EXTRACELLULAR-MATRIX MODULATES MACROPHAGE FUNCTIONS CHARACTERISTIC TOATHEROMA - COLLAGEN TYPE-I ENHANCES ACQUISITION OF RESIDENT MACROPHAGE TRAITS BY HUMAN PERIPHERAL-BLOOD MONOCYTES IN-VITRO

Activated resident macrophages sustain atheroma, and a high macrophage content is associated with plaque vulnerability. Factors leading to d ifferentiation and activation of these blood-derived cells remain larg ely unchacterized, We investigated the contribution of interaction wit h collagen type I, the predominant component oi atherosclerotic matrix , to differentiation and modulation of characteristic macrophage funct ions, including intracellular lipid accumulation and production of the typical matrix-degrading enzyme matrix metalloproteinase (MMP)-9. Whe n used as an adhesion substrate For human peripheral blood monocytes i n vitro, collegen type I increased monocyte differentiation, assessed by analysis of CD71 expression and cell spreading. Culturing on collag en type I doubled the number of differentiated monocytes at 24 hours ( 44.9+/-1.4% versus 18.4+/-1.7% on uncoated dishes, P<.001, n=3 indepen dent experiments) and was a stronger stimulus for differentiation than phorbol myristate acetate, a known inducer of monocyte differentiatio n. The effect of substrate on intracellular accumulation of modified l ipoproteins was assessed by quantitative confocal microscopy of monocy tes incubated with fluorescent acetylated LDL. The collagen type I sub strate also doubled the number of macrophages containing intracellular lipid and significantly increased the individual intracellular loadin g. Monocytes cultured on collagen type I also released more MMP-9 than did cells placed directly on plastic. The role of monocyte spreading was further assessed by treatment with colchicine, an inhibitor of cyt oskeletal function, or with genistein, a nonspecific inhibitor of tyro sine kinases shown to participate in cell adhesion, Cell spreading was inhibited in 77.3+/-6.7% of colchicine-treated and in 62.4+/-6.4% or genistein-treated monocytes (n=3, P<.01 in both cases). The same condi tion also decreased secretion of MMP-9, and genistein reduced the numb er of acetylated LDL-containing cells (from 286+/-7 to 184+/-8 cells/m m(2) with genistein, n=3, P<.001). Data showed a strong correlation (r >.98) between monocyte spreading on collagen type I and intracellular lipid accumulation. Our results indicate that interaction with vascula r matrix may play an important role in differentiation of peripheral b lood monocytes into resident lipid-laden macrophages, which act as cen tral stimulators throughout the natural history of atheroma.