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Anticancer T cells acquire a dysfunctional state characterized by poor effector function and expression of inhibitory receptors, such as PD-1. Blockade of PD-1 leads to T cell reinvigoration and is increasingly applied as an effective anticancer treatment. Recent work challenged the commonly held view that the phosphatase PTPN11 (known as SHP-2) is essential for PD-1 signaling in T cells, suggesting functional redundancy with the homologous phosphatase PTPN6 (SHP-1). Therefore, we investigated the effect of concomitant Ptpn6 and Ptpn11 deletion in T cells on their ability to mount antitumour responses. In vivo data show that neither sustained nor acute Ptpn6/11 deletion improves T cell-mediated tumor control. Sustained loss of Ptpn6/11 also impairs the therapeutic effects of anti-PD1 treatment. In vitro results show that Ptpn6/11-deleted CD8(+) T cells exhibit impaired expansion due to a survival defect and proteomics analyses reveal substantial alterations, including in apoptosis-related pathways. These data indicate that concomitant ablation of Ptpn6/11 in polyclonal T cells fails to improve their anticancer properties, implying that caution shall be taken when considering their inhibition for immunotherapeutic approaches.

MYC is a pleiotropic transcription factor involved in cancer, cell proliferation, and metabolism. Its regulation and function in NK cells, which are innate cytotoxic lymphocytes important to control viral infections and cancer, remain poorly defined. Here, we show that mice deficient for Myc in NK cells presented a severe reduction in these lymphocytes. Myc was required for NK cell development and expansion in response to the key cytokine IL-15, which induced Myc through transcriptional and posttranslational mechanisms. Mechanistically, Myc ablation in vivo largely impacted NK cells’ ribosomagenesis, reducing their translation and expansion capacities. Similar results were obtained by inhibiting MYC in human NK cells. Impairing translation by pharmacological intervention phenocopied the consequences of deleting or blocking MYC in vitro. Notably, mice lacking Myc in NK cells exhibited defective anticancer immunity, which reflected their decreased numbers of mature NK cells exerting suboptimal cytotoxic functions. These results indicate that MYC is a central node in NK cells, connecting IL-15 to translational fitness, expansion, and anticancer immunity.

The MHC class I pathway, presenting endogenously derived peptides to T lymphocytes, is hijacked in many pathological conditions. This affects MHC class I levels and peptide presentation at the cell surface leading to immune escape of cancer cells or microbes. It is therefore important to identify the molecular mechanisms behind MHC class I expression, processing and antigen presentation. The identification of NLRC5 as regulator of MHC class I transcription was a huge step forward in understanding the transcriptional mechanism involved. Nevertheless, many questions concerning MHC class I transcription are yet unsolved. Here we illuminate current knowledge on MHC class I and NLRC5 transcription, we highlight some remaining questions and discuss the use of quickly developing high-content screening tools to reveal unknowns in MHC class I transcription in the near future.

Regulatory factor X 7 (Rfx7) is an uncharacterized transcription factor belonging to a family involved in ciliogenesis and immunity. Here, we found that deletion of Rfx7 leads to a decrease in natural killer (NK) cell maintenance and immunity in vivo. Genomic approaches showed that Rfx7 coordinated a transcriptional network controlling cell metabolism. Rfx7(-/-) NK lymphocytes presented increased size, granularity, proliferation, and energetic state, whereas genetic reduction of mTOR activity mitigated those defects. Notably, Rfx7-deficient NK lymphocytes were rescued by interleukin 15 through engagement of the Janus kinase (Jak) pathway, thus revealing the importance of this signaling for maintenance of such spontaneously activated NK cells. Rfx7 therefore emerges as a novel transcriptional regulator of NK cell homeostasis and metabolic quiescence.

