Abstract.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 conditions. T HOUGH actin is one of the most conserved eukaryotic proteins, it is expressed in mammals and birds as six isoforms characterized by two-dimensional (2D)-PAGE and amino acid sequence analysis (14,49,56,57,59). Four of them represent differentiation markers of muscle tissues and two are found practically in all cells (56,57). Actin isoforms show >90% overall sequence homology, but only 50-60% homology in their 18 NH2-terminal residues (56, 57). The NH2-terminal region of actin appears to be a major antigenic region (2, 5, 34) and may be involved in the interaction of actin with other proteins such as myosin (31).Little is presently known about the function of actin isoforms. It has been shown that the relative proportions of actin isoforms are different in smooth muscles of different organs (12, 47; Skalli, O., J. Vandekerckhove, and G. Gabbiani, manuscript submitted for publication) and change within the same population of smooth muscle cells (SMCs) t during development (23, 24), pathological situations (11, 21), and different culture conditions (35,46,50). In noumuscle cells, a difference in the proportions of 13-and y-cytoplasmic actins has been reported between normal and tumoral T-lympho-1. Abbreviation used in this paper: SMC, smooth muscle cell. cytes (28). All these studies have been performed on cell populations by means ...

The cellular composition of human atherosclerotic plaques was analyzed by immunologic techniques. Plaques were removed from the internal carotid artery during surgery, and a panel of monoclonal antibodies was used to identify cell types.

A mouse monoclonal antibody (MAb) recognizing alpha-smooth-muscle actin has been used to study smooth-muscle differentiation features in the stromal cells of desmoplastic reactions accompanying mammary tumors. We have studied, by the same immunohistochemical technique, a series of malignant and non-malignant human breast tissues. Cells composing the desmoplastic reaction were found to express alpha-smooth-muscle actin in all the 11 breast carcinomas examined, whereas no immunostain was demonstrated in the stromal cells of 7 breast tissue samples histologically defined as normal. Three of 9 cases of fibrocystic disease showed a minority of positively stained stromal cells, generally in association with epithelial hyperplasia. All the 7 cases of sclerosing adenosis, 3 of 4 cases of diffuse papillomatosis and all 3 intraductal papillomas exhibited a majority of immunoreactive stromal cells. Numerous stromal cells in 3 of 11 circumscribed fibroadenomas analyzed expressed low amounts of alpha-smooth-muscle actin. The factor(s) responsible for smooth-muscle differentiation in stromal cells are presently unknown, but the detection of this previously unsuspected stromal cell phenotype in nonmalignant mammary tissues might help in characterizing the variant morphological aspects designated under the label "fibrocystic disease" and in understanding the biology of premalignant or early malignant lesions of the breast.

a-Smooth muscle (a-sm) actin, an isoform typical of smooth muscle cells (SMC) and present in high amounts in vascular SMC, was demonstrated in the cytoplasm ofpericytes of various rat and human organs by means of immunocytochemistry at the electron microscopic level. In SMC and pericytes, a-sm actin was localized in microfilament bundles, strength-I Supported by the Swiss National Science Foundation, Grants 3.107-0.85 and 3.404.86, and by the Fondation Centre de Recherches M#{233}dicales Carbos et Elsie de Reuter.

A large proportion of the cells of the human atherosclerotic plaque is assumed to be derived from medial smooth muscle cells. In contrast to these, the cells of the plaque have the capacity to accumulate lipid, and they also proliferate at a higher rate than medial cells. It has therefore been suggested that smooth muscle cells undergo a change of phenotype during atherogenesis, but there has been no evidence for such a change on the molecular level. We have now analyzed carotid artery plaques using a battery of antibodies against cell surface and cytoskeletal antigens, and found that most of the cells express the class II transplantation antigen (Ia antigen) HLA-DR. Also, the beta chain of HLA-DR was detected by immunoblotting of plaque extracts with the OKIal nmonoclonal antibody. HLA-DR is normally present on cells of the immune system, but only 60% of the DR-positive cells of the plaque reacted with monoclonal antibodies specific for macrophages and lymphocytes. Many of the remaining DR-positive cells contained the muscle-specific intermediate filament protein, desmin. This indicates that smooth muscle cells of atherosclerotic plaques express DR antigen. In contrast, very few DRpositive cells were found in normal human arteries. This suggests that expression of class II antigen is part of a phenotypic change in smooth muscle cells in atherosclerosis.