Expression, purification and biological activity assessment of romiplostim biosimilar peptibody.

BACKGROUND: Romiplostim is a peptibody analogue of thrombopoietin (TPO) which regulates platelet production. This molecule consists of two main parts: Peptide sequences which like wild type TPO, mimics stimulation of TPO receptor and IgG1Fc, (Peptide + Antibody = Peptibody). This drug is used in treatment of chronic Immune Thrombocytopenic Purpura (ITP). METHODS: In this project E. coli bacteria were transformed by a construct harboring peptibody fusion gene. This construct consisted of two repeated peptide sequences which have fused to Carboxyl group of IgG1Fc. Designed construct in E. coli host resulted in protein expression in cytoplasm as inclusion body. The inclusion bodies were separated, washed and after denaturation and solubilization, in the last stage the desired peptibodies were refolded and purified. The resulting peptibodies were characterized by SDS-PAGE and Western immunoblotting. The bioactivity were assessed in vivo using subcutaneous injection in mice. RESULTS: Results showed accurate molecules were produced and purified. Also, in vivo experiment showed significant increment (more than two fold) of platelets compared to control group. CONCLUSION: In this study laboratory scale production of recombinant romiplostim showed proper in-vivo bioactivity. This new approach in expression and purification of this recently introduced thrombopoietin receptor agonist drug may be followed by scale up of its production to response the chronic ITP patient's demand.

Expression of soluble and functional human neonatal Fc receptor in Pichia pastoris.

The neonatal Fc receptor (FcRn) is a non-covalently associated heterodimeric protein composed of a transmembrane anchored heavy chain (alphaFcRn) and a soluble light chain beta2-microglobulin (beta2m). In addition to its role in the transfer of maternal immunoglobulin Gs (IgGs) to the fetus, FcRn plays a key role in prolonging the serum half-life of IgGs in vivo. Herein, we report a strategy for functional expression of soluble human FcRn (shFcRn) in Pichia pastoris using a two-promoter vector system, where alphaFcRn and beta2m are co-expressed under their respective promoters in a single vector. The purified shFcRn from the culture supernatants correctly assembled to form the heterodimer with the typical secondary structures. At acidic pHs between 5.0 and 6.4, shFcRn exhibited substantial binding to the four subclasses of human IgGs at acidic pHs between 5.0 and 6.4, but at pHs between 6.8 and 8.0, its binding was negligible binding. No cross-reactivity with mouse IgG was exhibited even at acidic pH. This was consistent with the pH-dependent binding profiles of the shFcRn prepared from the mammalian cell expression. Furthermore, the shFcRn exhibited about 10-fold higher binding affinity with the tumor necrosis factor-alpha antagonists of monoclonal antibodies Infliximab and Adalimumab than that of Etanercept, providing a clue to their different serum half-lives in vivo. Our results suggest that the functionally expressed shFcRn from Pichia can be used for the biochemical and biological studies and as a screening probe for Fc engineering of human IgGs.

In search of the elusive mouse macrophage Fc-alpha receptor.

Human macrophages express an Fc receptor for IgA (FcalphaR, CD89) but so far no mouse counterpart or an alternative IgA receptor has been found. Given the biological importance of IgA in countering infections, and the extensive use of mouse experimental models for passive and active prophylactic strategies, it is somewhat surprising that this subject has received relatively little attention. So, what do we know so far?

Calnexin and ERp57 facilitate the assembly of the neonatal Fc receptor for IgG with beta 2-microglobulin in the endoplasmic reticulum.

The neonatal FcR (FcRn) consists of an MHC class I-like H chain in noncovalent association with beta(2)-microglobulin (beta(2)m). The proper folding of FcRn in the endoplasmic reticulum is essential for FcRn function. Using a low stringency immunoprecipitation of human FcRn, we observed the coprecipitation of an 88-kDa band. Mass spectrometry analysis revealed that this band was identical with calnexin (CNX). This association was verified by Western blotting the CNX or FcRn immunoprecipitates with either an anti-FcRn or anti-CNX Ab. In the beta(2)m-null FO-1 cell transfected with FcRn H chain alone or both FcRn H chain and beta(2)m, CNX bound to the FcRn H chain before the FcRn H chain association with beta(2)m. However, calreticulin only bound to the FcRn H chain-beta(2)m complex. Furthermore, the thiol oxidoreductase ERp57 was detected in FcRn-CNX complexes, suggesting its role in disulfide bond formation of the FcRn H chain. Removal of the N-linked glycosylation site from the FcRn H chain resulted in a decreased association of the FcRn H chain for beta(2)m. However, the absence of CNX did not significantly affect FcRn assembly as defined by the ability of FcRn to bind IgG and exit to the cell surface. This suggests that other chaperones compensate for the function of CNX in FcRn assembly. In addition, we found that tapasin and TAP were not involved in FcRn assembly, as shown by coimmunoprecipitation in THP-1 cells and IgG-binding assays in 721.220 (tapasin-deficient) and 721.174 (TAP-deficient) cells transfected with FcRn. These findings show the importance of chaperones in FcRn assembly.