The inflammasome–IL-1 axis and type I interferons (IFNs) have been shown to exert protective effects upon respiratory tract infections. Conversely, IL-1 has also been implicated in inflammatory airway pathologies such as asthma and chronic obstructive pulmonary disease (COPD). OM-85 is a bacterial extract with proved efficacy against COPD and recurrent respiratory tract infections, a cause of co-morbidity in asthmatic patients. We therefore asked whether OM-85 affects the above-mentioned innate immune pathways. Here we show that OM-85 induced interferon-β through the Toll-like receptor adaptors Trif and MyD88 in bone marrow-derived dendritic cells. Moreover, it exerted a dual role on IL-1 production; on the one hand, it upregulated proIL-1β and proIL-1α levels in a MyD88-dependent manner without activating the inflammasome. On the other hand, it repressed IL-1β secretion induced by alum, a well-known NLRP3 activator. In vivo, OM-85 diminished the recruitment of inflammatory cells in response to peritoneal alum challenge. Our findings therefore suggest that OM-85 favors a protective primed state, while dampening inflammasome activation in specific conditions. Taken together, these data bring new insights into the mechanisms of OM-85 action on innate immune pathways and suggest potential explanations for its efficacy in the treatment of virus-induced airway diseases.

SummaryOmega-3 fatty acids (ω-3 FAs) have potential anti-inflammatory activity in a variety of inflammatory human diseases, but the mechanisms remain poorly understood. Here we show that stimulation of macrophages with ω-3 FAs, including eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), and other family members, abolished NLRP3 inflammasome activation and inhibited subsequent caspase-1 activation and IL-1β secretion. In addition, G protein-coupled receptor 120 (GPR120) and GPR40 and their downstream scaffold protein β-arrestin-2 were shown to be involved in inflammasome inhibition induced by ω-3 FAs. Importantly, ω-3 FAs also prevented NLRP3 inflammasome-dependent inflammation and metabolic disorder in a high-fat-diet-induced type 2 diabetes model. Our results reveal a mechanism through which ω-3 FAs repress inflammation and prevent inflammation-driven diseases and suggest the potential clinical use of ω-3 FAs in gout, autoinflammatory syndromes, or other NLRP3 inflammasome-driven inflammatory diseases.

NLRC5 is a transcriptional regulator of MHC class I (MHCI), which maintains high MHCI expression particularly in T cells. Recent evidence highlights an important NK–T-cell crosstalk, raising the question on whether NLRC5 specifically modulates this interaction. Here we show that NK cells from Nlrc5-deficient mice exhibit moderate alterations in inhibitory receptor expression and responsiveness. Interestingly, NLRC5 expression in T cells is required to protect them from NK-cell-mediated elimination upon inflammation. Using T-cell-specific Nlrc5-deficient mice, we show that NK cells surprisingly break tolerance even towards ‘self' Nlrc5-deficient T cells under inflammatory conditions. Furthermore, during chronic LCMV infection, the total CD8+ T-cell population is severely decreased in these mice, a phenotype reverted by NK-cell depletion. These findings strongly suggest that endogenous T cells with low MHCI expression become NK-cell targets, having thus important implications for T-cell responses in naturally or therapeutically induced inflammatory conditions., NK cell tolerance to self-MHCI levels is calibrated during their development. Here the authors show that this tolerance is overcome by an inflammatory environment and that NLRC5 protects T cells from NK cell-mediated elimination by maintaining high MHCI expression.

Although contributing to inflammatory responses and to the development of certain autoimmune pathologies, type I interferons (IFNs) are used for the treatment of viral, malignant, and even inflammatory diseases. Interleukin-1 (IL-1) is a strongly pyrogenic cytokine and its importance in the development of several inflammatory diseases is clearly established. While the therapeutic use of IL-1 blocking agents is particularly successful in the treatment of innate-driven inflammatory disorders, IFN treatment has mostly been appreciated in the management of multiple sclerosis. Interestingly, type I IFNs exert multifaceted immunomodulatory effects, including the reduction of IL-1 production, an outcome that could contribute to its efficacy in the treatment of inflammatory diseases. In this review, we summarize the current knowledge on IL-1 and IFN effects in different inflammatory disorders, the influence of IFNs on IL-1 production, and discuss possible therapeutic avenues based on these observations.

