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and by CD40/CD40 ligand (CD40L and CD154) interactions. 18–22 Once adherent, the monocytes transmigrate into the tunica intima, the innermost layer of the arterial wall, passing between the ECs. This monocyte migration is directed along a concentration gradient of MCP-1, via interaction with the monocyte receptor CCR2. 23 Once within the arterial intima, the monocytes develop into macrophages and begin to express scavenger receptors, such as SR-A, CD36, and LOX-1, that internalize modified lipoproteins. 24, 25 Internalization of these lipoprotein particles gives rise to lipidladen macrophages or foam cells, which characterize early atherosclerotic lesions. Within the developing atheroma, the foam cells begin to secrete proinflammatory cytokines that maintain a chemotactic stimulus for adherent leukocytes, augment expression of scavenger receptors, and promote macrophage replication. 6

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