Cholesterol Lowering Alone Fails to Reverse Atherosclerotic Plaque Necrosis, Granulopoiesis, and Neurovascular Neutrophils in Middle‐Aged Mice

Cholesterol lowering through diet, lifestyle, and pharmacologic therapy remains central for limiting atherosclerosis and prevention of major adverse cardiovascular events. Yet, 33%-50% of individuals on lipid-lowering therapy continue to exhibit elevated inflammation. Middle age (MA) represents a critical window for disease acceleration, underscoring a need to better understand nonresolving inflammation in this time frame. Here, we rendered young (2 months) and MA mice (10 months) hypercholesterolemic with an AAV8-PCSK9 virus and western diet (WD). Following 20 weeks on WD, mice were switched to a chow diet for 6 weeks to induce lipid lowering. This design models dietary cholesterol reduction to dissect lipid-driven versus inflammation-driven pathways in atherosclerosis. At baseline atherosclerosis, we found that MA mice had increased plaque necrosis as well as increased circulating and bone marrow PMN compared to young mice. After chow switch, unlike in young mice, MA mice had increased plaque necrosis and reduced remodeling, as well as increased circulating white blood cells and bone marrow hematopoietic stem cell progenitors (HSPCs). In MA chow-switched mice, circulating neutrophils correlated with necrosis whereas young mice exhibited no correlation. Furthermore, MA atherosclerotic mice had bone marrow HSPCs and neutrophils that exhibited a more activated phenotype relative to young after chow switch. In addition, we observed elevated neutrophil-endothelial contacts in the hippocampal vasculature of MA mice after chow switch. While dietary intervention and lowered plasma cholesterol restrained atheroprogression in young, it was inadequate in MA mice, failing to reduce systemic inflammation and indicating the need for complementary therapies during this time frame.