Complement contributes to inflammatory tissue destruction in a mouse model of Ross River virus-induced disease

Arthritogenic alphaviruses, including Ross River virus (RRV) and chikungunya virus, are mosquito-borne viruses that cause significant human disease worldwide, including explosive epidemics that can result in thousands to millions of infected individuals. Similar to infection of humans, infection of C57BL/6 mice with RRV results in severe monocytic inflammation of bone, joint, and skeletal muscle tissues. We demonstrate here that the complement system, an important component of the innate immune response, enhances the severity of RRV-induced disease in mice. Complement activation products were detected in the inflamed tissues and in the serum of RRV-infected wild-type mice. Furthermore, mice deficient in C3 (C3^−/−), the central component of the complement system, developed much less severe disease signs than did wild-type mice. Complement-mediated chemotaxis is essential for many inflammatory arthritides; however, RRV-infected wild-type and C3^−/− mice had similar numbers and composition of inflammatory infiltrates within hind limb skeletal muscle tissue. Despite similar inflammatory infiltrates, RRV-infected C3^−/− mice exhibited far less severe destruction of skeletal muscle tissue. In addition to these studies, complement activation was also detected in synovial fluid from RRV-infected patients. Taken together, these findings indicate that complement activation occurs in the tissues of humans and mice infected with RRV and suggest that complement plays an essential role in the effector phase, but not the inductive phase, of RRV-induced arthritis and myositis.     

Complement receptor 3 promotes severe ross river virus-induced disease

Alphaviruses such as Ross River virus (RRV) and chikungunya virus are mosquito-transmitted viruses that cause explosive epidemics of debilitating arthritis and myositis affecting millions of humans worldwide. Previous studies using a mouse model of RRV-induced disease demonstrated that viral infection results in a severe inflammatory arthritis and myositis and that complement component 3 (C3) contributes to the destructive phase of the inflammatory disease but not the recruitment of cellular infiltrates to the sites of RRV-induced inflammation. Here, we demonstrate that mice deficient in complement receptor 3 (CR3) (CD11b^−/−), a signaling receptor activated by multiple ligands including the C3 cleavage fragment iC3b, develop less-severe disease signs and decreased tissue destruction compared to RRV-infected wild-type mice. CR3 deficiency had no effect on viral replication, nor did it diminish the magnitude, kinetics, and composition of the cellular infiltrates at the sites of inflammation. However, the genetic absence of CR3 diminished the expression of specific proinflammatory and cytotoxic effectors, including S100A9/S100A8 and interleukin-6, within the inflamed tissues, suggesting that CR3-dependent signaling at the sites of inflammation contributes to tissue damage and severe disease.     

Characterization of Ross River virus tropism and virus-induced inflammation in a mouse model of viral arthritis and myositis

Mosquito-borne alphaviruses are a significant cause of both encephalitic and arthritic disease in humans worldwide. In contrast to the encephalitic alphaviruses, the pathogenesis of alphavirus-induced arthritic disease is not well understood. Utilizing a mouse model of Ross River virus (RRV) disease, we found that the primary targets of RRV infection are bone, joint, and skeletal muscle tissues of the hind limbs in both outbred CD-1 mice and adult C57BL/6J mice. Moreover, histological analyses demonstrated that RRV infection resulted in severe inflammation of these tissues. Characterization of the inflammatory infiltrate within the skeletal muscle tissue identified inflammatory macrophages, NK cells, and CD4+ and CD8+ T lymphocytes. To determine the contribution of the adaptive immune system, the outcome of RRV-induced disease was examined in C57BL/6J RAG-1(-/-) mice, which lack functional T and B lymphocytes. RAG-1(-/-) and wild-type mice developed similar disease signs, infiltration of inflammatory macrophages and NK cells, and muscle pathology, suggesting that the adaptive immune response does not play a critical role in the development of disease. These results establish the mouse model of RRV disease as a useful system for the identification of viral and host factors that contribute to alphavirus-induced arthritis and myositis.     

