No abstract

Toxoplasma gondii (T. gondii) is the protozoan parasite that causes toxoplasmosis, a potentially fatal disease to immunocompromised patients, and which affects approximately 30% of the world’s population. Previously, we showed that purinergic signaling via the P2X7 receptor contributes to T. gondii elimination in macrophages, through reactive oxygen species (ROS) production and lysosome fusion with the parasitophorous vacuole. Moreover, we demonstrated that P2X7 receptor activation promotes the production of anti-parasitic pro-inflammatory cytokines during early T. gondii infection in vivo. However, the cascade of signaling events that leads to parasite elimination via P2X7 receptor activation remained to be elucidated. Here, we investigated the cellular pathways involved in T. gondii elimination triggered by P2X7 receptor signaling, during early infection in macrophages. We focused on the potential role of the inflammasome, a protein complex that can be co-activated by the P2X7 receptor, and which is involved in the host immune defense against T. gondii infection. Using peritoneal and bone marrow-derived macrophages from knockout mice deficient for inflammasome components (NLRP3−/−, Caspase-1/11−/−, Caspase-11−/−), we show that the control of T. gondii infection via P2X7 receptor activation by extracellular ATP (eATP) depends on the canonical inflammasome effector caspase-1, but not on caspase-11 (a non-canonical inflammasome effector). Parasite elimination via P2X7 receptor and inflammasome activation was also dependent on ROS generation and pannexin-1 channel. Treatment with eATP increased IL-1β secretion from infected macrophages, and this effect was dependent on the canonical NLRP3 inflammasome. Finally, treatment with recombinant IL-1β promoted parasite elimination via mitochondrial ROS generation (as assessed using Mito-TEMPO). Together, our results support a model where P2X7 receptor activation by eATP inhibits T. gondii growth in macrophages by triggering NADPH-oxidase-dependent ROS production, and also by activating a canonical NLRP3 inflammasome, which increases IL-1β production (via caspase-1 activity), leading to mitochondrial ROS generation.

Leishmaniasis is a neglected tropical disease affecting millions of individuals worldwide. P2X7 receptor has been linked to the elimination of Leishmania amazonensis . Biological responses evoked by P2X7 receptor activation have been well-documented, including apoptosis, phagocytosis, cytokine release, such as IL-1β. It was demonstrated that NLRP3 inflammasome activation and IL-1β signaling participated in resistance against L . amazonensis . Furthermore, our group has shown that L . amazonensis elimination through P2X7 receptor activation depended on leukotriene B 4 (LTB 4 ) production and release. Therefore, we investigated whether L . amazonensis elimination by P2X7 receptor and LTB 4 involved NLRP3 inflammasome activation and IL-1β signaling. We showed that macrophages from NLRP3 -/- , ASC -/- , Casp-1/11 -/- , gp91 phox-/- , and IL-1R -/- mice treated with ATP or LTB 4 did not decrease parasitic load as was observed in WT mice. When ASC -/- macrophages were treated with exogenous IL-1β, parasite killing was noted, however, we did not see parasitic load reduction in IL-1R -/- macrophages. Similarly, macrophages from P2X7 receptor-deficient mice treated with IL-1β also showed decreased parasitic load. In addition, when we infected Casp-11 -/- macrophages, neither ATP nor LTB 4 were able to reduce parasitic load, and Casp-11 -/- mice were more susceptible to L . amazonensis infection than were WT mice. Furthermore, P2X7 -/- L . amazonensis- infected mice locally treated with exogenous LTB 4 showed resistance to infection, characterized by lower parasite load and smaller lesions compared to untreated P2X7 -/- mice. A similar observation was noted when infected P2X7 -/- mice were treated with IL-1β, i.e., lower parasite load and smaller lesions compared to P2X7 -/- mice. These data suggested that L . amazonensis elimination mediated by P2X7 receptor and LTB 4 was dependent on non-canonical NLRP3 inflammasome activation, ROS production, and IL-1β signaling.