THYMOCYTE-DIRECTED ENHANCEMENT OF APOPTOSIS VIA SOLUBLE FACTOR(S) DERIVED FROM A CORTICAL AND A MEDULLARY THYMIC EPITHELIAL-CELL LINE

Apoptosis of murine thymocytes was examined either in intact fetal thy mus lobes or in thymus cell suspensions, both cultured alone or in the presence of either a cortical (TEC 1.4) or a medullary (TEC 2.3) thy mic epithelial cell line. Both TECs induced a pronounced increase of a poptosis in 24-h cultivated single thymus cell suspensions but not in spleen or bone mar row cell cultures. Go-culture of thymocytes with mu rine fibroblasts did not enhance apoptosis of the thymus cells, A simi lar enhancement of thymocyte apoptosis was observed with dialysed cult ure supernatants derived from both TEC lines, the active component(s) having a molecular weight of >30 kDa. In contrast, the cortical TEG 1. 4 had a pronounced apoptosis inducing effect on intact fetal thymus lo bes cultivated for six days, whereas the medullary TEC 2.3 had only a marginal influence. TEG 1.4 also induced a significant alteration in t he ratio of CD4(+)CD8(+) to CD4(-)CD8(-) cells. It is concluded that b oth the cortical and medullary epithelial cell lines are able to induc e thymocyte apoptosis but that a large proportion of the cells within the intact thymus stroma is refractory to the respective signal(s) of the medullary epithelial cell line.