A thymic epithelial cell line, IT-45R1, induces the differentiation of prethymic progenitor cells into postthymic cells through direct contact.

Differentiation of T lymphocyte in rat bone marrow was studied in vitro using the IT-45R1 line, which is an established pure epithelial cell line from normal rat thymus. Among the five fractions of rat bone marrow fractionated by bovine serum albumin (BSA) density gradient, two types of T-lymphocyte progenitors were separated; fraction 5 cells which are regarded as prethymic T-lymphocyte progenitors, and fraction 4 cells which are considered as postthymic. After the incubation with the culture supernatant of IT-45R1 (STEL), 6 to 7% of fraction 5 cells were killed with anti-thy 1.1 serum and complement., while about 3% of fraction 4 cells were induced to form rosettes with guinea pig erythrocytes. By STEL alone, no increase was encountered in the ratio of rosette-forming cells (RFC) in fraction 5 and in the cytotoxic indices in fraction 4. On the monolayer of IT-45R1 together with STEL, however, a small but significant number of fraction 5 cells were induced to form rosettes, indicating that IT-45R1 cells converted a part of prethymic fraction 5 cells into postthymic RFC. Thus, in addition to the effect of STEL, IT-45R1 cells were proved to have a distinct effect through direct contact upon the differentiation of T lymphocytes. This contact is considered essential for the differentiation of prethymic progenitors into postthymic cells.