Effects of recombinant murine (rm) interleukin-12 and rm interferon-gamma in mice infected with Bordetella pertussis

High dose interleukin-12 exacerbates Bordetella pertussis infection and is associated with suppression of cell-mediated immunity in a murine aerosol challenge model

The in-vivo clearance of Bordetella pertussis infections in murine models in naive mice and animals vaccinated with whole-cell vaccine is considered to be via a Th-1-dependent mechanism in which interleukin-12 (IL)-12 may play a prominent role. It has also been demonstrated clearly that the treatment of animals with macrophage-derived IL-12 administered with an acellular vaccine can increase the efficacy of this vaccine preparation to levels seen with the whole-cell vaccine. However, the effects of exogenously added IL-12 on immune responses in non-vaccinated B. pertussis-challenged mice remain unclear, with two studies giving contradictory findings. In this study we have treated mice with escalating doses of mIL-12 (0.1-10 microg/mouse) prior to challenge with B. pertussis (using an aerosol challenge model of infection). The ability of mice to clear infection was assessed in IL-12 treated and in phosphate buffered saline (PBS) control animals at days 6 and 13 post-challenge. Lymphoid cells were isolated from spleen and cell-mediated immune responses assessed at days 1, 6 and 13 post-challenge. In addition, the direct effects of high-dose IL-12 on challenged mice was assessed by checking natural killer (NK) activity from isolated lung and spleen lymphoid cells as well as interferon-gamma (IFN-gamma) generation from isolated cells and serum at day 1 post-challenge. The results from this study show that bacterial colonization of the lungs is actually enhanced following treatment with high-dose IL-12. This is associated with impaired cellular immune responses. The mechanisms associated with the immunosuppressive effects of IL-12 are discussed.

Interferon-gamma mediated immune effector mechanisms against Bordetella pertussis

The role of IFN-gamma in reducing the intracellular load of Bordetella pertussis in murine macrophages in vitro has been examined. The results demonstrate that exposure to IFN-gamma can reduce bacterial load in viable macrophages and that this is associated with production of nitric oxide (NO). These observations provide a mechanism by which IFN-gamma may mediate its antimicrobial effect and support an important role for activated alveolar macrophages in the elimination of B. pertussis from the respiratory tract. Using intracellular iron chelation, it is shown that intracellular survival of B. pertussis is dependent on iron availability and suggest that iron restriction may be an important mechanism by which IFN-gamma influences bacterial survival within mouse macrophages. It is also shown that IFN-gamma may mediate its effect through NO independent mechanisms and that B. pertussis is sensitive to agents that stimulate the respiratory burst. Finally, it is shown that the concentration of L-tryptophan may be a limiting step in the intracellular survival of B. pertussis and that the induction of tryptophan degrading enzymes may be an additional mechanism through which IFN-gamma exerts its antimicrobial effects against B. pertussis.