Immunohistochemical characterization of myofibroblasts appearing in isoproterenol-induced rat myocardial fibrosis

Fibrotic lesion is formed by myofibroblasts capable of producing collagens. The myofibroblasts are characterized by immunoexpressions of vimentin, desmin and α-smooth muscle actin (α-SMA) in varying degrees. The cellular characteristics remain investigated in myocardial fibrosis. We analyzed immunophenotypes of myofibroblasts appearing in isoproterenol-induced myocardial fibrosis in rats until 28 days after injection (10 mg/kg body weight); the lesions developed as interstitial edema and inflammatory cell reaction on 8 hr and days 1 and 3, and fibrosis occurred on days 1, 3, 7, 14, and 21 by gradual deposition of collagens, showing the greatest grade on day 14; the lesions gradually reduced with sporadic scar until day 28. Myofibroblasts expressing vimentin and α-SMA increased with a peak on day 3, and then, gradually decreased onwards. Interestingly, Thy-1 expressing cells appeared in the affected areas, apparently being corresponding to the grade similar to vimentin- and α-SMA-positive cells. Thy-1 is expressed in immature mesenchymal cells such as pericytes with pluripotent nature. The immunoreactivity for A3-antigen, a marker for immature mesenchymal cells, was seen in some surrounding cells. There were no cells reacting with antibodies to nestin or glial fibrillary acidic protein, although hepatic myofibroblats have been reported to react with these antibodies. Collectively, myofibroblasts appearing in rat myocardial fibrosis may have been derived from immature mesenchymal cells positive for Thy-1 or A3-antigen, with thereafter showing expressions of vimentin and α-SMA in differentiation.