NLRP3 and Potassium Efflux Drive Rapid IL-1β Release from Primary Human Monocytes during <i>Toxoplasma gondii</i> Infection

Abstract

IL-1β is produced by myeloid cells and acts as a critical mediator of host defense during infection and injury. We found that the intracellular protozoan parasite <i>Toxoplasma gondii</i> induced an early IL-1β response (within 4 h) in primary human peripheral blood monocytes isolated from healthy donors. This process involved upregulation of <i>IL-1β</i>, <i>IL-1RN</i> (IL-1R antagonist), and <i>NLRP3</i> transcripts, de novo protein synthesis, and the release of pro- and mature IL-1β from infected primary monocytes. The released pro-IL-1β was cleavable to mature bioactive IL-1β in the extracellular space by the protease caspase-1. Treatment of primary monocytes with the NLRP3 inhibitor MCC950 or with extracellular potassium significantly reduced IL-1β cleavage and release in response to <i>T. gondii</i> infection, without affecting the release of TNF-α, and indicated a role for the inflammasome sensor NLRP3 and for potassium efflux in <i>T. gondii</i>-induced IL-1β production. Interestingly, <i>T. gondii</i> infection did not induce an IL-1β response in primary human macrophages derived from the same blood donors as the monocytes. Consistent with this finding, <i>NLRP3</i> was downregulated during the differentiation of monocytes to macrophages and was not induced in macrophages during <i>T. gondii</i> infection. To our knowledge, these findings are the first to identify NLRP3 as an inflammasome sensor for <i>T. gondii</i> in primary human peripheral blood cells and to define an upstream regulator of its activation through the release of intracellular potassium.