Activation of murine macrophages and a bovine monocyte cell line by bovine lymphokines to kill the intracellular pathogens Eimeria bovis and Toxoplasma gondii

Abstract

Macrophage (MΦ)-activating lymphokines present in concanavalin A-stimulated bovine T-lymphocyte cultures (ConAS) were studied by assessing their effects on Eimeria bovis and Toxoplasma gondii growth in cultured bovine monocytes (BM) and mouse MΦ. The in vitro development of both parasites was assessed by incorporation of [3H]uracil and by microscopic examination of parallel cultures. Incorporation of [3H]uracil into infected cultures was an accurate indicator of growth of both E. bovis and T. gondii in BM and mouse MΦ. Sporozoites of E. bovis underwent merogony in untreated BM but not in mouse MΦ, whereas T. gondii developed in both cell types. Inhibition of T. gondii growth was greatest in ConAS-treated BM, whereas preincubation of mouse MΦ with ConAS resulted in about 80% growth inhibition. There was no significant difference between the inhibition of either T. gondii sporozoite- or tachyzoite-induced growth in ConAS-treated cells, showing that activation pathways are equally effective against both stages. Treatment of ConAS with glycine-hydrochloride buffer (pH 2) resulted in a total loss of antiviral activity mediated by gamma interferon (IFN-γ). When pH 2 dialyzed ConAS was used to activate BM, inhibition of T. gondii growth was only partially affected. Because bovine IFN-γ does not activate mouse MΦ and due to the partial effects of pH 2 on ConAS-induced growth inhibition, the major component(s) of ConAS responsible for T. gondii growth inhibition is distinct from IFN-γ. Furthermore, IFN-γ may act synergistically rather than being part of a priming sequence for MΦ responsiveness to other lymphokines. Murine recombinant granulocyte-macrophage colony-stimulating factor (rGM-CSF) was tested for any microbistatic activity against T. gondii sporozoites and tachyzoites. There was no significant difference in either colony formation or [3H]uracil incorporation between rGM-CSF-treated and control cultures, regardless of host cell type. Thus, rGM-CSF does not induce adequate MΦ activation to kill T. gondii and is not a major microbistatic component of ConAS. rGM-CSF also had no effect on T. gondii infection in vivo.