Stimulation of human neutrophils, monocytes, and platelets by modified C-reactive protein (CRP) expressing a neoantigenic specificity.

C-reactive protein (CRP) can be structurally modified by heat, acid, or urea-chelation to express a neoantigen designated by us as neo-CRP. This antigen is also expressed on the in vitro primary protein translation products of both human and rabbit CRP. Unmodified CRP and CRP complexed with pneumococcal C-polysaccharide (CPS) do not express neo-CRP. Forms of CRP expressing neo-CRP but not native CRP antigenicity (even in the presence of CPS) consistently and in a dose-dependent manner potentiated the respiratory burst response of human polymorphonuclear leukocytes and peripheral blood monocytes to heat-modified IgG. Forms of CRP expressing neo-CRP antigenicity also induced reactions of aggregation and secretion from isolated platelets and potentiated platelet activation stimulated by ADP in platelet-rich-plasma, while native CRP alone or complexed with CPS again did not. Unlike CRP-CPS complexes, forms of CRP expressing neo-CRP were not able to activate the complement system. These data emphasize the biologic potential inherent in this humoral acute-phase reactant, particularly in the activation of the formed elements of the blood important in the inflammatory response. Since these cell-activating properties are preferentially observed when CRP is structurally modified to express the neo-CRP antigen, such a molecular conversion may be central to the structure-function relationships of CRP at local sites of inflammation and tissue injury.

Influence of tuftsin-like synthetic peptides derived from C-reactive protein (CRP) on platelet behaviour.

C-reactive protein (CRP) is an acute-phase reactant that modifies platelet function differently, depending upon its physiochemical state. Aggregated and ligand-complexed forms of CRP initiate the activation of platelets, whereas naturally occurring CRP peptides inhibit platelet activation. The present study documents neutral proteases of the polymorphonuclear leucocyte (PMN) to cleave CRP into reaction products with the potential to inhibit platelet activation, and explore the structure-function relationships involved in the regulation of platelet activation by CRP using synthetic CRP peptides. Evidence was obtained that (i) a minimum of two linear functional domains exist within CRP that influence platelet activation; (ii) they reside in the mid-portion and at the C-terminus of the CRP molecule; (iii) the mid-portion domain inhibits platelet activation stimulated by adenosine diphosphate (ADP) or acid-soluble collagen, whereas the C-terminal domain initiates platelet activation; (iv) the functional expression of the C-terminal domain is maximized when the linear peptide is immobilized on latex; and (v) both CRP domains contain a homologue of the immunoregulatory signal peptide, tuftsin. These data suggest that the molecular mechanisms by which platelet processes are modulated by CRP may be related to the presence of tuftsin homologues in CRP.

Platelet response to aggregated C-reactive protein: fibrinogen-dependent and independent signals.

Fibrinogen augmented gel-filtered platelet (GFP) aggregation only during an intermediate level of platelet activation stimulated by aggregated C-reactive protein (CRP) (AggCRP), implying that a mechanism to utilize fibrinogen is not operative or does not require an exogenous source of fibrinogen at near-maximal or threshold levels of platelet activation. By contrast, inclusion of the tetrapeptide fibrinogen antagonist, Arg-Gly-Asp-Ser(RGDS), inhibited both intermediate and near-maximal, but not threshold, levels of platelet activation stimulated by AggCRP. These data suggest that AggCRP initially stimulates platelets independent of fibrinogen but, in so doing, activates a fibrinogen-dependent mechanism(s) capable of augmenting the overall extent of platelet stimulation.

Expression, detection and assay of a neoantigen (Neo-CRP) associated with a free, human C-reactive protein subunit.