Myd88-dependent toll-like receptor 7 signaling mediates protection from severe ross river virus-induced disease in mice

Arthralgia-associated alphaviruses, including chikungunya virus (CHIKV) and Ross River virus (RRV), pose significant public health threats because of their ability to cause explosive outbreaks of debilitating arthralgia and myalgia in human populations. Although the host inflammatory response is known to contribute to the pathogenesis of alphavirus-induced arthritis and myositis, the role that Toll-like receptors (TLRs), which are major regulators of host antiviral and inflammatory responses, play in the pathogenesis of alphavirus-induced arthritis and myositis has not been extensively studied. Using a mouse model of RRV-induced myositis/arthritis, we found that myeloid differentiation primary response gene 88 (Myd88)-dependent TLR7 signaling is involved in protection from severe RRV-associated disease. Infections of Myd88- and TLR7-deficient mouse strains with RRV revealed that both Myd88 and TLR7 significantly contributed to protection from RRV-induced mortality, and both mouse strains exhibited more severe tissue damage than wild-type (WT) mice following RRV infection. While viral loads were unchanged in either Myd88 or TLR7 knockout mice compared to WT mice at early times postinfection, both Myd88 and TLR7 knockout mice exhibited higher viral loads than WT mice at late times postinfection. Furthermore, while high levels of RRV-specific antibody were produced in TLR7-deficient mice, this antibody had very little neutralizing activity and had lower affinity than WT antibody. Additionally, TLR7- and Myd88-deficient mice showed defects in germinal center activity, suggesting that TLR7-dependent signaling is critical for the development of protective antibody responses against RRV.     

The Lectin Pathway of Complement Activation Is a Critical Component of the Innate Immune Response to Pneumococcal Infection

The complement system plays a key role in host defense against pneumococcal infection. Three different pathways, the classical, alternative and lectin pathways, mediate complement activation. While there is limited information available on the roles of the classical and the alternative activation pathways of complement in fighting streptococcal infection, little is known about the role of the lectin pathway, mainly due to the lack of appropriate experimental models of lectin pathway deficiency. We have recently established a mouse strain deficient of the lectin pathway effector enzyme mannan-binding lectin associated serine protease-2 (MASP-2) and shown that this mouse strain is unable to form the lectin pathway specific C3 and C5 convertases. Here we report that MASP-2 deficient mice (which can still activate complement via the classical pathway and the alternative pathway) are highly susceptible to pneumococcal infection and fail to opsonize Streptococcus pneumoniae in the none-immune host. This defect in complement opsonisation severely compromises pathogen clearance in the lectin pathway deficient host. Using sera from mice and humans with defined complement deficiencies, we demonstrate that mouse ficolin A, human L-ficolin, and collectin 11 in both species, but not mannan-binding lectin (MBL), are the pattern recognition molecules that drive lectin pathway activation on the surface of S. pneumoniae. We further show that pneumococcal opsonisation via the lectin pathway can proceed in the absence of C4. This study corroborates the essential function of MASP-2 in the lectin pathway and highlights the importance of MBL-independent lectin pathway activation in the host defense against pneumococci.     

Mannose-Binding Lectin Regulates Host Resistance and Pathology during Experimental Infection with Trypanosoma cruzi

Mannose-binding lectin (MBL) is a humoral pattern-recognition molecule important for host defense. Although recent genetic studies suggest an involvement of MBL/MASP2-associated pathways in Chagas’ disease, it is currently unknown whether MBL plays a role in host resistance to the intracellular protozoan Trypanosoma cruzi, the causative agent of Chagas’ disease. In this study we employed MBL^−/− mice to assess the role of MBL in resistance to experimental infection with T. cruzi. T. cruzi infection enhanced tissue expression of MBL both at the mRNA and protein level. Similarly, symptomatic acute Chagas’ disease patients displayed increased serum concentrations of MBL compared to patients with indeterminate, asymptomatic forms of the disease. Furthermore, increased parasite loads in the blood and/or tissue were observed in MBL^−/− mice compared to WT controls. This was associated with reduced systemic levels of IL-12/23p40 in MBL^−/− mice. Importantly, MBL^−/− mice infected with a cardiotropic strain of T. cruzi displayed increased myocarditis and cardiac fibrosis compared to WT controls. The latter was accompanied by elevated hydroxyproline content and mRNA levels of collagen-1 and -6 in the heart. These observations point to a previously unappreciated role for MBL in regulating host resistance and cardiac inflammation during infection with a major human pathogen.     