Fc alpha RI/CD89 circulates in human serum covalently linked to IgA in a polymeric state.

The FcR for IgA CD89/FcalphaRI, is a type I receptor glycoprotein, expressed on myeloid cells, with important immune effector functions. In vitro CD89 can be released from CD89-expressing cells upon activation. Little information is available on the existence of this soluble molecule in vivo. Using specific and sensitive ELISA techniques (detection limit 50 pg/ml), we were not able to detect circulating CD89 in human sera. However, using Western blotting, a 30-kDa soluble CD89 molecule was demonstrated in both serum and plasma. Moreover, using a specific semiquantitative dot-blot system, we found CD89 in all human sera tested (mean concentration 1900 ng/ml). Size fractionation of human serum using gel filtration chromatography showed that the CD89 molecule was predominantly present in larger molecular mass fractions. Direct complexes between IgA and CD89 were demonstrated by anti-IgA affinity purification, and when analyzed under nonreducing conditions appeared to be covalently linked. Size fractionation of affinity-purified IgA showed the presence of soluble CD89 only in the high molecular mass fractions of IgA, but not in monomeric IgA. High molecular mass complexes of CD89-IgA could be distinguished from J chain containing dimeric IgA. These data show that CD89 circulates in complex with IgA, and suggest that CD89 might contribute to the formation of polymeric serum IgA.

The IgA/IgM receptor expressed on a murine B cell lymphoma is poly-Ig receptor.

T560, a mouse B lymphoma that originated in gut-associated lymphoid tissue, expresses receptors that bind dimeric IgA and IgM in a mutually inhibitory manner but have little affinity for monomeric IgA. Evidence presented in this paper indicates that the receptor is poly-Ig receptor (pIgR) known in humans and domestic cattle to bind both IgA and IgM. The evidence includes the demonstration that binding of IgM is J chain dependent, and that pIg-precipitated receptor has an appropriate Mr of 116-120 kDa and can be detected on immunoblots with specific rabbit anti-mouse pIgR. Overlapping RT-PCR performed using template mRNA from T560 cells and oligonucleotide primer pairs designed from the published sequence of mouse liver pIgR indicate that T560 cells express mRNA virtually identical with that of the epithelial cell pIgR throughout its external, transmembrane, and intracytoplasmic coding regions. Studies using mutant IgAs suggest that the Calpha2 domain of dimeric IgA is not involved in high-affinity binding to the T560 pIgR. Inasmuch as this mouse B cell pIgR binds IgM better than IgA, it is similar to human pIgR and differs from rat, mouse, and rabbit epithelial cell pIgRs that bind IgA but not IgM. Possible explanations for this difference are discussed. All clones of T560 contain some cells that spontaneously secrete both IgG2a and IgA, but all of the IgA recoverable from the medium and from cell lysates is monomeric; it cannot be converted to secretory IgA by T560 cells.

Crosslinking of the human Fc receptor for IgA (FcalphaRI/CD89) triggers FcR gamma-chain-dependent shedding of soluble CD89.

CD89/FcalphaRI is a 55- to 75-kDa type I receptor glycoprotein, expressed on myeloid cells, with important immune effector functions. At present, no information is available on the existence of soluble forms of this receptor. We developed an ELISA for the detection of soluble CD89 (sCD89) forms and investigated the regulation of sCD89 production. PMA/ionomycin stimulation of monocytic cell lines (U937, THP-1, and MM6), but not of neutrophils, resulted in release of sCD89. Crosslinking of CD89 either via its ligand IgA or with anti-CD89 mAbs similarly resulted in sCD89 release. Using CD89-transfected cells, we showed ligand-induced shedding to be dependent on coexpression of the FcR gamma-chain subunit. Shedding of sCD89 was dependent on signaling via the gamma-chain and prevented by addition of inhibitors of protein kinase C (staurosporine) or protein tyrosine kinases (genistein). Western blotting revealed sCD89 to have an apparent molecular mass of 30 kDa and to bind IgA in a dose-dependent fashion. In conclusion, the present data document a ligand-binding soluble form of CD89 that is released upon activation of CD89-expressing cells. Shedding of CD89 may play a role in fine-tuning CD89 immune effector functions.

