From cells to signaling cascades: manipulation of innate immunity by Toxoplasma gondii.

The intracellular opportunistic protozoan Toxoplasma gondii is a potent stimulus for cell-mediated immunity, and IL-12-dependent IFN-gamma induction is vital in resistance to the parasite. Dendritic cells, neutrophils and macrophages are important sources of IL-12 during infection. T. gondii possesses two mechanisms for triggering IL-12. One is dependent upon the common adaptor protein MyD88, and is likely to involve Toll-like receptors. The other is a more unusual pathway that involves triggering through CCR5 by a parasite cyclophilin molecule. Countering these potent pro-inflammatory activities, T. gondii has several mechanisms to down-regulate immunity. Intracellular infection causes a blockade in the NFkappaB macrophage signaling pathway, correlating with reduced capacity for IL-12 and TNF-alpha production. The parasite also prevents STAT1 activity, resulting in decreased levels of IFN-gamma-stimulated MHC surface antigen expression. Furthermore, infection also induces resistance to apoptosis through inhibition of caspase activity. Extracellular pathways of suppression involve soluble mediators such as IL-10 and lipoxins that have potent IL-12 down-regulatory effects. The balance of pro-inflammatory and anti-inflammatory signaling which T. gondii engages is likely dictated by requirements for a stable host-parasite interaction. First, there is a need for Toxoplasma to induce an immune response robust enough to allow host survival and establish long-term chronic infection. Second, the parasite must avoid immune-elimination and induction of pro-inflammatory pathology that can cause lethality if unchecked. The widespread distribution of T. gondii and the normally innocuous nature of infection indicate the skill with which the parasite achieves the two seemingly contrary goals.