Summary Type I interferon (IFN) is a common therapy for autoimmune and inflammatory disorders, yet the mechanisms of action are largely unknown. Here we showed that type I IFN inhibited interleukin-1 (IL-1) production through two distinct mechanisms. Type I IFN signaling, via the STAT1 transcription factor, repressed the activity of the NLRP1 and NLRP3 inflammasomes, thereby suppressing caspase-1-dependent IL-1β maturation. In addition, type I IFN induced IL-10 in a STAT1-dependent manner; autocrine IL-10 then signaled via STAT3 to reduce the abundance of pro-IL-1α and pro-IL-1β. In vivo, poly(I:C)-induced type I IFN diminished IL-1β production in response to alum and Candida albicans , thus increasing susceptibility to this fungal pathogen. Importantly, monocytes from multiple sclerosis patients undergoing IFN-β treatment produced substantially less IL-1β than monocytes derived from healthy donors. Our findings may thus explain the effectiveness of type I IFN in the treatment of inflammatory diseases but also the observed "weakening" of the immune system after viral infection.

There is growing evidence that the maturation state of dendritic cells (DCs) is a critical parameter determining the balance between tolerance and immunity. We report that mouse CD4(+) effector memory T (T(EM)) cells, but not naive or central memory T cells, constitutively expressed CD40L at levels sufficient to induce DC maturation in vitro and in vivo in the absence of antigenic stimulation. CD4(+) T(EM) cells were excluded from resting lymph nodes but migrated in a CD62P-dependent fashion into reactive lymph nodes that were induced to express CD62P, in a transient or sustained fashion, on high endothelial venules. Trafficking of CD4(+) T(EM) cells into chronic reactive lymph nodes maintained resident DCs in a mature state and promoted naive T cell responses and experimental autoimmune encephalomyelitis (EAE) to antigens administered in the absence of adjuvants. Antibodies to CD62P, which blocked CD4(+) T(EM) cell migration into reactive lymph nodes, inhibited DC maturation, T cell priming, and induction of EAE. These results show that T(EM) cells can behave as endogenous adjuvants and suggest a mechanistic link between lymphocyte traffic in lymph nodes and induction of autoimmunity.

NLRC5, a member of the NOD-like receptor (NLR) protein family, has recently been characterized as the master transcriptional regulator of MHCI molecules in lymphocytes, in which it is highly expressed. However, its role in activated dendritic cells (DCs), which are instrumental to initiate T cell responses, remained elusive. We show in this study that, following stimulation of DCs with inflammatory stimuli, not only did NLRC5 level increase, but also its importance in directing MHCI transcription. Despite markedly reduced mRNA and intracellular H2-K levels, we unexpectedly observed nearly normal H2-K surface display in Nlrc5−/− DCs. Importantly, this discrepancy between a strong intracellular and a mild surface defect in H2-K levels was observed also in DCs with H2-K transcription defects independent of Nlrc5. Hence, alongside with demonstrating the importance of NLRC5 in MHCI transcription in activated DCs, we uncover a general mechanism counteracting low MHCI surface expression. In agreement with the decreased amount of neosynthesized MHCI, Nlrc5−/− DCs exhibited a defective capacity to display endogenous Ags. However, neither T cell priming by endogenous Ags nor cross-priming ability was substantially affected in activated Nlrc5−/− DCs. Altogether, these data show that Nlrc5 deficiency, despite significantly affecting MHCI transcription and Ag display, is not sufficient to hinder T cell activation, underlining the robustness of the T cell priming process by activated DCs.

International audience; Pattern recognition receptors (PRRs) are commonly known as sensor proteins crucial for the early detection of microbial or host-derived stress signals by innate immune cells. Interestingly, some PRRs are also expressed and functional in cells of the adaptive immune system. These receptors provide lymphocytes with innate sensing abilities; for example, B cells express Toll-like receptors, which are important for the humoral response. Strikingly, certain other NOD-like receptors are not only highly expressed in adaptive immune cells, but also exert functions related specifically to adaptive immune system pathways, such as regulating antigen presentation. In this review, we will focus particularly on the current understanding of PRR functions intrinsic to B and T lymphocytes; a developing aspect of PRR biology.

Inflammasomes are protein complexes that form in response to pathogen-derived or host-derived stress signals. Their activation leads to the production of inflammatory cytokines and promotes a pyrogenic cell death process. The massive release of inflammatory mediators that follows inflammasome activation is a key event in alarming innate immune cells. Growing evidence also highlights the role of inflammasome-dependent cytokines in shaping the adaptive immune response, as exemplified by the capacity of IL-1β to support Th17 responses, or by the finding that IL-18 evokes antigen-independent IFN-γ secretion by memory CD8+ T cells. A deeper understanding of these mechanisms and on how to manipulate this powerful inflammatory system therefore represents an important step forward in the development of improved vaccine strategies. © 2013 Elsevier Ltd.

