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Complement receptor of immunoglobulin superfamily (CRIg) is expressed on liver macrophages and directly binds complement component C3b or Gram-positive bacteria to mediate phagocytosis. CRIg plays important roles in several immune-mediated diseases, but it is not clear how its pathogen recognition and phagocytic functions maintain homeostasis and prevent disease. We previously associated cytolysin-positive Enterococcus faecalis with severity of alcohol-related liver disease. Here, we demonstrate that CRIg is reduced in liver tissues from patients with alcohol-related liver disease. CRIg-deficient mice developed more severe ethanol-induced liver disease than wild-type mice; disease severity was reduced with loss of toll-like receptor 2. CRIg-deficient mice were less efficient than wild-type mice at clearing Gram-positive bacteria such as Enterococcus faecalis that had translocated from gut to liver. Administration of the soluble extracellular domain CRIg–Ig protein protected mice from ethanol-induced steatohepatitis. Our findings indicate that ethanol impairs hepatic clearance of translocated pathobionts, via decreased hepatic CRIg, which facilitates progression of liver disease. CRIg is expressed on liver macrophages and binds Gram-positive bacteria to mediate phagocytosis, but it is not clear how its phagocytic functions contribute to liver homeostasis or disease. Here the authors report that ethanol impairs hepatic clearance of translocated pathobionts, via decreased hepatic CRIg, which facilitates progression of alcoholic liver disease.

Background The complement receptor immunoglobulin (CRIg), a key microbial pathogen phagocytosis-promoting receptor, responsible for intravascular clearance of bacteria, is purported to be expressed selectively on tissue-fixed macrophages such as Kupffer cells. However, recently it has been reported that neutrophils can also express functional CRIg following activation by inflammatory mediators. Monocytes have been reported not to express CRIg under non-activated conditions. Thus, investigations were undertaken to examine whether blood monocytes express CRIg under cell activation conditions and its role in anti-microbial immunity. Methods Monocytes CRIg expression in whole human and mouse blood or peripheral blood mononuclear cells and purified monocytes using density gradient centrifugation or an affinity purification kit was examined using PE/FITC-labelled anti-CRIg monoclonal antibody and flow cytometry. Characterization of CRIg isoforms in monocytes was determined by the detection of CRIg mRNA transcripts and protein using RT-PCR and Western blot, respectively. Gene-edited CRIg – and CD18 – monocytic THP-1 cell lines were generated to assess the role of CRIg and CD18 in cell adhesion, phagocytosis, and microbial killing. Functional assays were performed using Staphylococcus aureus as a model pathogen. Results CRIg was constitutively expressed, dynamically, on the surface of human and mouse blood monocytes. All three human monocyte subpopulations expressed CRIg, equally. The inability to demonstrate expression on monocytes cell surface by previous studies can be explained by its lability during blood storage and loss during monocyte isolation steps. Interestingly of the monocyte subpopulations only the classical and intermediate but not the non-classical showed a loss of CRIg expression. The data showed that loss from the surface was most likely due to relocation of the receptor intracellularly. Monocytes expressed 6 different CRIg mRNA transcripts and immunoreactive isoforms. Using CRIg – and CD18 – THP-1 monocytic cells, we found that both CRIg and CD18 (CR3/CR4) were critical for cell adhesion, but for phagocytosis and killing of S. aureus, either receptor was independently effective. Conclusion The data provide compelling evidence that monocytes express functional CRIg, relevant to the cells’ anti-microbial role of the ‘wandering’ phagocyte and consolidate a view that CRIg is widely expressed in our phagocytic cell system, similar to the classical complement receptors CR3 and CR4.

Many circumstantial evidences from human and animal studies suggest that complement cascade dysregulation may play an important role in pregnancy associated complications including preeclampsia. Deletion of rodent specific complement inhibitor gene, Complement Receptor 1-related Gene/Protein y (Crry) produces embryonic lethal phenotype due to complement activation. It is not clear if decreased expression of Crry during pregnancy produces hypertensive phenotype. We downregulated Crry in placenta by injecting inducible lentivialshRNA vectors into uterine horn of pregnant C57BL/6 mice at the time of blastocyst hatching. Placenta specific downregulation of Crry without significant loss of embryos was achieved upon induction of shRNA using an optimal doxycycline dose at mid gestation. Crry downregulation resulted in placental complement deposition. Late-gestation measurements showed that fetal weights were reduced and blood pressure increased in pregnant mice upon downregulation of Crry suggesting a critical role for Crry in fetal growth and blood pressure regulation.

