Genetics and biosynthesis of complement proteins

Generation of complement protein C3 deficient pigs by CRISPR/Cas9-mediated gene targeting

Complement protein C3 is the pivotal component of the complement system. Previous studies have demonstrated that C3 has implications in various human diseases and exerts profound functions under certain conditions. However, the delineation of pathological and physiological roles of C3 has been hampered by the insufficiency of suitable animal models. In the present study, we applied the clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated (Cas) system to target the C3 gene in porcine fetal fibroblasts. Our results indicated that CRISPR/Cas9 targeting efficiency was as high as 84.7%, and the biallelic mutation efficiency reached at 45.7%. The biallelic modified colonies were used as donor for somatic cell nuclear transfer (SCNT) technology to generate C3 targeted piglets. A total of 19 C3 knockout (KO) piglets were produced and their plasma C3 protein was undetectable by western blot analysis and ELISA. The hemolytic complement activity and complement-dependent cytotoxicity assay further confirmed that C3 was disrupted in these piglets. These C3 KO pigs could be utilized as a valuable large animal model for the elucidation of the roles of C3.

Complement: A primer for the coming therapeutic revolution

The complement system is an important part of the innate and adaptive immune systems. Originally characterized as a single serum component contributing to the killing of bacteria, we now know that there are close to sixty complement proteins, multiple activation pathways and a wide range of effector functions mediated by complement. The system plays a critical role in host defense against bacteria, viruses, fungi and other pathogens. However, inappropriate complement activation contributes to the pathophysiology of autoimmune diseases and many inflammatory syndromes. Over the last several decades, therapeutic approaches to inhibit complement activation at various steps in the pathways have met with initial success, particularly at the level of the terminal pathway. This success, combined with insight from animal model studies, has lead to an unprecedented effort by biotech and pharmaceutical companies to begin developing complement inhibitors. As a result, complement has been brought for the first time to the attention of pharmacologists, toxicologists, project managers and others in the drug development industry, as well as those in the investment world. The purpose of this primer is to provide a broad overview of complement immunobiology to help those new to complement understand the rationale behind the current therapeutic directions and the investment potential of these new therapeutics.

The complement genes

The structure, organization and control of the expression of complement genes is now being studied at the RNA and DNA levels. This work was prompted by recognized genetic polymorphisms and complement deficiencies in humans and animals. Here Harvey Colten discusses progress towards an understanding of the molecular basis of these variants and abnormalities.

Heightened measures of immune complex and complement function and immune complex-mediated granulocyte activation in human lymphatic filariasis

The presence of circulating immune complexes (CICs) is a characteristic feature of human lymphatic filariasis. However, the role of CICs in modulating granulocyte function and complement functional activity in filarial infection is unknown. The levels of CICs in association with complement activation in clinically asymptomatic, filarial-infected patients (INF); filarial-infected patients with overt lymphatic pathologic changes (CPDT); and uninfected controls (EN) were examined. Significantly increased levels of CICs and enhanced functional efficiency of the classical and mannose-binding lectin pathways of the complement system was observed in INF compared with CPDT and EN. Polyethylene glycol-precipitated CICs from INF and CPDT induced significantly increased granulocyte activation compared with those from EN, determined by the increased production of neutrophil granular proteins and a variety of pro-inflammatory cytokines. Thus, CIC-mediated enhanced granulocyte activation and modulation of complement function are important features of filarial infection and disease.

Selective intestinal decontamination increases serum and ascitic fluid C3 levels in cirrhosis

Selective intestinal decontamination for 7 days with norfloxacin was performed in 14 cirrhotic patients with ascites and low ascitic fluid total protein. Variations in serum and ascitic fluid of C3 and C4 and ascitic fluid total protein after therapy were compared with those of a control group of 14 untreated patients with similar characteristics. After oral norfloxacin administration, we saw a significant increase of C3 in serum (p less than 0.05) and ascitic fluid (p = 0.01). A significant increase was also observed in ascitic fluid total protein (p less than 0.05) but not in serum and ascitic fluid C4. There were no changes in serum C3, ascitic fluid C3, ascitic fluid C4 or in ascitic fluid total protein in group 2. These data demonstrate that selective intestinal decontamination increases serum and ascitic fluid C3 levels and, therefore, might be useful in preventing spontaneous infections in cirrhotic patients at high risk of infection.

