The story of complement factor I.

Factor I was first discovered in 1966. Its importance became apparent with the description of the original Factor I deficient patient in Boston in 1967. This patient presented with a hyperactive alternative complement pathway resulting in secondary complement deficiency due to continuous complement consumption. On the basis of these findings, the mechanism of the alternative pathway was worked out. In 1975, the surprise finding was made that elevating levels of Factor I in plasma down-regulated the alternative pathway. Attempts to exploit this finding for clinical use had a long and frustrating history and it was not until 2019 that the first patient was treated with the gene therapy vector for age related macular degeneration by Professor Sir Robert MacLaren in Oxford. This review follows the long and contorted course from initial observations to clinical use of complement Factor I.

Autoantibodies Against C3b-Functional Consequences and Disease Relevance.

The complement component C3 is at the heart of the complement cascade. It is a complex protein, which generates different functional activated fragments (C3a, C3b, iC3b, C3c, C3d). C3b is a constituent of the alternative pathway C3 convertase (C3bBb), binds multiple regulators, and receptors, affecting thus the functioning of the immune system. The activated forms of C3 are a target for autoantibodies. This review focuses on the discovery, disease relevance, and functional consequences of the anti-C3b autoantibodies. They were discovered about 70 years ago and named immunoconglutinins. They were found after infections and considered convalescent factors. At the end of the twentieth century IgG against C3b were found in systemic lupus erythematosus and recently in lupus nephritis, correlating with the disease severity and flare. Cases of C3 glomerulopathy and immune complex glomerulonephritis were also reported. These antibodies recognize epitopes, shared between C3(H2O)/C3b/iC3b/C3c and have overt functional activity. They correlate with low plasmatic C3 levels in patients. In vitro, they increase the activity of the alternative pathway C3 convertase, without being C3 nephritic factors. They perturb the binding of the negative regulators Complement Receptor 1 and Factor H. The clear functional consequences and association with disease severity warrant further studies to establish the link between the anti-C3b autoantibodies and tissue injury. Comparative studies with such antibodies, found in patients with infections, may help to uncover their origin and epitopes specificity. Patients with complement overactivation due to presence of anti-C3b antibodies may benefit from therapeutic targeting of C3.

[Vancomycin-resistant Enterococcal meningitis in patients with ventriculoperitoneal shunt: two cases report and review of the literature].

Enterococcal meningitis often occurs in patients with head trauma, cerebrospinal fluid leakage, or shunt devices. Herein, we describe two patients, a 12-year-old boy and a 57-year-old man, who developed meningitis due to vancomycin-resistant Enterococcus (VRE) gallinarum. Both of them had a ventriculoperitoneal shunt for hydrocephalus. The meningitis in one patient was due to fecal transmission of VRE derived from the gut and in the other from prolonged use of vancomycin. In addition, both patients were immunocompromised due to radiotherapy and steroid therapy. The mechanism of developing resistancy and the management of VRE meningitis is discussed.

Conglutinin and immunoconglutinin titers in stressed calves in a feedlot.

OBJECTIVE: To determine whether increased conglutinin titers are evident in stressed calves that do not develop respiratory tract disease in feedlots, compared with respiratory tract disease, and to determine the increase in immunoconglutinin titers. ANIMALS: 101 mixed-breed beef calves. PROCEDURE: Calves were processed at 4 farms of origin and allowed to remain with their dams for another 100 days. Calves from each farm were brought to a centrally located order-buyer barn. In a feedlot, 101 calves were assigned to pens and observed daily for clinical signs of acute respiratory tract disease. When sick calves were detected, they were treated with antibiotics and isolated in a pen for 4 days. Conglutinin and immunoconglutinin titers were determined for all calves. RESULTS: During the 28-day study, 73 calves developed respiratory tract disease, whereas 28 calves remained healthy. Mean conglutinin titers differed significantly among calves from the 4 farms. Significant differences were not detected in conglutinin titers among calves on the basis of sex, morbidity, or vaccination status against Mannheimia haemolytica at each farm, the order-buyer barn, or the feedlot on days 8, 15, and 28 after arrival. Immunoconglutinin titers in calves differed significantly among farms and morbidity status. CONCLUSIONS AND CLINICAL RELEVANCE: Mean conglutinin titers in calves do not appear to be associated with the incidence of acute respiratory tract disease; however, increased immunoconglutinin titers appear to be associated with recovery of stressed calves from respiratory tract disease during the first 15 days after arrival in a feedlot.