Nucleotide-binding domain and leucine-rich repeat containing receptors (NLRs) are intracellular proteins mainly involved in pathogen recognition, inflammatory responses, and cell death. Until recently, the function of the family member NLR caspase recruitment domain (CARD) containing 5 (NLRC5) has been a matter of debate. It is now clear that NLRC5 acts as a transcriptional regulator of the major-histocompatibility complex (MHC) class I. In this review we detail the development of our understanding of NLRC5 function, discussing both the accepted and the controversial aspects of NLRC5 activity. We give insight into the molecular mechanisms, and the potential implications, of NLRC5 function in health and disease.

Conditioning therapies before transplantation induce the release of uric acid, which triggers the NLRP3 inflammasome and IL-1β production contributing to graft-versus-host disease., The success of allogeneic hematopoietic cell transplantation is limited by acute graft-versus-host disease (GvHD), a severe complication accompanied by high mortality rates. Yet, the molecular mechanisms initiating this disease remain poorly defined. In this study, we show that, after conditioning therapy, intestinal commensal bacteria and the damage-associated molecular pattern uric acid contribute to Nlrp3 inflammasome–mediated IL-1β production and that gastrointestinal decontamination and uric acid depletion reduced GvHD severity. Early blockade of IL-1β or genetic deficiency of the IL-1 receptor in dendritic cells (DCs) and T cells improved survival. The Nlrp3 inflammasome components Nlrp3 and Asc, which are required for pro–IL-1β cleavage, were critical for the full manifestation of GvHD. In transplanted mice, IL-1β originated from multiple intestinal cell compartments and exerted its effects on DCs and T cells, the latter being preferentially skewed toward Th17. Compatible with these mouse data, increased levels of active caspase-1 and IL-1β were found in circulating leukocytes and intestinal GvHD lesions of patients. Thus, the identification of a crucial role for the Nlrp3 inflammasome sheds new light on the pathogenesis of GvHD and opens a potential new avenue for the targeted therapy of this severe complication.

Nucleotide-binding oligomerization domain-like receptors (NLRs) are intracellular proteins involved in innate-driven inflammatory responses. The function of the family member NLR caspase recruitment domain containing protein 5 (NLRC5) remains a matter of debate, particularly with respect to NF-κB activation, type I IFN, and MHC I expression. To address the role of NLRC5, we generated Nlrc5-deficient mice (Nlrc5Δ/Δ). In this article we show that these animals exhibit slightly decreased CD8+ T cell percentages, a phenotype compatible with deregulated MHC I expression. Of interest, NLRC5 ablation only mildly affected MHC I expression on APCs and, accordingly, Nlrc5Δ/Δ macrophages efficiently primed CD8+ T cells. In contrast, NLRC5 deficiency dramatically impaired basal expression of MHC I in T, NKT, and NK lymphocytes. NLRC5 was sufficient to induce MHC I expression in a human lymphoid cell line, requiring both caspase recruitment and LRR domains. Moreover, endogenous NLRC5 localized to the nucleus and occupied the proximal promoter region of H-2 genes. Consistent with downregulated MHC I expression, the elimination of Nlrc5Δ/Δ lymphocytes by cytotoxic T cells was markedly reduced and, in addition, we observed low NLRC5 expression in several murine and human lymphoid-derived tumor cell lines. Hence, loss of NLRC5 expression represents an advantage for evading CD8+ T cell-mediated elimination by downmodulation of MHC I levels—a mechanism that may be exploited by transformed cells. Our data show that NLRC5 acts as a key transcriptional regulator of MHC I in lymphocytes and support an essential role for NLRs in directing not only innate but also adaptive immune responses.

