Search PubMed⌕ Search

Biomedical subjects

H Gewurz

Publications and source records attributed to H Gewurz.

At least 19 recordsLinked to original sources
PubMed1998

Requirement for the alternative pathway as well as C4 and C2 in complement-dependent hemolysis via the lectin pathway.

Mannan-binding lectin (MBL) is a C1q-like molecule opsonic for several micro-organisms. MBL can activate C4, C2, and later acting complement components in the presence of serine proteases similar to but distinct from C1r and C1s via the lectin pathway of complement activation. We report here that mannan-coated MBL-sensitized erythrocytes are lysed via the lectin pathway in human serum-Mg-EGTA. The surprising occurrence of MBL-initiated lysis in the absence of calcium contrasts with the calcium requirement for C1q-initiated activation of C4 and C2. C2 is required, and lysis is significantly enhanced when indicator cells presensitized with C4 and then coated with mannan (EAC4-M) are used. The alternative pathway also is required, since lysis is lost when either factor D or factor B is removed and is restored upon reconstitution with the purified protein. Even though MBL is a C-type lectin, it is retained on mannan-coated erythrocytes in the absence of calcium. This contrasts with the absence of calcium-independent retention on mannan immobilized on polystyrene plates or beads, and helps explain the MBL-initiated hemolysis in Mg-EGTA. These investigations show that the alternative pathway as well as C4 and C2 of the classical pathway are required for complement-dependent hemolysis via the lectin pathway and provide a method for assay of lectin pathway-mediated complement activity in human serum that should be useful in unraveling the molecular interactions of this pathway.

Calcium↗
PubMed1998

Complement regulation in innate immunity and the acute-phase response: inhibition of mannan-binding lectin-initiated complement cytolysis by C-reactive protein (CRP).

Mannan-binding lectin (MBL) is an acute-phase protein which activates complement at the level of C4 and C2. We recently reported that the alternative pathway also is required for haemolysis via this 'lectin pathway' in human serum. CRP is another acute-phase reactant which activates the classical pathway, but CRP also inhibits the alternative pathway on surfaces to which it binds. Since serum levels of both proteins generally increase with inflammation and tissue necrosis, it was of interest to determine the effect of CRP on cytolysis via the lectin pathway. We report here that although CRP increases binding of C4 to MBL-sensitized erythrocytes, which in turn enhances lectin pathway haemolysis, it inhibits MBL-initiated cytolysis by its ability to inhibit the alternative pathway. This inhibition is characterized by increased binding of complement control protein H and decreased binding of C3 and C5 to the indicator cells, which in turn is attributable to the presence of CRP. Immunodepletion of H leads to greatly enhanced cytolysis via the lectin pathway, and this cytolysis is no longer inhibited by CRP. These results indicate that CRP regulates MBL-initiated cytolysis on surfaces to which both proteins bind by modulating alternative pathway recruitment through H, pointing to CRP as a complement regulatory protein, and suggesting a co-ordinated role for these proteins in complement activation in innate immunity and the acute-phase response.

Acute-Phase Reaction↗
PubMed1995

Structure and function of the pentraxins.

Over the past two years, the three-dimensional structure of the serum amyloid P component was defined by X-ray diffraction, the first such visualization of a pentraxin. Binding sites for calcium, ligands and complement were identified. New fusion proteins with amino acid sequence homology to the pentraxins were described, and new insights were gained into pentraxin phylogeny, biosynthesis, ligands, complement activation, leukocyte reactivity and biological functions in vivo.

Alzheimer Disease↗
PubMed1993

Human serum amyloid P component oligomers bind and activate the classical complement pathway via residues 14-26 and 76-92 of the A chain collagen-like region of C1q.

Serum amyloid P component (SAP) was polymerized using the cleavable cross-linker 3,3'-dithio-bis-(sulfo-succinimidylpropionate) to study its interaction with the C system. Dimers and trimers, but no larger oligomers, were observed; the trimers retained native SAP immunoreactivity (except for one calcium-dependent epitope) without displaying neo-SAP epitopes. The SAP trimers bound strongly to C1q, at the level of the collagen-like region (CLR). SAP bound to synthetic C1q A chain peptides 14-26 and 76-92, and these peptides inhibited the binding of SAP trimers to the CLR. When incubated in dilute human serum, SAP trimers consumed total C and C4, but not alternative pathway, hemolytic activities. Consumption of C4 by SAP trimers was inhibited by C1q A chain peptide 14-26. Thus, SAP oligomers bind C1q and activate the classical C pathway via the collagen-like region of C1q, at sites located within residues 14-26 and/or 76-92 of the C1q A chain.