Correlation between anti-C1q and immune conglutinin levels, but not between levels of antibodies to the structurally related autoantigens C1q and type II collagen in SLE or RA.

The simultaneous appearance of autoantibodies with either a functional or structural relationship to anti-C1q antibodies (anti-C1q) was investigated in 39 systemic lupus erythematosus (SLE) patients and in 28 rheumatoid arthritis (RA) patients, in both cross-sectional and longitudinal design. Levels of anti-C1q showed an isotype-specific correlation to levels of immune con-glutinin (IK) in SLE patients, whereas no correlation was evident to levels of antibodies to the structurally related antigen type II collagen (anti-CII) in SLE or RA patients. IgG anti-C1q levels correlated with serum levels of the terminal complement complex (sC5b-9) in SLE patients. In two longi-tudinally followed patients, the IK response preceded the anti-C1q response. Possibilities for regulation of the humoral anti-complement response are discussed.

Detection and characterization of immunoconglutinins in patients with systemic lupus erythematosus (SLE): serial analysis in relation to disease course.

The levels of IgA, IgG and IgM immunoconglutinins (IK) were assessed in sera from 20 patients with SLE which were followed for 8-month periods. At the time of the exacerbation, IgG IKs were significantly increased to 226 +/- 90 arbitrary units (mean +/- s.e.m.) compared with both the minimum value of 75 +/- 28 in the SLE patients and with 31 +/- 2 in healthy controls (P < 0.05). There was no difference between SLE patients and controls in the levels of IgM and IgA IKs. Most of the SLE patients in this material showed maximal IgG IK levels before exacerbation, but there was no correlation between the clinical disease index and the levels of IgG IK. The specificity of IgG IKs showed a broad diversity for microtitre-fixed C3b, iC3b, C3c and C3dg. The antibodies were of IgG1, IgG3 and in two patients, IgG4 subclass. IgG IKs were correlated to the C3d/C3 ratio which suggested that the IK responses were secondary to C3 activation. In summary, unlike other conditions associated with complement activation where elevated IgM IKs are common, an increase in IgG IK levels was observed. It is possible that this diverging IK response contributes to the pathophysiology of the disease.

Inhibition of human immunodeficiency virus-1 infection by human conglutinin-like protein: in vitro studies.

The lectin-like protein analogous to bovine conglutinin was purified from human serum. The carbohydrate-binding ability of conglutinin-like protein was inhibited by D-mannose, N-acetylglucosamine and L-fucose as well as by mannan-containing oligosaccharides. By applying a lectin-based ELISA system it was demonstrated that conglutinin-like protein binds to human immunodeficiency virus-1 (HIV-1) glycoprotein 120 (gp120) via its carbohydrate binding site. In vitro experiments with T-lymphoblastoid CEM cells revealed that conglutinin-like protein abolishes infection by HIV-1; a 50% cytoprotective concentration of 23.9 micrograms/ml was measured. These findings demonstrate that human conglutinin-like protein binds to HIV-gp120 and inhibits, under the described in vitro conditions, CEM cell infection.

Purification and characterization of IgG immunoconglutinins from patients with systemic lupus erythematosus: implications for a regulatory function.

