Alexander J. Szalai

C-Reactive Protein Accelerates the Progression of Atherosclerosis in Apolipoprotein E–Deficient Mice

Background—Plasma C-reactive protein (CRP) concentration is a strong predictor of atherosclerosis. However, to date, there is no in vivo evidence that CRP is proatherogenic. Methods and Results—We studied the effect of human CRP transgene (tg) expression, under basal and turpentine-stimulated conditions, on atherosclerosis in apolipoprotein (apo) E−/− mice. Aortic atherosclerotic lesions in 29-week-old male mice were 48% larger (P <0.02) in turpentine-treated mice and 34% larger (P <0.05) in untreated CRPtg+/0/apoE−/− mice. Turpentine treatment per se did not affect the extent of atherosclerosis in CRP transgenic or nontransgenic apoE−/− mice. Transgenic mice exhibited lower plasma complement C3 but increased deposition of CRP and C3 in the lesions, which suggests that CRP stimulated activation of complement within the lesion. There was more intense and widespread vascular cell adhesion molecule-1 and collagen staining in the lesions of CRPtg+/0/apoE−/− mice than in CRPtg0/0/apoE−/− littermates. Lesions of CRPtg+/0/apoE−/− mice contained increased angiotensin type 1 receptor (AT1-R) transcripts and displayed increased AT1-R immunostaining compared with those of CRPtg0/0/apoE−/− mice. There was no difference in blood pressure in the 2 types of mice, which indicates that the proatherogenic effect of CRP-associated AT1-R overexpression is local and not mediated by its hypertensive properties. Conclusions—Human CRP transgene expression causes accelerated aortic atherosclerosis in apoE−/− mice. CRP was detected in the lesion, which was associated with increased C3 deposition and increased AT1-R, vascular cell adhesion molecule-1, and collagen expression. These data document a proatherogenic role for CRP in vivo.

Increased Thrombosis After Arterial Injury in Human C-Reactive Protein–Transgenic Mice

Background—C-reactive protein (CRP), an acute-phase reactant long considered merely an innocent bystander in the inflammatory process, is now recognized as a powerful predictor of cardiovascular events. Emerging in vitro evidence suggests that CRP may have direct proinflammatory and prothrombotic effects on monocytes and endothelial cells. To determine whether CRP directly modulates vascular cell function in vivo, we subjected wild-type mice, which do not express CRP, and human CRP–transgenic (CRPtg) mice to 2 models of arterial injury. Methods and Results—Baseline serum CRP levels in CRPtg mice were 18±6 mg/L. CRP levels were undetectable in wild-type mice. Transluminal wire injury led to complete thrombotic occlusion of the femoral artery at 28 days in 75% of CRPtg arteries (6 of 8) compared with 17% (2 of 12) in wild-type mice (P <0.05). In a model of arterial photochemical injury, clot formation time was shortened in CRPtg mice; mean time to occlusion was 33±19 minutes compared with 59±19 minutes in wild-type mice (n=10; P <0.05). Conclusions—Arterial injury in CRPtg mice results in an expedited and higher rate of thrombotic occlusion. This is the first report of a prothrombotic phenotype directly attributable to the presence of human CRP in vivo. Investigation of the inflammatory-thrombotic axis in CRPtg mice may elucidate the prothrombotic actions of CRP in unstable arterial diseases and may pave the way for novel therapeutic interventions for preventing cardiovascular events.

Induction of leptin resistance through direct interaction of C-reactive protein with leptin

The mechanisms underlying leptin resistance are still being defined. We report here the presence in human blood of several serum leptin-interacting proteins (SLIPs), isolated by leptin-affinity chromatography and identified by mass spectrometry and immunochemical analysis. We confirmed that one of the major SLIPs is C-reactive protein (CRP). In vitro, human CRP directly inhibits the binding of leptin to its receptors and blocks its ability to signal in cultured cells. In vivo, infusion of human CRP into ob/ob mice blocked the effects of leptin upon satiety and weight reduction. In mice that express a transgene encoding human CRP, the actions of human leptin were completely blunted. We also found that physiological concentrations of leptin can stimulate expression of CRP in human primary hepatocytes. Recently, human CRP has been correlated with increased adiposity and plasma leptin. Thus, our results suggest a potential mechanism contributing to leptin resistance, by which circulating CRP binds to leptin and attenuates its physiological functions.

