Human macrophages synthesize type VIII collagen in vitro and in the atherosclerotic plaque

Type VIII collagen is a short-chain collagen that is present in increased a mounts in atherosclerotic lesions. Although the physiological function of t his matrix protein is unclear, recent data suggest an important role in tis sue remodeling. Type VIII collagen in the atherosclerotic lesion is mainly derived from smooth muscle cells. We now show that macrophages in the ather osclerotic vessel wall and monocytes in adjacent mural thrombi also express type VIII collagen. We demonstrated this using a novel combined fluorescen ce technique that simultaneously stains, within the same tissue section, sp ecific RNAs by in situ hybridization and proteins by indirect immunofluores cence. In culture, human monocyte/macrophages expressed type VIII collagen at all time points from 1 h to 3 wk after isolation. Western blotting and i mmunoprecipitation also revealed secretion of type VIII collagen into the m edium of 14-day-old macrophages. Because this is the first report of secret ion of a collagen by macrophages, we tested the effect of lipopolysaccharid e (LPS) and interferon gamma, substances that stimulate macrophages to secr ete lyric enzymes, on macrophage expression of type VIII collagen. LPS and interferon gamma decreased expression of type VIII collagen. By contrast, s ecretion of matrix metalloproteinase 1 (MMP 1) was increased, indicating a switch from a collagen-producing to a degradative phenotype. Double in situ hybridization studies of expression of type VIII collagen and MMP 1 in hum an coronary arteries showed that in regions important for plaque stability, the ratio of MMP 1 RNA to macrophage type VIII collagen RNA varies widely, indicating that the transition from one phenotype to the other that we obs erved in vitro may also occur in vivo.