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The biological role of the complement system and the clinical importance of complement measurements.

Properties of serum proteins belonging to the complement system, two pathways of the complement activation (classical and alternative pathway) as well as the physiological role of the complement system are discussed. Complement has essential importance in some physiological processes: In the induction of the humoral immune response, in the elimination of immune complexes and in the protection against bacterial and viral infections. After a short discussion of the genetics of the complement system, the principle and possibilities of clinical applications of the complement measurements are described. Finally, different approaches to the therapeutic manipulation of the complement system are discussed.

Angioedema↗
PubMed1983

[Complement components, whole complement activity, and circulating immune complexes in neoplastic diseases].

Serum levels of complement components(cc), whole complement activity (CH 50), and circulating immune complexes (IC) were measured in 41 patients with neoplastic diseases. The level of cc was higher than in healthy controls; the levels of C1q, C1INA, C4, C3c, C3ACT, C5, and C9 were statistically higher. In patients with lung cancer, the levels of cc were correlated with the clinical stage as well as the performance status. Both the IC serum level and the incidence of high serum IC levels in lung cancer were higher in stage III and IV than in stage I and II. Serum CH 50 was higher than in healthy controls, but not correlated with the clinical stage.

Aged↗
PubMed1981

Activation of the fifth and sixth component of the complement system: similarities between C5b6 and C(56)a with respect to lytic enhancement by cell-bound C3b or A2C, and species preferences of target cell.

Brief shift of purified C5 and C6 at 0 degrees C to pH 6.4, followed by immediate neutralization, results in the generation of a factor, designated C(56)a, that lyses erythrocytes together with C7, C8, and C9. We compared C(56)a and C5b6 generated by an alternative-pathway convertase, with regard to their action on different target cells. We found tht C(56)a is similar to C5b6 in the following properties: 1) Together with C7, C(56)a forms a stable intermediate on either sheep or guinea pig erythrocytes. 2) Membrane-bound C3b, or A2C incorporated in the membrane, enhances lysis by C(56)a-9, as well as lysis by C5b6-9. We also found that the lysis of EC(56)a7 or EC5b67 intermediates by C8 and C9 depends on the species of the erythrocytes and the species of C8 and C9. Thus, lysis of sheep erythrocytes is more efficient with guinea pig C8 and C9 than with human C8 and C9. In the case of guinea pig erythrocytes, this relationship is reversed, i.e., these cells lyse more efficiently when human C8 and C9 are used. Enhancement of lysis by membrane-bound C3b or A2C does not abrogate this species incompatibility pattern.

Animals↗
PubMed2005

The role of properdin in the assembly of the alternative pathway C3 convertases of complement.

Complement is a powerful host defense system that contributes to both innate and acquired immunity. There are three pathways of complement activation, the classical pathway, lectin pathway, and alternative pathway. Each generates a C3 convertase, a serine protease that cleaves the central complement protein, C3. Nearly all the biological consequences of complement are dependent on the resulting cleavage products. Properdin is a positive regulator of complement activation that stabilizes the alternative pathway convertases (C3bBb). Properdin is composed of multiple identical protein subunits, with each subunit carrying a separate ligand-binding site. Previous reports suggest that properdin function depends on multiple interactions between its subunits with its ligands. In this study I used surface plasmon resonance assays to examine properdin interactions with C3b and factor B. I demonstrated that properdin promotes the association of C3b with factor B and provides a focal point for the assembly of C3bBb on a surface. I also found that properdin binds to preformed alternative pathway C3 convertases. These findings support a model in which properdin, bound to a target surface via C3b, iC3b, or other ligands, can use its unoccupied C3b-binding sites as receptors for nascent C3b, bystander C3b, or pre-formed C3bB and C3bBb complexes. New C3bP and C3bBP intermediates can lead to in situ assembly of C3bBbP. The full stabilizing effect of properdin on C3bBb would be attained as properdin binds more than one ligand at a time, forming a lattice of properdin: ligand interactions bound to a surface scaffold.

Binding Sites↗
PubMed1995

Transforming growth factor beta modulates C3 and factor B biosynthesis and complement receptor 3 expression in cultured human monocytes.

