Proenzyme Form of Cathepsin L Produced by Thymic Epithelial Cells Promotes Proliferation of Immature Thymocytes in the Presence of IL-1, IL-7, and Anti-CD3 Antibody

A thymic epithelial cell line (TECI-4) established from newborn mouse thymus produced a cofactor to promote proliferation of double-negative (CD4-8-) thymocytes in the presence of IL-1, IL-7, and anti-CD3 antibody. Based upon the sequence of 8 amino acid residues near the N terminal portion, the factor was identified as the mouse proenzyme form of cathepsin L which is almost identical to the rat proenzyme form of cathepsin L (95% in homology). Production of the proenzyme form of cathepsin L was confirmed in the TECI-4 and mouse embryonic thymus by Western blotting analysis and Northern blotting analysis. Furthermore, immunohistologically, the presence of cathepsin L was observed in the epithelial cells of the subcapsular layer and around the small vessels in the thymic cortex. It has been concluded that a subpopulation of thymic cortical epithelial cells produces the proenzyme form of cathepsin L which can act as a cofactor to promote proliferation of DN immature thymocytes.