Studies on the histogenesis of myxomatous tissue of human coronary lesions

Myxomatous tissue is a characteristic component of human coronary artery le sions, found more often in restenotic lesions. It represents a bulky accumu lation of stellate-shaped cells of unknown histogenesis that are embedded i n a loose stroma. We analyzed 64 atherectomy specimens containing substanti al amounts of myxomatous tissue by using immunohistochemistry, in situ hybr idization, and electron microscopy techniques. Stellate cells represented a heterogeneous population, sharing features of smooth muscle cells (SMCs), macrophages, as well as antigen-presenting dendritic cells. Like quiescent medial SMCs, the stellate cells in all specimens expressed high levels of S M cr-actin message and protein and showed heterogeneity with respect to hea vy-chain myosin, SM22, desmin, and vimentin. Ultrastructurally, stellate ce lls resembled SMCs, with some peculiarities that distinguish them from both differentiated and dedifferentiated SMCs. In contrast to quiescent SMCs, t he stellate cells expressed high levels of acidic fibroblast growth factor mRNA and protein similar to cells of monocyte/macrophage lineage. However, stellate cells did not express the marker of mature macrophages, HAM56, and were heterogeneous with respect to CD68. Moreover, unlike SMCs, the stella te cells bore some of the major phenotypic markers of dendritic cells: they were S100-positive and showed various reactivity with respect to CD1a and human leukocyte antigen (HLA)-DR. Invasion of myxomatous tissue with CD45RO -positive T lymphocytes was correlated with strong expression of CD la in t hese specimens. Stellate cells also expressed a pericyte marker, high-molec ular-weight melanoma-associated antigen. We conclude that stellate cells of myxomatous tissue represent a specific phenotype of mesenchymal cells (pos sibly pericytes), which is activated to express some markers of antigen-pre senting cells. These findings suggest involvement of the stellate cells in a local immune response.