NLRP3 and Potassium Efflux Drive Rapid IL-1ß Release from Primary Human Monocytes during Toxoplasma gondii Infection.

IL-1ß 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 Toxoplasma gondii induced an early IL-1ß response (within 4 h) in primary human peripheral blood monocytes isolated from healthy donors. This process involved upregulation of IL-1ß, IL-1RN (IL-1R antagonist), and NLRP3 transcripts, de novo protein synthesis, and the release of pro- and mature IL-1ß from infected primary monocytes. The released pro-IL-1ß was cleavable to mature bioactive IL-1ß 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-1ß cleavage and release in response to T. gondii infection, without affecting the release of TNF-α, and indicated a role for the inflammasome sensor NLRP3 and for potassium efflux in T. gondii-induced IL-1ß production. Interestingly, T. gondii infection did not induce an IL-1ß response in primary human macrophages derived from the same blood donors as the monocytes. Consistent with this finding, NLRP3 was downregulated during the differentiation of monocytes to macrophages and was not induced in macrophages during T. gondii infection. To our knowledge, these findings are the first to identify NLRP3 as an inflammasome sensor for T. gondii in primary human peripheral blood cells and to define an upstream regulator of its activation through the release of intracellular potassium.

Type II Toxoplasma gondii induction of CD40 on infected macrophages enhances interleukin-12 responses.

Toxoplasma gondii is an obligate intracellular parasite that can cause severe neurological disease in infected humans. CD40 is a receptor on macrophages that plays a critical role in controlling T. gondii infection. We examined the regulation of CD40 on the surface of T. gondii-infected bone marrow-derived macrophages (BMdMs). T. gondii induced CD40 expression both at the transcript level and on the cell surface, and interestingly, the effect was parasite strain specific: CD40 levels were dramatically increased in type II T. gondii-infected BMdMs compared to type I- or type III-infected cells. Type II induction of CD40 was specific to cells harboring intracellular parasites and detectable as early as 6 h postinfection (hpi) at the transcript level. CD40 protein expression peaked at 18 hpi. Using forward genetics with progeny from a type II × type III cross, we found that CD40 induction mapped to a region of chromosome X that included the gene encoding the dense granule protein 15 (GRA15). Using type I parasites stably expressing the type II allele of GRA15 (GRA15II), we found that type I GRA15II parasites induced the expression of CD40 on infected cells in an NF-κB-dependent manner. In addition, stable expression of hemagglutinin-tagged GRA15II in THP-1 cells resulted in CD40 upregulation in the absence of infection. Since CD40 signaling contributes to interleukin-12 (IL-12) production, we examined IL-12 from infected macrophages and found that CD40L engagement of CD40 amplified the IL-12 response in type II-infected cells. These data indicate that GRA15II induction of CD40 promotes parasite immunity through the production of IL-12.

MicroRNA profiling identifies miR-34a and miR-21 and their target genes JAG1 and WNT1 in the coordinate regulation of dendritic cell differentiation.

MicroRNAs (miRNAs, miRs) modulate a multitude of cellular events. Here, we identify functional miRNA-protein networks that regulate human monocyte-derived dendritic cell (MDDC) differentiation. miRNA profiling revealed stage-specific differential expression of 20 miRNAs during days 1, 3, and 5 of MDDC differentiation. To identify and prioritize miRNA-protein networks for functional validation, we developed a target ranking algorithm that incorporates many features of miRNA regulatory networks. This system prioritized miR-21, miR-34a, and their cognate targets WNT1 and JAG1 for functional validation. Inhibition of both miR-21 and miR-34a stalled MDDC differentiation, as quantified by DC-SIGN/CD14 expression ratios, showing cooperative involvement of these miRNAs in MDDC differentiation. We confirmed that the 3' untranslated regions of WNT1 and JAG1 were functional targets of these miRNAs and provide evidence that these targets were translationally suppressed. Significantly, exogenously added Wnt-1 and Jagged-1 also stalled MDDC differentiation, suggesting that miRNA-mediated inhibition of endogenous WNT1 and JAG1 expression was important for proper MDDC differentiation. Finally, inhibition of miR-21 and miR-34a, or addition of Wnt-1 and Jagged-1, led to a decrease in endocytic capacity, a key function of immature DCs. Thus, our novel approach identified and validated some miRNA-protein networks involved in phenotypic and functional MDDC differentiation.