Complement biosynthesis in monocytes is stimulated by different pathogens and modulated by a variety of cytokines, but little is known about the possible effect of transforming growth factor beta (TGF-beta) on this monocyte function. We therefore studied the effect of TGF-beta 1 and TGF-beta 2 on constitutive, lipopolysaccharide (LPS)- and Candida albicans-induced monocyte biosynthesis of complement components C3 and factor B. Under all three conditions, both forms of TGF-beta (20 ng/ml) induced a two- to fourfold increase in C3 concentration in monocyte supernatants harvested after 2 or 5 days of cell culture, an effect that was abrogated by cycloheximide. In contrast, constitutive and pathogen-induced production of factor B was suppressed by TGF-beta. The effects of TGF-beta on complement production were neutralized by a monoclonal anti-TGF-beta antibody. Moreover, TGF-beta suppressed the pathogen-induced release of granulocyte-macrophage colony-stimulating factor and down-regulated the expression of complement receptor 3 (CD11b/CD18), while the expression of CD11a/CD18, a related beta 2 integrin, was unaffected. These novel effects of TGF-beta emphasize the immunomodulatory significance of this cytokine.

Adjuvants, Immunologic↗
PubMed1986

Complement fixation by Brazilian pemphigus foliaceus antibodies in vitro.

In order to examine the presence of intercellular substance antibodies (ICS Ab) which will fix complement in vitro, in the sera of patients with Brazilian pemphigus foliaceus (BPF), 7 BPF sera were tested using immunofluorescence (IF) and complement IF (CIF). All 7 sera possessed IgG ICS Ab at titers varying from 1/320 to 1/2,560 (normal human skin as substrates). Using CIF, 2 out of 7 sera proved to have a capacity to fix complement (C3) in vitro at a titer of 1/40. Although the titer of complement-fixing ICS Ab detected by CIF was lower than IgG ICS Ab detected by IF, these results with the evidence of frequent C3 deposition in the lesional skin of BPF suggest the necessity of further studies on the role of complement system in BPF.

Autoantibodies↗
PubMed1990

Association of C3 and C4A complement types with familial amyloidotic polyneuropathy.

A mutant variant of the serum protein transthyretin (TTR-met30) appears to be a necessary but not sufficient condition for the development of familial amyloidotic polyneuropathy (FAP). We have studied a number of serum protein markers (alpha 1-antitrypsin, properdin factor B, C3, C4A, C4B, haptoglobin, transferrin and group-specific component) in FAP patients and healthy controls in an attempt to identify additional pathogenic factors which may influence the risk for developing FAP in male and female patients as well as the age of onset of the disease. Statistically significant associations were found in the complement systems C3 and C4A. The C3F variant was significantly increased in all FAP patients with a relative risk (RR) of 2.0, more pronounced in female patients (RR = 2.6) and patients with an early onset of the disease (RR = 4.5). In the FAP patients only the variants A3 and A4 were found in the C4A system. C4A3 was found in all patients, which was significantly higher than in the controls. The remaining serum protein systems showed no statistically significant associations with FAP. The results suggest that genetic variants of complement factors C3 and C4A may interact with the mutant TTR-met30 by modifying the expression and onset of FAP.

Age Factors↗
PubMed1982

[Intranuclear alpha 1-antitrypsin in HBs-Ag-seronegative hepatitis B with PAS-negative globular hyaline bodies containing the complements components C4, C3, and C3-activator (author's transl)].

In the liver biopsy specimen of a 16 year old patient with HBsAg-seronegative hepatitis B PAS-negative globular hyaline bodies reacted with antibodies to the complement components C4, C3, and C3-activator, but not with antibodies to alpha 1-antitrypsin. However, alpha 1-antitrypsin could be identified in the liver cell nuclei, cytoplasm, and cell membrane. Probably, these findings are caused by an hereditary or acquired metabolic disorder.

Adolescent↗
PubMed1988

Monoclonal rheumatoid factor-IgG immune complexes. Poor fixation of opsonic C4 and C3 despite efficient complement activation.

Monoclonal IgM rheumatoid factor forms complexes with IgG in essential mixed cryoglobulinemia. We demonstrate that such complexes fix C3 and C4 poorly, although efficient fluid-phase C3 conversion can occur. Fixation of small amounts of C4 may be sufficient to generate a C3 convertase, but may prevent subsequent fixation of C3 by competing for binding sites on the complex. These complexes bind inefficiently to normal erythrocyte complement receptor type 1 (CR1) in vitro, and are undetectable on erythrocytes of patients with essential mixed cryoglobulinemia in vivo. Clearance of such phlogistic complexes from tissues by CR1-bearing cells may be inefficient.

Antibodies, Monoclonal↗
PubMed2002

A tumor-expressed inhibitor of the early but not late complement lytic pathway enhances tumor growth in a rat model of human breast cancer.