Pathogenic complement activation in collagen antibody-induced arthritis in mice requires amplification by the alternative pathway

Immune complex-induced inflammation can be mediated by the classical pathway of complement. However, using mice genetically deficient in factor B or C4, we have shown that the collagen Ab-induced model of arthritis requires the alternative pathway of complement and is not dependent on the classical pathway. We now demonstrate that collagen Ab-induced arthritis is not altered in mice genetically deficient in either C1q or mannose-binding lectins A and C, or in both C1q and mannose-binding lectins. These in vivo results prove the ability of the alternative pathway to carry out pathologic complement activation in the combined absence of intact classical and lectin pathways. C3 activation was also examined in vitro by adherent collagen-anti-collagen immune complexes using sera from normal or complement-deficient mice. These results confirm the ability of the alternative pathway to mediate immune complex-induced C3 activation when C4 or C1q, or both C1q and mannose-binding lectins, are absent. However, when all three activation pathways of complement are intact, initiation by immune complexes occurs primarily by the classical pathway. These results indicate that the alternative pathway amplification loop, with its ability to greatly enhance C3 activation, is necessary to mediate inflammatory arthritis induced by adherent immune complexes.     

Surviving Mousepox Infection Requires the Complement System

Poxviruses subvert the host immune response by producing immunomodulatory proteins, including a complement regulatory protein. Ectromelia virus provides a mouse model for smallpox where the virus and the host''s immune response have co-evolved. Using this model, our study investigated the role of the complement system during a poxvirus infection. By multiple inoculation routes, ectromelia virus caused increased mortality by 7 to 10 days post-infection in C57BL/6 mice that lack C3, the central component of the complement cascade. In C3^−/− mice, ectromelia virus disseminated earlier to target organs and generated higher peak titers compared to the congenic controls. Also, increased hepatic inflammation and necrosis correlated with these higher tissue titers and likely contributed to the morbidity in the C3^−/− mice. In vitro, the complement system in naïve C57BL/6 mouse sera neutralized ectromelia virus, primarily through the recognition of the virion by natural antibody and activation of the classical and alternative pathways. Sera deficient in classical or alternative pathway components or antibody had reduced ability to neutralize viral particles, which likely contributed to increased viral dissemination and disease severity in vivo. The increased mortality of C4^−/− or Factor B^−/− mice also indicates that these two pathways of complement activation are required for survival. In summary, the complement system acts in the first few minutes, hours, and days to control this poxviral infection until the adaptive immune response can react, and loss of this system results in lethal infection.     

Role of interferon in homologous and heterologous rotavirus infection in the intestines and extraintestinal organs of suckling mice

Recent studies demonstrated that viremia and extraintestinal rotavirus infection are common in acutely infected humans and animals, while systemic diseases appear to be rare. Intraperitoneal infection of newborn mice with rhesus rotavirus (RRV) results in biliary atresia (BA), and this condition is influenced by the host interferon response. We studied orally inoculated 5-day-old suckling mice that were deficient in interferon (IFN) signaling to evaluate the role of interferon on the outcome of local and systemic infection after enteric inoculation. We found that systemic replication of RRV, but not murine rotavirus strain EC, was greatly enhanced in IFN-α/β and IFN-γ receptor double-knockout (KO) or STAT1 KO mice but not in mice deficient in B- or T-cell immunity. The enhanced replication of RRV was associated with a lethal hepatitis, pancreatitis, and BA, while no systemic disease was observed in strain EC-infected interferon-deficient mice. In IFN-α/β receptor KO mice the extraintestinal infection and systemic disease were only moderately increased, while RRV infection was not augmented and systemic disease was not present in IFN-γ receptor KO mice. The increase of systemic infection in IFN-deficient mice was also observed during simian strain SA11 infection but not following bovine NCDV, porcine OSU, or murine strain EW infection. Our data indicate that the requirements for the interferon system to inhibit intestinal and extraintestinal viral replication in suckling mice vary among different heterologous and homologous rotavirus strains, and this variation is associated with lethal systemic disease.     

