Pentraxins and Fc Receptor-Mediated Immune Responses

C-reactive protein (CRP) is a member of the pentraxin family of proteins. These proteins are highly conserved over the course of evolution being present as far back as 250 million years ago. Mammalian pentraxins are characterized by the presence of five identical non-covalently linked subunits. Each subunit has a structurally conserved site for calcium-dependent ligand binding. The biological activities of the pentraxins established over many years include the ability to mediate opsonization for phagocytosis and complement activation. Pentraxins have an important role in protection from infection from pathogenic bacteria, and regulation of the inflammatory response. It was recognized early on that some of these functions are mediated by activation of the classical complement pathway through C1q. However, experimental evidence suggested that cellular receptors for pentraxins also play a role in phagocytosis. More recent experimental evidence indicates a direct link between pentraxins and Fc receptors. The Fc receptors were first identified as the major receptors for immunoglobulins. The avidity of the interaction between IgG complexes and Fc receptors is greatly enhanced when multivalent ligands interact with the IgG binding sites and activation of signaling pathways requires Fc receptor crosslinking. Human pentraxins bind and activate human and mouse IgG receptors, FcγRI and FcγRII, and the human IgA receptor, FcαRI. The affinities of the interactions between Fc receptors and pentraxins in solution and on cell surfaces are similar to antibody binding to low affinity Fc receptors. Crystallographic and mutagenesis studies have defined the structural features of these interactions and determined the stoichiometry of binding as one-to-one. Pentraxin aggregation or binding to multivalent ligands increases the avidity of binding and results in activation of these receptors for phagocytosis and cytokine synthesis. This review will discuss the structural and functional characteristics of pentraxin Fc receptor interactions and their implications for host defense and inflammation.

Pentraxins and IgA share a binding hot-spot on FcαRI

The pentraxins, C-reactive protein (CRP), and serum amyloid P component (SAP) have previously been shown to function as innate opsonins through interactions with Fcγ receptors. The molecular details of these interactions were elucidated by the crystal structure of SAP in complex with FcγRIIA. More recently, pentraxins were shown to bind and activate FcαRI (CD89), the receptor for IgA. Here, we used mutations of the receptor based on a docking model to further examine pentraxin recognition by FcαRI. The solution binding of pentraxins to six FcαRI alanine cluster mutants revealed that mutations Y35A and R82A, on the C-and F-strands of the D1 domain, respectively, markedly reduced receptor binding to CRP and SAP. These residues are in the IgA-binding site of the receptor, and thus, significantly affected receptor binding to IgA. The shared pentraxin and IgA-binding site on FcαRI is further supported by the results of a solution binding competition assay. In addition to the IgA-binding site, pentraxins appear to interact with a broader region of the receptor as the mutation in the C'-strand (R48A/E49A) enhanced pentraxin binding. Unlike Fcγ receptors, the H129A/I130A and R178A mutations on the BC- and FG-loops of D2 domain, respectively, had little effect on FcαRI binding to the pentraxins. In conclusion, our data suggest that the pentraxins recognize a similar site on FcαRI as IgA.

Pentraxins: structure, function, and role in inflammation

The pentraxins are an ancient family of proteins with a unique architecture found as far back in evolution as the Horseshoe crab. In humans the two members of this family are C-reactive protein and serum amyloid P. Pentraxins are defined by their sequence homology, their pentameric structure and their calcium-dependent binding to their ligands. Pentraxins function as soluble pattern recognition molecules and one of the earliest and most important roles for these proteins is host defense primarily against pathogenic bacteria. They function as opsonins for pathogens through activation of the complement pathway and through binding to Fc gamma receptors. Pentraxins also recognize membrane phospholipids and nuclear components exposed on or released by damaged cells. CRP has a specific interaction with small nuclear ribonucleoproteins whereas SAP is a major recognition molecule for DNA, two nuclear autoantigens. Studies in autoimmune and inflammatory disease models suggest that pentraxins interact with macrophage Fc receptors to regulate the inflammatory response. Because CRP is a strong acute phase reactant it is widely used as a marker of inflammation and infection.

C-reactive protein inhibits plasmacytoid dendritic cell interferon responses to autoantibody immune complexes

OBJECTIVE

C-reactive protein (CRP) is a serum pattern recognition molecule that binds to apoptotic cells and nucleoprotein autoantigens and Fcγ receptors (FcγR). In systemic lupus erythematosus (SLE), immune complexes (ICs) containing nucleoprotein autoantigens activate plasmacytoid dendritic cells (PDCs) to produce type I interferon (IFN), which contributes to disease pathogenesis. Autoantibody ICs are taken up by PDCs through FcγR type IIa into endosomes, where the nucleic acid components activate Toll-like receptor 7 (TLR-7) or TLR-9. The objective of this study was to investigate the effect of CRP on PDC and monocyte responses to nucleoprotein autoantigens and ICs.

