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PubMed1983

C3b acceptors on macrophages: inhibition of Fc gamma-receptor-mediated phagocytosis by acceptor-bound C3b.

The binding of nascent human C3b (i.e. the fragment of C3 just after trypsin cleavage) to mouse peritoneal macrophages was demonstrated by immune adherence. Acceptor-bound C3b could be detected longer than 24 h on the cell membrane. The rosette formation and phagocytosis of SRBC coated with anti-SRBC rat IgG was inhibited by preincubation of the cells with C3 and trypsin (15 min, 37 degrees C). However, the phagocytosis of opsonized yeast particles was not influenced by acceptor-bound C3b, proving that C3b-C3b acceptor interaction did not alter the function of C3b-receptors. Acceptor-bound C3b on the macrophages failed to mediate phagocytosis of human 0,Rh+ red cells having C3b-receptors.

Animals↗
PubMed1990

The leukocyte cell surface receptor(s) for the iC3b product of complement.

CR3 is probably the major adhesion molecule on monocytes and neutrophils. Its function as a phagocytic receptor for iC3b-coated particles has been well characterized. CR3 also has binding affinity for other ligands, including those that compete with iC3b such as fibrinogen, factor X, and beta-glucan, and those that do not such as bacterial LPS. CR3 binding to endothelial cells probably plays an important role in the extravascular migration of monocytes and neutrophils, but the ligand that it recognizes on endothelial cells has not been identified. Structurally CR3 belongs to the integrin family, and it shares a common subunit with p150,95 and LFA-1. The expression of these three membrane antigens appear to be limited to leukocytes, and they are sometimes referred to collectively as the leukocyte integrins. All three antigens have a common binding affinity for bacterial LPS. p150,95 also has affinity for iC3b, but p150,95/iC3b-dependent cellular responses has not been demonstrated. Its status as a complement receptor therefore awaits further experimental support.

Amino Acid Sequence↗
PubMed1989

[Indicators of specific adhesiveness of neutrophils in children with chronic hepatitis].

Altogether 58 children with chronic hepatitis were examined for specific C31- and Fc-dependent neutrophil adhesion. Considerable alterations were found as dependent on the disease pattern and the degree of the pathological process activity in the liver. The data obtained promote verification of the pathogenesis of chronic hepatitis in children.

Adhesiveness↗
PubMed1994

The leukocyte integrin p150,95 (CD11c/CD18) as a receptor for iC3b. Activation by a heterologous beta subunit and localization of a ligand recognition site to the I domain.

p150,95 is a member of the leukocyte integrin family of adhesion proteins. Compared with LFA-1 and Mac-1, p150,95 is less well functionally characterized. Although p150,95 has complement receptor activity for iC3b and has been designated complement receptor type 4, transfected cells expressing p150,95 do not bind iC3b-sensitized cells. We report that cells cotransfected with a human p150,95 alpha subunit and a chicken, but not human, beta subunit bind IgM-iC3b-coated erythrocytes, suggesting that interactions between the alpha and beta subunits can regulate p150,95 adhesiveness. Furthermore, purified human p150,95 binds to cell-bound iC3b-coated erythrocytes. Because binding to iC3b by cellular and purified p150,95 is specifically abolished by mAbs that localize to the I domain of p150,95, we suggest that the I domain of the p150,95 alpha subunit is an important ligand recognition site for iC3b.

Animals↗
PubMed1993

[The effect of heroin on red cell immunoadherence function of human body].

Red cell immunocompetence was studied in 38 cases of heroin addicts by measuring the rate of red blood cell C3b receptor rosette (RBC-C3bRR), and red blood cell immune complex rosette (RBC-ICR) the concentration of complement C3 and the content of circulating immune complex (CIC). The results showed that the rate of RBC-C3bRR and RBC-ICR and C3 concentration in the heroin addicts decreased significantly, while their CIC content increased significantly as compared with those of the controls. It was also found that the duration of drug taking was related with the changes of the rate of RBC-C3bRR and RBC-ICR, C3 concentration and CIC content. It is suggested that red cell immunoadherence function in heroin addicts decreases significantly as to influence the clearance of CIC.

Adolescent↗
PubMed1979

The role of C4-binding protein and beta 1H in proteolysis of C4b and C3b.

Two forms of C4-binding protein (C4-bp) (C4-bp low, C4-bp high), which differ slightly in net charge and apparent molecular weight, as determined by SDS- PAGE, were separated by ion-exchange chromatography and contaminants removed with specific antisera. Both forms of C4-bp served as cofactors for the cleavage of C4b in solution by C3b inactivator, and the resulting fragments of the a'-chain of C4b had identical molecular weights. In addition, similarly to beta1H, C4-bp low or high served as cofactors for the cleavage of fluid phase C3b by C3bINA. However, important quantitative differences between the activities of C4-bp and beta1H were observed. With regard to C3b in solution, the cofactor activity of beta1H was {approximately equal to}20 times greater than that of C4-bp on a weight basis. In relation to cell-bound C3b, the differences in activity were even more marked. Whereas beta1H enhanced the effects of C3bINA on the erythrocyte intermediate EC3b, inhibiting the assembly of EC3bBb, C4-bp was without effect even at concentrations {approximately equal to}300 times greater than beta1H. Therefore, under physiological conditions, it is likely that beta1H is the key protein which controls the function of C3b, and that C4-bp activity is directed mainly toward the cleavage of C4b. We also examined the relation between C4-bp and the C3b-C4bINA cofactor described by Stroud and collaborators (3, 4). By functional, physico-chemical and immunological criteria, they are the same protein.

