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 ...

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We have previously demonstrated that murine macrophages (Mphi) infected with Leishmania promastigotes, in contrast to Mphi infected with the amastigote stage of these parasites, are able to present the Leishmania antigen LACK (Leishmania homologue of receptors for activated C kinase) to specific, I-Ad-restricted T cell hybrids and to the T cell clone 9.1-2. These T cells react with the LACK (158-173) peptide, which is immunodominant in BALB/c mice. Here, we show that the level of stimulation of the LACK-specific T cell hybridoma OD12 by promastigote-infected Mphi is clearly dependent upon the differentiation state of the internalized parasites. Thus, shortly after infection with log-phase or stationary-phase promastigotes of L. major or of L. amazonensis, Mphi strongly activated OD12. The activity was transient and rapidly lost. However, under the same conditions, activation of OD12 by Mphi infected with metacyclic promastigotes of L. major or of L. amazonensis was barely detectable. At the extreme, Mphi infected with amastigotes were incapable to stimulate OD12. Thus, the presentation of LACK by infected Mphi correlates with the degree of virulence of the phagocytosed parasites, the less virulent being the best for the generation/expression of LACK (158-173)-I-Ad complexes. While the intracellular killing of the parasites appears to be an important condition for the presentation of LACK, it is not the only requisite. The partial or total destruction of intracellular L. amazonensis amastigotes does not allow the presentation of LACK to OD12. A preferential interaction of LACK (158-173) with recycling rather than newly synthesized MHC class II molecules does not explain the transient presentation of LACK by Mphi infected with log-phase or stationary-phase promastigotes because brefeldin A strongly inhibited the presentation of LACK to OD12. Taken together, these results suggest that virulent stages of Leishmania, namely metacyclics and amastigotes, have evolved strategies to avoid or minimize their recognition by CD4+ T lymphocytes.

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The establishment of Leishmania in mammals depends on the transformation of metacyclic promastigotes into amastigotes within macrophages. The kinetics of this process was examined using mouse macrophages infected with metacyclic promastigotes of L. amazonensis. The appearance of amastigote characteristics, including large lysosome-like organelles called megasomes, stage-specific antigens, high cysteine protease activity and sensitivity to L-leucine methyl ester, was followed over a 5-day period. Megasomes were observed at 48 h but probable precursors of these organelles were detected at 12h p.i. The promastigote-specific molecules examined were down-regulated within 5 to 12h after phagocytosis whereas the amastigote-specific antigens studied were detectable from 2 to 12-24 h. An increase in the cysteine protease activity and in sensitivity to L-leucine methyl ester of the parasites was detected from 24 h. The data indicate that at 48 h p.i., parasites exhibit several amastigote features but that complete differentiation requires at least 5 days. The appearance of megasomes or of megasome precursors and the rise in cysteine protease activity correlate quite well with the capacity of parasites to internalize and very likely degrade host MHC molecules. The fact that internalization by the parasites of host cell molecules occurs very early during the differentiation process argues for a role of this mechanism in parasite survival.

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