METHODS

Peripheral blood mononuclear cells (PBMCs), purified monocytes, and PDCs were isolated from healthy volunteers and stimulated with autoantibody ICs containing apoptotic cells, small nuclear RNPs (snRNPs), or DNA, or directly with TLR-7 and TLR-9 agonists. Supernatants were analyzed for IFNα and cytokine levels by enzyme-linked immunosorbent assay and multiplex assay. Small nuclear RNPs were fluorescence-labeled, and the effect of CRP on binding, uptake, and intracellular localization of autoantibody snRNP complexes was measured by flow cytometry and confocal microscopy.

RESULTS

CRP bound to autoantigen did not induce IFNα in PBMCs or PDCs, whereas complexes formed with autoantibody did. Significantly, CRP inhibited the IFNα response to both anti-U1 RNP-snRNP complexes and anti-DNA-DNA complexes, but not to other TLR-7 and TLR-9 agonists. CRP directly inhibited PDC IFNα release, promoted PDC differentiation, and increased late endosome localization of autoantigen in PDCs and monocytes.

CONCLUSION

CRP is a regulator of the type I IFN response to SLE ICs. CRP increased the intracellular processing of ICs in late endosomes, which is associated with decreased synthesis of type I IFN after intracellular TLR activation.

Pentraxins and Fc receptors

Pentraxins are innate pattern recognition molecules whose major function is to bind microbial pathogens or cellular debris during infection and inflammation and, by doing so, contribute to the clearance of necrotic cells as well as pathogens through complement activations. Fc receptors are the cellular mediators of antibody functions. Although conceptually separated, both pentraxins and antibodies are important factors in controlling acute and chronic inflammation and infections. In recent years, increasing experimental evidence suggests a direct link between the innate pentraxins and humoral Fc receptors. Specifically, both human and mouse pentraxins recognize major forms of Fc receptors in solution and on cell surfaces with affinities similar to antibodies binding to their low affinity Fc receptors. Like immune complex, pentraxin aggregation and opsonization of pathogen result in Fc receptor and macrophage activation. The recently published crystal structure of human serum amyloid P (SAP) in complex with FcγRIIA further illustrated similarities to antibody recognition. These recent findings implicate a much broader role than complement activation for pentraxins in immunity. This review summarizes the structural and functional work that bridge the innate pentraxins and the adaptive Fc receptor functions. In many ways, pentraxins can be regarded as innate antibodies.

Transforming growth factor-β, macrophage colony-stimulating factor and C-reactive protein levels correlate with CD14(high)CD16+ monocyte induction and activation in trauma patients

Severe injury remains a leading cause of death and morbidity in patients under 40, with the number of annual trauma-related deaths approaching 160,000 in the United States. Patients who survive the initial trauma and post-traumatic resuscitation are at risk for immune dysregulation, which contributes to late mortality and accounts for approximately 20% of deaths after traumatic injury. This post-traumatic immunosuppressed state has been attributed to over-expression of anti-inflammatory mediators in an effort to restore host homeostasis. We measured a panel of monocyte markers and cytokines in 50 severely injured trauma patients for 3 days following admission. We made the novel observation that the subpopulation of monocytes expressing high levels of CD14 and CD16 was expanded in the majority of patients. These cells also expressed CD163 consistent with differentiation into alternatively activated macrophages with potential regulatory or wound-healing activity. We examined factors in trauma plasma that may contribute to the generation and activation of these cells. The percentage of CD14(high)CD16(+) monocytes after trauma correlated strongly with plasma C-reactive protein (CRP) transforming growth factor-β (TGF-β), and macrophage colony-stimulating factor (M-CSF) levels. We demonstrate a role for TGF-β and M-CSF, but not CRP in generating these cells using monocytes from healthy volunteers incubated with plasma from trauma patients. CD16 is a receptor for CRP and IgG, and we showed that monocytes differentiated to the CD14(high)CD16(+) phenotype produced anti-inflammatory cytokines in response to acute phase concentrations of CRP. The role of these cells in immunosuppression following trauma is an area of ongoing investigation.

