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Chlamydia pneumoniae-Induced Foam Cell Formation Requires MyD88-Dependent and -Independent Signaling and Is Reciprocally Modulated by Liver X Receptor Activation¹

Division of Pediatric Infectious Diseases and Immunology, Cedars-Sinai Medical Center and David Geffen School of Medicine, University of California, Los Angeles, CA 90048

Chlamydia pneumoniae is detected by macrophages and other APCs via TLRs and can exacerbate developing atherosclerotic lesions, but how that occurs is not known. Liver X receptors (LXRs) centrally control reverse cholesterol transport, but also negatively modulate TLR-mediated inflammatory pathways. We isolated peritoneal macrophages from wild-type, TLR2, TLR3, TLR4, TLR2/4, MyD88, TRIF, MyD88/TRIF, and IFN regulatory factor 3 (IRF3) KO mice, treated them with live or UV-killed C. pneumoniae in the presence or absence of oxidized LDL, then measured foam cell formation. In some experiments, the synthetic LXR agonist GW3965 was added to macrophages infected with C. pneumoniae in the presence of oxidized LDL. Both live and UV-killed C. pneumoniae induced IRF3 activation and promoted foam cell formation in wild-type macrophages, whereas the genetic absence of TLR2, TLR4, MyD88, TRIF, or IRF3, but not TLR3, significantly reduced foam cell formation. C. pneumoniae-induced foam cell formation was significantly reduced by the LXR agonist GW3965, which in turn inhibited C. pneumoniae-induced IRF3 activation, suggesting a bidirectional cross-talk. We conclude that C. pneumoniae facilitates foam cell formation via activation of both MyD88-dependent and MyD88-independent (i.e., TRIF-dependent and IRF3-dependent) pathways downstream of TLR2 and TLR4 signaling and that TLR3 is not involved in this process. This mechanism could at least partly explain why infection with C. pneumoniae accelerates the development of atherosclerotic plaque and lends support to the proposal that LXR agonists might prove clinically useful in suppressing atherogenesis.

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ This work was supported by National Institutes of Health Grants AI 067995 and HL66436 (to M.A.).

² Address correspondence and reprint requests to Dr. Moshe Arditi, Cedars-Sinai Medical Center, Division of Pediatric Infectious Diseases and Immunology, 8700 Beverly Boulevard, Room 4221, Los Angeles, California, 90048. E-mail address: moshe.arditi{at}cshs.org

³ Abbreviations used in this paper: TRIF, Toll/IL-1R domain-containing adapter inducing IFN-β; ABCA1, ATP-binding cassette transporter 1; IRF, IFN regulatory factor; KO, knockout; LXR, liver X receptor; MOI, multiplicity of infection; ox-LDL, oxidized low-density lipoprotein.

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