Pcsk6 ablation increases atherosclerotic burden, but provides plaque stability by regulating th17 and smooth muscle cell content

Background and Aims: We have previously shown that PCSK6 is a key protease modulating smooth muscle cell activation in vascular disease. Its expression and localization also associated positively with typical markers of inflammatory cells in atherosclerotic plaques, leading to a hypothesis that PCSK6 may be involved in modulating immune responses. Methods: Detailed immunophenotyping using histology, FACS-, OLINK- and ELISA-based analyses and primary cell cultures was used to compare Pcsk6-/-mice and controls. Atherosclerosis was evaluated in a bone marrow transplant model. Results: Plasma from Pcks6-/- mice showed increased levels of pro-inflammatory cytokines CCL2, CCL3 and in particular IL-17A and IL-17F. Pcsk6-/- spleens had an increased number of germinal centers and contained more CD8+ T cells. Splenocytes isolated from Pcsk6-/- mice secreted higher levels of IFN-g, IL-2, IL-17 and IL-10 upon stimulation and were more proliferative in vitro. Peritoneal macrophages from Pcsk6-/- mice also secreted more cytokines, including TNF-a, CCL2, IL-6 and IL-10 upon stimulation in vitro, while bone marrow derived macrophages from Pcsk6-/- mice were prone to lipid uptake. Finally, in vivo transplantation of Pcsk6-/- bone marrow to Ldlr-/- mice led to increased atherosclerotic plaque burden compared to Ldlr-/- receiving control bone marrow. However, these plaques presented more stable features, attributed to increased collagen deposition and SMC presence, and increased content of the fibrogenic IL-17 cytokine. Conclusions: Pcsk6-/- ablation in bone marrow revealed its dichotomous role in atherogenesis, provoking higher atherosclerotic burden, but also increased plaque stability. Taken together, these results indicate that PCSK6 is a key regulator of the immune system.