C-reactive protein at the interface between innate immunity and inflammation

C-reactive protein (CRP), the prototypic acute-phase protein, increases rapidly in response to infection and inflammation. Although CRP was thought to be a passive, nonspecific marker of inflammation, recent studies indicate that CRP plays a key role in the innate immune system by recognizing pathogens and altered self determinants. Activation of complement and interaction with Fcgamma receptors by CRP provides a link between the innate and adaptive immune systems. Recent evidence suggests that CRP is a marker of atherosclerotic disease and may play a role in its induction. However, CRP has an anti-inflammatory role in autoimmune diseases, such as systemic lupus erythematosus. In this article, we review the biological mechanisms by which CRP exerts its effects on the immune system and discuss its role in infection, cardiovascular disease, malignancy and systemic lupus erythematosus.

Macrophages activated by C-reactive protein through Fc gamma RI transfer suppression of immune thrombocytopenia

C-reactive protein (CRP) is an acute-phase protein with therapeutic activity in mouse models of systemic lupus erythematosus and other inflammatory and autoimmune diseases. To determine the mechanism by which CRP suppresses immune complex disease, an adoptive transfer system was developed in a model of immune thrombocytopenic purpura (ITP). Injection of 200 microg of CRP 24 h before induction of ITP markedly decreased thrombocytopenia induced by anti-CD41. CRP-treated splenocytes also provided protection from ITP in adoptive transfer. Splenocytes from C57BL/6 mice were treated with 200 microg/ml CRP for 30 min, washed, and injected into mice 24 h before induction of ITP. Injection of 10(6) CRP-treated splenocytes protected mice from thrombocytopenia, as did i.v. Ig-treated but not BSA-treated splenocytes. The suppressive cell induced by CRP was found to be a macrophage by depletion, enrichment, and the use of purified bone marrow-derived macrophages. The induction of protection by CRP-treated cells was dependent on FcRgamma-chain and Syk activation, indicating an activating effect of CRP on the donor cell. Suppression of ITP by CRP-treated splenocytes required Fc gamma RI on the donor cell and Fc gamma RIIb in the recipient mice. These findings suggest that CRP generates suppressive macrophages through Fc gamma RI, which then act through an Fc gamma RIIb-dependent pathway in the recipient to decrease platelet clearance. These results provide insight into the mechanism of CRP regulatory activity in autoimmunity and suggest a potential new therapeutic approach to ITP.

C-reactive protein enhances immunity to Streptococcus pneumoniae by targeting uptake to Fc gamma R on dendritic cells

C-reactive protein (CRP) is an acute phase reactant with roles in innate host defense, clearance of damaged cells, and regulation of the inflammatory response. These activities of CRP depend on ligand recognition, complement activation, and binding to FcgammaR. CRP binds to phosphocholine in the Streptococcus pneumoniae cell wall and provides innate defense against pneumococcal infection. These studies examine the effect of this early innate defense molecule on the development of Abs and protective immunity to S. pneumoniae. Dendritic cells (DC) initiate and direct the adaptive immune response by integrating innate stimuli with cytokine synthesis and Ag presentation. We hypothesized that CRP would direct uptake of S. pneumoniae to FcgammaR on DC and enhance Ag presentation. CRP opsonization of the R36a strain of S. pneumoniae increased the uptake of bacteria by DC. DC pulsed with untreated or CRP-opsonized R36a were transferred into recipient mice, and Ab responses were measured. In mice challenged with free R36a, CRP opsonization resulted in higher secondary and memory IgG responses to both phosphocholine and pneumococcal surface protein A. Furthermore, mice immunized with DC that had been pulsed with CRP-opsonized R36a showed increased resistance to intranasal infection with virulent S. pneumoniae. The effects of CRP on Ag uptake, Ab responses, and protection from infection all required FcR gamma-chain expression on DC. The results indicate that innate recognition by CRP enhances effective uptake and presentation of bacterial Ags through FcgammaR on DC and stimulates protective adaptive immunity.

C-Reactive Protein Stimulated Spleen Cells Transfer Suppression of Experimental Immune Thrombocytopenia (53.9)

C-reactive protein (CRP) is an innate recognition molecule, which regulates immune responses by interaction with FcγR. CRP suppresses inflammation and autoimmunity in diverse experimental models. Idiopathic thrombocytopenic purpura (ITP) is an autoimmune disease in which platelets are cleared by splenic macrophages through FcγR. IVIG is used to treat human ITP. IVIG induces dendritic cells to transfer suppression of ITP (Siragam et al., 2006, Nat Med). In this study, splenocytes were treated with 200 μg/ml CRP or BSA or 1.8 mg/ml IVIG for 30 min, washed and transferred to naïve recipients. After 24 h, ITP was induced by injection of anti-CD41 mAb. Platelets were counted 24 h later. Anti-CD41 decreased platelet counts from 945±29 to 312±61 x 103/mm3. Platelet counts in recipients of 106 treated spleen cells were: BSA, 225±35; CRP, 672±25; and IVIG 590±35. Spleen cells from FcR γ-chain−/− or FcγRI−/− mice treated with CRP or IVIG were not protective and no protection was observed in FcγRIIb−/− mice. Macrophage depletion in vivo by Clodronate liposome treatment of the donor eliminated protection by CRP (363±9, p<0.001 vs CRP), but not IVIG (560±38). Thus, CRP treatment of spleen cells in vitro generates a cell that transfers suppression of ITP. Generation of this cell by CRP is dependent on FcγRI and macrophages. The findings allow for the characterization of the cells involved in the immunoregulatory properties of CRP.