The levels of IgG immunoconglutinins in plasma from patients with rheumatoid arthritis, systemic lupus erythematosus and primary biliary cirrhosis were monitored by ELISA. High levels of IgG immunoconglutinins were found mainly in plasma from patients with systemic lupus erythematosus. These immunoconglutinins bound to microtitre plate-fixed C3, C3b and C3c but poorly to C3d. This binding was inhibited by particle-bound C3b and iC3b but not by the corresponding soluble fragments. Furthermore, Western blot analysis revealed no immunoconglutinin-binding to reduced C3 peptides and no binding was shown to soluble C3 alpha and beta chain by ELISA. IgG immunoconglutinins were purified from three plasma specimens by affinity chromatography on activated thiol sepharose ATS/C3 fragments. Two immunoconglutinin preparations that preferentially recognize ATS-C3b, inhibited C5-convertase function by 50-100% while one immunoconglutinin that recognized ATS-C3d,g had no effect. The two former immunoconglutinins also inhibited all three factor I cleavages in C3 alpha chain but the latter inhibited only the third cleavage. None of the immunoconglutinins affected the binding of complement-coated anti-BSA/BSA complexes to CR1 (CD35) on human erythrocytes, but the two immunoconglutinins that inhibited all factor I cleavages also inhibited the factor I-induced release of anti-BSA/BSA complexes from CR1. The results show that immunoconglutinins recognize specific epitopes on bound C3 fragments and that they are able to modulate C3-mediated functions.

The pathogenesis of heartwater.

Hypotheses on the pathogenesis of heartwater, which have been published so far, are briefly reviewed. Attempts were made at counteracting the effects of vaso-active substances released by mast cells by treating mice infected with Cowdria ruminantium with antagonists to histamin and serotonin on one hand, and with mast cell stabilizers on the other, but were not successful. Preliminary findings suggest that a hypersensitive type of reaction, triggered by the release of pharmacologically active substances, may possibly be basic to the pathogenesis of heartwater. Complement, and the products of arachidonic acid metabolism, possibly play a role in the release of the vaso-active substances.

[Serum immunoconglutinin in patients with a respiratory allergy syndrome, before and after specific immunotherapy]. / L'immunoconglutinina serica in pazienti con sindrome allergica respiratoria, prima e dopo immunoterapia specifica.

Immunoconglutinins (Ic) are a group of predominantly IgM antibodies formed towards antigenic determinants exposed in fixed complement components (C3b and C4). Ic production is stimulated by bacterial and viral infections; elevated titers were also found in a number of other diseases involving completement-fixing reactions in vivo. High titers have been associated with the infectious morbidity within a particular population. The object of this investigation was to determine the correlations of autostimulated Ic titre with several parameters in 100 atopic nondesensitized subjects with respiratory allergies and in 140 non atopic, healthy, individuals. Eighty of them had rhinitis and/or conjunctivitis, 9 asthma and 11 rhinitis and asthma. All the atopics were studied also after at least 6 months of specific immunotherapy. None of the subjects received any other therapy at the time of study. Atopic subjects with high titers of Ic are significantly more numerous than controls, without significant correlation with the kind and beginning of atopic syndrome, and with immunotherapy. These results might be attributed to a higher incidence of infections in the atopic population. The Ic might play an important role in the immunoregulation involving complement system.

Human conglutinin-like protein.

The presence in human plasma of a molecule homologous to bovine conglutinin is indicated by the results of biological and immunochemical analysis. The human conglutinin-like protein shows calcium-dependent binding to complement-treated solid phase IgG and immunological cross-reaction with chicken anti-bovine conglutinin. The binding of the human protein to complement-treated IgG was inhibited by N-acetyl-D-glucosamine but not by other sugars. Analysis by SDS-PAGE and Western blotting showed reaction of anti-conglutinin with molecules of similar mobility to the monomer and hexamer of bovine conglutinin.

Complement reversal of immunosuppression induced with plasma of rats infected with Trypanosoma brucei rhodesiense.