Complements as new diagnostic tools of hepatocellular carcinoma in cirrhotic patients

Serum complements, C3, C4, and C3 proactivator (C3PA), were evaluated prospectively for their diagnostic power in detecting hepatocellular carcinoma (HCC) in patients with liver cirrhosis (LC). Sixty-three LC patients, including 36 LC with HCC patients, were recruited for this study during the period from March to November 1986. The cutoff values of the complements were set according to retrospective study including 17 LC patients and 20 HCC patients. The values with highest accuracy, 75 mg/dl C3, 16 mg/dl C4, and 18 mg/dl C3PA, were selected. Based on these data, the positive predictive values, negative predictive values, and the accuracies of complements were as follows: C3, 88.9%, 85.2%, and 87.3%; C4, 93.8%, 80.6%, and 87.3%; and C3PA, 66.7%, 90.9%, and 70.9%, respectively. The accuracy elevated to 93.7% when C3, C4, and alpha-fetoprotein were combined as the diagnostic criteria. This study supports the use of complement testing as a new diagnostic tool for HCC screening in LC patients and for follow-up evaluation in posttherapy patients of LC with HCC.

Allelic variants for complement factors C3, C4, and B in acute necrotizing ulcerative gingivitis

Impaired immune defense mechanisms and genetic factors appear to play a role in susceptibility to acute necrotizing ulcerative gingivitis (ANUG). Therefore, possible etiological mechanisms might involve genes at the Major Histocompatibility Complex, which include the complement factor loci. We have tested for a possible association between certain complement factor alleles and ANUG using a case-control study design. Specific alleles at complement factors C3 and C4, and properdin factor B (Bf) loci were determined indirectly by high voltage agarose gel electrophoresis in 58 subjects with a history of ANUG and in 58 age-sex-matched healthy controls. The highest relative risk of ANUG, as obtained by conditional logistic regression, for alleles at the C3 locus was 1.9 (90% confidence limits 0.8 to 4.8; p = 0.229) for C3*F-positive individuals. The highest relative risk for alleles at the C4 locus was 2.6 (0.5 to 14.9; p = 0.358) for C4A*3-positive individuals. There was no evidence for an association between Bf allotype and risk of ANUG, with a relative risk of 1.2 for Bf*F- and relative risk of 1.0 for B*S-positive individuals. None of our estimates was statistically significant. We conclude, therefore, that it is unlikely that there is any association between complement factor gene haplotype and susceptibility to ANUG.

An inherited deficiency of the third component of complement, C3, in guinea pigs

Hereditary deficiency of the third component of complement, C3, is found very seldom in the human. C3 deficiency is associated with severe bacterial infections revealing the central role of C3 in complement activation via the classical or alternative pathway. We describe a new hereditary C3 deficiency in strain 2 guinea pigs. Serum from these animals had a markedly reduced lytic activity in a standard assay for complement-dependent, antibody-mediated cytotoxicity. In functional assays of individual components, the hemolytic activity of the components C4, C2, C5 and of factors B, D and H was in the normal range. The functional C3 titer, and similarly C3 antigenic activity in the serum of these C3-deficient animals (C3D) was on average only 5.7% of normal activity. Typing the animals with alloantisera or monoclonal antibodies to guinea pig Ia-antigens revealed that the C3D animals had the major histocompatibility complex-haplotype of inbred strain 2 guinea pigs (B.1, Ia.2,4). The C3 defect is not linked to the major histocompatibility complex and, in addition, is not linked to a C3a receptor deficiency. Macrophages and hepatocytes of the C3D animals have an unimpaired capacity for synthesis and secretion of C3 as measured by enzyme-linked immunosorbent assay. There was no indication for hypercatabolism of normal C3 by the animals as shown by plasma clearance of 125I-radiolabeled C3. Thrombocytes of the C3D animals responded normally to stimulation with purified C3a in an ATP-release assay without an indication for a desensitization in vivo. Possibly the fault resides in an enhanced susceptibility of their own C3 to proteolysis. However, C3 partially purified from the plasma of the C3D animals or secreted by hepatocytes exhibited no obvious structural differences to purified normal C3 in sodium dodecyl sulfate-polyacrylamide gel electrophoresis or in immunoblotting. The C3D serum had a reduced bactericidal activity compared to normal or to C4-deficient serum. Nevertheless, the animals are apparently healthy without an indication for increased frequency of bacterial infections. These guinea pigs provide an unique model for analysis of the biological functions of C3 in vivo and in vitro without the need for artificial C3-depletion procedures with all their known and unknown side-effects.

