Background: Monocytes are critical mediators of atherogenesis. Deletion of individual chemokines or chemokine receptors leads to significant but only partial inhibition of lesion development, whereas deficiency in other signals such as CXCL16 or CCR1 accelerates atherosclerosis. Evidence that particular chemokine pathways may cooperate to promote monocyte accumulation into inflamed tissues, particularly atherosclerotic arteries, is still lacking.
Methods and results: Here, we show that chemokine-mediated signals critically determine the frequency of monocytes in the blood and bone marrow under both noninflammatory and atherosclerotic conditions. Particularly, CCL2-, CX3CR1-, and CCR5-dependent signals differentially alter CD11b(+) Ly6G(-) 7/4(hi) (also known as Ly6C(hi)) and CD11b(+) Ly6G(-) 7/4(lo) (Ly6C(lo)) monocytosis. Combined inhibition of CCL2, CX3CR1, and CCR5 in hypercholesterolemic, atherosclerosis-susceptible apolipoprotein E-deficient mice leads to abrogation of bone marrow monocytosis and to additive reduction in circulating monocytes despite persistent hypercholesterolemia. These effects are associated with a marked and additive 90% reduction in atherosclerosis. Interestingly, lesion size highly correlates with the number of circulating monocytes, particularly the CD11b(+) Ly6G(-) 7/4(lo) subset.
Conclusions: CCL2, CX3CR1, and CCR5 play independent and additive roles in atherogenesis. Signals mediated through these pathways critically determine the frequency of circulating monocyte subsets and thereby account for almost all macrophage accumulation into atherosclerotic arteries.