American Journal of Pathology, Vol. 167, No. 4, October 2005
Copyright © American Society for Investigative Pathology

Commentary
C-Reactive Protein and Atherogenesis
New Insights from Established Animal Models

Jan Torzewski
From the Department of Internal Medicine II-Cardiology,
University of Ulm, Ulm, Germany

In this issue of The American Journal of Pathology, Sun andcolleagues1 publish a potential key article in the field of
C-reactive protein (CRP) and atherosclerosis, which iscurrently a lively topic in cardiovascular research. The
reasons for this wide-reaching interest are briefly summarized as follows. First, CRP has been identified as a
powerful cardiovascular risk marker.2,3 Second, CRP
may not only be a cardiovascular risk marker but also arisk factor, ie, CRP may be causally involved in atherogenesis.4,5 Third, if CRP is indeed a cardiovascular risk
factor, the molecule may well be a target for drug development, and CRP inhibition might be a strategy for primary or secondary prevention of cardiovascular disease.6 The medical and economical impact of these
possibilities is evident. In analogy to other active areas inresearch, the role of CRP in cardiovascular disease iscontroversially discussed. Such controversial discussion
relates to each of the three points raised above.
The role of CRP as a cardiovascular risk marker isalmost generally accepted, and evidence is based on
several large and well-controlled trials.2,3 The American
Heart Association has already included CRP measurement into the guidelines for cardiovascular risk prediction.7 Few populations, however, have shown less strong
correlation of CRP plasma levels with cardiovascular
events,8 and consequently, some researchers doubt that
CRP measurement adds useful information to the Framingham risk score.
Concerning the second point, ie, the potential role of
CRP as a cardiovascular risk factor, there is definitely nointernational consensus. CRP was first hypothesized to
be involved in atherogenesis in 1982 when researchers
demonstrated that CRP interacts with low-density lipoprotein (LDL),9 a finding that has recently been confirmed byothers using various modified and nonmodified LDL molecules.10 –12 Scientific interest waned, however, when thesame group consecutively published that CRP is present

neither in human nor in rabbit arterial lesions.13 Twosmaller publications on CRP deposition in human lesions
were primarily ignored.14,15 Interest returned with theresults from epidemiology and the identification of CRP
as a cardiovascular risk marker.2,3 In search for complement-activating molecules in human atherosclerotic lesions, it was demonstrated in 1998 that CRP is indeed
ubiquitously present in all stages of human atherosclerosis and that it co-localizes with activated complement
fragments.16 This finding also has been confirmed byseveral groups,17 and since then various researchers
have investigated a potential active contribution of CRP
to atherosclerosis, suggesting that CRP activates monocytes/macrophages,18,19 endothelial cells,20,21 and vascular smooth muscle cells.22 A critical point in all thesestudies is the use of commercially available CRP preparations that may contain contaminants such as lipopolysaccharide and azide. Indeed, it has recently been demonstrated that almost all of the published effects of CRP
on endothelial cells are due to contamination of CRP
preparations with azide and lipopolysaccharide.23,24 In
this context it should be noted that CRP-mediated activation of endothelial cells and smooth muscle cells may
not be biologically plausible and may lack a convincing
molecular explanation.
To understand a molecule’s role in disease, the best
approach might be to reconsider its role in physiology.
CRP is an ancient immune molecule that shares many
functional properties with antibodies: CRP binds to avariety of ligands (including LDL or modified LDL), activates complement,25 opsonizes biological particles,26
and binds to and signals via Fc␥-receptors.27,28 CRP andcomplement appeared very early in the evolution of theimmune system and so did monocytes/macrophages. It
is thus likely that the target cell for CRP in vascularbiology and atherosclerosis is the monocyte/macrophage and not the endothelial cell or smooth muscle cell,
especially in view of the fact that CRP co-localizes withAccepted for publication July 12, 2005.
Address reprint requests to Jan Torzewski, M.D., M.Phil., University ofUlm, Department of Internal Medicine II–Cardiology, 89081 Ulm, Germany. E-mail: jan.torzewski@medizin.uni-ulm.de.

