Comparison of macrophage responses between high- and low-virulent strain Edwardsiella tarda infections in mouse macrophage RAW264.7 cells: interleukin-1β secretion as a key macrophage response to a high-virulent strain E. tarda.

Edwardsiella tarda, a major facultative intracellular pathogen affecting aquaculture, can cause hemorrhagic septicemia and hemolytic ascites disease in aquaculture fish. It is characterized by the ability to survive and replicate in macrophages. In this study, mouse macrophage cell line RAW264.7 cells were used as a model system to study macrophage responses toward E. tarda in vitro. We found that the high-virulent strain NUF251 can survive and multiply in RAW264.7 cells, but the low-virulent strain NUF194 has no such ability. These results were consistent with our previous study conducted in fish macrophages. Both strains induced an increase in intracellular reactive oxygen species and secretion of nitric oxide and tumor necrosis factor-α in RAW264.7 cells, but significantly higher levels of these mediators were induced by NUF194. In addition, DNA fragmentation concomitant with slight activation of caspase-3 was observed in NUF194-infected RAW264.7 cells, suggesting the occurrence of apoptosis, whereas no such clear typical apoptosis symptoms were detected in NUF251-infected RAW264.7 cells. Interestingly, a much higher level of interleukin (IL)-1β was secreted from NUF251-infected RAW264.7 cells as compared to those of NUF194-infected or LPS-treated cells, especially at 3 h post-infection, even though similar levels of IL-1β mRNA were detected in all these cells. Since the secretion of IL-1β is a characteristic feature of pyroptosis, our results suggest that induction of pyroptosis accompanied by IL-1β secretion may be a specific macrophage response linked with the virulence of E. tarda infection.