Molecular cloning of the bovine thymopoietin gene and its expression in different calf tissues: evidence for a predominant expression in thymocytes

Thymopoietin (TP), a 49 amino acid peptide, is regarded as a thymic hormone, secreted specifically by some epithelial cells in the thymic stroma and exerting a multitude of effects on maturation and function of T lineage cells. As part of our study on the molecular biology of TP, we isolated cDNA clone coding for a bovine TP precursor and used it as a probe to analyze the presence of mRNA coding for TP in different tissues. The cDNA clone reported here is 1.1 kb long and contains an open reading frame (ORF) of 741 bp which corresponds to 247 amino acids. The 147 bp coding for the entire bovine TP are at the 5′ end of the ORF. A DNA fragment coding for amino acids 1 - 42 of bovine TP was used as a probe to look for hybridizable RNA sequences, extracted from various calf tissues, by the S1 nuclease protection method. Our results indicate that the TP gene is expressed predominantly in lymphatic tissues. Lymphatic tissues with the highest levels observed were thymocytes and not thymic stroma. Lower, but still significant, amounts were present in tonsils, neck lymph nodes, and small intestine (probably because of its lymphatic part - the Peyer's patches), whereas cultured thymic stromal cells, spleen tissue and peripheral blood mononuclear cells displayed a low level of TP mRNA. The TP gene expression in all other (non-lymphatic) tissues tested, was weak, barely detectable or virtually absent. However, the cerebellum could be singled out with relatively strong expression of TP mRNA. These results clearly show that, although TP expression (at low level) can be monitored in many tissues, its expression in the immune system is peculiar in both its maximal intensity and its wide range, and that thymocytes, rather than epithelial cells, are the main cells which transcribe the TP gene in the thymus. These findings do not favor the prevalent notion of thymic epithelial cells being the source and T lineage cells being the targets of TP hormonal actions. Accordingly, TP function in the immune system should be reassessed.