C-reactive protein-mediated suppression of nephrotoxic nephritis: role of macrophages, complement, and Fcgamma receptors

C-reactive protein (CRP) is a member of the pentraxin family of proteins and an acute phase reactant. CRP modulates the response to inflammatory stimuli including LPS and C5a. We recently demonstrated that CRP prevents and reverses proteinuria in accelerated nephrotoxic nephritis (NTN). NTN is a model of active inflammatory immune complex-mediated nephritis induced by injection of antiglomerular basement membrane. CRP treatment prevented the induction of NTN in C57BL/6 (B6) mice, increased survival, and reversed ongoing nephritis. Protection was associated with a decrease in IL-1beta and chemokines in the kidney and peritoneal cells as measured by quantitative RT-PCR. However, IL-10(-/-) mice were not protected by CRP either when given before disease onset or when disease activity was maximal. FcgammaRI(-/-) mice developed NTN, but were only transiently protected by CRP treatment. This transient protection was abrogated by cobra venom factor depletion of complement from FcgammaRI(-/-) mice. However, complement depletion did not prevent CRP-mediated protection in B6 mice, and CRP was protective in C3(-/-) mice. The role of macrophages in the protection provided by CRP was tested by treating B6 mice with liposomes containing clodronate. Clodronate-containing liposomes deplete mice of splenic and hepatic macrophages for 5-7 days. Pretreatment of NTN mice with clodronate but not control liposomes completely prevented CRP-mediated protection. These studies suggest that CRP mediates protection from NTN through the induction of IL-10 and that macrophages are required. In addition, FcgammaRI plays an important role but is not the sole mediator of CRP-mediated protection.

The biological effects of CRP are not attributable to endotoxin contamination: evidence from TLR4 knockdown human aortic endothelial cells

C-reactive protein (CRP) is the prototypic marker of inflammation and a strong predictor of cardiovascular events in humans. There are questions regarding the validity of the biological effects reported for CRP, in spite of adherence to rigorous control measures minimizing endotoxin [lipopolysaccharide (LPS)] contamination in these in vitro studies. In this study, we addressed the key question of endotoxin contamination in CRP preparations using Toll-like receptor 4 (TLR4) knockdown endothelial cells. Human aortic endothelial cells (HAECs) transfected with prevalidated TLR4 small interfering RNA (siRNA) and scrambled siRNA controls were challenged with pleural fluid-derived CRP or LPS for 12-16 h. Secreted interleukin-6 (IL-6), IL-1beta, IL-8, and plasminogen activator inhibitor-1 (PAI-1) levels and endothelial Nitric oxide synthase (eNOS) activity were determined. TLR4 knockdown in HAECs significantly decreased LPS-induced IL-1beta, IL-6, and IL-8, whereas the stimulatory effects of CRP were similar in both scrambled control and TLR4 knockdown cells. Furthermore, CRP significantly stimulated PAI-1 levels in both control and TLR4-transfected cells and inhibited eNOS activity, whereas LPS effects were negated in TLR4-transfected cells. The data presented cogently demonstrate and further confirm that the biological effects of CRP on HAECs are independent of LPS and thus are attributable to native protein per se. This is the first study to positively authenticate the significance of earlier in vitro reports on CRP biological effects.

C-Reactive Protein Increases Cytokine Responses toStreptococcus pneumoniaethrough Interactions with Fcγ Receptors

