A potential Toxoplasma gondii lipoxygenase is necessary for virulence and associated with the host immune response

Abstract

While the asexual cycle of Toxoplasma gondii can occur in any warm-blooded animal, the sexual cycle is restricted to the feline intestine. We previously determined that because cats lack delta-6-desaturase activity in their intestines, they build up excess linoleic acid, which signals T. gondii to undergo sexual development. We hypothesized that T. gondii oxygenates linoleic acid to signal sexual development, so we examined the T. gondii genome for potential lipoxygenases (TgLOX) enzymes. We identified seven potential TgLOXs that were at least 100-fold more abundant in the cat intestinal versus the tissue culture tachyzoite stage. Parasites deleted in TgLOX1 (Tg{Delta}LOX1) had no significant growth differences in tissue culture fibroblast cells. Because the sexual development assay begins with brain cysts, we infected mice with Tg{Delta}LOX1 and were surprised to find that Tg{Delta}LOX1 had reduced virulence. The Tg{Delta}LOX1 parasitemia was reduced by 3 days postinfection and largely cleared by 7 days postinfection. At 3 days postinfection, the cytokines IFN{gamma}, IL-6, MCP-1, and TNF- were significantly reduced in Tg{Delta}LOX1-infected mice, which prompted us to examine Tg{Delta}LOX1 in IFN{gamma}KO mice. We found that IFN{gamma}KO mice infected with Tg{Delta}LOX1 succumbed to acute infection with the same kinetics as the parental and complemented strains, suggesting the role of TgLOX1 in mice was IFN{gamma} dependent. In tissue culture fibroblasts, TgLOX1 was localized within the parasite, but in leukocytes from infected mice and activated macrophages, TgLOX1 was localized in vesicular structures in the host cytoplasm. These results suggest that TgLOX1 in these vesicular structures modifies the host immune response. ImportanceLipoxygenases are enzymes that catalyze the dioxygenation of polyunsaturated fatty acids such as linoleic and arachidonic acid. These modifications create signaling molecules that are best characterized for modulating the immune response. Deletion of the first lipoxygenase characterized for Toxoplasma gondii (TgLOX1) generated a less virulent strain and infected mice showed a decreased immune response. This virulence defect was dependent on the mouse cytokine IFN{gamma}. TgLOX1 changes location from inside the parasite in tissue culture conditions to vesicular structures within the host immune cells during mouse infection. These results suggest that TgLOX1 plays a role in the modification of the host immune response in mice.