Leishmania guyanensis suppressed inducible nitric oxide synthase provoked by its viral endosymbiont

Inducible nitric oxide synthase (iNOS) is essential to the production of nitric oxide (NO), an efficient effector molecule against intracellular human pathogens such as Leishmania protozoan parasites. Some strains of Leishmania are known to bear a viral endosymbiont termed Leishmania RNA virus 1 (LRV1). Recognition of LRV1 by the innate immune sensor Toll-like receptor-3 (TLR3) leads to conditions worsening the disease severity in mice. This process is governed by type I interferon (type I IFNs) arising downstream of TLR3 stimulation and favoring the formation of secondary metastatic lesions. The formation of these lesions is mediated by the inflammatory cytokine IL-17A and occurs in the absence, or low level of, protective cytokine IFN-γ. Here, we described that the presence of LRV1 led to the initial expression of iNOS and low production of NO that failed to control infection. We subsequently showed that LRV1-triggered type I IFN was essential but insufficient to induce robust iNOS induction, which requires strong activation of nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB). Leishmania guyanensis carrying LRV1 (LgyLRV1+) parasites mitigated strong iNOS production by limiting NF-kB activation via the induction of tumor necrosis factor-alpha-induced protein 3 (TNFAIP3), also known as A20. Moreover, our data suggested that production of LRV1-induced iNOS could be correlated with parasite dissemination and metastasis via elevated secretion of IL-17A in the draining lymph nodes. Our findings support an additional strategy by which LRV1-bearing Leishmania guyanensis evaded killing by nitric oxide and suggest that low levels of LRV1-induced NO might contribute to parasite metastasis.

In and out: Leishmania metastasis by hijacking lymphatic system and migrating immune cells

The lymphatic system plays a crucial role in mounting immune response against intracellular pathogens, and recent studies have documented its role in facilitating tumor dissemination linked largely with cancer cells. However, in mucocutaneous leishmaniasis (MCL) caused by Leishmania Viannia subgenus showing infectious metastasis and resulting in severe distant secondary lesions, the route of escape of these parasites to secondary sites has not yet been investigated in detail. Our results demonstrated that when infection was associated with inflammation and additionally exacerbated by the presence of dsRNA viral endosymbiont (LRV1), lymphatic vessels could serve as efficient routes for infected cells to egress from the primary site and colonize distant organs. We challenged this hypothesis by using the intracellular Leishmania protozoan parasites Leishmania guyanensis (Lgy) associated with or without a dsRNA viral endosymbiont, exacerbating the infection and responsible for a strong inflammatory response, and favoring metastasis of the infection. We analyzed possible cargo cells and the routes of dissemination through flow cytometry, histological analysis, and in vivo imaging in our metastatic model to show that parasites disseminated not only intracellularly but also as free extracellular parasites using migrating immune cells, lymph nodes (LNs), and lymph vessels, and followed intricate connections of draining and non-draining lymph node to finally end up in the blood and in distant skin, causing new lesions.

Dissection of the macrophage response towards infection by the Leishmania-viral endosymbiont duo and dynamics of the type I interferon response

Leishmania RNA virus 1 (LRV1) is a double-stranded RNA virus found in some strains of the human protozoan parasite Leishmania, the causative agent of leishmaniasis, a neglected tropical disease. Interestingly, the presence of LRV1 inside Leishmania constitutes an important virulence factor that worsens the leishmaniasis outcome in a type I interferon (IFN)–dependent manner and contributes to treatment failure. Understanding how macrophages respond toward Leishmania alone or in combination with LRV1 as well as the role that type I IFNs may play during infection is fundamental to oversee new therapeutic strategies. To dissect the macrophage response toward infection, RNA sequencing was performed on murine wild-type and Ifnar-deficient bone marrow–derived macrophages infected with Leishmania guyanensis (Lgy) devoid or not of LRV1. Additionally, macrophages were treated with poly I:C (mimetic virus) or with type I IFNs. By implementing a weighted gene correlation network analysis, the groups of genes (modules) with similar expression patterns, for example, functionally related, coregulated, or the members of the same functional pathway, were identified. These modules followed patterns dependent on Leishmania, LRV1, or Leishmania exacerbated by the presence of LRV1. Not only the visualization of how individual genes were embedded to form modules but also how different modules were related to each other were observed. Thus, in the context of the observed hyperinflammatory phenotype associated to the presence of LRV1, it was noted that the biomarkers tumor-necrosis factor α (TNF-α) and the interleukin 6 (IL-6) belonged to different modules and that their regulating specific Src-family kinases were segregated oppositely. In addition, this network approach revealed the strong and sustained effect of LRV1 on the macrophage response and genes that had an early, late, or sustained impact during infection, uncovering the dynamics of the IFN response. Overall, this study contributed to shed light and dissect the intricate macrophage response toward infection by the Leishmania-LRV1 duo and revealed the crosstalk between modules made of coregulated genes and provided a new resource that can be further explored to study the impact of Leishmania on the macrophage response.