It has previously been reported that human C-reactive protein (CRP) can exist in at least two molecular conformations distinguished by antigenic, electrophoretic and ligand-binding reactivities. In the present study we describe the formation, detection and distinctiveness of a conformation expressing a CRP neoantigen (neo-CRP), and report that this form is characteristic in vitro of a free CRP subunit. Soluble native-CRP was found to express neo-CRP antigenicity upon treatment with acid; upon urea-chelation or heating in the absence of calcium; and upon adsorption onto uncoated polystyrene plates. Native-CRP bound by capture ELISA to phosphorylcholine-containing ligand or anti-native-CRP did not express neo-CRP antigenicity, suggesting that PC ligand- or antibody binding is not sufficient to induce expression of the neoantigen. Human CRP which expressed neo-CRP antigenicity had limited solubility and tended to aggregate in buffers of ionic strength 0.15, but remained soluble when the ionic strength was reduced to 0.015. Soluble urea-chelated or acid-treated CRP molecules expressing neo-CRP antigenicity chromatographed and electrophoresed as a single protein with a Mr of approx. 22,000, indicating that the CRP neoantigen can be expressed on free CRP subunits and this expression need not require proteolysis. Further, molecules expressing neo-CRP antigenicity were detected in the plasma of patients with rheumatoid arthritis. The identification and characterization of this CRP neoantigen should serve as a useful marker in studies of CRP subunits and biologically relevant forms of CRP, and should contribute to the elucidation of the role of CRP in the acute inflammatory response.

Further studies on the modulation of blood coagulation by human serum amyloid P component and its acute phase homologue C-reactive protein.

Serum amyloid P component (SAP), and its acute phase homologue C-reactive protein (CRP), prolonged activated partial thromboplastin times (APTT) in cell free plasma when assayed at physiological concentrations in the presence of heparin. SAP also inhibited clot formation initiated through the extrinsic and terminal phases of coagulation in heparinized cell free plasma, an activity not shared with CRP. When CRP and SAP were similarly evaluated in whole blood using the thromboelastograph (TEG), CRP delayed the onset of coagulation and the initial rate of fibrin formation/polymerization; final clot patency was unaltered. SAP suppressed the anticoagulant activity of heparin in the TEG assay, unlike results obtained in heparinized cell free plasma, by facilitating a more rapid onset of coagulation, increasing the rate of fibrin formation/polymerization, and correcting clot patency. The data provided offer further evidence that these homologues can intercede in blood coagulation.

Cleaved forms of C-reactive protein are associated with platelet inhibition.

C-reactive protein (CRP) is the prototypic acute phase reactant and serves clinically as a marker of inflammation and tissue destruction. When native CRP pentamer was incubated with Streptomyces griseus protease, a newly formed and transient ability to inhibit platelet aggregation stimulated by adenosine diphosphate or collagen was often elicited early during the course of enzymatic digestion. Sodium dodecyl sulfate polyacrylamide gel electrophoresis analyses of the digests revealed that platelet inhibitory activity correlated with altered electrophoretic mobility and reductions in subunit and pentameric m.w. Minimally degraded forms of CRP were also isolated "de novo" from inflammatory fluids and, like their enzyme degraded counterparts, inhibited platelet activation. Dissociation of degraded CRP with SDS followed by the removal of SDS resulted in the separation fragments which inhibited platelet function. We propose that in a degradative environment, such as at sites of inflammation/tissue damage or through the action of serum proteases, CRP may transitorily down-regulate the platelet.

Platelet agonist synergism by the acute phase reactant C-reactive protein.

C-reactive protein is the prototypic acute phase reactant. A self-complexed form (H-CRP) can induce isolated platelets to undergo aggregation, secretion of dense and alpha-granule constituents, and generation of thromboxane A2, but fails to function in platelet-rich plasma (PRP) as a direct agonist. In contrast, when PRP was activated with an amount of adenosine diphosphate (ADP) that produced only reversible platelet aggregation, the presence of H-CRP resulted in irreversible aggregation and the secretion of adenosine triphosphate (ATP). Following a maximum stimulus with ADP alone, where platelet secretion occurred late during the aggregation response, the presence of H-CRP shifted and increased the secretory burst to a time simultaneous with the onset of aggregation. This hypersecretion required H-CRP to be present prior to platelet stimulation or to be added within 15 to 30 seconds following the addition of ADP. H-CRP also potentiated platelet activation stimulated with epinephrine, thrombin, and collagen. When the synergism generated in PRP by H-CRP in the presence of ADP or epinephrine was compared to the synergism similarly produced by aggregated human IgG, collagen, or thrombin, it more closely resembled that of collagen, as reflected by the kinetics and characteristics of synergism and sensitivity to creatine phosphate/creatine phosphokinase or 5,8,11,14-eicosatetraynoic acid. These data provide a philosophically ideal niche for the acute phase (and C-reactive protein) in that a platelet-directed activity associated with this acute phase reactant is not utilized unless platelets are otherwise challenged.