Isolation and purification of an early pregnancy factor-like molecule from culture supernatants obtained from lymphocytes of pregnant women: II. Identification of the molecule as a Fc-receptor-like molecule: a preliminary report.

PURPOSE: Early pregnancy factor (EPF)-like activity from culture supernatants obtained from stimulated lymphocytes of pregnant women was characterized and identified. METHODS: The enzyme-linked immunosorbent assay depending on the presence of "Fc" receptors on bovine spermatozoa was used to identify the EPF-like molecule purified by gel filtration and reverse-phase high-performance liquid chromatography. RESULTS: The results indicated that the crude lymphocyte culture supernatant, the EPF-positive G IV fraction obtained on gel filtration, and the EPF-positive reverse-phase high-performance liquid chromatography protein readily bound with the different concentrations of aggregated human gamma-globulin in a manner similar to that in which the standard control of aggregated human gamma-globulin binds to the bovine spermatozoa. CONCLUSIONS: EPF-like activity synthesized and secreted by lymphocytes during pregnancy may be a Fc-receptor-like molecule.

The polymeric immunoglobulin receptor is the major calmodulin-binding protein in an endosome fraction from rat liver enriched in recycling receptors.

Rat liver endosomes contain one major high-affinity calmodulin-binding protein (CaMBP) that now has been identified as the polymeric immunoglobulin receptor (pIgR). In isolated endosomes pIgR was enriched in the receptor-recycling compartment (RRC); lesser enrichment was found in 'early' endosome (CURL) and much less in 'late' endosome fractions (multivesicular bodies, MVB). The distribution of the major CaMBP, shown by Western blotting or by overlay with I125-calmodulin in the isolated fractions, was consistent with rapid accumulation of I125-immunoglobulin A (IgA) in RRC and CURL after intravenous injection into rats. The receptor was also found in sinusoidal plasma membranes but not in cell fractions containing apical (bile canalicular) or lateral plasma membrane domains of the hepatocyte. The interaction of pIgR with calmodulin was shown by direct binding assays and by affinity chromatography. Thus, calmodulin is the first cytoplasmic protein shown to interact with the pIgR. We postulate that calmodulin regulates pIgA trafficking in rat liver. In addition, the receptor recycling fraction emerges as an endosomal subcompartment involved in pIgA transport via pIgR.

Functional association between the human myeloid immunoglobulin A Fc receptor (CD89) and FcR gamma chain. Molecular basis for CD89/FcR gamma chain association.

FcR gamma chain has previously been shown to interact with the TCR-CD3 complex, the IgE Fc receptor I (Fc epsilon RI), and the class I and IIIA IgG receptors (Fc gamma RI and Fc gamma RIIIa). Here, we demonstrate that the Fc receptor gamma chain associates with Fc alpha R in transfected IIA1.6 B lymphocytes. Fc alpha R could be expressed at the surface of IIA1.6 B cells by itself, but was devoid of signaling capacity. Upon co-expression of FcR gamma chain, a physical interaction with Fc alpha R could be demonstrated. This association proved crucial for the triggering of both proximal (intracellular calcium increase and tyrosine phosphorylation), as well as distal (IL-2 release), signal transduction responses. We next tested the hypothesis that a positively charged arginine residue (Arg209) within the transmembrane domain of Fc alpha R promotes association with FcR gamma chain. We therefore constructed Fc alpha R molecules where Arg209 was mutated to either a positively charged histidine, a negatively charged aspartic acid, or an uncharged leucine. A functional association between Fc alpha R and FcR gamma chain was observed only with a positively charged residue (Arg209 or His209) present within the Fc alpha R transmembrane domain. These data show that transmembrane signal transduction by the Fc alpha R is mediated via FcR gamma chain, and that Fc alpha R requires a positively charged residue within the transmembrane domain to promote functional association.

Placental alkaline phosphatase has a binding site for the human immunoglobulin-G Fc portion.

