Infection with Leishmania, an obligate intracellular parasite of mononuclear phagocytes, stimulates the production of IFN-gamma from NK cells, via a pathway which is dependent upon IL-12 and IL-2. IL-12 is also essential for the development of host protective T cell responses to this parasite. However, previous in vitro studies have indicated that macrophages fail to make IL-12 following infection with Leishmania, and that subsequent to infection, macrophages become refractory to normal IL-12-inducing stimuli. We have used an in situ approach to attempt to resolve this apparent paradox, and by immunostaining for IL-12 p40 protein, we now demonstrate for the first time, that dendritic cells (DC) are the critical source of early IL-12 production following Leishmania infection. IL-12 production by DC is transient, peaking at 1 day post infection and returning to the levels seen in uninfected mice by day 3. Although resident tissue macrophages fail to produce IL-12 after Leishmania infection, these cells are not totally refractory to cytokine inducing stimuli, as TNF-alpha production is induced by day 3 post infection. Not only do these data satisfactorily explain the differences between in vivo and in vitro data by identifying the cellular source of IL-12, but they also suggest a novel model for NK cell activation; namely that in response to pathogens which fail to trigger IL-12 production by macrophages, DC-T cell clusters provide the microenvironment for initial NK cell activation.

Resolution of Leishmania infection is T cell-dependent, and B lymphocytes have been considered to play a minimal role in host defense. In this study, the contribution of B lymphocytes to the response against Leishmania donovani was investigated using genetically modified IgM transmembrane domain (muMT) mutant mice, which lack mature B lymphocytes. When compared with wild-type mice, muMT mice cleared parasites more rapidly from the liver, and infection failed to establish in the spleen. The rapid clearance of parasites in muMT mice was associated with accelerated and more extensive hepatic granuloma formation compared with wild-type mice. However, the liver of infected muMT mice also showed signs of destructive pathology, associated with the presence of increased numbers of neutrophils. The role of neutrophils in controlling parasite growth in the viscera was determined by depletion with the mAb RB6-8C5. This treatment led to a dramatic enhancement of parasite growth in both the liver and spleen of muMT and wild-type mice. As assessed by transfer of both normal and chronic-infection serum, Ig protects microMT mice from destructive hepatic pathology, but minimally alters their resistance compared with wild-type mice. However, adoptive transfer of CD4+ and CD8+ T cells into recombinase activating gene 1 (RAG1-/-) recipients, suggested that T cell function was not altered by maturation in a B cell-deficient environment. Taken together, these data suggest an inhibitory role for B lymphocytes in resistance to L. donovani unrelated to the presence or absence of Ig. However, Ig protects muMT mice from the exaggerated pathology that occurs during infection.

The development of secondary lymphoid organs is a highly regulated process, mediated by tumor necrosis factor (TNF) family cytokines. In contrast, the mechanisms controlling changes in lymphoid architecture that occur during infectious disease are poorly understood. Here we demonstrate that during infection with Leishmania donovani, the marginal zone of mice undergoes extensive remodeling, similar in extent to developmental abnormalities in mice lacking some TNF family cytokines. This process is selective, comprising a dramatic and rapid loss of marginal zone macrophages (MZMs). As a functional consequence, lymphocyte traffic into the white pulp is impaired during chronic leishmaniasis. Significantly, MZMs were preserved in L. donovani-infected B6.TNF-alpha(-/-) mice or mice that received anti-TNF-alpha antibodies, whereas studies in CD8(+) T-cell-deficient mice and in mice lacking functional CD95L, excluded a direct role for either cytotoxic T lymphocyte activity or CD95-mediated apoptosis in this process. Loss of MZMs was independent of parasite burden, yet could be partially prevented by chemotherapy, which in turn reduced endogenous TNF-alpha levels. This is the first report of an infectious agent causing selective and long-lasting changes to the marginal zone via TNF-alpha-mediated mechanisms, and illustrates that those cytokines involved in establishing lymphoid architecture during development, may also play a role in infection-induced lymphoid tissue remodeling.

No abstract

Cerebral malaria (CM) is a serious complication of Plasmodium falciparum infection, causing significant morbidity and mortality among young children and nonimmune adults in the developing world. Although previous work on experimental CM has identified T cells as key mediators of pathology, the APCs and subsets therein required to initiate immunopathology remain unknown. In this study, we show that conventional dendritic cells but not plasmacytoid dendritic cells are required for the induction of malaria parasite-specific CD4+ T cell responses and subsequent experimental CM. These data have important implications for the development of malaria vaccines and the therapeutic management of CM.