Interleukin 1beta (IL 1beta) is a potent inflammatory cytokine that is usually cleaved and activated by inflammasome associated caspase 1. To determine whether IL 1beta activation is regulated by inhibitor of apoptosis (IAP) proteins we treated macrophages with an IAP antagonist "Smac mimetic" compound or genetically deleted the genes that encode the three IAP family members cIAP1 cIAP2 and XIAP. After Toll like receptor priming IAP inhibition triggered cleavage of IL 1beta that was mediated not only by the NLRP3 caspase 1 inflammasome but also by caspase 8 in a caspase 1 independent manner. In the absence of IAPs rapid and full generation of active IL 1beta by the NLRP3 caspase 1 inflammasome or by caspase 8 required the kinase RIP3 and reactive oxygen species production. These results demonstrate that activation of the cell death inducing ripoptosome platform and RIP3 can generate bioactive IL 1beta and implicate them as additional targets for the treatment of pathological IL 1 driven inflammatory responses.

Although the importance of the NOD-like receptor family, pyrin domain containing 3 (NLRP3) inflammasome in health and disease is well appreciated, a precise characterization of NLRP3 expression is yet undetermined. To this purpose, we generated a knock-in mouse in which the Nlrp3 coding sequence was substituted for the GFP (enhanced GFP [egfp]) gene. In this way, the expression of eGFP is driven by the endogenous regulatory elements of the Nlrp3 gene. In this study, we show that eGFP expression indeed mirrors that of NLRP3. Interestingly, splenic neutrophils, macrophages, and, in particular, monocytes and conventional dendritic cells showed robust eGFP fluorescence, whereas lymphoid subsets, eosinophils, and plasmacytoid dendritic cells showed negligible eGFP levels. NLRP3 expression was highly inducible in macrophages, both by MyD88- and Trif-dependent pathways. In vivo, when mice were challenged with diverse inflammatory stimuli, differences in both the number of eGFP-expressing cells and fluorescence intensity were observed in the draining lymph node. Thus, NLRP3 levels at the site of adaptive response initiation are controlled by recruitment of NLRP3-expressing cells and by NLRP3 induction.

Nanoparticles are increasingly used in various fields, including biomedicine and electronics. One application utilizes the opacifying effect of nano-TiO 2 , which is frequently used as pigment in cosmetics. Although TiO 2 is believed to be biologically inert, an emerging literature reports increased incidence of respiratory diseases in people exposed to TiO 2 . Here, we show that nano-TiO 2 and nano-SiO 2 , but not nano-ZnO, activate the NLR pyrin domain containing 3 (Nlrp3) inflammasome, leading to IL-1β release and in addition, induce the regulated release of IL-1α. Unlike other particulate Nlrp3 agonists, nano-TiO 2 –dependent-Nlrp3 activity does not require cytoskeleton-dependent phagocytosis and induces IL-1α/β secretion in nonphagocytic keratinocytes. Inhalation of nano-TiO 2 provokes lung inflammation which is strongly suppressed in IL-1R– and IL-1α–deficient mice. Thus, the inflammation caused by nano-TiO 2 in vivo is largely caused by the biological effect of IL-1α. The current use of nano-TiO 2 may present a health hazard due to its capacity to induce IL-1R signaling, a situation reminiscent of inflammation provoked by asbestos exposure.

Interleukin-1beta (IL-1beta) is a potent inflammatory cytokine, which is implicated in acute and chronic inflammatory disorders. The activity of IL-1beta is regulated by the proteolytic cleavage of its inactive precursor resulting in the mature, bioactive form of the cytokine. Cleavage of the IL-1beta precursor is performed by the cysteine protease caspase-1, which is activated within protein complexes termed 'inflammasomes'. To date, four distinct inflammasomes have been described, based on different core receptors capable of initiating complex formation. Both the host and invading pathogens need to control IL-1beta production and this can be achieved by regulating inflammasome activity. However, we have, as yet, little understanding of the mechanisms of this regulation. In particular the negative feedbacks, which are critical for the host to limit collateral damage of the inflammatory response, remain largely unexplored. Recent exciting findings in this field have given us an insight into the potential of this research area in terms of opening up new therapeutic avenues for inflammatory disorders.

