Synergistic toxicity of IFN-gamma-producing Escherichia coli K12 cells.

Genetically modified microorganisms (GMMs) are frequently used as producers of mammalian immunomodulatory proteins, e.g. interferons and interleukins. Here we have examined the question of whether such GMMs interact in a way different from that of their non-modified parent micro-organisms with mammalian antimicrobial defence systems. As a typical GMM host micro-organism we used Escherichia coli K12, and as a typical immunomodulatory protein produced by a GMM we used mouse interferon-gamma (MuIFN-gamma). Two experimental systems are described in which synergistic "toxic" biological effects are induced by a combined treatment with E. coli and MuIFN-gamma but not, or less so, by the parental strain and the recombinant protein separately. First, it is shown that the IFN-gamma-producing GMM, or mixtures of E. coli cells and IFN-gamma, are cytolytic for mouse embryo fibroblastoid cells (MEF), whereas no cell killing occurs in MEF cultures treated with control E. coli cells or in those treated with bacteria-free recombinant IFN-gamma. Second, it is demonstrated that intraperitoneal injection in mice of high but not low numbers of control E. coli K12 cells induces a shock-like mortality, whereas co-injection with IFN-gamma induces killing at low numbers. IFN-gamma-producing E. coli cells cause a mortality rate that does not differ from that of control E. coli cells, probably because in these experimental conditions the level of recombinant MuIFN-gamma per cell is insufficiently high. Taken together, these data indicate that synergistic toxic effects induced by bacteria and their recombinant products can occur and may in certain situations enhance the intrinsic toxic capacity of the GMM. Synergistic toxic effects may thus be of relevance for identifying the safety level that should be employed when working with GMMs.