Enhanced skin reactivity to platelet-derived permeability factor (PDPF) and exogenous histamine in an acute phase rabbit model.

Rabbits made acute phase by sub-cutaneous trauma with 2% croton oil (in mineral oil) were tested by intradermal (ID) injection with platelet-granule extracts containing platelet-derived permeability factor (PDPF). Compared with controls, skin reactivity to PDPF was enhanced in acute phase animals 3-7 days post-trauma, a period of acute inflammation as reflected by the occurrence in the circulation of C-reactive protein; maximal skin responses were observed 3-4 days post-trauma. Individual skin sites reached maximum intensity 15 min-1 hour post-ID injection of PDPF and were sensitive to chlorpheniramine maleate, suggesting a major role for histamine. Intradermal injection of histamine revealed that acute phase animals yielded an initially more intense skin reaction, and were markedly less capable of recovering from the effects of histamine. These data suggest that in the acute phase, there exists a heightened and prolonged sensitivity to the action of histamine which can be exploited by pro-inflammatory agents such as PDPF.

Selective inhibition of platelet activation by the amyloid P-component of serum.

One component of amyloid, protein AP, has a characteristic pentameric structure and is identical with a 9.5s serum alpha 1-globulin designated serum amyloid P-component or SAP. Another pentameric molecule, the acute-phase reactant C-reactive protein (CRP), shares major amino acid sequence homology with SAP although, in man, SAP is not an acute-phase reactant. Recently, we demonstrated that heat-aggregated CRP (H-CRP), like heat-aggregated IgG, activates platelets to reactions of aggregation, secretion, and generation of thromboxane A2. We report here that physiologic concentrations of SAP inhibit platelet aggregation stimulated by H-CRP. SAP must be present before platelet challenge with H-CRP to be effective. Native (unaggregated) CRP does not inhibit platelet activation induced by H-CRP, and the platelet inhibitory effect of SAP is restricted because platelet responses to each heat-aggregated IgG, acid-soluble collagen, DNA, ADP, and thrombin remain unaltered in the presence of SAP. Thus, human SAP seems to selectively modulate platelet reactivity to modified CRP, and as such to down-regulate at least one aspect of the biologic capacity of its acute-phase homologue.

Modulation of fibrin clot formation by human serum amyloid P component (SAP) and heparin.

Serum amyloid P-component (SAP) is a normal plasma constituent in man with a circulating concentration of approximately 40 micrograms/ml. Supraphysiological amounts of SAP (150-300 micrograms/ml) have been reported to affect coagulation. We have investigated this further by studying the effect of SAP upon clot times in both the absence and presence of heparin, a suggested ligand for SAP and itself a modulator of coagulation processes. In the absence of heparin, SAP (5-125 micrograms/ml) had no effect on clot times generated by Activated Thrombofax Reagent, brain thromboplastin, Russell's Viper Venom or thrombin when assessed in normal citrated plasma. However, in the presence of amounts of heparin that had only a minor effect upon clot times, SAP (5-40 micrograms/ml) greatly prolonged clot formation, with the thrombin time the most sensitive to SAP. This suggested that the primary effect of SAP was at this distal level of the coagulation pathway. Evaluation by radioimmunoassay revealed that supraphysiological concentrations of SAP (150-300 micrograms/ml) alone reduced by approximately 25% the release of fibrinopeptide A (FPA) from fibrinogen. In the presence of heparin, substantial synergism was observed with maximal reductions of approximately 70% in FPA production requiring only 25-50 micrograms/ml SAP. This inhibition correlated with increased thrombin clot time but was unrelated to any direct modulation in either the activities of anti-thrombin III or activated Factor XIII, and was independent of an alteration in the rate of fibrinolysis. Further, while SAP itself did not interfere with the process of spontaneous fibrin polymerization, in the presence of heparin a prolonged polymerization time (greater than 145%) was observed. We believe that these data reflect the primary mechanisms by which serum amyloid P component influences blood coagulation.

