Influence of processing by erythrocyte C3b/C4b receptors (CR1) on binding of immune complexes to Raji cells and polymorphonuclear granulocytes

The role of complement receptor type 1 (CR1, CD35) in determining the cellular distribution of opsonized immune complexes between whole blood cells: kinetic analysis of the buffering capacity of erythrocytes

Erythrocytes (E) express complement receptor, type 1 (CR1, CD35), by which they bind opsonized immune complexes (IC) in competition with leucocytes expressing higher numbers of CR1 as well as other complement- and Fc-receptors. This may prevent inappropriate activation of phagocytic cells. We examined the distribution on whole blood cells of preformed tetanus toxoid (TT)/human anti-TT IC, opsonized in situ in 80% autologous serum. Binding to E occurred rapidly and reflected the kinetics of C3-fragment incorporation into the IC. Among eight donors, expressing 180-361 CR1 per E. > 90% of the cell-bound IC were associated with E from 1 to 5 min of incubation, decreasing to 12 +/- 13% after 40 min. Upon comparison of the IC-binding to leucocytes in whole blood with that of isolated leucocytes we found that E, despite their extensive early complex uptake, only reduced the IC-deposition on polymorphonuclear leucocytes (PMN) by 61 +/- 26% after 30 seconds of incubation and 47 +/- 14% after 5 min. During the subsequent 10 min, this buffering capacity of E was essentially abolished E restricted the initial IC-binding to B cells by 73 +/- 19%, but from 3 min of incubation the presence of E promoted, in a CR1-dependent manner, a progressive uptake via CR2 by the B cells. CR1 was the dominant receptor in the early IC-uptake by B cells as well as PMN and monocytes, since CR1-blockade inhibited the initial IC-uptake by these populations in a preparation of isolated leucocytes suspended in serum by > or = 84% after 30 seconds of incubation. We conclude, that E exert a substantial buffering effect on the IC-deposition on PMN, monocytes and B cells, while CR1 is the dominant receptor in the uptake by these cells. However, this effect is short-lived and less than expected from the proportion of IC bound to E. Moreover, E are efficient processors of IC-attached C3b/iC3b fragments to C3dg as indicated by a pronounced enhancement by E of IC-uptake via CR2 on B cells. We propose that this mechanism may play a role in preventing phagocyte activation via CR3.

Interactions of opsonized immune complexes with whole blood cells: binding to erythrocytes restricts complex uptake by leucocyte populations

The binding of opsonized, fluorescein-labelled bovine serum albumin (BSA)/rabbit anti-BSA complexes (IC) to washed human whole blood cells and isolated leucocytes in the presence of autologous serum was investigated by flow cytometry. In the presence of erythrocytes (E), the IC-binding to granulocytes (PMN), monocytes and lymphocytes was inhibited by up to 46%, 61% and 48%, respectively, depending on the incubation time and the IC-concentration tested. The E-mediated inhibition of the binding to PMN was found to correlate with the average numbers of CR1 per E during the initial 15 min of incubation. Thereafter, the difference between IC binding to PMN in absence and presence of E, decreased in accordance with decreasing binding to E. IC-uptake by PMN induced a drop in side-scatter characteristics, attributable to degranulation, which could be prevented by the presence of E. In contrast to the findings for PMN, the difference between IC-binding to monocytes in the absence and presence of E increased progressively over the 90 min observation period, suggesting that different mechanisms are involved in the late-phase IC uptake by monocytes and PMN. Lymphocytes were heterogeneous with respect to IC binding, the main contributors being B cells. E initially inhibited and then later enhanced the IC binding to lymphocytes, suggesting that E promote B cell uptake of C3d,g-covered IC via CR2. Our findings, that E can restrict the IC uptake by circulating leucocytes, and that an IC-induced degranulation of PMN may be prevented by E, indicate that E may act as a high capacity buffer limiting inappropriate activation of phagocytes by circulating IC.

Reciprocal changes in complement activity and immune-complex levels during plasma infusion in a C2-deficient SLE patient

Although systemic lupus erythematosus (SLE) is abnormally common in individuals with complement deficiency, conclusive evidence has been lacking for a direct causal relationship between disease manifestations and a missing complement component. A patient with C2 deficiency and SLE has been treated with 56 courses of fresh frozen plasma (FFP) infusions over a period of 8 years. Each infusion, involving a total of 12 units of FFP administered in equal doses over 4 consecutive days, has consistently resulted in a transient restoration of the classical pathway of complement, and a full clinical remission lasting 6-8 weeks. This report is concerned with changes in the levels of immune complexes, C2 and C3d during an infusion cycle. Four progressively rising peaks in C2 and C3d were observed during the 4 days of the plasma infusion, and these peaks coincided with four reciprocally descending troughs in the levels of immune complexes. Identical fluctuations have been consistent in all the plasma-infusion cycles that have so far been monitored, and their consistent association with clinical remissions indicates a causal relationship between the C2 restoration and clinical remissions in this C2-deficient SLE patient.

