cDNA clones from autocrine thymic lymphoma cells encode two mitogenic proteins, a serine protease and a truncated T-cell receptor beta-chain.

Cell lines derived from primary X-ray induced T cell lymphomas (PXTL) of C57BL/6 mice secrete into the medium factor(s) required for their growth. These autocrine factor(s) are distinct from previously described growth factors. cDNA cloning experiments were performed in an attempt to identify these autocrine factor(s). cDNA clones were selected by mRNA size, differential expression, and mitogenic activity of their translation products (Xenopus expression system) on PXTL cells. Two different cDNA clones yielded distinct mitogenic proteins. One clone encodes an altered form of the T cell receptor beta-chain which is truncated at the N-terminus to amino acid 49 of the constant region beta 2. The second clone encodes a serine protease which is identical to factor H or granzyme A from cytotoxic T cells. The 5' portion of the cDNA encoding the serine protease derived from PXTL cells differs from that derived from cytotoxic T cells. This difference results in distinct signal peptides. Unlike cytotoxic T cells, PXTL cells do not store the serine protease intracellularly but secrete it.