Amino Acid Sequence↗
PubMed1992

DNA binds and activates complement via residues 14-26 of the human C1q A chain.

The mechanism by which DNA activates the classical complement pathway was investigated, with emphasis upon the C1q binding sites involved. DNA bound to both the collagen-like and globular regions of C1q. Binding reactivity with DNA was retained after reduction/alkylation and sodium dodecyl sulfate treatment of C1q. DNA bound preferentially to the A chain of C1q. Binding sites for DNA were localized by using synthetic C1q A chain peptides to two cationic regions within residues 14-26 and 76-92, respectively. Peptides 14-26 and 76-92 avidly bound DNA in enzyme-linked immunosorbent and gel shift assays. Peptide 14-26 also precipitated with DNA and blocked its ability to bind C1q and activate C. Replacement of the two prolines with alanines or scrambling the order of the amino acids resulted in loss of ability of peptide 14-26 to inhibit C1q binding and complement activation by DNA; similar investigations showed a sequence specificity for peptide 76-92 as well. These experiments identify C1q A chain residues 14-26 as the major site, and residues 76-92 as a secondary site, through which DNA binds C1q and activates the classical complement pathway, and demonstrate that a peptide identical to residues 14-26 can modulate C1q binding and complement activation by DNA.

Amino Acid Sequence↗
PubMed1992

Localization of sites through which C-reactive protein binds and activates complement to residues 14-26 and 76-92 of the human C1q A chain.

Studies were initiated to localize the C-reactive protein (CRP) binding site on the collagen-like region (CLR) of C1q. CRP bound preferentially to the A chain of reduced C1q, in contrast to aggregated immunoglobulin G (Agg-IgG), which reacted preferentially with the C chain. A group of C1q A chain peptides, including peptides identical to residues 81-97, 76-92, and 14-26, respectively, were synthesized from predicted binding regions. Peptide 76-92 contained two proximal lysine groups, and peptide 14-26 contained four proximal arginine groups. CRP-trimers and CRP-ligand complexes did not bind to immobilized peptide 81-97, but bound avidly to immobilized peptides 76-92 and 14-26. Agg-IgG did not bind to any of the peptides. Peptide 76-92 partially, and peptide 14-26 completely, inhibited binding of CRP to intact C1q. Peptide 14-26 also blocked C consumption initiated by CRP, but not by IgG. Replacement of the two prolines with alanines, or scrambling the order of the amino acids, resulted in loss of ability of peptide 14-26 to inhibit C1q binding and C activation by CRP, indicating a sequence specificity, and not a charge specificity alone, as the basis for the inhibitory activity of the peptide. Similar investigations with scrambled peptides showed a sequence specificity for the effects of peptide 76-92 as well. DNA and heparin inhibited binding of CRP trimers to intact C1q, as well as to each peptide 14-26 and 76-92, suggesting involvement of these regions in C1q-CLR binding reactions generally. Collectively, these data identify two cationic regions within residues 14-26 and 76-92 of the C1q A chain CLR as sites through which CRP binds and activates the classical C pathway, and suggest that these residues represent significant regions for C1q CLR binding reactions generally. To our knowledge, this represents the first delineation of sites on C1q through which binding and activation of the classical C pathway can occur.

Amino Acid Sequence↗
PubMed1992

A protease-sensitive site in the proposed Ca(2+)-binding region of human serum amyloid P component and other pentraxins.

