Acute and lethal ileitis can be elicited in certain strains of inbred mice after oral infection with the intracellular protozoan parasite Toxoplasma gondii. The development of this inflammatory process is dependent upon the induction of a robust Th1 response, including overproduction of IFN-γ, TNF-α, and NO, as has been reported in other experimental models of human inflammatory bowel disease. In this study we have investigated the role of CD4+ T cells from the lamina propria (LP) in the early inflammatory events after T. gondii infection using isolated and primary cultured intestinal cells from infected mice and immortalized mouse mICcl2 intestinal epithelial cells. Primed LP CD4+ T cells isolated from parasite-infected mice produce substantial quantities of both IFN-γ and TNF-α. IFN-γ- and TNF-α-producing LP CD4+ T cells synergize with infected mICcl2 and enhance the production of several inflammatory chemokines including macrophage-inflammatory protein-2, monocyte chemoattractant protein-1, monocyte chemoattractant protein-3, macrophage-inflammatory protein-1αβ, and IFN-γ-inducible protein-10. Furthermore, primed LP CD4+ T cells cocultured with infected mICcl2 inhibited replication of the parasite in the intestinal epithelial cells. Thus, LP CD4+ T cells can interact with parasite-infected intestinal epithelial cells and alter the expression of several proinflammatory products that have been associated with the development of intestinal inflammation. The interaction between these two components of the gut mucosal compartment (CD4+ T cells and enterocytes) may play a role in the immunopathogenesis of this pathogen-driven experimental inflammatory bowel disease model.

In its vertebrate host, Leishmania encounters cells that express TLRs. Using genetically resistant C57BL/6 mice deficient in either TLR2, 4, or 9, we show in this study that only TLR9-deficient mice are more susceptible to infection with Leishmania major. TLR9-deficient mice resolved their lesions and controlled parasites growth with much lower efficiency than wild-type C57BL/6 mice. The absence of TLR9 also transiently inhibited the development of curative Th1 response. In an attempt to analyze the possible basis for such aberrant response in TLR9(-/-) mice, we have studied the importance of TLR9 for the activation of dendritic cells (DCs) by L. major. Results show that DCs in the draining lymph nodes are activated following infection with L. major. Furthermore, bone marrow-derived DCs as well as DCs freshly isolated from the spleen of C57BL/6 mice can be activated by either heat-killed or live L. major in vitro. In sharp contrast, L. major failed to activate DCs from TLR9(-/-) mice. Noteworthily, activation of DCs was abolished either following treatment of the parasites with DNase or after acidification of the endosomal compartment of DCs by chloroquine, pinpointing the DNA of L. major as the possible ligand of TLR9 leading to the activation of DCs. Results showed that DNA purified from L. major was indeed capable of activating DCs in a strictly TLR9-dependent manner. Moreover we showed that the L. major DNA-induced TLR9 signaling in DCs condition these cells to promote IFN-gamma production by CD4(+) T cells.

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The role of specific microbial Ags in the induction of experimental inflammatory bowel disease is poorly understood. Oral infection of susceptible C57BL/6 mice with Toxoplasma gondii results in a lethal ileitis within 7–9 days postinfection. An immunodominant Ag of T. gondii (surface Ag 1 (SAG1)) that induces a robust B and T cell-specific response has been identified and a SAG1-deficient parasite (Δsag1) engineered. We investigated the ability of Δsag1 parasite to induce a lethal intestinal inflammatory response in susceptible mice. C57BL/6 mice orally infected with Δsag1 parasites failed to develop ileitis. In vitro, the mutant parasites replicate in both enterocytes and dendritic cells. In vivo, infection with the mutant parasites was associated with a decrease in the chemokine and cytokine production within several compartments of the gut-associated cell population. RAG-deficient (RAG1−/−) mice are resistant to the development of the ileitis after T. gondii infection. Adoptive transfer of Ag-specific CD4+ effector T lymphocytes isolated from C57BL/6-infected mice into RAG−/− mice conferred susceptibility to the development of the intestinal disease. In contrast, CD4+ effector T lymphocytes from mice infected with the mutant Δsag1 strain failed to transfer the pathology. In addition, resistant mice (BALB/c) that fail to develop ileitis following oral infection with T. gondii were rendered susceptible following intranasal presensitization with the SAG1 protein. This process was associated with a shift toward a Th1 response. These findings demonstrate that a single Ag (SAG1) of T. gondii can elicit a lethal inflammatory process in this experimental model of pathogen-driven ileitis.