Platelet interactions with C1q in whole blood and in the presence of immune complexes or aggregated IgG

Contribution of chondroitin sulfate A to the binding of complement proteins to activated platelets

Background

Exposure of chondroitin sulfate A (CS-A) on the surface of activated platelets is well established. The aim of the present study was to investigate to what extent CS-A contributes to the binding of the complement recognition molecule C1q and the complement regulators C1 inhibitor (C1INH), C4b-binding protein (C4BP), and factor H to platelets.

Principal Findings

Human blood serum was passed over Sepharose conjugated with CS-A, and CS-A-specific binding proteins were identified by Western blotting and mass spectrometric analysis. C1q was shown to be the main protein that specifically bound to CS-A, but C4BP and factor H were also shown to interact. Binding of C1INH was dependent of the presence of C1q and then not bound to CS-A from C1q-depleted serum. The specific interactions observed of these proteins with CS-A were subsequently confirmed by surface plasmon resonance analysis using purified proteins. Importantly, C1q, C4BP, and factor H were also shown to bind to activated platelets and this interaction was inhibited by a CS-A-specific monoclonal antibody, thereby linking the binding of C1q, C4BP, and factor H to exposure of CS-A on activated platelets. CS-A-bound C1q was also shown to amplify the binding of model immune complexes to both microtiter plate-bound CS-A and to activated platelets.

Conclusions

This study supports the concept that CS-A contributes to the binding of C1q, C4BP, and factor H to platelets, thereby adding CS-A to the previously reported binding sites for these proteins on the platelet surface. CS-A-bound C1q also seems to amplify the binding of immune complexes to activated platelets, suggesting a role for this molecule in immune complex diseases.

Platelet receptors for the complement component C1q: implications for hemostasis and thrombosis

Platelets participate in a variety of responses of the blood to injury (1). In addition to their well known role in hemostasis and thrombosis, platelets play a role in inflammation and react with components of the immune system. Immune complexes and aggregated IgG, for example, are known to activate platelets via ligation of Fc gamma RII receptors and induce the release of platelet granule contents, including biogenic amines and adenine nucleotides (2). Platelets also interact with the complement subcomponent C1q utilizing binding sites that are unrelated to C1s, a complement subcomponent which was originally suggested to support C1q binding to thrombocytes (3). The physiologic and pathologic consequences of platelet C1q receptor occupancy are incompletely understood. Platelet C1q receptors may contribute to immune complex localization and clearance, as has been suggested for C1q receptors on phagocytic cells (4), but considerable evidence is emerging to suggest that the interaction between C1q and platelets may influence hemostasis and perhaps, more profoundly, thrombotic complications resulting from immune injury. This review will summarize current concepts in C1q receptor biology as it relates to human platelet function and blood coagulation.

Platelet adhesion onto artificial surfaces: inhibition by benzamidine, pentamidine, and pyridoxal-5-phosphate as demonstrated by flow cytometric quantification of platelet adhesion to microspheres

An appreciable effort is directed toward designing strategies to minimize platelet interactions with artificial surfaces, because their reactivity is thought to promote thrombus formation and lead to materials failure. Although platelet glycoprotein Ib/IX (GPIb/IX) and glycoprotein IIb/IIa (GPIIb/IIIa) receptors are thought to mediate adhesion, whether GPIIb/IIIa receptors are activated and how this might occur are largely unknown and are the focus of this article. There are a few ways, other than thrombin generation, that blood contact with artificial surfaces can lead to GPIIb/IIIa activation. Complement activation can lead to products capable of activating platelets (C1q, C5b-9), and contact between platelet CD32 (FcgammaRII) receptors and immobilized immunoglobulin G could also activate platelets. In this article the potential role of these processes was evaluated by using various inhibitors in a microsphere-based platelet adhesion immunoassay. Polystyrene microspheres (10 microm) were incubated in platelet-rich plasma before flow cytometric analysis of beads for adherent platelets. The data eliminated occupancy of the FcgammaRII receptor (by use of IV.3 blocking antibody), C5b-9 production (by use of sCR1), and the indirect action of factor XIIa on complement components (by use of corn trypsin inhibitor) as playing roles in supporting platelet adhesion. Agents directed against the first complement component (benzamidine, pentamidine, pyridoxal-5-phosphate) were effective inhibitors of platelet adhesion and were also demonstrated to inhibit SC5b-9 and C3d levels on the bead surface after serum incubations. Because these agents are not highly specific, it can not be concluded that C1q is a mediator of adhesion. These agents were also demonstrated to inhibit fluorescein isothiocyanate-fibrinogen binding to activated washed platelets, therefore indicating that fibrinogen receptor expression is a requirement for platelet adhesion.

