Characterization of a 50 kDa surface membrane protein on thymic stromal cells as an important factor for early T cell development.

We previously reported that the nude mouse-derived splenic T cell clone, N-9F, exhibits a proliferative response to the SL10.3 thymic epithelial cell clone. In the present study we generated an Armenian hamster mAb, HS9, specific for SL10.3, which inhibited the N-9F's proliferative response to SL10.3. We performed thymocyte repopulation experiments using fetal liver cells and 2'-deoxyguanosine-treated thymic rudiments. After 14 days of culture, donor fetal liver cells proliferated and differentiated to CD4+CD8+ and CD4-CD8+ with some CD4+CD8- cells in the host thymic rudiments. However, most of the thymocytes remained at a CD4-CD8- immature stage in the presence of HS9 and the cell recovery was reduced to 30% of the control. Immunohistostaining and flow cytometry studies revealed that HS9 reacted with stromal cells of fetal thymus at the earliest from day 14 gestation. Neither thymocytes nor lymph node T cells were stained with HS9. HS9 antigen was distributed not only on thymic subcapsular and cortical stromal cells, but also on peripheral B cells in adult mice. The antigen that HS9 detected was found to be a 50 kDa surface membrane protein on thymic stromal cells. On the other hand, the 50 kDa molecule is associated with two other molecules of 80 and 100 kDa on the B cells. These data indicate that the HS9 antigen may have an important role for early T cell development, especially at a stage from CD4-CD8- to CD4+CD8+, and may have some unknown function on B cells.