A model of Leishmania major infection in C57BL/6 mice has been established that combines two main features of natural transmission: low dose (100 metacyclic promastigotes) and inoculation into a dermal site (the ear dermis). The evolution of the dermal lesion could be dissociated into two distinct phases. The initial “silent” phase, lasting 4–5 wk, favored establishment of the peak load of parasites in the dermis in the absence of lesion formation or any overt histopathologic changes in the site. The second phase corresponds to the development of a lesion associated with an acute infiltration of neutrophils, macrophages, and eosinophils into the dermis and was coincident with the killing of parasites in the site. The onset of immunity/pathology was correlated with the appearance of cells staining for IL-12p40 and IFN-γ in the epidermal compartment, and an expansion of T cells capable of producing IFN-γ in the draining lymph node. Parasite growth was not enhanced over the first 4.5 wk in anti-CD4-treated mice, SCID mice, or C57BL/6 mice deficient in IL-12p40, IFN-γ, CD40 ligand, or inducible NO synthase. These mice all failed to ultimately control infection in the site, but in some cases (anti-CD4 treated, IL-12p40−/−, CD40 ligand−/−, and SCID) high dermal parasite loads were associated with little or no pathology. These results extend to a natural infection model a role for Th1 cells in both acquired resistance and lesion formation, and document the remarkable avoidance of this response during a prolonged phase of parasite amplification in the skin.

The protozoan parasite Toxoplasma gondii enters hosts through the intestinal mucosa and colonizes distant tissues such as the brain, where its progeny persists for a lifetime. We investigated the role of CD11c-and CD11b-expressing leukocytes in T gondii transport during the early step of parasitism from the mouse small intestine and during subsequent parasite localization in the brain. Following intragastric inoculation of cystcontaining parasites in mice, CD11c ؉ dendritic cells from the intestinal lamina propria, the Peyer patches, and the mesenteric lymph nodes were parasitized while in the blood, parasites were associated with the CD11c ؊ CD11b ؉ monocytes. Using adoptive transfer experiments, we demonstrated that these parasitized cells triggered a parasitic process in the brain of naive recipient mice. Ex vivo analysis of parasitized leukocytes showed that single tachyzoites remained at the cell periphery, often surrounded by the host cell plasma membrane, but did not divide. Using either a dye that labels circulating leukocytes or an antibody known to prevent CD11b ؉ circulating leukocytes from leaving the microvascular bed lumen, and chimeric mice in which the hematopoietic cells expressed the green fluorescent protein, we established that T gondii zoites hijacked CD11b ؉ leukocytes to reach the brain extravascular space. IntroductionMany microorganisms initiate interactions with vertebrate hosts through digestive mucosa. Some will develop only in these peripheral host tissues, whereas others, including the protozoan parasite Toxoplasma gondii, disseminate throughout the host organism and colonize distant nonmucosal tissues. When hosts, including humans, ingest T gondii-containing cysts or oocysts, free T gondii zoites are released in the gut lumen. They subsequently enter enterocytes, where they multiply and initiate the parasitic process per se. [1][2][3] Enterocytes loaded with zoites secrete chemokines such as monocyte chemotactic protein 1 (MCP-1/CCL-2), macrophage inflammatory protein 1␣, and ␤ (MIP-1␣ and ␤/CCL3 and CCL4), as well as MIP-2/CXCL2, 4,5 which recruit leukocytes in the lamina propria (LP) extravascular space. Parasites then disseminate to several distant tissues, including the brain, a major site supporting T gondii progeny latency. 6,7 Such features have important clinical implications since T gondii can remain, for a lifetime, cryptic as bradyzoite, a slowly replicative intracellular stage under the control of unique host-dependent immune signals. However, upon rupture of this equilibrium, such as the one occurring in HIV-loaded individuals or following tissue transplantation and cancer therapy, bradyzoites can differentiate into tachyzoites, which massively replicate. This process of parasite reactivation is assessed by pathology, especially severe to fatal cerebral toxoplasmosis. 8,9 To date, whether they are acting at the cell lineage or tissue levels, the mechanisms underlying T gondii dissemination and entry into the brain remain largely unknown.T gondii zoites display unique ...