Replication of Toxoplasma gondii, but not Trypanosoma cruzi, is regulated in human fibroblasts activated with gamma interferon: Requirement of a functional JAK/STAT pathway

To study the role of tryptophan degradation by indoleamine 2,3-dioxygenase (INDO) in the control of Trypanosoma cruzi or Toxoplasma gondii replication , we used human fibroblasts and a fibrosarcoma cell line (2C4), The cells w ere cultured in the presence or absence of recombinant gamma interferon (rI FN-gamma) and/or recombinant tumor necrosis factor alpha (rTNF-alpha) for 2 4 h and were then infected with either T. cruzi or T. gondii, Intracellular parasite replication was evaluated 24 or 48 h after infection. Treatment w ith rIFN-gamma and/or rTNF-alpha had no inhibitory effect on T. cruzi repli cation. In contrast, 54, 73, or 30% inhibition of T. gondii replication was observed in the cells treated with rIFN-gamma alone, rIFN-y plus rTNF-alph a, or TNF-alpha alone, respectively. The replication of T. gondii tachyzoit es in cytokine-activated cells was restored by the addition of extra trypto phan to the culture medium. Similarly, T. gondii tachyzoites transfected wi th bacterial tryptophan synthase were not sensitive to the microbiostatic e ffect of rIFN-gamma. We also investigated the basis of the cytokine effect on parasite replication by using the three mutant cell lines B3, B9, and BI O derived from 2C4 and expressing defective STAT1 alpha (signal transducer and activator of transcription), JAK2 (Janus family of cytoplasmic tyrosine kinases), or JAK1, respectively, three important elements of a signaling p athway triggered by rIFN-gamma, We found that rTNF-alpha was able to induce low levels expression of INDO mRNA in the parental cell line, as well as t he cell line lacking functional JAK2, In contrast to the parental cell line (2C4), rIFN-gamma was not able to induce the expression of INDO mRNA or mi crobiostatic activity in any of the mutant cell lines. These findings indic ate the essential requirement of the JAK/STAT pathway for the induction of high levels of INDO mRNA, tryptophan degradation, and the anti-Toxoplasma a ctivity inside human nonprofessional phagocytic cells.