Toxoplasma gondiiTachyzoites Inhibit Proinflammatory Cytokine Induction in Infected Macrophages by Preventing Nuclear Translocation of the Transcription Factor NF-κB

Abstract

Control of microbial infection requires regulated induction of NF-κB-dependent proinflammatory cytokines such as IL-12 and TNF-α. Activation of this important transcription factor is driven by phosphorylation-dependent degradation of the inhibitory IκB molecule, an event which enables NF-κB translocation from the cytoplasm to the nucleus. In this study, we show that intracellular infection of macrophages with the protozoan parasite Toxoplasma gondii induces rapid IκB phosphorylation and degradation. Nevertheless, NF-κB failed to translocate to the nucleus, enabling the parasite to invade cells without triggering proinflammatory cytokine induction. Infected cells subsequently subjected to LPS triggering were severely crippled in IL-12 and TNF-α production, a result of tachyzoite-induced blockade of NF-κB nuclear translocation. Our results are the first to demonstrate the ability of an intracellular protozoan to actively interfere with the NF-κB activation pathway in macrophages, an activity that may enable parasite survival within the host.

Heat Shock Protein 70 Is a Potential Virulence Factor in MurineToxoplasmaInfection Via Immunomodulation of Host NF-κB and Nitric Oxide

“…However, within the next phase of activation, HSP70 levels are sufficient to reduce NF-B uptake to the nucleus. This timing fits very nicely with the timings of suppression of immune activities observed by Butcher et al (13). Furthermore, our observation that the inhibition of NF-B translocation was not complete (an observation also made by Butcher et al, Ref.…”

“…This activity could result in prevention of I B phosphorylation and subsequent dissociation of NF-B. However, this explanation seems unlikely to account for the effect of T. gondii HSP70, because it has recently been shown that although tachyzoites do invade macrophages without activating NF-B, they trigger normal phosphorylation-dependent degradation of I B, thus indicating that the block on NF-B translocation occurs downstream of these events (13).…”

“…Thus, this is a plausible explanation for the immunomodulatory effects of T. gondii HSP70, but it begs the question of how the parasite HSP70 gains access to the host cytoplasm and nucleus. It will be interesting to see whether HSP70 is released during, or shortly after, host cell invasion by virulent tachyzoites, along with several other proteins that could conceivably interfere with host signaling pathways (13). We do have preliminary evidence that HSP70 is indeed secreted by T. gondii; analysis of precipitated proteins separated by two-dimensional electrophoresis and probed with a parasite specific anti-HSP70 Ab detected parasite HSP70 in the culture supernatant at 4 h postinfection (data not shown).…”

Toxoplasma gondii Virulence Factor ROP18 Inhibits the Host NF-κB Pathway by Promoting p65 Degradation

“…However, despite the initiation of NF-B signaling, infection with T. gondii did not lead to the activation of NF-B but to its termination. The reason for disabling NF-B is associated with blocking of p65 translocation to the nucleus (10,12,13). Other studies have demonstrated that T. gondii activates NF-B, which up-regulates the expression of anti-apoptotic genes to facilitate the replication of the pathogen (15)(16)(17).…”

Host cell manipulation by the human pathogen Toxoplasma gondii

“…This observation demonstrated the importance of NF-κB p65 component as a key factor in the protection of the infected fibroblast against apoptosis [99]. It should be noted, however, that movement of NF-κB to the nucleus is not observed in all types of infected cells including macrophages, which do not exhibit NF-κB translocation early after invasion [105][106][107]. This may be due to inherent differences between cell types or a result of examining translocation at different time points post-invasion.…”