Background Toxoplasma gondii can invade and replicate in all nucleated cells in a wide range of host species, and infection induces IL-1β production. IL-1β plays central roles in the stimulation of the innate immune system and inflammation. However, little is known of the innate immune responses in human fetal small intestinal epithelial cells (FHs 74 Int cells) after T. gondii infection.MethodsFHs 74 Int cells were infected with the T. gondii GFP-RH strain. Then, IL-1β production and its mechanisms of action were evaluated using ELISA, MTT cell viability assays, Western blotting, immunofluorescence, quantitative real-time polymerase chain reaction (qRT-PCR), and gene-specific small interfering RNA (siRNA) transfection.ResultsInfection of FHs 74 Int cells by T. gondii triggered significant time- and dose-dependent IL-1β production. Although T. gondii activated NLRP1, NLRP3, NLRC4 and AIM2 inflammasomes in FHs 74 Int cells, NLRP3 levels were consistently and significantly time-dependently increased, while the other inflammasomes were not. Transfection with siRNA targeting NLRP3, cleaved caspase-1 (Casp-1) or ASC significantly reduced T. gondii-induced IL-1β production, whereas T. gondii proliferation was markedly increased. Toxoplasma gondii infection activated P2X7 receptor (P2X7R) levels in FHs 74 Int cells in a time-dependent manner; however, transfection with siRNA targeting P2X7R significantly reduced T. gondii-induced IL-1β secretion and substantially increased T. gondii proliferation, which is mediated by decreased protein expression levels of NLRP3, cleaved Casp-1 and ASC. Collectively, NLRP3-dependent IL-1β secretion is mediated by P2X7R in small intestinal epithelial cells in response to T. gondii infection, thereby controlling parasite proliferation.ConclusionsThis study revealed that the P2X7R/NLRP3 pathway plays important roles in IL-1β secretion and inhibition of T. gondii proliferation in small intestinal epithelial cells. These results not only contribute to our understanding of the mucosal immune mechanisms of T. gondii infection but also offer new insight into the identification of innate resistance in the gut epithelium.Electronic supplementary materialThe online version of this article (10.1186/s13071-017-2573-y) contains supplementary material, which is available to authorized users.

The nucleotide-binding domain and leucine-rich repeats containing proteins (NLRs) serve as immune receptors in both plants and animals. Overaccumulation of NLRs often leads to autoimmune responses, suggesting that the levels of these immune receptors must be tightly controlled. However, the mechanism by which NLR protein levels are regulated is unknown. Here we report that the F-box protein CPR1 controls the stability of plant NLR resistance proteins. Loss-of-function mutations in CPR1 lead to higher accumulation of the NLR proteins SNC1 and RPS2, as well as autoactivation of immune responses. The autoimmune responses in cpr1 mutant plants can be largely suppressed by knocking out SNC1. Furthermore, CPR1 interacts with SNC1 and RPS2 in vivo, and overexpressing CPR1 results in reduced accumulation of SNC1 and RPS2, as well as suppression of immunity mediated by these two NLR proteins. Our data suggest that SKP1-CULLIN1-F-box (SCF) complex-mediated stability control of plant NLR proteins plays an important role in regulating their protein levels and preventing autoimmunity.

Extreme weather conditions associated with climate change affect many aspects of plant and animal life, including the response to infectious diseases. Production of salicylic acid (SA), a central plant defence hormone1–3, is particularly vulnerable to suppression by short periods of hot weather above the normal plant growth temperature range via an unknown mechanism4–7. Here we show that suppression of SA production in Arabidopsis thaliana at 28 °C is independent of PHYTOCHROME B8,9 (phyB) and EARLY FLOWERING 310 (ELF3), which regulate thermo-responsive plant growth and development. Instead, we found that formation of GUANYLATE BINDING PROTEIN-LIKE 3 (GBPL3) defence-activated biomolecular condensates11 (GDACs) was reduced at the higher growth temperature. The altered GDAC formation in vivo is linked to impaired recruitment of GBPL3 and SA-associated Mediator subunits to the promoters of CBP60g and SARD1, which encode master immune transcription factors. Unlike many other SA signalling components, including the SA receptor and biosynthetic genes, optimized CBP60g expression was sufficient to broadly restore SA production, basal immunity and effector-triggered immunity at the elevated growth temperature without significant growth trade-offs. CBP60g family transcription factors are widely conserved in plants12. These results have implications for safeguarding the plant immune system as well as understanding the concept of the plant–pathogen–environment disease triangle and the emergence of new disease epidemics in a warming climate.

SUMMARYHeat shock proteins (HSPs) serve as molecular chaperones for diverse client proteins in many biological processes. In plant immunity, cytosolic HSP90s participate in the assembly, stability control and/or activation of immune receptor complexes. In this paper we report that in addition to the well-established positive roles that HSP90 isoforms play in plant immunity, they are also involved in the negative regulation of immune receptor accumulation. Point mutations in two HSP90 genes, HSP90.2 and HSP90.3, were identified from a forward genetic screen designed to isolate mutants with enhanced disease resistance. We found that specific mutations in HSP90.2 and HSP90.3 lead to heightened accumulation of immune receptors, including SNC1, RPS2 and RPS4. HSP90s may assist SGT1 in the formation of SCF E3 ubiquitin ligase complexes that target immune receptors for degradation. Such regulation is critical for maintaining appropriate levels of immune receptor proteins to avoid autoimmunity.