Sex-Biased Control of Inflammation and Metabolism by a Mitochondrial Nod-Like Receptor

Mitochondria regulate steroid hormone synthesis, and in turn sex hormones regulate mitochondrial function for maintaining cellular homeostasis and controlling inflammation. This crosstalk can explain sex differences observed in several pathologies such as in metabolic or inflammatory disorders. Nod-like receptor X1 (NLRX1) is a mitochondria-associated innate receptor that could modulate metabolic functions and attenuates inflammatory responses. Here, we showed that in an infectious model with the human protozoan parasite, Leishmania guyanensis, NLRX1 attenuated inflammation in females but not in male mice. Analysis of infected female and male bone marrow derived macrophages showed both sex- and genotype-specific differences in both inflammatory and metabolic profiles with increased type I interferon production, mitochondrial respiration, and glycolytic rate in Nlrx1-deficient female BMDMs in comparison to wild-type cells, while no differences were observed between males. Transcriptomics of female and male BMDMs revealed an altered steroid hormone signaling in Nlrx1-deficient cells, and a “masculinization” of Nlrx1-deficient female BMDMs. Thus, our findings suggest that NLRX1 prevents uncontrolled inflammation and metabolism in females and therefore may contribute to the sex differences observed in infectious and inflammatory diseases.

The Dangerous Liaisons in the Oxidative Stress Response to Leishmania Infection

Leishmania parasites preferentially invade macrophages, the professional phagocytic cells, at the site of infection. Macrophages play conflicting roles in Leishmania infection either by the destruction of internalized parasites or by providing a safe shelter for parasite replication. In response to invading pathogens, however, macrophages induce an oxidative burst as a mechanism of defense to promote pathogen removal and contribute to signaling pathways involving inflammation and the immune response. Thus, oxidative stress plays a dual role in infection whereby free radicals protect against invading pathogens but can also cause inflammation resulting in tissue damage. The induced oxidative stress in parasitic infections triggers the activation in the host of the antioxidant response to counteract the damaging oxidative burst. Consequently, macrophages are crucial for disease progression or control. The ultimate outcome depends on dangerous liaisons between the infecting Leishmania spp. and the type and strength of the host immune response.

Latrocimicinae completes the phylogeny of Cimicidae: meeting old morphologic data rather than modern host phylogeny

The family Cimicidae includes obligate hematophagous ectoparasites (bed bugs and their relatives) with high veterinary/medical importance. The evolutionary relationships of Cimicidae and their hosts have recently been reported in a phylogenetic context, but in the relevant study, one of the six subfamilies, the bat-specific Latrocimicinae, was not represented. In this study the only known species of Latrocimicinae, i.e., Latrocimex spectans, was analyzed with molecular and phylogenetic methods based on four (two nuclear and two mitochondrial) genetic markers. The completed subfamily-level phylogeny of Cimicidae showed that Latrocimicinae is most closely related to Haematosiphoninae (ectoparasites of birds and humans), with which it shares systematically important morphologic characters, but not hosts. Moreover, in the phylogenetic analyses, cimicid bugs that are known to infest phylogenetically distant bat hosts clustered together (e.g., Leptocimex and Stricticimex within Cacodminae), while cimicid subfamilies (Latrocimicinae, Primicimicinae) that are known to infest bat hosts from closely related superfamilies clustered distantly. In conclusion, adding Latrocimicinae significantly contributed to the resolution of the phylogeny of Cimicidae. The close phylogenetic relationship between Latrocimicinae and Haematosiphoninae is consistent with long-known morphologic data. At the same time, phylogenetic relationships of genera within subfamilies are inconsistent with the phylogeny of relevant hosts.

The antioxidant response favors Leishmania parasites survival, limits inflammation and reprograms the host cell metabolism

The oxidative burst generated by the host immune system can restrict intracellular parasite entry and growth. While this burst leads to the induction of antioxidative enzymes, the molecular mechanisms and the consequences of this counter-response on the life of intracellular human parasites are largely unknown. The transcription factor NF-E2-related factor (NRF2) could be a key mediator of antioxidant signaling during infection due to the entry of parasites. Here, we showed that NRF2 was strongly upregulated in infection with the human Leishmania protozoan parasites, its activation was dependent on a NADPH oxidase 2 (NOX2) and SRC family of protein tyrosine kinases (SFKs) signaling pathway and it reprogrammed host cell metabolism. In inflammatory leishmaniasis caused by a viral endosymbiont inducing TNF-α in chronic leishmaniasis, NRF2 activation promoted parasite persistence but limited TNF-α production and tissue destruction. These data provided evidence of the dual role of NRF2 in protecting both the invading pathogen from reactive oxygen species and the host from an excess of the TNF-α destructive pro-inflammatory cytokine.