Serum amyloid P component (SAP) is a decamer of 10 identical 25.5-kDa subunits. Limited proteolysis of SAP with alpha-chymotrypsin cleaves the subunit into two fragments of 18 and 7.5 kDa, although the fragments stay together in the decamer under nondenaturing conditions. Proteolysis does not occur in the presence of Ca2+ (10 mM). Cleavage with alpha-chymotrypsin prevents the Ca(2+)-dependent binding of SAP to zymosan extract, nucleosomes, and DNA. The alpha-chymotrypsin cleavage site identified is in a region of SAP that is highly conserved in members of the human C-reactive protein (CRP) family of proteins (pentraxins) to which SAP belongs and is similar to the Ca(2+)-binding site in calmodulin and related Ca(2+)-binding proteins (Nguyen, N.Y., Suzuki, A., Boykins, R.A., & Liu, T.-Y., 1986, J. Biol. Chem. 261, 10456-10465). Treatment of SAP with other proteases (trypsin, Pronase, and Nagarse protease) yields fragmentation patterns upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) that are similar to those obtained with alpha-chymotrypsin. Two other members of the pentraxin family of proteins, hamster female protein and rabbit CRP, also exhibit similar fragmentation patterns on SDS-PAGE when treated with the various proteases. Recently, it has been shown that the homologous protein, human CRP, is cleaved in the same homologous position as cleavage of SAP by alpha-chymotrypsin, resulting in the loss of Ca(2+)-binding (as shown by equilibrium dialysis) and Ca(2+)-dependent binding reactivities (Kinoshita, C.M., Ying, S.-C., Hugli, T.E., Siegel, J.N., Potempa, L.A., Jiang, H.J., Houghten, R.A., & Gewurz, H., 1989, Biochemistry 28, 9840-9848).(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗
PubMed1992

Localization of sequence-determined neoepitopes and neutrophil digestion fragments of C-reactive protein utilizing monoclonal antibodies and synthetic peptides.

We recently described 17 anti-CRP mAb, seven to native- (or conformational) and 10 to neo- (or sequence-determined) epitopes, including several anti-neo-CRP mAb specific for CRP peptide 199-206. In the present study, four new anti-native- and four new anti-neo-CRP mAb were generated and characterized by ELISA reactivity with native and modified human and rabbit CRP, as well as binding to pronase fragments of human CRP in Western blots. Assays with 17 synthetic CRP peptides identified anti-neo-CRP mAb specific for peptides 1-16, 14-24 and 137-152, respectively. The anti-neo-CRP mAb were reacted with fragments obtained by digesting CRP with multiple additional enzymes, including Staphylococcal V8 protease, trypsin, elastase, plasmin, thrombin and alpha-chymotrypsin. Native CRP was remarkably resistant to enzymic digestion, particularly in the presence of calcium, but was readily cleavable upon denaturation. Twenty-three informative fragments served to further distinguish mAb reactivity with at least four additional neo-CRP epitopes, which presumptively included residues in the regions of amino acids 22-45, 41-61, 114-121 and 130-138, respectively. The eight epitopes identified corresponded well with predicted regions of CRP antigenicity. In addition, at least six distinct native or conformation-determined epitopes were delineated. Reactivity of the anti-neo-CRP mAb with fragments of CRP generated by PMN enzymes indicated that regions sensitive to cleavage by neutrophil enzymes are located at approximately 3, 10 and 16 kD from the amino terminus of the CRP subunit. We expect that the anti-CRP mAb described and mapped herein will be useful tools for the elucidation of CRP structure and function.

Antibodies, Monoclonal↗
PubMed1992

Reactivity of anti-human C-reactive protein (CRP) and serum amyloid P component (SAP) monoclonal antibodies with limulin and pentraxins of other species.

Limulus polyphemus C-reactive protein (CRP) (limulin) has approximately 30% amino acid sequence homology and shares at least one idiotypic determinant associated with ligand-binding activity with human CRP (hCRP); limulin also shares amino acid sequence homology and lectin activity with human serum amyloid P component (hSAP). In the present study panels of 14 anti-hCRP monoclonal antibodies (mAb) directed to distinct hCRP epitopes and 11 anti-hSAP mAb directed to distinct epitopes of hSAP were tested for reactivity with limulin and pentraxins of other species including rabbit CRP (raCRP), rat CRP and hamster female protein (FP) by ELISA and Western blot analyses. None of the anti-human pentraxin mAb showed strong cross-reactivity with limulin; only five mAb reacted with limulin at all, and cross-reactivities of these mAb with the other pentraxins, when present, also were weak. Cross-reactivity of limulin with hCRP and hSAP was similar, and in light of comparable amino acid sequence homology, suggests this molecule can be considered the limulus SAP as well as the limulus CRP. Several anti-hCRP mAb cross-reacted strongly with rabbit CRP and rat CRP; a few anti-hSAP cross-reacted strongly with FP; and weak cross-reactions were observed between hCRP and hSAP, but cross-reactivities between the pentraxins generally were limited and weak. A rabbit polyclonal antibody raised to highly conserved limulin peptide 141-156 and strongly reactive with limulin reacted weakly with hCRP and raCRP but failed to react with rat CRP, hSAP or FP. These studies emphasize a limited but distinct antigenic similarity between limulin, hCRP and other pentraxins, and identify mAb reactive with potential regions of shared structure and/or function between pentraxins of different species.

