Burkholderia pseudomallei is the causative agent for melioidosis. For many bacterial infections, cytokine dysregulation is one of the contributing factors to the severe clinical outcomes in the susceptible hosts. The C57BL/6 and BALB/c mice have been established as a differential model of susceptibility in murine melioidosis. In this study, we compared the innate IFN-γ response to B. pseudomallei between the C57BL/6 and BALB/c splenocytes and characterized the hyperproduction of IFN-γ in the relatively susceptible BALB/c mice in vitro.

Naïve BALB/c splenocytes were found to produce more IFN-γ in response to live bacterial infection compared to C57BL/6 splenocytes. Natural killer cells were found to be the major producers of IFN-γ, while T cells and Gr-1^intermediate cells also contributed to the IFN-γ response. Although anti-Gr-1 depletion substantially reduced the IFN-γ response, this was not due to the contribution of Gr-1^high, Ly-6G expressing neutrophils. We found no differences in the cell types making IFN-γ between BALB/c and C57BL/6 splenocytes. Although IL-12 is essential for the IFN-γ response, BALB/c and C57BL/6 splenocytes made similar amounts of IL-12 after infection. However, BALB/c splenocytes produced higher proinflammatory cytokines such as IL-1β, TNF-α, IL-6, IL-18 than C57BL/6 splenocytes after infection with B. pseudomallei.

Higher percentages of Gr-1 expressing NK and T cells, poorer ability in controlling bacteria growth, and higher IL-18 could be the factors contributing to IFN-γ hyperproduction in BALB/c mice.