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monocytes/macrophages and foam cells in the humanatherosclerotic lesion.16 –18 In addition CRP and complement fragments are chemotactic for monocytes/macrophages,18,29 which express Fc␥ receptors and complement receptors at high levels. Fc␥ receptors andcomplement seem to be involved in CRP-mediated opsonization of LDL.19 Therefore, it might be hypothesizedthat LDL, CRP, complement, and macrophages orchestrate an inflammatory process in the arterial wall that
promotes atherogenesis.
In vitro studies have been supplemented by in vivo
investigations, with contradictory results. It is important tonote here that the most broadly available animal model,ie, the mouse, is considered useless regarding the studyof CRP functions because CRP is not an acute phasereactant in mice. To overcome this problem, a transgenic
mouse that overexpresses human CRP was generated,
and this model has been used to study the role of CRP in
cardiovascular disease.30 –33 Whereas two initial publications suggested an active contribution of CRP to atherosclerosis and acute cardiovascular events in this model,30,31 two more recent studies have detected noeffect.32,33 It is difficult to determine which of the reports
is right or wrong, valid or invalid, because the model itself
encounters several problems. Human CRP is a foreign
antigen in the mouse with many uncertainties concerning
its functional role in the immune system of these animals.
The situation becomes even more complicated whenthese mice are crossed with ApoE-deficient mice that
obviously lack a fully functional complement system.34 To
cut a long discussion short, it may be appropriate to saythat it was worth generating these model systems but
hardly possible to answer definitively whether CRP actively contributes to human atherogenesis or not.
In this context the article by Sun and colleagues1 published in this issue of The American Journal of Pathology
may be of considerable interest. The authors use wellestablished animal atherosclerosis models, ie, both cholesterol-fed and Watanabe heritable hyperlipidemic(WHHL) rabbits, as models to study the role of CRP in
atherogenesis. Interestingly, it is the rabbit again, andthus the same species that stopped scientific interest in
the matter more than 20 years ago,13 that attracts ourattention today. Being experts in working with this animalmodel, the authors elaborate three major results. First,
CRP levels are significantly elevated in hypercholesterolemic rabbits. Second, elevated CRP levels strongly
correlate with the extent of atherosclerosis in these animals. Third, CRP is ubiquitously present in atherosclerotic lesions in rabbits, and this lesional CRP is derived
from the circulation rather than being synthesized locallyin the arterial wall. These results are similar in both cholesterol-fed and WHHL rabbits, and each point is wellestablished through analysis of a large number of animals. The article certainly does not prove a causal involvement of CRP in atherogenesis, and even though
CRP is an acute-phase reactant in rabbits, many questions concerning CRP functions in these animals remain
to be resolved.1
The article does, however, describe models that may
help address important issues that hint to the third point

we raised in the introduction to this commentary: is CRP
inhibition with specific drugs a modality to treat atherosclerosis? Four major strategies of CRP inhibition arefeasible: 1) transcriptional inhibition of hepatic CRP synthesis,6 2) anti-sense strategies,35 3) blockage of CRPmediated complement activation, and 4) blockage of
CRP receptor(s). Pharmaceutical companies are currently examining these strategies. The article by Sun andcolleagues1 in this issue of The American Journal of Pathology provides in vivo models that may be appropriate
to test future CRP inhibitors. If these compounds do not,in parallel, affect LDL levels, discrimination between LDL
and CRP effects on atherogenesis may be possible. Finally, these rabbit models might be appropriate for examining bioavailability, specificity, and toxicology of various treatments.
In awareness of the controversial discussion and thecontroversial data on CRP and atherosclerosis, I would
like to finish with a personal opinion from a cardiologist’s
perspective. We should try to inhibit CRP for the treatment of atherosclerosis; otherwise, we might miss achance to help our patients suffering from cardiovasculardisease.

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