Membrane-bound complement inhibitors protect host cells from inadvertent complement attack, and complement inhibitors are often up-regulated on tumors, possibly representing a selective adaptation by tumors to escape elimination by a host antitumor immune response. Relevant in vivo studies using rodent models of human cancer have been hampered by the fact that human complement inhibitors are not effective against rodent complement. Using nude rats and MCF7 cells expressing different rat complement inhibitors, a model of human breast cancer was established to investigate the role of complement and complement inhibitors in tumor progression. Expression of rat CD59, an inhibitor of the terminal cytolytic membrane attack complex of complement, had no effect on the incidence or growth rate of MCF7 tumors. In contrast, expression of rat Crry, an inhibitor of complement activation, dramatically enhanced the tumorigenicity of MCF7 cells. The expression of rat Crry on MCF7 inhibited the in vivo deposition of complement C3 fragments that serve as opsonins for receptors on phagocytes and natural killer cells. These data provide direct in vivo evidence that an inhibitor of complement activation can facilitate tumor growth by modulating C3 deposition. These data indicate an important role for complement opsonization in promoting cell-mediated antitumor immune function, a conclusion further supported by the demonstration that expression of rat Crry, but not rat CD59, on MCF7 cells inhibits rat cell-mediated cytotoxicity in vitro. Rat complement activation on MCF7 tumors was mediated by tumor-reactive antibodies present in the serum of naïve nude rats, but there was also an IgM response to MCF7 tumors, a situation with similarities to some human cancers. These data support a hypothesis that blocking complement inhibitor function on tumor cells will not only enhance monoclonal antibody-mediated immunotherapy but may also be effective at enhancing a normally ineffective humoral immune response in the absence of administered antitumor antibody.

Animals↗
PubMed1985

Biosynthesis of the third (C3), eighth (C8), and ninth (C9) complement components by guinea pig hepatocyte primary cultures.

In the present report guinea pig hepatocyte primary cultures were established in order to study the synthesis of the eighth (C8) and ninth (C9) complement component. As reference-protein, the third complement component (C3) was measured antigenetically and hemolytically. Synthesis of C8 and C9 was determined by means of the hemolytic activity of the culture supernatant harvested every 24 h during a 6-day incubation period in vitro. The data to be reported demonstrated that the hepatocytes are able to synthesize spontaneously and secrete C8 and C9; in their culture medium a hemolytic activity of about 15-25 X 10(8) em/10(6) cells/24 h for C8 and of 25-90 x 10(8) em/10(6) cells/24 h for C9 were found. The same hepatocyte cultures produced 2500-6000 micrograms/10(6) cells/24 h of C3. Hemolytic C3 activity was also found in the culture media. The synthesis of C8 and C9 could be reversibly inhibited by addition of 30-50 micrograms cycloheximide per ml of culture medium. The kinetics of synthesis show a slight decrease after the first day of culture and a recovery in the following days up to a rate that is two- to threefold higher than that of the first day. The data suggest that hepatocytes could contribute to the production of C8 and C9 present in the plasma.

Animals↗
PubMed2002

A complement component C3-like protein from the tunicate, Styela plicata.

This study identifies a complement component C3-like protein in the solitary tunicate, Styela plicata. Three different polyclonal antibodies raised against C3 molecules from two species (humans and the tunicate, Halocynthia roretzi) were used to identify the C3-like protein in S. plicata hemolymph. The C3 cross-reactive protein is a 170kDa heterodimer composed of polypeptides (116 and 80kDa) that have molecular weights comparable to those of C3alpha and C3beta from other species. Amino acid sequencing and amino acid composition analysis confirmed that the C3-like protein from S. plicata is closely related to C3 from H. roretzi.

Amino Acid Sequence↗
PubMed1978

C3 component of complement secreted by established cell lines.

The hamster cell line NIL8 secretes the C3 component of complement as well as collagenous molecules and fibronectin (LETS protein). The C3 is found in the culture medium as a disulfidebonded complex of two polypeptides of 130,000 daltons (alpha) and 65,000 daltons (beta). The secreted C3 can be quantitatively cleaved to C3b and further cleaved by C3 inactivators. The activation of C3 to C3b is promoted by zymosan or by antibody-coated erythrocytes, demonstrating participation in both the classical and alternative complement pathways. The availability of this culture system has enabled us to show that C3 is synthesized as a 185,000 dalton precursor (proC3) which is biologically inactive and becomes cleaved to active C3. Some other established cell lines (NIL1 and BALB/c 3T3) also secrete C3, but some others do not.

Animals↗