Roles of the small intestine for induction of toll-like receptor 4-mediated innate resistance in naturally acquired murine toxoplasmosis

Peroral infection of Toxoplasma gondii is thought to reflect the typical infection route of naturally acquired toxoplasmosis in humans. We have investigated possible differential roles of toll-like receptor 2 (TLR2) and TLR4 in host defense against naturally acquired murine toxoplasmosis. After peroral inoculation of T. gondii ME49 cysts, TLR4-deficient C3H/HeJ mice were more susceptible to infection than wild-type (WT) C3H/HeN mice, as shown by increased cyst number and low production of cytokines, which are the key factors in protective immunity. When mice were inoculated by intra-peritoneal inoculation of T. gondii, there were no significant differences in the number of brain cysts and cytokine productions between C3H/HeJ and C3H/HeN mice. Histopathologic examination revealed severe inflammation in the small intestine of C3H/HeJ (TLR4-deficient) mice, while an increased number of TLR4-positive mononuclear cells was found in C3H/HeN (WT) mice. To confirm these phenomena, TLR2−/− or TLR4−/− mice were infected perorally with T. gondii cysts. TLR4−/− mice were more susceptible to infection compared with TLR2−/− and C57BL/6 mice. Nuclear factor-kappa B activation through TLR4 agonistic activity of T. gondii ME49 was demonstrated by luciferase assay using stably expressing mouse (m) TLR2 or mTLR4/mMD-2 transfectants. We demonstrate here for the first time that innate immune recognition by TLR4 is involved in protective mechanisms against peroral infection with T. gondii ME49. These results suggest that the small intestine plays an important role in the induction of innate immunity in naturally acquired toxoplasmosis.