Nucleocytoplasmic trafficking is emerging as an important aspect of plant immunity. The three related pathways affecting plant immunity include Nuclear Localization Signal (NLS)–mediated nuclear protein import, Nuclear Export Signal (NES)–dependent nuclear protein export, and mRNA export relying on MOS3, a nucleoporin belonging to the Nup107–160 complex. Here we report the characterization, identification, and detailed analysis of Arabidopsis modifier of snc1, 11 (mos11). Mutations in MOS11 can partially suppress the dwarfism and enhanced disease resistance phenotypes of snc1, which carries a gain-of-function mutation in a TIR-NB-LRR type Resistance gene. MOS11 encodes a conserved eukaryotic protein with homology to the human RNA binding protein CIP29. Further functional analysis shows that MOS11 localizes to the nucleus and that the mos11 mutants accumulate more poly(A) mRNAs in the nucleus, likely resulting from reduced mRNA export activity. Epistasis analysis between mos3-1 and mos11-1 revealed that MOS11 probably functions in the same mRNA export pathway as MOS3, in a partially overlapping fashion, before the mRNA molecules pass through the nuclear pores. Taken together, MOS11 is identified as a new protein contributing to the transfer of mature mRNA from the nucleus to the cytosol.

Plant nucleotide-binding leucine-rich repeat (NB-LRR) proteins serve as intracellular sensors to detect pathogen effectors and trigger immune responses. Transcription of the NB-LRR-encoding Resistance (R) genes needs to be tightly controlled to avoid inappropriate defense activation. How the expression of the NB-LRR R genes is regulated is poorly understood. The Arabidopsis (Arabidopsis thaliana) suppressor of npr1-1, constitutive 1 (snc1) mutant carries a gain-of-function mutation in a Toll/Interleukin1 receptor-like (TIR)-NB-LRR-encoding gene, resulting in the constitutive activation of plant defense responses. A snc1 suppressor screen identified modifier of snc1,9 (mos9), which partially suppresses the autoimmune phenotypes of snc1. Positional cloning revealed that MOS9 encodes a plant-specific protein of unknown function. Expression analysis showed that MOS9 is required for the full expression of TIR-NB-LRR protein-encoding RECOGNITION OF PERONOSPORA PARASITICA 4 (RPP4) and SNC1, both of which reside in the RPP4 cluster. Coimmunoprecipitation and mass spectrometry analyses revealed that MOS9 associates with the Set1 class lysine 4 of histone 3 (H3K4) methyltransferase Arabidopsis Trithorax-Related7 (ATXR7). Like MOS9, ATXR7 is also required for the full expression of SNC1 and the autoimmune phenotypes in the snc1 mutant. In atxr7 mutant plants, the expression of RPP4 is similarly reduced, and resistance against Hyaloperonospora arabidopsidis Emwa1 is compromised. Consistent with the attenuated expression of SNC1 and RPP4, trimethylated H3K4 marks are reduced around the promoters of SNC1 and RPP4 in mos9 plants. Our data suggest that MOS9 functions together with ATXR7 to regulate the expression of SNC1 and RPP4 through H3K4 methylation, which plays an important role in fine-tuning their transcription levels and functions in plant defense.

Proteins containing nucleotide-binding and leucine-rich repeat domains (NB-LRRs) serve as immune receptors in plants and animals. Negative regulation of immunity mediated by NB-LRR proteins is crucial, as their overactivation often leads to autoimmunity. Here we describe a new mutant, snc1-enhancing (muse) forward genetic screen, targeting unknown negative regulators of NB-LRR-mediated resistance in Arabidopsis. From the screen, we identify MUSE5, which is renamed as AtPAM16 because it encodes the ortholog of yeast PAM16, part of the mitochondrial inner membrane protein import motor. Consistently, AtPAM16-GFP localizes to the mitochondrial inner membrane. AtPAM16L is a paralog of AtPAM16. Double mutant Atpam16-1 Atpam16l is lethal, indicating that AtPAM16 function is essential. Single mutant Atpam16 plants exhibit a smaller size and enhanced resistance against virulent pathogens. They also display elevated reactive oxygen species (ROS) accumulation. Therefore, AtPAM16 seems to be involved in importing a negative regulator of plant immunity into mitochondria, thus protecting plants from over-accumulation of ROS and preventing autoimmunity.

Activation of cell-autonomous defense by the immune cytokine interferon-γ (IFN-γ) is critical to the control of life-threatening infections in humans. IFN-γ induces the expression of hundreds of host proteins in all nucleated cells and tissues, yet many of these proteins remain uncharacterized. We screened 19,050 human genes by CRISPR-Cas9 mutagenesis and identified IFN-γ–induced apolipoprotein L3 (APOL3) as a potent bactericidal agent protecting multiple non–immune barrier cell types against infection. Canonical apolipoproteins typically solubilize mammalian lipids for extracellular transport; APOL3 instead targeted cytosol-invasive bacteria to dissolve their anionic membranes into human-bacterial lipoprotein nanodiscs detected by native mass spectrometry and visualized by single-particle cryo–electron microscopy. Thus, humans have harnessed the detergent-like properties of extracellular apolipoproteins to fashion an intracellular lysin, thereby endowing resident nonimmune cells with a mechanism to achieve sterilizing immunity.