Alpha-Globulins↗
PubMed1991

An appraisal of polystyrene-(ELISA) and nitrocellulose-based (ELIFA) enzyme immunoassay systems using monoclonal antibodies reactive toward antigenically distinct forms of human C-reactive protein.

The purpose of this study was to compare and contrast two enzyme immunoassay systems: the enzyme-linked immunosorbent assay (ELISA), which utilizes polystyrene microtiter plates as the adsorptive surface and the enzyme-linked immunoflow assay (ELIFA), which utilizes nitrocellulose membranes. The principal parameter under scrutiny was the denaturing or unfolding effects caused by the interaction of the protein with the adsorptive surfaces in each assay system. These effects were monitored by utilizing two conformationally distinct forms of human C-reactive protein (CRP), the native form of CRP and a denatured form (M-CRP), with a corresponding panel of monoclonal antibodies (MAbs) specific to either CRP or M-CRP. The results show that the ELIFA system was less sensitive than the ELISA system but that the ELIFA assay can be completed in less time than the ELISA. Also, adsorption of native CRP to the polystyrene surface in the ELISA system resulted in conformational changes of the adsorbed native CRP protein such that M-CRP reactive determinants were available for binding with anti-M-CRP MAbs, whereas native CRP adsorbed to the nitrocellulose membrane in the ELIFA system resulted in very limited conversion of CRP to M-CRP reactive epitopes. These results have important implications for development of immunoassays and screening of MAbs for proteins whose conformations may be affected by adsorption to various surfaces.

Antibodies, Monoclonal↗
PubMed1991

Binding and complement activation by C-reactive protein via the collagen-like region of C1q and inhibition of these reactions by monoclonal antibodies to C-reactive protein and C1q.

Ligand-complexed C-reactive protein (CRP), like aggregated or complexed IgG, can react with C1q and activate the classical C pathway. Whereas IgG is known to bind to the globular region and not to the collagen-like region (CLR) of C1q, the site of interaction of C1q with CRP has not been defined. CRP-trimers were prepared by cross-linking and found to bind to C1q and to activate the C system. Heat-aggregated IgG (Agg-IgG) did not block the binding of CRP-trimers to C1q, nor did CRP-trimers block binding of Agg-IgG to C1q, suggesting that CRP and IgG bind at different sites. ELISA and Western blot analysis showed that CRP-trimers bound to the CLR, whereas Agg-IgG bound only to the globular region; similarly, anti-CLR mAb inhibited binding of CRP-trimers to C1q whereas anti-globular region mAb did not. Reactivity with CRP-trimers as well as with Agg-IgG was retained after reduction/alkylation and SDS treatment of C1q. A group of 22 anti-CRP mAb directed against at least six distinct native-CRP epitopes and eight distinct neo-CRP epitopes was tested for ability to inhibit the CRP-CLR interaction; one mAb, anti-native CRP mAb 8D8, with strong inhibitory activity was identified. Fab' of 8D8 blocked binding of CRP-trimers to intact C1q as well as CLR, and also inhibited CRP (CRP-trimers and CRP-protamine complexes) induced C activation, but had no effect on C1q binding or C activation by Agg-IgG. These results indicate that a conformation-determined region on CRP binds to a sequence-determined region on the CLR of C1q in an interaction which leads to C activation. Anti-CRP and anti-C1q mAb that specifically inhibit this interaction are described.

Antibodies, Monoclonal↗
PubMed1991

Preferential binding and aggregation of rabbit C-reactive protein with arginine-rich proteins.

Human and rabbit C-reactive proteins (CRP) are similar in mol. wt, structure and amino acid sequence. In addition to the characteristic phosphoryline (PC)-binding specificity, both CRP molecules bind arginine- and lysine-rich proteins. The human CRP-cationic protein reactivity has been reported to be inhibited by calcium and promoted by PC in the presence of calcium. The present study compares binding and precipitation reactions of rabbit CRP (raCRP) with arginine- and lysine-rich proteins, and demonstrates the differential modulation of these interactions by calcium and acidic pH. Rabbit CRP shows preferential binding and precipitation reactivities with arginine-rich cationic molecules. Binding of raCRP to poly-L-arginine (PLA) and arginine-rich histone (ARH) occurs at pH 6.0, in the presence of calcium and is inhibitable by phosphorylcholine (PC) suggesting an interaction at or near the calcium-modulated PC binding site. The in vitro precipitation of raCRP and arginine-rich cationic molecules is significantly inhibited at pH 6.0, by the non-precipitating lysine-rich ligand PLL, and by physiological levels of calcium, and may reflect the participation of distinct "self-aggregation" sites on CRP in the precipitation response. The significance of the preferential arginine reactivity of raCRP to in vivo functions as a scavenger of chromatin during cell death and/or as a modulator of lipoprotein metabolism during the acute phase response is discussed.

