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

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. 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. 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. 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.