Date Published: March 13, 2014
Publisher: Public Library of Science
Author(s): Kimberly M. Cirelli, Gezahegn Gorfu, Musa A. Hassan, Morton Printz, Devorah Crown, Stephen H. Leppla, Michael E. Grigg, Jeroen P. J. Saeij, Mahtab Moayeri, Christopher M. Sassetti.
Toxoplasma gondii is an intracellular parasite that infects a wide range of warm-blooded species. Rats vary in their susceptibility to this parasite. The Toxo1 locus conferring Toxoplasma resistance in rats was previously mapped to a region of chromosome 10 containing Nlrp1. This gene encodes an inflammasome sensor controlling macrophage sensitivity to anthrax lethal toxin (LT) induced rapid cell death (pyroptosis). We show here that rat strain differences in Toxoplasma infected macrophage sensitivity to pyroptosis, IL-1β/IL-18 processing, and inhibition of parasite proliferation are perfectly correlated with NLRP1 sequence, while inversely correlated with sensitivity to anthrax LT-induced cell death. Using recombinant inbred rats, SNP analyses and whole transcriptome gene expression studies, we narrowed the candidate genes for control of Toxoplasma-mediated rat macrophage pyroptosis to four genes, one of which was Nlrp1. Knockdown of Nlrp1 in pyroptosis-sensitive macrophages resulted in higher parasite replication and protection from cell death. Reciprocally, overexpression of the NLRP1 variant from Toxoplasma-sensitive macrophages in pyroptosis-resistant cells led to sensitization of these resistant macrophages. Our findings reveal Toxoplasma as a novel activator of the NLRP1 inflammasome in rat macrophages.
Toxoplasma gondii is an obligate intracellular parasite, for which different host species or strains within a species display variable susceptibilities. Different Toxoplasma strains also differ in virulence within the same host, suggesting variation in effectors among parasite strains and/or their impact in various hosts. Host innate immunity is known to play a critical role in susceptibility to infection. In mice, for example, resistance to Toxoplasma infection is critically dependent on the induction of IL-12, which subsequently induces IFN-γ, the main mediator of toxoplasmicidal activities (for review, see ).
The Toxo1 locus that controls rat susceptibility to toxoplasmosis  was previously mapped to a region of rat chromosome 10 containing the inflammasome sensor Nlrp1. In this work we identify Toxoplasma as a novel pathogen activator of the NLRP1 inflammasome. Until this work, anthrax LT was the only known activator of this inflammasome sensor , , . We now demonstrate that like LT, rapid Toxoplasma-induced rat macrophage cell death is a pyroptotic event for which sensitivity correlates to NLRP1 sequence. Type I, Type II and a variety of genetically diverse T. gondii strains induce rapid pyroptosis in macrophages derived from inbred rats expressing NLRP1variant 5, while macrophages from BMDMs expressing NLRP1variant 1,2 are resistant to the parasite. This is the inverse of what is known for LT, where NLRP1variant 1,2 confers sensitivity . In rats, macrophage sensitivity to Toxoplasma-induced cell death inversely correlates with whole animal resistance to infection. Rat strains historically susceptible to chronic Toxoplasma infection (e.g., CDF, BN, SD; NLRP1variant 1,2) have pyroptosis-resistant macrophages whereas resistant rats that cure infection (e.g., LEW, SHR; NLRP1variant 5) harbor macrophages that undergo parasite-induced pyroptosis. This suggests that the ability of the macrophage to allow parasite proliferation and possibly dissemination is linked to resistance to parasite-induced macrophage pyroptosis. Similar findings were previously described for mouse Nlrp1b-mediated control of anthrax infection. Mice resistant to Bacillus anthracis have macrophages expressing Nlrp1b variants which confer macrophage sensitivity to anthrax LT, and resistance is linked to the IL-1β response induced by toxin , . The idea of control of parasite proliferation at the macrophage level is supported by findings that macrophages are among the first cell types to be infected when an animal ingests Toxoplasma cysts or oocysts ,  and innate immune cells are used to traffic from the site of infection to distant sites such as the brain .