Transfected Leishmania expressing biologically active IFN-gamma.

Infection of susceptible BALB/c mice with Leishmania major leads to progressive infection with the failure to expand and activate Th1 CD4+ T cells that elaborate IFN-gamma, a critically implicated cytokine for control of disease. We used the recently described capacity to express foreign genes in trypanosomatids to introduce into Leishmania the murine IFN-gamma gene on a drug-selectable plasmid under the constitutive control of intergenic tubulin sequences. Several clones of L. major were established and demonstrated to contain IFN-gamma DNA and IFN-gamma RNA that was appropriately trans-spliced with the Leishmania-specific leader sequence, and to secrete IFN-gamma into the media. The secreted IFN-gamma was biologically active as assessed by up-regulation of class II MHC Ag and induction of macrophage nitric oxide synthase activity in a macrophage cell line. Infection of nude mice with IFN-gamma-containing organisms resulted in significantly slower progression of disease as compared to infection with organisms containing the empty plasmid, suggesting that biologically important activation of infected macrophages might be occurring in vivo. Infection of genetically susceptible BALB/c mice, however, did not impede the expansion of Th2 cells and the inexorable progression of disease. Despite the demonstration of increased levels of IFN-gamma transcription in vivo, induction of nitric oxide synthase in macrophages and expression of Ly-6, and IFN-gamma-inducible Ag, on CD4+ lymphocytes could not be shown. In all cases, organisms recovered from tissue amastigotes contained the IFN-gamma plasmid and secreted active IFN-gamma. The data confirm earlier studies that IFN-gamma alone is not sufficient to impede activation and maturation of Th2 cells in susceptible mice, even when targeted directly to the infected cell.