Monocytes participate critically in atherosclerosis. There are 2 major subsets expressing different chemokine receptor patterns: CCR2+CX3CR1+Ly-6Chi and CCR2–CX3CR1++Ly-6Clo 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-6Chi monocytes in apoE–/– mice fed a high-fat diet. CCR2+Ly-6Chi monocytes efficiently accumulated in plaques, whereas CCR2–Ly-6Clo 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-6Chi 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.
Frank Tacke, David Alvarez, Theodore J. Kaplan, Claudia Jakubzick, Rainer Spanbroek, Jaime Llodra, Alexandre Garin, Jianhua Liu, Matthias Mack, Nico van Rooijen, Sergio A. Lira, Andreas J. Habenicht, Gwendalyn J. Randolph
Labeling of monocyte subsets in apoE–/– mice.