Streptococcus pneumoniae is the most common organism responsible for community acquired pneumonia and meningitis. In pneumococcal pneumonia, a strong local inflammatory cytokine response reduces the frequency of bacteremia and increases survival. The initiation of this cytokine response by innate recognition of bacterial cell wall components through TLR has been described, but the role of soluble innate mediators has received limited attention. C-reactive protein (CRP) is an acute phase protein that binds phosphocholine residues on S. pneumoniae cell walls. CRP interacts with phagocytic cells through FcgammaRI and FcgammaRII and activates the classical complement pathway. CRP is protective in mouse pneumococcal bacteremia by increasing complement-dependent clearance and killing of bacteria. We studied the cytokine response of PBMC stimulated with CRP-opsonized S. pneumoniae to determine the effect of CRP interaction with FcgammaR. CRP dramatically increased the production of TNF-alpha and IL-1beta in response to S. pneumoniae. These increases were blocked by phosphocholine, which inhibits CRP binding to S. pneumoniae, by inhibitors of FcgammaR signaling, and by mAb to FcgammaRI and FcgammaRII. A mutated rCRP with decreased FcgammaR binding had a decreased ability to stimulate TNF-alpha release, compared with wild-type CRP. Individuals who were homozygous for the R-131 allele of FcgammaRIIA, which has a higher affinity for CRP, showed higher responses to CRP-opsonized bacteria than did individuals homozygous for the H-131 allele, further implicating this receptor. The results indicate that CRP recognition of S. pneumoniae and binding to FcgammaR may enhance the early protective cytokine response to infection.

C-reactive protein: Ligands, receptors and role in inflammation

C-reactive protein (CRP) is the prototypical acute phase serum protein, rising rapidly in response to inflammation. CRP binds to phosphocholine (PC) and related molecules on microorganisms and plays an important role in host defense. However, a more important role may be the binding of CRP to PC in damaged membranes. CRP increases clearance of apoptotic cells, binds to nuclear antigens and by masking autoantigens from the immune system or enhancing their clearance, CRP may prevent autoimmunity. CRP binds to both the stimulatory receptors, FcgammaRI and FcgammaRIIa, increasing phagocytosis and the release of inflammatory cytokines; and to the inhibitory receptor, FcgammaRIIb, blocking activating signals. We have shown that, in two animal models of systemic lupus erythematosus (SLE), the (NZB x NZW)F1 mouse and the MRL/lpr mouse, a single injection of CRP before onset of proteinuria delayed disease development and late treatment reversed proteinuria. Thus, in these models, CRP plays an anti-inflammatory role.

Binding and Internalization of C-Reactive Protein by Fcgamma Receptors on Human Aortic Endothelial Cells Mediates Biological Effects

Objective— In addition to being a cardiovascular risk marker, recent studies support a role for CRP in atherothrombosis. Several investigators have reported that CRP binds to Fcgamma receptors on leukocytes. The aim of the study is to determine the processing of CRP by human aortic endothelial cells (HAECs).

Methods and Results— Binding studies were performed by incubation of HAECs with biotinylated CRP (B-CRP, 25 to 200 μg/mL) for 30 to 180 minutes at 4°C. B-CRP binding was quantitated using streptavidin-fluorescein isothiocyanate followed by flow cytometry. Saturable binding of CRP was obtained at 60 minutes with a CRP concentration between 100 to 150 μg/mL and Kd of 88 nM. CRP binding was inhibited by 10× cold CRP (58%). CRP (100 μg/mL) significantly upregulated surface expression of Fcgamma receptors, CD32, as well as CD64 on HAECs ( P <0.01). Also, preincubation with anti-CD32 and CD64 antibodies significantly inhibited maximal binding of CRP to HAECs 64% and 30%, respectively, whereas antibodies to CD16 had no effect. Internalization of CRP, as determined by loss of surface expression, was 50%. Also, binding and internalization of biotinylated CRP was confirmed by confocal microscopy and CRP colocalized with CD32 and CD64. Most importantly, the increase in interleukin-8, intercellular adhesion molecule 1, and vascular cell adhesion molecule-1 and the decrease in eNOS and prostacyclin induced by CRP was abrogated with antibodies to CD32 and CD64.

Conclusions— We demonstrate that CRP mediates its biological effects on HAECs via binding and internalization through Fcgamma receptors, CD32 and CD64.

Analysis of binding sites in human C-reactive protein for Fc{gamma}RI, Fc{gamma}RIIA, and C1q by site-directed mutagenesis

Human C-reactive protein (CRP) is a classical, acute phase serum protein synthesized by the liver in response to infection, inflammation, or trauma. CRP binds to microbial antigens and damaged cells, opsonizes particles for phagocytosis and regulates the inflammatory response by the induction of cytokine synthesis. These activities of CRP depend on its ability to activate complement and to bind to Fcgamma receptors (FcgammaR). The goal of this study was to elucidate amino acid residues important for the interaction of CRP with human FcgammaRI (CD64) and FcgammaRIIa (CD32). Several mutations of the CRP structure were studied based on the published crystal structure of CRP. Mutant and wild-type recombinant CRP molecules were expressed in the baculovirus system and their interactions with FcgammaR and C1q were determined. A previous study by our laboratory identified an amino acid position, Leu(176), critical for CRP binding to FcgammaRI and work by others (Agrawal, A., Shrive, A. K., Greenhough, T. J., and Volanakis, J. E. (2001) J. Immunol. 166, 3998-4004) determined several residues important for C1q binding. The amino acid residues important to CRP binding to FcgammaRIIa were previously unknown. This study newly identifies residues Thr(173) and Asn(186) as important for the binding of CRP to FcgammaRIIa and FcgammaRI. Lys(114), like Leu(176), was implicated in binding to FcgammaRI, but not FcgammaRIIa. Single mutations at amino acid positions Lys(114), Asp(169), Thr(173), Tyr(175), and Leu(176) affected C1q binding to CRP. These results further identify amino acids involved in the binding sites on CRP for FcgammaRI, FcgammaRIIa, and C1q and indicate that these sites are overlapping.