The aim of the present study was to evaluate differences in the expression of complement system genes, and serum levels of CH50, C3 and C4 in peripheral blood mononuclear cells from patients with myocardial infarction (AMI), stable angina pectoris (SA) and controls. A total of 100 patients with AMI, 100 with SA and 100 clinical controls were recruited in the present study. In each group, 20 randomly selected individuals were examined using whole human genome microarray analysis to detect the expression of genes of the complement system. The serum levels of CH50, C3 and C4 were measured in all 300 subjects. In the patients with AMI, the expression levels of genes encoding C1qα, C1qβ, C1qγ, C1r, Factor P, C5a (complement component), CR1, integrin αM, integrin αX, integrin β2, C5aR, CRIg (complement receptors) and CD46, CD55 and CD59 (complement regulators) were significantly higher, compared with the respective genes in the SA patients and controls (P<0.05), whereas the mRNA levels of C1s, C7, C8β and C9 were the lowest in this group (P<0.05). No statistically significant differences were found in the gene expression levels of complement components or regulators between the SA and control groups. The serum levels of CH50, C3 and C4 were significantly increased in the AMI and SA groups, compared with the controls. In the AMI and SA groups, the complement system was activated. However, the differential mRNA expression of complement components, receptors and regulators in the AMI group suggested the dysfunction of the C5b-9 complex. The depression of complement system immunity in the patients with AMI may be associated with the pathogenesis of AMI.

IgG3, passively administered together with small proteins, induces enhanced primary humoral responses against these proteins. We previously found that, within 2 h of immunization, marginal zone (MZ) B cells capture IgG3-antigen complexes and transport them into splenic follicles and that this requires the presence of complement receptors 1 and 2. We have here investigated the localization of IgG3 anti-2, 4, 6-trinitrophenyl (TNP)/biotin-ovalbumin-TNP immune complexes in the follicles and the involvement of classical versus total complement activation in this process. The majority (50–90%) of antigen inside the follicles of mice immunized with IgG3-antigen complexes co-localized with the follicular dendritic cell (FDC) network. Capture of antigen by MZ B cells as well as antigen deposition on FDC was severely impaired in mice lacking C1q or C3, and lack of either C1q or C3 also impaired the ability of IgG3 to enhance antibody responses. Finally, IgG3 efficiently primed for a memory response against small proteins as well as against the large protein keyhole limpet hemocyanine.

Previous studies indicate a pivotal role for complement in mediating both local and remote injury following ischemia and reperfusion of the intestine. Here, we report on the use of a mouse model of intestinal ischemia/reperfusion injury to investigate the strategy of targeting complement inhibition to sites of complement activation by linking an iC3b/C3dg-binding fragment of mouse complement receptor 2 (CR2) to a mouse complement-inhibitory protein, Crry. We show that the novel CR2-Crry fusion protein targets sites of local and remote (lung) complement activation following intestinal ischemia and reperfusion injury and that CR2-Crry requires a 10-fold lower dose than its systemic counterpart, Crry-Ig, to provide equivalent protection from both local and remote injury. CR2-Crry has a significantly shorter serum half-life than Crry-Ig and, unlike Crry-Ig, had no significant effect on serum complement activity at minimum effective therapeutic doses. Furthermore, the minimum effective dose of Crry-Ig significantly enhanced susceptibility to infection in a mouse model of acute septic peritonitis, whereas the effect of CR2-Crry on susceptibility to infection was indistinguishable from that of PBS control. Thus, compared with systemic inhibition, CR2-mediated targeting of a complement inhibitor of activation improved bioavailability, significantly enhanced efficacy, and maintained host resistance to infection.

