| Summary: | The examination of impression smears stained with Giemsa's stain showed schizonts and piroplasms were present in all lymphoid and non lymphoid organs in calves infected with <I>T. annulata </I>or <I>T. parva. </I>Analysis of tissue sections immunocytochemically labelled with monoclonal antibodies, reported to recognise schizonts, and counterstained with Harris's Haematoxylin detected schizonts within lesions in all calves and distinguished macroschizonts from microschizonts. The phenotypic analysis of schizont infected cells by immunocytochemical labelling of leucocyte surface markers in tissues indicated <I>T. annulata </I>resided within cells from the myeloid lineage which expressed CD11b, the membrane receptor for iC3b. <I>T. parva </I>resided within cells from the T cell lineage which expressed CD3; this parasite was also shown to inhabit CD11b cells which may have been B cells. The phenotypic analysis of uninfected cells in parasitised lymphoid organs in <I>T. annulata </I>or <I>T. parva </I>infected calves showed uninfected CD11b cells were markedly increased in medulla areas whereas uninfected CD3 cells were markedly decreased in thymic dependent areas as compared with lymphoid organs in normal cattle. The examination of parasitised non lymphoid organs showed an infiltration of larger numbers of uninfected CD11b cells and uninfected CD3<SUP>+</SUP> cells as compared with non lymphoid organs in normal cattle. The assessment of apoptosis in a prescapular lymph node draining the site of inoculation in a calf infected with <I>T. annulata </I>(Hisar) with TdT-mediated-dUTP-biotin nick end labelling showed more apoptotic cells were detected in this lymph node than in a prescapular lymph node in a normal calf. The effect of nitric oxide on uninfected and <I>T. annulata </I>infected bovine macroschizont cell lines cultured <I>in vitro </I>was examined because peripheral blood mononuclear cells harvested from calves infected with <I>T. annulata </I>(Hisar) had produced nitric oxide <I>in vitro, </I>nitric oxide had been reported to induce apoptosis under other circumstances and damaged tissues in calves infected with <I>T. annulata </I>had been infiltrated with macrophages, a potential source of nitric oxide. Incubation of uninfected and parasitised bovine macroschizont cell lines with S-nitroso-N-acetyl-DL-penicillamine, a nitric oxide donor, resulted in an increase of apoptosis in uninfected and parasitised bovine macroschizont cell lines and caused malformation and elimination of schizonts as compared with uninfected and parasitised bovine macroschizont cell lines incubated in medium alone. It was postulated that macrophage derived nitric oxide could be detrimental to the host by inducing tissue damage and beneficial to the host by controlling the parasite.
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