Mannose-binding lectin is a regulator of inflammation that accompanies myocardial ischemia and reperfusion injury

The mannose-binding lectin (MBL), a circulating pattern recognition molecule, recognizes a wide range of infectious agents with resultant initiation of the complement cascade in an Ab-independent manner. MBL recognizes infectious non-self and altered self in the guise of apoptotic and necrotic cells. In this study, we demonstrate that mice lacking MBL, and hence are devoid of MBL-dependent lectin pathway activation but have fully active alternative and classical complement pathways, are protected from cardiac reperfusion injury with resultant preservation of cardiac function. Significantly, mice that lack a major component of the classical complement pathway initiation complex (C1q) but have an intact MBL complement pathway, are not protected from injury. These results suggest that the MBL-dependent pathway of complement activation is a key regulator of myocardial reperfusion ischemic injury. MBL is an example of a pattern recognition molecule that plays a dual role in modifying inflammatory responses to sterile and infectious injury.     

Intradermal Immunization with Wall Teichoic Acid (WTA) Elicits and Augments an Anti-WTA IgG Response that Protects Mice from Methicillin-Resistant Staphylococcus aureus Infection Independent of Mannose-Binding Lectin Status

The objectives of this study were to investigate the immune response to intradermal immunization with wall teichoic acid (WTA) and the effect of MBL deficiency in a murine model of infection with methicillin-resistant Staphylococcus aureus (MRSA). WTA is a bacterial cell wall component that is implicated in invasive infection. We tested susceptibility to MRSA infection in wild type (WT) and MBL deficient mice using two strains of MRSA: MW2, a community-associated MRSA (CA-MRSA); and COL, a healthcare-associated MRSA (HA-MRSA). We also performed in vitro assays to investigate the effects of anti-WTA IgG containing murine serum on complement activation and bacterial growth in whole blood. We found that MBL knockout (KO) mice are relatively resistant to a specific MRSA strain, MW2 CA-MRSA, compared to WT mice, while both strains of mice had similar susceptibility to a different strain, COL HA-MRSA. Intradermal immunization with WTA elicited and augmented an anti-WTA IgG response in both WT and MBL KO mice. WTA immunization significantly reduced susceptibility to both MW2 CA-MRSA and COL HA-MRSA, independent of the presence of MBL. The protective mechanisms of anti-WTA IgG are mediated at least in part by complement activation and clearance of bacteria from blood. The significance of these findings is that 1) Intradermal immunization with WTA induces production of anti-WTA IgG; and 2) This anti-WTA IgG response protects from infection with both MW2 CA-MRSA and COL HA-MRSA even in the absence of MBL, the deficiency of which is common in humans.     

Complement component C3 is not required for full expression of immune complex glomerulonephritis in MRL/lpr mice

