A naturally occurring complement dependent human IgM macroglobulin inhibitory to Ehrlich ascites tumor cells

Human serum and the sera of a number of other mammalian species contain a naturally occurring macroglobulin with antibody-like properties which, in the presence of complement, is cytotoxic to murine Ehrlich ascites tumor cells in vitro. The serum of guinea pigs, rats and mice lack this macroglobulin activity but serve well as complement sources. The cytotoxic activity was demonstrated in short-term tissue cultures of Ehrlich ascites tumor cells by the marked and immediate inhibition of nucleic acid and protein synthesis. The cytotoxic activity is independent of the lytic antibody system which reacts against neuraminidase-treated lymphocytes.