Objective—Thrombospondin-1 (TSP1) is described as a positive regulator of vascular smooth muscle growth in cell culture. However, insight into the in vivo effects of TSP1 on smooth muscle cell (SMC) function is lacking.

Methods and Results—We analyzed wild-type (WT) and TSP1-deficient (Tsp1^-/-) mice in a carotid artery ligation model, in which neointimal lesions form without overt mechanical damage to the endothelium. On ligation, the expression of TSP1 increased strongly in the matrix of neointima and adventitia. In the early phase after ligation (day 3 to 7), activation, proliferation, and migration of medial SMCs were delayed and impaired in Tsp1^-/- mice, in parallel with defective upregulation of metalloproteinase (MMP)-2 activity. As a result, Tsp1^-/- arteries developed smaller neointimal lesions, a thicker media but comparably attenuated patency as in WT arteries, 28 days after ligation. Furthermore, medial and neointimal SMCs in Tsp1^-/- mice produced more collagen, more osteopontin, and displayed weaker smooth muscle actin staining than WT SMCs, indicative of a modified SMC phenotype in Tsp1^-/- mice.

Conclusions—Arterial SMC activation in the absence of TSP1 is delayed and dysregulated, reducing neointima formation, on mild vascular injury.