Complement activation and tissue deposition of complement fragments occur during disease progression in lupus nephritis. Genetic deficiency of some complement components (e.g., Factor B) and infusion of complement inhibitors (e.g., Crry, anti-C5 Ab) protect against inflammatory renal disease. Paradoxically, genetic deficiencies of early components of the classical complement pathway (e.g., C1q, C4, and C2) are associated with an increased incidence of lupus in humans and lupus-like disease in murine knockout strains. Complement protein C3 is the converging point for activation of all three complement pathways and thus plays a critical role in biologic processes mediated by complement activation. To define the role of C3 in lupus nephritis, mice rendered C3 deficient by targeted deletion were backcrossed for eight generations to MRL/lpr mice, a mouse strain that spontaneously develops lupus-like disease. We derived homozygous knockout (C3(-/-)), heterozygous (C3(+/-)), and C3 wild-type (C3(+/+)) MRL/lpr mice. Serum levels of autoantibodies and circulating immune complexes were similar among the three groups. However, there was earlier and significantly greater albuminuria in the C3(-/-) mice compared with the other two groups. Glomerular IgG deposition was also significantly greater in the C3(-/-) mice than in the other two groups, although overall pathologic renal scores were similar. These results indicate that C3 and/or activation of C3 is not required for full expression of immune complex renal disease in MRL/lpr mice and may in fact play a beneficial role via clearance of immune complexes.     

Complement-dependent injury and protection in a murine model of acute dextran sulfate sodium-induced colitis

Complement plays a key role in the pathophysiology of many inflammatory diseases, and in this study, we investigated the role of complement in the pathogenesis of inflammatory bowel disease. Compared to wild-type mice, mice deficient in C3 or factor B were protected from acute dextran sulfate sodium (DSS)-induced colitis. C1q/mannose-binding lectin (MBL) double-deficient mice, however, exhibited more severe colitis than wild-type mice. When mice were allowed to recover after DSS treatment, all C1q/MBL(-/-) mice died by day 2 of recovery period, and, surprisingly, all C3(-/-) and factor B(-/-) mice died by day 5. Serum endotoxin levels were significantly increased in complement-deficient mice prior to death, particularly in C1q/MBL(-/-) mice, and antibiotic treatment prevented the lethal effect of DSS in all complement-deficient mice. In contrast to complement deficiency, targeted complement inhibition with either complement receptor 2 (CR2)-Crry (blocks all pathways at C3 activation) or CR2-factor H (blocks alternative pathway) was highly protective at treating established acute colitis. Endotoxin levels remained low in complement-inhibited mice, and complement inhibition also reduced inflammatory cytokines, leukocyte infiltration, and tissue injury while improving wound repair and mucosal healing. CR2-factor H provided more effective protection than CR2-Crry. Thus, complement has both pathogenic and protective roles in acute DSS-induced colitis, and whereas the alternative pathway appears to play a key role in tissue inflammation and injury, the classical/lectin pathway provides important protection in terms of host defense and wound repair. Targeted inhibition of the alternative pathway may represent a therapeutic modality for treating acute phases of inflammatory bowel disease.     

Rotavirus Replication in the Cholangiocyte Mediates the Temporal Dependence of Murine Biliary Atresia

Biliary atresia (BA) is a neonatal disease that results in obliteration of the biliary tree. The murine model of BA, which mirrors the human disease, is based upon infection of newborn mice with rhesus rotavirus (RRV), leading to an obstructive cholangiopathy. The purpose of this study was to characterize the temporal relationship between viral infection and the induction of this model. BALB/c mice were infected with RRV on day of life (DOL) 0, 3, 5, and 7. Groups were characterized as early-infection (infection by DOL 3) or late-infection (infection after DOL 5). Early RRV infection induced symptoms in 95% of pups with a mortality rate of 80%. In contrast, late infection caused symptoms in only 50% of mice, and 100% of pups survived. The clinical findings correlated with histological analysis of extrahepatic biliary trees, cytokine expression, and viral titers. Primary murine cholangiocytes isolated, cultured, and infected with RRV yielded higher titers of infectious virus in those harvested from DOL 2 versus DOL 9 mice. Less interferon alpha and beta was produced in DOL 2 versus DOL 9 RRV infected primary cholangiocytes. Injection of BALB/c interferon alpha/beta receptor knockout (IFN-αβR^−/−) pups at DOL 7 showed increased symptoms (79%) and mortality (46%) when compared to late infected wild type mice. In conclusion, the degree of injury sustained by relatively immature cholangiocytes due to more robust RRV replication correlated with more severe clinical manifestations of cholangiopathy and higher mortality. Interferon alpha production by cholangiocytes appears to play a regulatory role. These findings confirm a temporal dependence of RRV infection in murine BA and begin to define a pathophysiologic role of the maturing cholangiocyte.     

