identical A chains, whereas they have individual B chains, encompassing the serine protease domain. MASP2 and MAp19 are alternative splice products of the MASP-2 gene, with MAp19 consisting of the first two domains of MASP-2 plus additional four amino acid residues. MASP-2 is the protease responsible for activating C4 and C2 to generate the C3 convertase, C4bC2b. The biological function of the remaining three proteins has not yet been resolved.

Role of the classical pathway of complement activation in experimentally induced polymicrobial peritonitis

The complement system and the natural antibody repertoire provide a critical first-line defense against infection. The binding of natural antibodies to microbial surfaces opsonizes invading microorganisms and activates complement via the classical pathway. Both defense systems cooperate within the innate immune response. We studied the role of the complement system in the host defense against experimental polymicrobial peritonitis using mice lacking either C1q or factor B and C2. The C1q-deficient mice lacked the classical pathway of complement activation. The factor B- and C2-deficient mice were known to lack the classical and alternative pathways, and we demonstrate here that these mice also lacked the lectin pathway of complement activation. Using inoculum doses adjusted to cause 42% mortality in the wild-type strain, none of the mice deficient in the three activation routes of complement (factor B and C2 deficient) survived (mortality of 100%). Mortality in mice deficient only in the classical pathway of complement activation (C1q deficient) was 83%. Application of further dilutions of the polymicrobial inoculum showed a dose-dependent decrease of mortality in wild-type controls, whereas no changes in mortality were observed in the two gene-targeted strains. These results demonstrate that the classical activation pathway is required for an effective antimicrobial immune defense in polymicrobial peritonitis and that, in the infection model used, the remaining antibody-independent complement activation routes (alternative and lectin pathways) provide a supporting line of defense to gain residual protection in classical pathway deficiency.