A Monoclonal Antibody against a-Smooth Muscle Actin: A New Probe for Smooth Muscle Differentiation

A monoclonal antibody (anti-ctsm-1) recognizing exclusively a-smooth muscle actin was selected and characterized after immunization of BALB/c mice with the NH2-terminal synthetic decapeptide of a-smooth muscle actin coupled to keyhole limpet hemocyanin. Anti-ctsm-1 helped in distinguishing smooth muscle cells from fibroblasts in mixed cultures such as rat dermal fibroblasts and chicken embryo fibroblasts. In the aortic media, it recognized a hitherto unknown population of cells negative for a-smooth muscle actin and for desmin. In 5-d-old rats, this population is about half of the medial cells and becomes only 8 + 5 % in 6-wk-old animals. In cultures of rat aortic media SMCs, there is a progressive increase of this cell population together with a progressive decrease in the number of a-smooth muscle actin-containing stress fibers per cell. Double immunofluorescent studies carried out with anti-ctsm-1 and anti-desmin antibodies in several organs revealed a heterogeneity of stromal cells. Desmin-negative, a-smooth muscle actin-positive cells were found in the rat intestinal muscularis mucosae and in the dermis around hair follicles. Moreover, desmin-positive, a-smooth muscle actin-negative cells were identified in the intestinal submucosa, rat testis interstitium, and uterine stroma, a-Smooth muscle actin was also found in myoepithelial cells of mammary and salivary glands, which are known to express cytokeratins. FinaUy, ~t-smooth muscle actin is present in stromal cells of mammary carcinomas, previously considered fibroblastic in nature. Thus, anti-asm-1 antibody appears to be a powerful probe in the study of smooth muscle differentiation in normal and pathological