Smooth muscle-specific SM22 protein is expressed in the adventitial cells of balloon-injured rabbit carotid artery

During the "response-to-injury" process after a mechanical insult to the po rcine coronary arteries, the adventitial cells acquire the structural chara cteristics of myofibroblasts before being incorporated into smooth muscle ( SM) layer. We assessed whether the SM-specific SM22 protein can be used as a tracer of adventitial cell-myofibroblast differentiation in the mild ball oon injury of rabbit carotid artery. To achieve this goal, we used 2 monocl onal anti-SM22 antibodies (E-ll and 1-B8) and a molecular probe for the SM2 2 alpha mRNA isoform in immunocytochemical and in situ hybridization experi ments. The differentiation profile and the migratory and proliferative abil ity of activated adventitial cells were evaluated by a panel of antibodies to some SM and nonmuscle antigens and pulse- and end-labeling with bromo-de oxyuridine, respectively. In adventitial cells, SM22 antigenicity and SM22a mRNA were detectable at days 2 and 4 and, to a lesser extent, at days 7 an d 21 after injury, particularly near the adventitia-media interface and mos tly colocalizing with bromo-deoxyuridine-positive cells. The pulse-labeling experiments showed that the large majority of these cells penetrated the o utermost layer of the tunica media without migrating to the subendothelial region. The phenotypic features of activated migrating and nonmigrating adv entitial cells resembled those of vimentin-actin myofibroblast subtype and fetal-type SM cells, These findings indicate that a direct exposure of adve ntitia to the lumen is not required for phenotypic changes and proliferatio n/migration of these cells. After comparison of the SM22 expression in arte rial vessels during early stages of development, we hypothesize that in the injured carotid artery the mural incorporation of adventitial cells and th e spatiotemporal activation of SM22 expression are reminiscent of the vascu lar morphogenetic process and suggest the existence of a stem cell-like res ervoir in adventitia, The early adventitial upregulation of SM22 expression in the injured vessel might be related to a multistep transition process i n which nonmuscle cells are converted to myofibroblasts and, possibly, to S M cells.