Complement in the pathophysiology and diagnosis of human diseases

Complement is a humoral effector system composed of 21 plasma proteins that was identified initially because of its cytolytic effects. In addition to cytolysis, complement has a number of different functions related to inflammatory and other host defense processes. The description of the reaction mechanism includes: (1) activation of the classical pathway through recognition of IgG and IgM antibodies by C1q, (2) activation of the alternative pathway which is usually achieved without participation of immunoglobulins, (3) generation of proteolytic enzymes composed of heteropolymers that cleave certain precursor proteins, (4) formation of the membrane attack complex (MAC), and (5) participation of control mechanisms. Methodologies for studying protein concentration and functional activities of complement components include not only the classical hemolytic techniques but also the extremely sensitive new radioimmunoassays and enzyme immunoassays for measuring the products of complement activation that are generated in vivo. Examples of genetically controlled complement deficiencies have been published for most complement components. The symptomatology of some of these patients serves to emphasize the protective role of complement. Acquired deficiencies are significant not only as laboratory aids in diagnosis and to evaluate the course of certain diseases, but also to indicate possible pathogenic disease mechanisms. Recently, it has been recognized that the complement proteins with genes located in the HLA region are polymorphic. Certain variants of proteins C2, C4, and factor B occur with higher frequencies in certain diseases than in the general population, which appears to be of great practical importance in laboratory medicine.

Host defense abnormalities predisposing the patient to infection

Considerable progress has recently been made in defining the cellular and molecular mechanisms involved in host resistance to infection. Virtually every decision related to antibiotic therapy is influenced by an assessment of the integrity of these resistance mechanisms. Defects in each major aspect of host defense, that is, humoral immunity, polymorphonuclear leukocyte defense, and cell-mediated immunity, increase the risk of infection caused by specific groups of microorganisms. Knowledge of these defects will guide the initial (empiric) selection of antibiotics, the dosage and duration of antibiotic therapy, and decisions regarding antibiotic prophylaxis. In the severely immunocompromised patient, antimicrobial therapy frequently involves both the administration of antibiotics and the use of treatment modalities that are likely to augment host defenses.

Genetic deficiency of C4 presenting with recurrent infections and a SLE-like disease. Genetic and immunologic studies

A young girl presenting with recurrent pulmonary infections and atypical lupus erythematosus was totally deficient in C4. In one sister, also deficient in C4, the same symptoms developed. Results of family studies were consistent with an autosomal recessive mode of transmission and with linkage of the genes determining C4 deficiency to those of the major histocompatibility complex. The patient's serum and red cells were Chido- and Rodgers-negative. Humoral and cellular immunity were normal, except for a low lymphocyte response in mixed lymphocyte culture. The cellular function of the patient's polymorphonuclear leukocytes was normal, for both phagocytosis and bactericidal activity using Candida albicans. However, in the presence of C4-deficient serum, opsonin generation and bactericidal indexes were diminished. These defects were completely reversible upon addition of purified C4.

Identification of a monocyte phagocytic defect in a subpopulation of patients with nephritis

Experimental studies have demonstrated the cardinal role played by the mononuclear phagocyte system in the removal of antigen-antibody complexes. To assess the functional capacity of phagocytes in patients with renal disease, 33 normal subjects, 10 patients with mesangial proliferative glomerulonephritis, 8 patients with membranous nephropathy, and 8 patients with moderately severe chronic renal failure were studied by an in vitro assay, measuring the ability of isolated monocytes to ingest sheep erythrocytes coated with IgG antibody and to phagocytize latex beads. Monocytes from four patients with mesangial proliferative glomerulonephritis and one patient with membranous nephropathy exhibited a subnormal capacity to ingest the antibody-coated erythrocytes. Additionally, monocytes from two of the four patients with mesangial proliferative glomerulonephritis and a defect in ingesting sensitized erythrocytes had a subnormal capacity to phagocytize latex beads. The results are interpreted in the context of a hypothesis which suggests that patients with immune nephritis show various forms of immune deficit.

alpha 2-Macroglobulin production by cultured human fibroblasts

Alpha 2-macroglobulin (alpha 2M), a high-molecular-weight plasma protease inhibitor has been shown, by both immunological and functional methods, to be produced by cultured adult lung fibroblasts. Cultured skin fibroblasts synthesized approximately one tenth as much alpha 2M as lung fibroblasts. This quantitative difference in alpha 2M production was also demonstrated in fibroblasts of isogenic origin. There was no difference in the amount of alpha 2M produced between adult and fetal fibroblasts of the same tissue type (i.e., of lung or of skin origin). alpha 2M was produced in culture during log-phase growth as well as at confluence. Two other plasma protease inhibitors, C1-esterase inhibitor and a substance immunologically crossreacting with human inter-alpha-trypsin inhibitor, were also made by the cultured fibroblasts. Plasma protease inhibitors not detectable in culture supernatants were alpha 1-antitrypsin, alpha 1-antichymotrypsin, and antithrombin III.