Caspases are best known for their role in apoptosis. More recently, they have gained prominence as critical mediators of innate immune responses. The so-called ‘inflammatory caspases’ include human caspase-1, -4, -5 and -12 and murine caspase-1, -11 and -12. Of these, caspase-1 is best characterized and serves as the prototype for our understanding of the processing, activation and function of inflammatory caspases. Like their apoptotic counterparts, inflammatory caspases are produced as inactive zymogens and require activation to become proteolytically active. Caspase-1 is activated within the inflammasome, a large cytosolic protein complex that is induced by a growing number of endogenous, microbial, chemical or environmental stimuli. The importance of caspase-1 in initiating innate immune responses is demonstrated by its role in cleaving pro-IL-1β and pro-IL-18 to their biologically active forms. New functions have also been implicated, as these proteases and the mechanisms underlying their activation and regulation emerge as important mediators of human health and disease.

Mouse memory T cell precursors express IL-7 receptor-alpha (IL-7R), proliferate with homeostatic cytokines and undergo secondary expansions with antigen. Here, we analyzed how the strength of antigenic stimulation regulates IL-7R expression, cytokine responsiveness and expansion potential of DC-primed human CD4(+ )T cells. IL-7R expression on proliferating T cells was highest at intermediate strength of stimulation, and purified CCR7(+)IL-7R(hi) and CCR7(-)IL-7R(lo) subsets had characteristics of memory and effector cells, respectively. However, CCR7(+)IL-7R(hi) cells generated under different priming conditions had strikingly different properties. Thus, increasing strength of stimulation promoted IL-7 responsiveness that correlated with reduced phosphatase and tensin homologue deleted on chromosome 10 (PTEN) expression and enhanced s6 kinase activation, suggesting a tunable IL-7R coupling to PI3 kinase-dependent signaling pathways. Furthermore, functional and gene expression analysis revealed that intermediate-stimulated CCR7(+)IL-7R(hi) cells were similar to non-polarized central memory cells with high expansion potential. Conversely, high-stimulated CCR7(+)IL-7R(hi) cells shared characteristics with circulating pre-Th1 cells and differentiated spontaneously to Th1 effector cells. These results show that the strength of stimulation determines properties of activated IL-7R(hi) T cells, and suggest that memory T cell subsets could be derived from CCR7(+) precursors that received different strengths of stimulation.

MHC class II (MHCII) genes are transactivated by the NOD-like receptor (NLR) family member CIITA, which is recruited to SXY enhancers of MHCII promoters via a DNA-binding “enhanceosome” complex. NLRC5, another NLR protein, was recently found to control transcription of MHC class I (MHCI) genes. However, detailed understanding of NLRC5’s target gene specificity and mechanism of action remained lacking. We performed ChIP-sequencing experiments to gain comprehensive information on NLRC5-regulated genes. In addition to classical MHCI genes, we exclusively identified novel targets encoding non-classical MHCI molecules having important functions in immunity and tolerance. ChIP-sequencing performed with Rfx5−/− cells, which lack the pivotal enhanceosome factor RFX5, demonstrated its strict requirement for NLRC5 recruitment. Accordingly, Rfx5-knockout mice phenocopy Nlrc5 deficiency with respect to defective MHCI expression. Analysis of B cell lines lacking RFX5, RFXAP, or RFXANK further corroborated the importance of the enhanceosome for MHCI expression. Although recruited by common DNA-binding factors, CIITA and NLRC5 exhibit non-redundant functions, shown here using double-deficient Nlrc5−/−CIIta−/− mice. These paradoxical findings were resolved by using a “de novo” motif-discovery approach showing that the SXY consensus sequence occupied by NLRC5 in vivo diverges significantly from that occupied by CIITA. These sequence differences were sufficient to determine preferential occupation and transactivation by NLRC5 or CIITA, respectively, and the S box was found to be the essential feature conferring NLRC5 specificity. These results broaden our knowledge on the transcriptional activities of NLRC5 and CIITA, revealing their dependence on shared enhanceosome factors but their recruitment to distinct enhancer motifs in vivo. Furthermore, we demonstrated selectivity of NLRC5 for genes encoding MHCI or related proteins, rendering it an attractive target for therapeutic intervention. NLRC5 and CIITA thus emerge as paradigms for a novel class of transcriptional regulators dedicated for transactivating extremely few, phylogenetically related genes., Author Summary Major histocompatibility complex class I (MHCI) molecules are central to immunity and immunological disorders, and constitute a major obstacle in organ transplantation. It is therefore vital to gain insight into the regulation of their expression. NLRC5 was recently found to regulate MHCI gene transcription. However, we lack a thorough understanding of its target gene specificity and mechanism of action. Our work addresses these questions, delineating the unique consensus sequence required for NLRC5 recruitment and pinpointing conserved features conferring its specificity. Furthermore, through genome-wide analyses, we confirm that NLRC5 regulates classical MHCI genes and identify novel target genes, all encoding non-classical MHCI molecules exerting an array of functions in immunity and tolerance. We thereby demonstrate that NLRC5 exclusively transactivates genes of the MHCI pathway, rendering it an attractive target for future therapeutic intervention. The most striking feature of NLRC5 is its restricted and highly focused transcriptional activity, which has been described so far only for one related factor, CIITA. NLRC5 and CIITA therefore emerge as prototypes for a novel kind of extremely specific transcriptional regulator.