C-reactive protein: an activator of innate immunity and a modulator of adaptive immunity

C-reactive protein (CRP) is an acute-phase serum protein and a member of the pentraxin protein family. Its host defense functions predate the adaptive immune system by millions of years. Our current understanding of CRP interactions with complement and with Fcgamma receptors (FcgammaR) have led to an increased appreciation of the regulatory role of CRP in inflammation and autoimmunity. This review outlines the role of CRP in infection, inflammation, and autoimmune disease. We provide a description of recent studies, which suggest that CRP acts through FcgammaR to reduce inflammation and protect from certain autoimmune diseases. A general description of the proposed function of CRP is provided as a framework for future investigation.

Studies of serum C-reactive protein in systemic lupus erythematosus

OBJECTIVE

To examine the relationship of serum C-reactive protein (CRP) levels to other indicators of disease activity during the course of systemic lupus erythematosus (SLE).

METHODS

In 124 patients serum CRP was measured retrospectively by ELISA and in some instances by radial immunodiffusion. Serum CRP levels were compared to laboratory, clinical, and radiographic assessments of disease activity. In many patients, serial CRP levels were measured over months or years to determine whether elevations of serum CRP reflected apparent changes in other disease activity variables. CRP was also measured in lyophilized aliquots of 24 h urine samples from SLE patients and controls with other renal disorders. Parallel determinations of interleukin 6 (IL-6) were made by ELISA in healthy controls and SLE patients.

RESULTS

Of the 124 SLE patients studied, most showed elevations in serum CRP levels in the course of their disease. No inverse or direct correlation was noted between serum CRP and levels of nucleosome antigen or serum IgM or IgG anti-DNA antibody. In patients with renal involvement and proteinuria, CRP was often detected in 24-h urine samples. A strong correlation (p < 0.001) was noted between CRP and IL-6 levels in healthy subjects, but no correlation was recorded between serum CRP and IL-6 in SLE.

CONCLUSION

Contrary to previous reports, most patients with SLE in our study showed elevations of serum CRP during the course of their illness, and extremely high serum CRP was recorded in some patients. CRP was also found in concentrated urine samples from patients with renal involvement and often paralleled elevated serum levels. In patients, no correlation was seen between CRP serum levels and serum IL-6, whereas a strong correlation between CRP level and IL-6 was recorded in healthy subjects.

Prevention and reversal of nephritis in MRL/lpr mice with a single injection of C-reactive protein

OBJECTIVE

C-reactive protein (CRP) is an acute-phase serum protein with binding reactivity to nuclear autoantigens and immunomodulatory function. The MRL/lpr mouse is an important model of human systemic lupus erythematosus (SLE). These mice develop high-titer anti-DNA antibodies and immune complex-mediated nephritis and exhibit progressive lymphadenopathy. The mortality rate among these mice is 50% by age 18-20 weeks; the most frequent cause of death is glomerulonephritis. The present study was undertaken to determine whether treatment of mice with CRP would affect the course of lupus nephritis.

METHODS

MRL/lpr mice were treated with a single 200-mug injection of CRP at either age 6 weeks (before disease onset) or age 13 or 15 weeks (when proteinuria had reached high levels). Proteinuria was measured weekly, and levels of anti-double-stranded DNA autoantibodies and blood urea nitrogen were determined monthly. Glomerular immune complex deposition and renal pathology were assessed in mice ages 15 weeks and 17 weeks.

RESULTS

Early CRP treatment markedly delayed the onset of proteinuria and lymphadenopathy, increased survival, and reduced levels of autoantibodies to DNA. Treatment of mice with active disease reversed proteinuria and prolonged survival. Renal disease was decreased in CRP-treated mice, with a marked suppression of glomerular pathology, tubular degeneration, and interstitial inflammation, which correlated with the decrease in proteinuria and azotemia.