Alternative complement pathway activation is essential for inflammation and joint destruction in the passive transfer model of collagen-induced arthritis

Activation of each complement initiation pathway (classical, alternative, and lectin) can lead to the generation of bioactive fragments with resulting inflammation in target organs. The objective of the current study was to determine the role of specific complement activation pathways in the pathogenesis of experimental anti-type II collagen mAb-passive transfer arthritis. C57BL/6 mice were used that were genetically deficient in either the alternative pathway protein factor B (Bf(-/-)) or in the classical pathway component C4 (C4(-/-)). Clinical disease activity was markedly decreased in Bf(-/-) compared with wild-type (WT) mice (0.5 +/- 0.22 (n = 6) in Bf(-/-) vs 8.83 +/- 0.41 (n = 6) in WT mice (p < 0.0001)). Disease activity scores were not different between C4(-/-) and WT mice. Analyses of joints showed that C3 deposition, inflammation, pannus, cartilage, and bone damage scores were all significantly less in Bf(-/-) as compared with WT mice. There were significant decreases in mRNA levels of C3, C4, CR2, CR3, C3aR, and C5aR in the knees of Bf(-/-) as compared with C4(-/-) and WT mice with arthritis; mRNA levels for complement regulatory proteins did not differ between the three strains. These results indicate that the alternative pathway is absolutely required for the induction of arthritis following injection of anti-collagen Abs. The mechanisms by which these target organ-specific mAbs bypass the requirements for engagement of the classical pathway remain to be defined but do not appear to involve a lack of alternative pathway regulatory proteins.     

Genetically-Defined Deficiency of Mannose-Binding Lectin Is Associated with Protection after Experimental Stroke in Mice and Outcome in Human Stroke

Background

The complement system is a major effector of innate immunity that has been involved in stroke brain damage. Complement activation occurs through the classical, alternative and lectin pathways. The latter is initiated by mannose-binding lectin (MBL) and MBL-associated serine proteases (MASPs). Here we investigated whether the lectin pathway contributes to stroke outcome in mice and humans.

Methodology/Principal Findings

Focal cerebral ischemia/reperfusion in MBL-null mice induced smaller infarctions, better functional outcome, and diminished C3 deposition and neutrophil infiltration than in wild-type mice. Accordingly, reconstitution of MBL-null mice with recombinant human MBL (rhMBL) enhanced brain damage. In order to investigate the clinical relevance of these experimental observations, a study of MBL2 and MASP-2 gene polymorphism rendering the lectin pathway dysfunctional was performed in 135 stroke patients. In logistic regression adjusted for age, gender and initial stroke severity, unfavourable outcome at 3 months was associated with MBL-sufficient genotype (OR 10.85, p = 0.008) and circulating MBL levels (OR 1.29, p = 0.04). Individuals carrying MBL-low genotypes (17.8%) had lower C3, C4, and CRP levels, and the proinflammatory cytokine profile was attenuated versus MBL-sufficient genotypes.

Conclusions/Significance

In conclusion, genetically defined MBL-deficiency is associated with a better outcome after acute stroke in mice and humans.     

