In vivo and in vitro expression of tenascin by human thymic microenvironmental cells.

Increasing evidence reveals that extracellular matrix components can be regarded as a group of mediators in intrathymic T-cell migration and/or differentiation. Yet, little is known about the expression and putative function of one particular extracellular matrix protein, namely, tenascin in the thymus. Herein we investigated, by means of immunocytochemistry, tenascin expression in normal infant and fetal human thymuses, as well as in cultures of thymic microenvironmental cells. In situ, tenascin distribution is restricted to the medulla and cortico-medullary regions of normal thymuses. This pattern thus differed from that of fibronectin, laminin and type IV collagen, in which subseptal basement membranes were strongly labeled. Interestingly, tenascin did not co-localize with the cytokeratin-defined thymic epithelial cell network. This was in keeping with the in vitro data showing that tenascin-bearing cells were nonepithelial (and probably nonfibroblastic) microenvironmental elements. Studies with fetal thymuses revealed a developmentally regulated expression of tenascin, with a faint but consistent network labeling, in thymic rudiments as early as 12 weeks of gestational age, that progressed to a strong TN expression at 18 weeks of fetal development, which was similar to the distribution pattern observed thereafter, including postnatally. Our results clearly indicated that tenascin is constitutively expressed in the human thymus, since early stages of thymic ontogeny, and suggest that the cell type responsible for its secretion is a nonepithelial microenvironmental cell.