Dear Editor, NLRX1/NOD5 belongs to the NOD clade of the NOD-like receptor (NLR) family. NLRs have emerged as key intracellular sensors for pathogen-derived molecules and endogenous danger signals to activate innate immune responses and inflammation. However, the function of most of these receptors still remains unclear.1 Beside the NACHT and LRR domains present in most NLRs, NLRX1/NOD5 possesses a N-terminal mitochondrial targeting sequence.2, 3 The function of NLRX1/NOD5 is still controversial; one study proposed that NLRX1/NOD5 inhibits the RIG-like receptor (RLR) antiviral pathways by binding the adaptor protein MAVS/IPS-1/Cardif/VISA,2 whereas another report implicated NLRX1/NOD5 in the generation of reactive oxygen species (ROS) and the amplification of NF-κB and JNK activation triggered by different stimuli such TNF-α, poly (I:C) and Shigella infection.3 Interestingly, a recent study proposed that NLRX1/NOD5 is imported to the mitochondrial matrix, making NLRX1/NOD5 interaction with the outer mitochondrial membrane protein MAVS difficult to explain.4 To clarify the role of NOD5 in vivo, we generated NOD5-deficient mice by replacing the first four coding exons with a neomycin selection cassette. Mutant mice are viable and born at the expected Mendelian ratios (data not shown). Considering the proposed role for NOD5 in modulating RLR–MAVS signalling,2 we tested whether NOD5 deficiency potentiates responses triggered by intracellular poly (I:C). However, when primary NOD5-deficient mouse embryonic fibroblasts (MEFs) were transfected with poly (I:C), we did not observe any major difference in MAVS-dependent IRF3 phosphorylation and IFN-β or IP-10 mRNA induction (Supplementary Figure 1 and data not shown). Moreover, IFN-β and IP-10 mRNA induction upon Sendai virus infection was normal in NOD5-deficient bone marrow-derived macrophages (BMDMs) (Supplementary Figure 1 and data not shown). NOD5-deficient MEFs also produced normal levels of IFN-β and IL-6 upon poly (I:C) stimulation (Supplementary Figure 1 and data not shown). To corroborate these observations in vivo, we assessed the effect of NOD5 deficiency on serum levels of IFN-β and IL-6 induced by intravenous injection of poly (I:C), responses largely dependent on MAVS (data not shown and5). Importantly, production of these cytokines was not affected by NOD5 deficiency (Supplementary Figure 1 and data not shown). Thus, NOD5 deficiency does not affect MAVS-dependent responses. A second report links NOD5 to ROS production and amplification of NF-κB and JNK signalling upon TNF-α stimulation or Shigella infection.3 Interestingly, no major differences in TNF-α-induced NF-κB, JNK and p38 activation were observed in NOD5 knockout cells (data not shown). In a complementary approach to define NOD5 function, we carried out a proteomic screen to identify new NOD5 interaction partners. As a significant hit, we found UQCRC2 (ubiquinol-cytochrome-c reductase complex core protein 2), a subunit of the complex III of the mitochondrial respiratory chain. UQCRC2 was previously identified as an NOD5-binding partner.4 In contrast, we failed to detect MAVS in the NOD5 immunoprecipitates. In keeping with this, endogenous UQCRC2, but not MAVS, could be co-immunoprecipitated with NOD5 (Supplementary Figure 1 and data not shown). Collectively, our data indicate that NOD5 deficiency does not affect RLR signalling in vitro and in vivo, at least under the conditions tested. The reasons for the discrepancy between our findings and the in vitro characterisation of NOD5 as a RLR inhibitor are unclear, but are reminiscent of reports investigating NLRC5/NOD4 function; NLRC5/NOD4 was also proposed as RLR inhibitor based on in vitro studies, but no alteration in RLR responses were observed in NLRC5/NOD4-deficient mice.6, 7 Although our findings indicate that NOD5 deficiency does not affect TNF-induced signalling, the confirmation of UQCRC2 as a ‘bonafide' interaction partner of NOD5 is consistent with the proposed link between NOD5 and ROS generation.3, 4, 8 Mitochondrial ROS has been recently shown to trigger NLRP3 inflammasome activation, and thus modulation of ROS would be an attractive function of NOD5. With the availability of NOD5-deficient mice, this hypothesis is now testable.