The role of mannose-binding lectin-associated serine protease-3 in activation of the alternative complement pathway

Mannose-binding lectin (MBL)-associated serine proteases (MASPs) are responsible for activation of the lectin complement pathway. Three types of MASPs (MASP-1, MASP-2, and MASP-3) are complexed with MBL and ficolins in serum. Although MASP-1 and MASP-2 are known to contribute to complement activation, the function of MASP-3 remains unclear. In this study, we investigated the mechanism of MASP-3 activation and its substrate using the recombinant mouse MASP-3 (rMASP-3) and several different types of MASP-deficient mice. A proenzyme rMASP-3 was obtained that was not autoactivated during preparation. The recombinant enzyme was activated by incubation with Staphylococcus aureus in the presence of MBL-A, but not MBL-C. In vivo studies revealed the phagocytic activities of MASP-1/3-deficient mice and all MASPs (MASP-1/2/3)-deficient mice against S. aureus and bacterial clearance in these mice were lower than those in wild-type and MASP-2-deficient mice. Sera from all MASPs-deficient mice showed significantly lower C3 deposition activity on the bacteria compared with that of wild-type serum, and addition of rMASP-3 to the deficient serum restored C3 deposition. The low C3 deposition in sera from all MASPs-deficient mice was probably caused by the low level factor B activation that was ameliorated by the addition of rMASP-3. Furthermore, rMASP-3 directly activated factors B and D in vitro. These results suggested that MASP-3 complexed with MBL is converted to an active form by incubation with bacterial targets, and that activated MASP-3 triggered the initial activation step of the alternative complement pathway.     

Heterocomplexes of mannose-binding lectin and the pentraxins PTX3 or serum amyloid P component trigger cross-activation of the complement system

The long pentraxin 3 (PTX3), serum amyloid P component (SAP), and C-reactive protein belong to the pentraxin family of pattern recognition molecules involved in tissue homeostasis and innate immunity. They interact with C1q from the classical complement pathway. Whether this also occurs via the analogous mannose-binding lectin (MBL) from the lectin complement pathway is unknown. Thus, we investigated the possible interaction between MBL and the pentraxins. We report that MBL bound PTX3 and SAP partly via its collagen-like domain but not C-reactive protein. MBL-PTX3 complex formation resulted in recruitment of C1q, but this was not seen for the MBL-SAP complex. However, both MBL-PTX3 and MBL-SAP complexes enhanced C4 and C3 deposition and opsonophagocytosis of Candida albicans by polymorphonuclear leukocytes. Interaction between MBL and PTX3 led to communication between the lectin and classical complement pathways via recruitment of C1q, whereas SAP-enhanced complement activation occurs via a hitherto unknown mechanism. Taken together, MBL-pentraxin heterocomplexes trigger cross-activation of the complement system.     

B Cell Deficient Mice Are Protected from Biliary Obstruction in the Rotavirus-Induced Mouse Model of Biliary Atresia

A leading theory regarding the pathogenesis of biliary atresia (BA) is that bile duct injury is initiated by a virus infection, followed by an autoimmune response targeting bile ducts. In experimental models of autoimmune diseases, B cells have been shown to play an important role. The aim of this study was to determine the role of B cells in the development of biliary obstruction in the Rhesus rotavirus (RRV)-induced mouse model of BA. Wild-type (WT) and B cell-deficient (Ig-α^-/-) mice received RRV shortly after birth. Ig-α^-/- RRV-infected mice had significantly increased disease-free survival rate compared to WT RRV-infected BA mice (76.8% vs. 17.5%). In stark contrast to the RRV-infected BA mice, the RRV-infected Ig-α^-/- mice did not have hyperbilirubinemia or bile duct obstruction. The RRV-infected Ig-α^-/- mice had significantly less liver inflammation and Th1 cytokine production compared to RRV-infected WT mice. In addition, Ig-α^-/- mice had significantly increased numbers of regulatory T cells (Tregs) at baseline and after RRV infection compared to WT mice. However, depletion of Tregs in Ig-α^-/- mice did not induce biliary obstruction, indicating that the expanded Tregs in the Ig-α^-/- mice were not the sole reason for protection from disease. Conclusion: B cell deficient Ig-α^-/- mice are protected from biliary obstruction in the RRV-induced mouse model of BA, indicating a primary role of B cells in mediating disease pathology. The mechanism of protection may involve lack of B cell antigen presentation, which impairs T-cell activation and Th1 inflammation. Immune modulators that inhibit B cell function may be a new strategy for treatment of BA.