The induction of interferon production in fibroblasts by invasive bacteria: a comparison of Salmonella and Shigella species.

As the role of interferon (IFN) in host defense against facultative intracellular bacterial infections continues to expand, it has become increasingly important to understand what cell types can produce IFN following infection and/or interaction with these invasive bacteria. We have demonstrated previously that Shigella flexneri was able to induce high levels of IFN in primary cultures of human and murine fibroblasts following bacterial invasion. In this study, we examined the ability of Salmonella typhimurium to induce IFN production in different cell lines. S. typhimurium-infected primary cell cultures of mouse embryo-fibroblasts (MEF) were shown to produce high levels of IFN following bacterial challenge. In contrast to Shigella, Salmonella required a much lower multiplicity of infection for optimal IFN induction. Examination at the RNA level of IFN production by MEF following challenge with either bacteria revealed that the IFN produced was a mixture of IFN alpha and IFN beta (IFN alpha/beta), with IFN beta 1 as the predominant species. As previously demonstrated for Shigella, bacterial invasion of cells appeared to be required for the induction of IFN production by S. typhimurium. Salmonella rendered non-invasive by UV-treatment failed to induce IFN production in MEF. Furthermore, Salmonella LPS, when tested over a wide range of concentrations, was unable to induce IFN production in these cells. In contrast to MEF, human and murine continuous cell lines did not produce IFN following Salmonella challenge. These results taken together suggest that IFN may be a common factor involved in Salmonella and Shigella infections. Furthermore, IFN may play an important role in the front line host defense against these types of infections.