Introduction: Recent studies suggest that Nlrp3 inflammasome activation plays a critical role in the development of atherosclerosis and Chlamydia pneumoniae (Cpn) infection has been shown to accelerate atherogenesis.
Hypothesis: Herein, we asked whether Cpn infection induced acceleration is via Nlrp3 inflammasome activation in hypercholestrolemia mouse model.
Methods and Results:Nlrp3-/-Ldlr-/-, Casp1-/-Ldlr-/- and Ldlr-/- mice were infected intranasally with Cpn followed by western diet (WD) for 16 weeks. Ldlr-/- mice infected with Cpn infection had markedly increased lesion size in the aortic sinus and aorta en face compared to WD only group. Casp1 activation in lesion macrophages in Ldlr-/- mice was also increased in Cpn group vs controls. Nlrp3-/-Ldlr-/- or Casp1-/-Ldlr-/- mice with and without Cpn resulted in significantly smaller of plaques in aortic root and aorta compare to Ldlr-/- mice. However, no difference was observed between Cpn infected and uninfected groups in the double knockout animals. Furthermore, foam cell formation of Nlrp3-/-, and Casp1-/- peritoneal macrophages after treatment with OxLDL and Cpn was significantly reduced when compared with WT cells. Interestingly, expression levels of the cholesterol efflux transporter, ATP-binding cassette A1 (ABCA1), was increased by RT PCR and western analysis in the KO macrophages. Further investigations found that the niacin receptor Gpr109a, a known positive regulator of ABCA1, was upregulated in Nlrp3 KO macrophages during foam cell formation. Hydroxy-butyrate, produced by infected macrophages, signaled through GPR109, thereby upregulating ABCA1. Intact IL-1 signaling suppressed Gpr109a expression suggesting a pathway by which the inflammasome and IL-1β could enhance foam cell formation. In aortic root lesions, macrophage expression of Gpr109a was increased in Nlrp3-/-Ldlr-/- mice compare with Ldlr-/- mice on WD and infected with Cpn. Additionally, this regulatory pathway was observed in human macrophages.
Conclusions: The activation of the NLRP3 inflammasome negatively regulates the Gpr109a receptor and its downstream cholesterol efflux transporter ABCA1, which leads to greater foam cell formation and acceleration of atherosclerosis in Ldlr-/- mice.