Interaction of platelet fc and complement receptors with circulating immune complexes involving the AB0 system

When platelets that are AB0-nonidentical are transfused, circulating immune complexes (CIC) are formed. In the present study we examined the ability of polyclonal antibodies to two C1q receptors on platelets, cC1q-R and gC1q-R, and a monoclonal IgG Fc gamma RII (CD32) antibody directed against the platelet Fc receptor to inhibit the uptake of CIC involving the AB0 blood group system by normal platelets. Four types of immune complexes of varying purity were made in vitro. In addition serum from 5 refractory group A patients who had demonstrable AB0 CIC, 5 patients who had received only AB0-identical platelets and had no AB0 CIC and 6 normal donors were evaluated. After exposure of normal platelets to serum of patients with demonstrable AB0 CIC there were increased levels of platelet-associated IgG. This binding was partially inhibited by preincubation of the platelets with either anti-cC1q-R (3/5 patients), gC1q-R (3/5 patients) or IgG Fc gamma RII in 4/5 patients. However, the pattern of inhibition by the three antibodies was variable. Using the artificial immune complexes a more consistent pattern was obtained. The binding of four types of artificial immune complexes to platelets was reduced by 67-99% after preincubation of the platelets with antibodies to the complement and Fc receptors. The present work supports the hypothesis that AB0 CIC bind to Fc and complement receptors on the platelet and if confirmed would suggest a pathophysiological mechanism for the clinical observations of the important role of AB0 in platelet transfusion.

Are there options for donor-derived i.m. anti-D IgG preparations other than to prevent Rh(D) sensitization? The intravenous route

The use of i.v. anti-Rh(D) IgG for conditions other than prevention of Rh(D) sensitization is discussed. Besides highlighting the platelet response in ATP, a putative distinctive effect on the hemorrhagic threshold is suggested. Accordingly, we have included discussion of cases of aplastic anemia, myelodysplasia, heavy chemotherapy, coagulation deficiency, and senile vascular atrophy. We have also considered attempts to replace the high-dose pooled i.v. IgG (IVIG) with the much smaller amounts of IgG present in anti-Rh(D) preparations used to prevent Rh-sensitization in pregnancy. Both Fc and variable fragments of the IgG molecule may play a role, the former potentiated by manufacture-induced IgG aggregation and the latter by donor hypersensitization. A role for IgG-anti-F(ab')2 molecules cannot be ruled out.

Identification of C1q as the heat-labile serum cofactor required for immune complexes to stimulate endothelial expression of the adhesion molecules E-selectin and intercellular and vascular cell adhesion molecules 1

To examine the role of complement components as regulators of the expression of endothelial adhesive molecules in response to immune complexes (ICs), we determined whether ICs stimulate both endothelial adhesiveness for leukocytes and expression of E-selectin and intercellular and vascular cell adhesion molecules 1 (ICAM-1 and VCAM-1). We found that ICs [bovine serum albumin (BSA)-anti-BSA] stimulated endothelial cell adhesiveness for added leukocytes in the presence of complement-sufficient normal human serum (NHS) but not in the presence of heat-inactivated serum (HIS) or in tissue culture medium alone. Depletion of complement component C3 or C8 from serum did not prevent enhanced endothelial adhesiveness stimulated by ICs. In contrast, depletion of complement component C1q markedly inhibited IC-stimulated endothelial adhesiveness for leukocytes. When the heat-labile complement component C1q was added to HIS, the capacity of ICs to stimulate endothelial adhesiveness for leukocytes was completely restored. Further evidence for the possible role of C1q in mediating the effect of ICs on endothelial cells was the discovery of the presence of the 100- to 126-kDa C1q-binding protein on the surface of endothelial cells (by cytofluorography) and of message for the 33-kDa C1q receptor in resting endothelial cells (by reverse transcription-PCR). Inhibition of protein synthesis by cycloheximide blocked endothelial adhesiveness for leukocytes stimulated by either interleukin 1 or ICs in the presence of NHS. After stimulation with ICs in the presence of NHS, endothelial cells expressed increased numbers of adhesion molecules (E-selectin, ICAM-1, and VCAM-1). Endothelial expression of adhesion molecules mediated, at least in part, endothelial adhesiveness for leukocytes, since leukocyte adhesion was blocked by monoclonal antibodies directed against E-selectin. These studies show that ICs stimulate endothelial cells to express adhesive proteins for leukocytes in the presence of a heat-labile serum factor. That factor appears to be C1q.

Clustering of neutrophil leucocytes in serum: possible role of C1q-containing immune complexes

Clustering activity for neutrophil granulocytes was generated in pooled normal human serum (NHS) by incubation of the serum with preformed IgG aggregates, but not in heat-treated NHS (56 degrees C, 30 min), indicating that the function was complement-dependent. Judging from results of experiments with complement-deficient sera, and serum depleted of C1q, factor D and properdin, recruitment of the complement system beyond C1 was not required for induction of the activity. Zymosan treatment of NHS resulted in some neutrophil clustering activity, but recombinant C5a had a limited effect. C1q added to heat-treated NHS in conjunction with performed IgG aggregates supported neutrophil clustering in a dose-dependent manner. The serum C1q inhibitor, a chondroitin 4-sulphate proteoglycan known to interact with the collagenous part of C1q, clearly reduced neutrophil clustering in heat-treated NHS supplemented with C1q and IgG aggregates. The C1q inhibitor also reduced the inherent neutrophil clustering activity of some sera from patients with systemic lupus erythematosus (SLE). Neutrophil clustering activity in SLE serum was earlier shown to be inversely related to the number of circulating neutrophils in vivo. Although the precise mechanisms remain unclear, we propose that C1q-containing immunoglobulin complexes mediate neutrophil clustering through C1q receptors, and that this might contribute to pathogenesis of immune complex diseases such as SLE.