Elucidating the Role of Macrophages in Visceral Leishmaniasis
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Series/Report no.:2007 Edward F. Hayes Graduate Research Forum. 21st
Background: Leishmaniasis is a tropical disease caused by protozoan parasite species belonging to the genus Leishmania. Depending on the infective species, leishmaniasis can manifest as one of three major disease states – cutaneous, mucocutaneous or visceral. L. donovani causes visceral disease with tropism for reticuloendothelial organs. Left untreated, L. donovani infections are fatal, often due to secondary bacterial infections and complications resulting from profound hepatosplenomegaly. Leishmania species parasitize host cell macrophages, the cell type that is ultimately responsible for killing parasites under correct immune conditions. Experimental mouse models that mount type I CD4+ T cell (TH1) responses are capable of activating macrophage populations to effectively kill parasites and exhibit a healing response. However, mice that mount type II CD4+ T cell (TH2) responses are unable to effectively activate macrophage populations and experience chronic, non-healing disease. Purpose: Recently our laboratory found that two important transcription factors involved in TH1 cell signaling, STAT-1 and T-bet, unexpectedly play distinct roles in determining the outcome of visceral leishmaniais caused by L. donovani. We showed that STAT-1 knock-out (KO) mice, which are highly resistant to L. donovani infection, recruit fewer macrophages into their livers following infection compared to susceptible wild-type and T-bet KO mice. This result led us to study the effect of treating mice with clodronate, a drug that selectively depletes macrophages, both pre- and post-infection with L. donovani. The purpose of this study was to determine the effect of macrophage depletion in mice infected with L. donovani. Research Methods: Mice were treated with clodronate 2 days prior to infection, or at either 1 day or 6 days post-infection. At timepoints ranging from day 7 to day 22 post-infection, livers and spleens of mice were harvested. At each timepoint, impression smears of the livers were taken to determine parasite burdens, sections of livers were prepared for immunohistological analysis, and liver lymphocytes were isolated and analyzed via flow cytometry. In addition, splenocytes were isolated, stimulated with soluble L donovani antigen, and cytokine production was determined using ELISA. Findings: Mice treated with clodronate both pre- and post- infection showed markedly reduced parasite burdens compared to PBS treated controls. In addition, mice that were treated with clodronate both pre-infection and one day post-infection showed little or no immunopathology compared with PBS controls. However, mice treated 6 days post-infection showed immunopathology similar to PBS controls, despite having significantly reduced parasite burdens. In addition, mice treated pre-infection had reduced levels of all cytokines measured (IL-4, IL-10, IL-12, and IFN-gamma) compared to PBS treated controls, whereas mice treated post-infection had reduced levels of all cytokines except IL-4 when compared to PBS treated controls. Implications: This research indicates that the depletion of macrophages at early timepoints post- L. donovani infection is able to reduce both parasite burdens and immunopathology associated with disease. This research may serve as a foundation in determining whether macrophage selective-drugs represent viable treatment options for human diseases that chronically infect host cell macrophages, such as leishmaniasis and tuberculosis.
Poster: 1st Place (The Ohio State University Edward F. Hayes Graduate Research Forum)
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