Suppression of antibody production by splenic lymphocytes from rats immunized with sheep red blood cells (SRBC) after incubation with plasma from rats infected with Trypanosoma brucei rhodesiense was confirmed. Suppressive activity became evident in plasma after the sixth day of infection and was manifested by reduction in the number of hemolytic Jerne plaques produced by the treated cells. The activity was temporally associated with increased amounts of soluble immune complex (SIC) reduced titers of lytic complement, elevated titers of immunoconglutinin (IK) and anemia. Treatment of suppressive plasma with hemolysin sensitized SRBC alexinated with horse complement to reduce IK did not reduce suppressive activity, and the activity appeared to have been enhanced when the plasma was heated to inactivate the remaining complement (C'). When fresh rat C' was added to the treated cells, the suppression was largely, though not completely, reversed. Treatment of spleen cells with SIC prepared in vitro from bovine serum albumin (BSA) and rabbit antiBSA also suppressed the plaque forming capacity of the cells. Complexes of BSA-antiBSA-C' and complexes of BSA-antiBSA-C'-IK were equally suppressive. Again, addition of fresh C' to cells treated with these complexes largely, though not completely, reversed the suppressive effect on the cells. From the results it is suggested that immunosuppression associated with experimental T. b. rhodesiense infection may be in part a suppression of the capacity of induced lymphocytes to produce antibody. It is possible that the suppression was mediated by SIC present in the plasma of the infected rats and this effect was probably enhanced by reduced levels of complement in the suppressive plasma.

Binding capacity of sera from systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA) to C3c. Evaluation by enzyme linked immunosorbent assay (ELISA).

Immunoconglutinins (IKs) are autoantibodies directed against antigenic determinants on C3 complement component. An ELISA was performed to detect IKs in sera from 50 RA patients, 50 SLE patients and 50 normal subjects. Comparison showed significantly higher levels in patients than in normal subjects (p less than 0.001) and higher IKs levels in RA than in SLE (p less than 0.001). IKs were not related to others biological tests, except a statistically significant inverse correlation between IKs and circulating immune complexes levels detected by conglutinin binding assay in RA.

A new enzyme-linked immunosorbent assay (ELISA) for measuring immunoconglutinins directed against the third component of human complement. Findings in systemic lupus erythematosus.

A new enzyme-linked immunosorbent assay, using purified activation product of the third component of human complement (C3b), detects immunoglobulins of the IgG isotype which demonstrate affinity for C3b (C3b-immunoconglutinins; C3b-IK). This assay offers significantly improved specificity compared to previous immunoconglutinin (IK) assays in that it not only defines the isotype and antigenic specificity of the IK but also eliminates false positive results associated with immune complexes or aggregated human gamma globulin, or with natural antibodies directed at heterologous reagents. Using this assay, we observed elevated C3b-IK levels in serum of 34 systemic lupus erythematosus (SLE) patients when compared to serum of 13 healthy controls. Comparing sera from patients with clinically active and clinically inactive lupus showed greater immunoconglutinin levels in the active group. Immunoconglutinin levels did not correlate with the erythrocyte sedimentation rate, total hemolytic complement, or with circulating immune complex levels by the Raji cell and C1q-binding assays.

Immunoconglutinin and suppression of an induced immune response by plasma from rats infected with Trypanosoma brucei rhodesiense.

Fresh plasma from rats infected with Trypanosoma brucei rhodesiense, incubated with splenic lymphocytes from rats previously immunized with sheep blood cells, suppressed the capacity of the splenic lymphocytes to produce antibody as was indicated by reductions in the numbers of hemolytic Jerne plaques produced by the treated cells. The effect was maximal in plasma samples drawn on the sixth to eighth day of infection when they contained elevated amounts of soluble immune complex, high titers of immunoconglutinin (IK), and reduced titers of lytic complement. We suggest that the active plasma may have affected the antibody-producing cells by one or both of two mechanisms. Soluble antigen-antibody complexes may have interacted with Fc receptors of activated lymphocytes to suppress antibody production. Alternatively, complement-fixing soluble immune complexes may have reacted with C3b receptors of the lymphocytes. These lymphocytes coated with the antigen for IK could then be injured by immunoconglutination.