Animals↗
PubMed1991

Direct binding of complement component C1q to human immunodeficiency virus (HIV) and human T lymphotrophic virus-I (HTLV-I) coinfected cells.

Previous studies have shown that coinfection of the human T lymphotrophic virus type I (HTLV-I) chronically infected cell line MT4 with human immunodeficiency virus type 1 (HIV-1) results in cells which spontaneously activate complement via the classical pathway. This complement activation was antibody independent, yet required C2, suggesting either direct C1, C4, or C2 activation. Because some animal retroviruses have been shown to bind human C1q directly, the present study investigated the possible direct binding of C1q by HIV coinfected MT4 cells. Coinfected cells bound both C1q present in serum and highly purified C1q. Binding of C1q resulted in formation of active C1 on the cell surface, which could in turn activate complement as shown by C4 consumption. The C1q binding was not HIV-isolate specific since infection of MT4 cells with any of three diverse isolates all induced C1q binding. Purified collagen-like region (CLR) and globular region (GR) fragments of C1q both bound to coinfected cells, suggesting a mechanism of binding by C1q similar to that of fibronectin-C1q binding. However, culture of coinfected cells in serum-free (fibronectin-free) medium did not reduce C1q binding. A second HTLV-I chronically infected line, SLB-1, also displayed increased binding of C1q after HIV infection. The H9 cell line, which is not HTLV-I infected, did not bind C1q after HIV infection. These results suggest that a retrovirus protein expressed by coinfected cells directly binds C1q resulting in classical complement activation. This type of activation may have profound biological effects in persons coinfected with HIV-1 and HTLV-I.

Cell Line↗
PubMed1991

Defined chemically cross-linked oligomers of human C-reactive protein: characterization and reactivity with the complement system.

Chemically cross-linked C-reactive protein (CRP) oligomers were prepared and characterized, and C1q binding and C activation were investigated. Purified human CRP was polymerized in the presence of both non-cleavable and cleavable cross-linking agents and further separated by Superose 12 analytical FPLC column chromatography into fractions of 110 KDa (pentameric monomers), 220 KDa (dimers) and 330 KDa (trimers); virtually no larger oligomers were formed under a variety of experimental conditions. CRP subunits were cross-linked both within and between CRP pentamers. CRP trimers retained native CRP antigenicity without expression of neo-CRP epitopes. CRP trimers showed maximal binding and CRP dimers showed partial binding of solid phase C1q while CRP monomers bound virtually no C1q at all; CRP trimers also bound to fluid phase C1q. Binding was Ca++ independent and increased as the ionic strength or pH were lowered, characteristics comparable to binding of aggregated IgG to C1q; it was not inhibited by phosphorylcholine. CRP trimers consumed total C, C1 and C2 haemolytic activities upon incubation in fresh human serum, but much less efficiently than did CRP-protamine complexes or Agg-IgG. CRP trimers failed to deplete alternative C pathway haemolytic activity at all. The stable, chemically defined CRP oligomers described in this report, which bind C1q efficiently but display poor ability to activate the classical C pathway in the absence of an appropriate ligand, should be valuable in further studies of the interactions between CRP and the C system.

C-Reactive Protein↗
PubMed1989

Elucidation of a protease-sensitive site involved in the binding of calcium to C-reactive protein.

