Serum lysozyme as a marker of host resistance. I. Production by macrophages resident in rat sarcomata.

With progressive growth of syngeneic sarcomata in rats there was a rise in serum levels of lysozyme which correlated with their immunogenicity and their macrophage content. By an examination of lymph/blood differences in normal and in tumour bearing rats and of the production of lysozyme by cells obtained from the tumours and maintained in vitro, it is apparent that the macrophages resident in a tumour mass make a massive contribution to the elevation in serum lysozyme concentrations. Tumour cells did not release detectable lysozyme activity. Tumour amputation led to a rapid fall in lysozyme levels. Irradiation of the host rats abolished the lysozyme response and the subsequent development of metastases in these rats was associated with a rise in serum lysozyme. The serum concentration of this enzyme reflects the macrophage content of a tumour mass and the draining lymph nodes. We conclude that under well defined conditions serum lysozyme activity may be a useful marker of macrophage mediated host responses to a tumour.