Affinity chromatography of human plasma on placental-alkaline-phosphatase-Sepharose columns (placental alkaline phosphatase, PLAP) yielded consistently a pure protein which was identified as IgG on the basis of electrophoretical and immunological comparisons with authentic human IgG. SDS/PAGE of the protein revealed, under reducing conditions, two polypeptides of 55 kDa and 25 kDa. The N-terminal amino acid sequence (12 residues) of the 55-kDa subunit presented high similarity (83-100%) with known sequences of immunoglobulin gamma chains. The IgG binds by its Fc portion to a fully exposed domain in the plasma-membrane-anchored PLAP. Scatchard analysis of the interaction gave a dissociation constant of 3.68 microM, a value close to those found for haematopoietic cells and syncytiotrophoblast Fc receptors. The latter was affinity purified from human placenta as the major IgG-binding component and presented cross-immunoreactivity with anti-PLAP antibodies, indicating that PLAP and the putative placental Fc receptor could be identical molecules.

Placental alkaline phosphatase is related to human IgG internalization in HEp2 cells.

The biological function(s) of placental alkaline phosphatase has not yet been unraveled. The low catalytic activity of the enzyme at physiological pH, and the lack of "natural substrates", bring about the necessity of a more structure-related conception of its role. We have observed an interaction between placental alkaline phosphatase and human IgG. In this report we show that this isozyme is the major membrane protein able to bind IgG in a IgG-internalizing cell line (HEp2). Pretreatment of these cells with Fab fragments of anti placental alkaline phosphatase antibodies blocks the internalization of IgG without perturbing the endocytosis of other ligands. Our results indicate that placental alkaline phosphatase has the ability not only to bind human IgG, but also to promote its internalization in HEp2 cells.

Phosphorylation and dephosphorylation of the high-affinity receptor for immunoglobulin E immediately after receptor engagement and disengagement.

Triggering of mast cells and basophils by immunoglobulin E (IgE) and antigen induces various biochemical signals, including tyrosine kinase activation, which lead to cell degranulation and the release of mediators of the allergic reaction. The high-affinity receptor for IgE (Fc epsilon RI) responsible for initiating these events is a complex structure composed of an IgE-binding alpha-chain, a beta-chain and a homodimer of gamma-chains. It has been assumed that beta and gamma, which have extensive cytoplasmic domains, play an important but undefined role in coupling Fc epsilon RI to signal transduction mechanisms. Here we show that Fc epsilon RI engagement induces immediate in vivo phosphorylation on beta (tyrosine and serine) and gamma (tyrosine and threonine) by at least two different non-receptor kinases. We take advantage of unique features of this receptor system to demonstrate that the phosphorylation signal is restricted to activated receptors and is immediately reversible upon receptor disengagement by undefined phosphatases. Rapid phosphorylation and dephosphorylation may be a general mechanism to couple and uncouple activated receptors to other effector molecules. This could be particularly relevant to other multimeric receptors containing Fc epsilon RI gamma-chains or the related zeta and eta chains such as the T-cell antigen receptor (TCR) and the low-affinity receptor for immunoglobulin G (Fc gamma RIII, CD16).

Properties of an immunoglobulin binding factor from human seminal plasma.

A soluble factor (IBF) in human seminal plasma that binds serum immunoglobulins (Ig) of various species was purified to homogeneity by ammonium sulfate precipitation, preparative isoelectrofocusing, and gel filtration chromatography. The purified IBF interacted weakly with Fc and F(ab')2 fragments and not with Fab. It interacted with anti-Leu 11b and polyclonal anti-Fc gamma RIII antibodies, but not with other anti-Fc gamma R antibodies (32.2, IV.3 and 3G8). IBF is probably a non-glycosylated protein with isoelectric point ranging from 5.1 to 5.8. The estimated Mr of the purified native IBF is 27 kD, determined by sodium dodecyl sulfate polyacrylamide electrophoresis (SDS-PAGE) under non-reducing condition. In its native form, IBF did not bind Ig or interact with anti-Fc gamma R antibodies. Following SDS-PAGE under reducing condition, IBF migrated as a single protein with an estimated Mr of 16 kD and interacted with Ig of various species and with anti-Leu 11b antibodies. When carboxymethylated, however, IBF did not bind IgG. The present results suggest that free sulfhydryl groups of IBF is required for Ig binding.

Binding of IgG to MoFc gamma RII purified and reconstituted into supported planar membranes as measured by total internal reflection fluorescence microscopy.

