ATTENUATION OF GAMMA-INTERFERON-INDUCED TYROSINE PHOSPHORYLATION IN MONONUCLEAR PHAGOCYTES INFECTED WITH LEISHMANIA-DONOVANI - SELECTIVE-INHIBITION OF SIGNALING THROUGH JANUS KINASES AND STAT1

The induction of gene transcription in response to gamma interferon is impaired in mononuclear phagocytes infected with Leishmania donovani, and the mechanisms involved are not fully understood. The changes in gene expression brought about by gamma interferon are thought to invol ve transient increases in the activities of cellular protein tyrosine kinases, including the Janus kinases Jak1 and Jak2, leading to tyrosin e phosphorylation of the transcription factor Stat1. To investigate th e mechanisms accounting for the impaired responses to gamma interferon , a model system for examining overall changes in protein tyrosine pho sphorylation, activation of Jak1 and Jak2, and phosphorylation of Stat 1 was developed in phorbol 12-myristate 13-acetate-differentiated U-93 7 cells. Analysis of whole-cell lysates by antiphosphotyrosine immunob lotting showed that incubation with gamma interferon brought about spe cific increases in phosphotyrosine labeling of several proteins. Incre ased labeling of these proteins occurred to similar extents in control cells and in cells that had been infected with L. donovani for 16 h. Jak1, Jak2, and Stat1 were immunoprecipitated from control and interfe ron-treated cells, and tyrosine phosphorylation of these proteins, det ected by antiphosphotyrosine immunoblotting, was used to measure their activation. Tyrosine phosphorylation of Jak1, Jak2, and Stat1 increas ed markedly, in a dose-dependent manner, in U-937 cells incubated with gamma interferon. In contrast, in cells infected with L. donovani, ty rosine phosphorylation of Jak1, Jak2, and Stat1 was markedly impaired. This effect was dependent upon the duration of exposure to L. donovan i and was maximal and complete at 16 h. Results similar to those obser ved with U-937 cells were also obtained with human peripheral blood mo nocytes. These findings indicate that infection of human mononuclear p hagocytes with L. donovani leads to impaired gamma interferon-mediated tyrosine phosphorylation and selective effects on the Jak-Stat1 pathw ay. Unresponsiveness to gamma interferon for activation of this pathwa y may explain impaired transcriptional responses in leishmania-infecte d cells.