CONCLUSION

These findings demonstrate that systemic suppression of autoimmunity is initiated by a single injection of CRP. Long-term maintenance of CRP-mediated protection was reversed by injection of an anti-CD25 monoclonal antibody but not by macrophage depletion, suggesting that disease suppression is maintained by CD25-bearing T cells.

Reversal of ongoing proteinuria in autoimmune mice by treatment with C-reactive protein

OBJECTIVE

To examine the ability of injection of C-reactive protein (CRP) to treat systemic lupus erythematosus (SLE) in the (NZB x NZW)F(1) (NZB/NZW) mouse and to use a nephrotoxic nephritis (NTN) model to further examine the mechanism of this activity.

METHODS

NZB/NZW mice were given a single injection of 200 mug of CRP prior to disease onset or after the onset of high-grade proteinuria. Mice were monitored weekly for proteinuria and monthly for anti-double-stranded DNA (anti-dsDNA) antibodies. NTN was induced by immunization with rabbit IgG followed by rabbit anti-mouse glomerular basement membrane. Proteinuria was measured daily, and renal pathology was scored. CRP was injected at the time of disease induction or 9 days later.

RESULTS

Treatment of NZB/NZW mice with CRP prior to disease onset delayed the onset of high-grade proteinuria by 16 weeks (P < 0.0001) and prolonged survival by 13 weeks (P < 0.002). CRP treatment of NZB/NZW mice during acute disease rapidly decreased proteinuria, and the treated mice remained aproteinuric for at least 10 weeks. Control and CRP-treated mice developed similar levels of anti-dsDNA. In C57BL/6 mice, injection of CRP either before or after induction of NTN suppressed proteinuria and glomerular pathology. CRP was completely ineffective in treating NTN in interleukin-10 (IL-10)-deficient mice.

CONCLUSION

CRP injection suppresses inflammation in the kidney in SLE and NTN. The requirement for IL-10 in this protection suggests that CRP must rapidly initiate an IL-10-dependent antiinflammatory process. These findings suggest that a major function of CRP during the acute-phase response is to limit tissue damage and modulate acute inflammation.

C-reactive protein mediates protection from lipopolysaccharide through interactions with Fc gamma R

C-reactive protein (CRP) is a component of the acute phase response to infection, inflammation, and trauma. A major activity of acute phase proteins is to limit the inflammatory response. It has been demonstrated that CRP protects mice from lethal doses of LPS. In the mouse, CRP binds to the regulatory receptor, FcgammaRIIb, and to the gamma-chain-associated receptor, FcgammaRI. The goal ofthis study was to determine whether FcgammaRs are necessary for the protective effect of CRP. The ability of CRP to protect mice from a lethal dose of LPS was confirmed using injections of 500 and 250 micro g of CRP at 0 and 12 h. CRP treatment of FcgammaRIIb-deficient mice increased mortality after LPS challenge and increased serum levels of TNF and IL-12 in response to LPS. CRP did not protect FcR gamma-chain-deficient mice from LPS-induced mortality. Treatment of normal mice, but not gamma-chain-deficient mice, with CRP increased IL-10 levels following LPS injection. In vitro, in the presence of LPS, CRP enhanced IL-10 synthesis and inhibited IL-12 synthesis by bone marrow macrophages from normal, but not gamma-chain-deficient mice. The protective effect of CRP appears to be mediated by binding to FcgammaRI and FcgammaRII resulting in enhanced secretion of the anti-inflammatory cytokine IL-10 and the down-regulation of IL-12. These results suggest that CRP can alter the cytokine profile of mouse macrophages by acting through FcgammaR leading to a down-regulation of the inflammatory response.

Serum amyloid P component and C-reactive protein opsonize apoptotic cells for phagocytosis through Fcgamma receptors

Serum amyloid P component (SAP) and C-reactive protein (CRP) are opsonins that react with nuclear autoantigens targeted in systemic autoimmunity. CRP and SAP bind to apoptotic and necrotic cells, which are potential sources of these autoantigens. We have recently determined that the receptors for CRP on phagocytic cells are Fcgamma receptors. The goal of this study was to determine whether CRP and SAP promote phagocytosis of apoptotic cells and to identify the receptors involved. Apoptosis was induced in human neutrophils (PMN) and the Jurkat T-cell line by UV-irradiation. SAP treatment of apoptotic human PMN increased ingestion by autologous macrophages. Both SAP and CRP increased ingestion of apoptotic, but not normal Jurkat cells by J-774 macrophages and mouse peritoneal macrophages. Neither SAP nor CRP increased ingestion of apoptotic Jurkat cells by macrophages from FcR gamma-chain deficient mice, which lack FcgammaRI and FcgammaRIII. Inhibition of FcgammaRIII-mediated uptake using mAb 2.4G2 eliminated opsonization by SAP, but not by CRP. These results indicate that pentraxins promote uptake of apoptotic cells through FcgammaRI and/or FcgammaRIII. Ingestion through these receptors is expected to alter the pattern of cytokine production and antigen presentation in response to apoptotic cells.

