PCSK6 ablation in blood circulating cells increases atherosclerotic burden, but improves plaque stability by activating Th17-smooth muscle cell modulatory axis

Background: Proprotein convertase subtilisins/kexins (PCSKs) have been implicated in cancers and cardiovascular disease. We have shown that PCSK6 is a key protease regulating smooth muscle cell (SMC)-mediated vascular remodeling, but also that it can be expressed by T cells and macrophages in atherosclerotic plaques. Whether PCSK6 regulates innate and adaptive immune responses in the context of vascular inflammation is still unknown. Methods: In this study, detailed immunophenotyping of constitutive Pcsk6^−/− mice was performed. Bone marrow transplantation into high-cholesterol diet fed Ldlr^−/− mice was used to investigate PCSK6-mediated immune effects in atherogenesis and plaque stability. Results: Compared to controls, Pcsk6^−/− mice showed higher plasma levels of the chemoattractants CCL2 and CCCL3, and Th17 cytokines IL-17 A and IL-17F. Pcsk6 ablation led to increased naïve and effector-memory CD4+ and CD8+ cell numbers in the spleen, and increased release of IL-17 A, IFN-γ and IL-10 as well as proliferation by spleenocytes in vitro. Lack of Pcsk6 also affected innate immunity as macrophages from Pcsk6^−/− mice secreted more cytokines, including TNF-α, CCL2, IL-6 and IL-10 upon LPS stimulation in vitro, and were more prone to oxLDL uptake. In line with a pro-inflammatory phenotype, Pcsk6^−/−➔Ldlr^−/− transplanted mice presented a higher atherosclerotic plaque burden compared to Ldlr^−/− receiving control bone marrow. Although larger, Pcsk6^−/−➔Ldlr^−/− plaques showed increased stability features, including collagen deposition and SMC presence coinciding with significantly increased local levels of the fibrogenic cytokine IL-17. Conclusions: Global Pcsk6 ablation leads to the activation of both adaptive and innate immune systems. Interestingly, Pcsk6^−/− ablation in bone marrow of hyperlipidemic mice revealed its dual role in atherogenesis, activating a Th17-SMC modulatory axis that promotes plaque stability, despite increased atherosclerotic burden.