Complement C3↗
PubMed1985

Herpes simplex virus type 1 infection of endothelial, epithelial, and fibroblast cells induces a receptor for C3b.

We recently demonstrated that herpes simplex virus type 1 (HSV 1) induces a receptor on human umbilical vein endothelial cells for complement component C3b (C3bR). We assigned this receptor function to HSV 1 viral glycoprotein C (gC) based on several observations: tunicamycin, which prevents glycosylation and expression of N-linked glycoproteins on the surface of infected cells, markedly reduced expression of the C3bR; monoclonal antibodies to HSV 1 gC blocked detection of the C3bR, whereas monoclonal antibodies to other HSV 1 glycoproteins (gB, gD, gE) had no effect; and the MP mutant of HSV 1, which fails to express gC, did not induce C3bR. We now report that HSV 1 induces C3bR on a wide variety of cell types including bovine thoracic aorta and pulmonary artery endothelial cells, human embryonic lung and embryonic foreskin fibroblasts, and human embryonic kidney cells. To date, all cells studied that are permissive to HSV 1 express C3bR, although the pattern of rosetting of C3b-coated erythrocytes varies among the cell strains examined. We also demonstrate that C3bR expression is not a general response of human umbilical vein endothelial cells to injury, because three other viruses (adenovirus 7, measles, and mumps) do not induce C3bR after infection of these cells. Previously we had shown that among herpes simplex viruses, a variety of HSV 1 strains induce C3bR, whereas HSV 2 strains do not. We now demonstrate that other herpes family viruses (CMV and VZV) do not express C3bR. Therefore, C3bR expression appears to be unique for HSV 1 and occurs on a wide variety of cells permissive to this virus.

Adenovirus Infections, Human↗
PubMed1984

Lack of adherence of IgG-, IgM-, and C3b-sensitized erythrocytes to human heart valves.

In a search for receptors for immunoglobulin Fc fragment and activated complement component C3 (C3b) in normal or rheumatic heart valve tissues, sheep erythrocytes sensitized with IgM (IgM EA), IgG (IgG EA) and IgM plus activated complement (IgM EA C3b) were used in a closed-chamber immune adherence technique. Neither this method nor a separate immunofluorescence technique revealed the presence of such immunologic receptors in frozen sections of heart valves.

Adult↗
PubMed1990

Peripheral nerve CR1 express in situ cofactor activity for degradation of C3b.

Adsorption of sheep erythrocytes (E) sensitized with IgM antibodies (A) and C3b (EAC3b) to C3b/C4b receptors (CR1) in cryostat sections of human myelinated nerves was studied using the closed chamber technique. The adsorption was stable for at least 3 h at 37 degrees C. In the presence of purified factor I, the indicator cells detached from the sections after 40 min at 37 degrees C. Factor H was not required. The release was not due to loss of CR1 activity in the sections. The detached indicator cells were negative in the immune adherence test and were agglutinated by antibody to C3d, but not by antibody to C3c. Western blot of the detached indicator cells revealed the presence of C3d and C3c was found in the chamber fluid. Accordingly, detachment of the indicator cells was due to degradation of C3b to C3d with the release of C3c into the chamber fluid. Protease inhibitors did not prevent the detachment of the indicator cells. EAC3b incubated with sections of myelinated nerves pre-incubated with anti-CR1 antibody or with sections of unmyelinated nerves which contain functionally inactive CR1 were not degraded. The results therefore indicate that CR1 in situ in myelinated nerves can provide the necessary cofactor activity for factor I-mediated degradation of C3b to C3d and C3c.

Animals↗
PubMed1999

Functional properties of complement factor H-related proteins FHR-3 and FHR-4: binding to the C3d region of C3b and differential regulation by heparin.

The human factor H-related proteins FHR-3 and FHR4 are members of a family of proteins related to the complement factor H. Here, we report that the two proteins bind to the C3d region of complement C3b. The apparent K(A) values for the interactions of FHR-3 and FHR-4 with C3b are 7.5 x 10(6) M(-1) and 2.9 x 10(6) M(-1), respectively. Binding studies performed with C3b-coated pneumococci confirmed the results obtained with the biosensor system. A C-terminal construct of factor H showed similar binding characteristics. The interaction of FHR-3, but not of FHR4, with opsonised pneumococci was inhibited by heparin.

Apolipoproteins↗
PubMed1985

Characterization of a soluble form of the C3b/C4b receptor (CR1) in human plasma.