C-reactive protein (CRP) is a Ca2+-binding protein composed of five identical 23-kDa subunits arranged as a cyclic pentamer, present in greatly elevated concentration in the blood during the acute phase of processes involving tissue injury and necrosis. In the present work, it was found that treatment of human CRP with Pronase or Nagarse protease produces two major fragments which remain associated in physiological buffers but are separable under denaturing conditions. To localize the cleavage site(s), the fragments were characterized according to molecular mass, amino acid composition, partial amino acid sequence, and reactivity with monoclonal antibodies specific for the fragments and for defined CRP epitopes including residues 147-152 and 199-206. Nagarse protease cleaves the CRP subunit between residues 145 and 146, producing two fragments, 16 and 6.5 kDa (calculated molecular mass). Pronase cleaves the CRP subunit between residues 146 and 147, producing a 16-kDa fragment (A1) and a 6.5-kDa fragment (B); an additional fragment (A2) approximately 1 kDa smaller than fragment A1 is also apparently produced due to a secondary cleavage site in fragment A1. Cleavage appears to be completely inhibited in the presence of 1 mM CaCl2. Ca2+ does not protect cleaved CRP from heat-induced aggregation (i.e., precipitation) as it does the intact protein. Protease-cleaved CRP loses the ability to bind to the Ca2+-dependent ligand phosphorylcholine but remains the ability to bind to the Ca2+-independent ligand arginine-rich histone. Equilibrium dialysis indicates that intact CRP binds 2 mol of Ca2+/mol of subunit with a Kd of 6 X 10(-5) M.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗
PubMed1989

Identification and partial characterization of multiple native and neoantigenic epitopes of human C-reactive protein by using monoclonal antibodies.

Multiple mAb to human C-reactive protein (CRP) were prepared which reacted preferentially with either native CRP, modified CRP (expressing "neo-CRP" determinants) or both forms of the molecule. These mAb were divided into four groups according to their binding characteristics to various CRP preparations and CRP peptides by using a combination of ELISA, dot blot, and Western blot assays; they were further characterized based upon their reactivity with CRP in the presence of calcium and inhibition by phosphorylcholine. The first group consisted of mAb that reacted only with native CRP, and served to define four distinct native CRP epitopes. The second group consisted of mAb that reacted with native CRP and also with CRP modified by direct immobilization on polystyrene plates, urea-chelation or SDS treatment in the absence of calcium, thus identifying a fifth native CRP epitope; these mAb displayed significantly greater reactivity with native than with modified CRP. The third group included mAb that reacted only with modified CRP and with the larger amino-terminal fragment (residues 1-146) of pronase-cleaved CRP. The fourth group included mAb that reacted only with modified CRP and with the smaller carboxyl-terminal fragment (residues 147-206) of pronase-cleaved CRP; most of these antibodies also reacted with the carboxyl-terminal octapeptide (residues 199-206) of CRP. These experiments have identified mAb that react preferentially with distinct conformational and sequence-determined epitopes of native and modified forms of the CRP molecule, respectively; provide partial identification of the epitopes with which they interact; point to the presence of at least five epitopes on native CRP and at least three epitopes on modified CRP; and provide antibodies suitable for identification and quantitation of native and modified forms of CRP. The mAb directed against neo-CRP epitopes may help identify the presence of this pentraxin and antigenically-related proteins at previously unappreciated sites.

Animals↗
PubMed1989

Binding sites for C-reactive protein on human monocytes are distinct from IgG Fc receptors.

Previous investigations have provided evidence to suggest that C-reactive protein (CRP), an acute-phase reactant, binds to human monocytes at a membrane site that is either identical to or physically associated with IgG Fc receptors. To characterize further the relationship between monocyte CRP binding sites and IgG Fc receptors, monocytes were allowed to attach to surfaces coated with IgG or CRP and binding-site redistribution was assessed. Binding was measured by using protein-coated sheep erythrocytes (E). When attached to control (gelatin or albumin) surfaces, greater than 60% and 43% of monocytes formed rosettes with E-IgG and E-CRP, respectively. Following adherence to surface immobilized CRP, the proportion of cells binding E-IgG was unchanged; however, fewer than 20% of monocytes bound E-CRP. When attached to IgG-coated surfaces, fewer than 20% of monocytes formed rosettes with either E-IgG or E-CRP. In order to determine whether the unidirectional modulation of CRP and IgG binding sites was the result of CRP binding directly to a subclass of IgG Fc receptors, fluid-phase IgG-blocking studies were performed. When monocyte monolayers were preincubated with either monomeric or heat-aggregated IgG, a dose-dependent reduction in E-IgG binding was observed. In contrast, all concentrations of fluid-phase IgG failed to inhibit monocyte binding of E-CRP. These data indicate that CRP binds to human monocytes at a site physically associated with but distinct from IgG Fc receptors.

Antigens, Differentiation↗