Total internal reflection fluorescence microscopy (TIRFM) has been combined with functional reconstitution of the mouse IgG receptor moFc gamma RII in substrate-supported planar membranes to quantitatively probe IgG-moFc gamma RII interactions. MoFc gamma RII was purified from the macrophage-related cell line J774A.1 using affinity chromatography with Fab fragments of the anti-moFc gamma RII monoclonal antibody 2.4G2. Purified moFc gamma RII was reconstituted into liposomes by detergent dialysis, and the liposomes were fused on quartz substrates to form supported planar membranes containing moFc gamma RII. TIRFM measurements showed that fluorescently labeled 2.4G2 Fab specifically bound to the planar membranes, confirming the presence of moFc gamma RII. The receptor density in the planar membranes was sufficiently high to allow direct detection of bound, fluorescently labeled polyclonal and monoclonal mouse IgG with TIRFM, demonstrating that moFc gamma RII retained Fc-mediated IgG binding activity after planar membrane formation and permitting direct measurement of bound IgG as a function of the IgG solution concentration. Cross-inhibition measurements showed that polyclonal mouse IgG blocked the binding of labeled 2.4G2 Fab and that 2.4G2 Fab blocked the binding of labeled polyclonal IgG. This work provides a direct measure of the relatively weak IgG-moFc gamma RII association constant and demonstrates a new model system in which the chemical and physical properties of IgG-moFc gamma RII interactions can be quantitatively characterized as a function of membrane, antibody, and solution properties.

Identification and structural characterization of Fc gamma-receptors on pulmonary alveolar macrophages.

Little is known about the structure of the cell surface receptors for the Fc portion of immunoglobulin G (Fc gamma R) on tissue macrophages. Studies on leukocytes indicate the existence of three classes of Fc gamma R, denoted I, II, and III. The purpose of this study was to structurally characterize Fc gamma R on alveolar macrophages obtained by bronchoalveolar lavage, comparing them with Fc gamma R of monocytes, neutrophils, and U937 cells. Flow-cytometric evaluation, utilizing anti-Fc gamma R class-specific monoclonal antibodies, showed that alveolar macrophages expressed three Fc gamma R classes. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of the immunopurified Fc gamma R molecules revealed the following apparent molecular mass for each Fc gamma R class: Fc gamma RI, 70 kDa; Fc gamma RII, 44 kDa; and Fc gamma RIII, 57-65 kDa. RNA blot analysis demonstrated a 1.7-kb transcript for Fc gamma RI, 2.5 and 1.6 kb transcripts for Fc gamma RII, and a 2.2 kb mRNA for Fc gamma RIII. Fc gamma RII displayed the high-responder/low-responder polymorphism. Fc gamma RIII did not express the neutrophil antigen-type specific structural polymorphism of the deglycosylated Fc gamma R molecule and appeared to be a transmembrane molecule.

Inhibition of platelet GPIIb/IIIa binding to fibrinogen by serum factors: studies of circulating immune complexes and platelet antibodies in patients with hemophilia, immune thrombocytopenic purpura, human immunodeficiency virus-related immune thrombocytopenic purpura, and systemic lupus erythematosus.

We have studied the conditions of in vitro binding of platelet glycoprotein IIb/IIIa (GPIIb/IIIa) to fibrinogen and applied the results to identify and measure the serum inhibitors to the binding. For the enzyme-linked immunosorbent assay, platelet extract was delivered to a fibrinogen-coated microtiter plate that was incubated for 2 hours, followed by incubation with anti-GPIIb/IIIa monoclonal antibody for another 2 hours. The plate was then incubated with peroxidase-conjugated anti-mouse IgG for color development. The binding was shown to be calcium-dependent. The binding was partially blocked by treating the coated fibrinogen with anti-fibrinogen antibody. Reduction or dissociation of GPIIb/IIIa resulted in the total loss of its ability to bind to fibrinogen. Platelet extracts of patients with hemophilia showed decreased binding (25% and 14%, compared with control platelet extract), and an extract from a patient with Glanzmann's thrombasthenia showed no binding. With the enzyme-linked immunosorbent assay we have measured serum inhibitors to GPIIb/IIIa binding to fibrinogen in 35 hemophilia A, 17 immune thrombocytopenic purpura, 22 human immunodeficiency virus-related immune thrombocytopenic purpura, and 29 systemic lupus erythematosus serum samples. In those patients with inhibition by serum, polyethylene glycol precipitation of circulating immune complexes (CICs) decreased the inhibition by the supernatants, and all the resolubilized CIC precipitates demonstrated inhibition, which indicates that CICs play a major role in the inhibition of GPIIb/IIIa binding to fibrinogen. This, then, provides evidence of CIC-mediated impaired GPIIb/IIIa binding to fibrinogen in hemophilia A, HIV-ITP, and SLE.

