Replication ofToxoplasma gondii, but NotTrypanosoma cruzi, Is Regulated in Human Fibroblasts Activated with Gamma Interferon: Requirement of a Functional JAK/STAT Pathway

Abstract

To study the role of tryptophan degradation by indoleamine 2,3-dioxygenase (INDO) in the control of Trypanosoma cruzior Toxoplasma gondii replication, we used human fibroblasts and a fibrosarcoma cell line (2C4). The cells were cultured in the presence or absence of recombinant gamma interferon (rIFN-γ) and/or recombinant tumor necrosis factor alpha (rTNF-α) for 24 h and were then infected with either T. cruzi or T. gondii. Intracellular parasite replication was evaluated 24 or 48 h after infection. Treatment with rIFN-γ and/or rTNF-α had no inhibitory effect on T. cruzi replication. In contrast, 54, 73, or 30% inhibition of T. gondii replication was observed in the cells treated with rIFN-γ alone, rIFN-γ plus rTNF-α, or TNF-α alone, respectively. The replication of T. gondii tachyzoites in cytokine-activated cells was restored by the addition of extra tryptophan to the culture medium. Similarly,T. gondii tachyzoites transfected with bacterial tryptophan synthase were not sensitive to the microbiostatic effect of rIFN-γ. We also investigated the basis of the cytokine effect on parasite replication by using the three mutant cell lines B3, B9, and B10 derived from 2C4 and expressing defective STAT1α (signal transducer and activator of transcription), JAK2 (Janus family of cytoplasmic tyrosine kinases), or JAK1, respectively, three important elements of a signaling pathway triggered by rIFN-γ. We found that rTNF-α was able to induce low levels expression of INDO mRNA in the parental cell line, as well as the cell line lacking functional JAK2. In contrast to the parental cell line (2C4), rIFN-γ was not able to induce the expression of INDO mRNA or microbiostatic activity in any of the mutant cell lines. These findings indicate the essential requirement of the JAK/STAT pathway for the induction of high levels of INDO mRNA, tryptophan degradation, and the anti-Toxoplasma activity inside human nonprofessional phagocytic cells.

Interaction of natural killer cells withTrypanosoma cruzi-infected fibroblasts

“…However, although iNOS levels decreased they appeared to be higher, as in control experiments, in the absence of spleen cells. Interestingly, in vitro experiments with T. cruzi infected human fibroblasts revealed that even the addition of recombinant IFN-γ could not affect the parasitic load in these cells [28].…”

ToxoplasmaEffectors Targeting Host Signaling and Transcription

“…Humans are relatively resistant to infection despite the occasional occurrence of disease, as described above. Human cells also rely on IFN-␥ and STAT1 signaling to control parasite replication in vitro (196). Although many of the ISGs regulated by IFN-␥ are similar in mouse and human, human cells control intracellular T. gondii by very different mechanisms than those described for mouse cells (197).…”

STAT1 and pathogens, not a friendly relationship

“…The strong Th1-type immune response raised by Toxoplasma involves an IL-12-driven IFNg secretion by lymphocytes and the activation of STAT1. In non-professional phagocytic cells that do not express STAT1 there is no anti-Toxoplasma activity [257], and STAT1 À/À mice die of infection although they produce normal levels of IFNg [258], pointing to a key function of STAT1 in the anti-parasitic function itself. Indeed, IFNg-inducible genes include genes such as inducible nitric oxyde synthase (iNOS), which is under the control of STAT1 (see: [256]).…”