Intracellular infection by Leishmania donovani inhibits macrophage apoptosis

The phagocytic macrophage plays a critical role in host immune responses to microbial infection, and represents a major source of inflammatory and growth cytokines. Intramacrophage infection by the protozoan parasite Leishmania donovani results in increased viability of the host cell in the absence of exogenous growth factor. We demonstrate that infection of bone marrow-derived macrophages (BMMs) by L. donovani promastigotes or treatment of BMMs with lipophosphoglycan LPG, the major surface molecule of the promastigote, inhibits apoptosis in the macrophage induced by the removal of macrophage (M)-CSF. This effect was also achieved by supernatants collected from L. donovani-infected macrophages, implicating the elaboration of a soluble factor by infected cells as the mediator of this inhibition. To identify candidate factors, reverse transcription PCR was employed to characterize the mRNA cytokine profile of infected macrophages. L. donovani infection of BMMs was found to induce gene expression for granulocyte-macrophage CSF, TNF-alpha, TGF-beta, and IL-6, but not M-CSF or IL-1 beta. Of the cytokines induced by L. donovani, rTNF-alpha and recombinant granulocyte-macrophage CSF were shown to inhibit apoptosis of BMMs induced by the removal of M-CSF. The amount of these cytokines in L. donovani-infected cell supernatants was quantified by ELISA. The mechanism by which L. donovani may inhibit apoptosis is discussed.