BACKGROUND: Characteristic symptoms of malaria include recurrent fever attacks and neurodegeneration, signs that are also found in patients with a hyperactive Nalp3 inflammasome. Plasmodium species produce a crystal called hemozoin that is generated by detoxification of heme after hemoglobin degradation in infected red blood cells. Thus, we hypothesized that hemozoin could activate the Nalp3 inflammasome, due to its particulate nature reminiscent of other inflammasome-activating agents. METHODOLOGY/PRINCIPAL FINDINGS: We found that hemozoin acts as a proinflammatory danger signal that activates the Nalp3 inflammasome, causing the release of IL-1beta. Similar to other Nalp3-activating particles, hemozoin activity is blocked by inhibiting phagocytosis, K(+) efflux and NADPH oxidase. In vivo, intraperitoneal injection of hemozoin results in acute peritonitis, which is impaired in Nalp3-, caspase-1- and IL-1R-deficient mice. Likewise, the pathogenesis of cerebral malaria is dampened in Nalp3-deficient mice infected with Plasmodium berghei sporozoites, while parasitemia remains unchanged. SIGNIFICANCE/CONCLUSIONS: The potent pro-inflammatory effect of hemozoin through inflammasome activation may possibly be implicated in plasmodium-associated pathologies such as cerebral malaria.

Through their capacity to sense danger signals and to generate active interleukin 1beta (IL 1beta) inflammasomes occupy a central role in the inflammatory response. In contrast to IL 1beta little is known about how IL 1alpha is regulated. We found that all inflammasome activators also induced the secretion of IL 1alpha leading to the cosecretion of both IL 1 cytokines. Depending on the type of inflammasome activator release of IL 1alpha was inflammasome dependent or independent. Calcium influx induced by the opening of cation channels was sufficient for the inflammasome independent IL 1alpha secretion. In both cases IL 1alpha was released primarily in a processed form resulting from intracellular cleavage by calpain like proteases. Inflammasome caspase 1 dependent release of IL 1alpha and IL 1beta was independent of caspase 1 catalytic activity defining a mode of action for caspase 1. Because inflammasomes contribute to the pathology of numerous chronic inflammatory diseases such as gout and diabetes IL 1alpha antagonists may be beneficial in the treatment of these disorders.

The recent use of T cell-based cancer immunotherapies, such as adoptive T-cell transfer and checkpoint blockade, yields increasing clinical benefit to patients with different cancer types. However, decrease of MHC class I expression is a common mechanism transformed cells take advantage of to evade CD8(+) T cell-mediated antitumor responses, negatively impacting on the outcome of immunotherapies. Hence, there is an urgent need to develop novel approaches to overcome this limitation. NLRC5 has been recently described as a key transcriptional regulator controlling expression of MHC class I molecules. In this commentary, we summarize and put into perspective a study by Rodriguez and colleagues recently published in Oncoimmunology, addressing the role of NLRC5 in melanoma. The authors demonstrate that NLRC5 overexpression in B16 melanoma allows to recover MHC class I expression, rising tumor immunogenicity and counteracting immune evasion. Possible ways of manipulating NLRC5 activity in tumors will be discussed. Highlighting the therapeutic potential of modulating NLRC5 levels, this publication also encourages evaluation of NLRC5, and by extension MHC class I pathway, as clinical biomarker to select personalized immunotherapeutic strategies.