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

Nitric oxide-induced expression of C-reactive protein in islet cells as a very early marker for islet stress in the rat pancreas

In searches for marker molecules specifically expressed in nitric oxide-treated islet cells as a means to recognize early events in islet destruction, we now establish the presence of neo-C-reactive protein (neoCRP) in rat islet cells as early as 2 hr after treatment. We detected this altered molecular form of the acute-phase-reactant C-reactive protein (CRP) using immunocytochemistry with an anti-neoCRP-specific monoclonal antibody as well as reverse transcription-polymerase chain reaction with CRP-specific primers and in situ hybridization to demonstrate the presence of CRP-specific mRNA. After induction of a generalized inflammatory reaction in rats with heat-inactivated Corynebacterium parvum in vivo, neoCRP expression in islets is also found and within the pancreas restricted to pancreatic islet cells only. Our findings suggest an early heat-shock-like expression of this molecule in response to local nitrite oxide production or to exogeneously added nitric oxide in islet cells.

Binding site on human C-reactive protein (CRP) recognized by the leukocyte CRP-receptor

C-reactive protein (CRP), the prototypical inflammatory acute phase reactant in humans, interacts with monocytes and neutrophils via a specific receptor. To map the site on CRP recognized by the CRP receptor (CRP-R), synthetic peptides corresponding to the surface region on each of the five identical subunits were tested as competitors vs. [125I]-CRP for cell binding. A peptide of residues 27-38 (TKPLKAFTVCLH) efficiently inhibited CRP binding when compared to other nonoverlapping peptides. This peptide was termed the cell-binding peptide (CB-Pep). The F(ab')2 of an IgG Ab to the CB-Pep specifically inhibited CRP binding upon reacting with the ligand. Competitive binding studies with synthetic peptides truncated from either the NH2- or COOH-terminus of the CB-Pep revealed that the minimum length recognized by the CRP-R consisted of residues 31-36: KAFTVC. Conservative substitutions of residues within the CB-Pep indicated that the four residues AFTV were critical for CRP-R binding. The CB-Pep also inhibited induced superoxide generation by HL-60 granulocytes. The minimum length required for the inhibition was also KAFTVC; however, only Phe-33 and Leu-37 were critical residues in this assay. Anti-CB-Pep IgG Ab reacted more extensively with heat-modified CRP, suggesting that an altered conformation of CRP is preferentially recognized by the CRP-R. The results suggest that this contiguous sequence on a beta-strand on one face of each of five subunits of the CRP pentamer serves as a unique recognition motif for inflammatory leukocytes.

A hypothesis resolving the apparently disparate activities of native and altered forms of human C-reactive protein

Although C-reactive protein (CRP) has been studied for over 60 years, the in vivo function of this acute-phase reactant has not been clearly defined. The literature on CRP has been divided here into three categories: the cyclic, pentameric blood-borne form of CRP termed 'native' CRP which has activities mainly associated with the resolution of inflammation, conformationally altered and aggregated forms of CRP which display pro-inflammatory properties, and proteolytic forms of CRP exhibiting mixed activities. Since the activities of certain forms of CRP in some cases contradict others, a hypothesis has been developed which reconciles these differences. It is proposed that distinct species of CRP are formed which have unique activities at an inflammatory site; conformationally altered and proteolytic forms of CRP are created in succession from bound native CRP at the inflammatory site due to local conditions (e.g. lowered pH, oxygen radicals, or possibly enzymes). Aggregated and/or conformationally altered forms of CRP initially promote inflammation, and subsequently produced peptide products either up or down regulate different leukocyte activities to aid in the progression of the inflammatory event. As the local conditions favoring the conversion of native CRP to altered forms begin to subside, native CRP then predominates at the site, facilitating the removal of cellular debris and resolution of the lesion.

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.

Preferential expression of neo-CRP epitopes on the surface of human peripheral blood lymphocytes

Antibodies specific for C-reactive protein (CRP) have been reported to react with certain human peripheral blood lymphocytes (PBL); however, the nature of the antigen has not been clearly defined. In the present study we identified the CRP antigenicity on PBL as a CRP neoepitope not seen on the native-CRP molecule. Neo-CRP epitopes are expressed when the native pentameric form of CRP is dissociated into free subunits. Commercial anti-CRP antisera were found to possess a significant proportion of specificities (up to 16% of the total reactivity) directed against neo-CRP antigenicity. Since similar reagents had been used in previous studies on the reactivity of anti-CRP antisera with PBL, we set out to determine if either native- or neo-CRP epitopes were preferentially expressed on PBL. We prepared antisera monospecific for native-CRP and neo-CRP, respectively, and characterized these reactivities in both direct and indirect enzyme immunoassays. When analyzed by flow cytometry, anti-neo-CRP but not anti-native-CRP antiserum was found to react with normal PBL. F(ab')2 fragments of affinity-purified anti-neo-CRP had identical activity, and the reactivity against CRP was absorbed by reagents expressing neo-CRP but not native-CRP epitopes. Flow cytometric analyses of monocyte-depleted PBL from 25 normal donors detected a mean of 23.8 +/- 5.8% anti-neo-CRP-positive cells, a higher proportion of PBL expressing the CRP antigen than previously reported. Our findings indicate that a molecule identical to, or cross-reactive with, a neo-antigenic form of CRP is present on the surface of a significant proportion of normal human PBL.

Expression of a C-reactive protein neoantigen (neo-CRP) in inflamed rabbit liver and muscle

We have reported that human C-reactive protein (huCRP) can exist in two antigenically distinct forms which are observed as the native, pentameric, phosphorylcholine (PC)-binding CRP ("native-huCRP antigen"), and as a conformationally and physicochemically distinct free huCRP subunit ("neo-huCRP antigen"), respectively. We have demonstrated that forms of huCRP which preferentially express neo-huCRP antigenicity are naturally present on the surface of both normal human peripheral blood B lymphocytes and lymphocytes with natural killer cell activity. We have also reported that forms of huCRP expressing the neo-huCRP antigen have potent in vitro activities in platelet, polymorphonuclear leukocyte, and monocyte assays. In this study, we demonstrate a rabbit-CRP (raCRP) neoantigen can be expressed when isolated PC-binding raCRP is modified in analogy to huCRP. This "neo-raCRP" is cross-reactive with the neo-huCRP antigen and occurs naturally in acute phase but not normal rabbit liver and muscle. The relative distribution and localization of both antigens were comparable in tissue sections taken at 24 and 48 hr postinflammation elicited with typhoid vaccine. These data support the concept that CRP molecules expressing a structure and antigenicity which are distinct from native, pentagonal CRP do occur in vivo, and that such molecules accumulate at tissue sites of inflammation and necrosis.