Effects of PspA and Antibodies to PspA on Activation and Deposition of Complement on the Pneumococcal Surface

ABSTRACT Streptococcus pneumoniae infection is a frequent cause of pneumonia, otitis media, meningitis, and septicemia. Pneumococcal surface protein A (PspA) is an important virulence factor on the pathogen surface, and it is known to interfere with complement activation. In this study, flow cytometry was used to study the effects of PspA and antibodies to PspA on the deposition of complement C3 on the surface of a capsular type 3 strain, WU2, and its PspA− mutant, JY1119. Using naive mouse serum as a complement source, measurable deposition of C3 was observed within 4 min on PspA− pneumococci, and the amount of surface-bound C3 accumulated rapidly as the amount of serum was increased. In contrast, very little C3 was deposited on the PspA+ strain. In nonimmune mouse serum, the classical pathway was the dominant activation pathway triggered by PspA− pneumococci. Accordingly, EGTA blocked almost all of the complement activation. Moreover, a significant amount of C3 was still deposited on the PspA− strain when serum from factor B-deficient mice was used. This deposition was not observed on the PspA+ pneumococci, indicating that PspA may inhibit complement deposition via the classical pathway. Furthermore, under the conditions we tested, PspA also inhibited C3 deposition when the classical pathway was initiated by antibodies to capsular polysaccharide. Antibodies to PspA could overcome the anticomplementary effect of PspA, allowing for increased complement activation and C3 deposition onto PspA+ bacteria.

Association between baseline levels of C-reactive protein (CRP) and a dinucleotide repeat polymorphism in the intron of the CRP gene.

Elevation of baseline C-reactive protein (CRP) is associated with increased risk of cardiac disease. This increase might reflect low-grade inflammation, but differences in CRP serum levels might also have a genetic component. To test this possibility, we investigated whether a polymorphic GT-repeat in the intron of the CRP gene contributes to variation in baseline CRP. We found that the polymorphism was associated with differences in baseline CRP in both normal individuals and in patients with the inflammatory disease systemic lupus erythematosus, viz. donors carrying two GT16 alleles, two GT21alleles, or GT16/21 heterozygotes had two-fold lower serum CRP than those with other genotypes. The frequency of GT16 and GT21 was two-fold higher in Caucasians than in African-Americans, but there was no difference in allele distribution between patients and controls. It is not yet known how this genetic polymorphism mediates its effect on CRP expression, and it probably is not a systemic lupus erythematosus susceptibility factor. Rather, the CRP intron polymorphism likely modifies the disease phenotype. On the other hand, the fact that baseline CRP does have a genetic component suggests that in coronary disease, stratification of risk assessment based on CRP levels might be enhanced by consideration of this polymorphism.

Attenuation of Experimental Autoimmune Demyelination in Complement-Deficient Mice

The exact mechanisms leading to CNS inflammation and myelin destruction in multiple sclerosis and in its animal model, experimental allergic encephalomyelitis (EAE) remain equivocal. In both multiple sclerosis and EAE, complement activation is thought to play a pivotal role by recruiting inflammatory cells, increasing myelin phagocytosis by macrophages, and exerting direct cytotoxic effects through the deposition of the membrane attack complex on oligodendrocytes. Despite this assumption, attempts to evaluate complement’s contribution to autoimmune demyelination in vivo have been limited by the lack of nontoxic and/or nonimmunogenic complement inhibitors. In this report, we used mice deficient in either C3 or factor B to clarify the role of the complement system in an Ab-independent model of EAE. Both types of complement-deficient mice presented with a markedly reduced disease severity. Although induction of EAE led to inflammatory changes in the meninges and perivascular spaces of both wild-type and complement-deficient animals, in both C3−/− and factor B−/− mice there was little infiltration of the parenchyma by macrophages and T cells. In addition, compared with their wild-type littermates, the CNS of both C3−/− and factor B−/− mice induced for EAE are protected from demyelination. These results suggest that complement might be a target for the therapeutic treatment of inflammatory demyelinating diseases of the CNS.

No Effect of C-Reactive Protein on Early Atherosclerosis Development in Apolipoprotein E*3-Leiden/Human C-Reactive Protein Transgenic Mice

Objective—C-reactive protein (CRP) has been associated with risk of cardiovascular disease. It is not clear whether CRP is causally involved in the development of atherosclerosis. Mouse CRP is not expressed at high levels under normal conditions and increases in concentration only several-fold during an acute phase response. Because the dynamic range of human CRP is much larger, apolipoprotein E*3-Leiden (E3L) transgenic mice carrying the human CRP gene offer a unique model to study the role(s) of CRP in atherosclerosis development. Methods and Results—Atherosclerosis development was studied in 15 male and 15 female E3L/CRP mice; E3L transgenic littermates were used as controls. The mice were fed a hypercholesterolemic diet to induce atherosclerosis development. Cholesterol exposure did not differ between E3L/CRP and E3L mice. Plasma CRP levels were on average 10.2±6.5 mg/L in male E3L/CRP mice, 0.2±0.1 mg/L in female E3L/CRP mice, and undetectable in E3L mice. Quantification of atherosclerosis showed that lesion area in E3L/CRP mice was not different from that in E3L mice. Conclusion—This study demonstrates that mildly elevated levels of CRP in plasma do not contribute to the development of early atherosclerosis in hypercholesterolemic E3L/CRP mice.