Effects of heparin and alpha 1-acid glycoprotein on thrombin or Activated Thrombofax Reagent-induced platelet aggregation and clot formation.

alpha 1-Acid glycoprotein (AAG) obtained from ascites and/or pleural fluids exhibited anti-heparin effects in platelet-poor plasma when evaluated by activated partial thromboplastin (Activated Thrombofax Reagent) and heparin-thrombin clotting time assays. An anti-heparin effect for AAG was also demonstrable in platelet-rich plasma (PRP) challenged with thrombin, but only over a limited range of heparin concentrations; at elevated heparin concentrations, and only in the presence of AAG, both platelet aggregation and clot formation were substantially inhibited. However, no detectable anti-heparin effect was observed following challenge of PRP with Activated Thrombofax Reagent; indeed, the anti-coagulant effect of heparin appeared synergistically amplified in these systems containing AAG, and AAG exhibited platelet pro-aggregate and pro-coagulant properties in the absence of heparin. The platelet pro-aggregating activity of AAG, though independent of heparin, appeared to require the onset of the coagulation cascade prior to the generation of thrombin; in the absence of such initiation, AAG remained a potent inhibitor of platelet activation.

Comparison of the enzymatic sensitivities of the platelet receptor for human C-reactive protein and its functional relationship to the platelet IgG Fc receptor.

Thermally modified human C-reactive protein (H-CRP) and IgG (AHGG) each activate isolated human platelets to reactions of aggregation and secretion. As these molecules exhibit many functional similarities, we questioned whether they might also share a receptor on the platelet membrane. Neither plasmin nor phospholipase C altered the platelet response to H-CRP or AHGG, although these reagents enhanced the platelet expression to acid soluble collagen (ASC). Conversely, chymotrypsin treatment of platelets resulted in an elevated response to each H-CRP and AHGG, but not to ASC. These data suggest that the H-CRP and AHGG platelet receptors share characteristics which contrast with those of the receptor for collagen. However, monomeric IgG, which can bind with the platelet and inhibit the response to AHGG, exerted no effect on the platelet response to H-CRP. Further, a functional receptor for thermally modified human or rabbit CRP was detected on rabbit platelets in the absence of a demonstrable Fc receptor for aggregated IgG. These data indicate that the platelet receptors for the modified forms of CRP and IgG are distinct.

Generation of thromboxane A2 and aorta-contracting activity from platelets stimulated with modified C-reactive protein.

The classical acute phase reactant, C-reactive protein (CRP), appears in markedly elevated concentration in the sera of individuals undergoing reactions of acute inflammation and tissue degradation. We previously demonstrated that like IgG, appropriately purified CRP could be thermally modified (H-CRP) such that it enhanced platelet activation in plasma and initiated platelet responses in isolated systems. We now report that this direct platelet activation by modified CRP results in the secretion of both platelet dense body and alpha-granule constituents, and is sensitive to non-steroidal anti-inflammatory drugs as well as the adenosine diphosphate (ADP)-removing enzyme system creatine phosphate/creatine phosphokinase. Thin-layer chromatographic (TLC) analysis of prostanoate endproducts following platelet activation with H-CRP revealed the formation of thromboxane B2 (the hydrated endproduct of thromboxane A2), an important endogenous platelet activator and contractor of vascular tissue; bioassay on rabbit aorta strips of supernatants obtained from platelets undergoing challenge with H-CRP supported the TLC analysis. Complexes formed between CRP and one major ligand, the polycation, were found to share certain platelet activating properties with H-CRP, as does latex-aggregated CRP. These data imply a potential agonist role for this acute phase reactant in platelet physiology and suggest that the interaction of modified forms of CRP with the platelet at sites of vascular damage could have pathological significance.

Activation of platelets by modified C-reactive protein.

The functional similarities between C-reactive protein (CRP) and immunoglobulin raised the possibility that modified CRP might resemble immunoglobulin in its activating effects upon the human platelet. Thermally-aggregated CRP (H-CRP), but not unmodified CRP, induced reactions of aggregation and secretion from isolated platelets; maximum responses occurred with less than 50 microgram/ml H-CRP and were similar to responses mediated by thermally-aggregated human IgG (AHGG). Platelet activation induced by H-CRP was sensitive to the presence of EDTA and dibucaine, required metabolic energy and was inhibited by increased levels of cAMP. Like AHGG, H-CRP acted synergistically with other platelet stimulators, although on a weight basis H-CRP appeared approximately ten- to twenty-fold more effective than AHGG. Complexes formed between CRP and certain of its polycationic ligands (PLL and protamine) shared platelet activating properties with H-CRP, whereas complexes of CRP and CPS did not. These data point to the ability of appropriately modified CRP to stimulate or enhance platelet responsiveness, and taken together with those reactivities described previously between modified CRP and certain lymphocytes, phagocytes, and the complement system, support the concept that CRP can initiate biological activities similar to those mediated by immunoglobulin.