Protection from Streptococcus pneumoniae infection by C-reactive protein and natural antibody requires complement but not Fc gamma receptors

Streptococcus pneumoniae is an important human pathogen and the most common cause of community-acquired pneumonia. Both adaptive and innate immune mechanisms provide protection from infection. Innate immunity to S. pneumoniae in mice is mediated by naturally occurring anti-phosphocholine (PC) Abs and complement. The human acute-phase reactant C-reactive protein (CRP) also protects mice from lethal S. pneumoniae infection. CRP and anti-PC Ab share the ability to bind to PC on the cell wall C-polysaccharide of S. pneumoniae and to activate complement. CRP and IgG anti-PC also bind to Fc gamma R. In this study, Fc gamma R- and complement-deficient mice were used to compare the mechanisms of protection conferred by CRP and anti-PC Ab. Injection of CRP protected wild-type, FcR gamma-chain-, Fc gamma RIIb-, and Fc gamma RIII-deficient mice from infection. Complement was required for the protective effect of CRP as cobra venom factor treatment eliminated the effect of CRP in both gamma-chain-deficient and wild-type mice, and CRP failed to protect C3- or C4-deficient mice from infection. Unexpectedly, gamma-chain-deficient mice were extremely sensitive to pneumococcal infection. This sensitivity was associated with low levels of natural anti-PC Ab. Gamma-chain-deficient mice immunized with nonencapsulated S. pneumoniae produced both IgM- and IgG PC-specific Abs, were protected from infection, and were able to clear the bacteria from the bloodstream. The protection provided by immunization was eliminated by complement depletion. The results show that in this model of systemic infection with highly virulent S. pneumoniae, protection from lethality by CRP and anti-PC Abs requires complement, but not Fc gamma R.

The major receptor for C-reactive protein on leukocytes is fcgamma receptor II

C-reactive protein (CRP) is an acute phase serum protein that shares several functions with immunoglobulin (Ig)G including complement activation and binding to receptors on monocytes and neutrophils. The identity of the receptor for CRP has been the target of extensive research. We previously determined that CRP binds to the high affinity receptor for IgG, FcgammaRI (CD64). However, this interaction could not account for the majority of binding of CRP to neutrophils or monocytic cells. We now determine that CRP also interacts with FcgammaRIIa (CD32), the low affinity receptor for IgG on monocytes and neutrophils. COS-7 cells were transfected with a construct containing the human FcgammaRIIA cDNA. CRP binding and the presence of CD32 were detected by mAb and analyzed by two-color flow cytometry. Cells expressing CD32 bound CRP in a dose-dependent and saturable manner consistent with receptor binding. CRP bound to transfectants and K-562 cells with similar kinetics, and in both cases binding was completely inhibited by aggregated IgG. On monocytic cell lines, treatment with Bt(2)cAMP increased FcgammaRII expression and enhanced CRP binding. CRP also specifically precipitated FcgammaRI and FcgammaRII from the monocytic cell line, THP-1. It is suggested that the major receptor for CRP on phagocytic cells is FcgammaRII.

Suppressor T Cells: Presence in Mice Rendered Tolerant by Neonatal Treatment with Anti-Receptor Antibody or Antigen

Specific tolerance to phosphorylcholine (PC) was induced in BALB/c mice by two methods. Neonatal mice received a single injection of either: 1) PnC, the C-polysaccharide from S. pneumoniae, R36a vaccine which has PC as a major antigenic determinant or 2) ARA, an homologous antibody directed against the receptor for PC. Spleen cells from animals treated as neonates with either PnC or ARA were specifically suppressed for the response to PC antigens in vitro. In addition, cells from either group of unresponsive animals co-cultured with spleen cells of normal BALB/c mice markedly suppressed the response of the normal cells to PC. Greater than 90% inhibition of the plaque-forming cell response was obtained when unresponsive cells were mixed with normal cells in ratios of 1:1 or greater. Equal numbers of cells from animals made unresponsive by PnC or ARA produced an equivalent degree of suppression. Neither supernatants of cultures nor sera of animals of either unresponsive group suppressed the response of normal spleen cells to PC. Suppression by cells from both groups of tolerant mice was eliminated by treatment with anti-Thy 1.2 serum and C. Presumably, a common cell is responsible for suppression caused by cells from mice made unresponsive by either procedure.