Genome-wide association studies (GWAS) have identified several susceptibility loci of Alzheimer’s disease (AD), which were mainly located in noncoding regions of the genome. Meanwhile, the putative biological mechanisms underlying AD susceptibility loci were still unclear. At present, identifying the functional variants of AD pathogenesis remains a major challenge. Herein, we first used summary data-based Mendelian randomization (SMR) with AD GWAS summary and expression quantitative trait loci (eQTL) data to identify variants who affects expression levels of nearby genes and contributed to the risk of AD. Using the SMR integrative analysis, we totally identified 14 SNPs significantly affected the expression level of 16 nearby genes in blood or brain tissues and contributed to the AD risk. Then, to confirm the results, we replicated the GWAS and eQTL results across multiple samples. Totally, four risk SNP (rs11682128, rs601945, rs3935067, and rs679515) were validated to be associated with AD and affected the expression level of nearby genes ( BIN1, HLA-DRA, EPHA1-AS1, and CR1 ). Besides, our differential expression analysis showed that the BIN1 gene was significantly downregulated in the hippocampus ( P = 2.0 × 10 −3 ) and survived after multiple comparisons. These convergent lines of evidence suggest that the BIN1 gene identified by SMR has potential roles in the pathogenesis of AD. Further investigation of the roles of the BIN1 gene in the pathogenesis of AD is warranted.

Maturation of B cells within germinal centers (GCs) generates diversified B cell pools and high-affinity B cell antigen receptors (BCRs) for pathogen clearance. Increased receptor affinity is achieved by iterative cycles of T cell–dependent, affinity-based B cell positive selection and clonal expansion by mechanisms hitherto incompletely understood. Here we found that, as part of a physiologic program, GC B cells repressed expression of decay-accelerating factor (DAF/CD55) and other complement C3 convertase regulators via BCL6, but increased the expression of C5b-9 inhibitor CD59. These changes permitted C3 cleavage on GC B cell surfaces without the formation of membrane attack complex and activated C3a- and C5a-receptor signals required for positive selection. Genetic disruption of this pathway in antigen-activated B cells by conditional transgenic DAF overexpression or deletion of C3a and C5a receptors limited the activation of mechanistic target of rapamycin (mTOR) in response to BCR–CD40 signaling, causing premature GC collapse and impaired affinity maturation. These results reveal that coordinated shifts in complement regulation within the GC provide crucial signals underlying GC B cell positive selection. Heeger and colleagues report that activated B cells dynamically regulate the expression of complement regulatory proteins via the transcription factor BCL6. C3 convertase activity and C3aR1–C5aR1 signaling were both necessary for optimal B cell activation and germinal center formation.

The complement cascade defines an important link between the innate and the specific immune system. Here we show that mice deficient for the third component of complement ( C3 −/− mice) are highly susceptible to primary infection with influenza virus. C3 −/− mice showed delayed viral clearance and increased viral titers in lung, whereas mice deficient for complement receptors CR1 and CR2 ( Cr2 −/− mice) cleared the infection normally. Priming of T-helper cells and cytotoxic T cells (CTLs) in lung-draining lymph nodes was reduced, and the recruitment into the lung of virus-specific CD4 + and CD8 + effector T cells producing interferon-γ was severely impaired in C3 −/− but not in Cr2 −/− mice. Consequently, T-helper cell–dependent IgG responses were reduced in C3 −/− mice but remained intact in Cr2 −/− mice. These results demonstrate that complement induces specific immunity by promoting T-cell responses.

Fungal dissemination into the bloodstream is a critical step leading to invasive fungal infections. Here, using intravital imaging, we show that Kupffer cells (KCs) in the liver have a prominent function in the capture of circulating Cryptococcus neoformans and Candida albicans , thereby reducing fungal dissemination to target organs. Complement C3 but not C5, and complement receptor CRIg but not CR3, are involved in capture of C. neoformans . Internalization of C. neoformans by KCs is subsequently mediated by multiple receptors, including CR3, CRIg, and scavenger receptors, which work synergistically along with C5aR signaling. Following phagocytosis, the growth of C. neoformans is inhibited by KCs in an IFN-γ independent manner. Thus, the liver filters disseminating fungi from circulation via KCs, providing a mechanistic explanation for the enhanced risk of cryptococcosis among individuals with liver diseases, and suggesting a therapeutic strategy to prevent fungal dissemination through enhancing KC functions. Patients with liver diseases are at increased risk of fungal infections. Here the authors show that Kupffer cells are critical for the filtration of fungi out of the blood and thereby for liver-mediated protection against disseminating fungal infection.