Dysregulation of macrophage signal transduction by Toxoplasma gondii: past progress and recent advances

Abstract

The opportunistic protozoan parasite Toxoplasma gondii is well known as a strong inducer of cell-mediated immunity, largely as a result of proinflammatory cytokine induction during in vivo infection. Yet, during intracellular infection the parasite suppresses signal transduction pathways leading to these proinflammatory responses. The opposing responses are likely to reflect the parasite's need to stimulate immunity allowing host survival and parasite persistence, and at the same time avoiding excessive responses that could result in parasite elimination and host immunopathology. This Review summarizes past and present investigations into the effects of Toxoplasma on host cell signal transduction. These studies reveal insight into the profound suppression of proinflammatory cytokine responses that occurs when the parasite infects macrophages and other cells of innate immunity.

Figure 1

The Stat1 signaling pathway and modulation by T. gondii. 1, IFN-γ binding causes receptor dimerization. 2, This causes receptor associated Jak molecules to tyrosine phosphorylate each other, leading in turn to recruitment and tyrosine phosphorylation of Stat1 (step 3). Activated Stat1 dissociates from the receptor complex and dimerizes in the cytosol (step 5). Stat1 dimers translocate to the nucleus (step 6) where they bind to IFN-γ responsive promoter elements. Full Stat1 transcriptional activity requires serine phosphorylation dependent upon phosphatidylinositiol 3-kinase (PI3-K) and protein kinase B/Akt (step 7). Multiple studies suggest that Toxoplasma interferes with Stat1 signaling, but the control points are less clear. There is evidence that T. gondii prevents Stat1 phosphorylation and causes its degradation. Other data suggest that the parasite prevents Stat1 nuclear translocation. Finally, some studies suggest that Stat1 is subject to dephosphorylation following nuclear translocation.

Figure 2

The Toll-like receptor signaling cascade: Impact of Toxoplasma infection. In a generalized pathway, TLR binding to its ligand (step 1) results in recruitment of the MyD88 adaptor molecule (step 2). In turn, this mediates recruitment of IRAK1 and IRAK4, which form a complex with TRAF6 (step 3). In step 4, the TRAF6 molecule interacts with Uva1 and Ubc13, triggering ubiquitination of TRAF6. This stimulates activation of TAK1 which is associated with TAB1 and TAB2 (step 5). Activated TAK1 possesses MAPK kinase kinase activity, triggering MAPK activation (step 6). The TAK1 molecule also activates the IKK complex (step 7) leading in turn to degradation of IκB and nuclear translocation of NFκB p50:p65. Toxplasma blocks most, but not all TLR-induced cytokine responses. There is evidence that the parasite blocks nuclear NFκB translocation, although this is a temporary effect. There are also data to suggest that the parasite interferes with TLR-induced MAPK activation.

Figure 3

T. gondii down-regulates MyD88-independent signaling mediated by poly I:C/TLR3. A, bone marrow-derived MyD88^+/+ and MyD88^−/− macrophages were infected, subjected to 2 hr poly I:C stimulation, then cells were harvested and RNA prepared for RT-PCR analysis of IFN-γ, IL-12p40 and GAPDH. M, cells cultured in medium for the entire course of the experiment; P, uninfected cells stimulated with poly I:C; R, cells infected with RH strain tachyzoites, no subsequent poly I:C stimulation; R+P, cells infected with Toxoplasma RH strain, then 6 hr later subjected to poly I:C stimulation (2 hr). B, wild-type and MyD88^−/− bone marrow-derived macrophages were infected with RH strain tachyzoites, then 6 hr later subjected to poly I:C stimulation. Supernatants were collected for IFN-γ ELISA 6 hr later.

Figure 4

Targetting histone modification as a possible unifying mechanism of suppression. Macrophage activation through receptors such as TLR4 induces production of multiple proinflammatory cytokines, but two general information pathways are needed. Activation of transcription factors and binding to promoter elements is required to activate specific genes. However, prior to binding, signal-induced histone modification is needed in many cases to re-model chromatin into an open configuration that enables transcription factors to act at target promoter regions. There is evidence that T. gondii targets the chromatin remodeling pathway, offering an explanation for the parasite's ability to simultaneously down-regulate a large panel of proinflammatory cytokines and chemokines. In particular, there is a block in TLR4-induced histone H3 phosphorylation. Whether this is because upstream histone H3 kinases such as MSK-1 are deactivated, or whether the parasite induces histone H3-directed phosphatase activity is not known at present.