Mechanism of transfer of immune complexes from red blood cell CR1 to monocytes

Complement receptor 1 (CR1) on primate red blood cells (RBC) binds most complement-fixing immune complexes in the circulation. It has been postulated that by binding them, RBC keep immune complexes in the intravascular space and deliver them to the tissue macrophages of the mononuclear phagocyte system. We have developed an in vitro model to study the transfer of RBC-bound immune complexes (heat-aggregated IgG and DNA-anti-DNA) to phagocytic cells (human monocytes). Transfer of immune complexes from RBC to monocytes occurred significantly more rapidly than monocyte uptake of the same immune complexes from solution. In the transfer process, complex-bearing RBC were not bound or sequestered by the monocytes. To define the monocyte receptors involved in binding immune complexes from the RBC surface, monocyte receptors were blocked with MoAbs (anti-CR1, anti-FcRII) or EDTA (to block CR3). Monocyte binding of immune complexes primarily used CR1 with a small contribution from FcRII, and with little or no contribution from CR3 and FcRI. Uptake of immune complexes from solution employed the same monocyte receptors as binding of complexes from the RBC surface. Immune complexes in solution bound to RBC and to monocytes with equally high avidity (approximately 1 x 10(11) l/M), but monocytes expressed a 15-20-fold greater number of immune complex binding sites. We propose that immune complexes distribute between RBC and monocytes according to the binding capacity of these cells, such that at equal or high RBC/monocyte ratios as would be seen in the circulation immune complexes bind to RBC, but at low RBC/monocyte ratios (as would be seen in the sinusoidal circulation of the liver and spleen), most immune complexes bind to monocytes. To define the pathway by which immune complexes move from RBC to monocytes, their release from RBC CR1 was examined. Under various conditions, the dissociation rate was extremely slow, and did not increase with the addition of monocyte supernatants. To examine whether factor I-mediated processing of immune complexes enhances binding of immune complexes to monocytes, RBC-bound complexes were released with factor I, and binding of these 'processed' immune complexes to monocytes was examined. Monocyte binding of these processed immune complexes was slower than of control ones; furthermore, performance of transfer experiments at 4 degrees C, which significantly shows enzymatic processes, did not decrease the rate of immune complex transfer from RBC to monocytes.(ABSTRACT TRUNCATED AT 400 WORDS)

Zinc ions inhibit factor I-mediated release of CR1-bound immune complexes and degradation of cell-bound complement factors C3b and C4b

ZnCl2 exerted a dose-dependent inhibition of citrate-phosphate-dextrose (CPD) plasma-induced release of 125I-labelled BSA-anti-BSA immune complexes (IC) bound to complement receptor type 1 (CR1, CD35) in human whole blood. Maximal inhibition was observed at 10 mM of ZnCl2. Furthermore, the release of IC bound to erythrocyte (E)-CR1 by purified factor I, factor I-deficient serum plus purified factor I, or normal human serum was reduced by approximately 90%, 64%, and 52%, respectively, in the presence of 10 mM ZnCl2. The effect of ZnCl2 on factor I-mediated degradation of cell-bound C3b/C4b was also investigated employing CPD blood or E from a factor I-deficient donor. These cells expressed covalently bound C3b and C4b as demonstrated by a simple agglutination technique. Upon incubation of CPD whole blood with purified factor I, or of E with purified factor I or normal CPD plasma, the C-fragments were cleaved and the cells were no longer agglutinated by antibodies to C3c and C4c. The presence of ZnCl2 prevented this factor I-mediated degradation of C3b and C4b, as evidenced by the unaffected agglutination of the cells by the antibodies. We conclude that ZnCl2 inhibited factor I activity since: (1) release of complement-preopsonized IC from E-CR1 by purified factor I was markedly inhibited (90%) in the presence of ZnCl2, (2) preincubation of the cells with ZnCl2 caused only a moderate inhibition (32-38%) of the IC release, and (3) degradation by purified factor I of covalently cell-bound C3b and C4b was abrogated in the presence of 10 mM ZnCl2.

