Matrix metalloproteinase-9 (MMP-9) has been proposed to be an important modulator of atherosclerotic plaque vulnerability. We generated a transgenic (tg) model expressing human proMMP-9 in macrophages, using the scavenger receptor enhancer/promoter A. This model was crossed into the double Apoe/Timp-1 knockout background. After 16 weeks of a high-fat diet, there were no significant changes in plaque size in the proximal aortas between the four groups of the study population (Apoe(-/-), Apoe(-/-)/MMP-9tg, Apoe(-/-)/Timp-1(-/-), and Apoe(-/-)/MMP-9tg/Timp-1(-/-)), and, in the Timp-1 knockout background, MMP-9 transgenic mice and control littermates had similar micro-aneurysm formation. However, lesions in Apoe(-/-)/MMP-9tg/Timp-1(-/-) mice contained significantly more collagen compared to the three other groups (P<0.005). Culture supernatants from elicited Apoe(-/-)/MMP-9tg/Timp-1(-/-) macrophages contained higher levels of active TGF-beta than the three other groups (P<0.05), suggesting that augmented collagen deposition resulted from an increase in TGF-beta activation due to transgenic MMP-9 in the Timp-1(-/-) background. This study indicates that, in human atherosclerosis, increased MMP-9 activity could up-regulate collagen deposition, possibly through TGF-beta activation.