Platelet inhibitory effects of CRP preparations are due to a co-isolating low molecular weight factor.

We previously reported that C-reactive protein (CRP), an acute phase reactant, inhibits platelet activation through an effect upon a factor(s) critical to ADP-mediated secondary wave platelet aggregation but independent of a direct effect upon platelet contractile elements. However, a role for an accessory factor in this inhibitory effect became of concern because of an inconsistency in the effects of CRP preparations upon the platelet: inhibition was lost upon storage and CRP preparations differed, on a weight basis, in inhibitory capacity and sensitivity to the presence of the CRP ligand C-polysaccharide (CPS(. The studies presented herein were thus intended to assess whether an accessory factor was involved in the inhibition of platelet activation observed with CRP. We report that the activity of the inhibitory CRP preparations resulted from association with a low molecular weight factor (LMF) with an apparent nominal molecular weight of 8300-12,500 and an A280:A260 ratio of approximately 0.4. Purified CRP did not inhibit platelet responsiveness but CRP with associated LMF (CRP-LMF) did. Moreover, the inhibitory capacity of CRP-LMF but not LMF was substantially reversed in the presence of CPS. These studies indicate that the platelet inhibitory properties of CRP preparations result from and are contingent upon the presence of a co-isolating low molecular weight factor.

Modulation of platelet aggregation by native DNA - initial description of platelet receptor type, number and discrimination for native DNA.

Native DNA (dsDNA) induces the aggregation of isolated human platelets. Using isotopically labeled dsDNA (125I-dsDNA) and Scatchard analysis, a single class of platelet receptor was detected with a KD = 190 pM and numbering approximately 275/platelet. This receptor was discriminatory in that heat denatured dsDNA, poly A, poly C, poly C x I and poly C x poly I failed to substantially inhibit either the platelet binding of, or platelet aggregation induced by, dsDNA; by themselves, these polynucleotides were ineffective as platelet agonists. However, poly G, poly I and poly G x I effectively and competitively inhibited platelet binding of the radioligand, independently activated the platelet and when used at a sub-activating concentration decreased the extent of dsDNA stimulated platelet aggregation. These data depict a receptor on human platelets for dsDNA and perhaps certain additional polynucleotides and relate receptor-ligand interactions to a physiologic platelet function.

Histamine secretion from human leucocytes stimulated by basophil-derived platelet-activating factor.

Crude preparations of platelet-activating factors (PAF) derived by methanolic extraction of supernatants from antigen-challenged basophils generally stimulated the secretion of histamine from human peripheral blood leucocytes when co-incubated with these cells in the absence of an additional stimulator. When these crude preparations were analysed by a preparative thin-layer chromatography technique established for the isolation of purified native PAF, multiple lipid components were identified. The component that migrated in accordance with native PAF, and which activated platelets to aggregation/secretion, was responsible for the induction of histamine release previously seen with the crude PAF preparations and could augment histamine secretion from leucocytes undergoing stimulation with either the biologically active peptide F-Met-Leu-Phe or with antigen E. These data suggest that basophil-derived PAF may serve as an important endogenous feedback mechanism during basophil activation in addition to its modulatory role upon the platelet.

Platelet activating factor: an inhibitor of neutrophil activation?

Preparations of platelet activating factor (PAF) derived by methanolic extraction of supernatants from antigen-challenged rabbit basophils proved capable of activating platelets while concurrently inhibiting neutrophil aggregation/secretion stimulated by biologically active F-Met peptides, ionophore A23187, or zymosan-treated serum. This inhibition was non-cytotoxic and species-non-specific. When these PAF preparations were analysed using thin-layer chromatography, multiple lipids were detected. Both platelet-stimulating as well as neutrophil-inhibitory activity was present in a lipid component migrating at an RF consistent with native PAF; however, these biological activities were not limited only to PAF and, indeed, could also be detected in lipid with solubility characteristics more closely related to a lysophosphatide than to native PAF. These data are compatible with the belief that native PAF may belong to a family of biologically active lipids differing somewhat in physico-chemical properties. Moreover, these data illustrate that PAF and/or PAF-like molecules may also demonstrate a biological activity distinct from their effects upon the platelet.