Human platelet Fc receptor for immunoglobulin G. Identification as a 40,000-molecular-weight membrane protein shared by monocytes.

Abstract

We have recently shown that human monocytes and U937 cells possess two molecular classes of Fcy receptor. One, a 72,000-mol-wt sialoglycoprotein, has high affinity for certain subclasses of human and murine monomeric IgG. The other is a 40,000-mol-wt protein (p40) with low affinity for monomeric IgG but with the capacity to bind IgG aggregates or IgG-coated particles.In the present study, a 40,000-mol-wt single chain protein, apparently identical to p40 from U937 cells, was isolated from surface-radioiodinated human platelets by affinity purification using a murine IgG2b monoclonal antibody to p40. This 40,000-mol-wt protein was not seen when control IgG2b or unrelated murine monoclonal antibodies were employed in place of antip40. The same 40,000-mol-wt protein was also recovered from an IgG-Sepharose affinity adsorbent, but not from ovalbuminor myoglobin-Sepharose. The 72,000-mol-wt Fcvy receptor of monocytes was not identified on platelets.Monoclonal anti-p4O and Fab fragments derived from this antibody blocked platelet aggregation by heat-aggregated human IgG, whereas a control murine IgG2b protein had little or no inhibitory effect at 500-1,000-fold higher concentrations. A murine IgG1 monoclonal antibody, reactive with an unrelated platelet-specific membrane antigen, did not inhibit platelet responses to aggregated IgG. Anti-p4O did not affect platelet aggregation by thrombin, collagen, or fibrinogen plus ADP. Although antip40 did not directly aggregate platelets in the concentrations employed, cross-linking with F(ab'`) goat anti-murine Ig induced apyrase-sensitive aggregation of anti-p40-treated platelets. This indicates that p40 possesses transmembrane linkage for platelet activation and secretion.These observations strongly suggest that this newly recognized 40,000-mol-wt platelet membrane protein serves as an Fcy receptor.

Allergen-specific IgG1 and IgG3 through Fc gamma RII induce eosinophil degranulation.

“…-15000 Fourth, to determine which FcR was mediating eosinophil P degranulation, we studied the effects of the blocking of FcR by -10000 < various anti-FcR antibodies on eosinophil degranulation. All 'c antibodies we used are reported to inhibit the binding of their -5000 respective Ig to cell surfaces (17,25,(35)(36)(37). As shown in Table IV, only anti-FcyRII antibodies showed potent inhibitory 0 effects on degranulation in the presence and absence of PBMC.…”

Analysis of CD20-dependent cellular cytotoxicity by G-CSF-stimulated neutrophils

“…CD32 mAbs (IV.3 to Fc␥RII) were purified from hybridoma culture supernatant in our own laboratory. 20 FabЈ fragments of IV.3 and F(abЈ) 2 fragments of CLB-FcRgran1 were produced in our own laboratory.…”

Autoimmune Thrombocytopenia

“…Yet, in the diagnosis of autoimmune thrombocytopenia detection of PA-IgG suffers from lack of specificity (von dem Borne et al, 1986b;Rosenfeld et al, 1987a). However, complications in the measurement of platelet antibodies may be caused by naturally occurring antibodies, internal platelet IgG and aggregated IgG of immune complexes bound to the platelet or antibodies against platelet cryptantigens (Rosenfeld et al, 1985;Muniz-Diaz et al, 1995).…”

“…FcγRIIA is the only FcγR expressed on platelets (Rosenfeld et al, 1985;Cassel et al, 1993;Wu et al, 1996). The number of FcγRIIA molecules on the platelet surface may vary considerably between individuals; the number of binding sites per platelet has been suggested to vary between 600 and 8000 (Karas et al, 1982;Rosenfeld et al, 1987b;Anderson et al, 1995; Table 1).…”

“…& Schreiber, 1994;Anderson et al, 1995;Kimberley et al, 1995;Koene et al, 1997;Norris et al, 1998;Lehrnbecher et al, 1999 The most widely expressed of all the FcγRs is FcγRII. The only FcγR expressed on platelets is FcγRIIA (Rosenfeld et al, 1985). FcγRII is an integral membrane glycoprotein of 40 kDa with two extracellular Ig-like domains, which is found on phagocytes, neutrophils, monocytes and macrophages and also at low levels on B cells.…”