Immune complexes and immunoconglutinin interactions associated with altered lymphocyte activity in Plasmodium chabaudi infections.

Fresh plasma from rats infected with Plasmodium chabaudi, incubated with splenic lymphocytes from rats immunized 5 days previously with sheep blood cells, suppressed the capacity of the spleen cells to produce antibody against the sheep cells as was indicated by reductions in the numbers of hemolytic Jerne plaques formed by the treated cells. The effect was maximal in plasma of rats drawn on the 7th day of infection at a time the rats experienced a hemolytic crisis. Serologic studies indicated that the active plasma contained elevated titers of antibody against fibrinogen products, antibody against the soluble serum antigens elaborated during blood infections and antibody against the third component of fixed complement (C3) or immunoconglutinin. Titers of lytic complement were reduced and amounts of soluble immune complex precipitated with polyethylene glycol 6000 were elevated. The active plasma may have affected the antibody producing cells by one or both of two mechanisms. Soluble antigen-antibody complexes could have interacted with Fc receptors of activated lymphocytes to alter their function. Alternatively, the complexes may have fixed complement and interacted with receptors for fixed C3 on the lymphocyte membrane. Such cells, being coated with the antigen for immunoconglutinin, could be altered by immunoconglutination. Inasmuch as the immune complexes in the active plasma were generated in vivo, it would seem unlikely that the plasma would contain significant amounts of complex that had not fixed complement. With immunoconglutinin present in the plasma, alteration of the cells by immunoconglutination seems a more likely possibility.

Interrelationships of immunoconglutinin, immune complexes, and complement in anemia, thrombocytopenia, and parasitemia of acute and chronic malaria in rats.

Anemia, thrombocytopenia and reduction in parasitemia in P. chabaudi infection of rats were associated with appearance in blood of soluble immune complexes, immunoconglutinin (IK), and by reductions in titers of lytic complement. With reduction of parasitemia to subpatent levels, anemia, but not thrombocytopenia, persisted and erythrocyte counts did not return to preinfection levels for several weeks. This chronic anemia was accompanied by elevated amounts of soluble immune complex, depressed titers of lytic complement and persistence of IK. Evidence was presented indicating that the infections of the rats persisted in a chronic form. It was thus indicated that immune interactions, related to anemia and clearance of parasitemia, persisted in absence of microscopically evident parasitemia, and may have been in part responsible for the persistent anemia. Based on the evidence cited, it is suggested that complement-fixing immune complexes attach to blood cells, infected as well as uninfected, and that these cells are sequestered and phagocytized in the spleen after immunoconglutination by IK. It is also suggested that this interaction continued after the clearance of patent parasitemia and accounted for the persistence of anemia in the chronic phase of infection.

Conglutinin, immunoconglutinins and heterophile antibodies in the sera of apparently healthy moose (Alces alces) and caribou (Rangifer tarandus) in Quebec.

Serum samples from apparently healthy wild populations of moose and caribou in the province of Quebec, Canada were screened for the presence of conglutinin (K), immunoconglutinins (IKS) and heterophile antibodies. The conglutinating factor in moose and caribou sera was characterized utilizing the necessity of calcium ions for its reaction with sensitized sheep erythrocytes which had been alexinated with horse complement. The conglutinating substance in these animals did not require calcium ions for its activity. The conglutinating activity in both moose and caribou sera was characterized due to IKS as those present in sheep, dog and rabbit sera. Both moose and caribou had non-agglutinating type of heterophile antibodies. Their titres varied from 0 to 80. None of the animals tested had K in their blood. The titre of IKS varied from 0 to 640 with a mean value of 41 in moose, whereas it varied from 0 to 80 with a mean value of 18 in caribou. About 75% of all the animals in both the groups were positive for IKS. The specificity of IKS was demonstrated by the total removal of its activity on absorption with alexinated cells. Presence of IKS in these animals is suggestive of latent infection(s) possibly of bacterial, viral or parasitic origin.