A Highly Conserved Toxo1 Haplotype Directs Resistance to Toxoplasmosis and Its Associated Caspase-1 Dependent Killing of Parasite and Host Macrophage

Abstract

Natural immunity or resistance to pathogens most often relies on the genetic make-up of the host. In a LEW rat model of refractoriness to toxoplasmosis, we previously identified on chromosome 10 the Toxo1 locus that directs toxoplasmosis outcome and controls parasite spreading by a macrophage-dependent mechanism. Now, we narrowed down Toxo1 to a 891 kb interval containing 29 genes syntenic to human 17p13 region. Strikingly, Toxo1 is included in a haplotype block strictly conserved among all refractory rat strains. The sequencing of Toxo1 in nine rat strains (5 refractory and 4 susceptible) revealed resistant-restricted conserved polymorphisms displaying a distribution gradient that peaks at the bottom border of Toxo1, and highlighting the NOD-like receptor, Nlrp1a, as a major candidate. The Nlrp1 inflammasome is known to trigger, upon pathogen intracellular sensing, pyroptosis programmed-cell death involving caspase-1 activation and cleavage of IL-1β. Functional studies demonstrated that the Toxo1-dependent refractoriness in vivo correlated with both the ability of macrophages to restrict T. gondii growth and a T. gondii-induced death of intracellular parasites and its host macrophages. The parasite-induced cell death of infected macrophages bearing the LEW-Toxo1 alleles was found to exhibit pyroptosis-like features with ROS production, the activation of caspase-1 and IL1-β secretion. The pharmacological inactivation of caspase-1 using YVAD and Z-VAD inhibitors prevented the death of both intravacuolar parasites and host non-permissive macrophages but failed to restore parasite proliferation. These findings demonstrated that the Toxo1-dependent response of rat macrophages to T. gondii infection may trigger two pathways leading to the control of parasite proliferation and the death of parasites and host macrophages. The NOD-like receptor NLRP1a/Caspase-1 pathway is the best candidate to mediate the parasite-induced cell death. These data represent new insights towards the identification of a major pathway of innate resistance to toxoplasmosis and the prediction of individual resistance.

Inherent Oxidative Stress in the Lewis Rat Is Associated with Resistance to Toxoplasmosis

“…In the case of intracellular T. gondii, lysosomal fusion of the nonfusogenic parasitophorous vacuole leads to killing of the intracellular parasites (34). Consistent with our observation that scavenging ROS augments parasite survival and growth in the LEW rat peritoneal cells, others (11) have demonstrated the involvement of ROS and caspase-1 in a T. gondii killing mechanism that is linked to the Toxo1 locus in the LEW rat.…”

“…The resistance phenotype of the LEW rat peritoneal macrophages to T. gondii infection has been associated with the rapid death of both parasites and infected host cells (10,11). Thus, we endeavored to investigate whether the increase in T. gondii growth in LEW rat peritoneal cells treated with MnTBAP would lead to a higher cell death rate in the LEW than in the BN rat peritoneal cells.…”

“…Pursuant to this, two genes called NLRP1 and ALOX1 in the orthologous Toxo1 locus on chromosome 17 in the human genome have been demonstrated to possess alleles linked to susceptibility to human congenital toxoplasmosis (3,4). The inhibition of intracellular T. gondii growth in LEW rat peritoneal macrophages (10) has been linked to rapid death of both parasites and infected host cells (11). This mode of clearance of parasites in LEW rat cells suggests the involvement of a rapid and vigorous killing response at the site of infection, thus impeding the dissemination of the parasites in the host animal.…”

Toxoplasma gondii GRA7-Induced TRAF6 Activation Contributes to Host Protective Immunity

“…Additionally, a recent study showed that T. gondii induced the NOX1-dependent production of excessive ROS, which contributed to mitochondrial structural damage and mitochondrial dysfunction in placentas, as well as the cleavage of caspase-9 and caspase-3, which resulted in the apoptosis of trophoblasts (35). T. gondii also induced pyroptosis-like features with ROS production, as well as the activation of caspase-1 and IL-1␤ secretion, in macrophages (36). Our results correlate with those of previous studies that showed that NOX is an important source of GRA7-induced ROS that initiate intracellular signaling cascades.…”

Distinct Regions of NLRP1B Are Required To Respond to Anthrax Lethal Toxin and Metabolic Inhibition

“…Toxoplasma gondii infection has been shown to induce inflammasome activation that is dependent on NLRP1B in mice and on NLRP1A in rats (15)(16)(17)23). There are two NLRP1 paralogs in rats (Nlrp1a and Nlrp1b) (24): the Nlrp1a paralog is associated with rat macrophage susceptibility to LeTx (25), but little is known about the Nlrp1b paralog.…”