Monocyte subsets differentially employ CCR2, CCR5, and CX3CR1 to accumulate within atherosclerotic plaques

Abstract

Monocytes participate critically in atherosclerosis. There are 2 major subsets expressing different chemokine receptor patterns: CCR2⁺CX3CR1⁺Ly-6C^hi and CCR2^–CX3CR1⁺⁺Ly-6C^lo monocytes. Both C-C motif chemokine receptor 2 (CCR2) and C-X₃-C motif chemokine receptor 1 (CX3CR1) are linked to progression of atherosclerotic plaques. Here, we analyzed mouse monocyte subsets in apoE-deficient mice and traced their differentiation and chemokine receptor usage as they accumulated within atherosclerotic plaques. Blood monocyte counts were elevated in apoE^–/– mice and skewed toward an increased frequency of CCR2⁺Ly-6C^hi monocytes in apoE^–/– mice fed a high-fat diet. CCR2⁺Ly-6C^hi monocytes efficiently accumulated in plaques, whereas CCR2^–Ly-6C^lo monocytes entered less frequently but were more prone to developing into plaque cells expressing the dendritic cell–associated marker CD11c, indicating that phagocyte heterogeneity in plaques is linked to distinct types of entering monocytes. CCR2^– monocytes did not rely on CX3CR1 to enter plaques. Instead, they were partially dependent upon CCR5, which they selectively upregulated in apoE^–/– mice. By comparison, CCR2⁺Ly-6C^hi monocytes unexpectedly required CX3CR1 in addition to CCR2 and CCR5 to accumulate within plaques. In many other inflammatory settings, these monocytes utilize CCR2, but not CX3CR1, for trafficking. Thus, antagonizing CX3CR1 may be effective therapeutically in ameliorating CCR2⁺ monocyte recruitment to plaques without impairing their CCR2-dependent responses to inflammation overall.