Both Family 1 and Family 2 PspA Proteins Can Inhibit Complement Deposition and Confer Virulence to a Capsular Serotype 3 Strain of Streptococcus pneumoniae

ABSTRACT Pneumococcal surface protein A (PspA), a virulence factor of Streptococcus pneumoniae, is exceptionally diverse, being classified into two major families which are over 50% divergent by sequence analysis. A family 1 PspA from strain WU2 was previously shown to impede the clearance of pneumococci from mouse blood and to interfere with complement deposition on the bacterial surface. To determine whether a family 2 PspA can perform the same role as family 1 PspA, the family 1 PspA (from strain WU2) was replaced with a family 2 PspA (from strain TIGR4) by molecular genetic methods to make an isogenic pair of strains expressing different PspA proteins. Surface binding of lactoferrin and interference with C3 deposition by the two types of PspA proteins were determined by flow cytometry, and virulence was assessed in a mouse bacteremia model. Although the family 2 PspA appeared to bind less human lactoferrin than did the family 1 PspA, both PspA proteins could interfere with complement deposition on the pneumococcal surface and could provide full virulence in the mouse infection model. A mutant form of the family 2 PspA with a deletion within the choline-binding region was also produced. Pneumococci with this mutant PspA failed to bind human lactoferrin even though the PspA was present on the pneumococcal surface. The mutant PspA only partially interfered with complement deposition and moderately attenuated virulence. These results suggest that family 1 and family 2 PspA proteins play similar roles in virulence and that surface accessibility of PspA is important for their function.

The Virulence Function of Streptococcus pneumoniae Surface Protein A Involves Inhibition of Complement Activation and Impairment of Complement Receptor-Mediated Protection

Complement is important for elimination of invasive microbes from the host, an action achieved largely through interaction of complement-decorated pathogens with various complement receptors (CR) on phagocytes. Pneumococcal surface protein A (PspA) has been shown to interfere with complement deposition onto pneumococci, but to date the impact of PspA on CR-mediated host defense is unknown. To gauge the contribution of CRs to host defense against pneumococci and to decipher the impact of PspA on CR-dependent host defense, wild-type C57BL/6J mice and mutant mice lacking CR types 1 and 2 (CR1/2−/−), CR3 (CR3−/−), or CR4 (CR4−/−) were challenged with WU2, a PspA+ capsular serotype 3 pneumococcus, and its PspA− mutant JY1119. Pneumococci also were used to challenge factor D-deficient (FD−/−), LFA-1-deficient (LFA-1−/−), and CD18-deficient (CD18−/−) mice. We found that FD−/−, CR3−/−, and CR4−/− mice had significantly decreased longevity and survival rate upon infection with WU2. In comparison, PspA− pneumococci were virulent only in FD−/− and CR1/2−/− mice. Normal mouse serum supported more C3 deposition on pneumococci than FD−/− serum, and more iC3b was deposited onto the PspA− than the PspA+ strain. The combined results confirm earlier conclusions that the alternative pathway of complement activation is indispensable for innate immunity against pneumococcal infection and that PspA interferes with the protective role of the alternative pathway. Our new results suggest that complement receptors CR1/2, CR3, and CR4 all play important roles in host defense against pneumococcal infection.

FcγRs Modulate Cytotoxicity of Anti-Fas Antibodies: Implications for Agonistic Antibody-Based Therapeutics

Development of anti-Fas Abs to treat diseases with insufficient Fas-mediated apoptosis has been limited by concern about hepatotoxicity. We report here that hepatotoxicity elicited by anti-Fas Ab Jo2 is dependent on FcγRIIB. Thus, following Jo2 treatment, all FcγRIIB−/− mice survived while 80% of wild-type and all FcR-γ−/− mice died from acute liver failure. Microscopic examination suggests that FcγRIIB deficiency protects the hepatic sinusoidal endothelium, a cell type that normally coexpresses Fas and FcγRIIB. In vitro studies showed that FcγRIIB, but not FcγRI and FcγRIII, on neighboring macrophages substantially enhanced Jo2 mediated apoptosis of Fas expressing target cells. However, FcγRI and FcγRIII appeared essential for apoptosis-inducing activity of a non-hepatotoxic anti-Fas mAb HFE7A. These findings imply that by interacting with the Fc region of agonistic Abs, FcγRs can modulate both the desired and undesired consequences of Ab-based therapy. Recognizing this fact should facilitate development of safer and more efficacious agonistic Abs.