Affinity-purified soluble Fc epsilon RII/CD23 derived from RPMI-8866 cells induces histamine release from human nasal polyp mast cells through a non-IgE-mediated mechanism.

Soluble Fc epsilon RII/CD23 (IgE-binding factor) is released spontaneously from activated B cells and most EBV-immortalised B cell lines. We have purified soluble Fc epsilon RII/CD23 from culture supernatants of RPMI-8866 cells on an IgE Sepharose column, and studied its ability to release histamine from human nasal polyp mast cells. Soluble Fc epsilon RII/CD23 induces release of a significant amount of histamine from nasal polyp mast cells in a dose-dependent manner. IgE, and a monoclonal antibody specific for the soluble form of this receptor, were shown to neutralise this effect. It was found that soluble Fc epsilon RII/CD23 was still capable of triggering histamine release from nasal polyp mast cells from which IgE had been eluted by incubation in a low pH buffer, suggesting that a non-IgE mediated mechanism was responsible for this effect.

Isolation and characterization of the Fc receptor from the fetal yolk sac of the rat.

The yolk sac of the fetal rat and the proximal small intestine of the neonatal rat selectively transport maternal IgG. IgG-Fc receptors are thought to mediate transport across the epithelium of both tissues. We used a mouse mAb (MC-39) against the 45-54-kD component of the Fc receptor of the neonatal intestine to find an antigenically related protein that might function as an Fc receptor in fetal yolk sac. In immunoblots of yolk sac, MC-39 recognized a protein band with apparent molecular mass of 54-58 kD. MC-39 bound to the endoderm of yolk sac in immunofluorescence studies. In immunogold-labeling experiments MC-39 was associated mainly with small vesicles in the apical cytoplasm and in the region near the basolateral membrane of endodermal cells. The MC-39 cross-reactive protein and beta 2-microglobulin, a component of the intestinal Fc receptor, were copurified from detergent-solubilized yolk sac by an affinity purification that selected for proteins which, like the intestinal receptor, bound to IgG at pH 6.0 and eluted at pH 8.0. In summary, the data suggest that we have isolated the Fc receptor of the yolk sac and that this receptor is structurally and functionally related to the Fc receptor of the neonatal intestine. An unexpected finding is that, unlike the intestinal receptor which binds maternal IgG on the apical cell surface, the yolk sac receptor appears to bind IgG only within apical compartments which we suggest represent the endosomal complex.

Characterization of the Fc gamma receptor on human platelets.

IgG-containing immune complexes may play a role in the immune destruction of human platelets by interacting with an Fc gamma receptor on the platelet surface. We studied the platelet Fc gamma receptor and characterized its interaction with IgG ligand and anti-Fc gamma receptor monoclonal antibodies. Oligomers of IgG, but not monomeric IgG, bound to platelets and the number of binding sites was significantly increased at low ionic strength. Ligand-binding studies indicated that normal human platelets express a single Fc gamma receptor (Fc gamma RII) with 8559 +/- 852 sites per cell, Kd = 12.5 +/- 1.7 X 10(-8) M using trimeric IgG. Results of studies with bivalent and Fab monoclonal anti-Fc gamma RII were consistent with each Fc gamma receptor expressing two epitopes recognized by the antibody. The number of Fc gamma binding sites and affinity of binding were unchanged by the presence of 2.0 mM Mg2+ or 10 micrograms/ml cytochalasin B. Platelet stimulation with thrombin or ADP in the presence of fibrinogen also did not alter the number of Fc gamma binding sites or the affinity of binding. However, platelets preincubated with 5 microM dexamethasone expressed a decreased number of Fc gamma binding sites as well as decreased IgG-dependent platelet aggregation. Platelets from patients with Glanzmann's thrombasthenia and from patients with the Bernard Soulier syndrome expressed a normal number and affinity of Fc gamma binding sites. The data suggest that platelet Fc gamma RII binding of trimeric IgG occurs independent of actin filament interaction, Mg2+, ADP, or thrombin and does not require GPIIb/IIIa or GPIIb/IIIa-fibrinogen interaction. Furthermore, this receptor appears to be normally expressed on GPIb-deficient platelets and susceptible to modulation by glucocorticoids. Finally, the Fc gamma-binding protein was isolated from whole platelets as a 220-kDa protein which upon reduction dissociates into 50,000 Mr subunits.