A radioimmunoassay with the use of soluble 125I-Fab monoclonal anti-CR1 and rabbit IgG anti-CR1 bound to Staphylococcus aureus particles was employed to detect and quantitate CR1 antigen in human plasma. Among 16 normal individuals the concentration of soluble CR1 in plasma ranged from 13 to 81 ng/ml, and a similar range of concentration was found in plasma from 15 patients having systemic lupus erythematosus (SLE). The amount of plasma CR1 in normal donors, but not in SLE patients, significantly correlated with the number of CR1 sites on erythrocytes (r = 0.90, p less than 0.001), and was 7.1% of the amount of receptor that was present on erythrocytes in blood. The concentration of soluble CR1 was not diminished by ultracentrifugation or ultrafiltration of plasma, was not affected by various modes of anti-coagulation or even by clotting of blood, and did not change during incubation of blood at 4 degrees C for up to 4 hr. On sucrose density gradient ultracentrifugation of plasma the CR1 was distributed as a broad peak that overlapped the plasma protein profile. The Mr of plasma CR1 was identical to that of erythrocyte CR1 when assessed by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate and immunoblotting. In addition, the plasma form of CR1 exhibited the same structural phenotype as did receptor from erythrocytes of the same individual. CR1 antigen purified from plasma was as active as CR1 from erythrocytes in promoting the cleavage by factor I of C3b to iC3b, C3c, and C3dg. Therefore, a functionally and structurally intact form of soluble CR1 resides in plasma.

Animals↗
PubMed1980

Control of serum C3 levels by beta 1H and C3b inactivator.

In serum specimens from 50 normal adults, a high correlation was found between the sum of the levels of the control proteins of the C3b amplification loop, C3b inactivator and beta 1H, and the sum of the component proteins, C3 and factor B (R = 0.83), when the concentrations are expressed as percent of the normal adult level. The slope for this regression was 0.98 and the y intercept practically zero, indicating that under normal conditions, beta 1H + C3b inactivator = C3 + factor B +/- 35% (95% error of the prediction). Biological significance of the combined values was suggested by the fact that their equality was largely preserved in serum specimens from cord blood, from infants, and from patients with idiopathic nephrotic syndrome, malnutrition, chronic illness, and partial C3b inactivator deficiency even though, individually, control and component protein concentrations deviated from normal in many of these conditions and, as single proteins, did not correlate with each other as in normal subjects. On the other hand, C3 + factor B was disproportionately lower than beta 1H + C3b inactivator in subjects heterozygously deficient in C3 and in those with hypocomplementemic glomerulonephritis and was often disproportionately high in those with an acute-phase reaction. The data indicate that for meaningful interpretation of the serum C3 level, the concurrent levels of factor B, C3b inactivator, and beta 1H must be taken into consideration.

Adolescent↗
PubMed1982

Tumor-promoting phorbol esters stimulate C3b and C3b' receptor-mediated phagocytosis in cultured human monocytes.

Monocytes were isolated in high yield (approximately 80%) and purity (greater than 90%) by Percoll gradient centrifugation and incubated in Teflon culture vessels. Using this culture method, we routinely recovered 80% of the cells originally placed into culture. Studies of the C3b and C3b' receptors of these monocytes showed that the function of both receptors could be dramatically altered by treating the cells with tumor-promoting phorbol esters. Both C3b and C3b' receptors of human monocytes efficiently mediate attachment of erythrocytes coated with the corresponding ligands, but do not promote their ingestion. However, monocytes treated with phorbol myristate acetate (PMA) or phorbol didecanoate ingest C3b- and C3b'-coated erythrocytes. Phorbol esters that are inactive as tumor promoters do not stimulate C3 receptor-mediated phagocytosis. The ability of monocytes to respond to PMA by activation of C3 receptors is developmentally regulated. Freshly isolated monocytes do not take up C3b- or C3b'-coated erythrocytes in response to PMA, but after 3 d of culture they show strong PMA-stimulated uptake. The stimulatory effect of PMA on monocyte C3b and C3b' receptor function occurs within minutes, is stable for hours, is cycloheximide insensitive, and can be inhibited with colchicine. Several lines of evidence indicates that phagocytosis of C3b or C3b'-coated erythrocytes is specifically mediated by the monocytes' C3b and C3b' receptors. First, erythrocytes attached to monocytes with concanavalin A are not ingested when the monocytes are treated with PMA. Second, monocytes plated on IgG-bearing substrates lose Fc receptor activity on their nonadherent surfaces but retain the capacity to ingest C3b- or C3b'-coated erythrocytes after PMA treatment. Third, PMA-treated monocytes plated on C3b-coated surfaces lose C3b receptor activity on their nonadherent surfaces but retain the capacity to ingest C3b'-coated erythrocytes. Conversely, PMA-treated monocytes plated on C3b'-coated surfaces show reduced C3b' receptors activity on their nonadherent surfaces but retain the capacity to ingest C3b-coated erythrocytes.

Cations↗