The influence of C3-coated homologous erythrocytes on pokeweed-mitogen-induced polyclonal differentiation of human B cells

The present study was undertaken in order to test the hypothesis that homologous erythrocytes (E) coated in vivo with C3d could modulate the immunoglobulin (Ig) synthesis of human peripheral blood mononuclear cells (PBMC), stimulated with pokeweed mitogen (PWM) in vitro. E from healthy individuals were found to enhance markedly the Ig synthesis of PMBC cultures stimulated with suboptimal doses (0.01 microgram/ml) of PWM. E coated in vivo with increasing amounts of C3d (1.4-6.3 times the amounts on normal E), obtained from patients with systemic lupus erythematosus, failed to induce any significant increase in Ig synthesis of PBMC cultures stimulated with suboptimal PWM doses, compared with cultures co-stimulated in parallel with normal E. In contrast, an increase in IgM and IgG synthesis was observed in about 50% of PBMC cultures from different donors when stimulated with PWM in the presence of E coated with C3b in vivo (from a patient with congenital factor I deficiency), compared with the Ig synthesis in cultures co-stimulated in parallel with normal E. In contrast to the inability of C3d-coated E to modulate B-cell proliferation, the monoclonal anti-CR2 antibody OKB7 was found to be mitogenic for unstimulated peripheral B cells.

Enhanced binding of immune complexes processed by erythrocyte CR1 (CD 35) receptors to purified CR2 (CD 21) receptors from tonsillar mononuclear cells

The binding of immune complexes (IC) opsonized by serum complement (C) and IC processed by CR1 (CD 35) receptors on human erythrocytes (E) to purified CR2 (CD 21) receptors was compared. Soluble CR2 was prepared from tonsillar mononuclear cells and purified by antibody affinity chromatography. Solid phase CR2 as well as CR2 subjected to PAGE and blotted onto nitro-cellulose membranes bound 125I-labelled BSA anti-BSA IC which had been opsonized by C and processed by CR1 up to ten times more efficiently than IC reacted with serum only. Radiolabelled monomeric C3d also bound to solid phase CR2. The binding of IC to purified and solid phase bound CR2 could be inhibited by anti-CR2 antibodies or by preincubation of the IC with polyclonal antibodies reacting with C3d or C3b/iC3b. Thus, both C3dg and iC3b appeared to mediate binding of IC to CR2. Preincubation of solid phase CR2 with purified monomeric C3d did not inhibit the subsequent binding of E-CR1 processed IC. The data indicate that E-CR1 have an important role in generating IC which bind effectively to CR2 receptors on B lymphocytes.

Reduced erythrocyte CR1 (CD 35) receptor function and complement opsonization in factor I-deficient patients is restored by plasma infusion

Erythrocytes (E) from three factor I-deficient patients were investigated for surface-bound complement factors and CR1 (CD 35) expression and function. The E were coated with C4b, C3b, and factor H. Following plasma infusion or in vitro incubation of the patients' E with normal human serum (NHS) or purified factor I, cell-bound C4b and C3b could no longer be detected. The E now expressed C3d, and factor H was unaffected, indicating that factor H was bound to the C3d part of the C3b molecules, providing the co-factor for effective cleavage of E-bound C3b when purified factor I was added. The binding of monoclonal anti-CR1 antibodies (M710) to the patients' E was markedly reduced compared with control E, and was not normalized by treatment with NHS, probably because covalently bound C3d/factor H interfered with the binding of M710. By contrast, the reduced ability of the patients' E-CR1 to bind complement-opsonized immune complexes (IC) was normalized after plasma infusion. This shows that the impaired CR1 function was acquired and emphasizes the importance of performing functional CR1 assays. Complement opsonization of IC for binding to normal E was severely compromised in the patients' sera due to consumption of factor B and C3. After plasma infusion the opsonization capacity of the patients' sera was restored. Thus, two mechanisms of importance for normal clearance of IC were compromised in factor I-deficient patients: (1) the opsonization of IC for binding to E-CR1, and (2) the capacity of E-CR1 to bind opsonized complexes. Both dysfunctions were temporarily corrected by plasma infusion.

Inefficient binding of IgM immune complexes to erythrocyte C3b-C4b receptors (CR1) and weak incorporation of C3b-iC3b into the complexes

The binding of soluble complement-reacted IgM immune complexes (IC) to erythrocyte (E) C3b-C4b receptors (CR1) and the incorporation of C3b-iC3b into solid phase IgM-IC was investigated. The optimal binding of liquid phase IgM-IC to E-CR1 was obtained with IC formed at moderate antibody excess, but the binding was low (2-3%) when compared to the binding of the corresponding IgG-IC (50-60%). Solid phase IC were prepared by coating microwells with heat-aggregated bovine serum albumin (BSA) followed by incubation with rabbit IgM anti-BSA antibody. The IC were reacted with human serum at 37 degrees C. The binding of C3b-iC3b was determined by use of biotinylated F(ab')2 antibodies to C3b-C3c and avidin-coupled alkaline phosphatase. The incorporation of C3b-iC3b into solid-phase IgM-IC increased when increasing amounts of IgM antibody were reacted with the antigen. The binding reaction was slow, reaching a maximum after about 2 h at 37 degrees C. The binding of C3b-iC3b to the IgM-IC was remarkably inefficient when compared to the incorporation into IgG-IC reacted with the same amounts of BSA-precipitating antibody.