Behind the Scenes: Nod-Like Receptor X1 Controls Inflammation and Metabolism

Regulatory Nod-like receptors (NLRs) are a subgroup of the cytosolic NLR family of pathogen recognition receptors (PRRs). These receptors can tune the innate immune responses triggered by the activation of other PRRs by either augmenting or attenuating the activated pro-inflammatory signaling cascades. Nod-like receptor X1 (NLRX1) is the only known mitochondria-associated negative regulatory NLR. NLRX1 attenuates several inflammatory pathways and modulates cellular processes such as autophagy and mitochondrial function following infection or injury. Using both in vitro expression and in vivo experimental models, NLRX1 is extensively described in the context of anti-viral signaling and host-defense against invading pathogens. More recently, NLRX1 has also gained interest in the field of cancer and metabolism where NLRX1 functions to attenuate overzealous inflammation in various inflammatory and autoimmune diseases. However, the exact function of this novel receptor is still under debate and many, often contradictory, mechanisms of action together with cellular localizations have been proposed. Thus, a better understanding of the underlying mechanism is crucial for future research and development of novel therapeutical approaches. Here, we summarize the current findings on NLRX1 and discuss its role in both infectious and inflammatory context.

Viruses of protozoan parasites and viral therapy: Is the time now right?

Infections caused by protozoan parasites burden the world with huge costs in terms of human and animal health. Most parasitic diseases caused by protozoans are neglected, particularly those associated with poverty and tropical countries, but the paucity of drug treatments and vaccines combined with increasing problems of drug resistance are becoming major concerns for their control and eradication. In this climate, the discovery/repurposing of new drugs and increasing effort in vaccine development should be supplemented with an exploration of new alternative/synergic treatment strategies. Viruses, either native or engineered, have been employed successfully as highly effective and selective therapeutic approaches to treat cancer (oncolytic viruses) and antibiotic-resistant bacterial diseases (phage therapy). Increasing evidence is accumulating that many protozoan, but also helminth, parasites harbour a range of different classes of viruses that are mostly absent from humans. Although some of these viruses appear to have no effect on their parasite hosts, others either have a clear direct negative impact on the parasite or may, in fact, contribute to the virulence of parasites for humans. This review will focus mainly on the viruses identified in protozoan parasites that are of medical importance. Inspired and informed by the experience gained from the application of oncolytic virus- and phage-therapy, rationally-driven strategies to employ these viruses successfully against parasitic diseases will be presented and discussed in the light of the current knowledge of the virus biology and the complex interplay between the viruses, the parasite hosts and the human host. We also highlight knowledge gaps that should be addressed to advance the potential of virotherapy against parasitic diseases.

Viral Double-Stranded RNA Detection by DNase I and Nuclease S1 digestions in Leishmania parasites

Many RNA viruses are found in protozoan parasites. They can be responsible for more serious pathology or treatment failure. For the detection of viral double-stranded RNA (dsRNA), sequence-dependent and -independent methods are available, such as quantitative real-time PCR and immunofluorescence, dot blot, ELISA or sequencing. The technique presented here is sequence-independent and is well detailed in the following protocol, taking the example of Leishmania RNA virus (LRV) in Leishmania guyanensis (Lgy) species. To summarise, the protocol is divided into four major steps: RNA extraction from the parasites, RNA purification, enzymatic digestions with DNase I and Nuclease S1, and visualization by gel electrophoresis. This method can be used to detect other viral dsRNA in other parasites. It provides an additional tool, complementary to other techniques previously cited and it is easy and quite fast to achieve.

Morphological and physiological consequences of a dietary restriction during early life in bats

Early life adverse conditions can have major consequences on an individual’s life history traits. Oxidative stress has been hypothesized to be one main mechanism underlying the negative consequences of early life adverse conditions. To test this hypothesis, we restricted the food availability of Seba’s short-tailed bat (Carollia perspicillata) mothers of unweaned pups for 10 days, followed by ad libitum provisioning. We also had a control, unrestricted group. We explored the morphological consequences of dietary restriction during early life by measuring growth rate. We also measured four markers of blood oxidative balance during growth. We assessed the level of cortisol, and its inactive form cortisone, in the hair of the pups at the end of growth. Finally, we monitored survival during the first year. Food restriction triggered a slowdown in growth followed by catch-up growth when ad libitum feeding was restored which did not lead to full compensation in size or mass compared to control individuals. We found that higher growth rate was associated with elevated oxidative damage, suggesting an oxidative cost to growth. However, we found no clear evidence for physiological costs specific to the catch-up growth. Survival after a year was not impacted by the treatment, the oxidative balance or the level of glucocorticoids at the end of growth. In conclusion, our results show that individuals were able to efficiently mitigate the short-term consequences of adverse early life conditions. However, consequences might arise in the long-term, and could impact reproductive success or lifespan.

Leishmania Parasite Quantification by Bioluminescence in Murine Models

Leishmaniasis remains a major public health problem worldwide with a prevalence of 12 million, an incidence of 1 million persons, and 350 million people being at risk. Murine models have been largely used for studying the host-pathogen relationship and developing effective chemotherapies against Leishmania parasites. Thus, preclinical imaging is crucial for monitoring the disease outcome. The aim of this protocol is to quantify parasite burden using bioluminescence in vivo imaging. Here, we describe a high-throughput imaging workflow, together with data acquisition and analysis ideal to assess in vivo parasite load in mouse models.

Amazonian Phlebovirus (Bunyaviridae) potentiates the infection of Leishmania (Leishmania) amazonensis: Role of the PKR/IFN1/IL-10 axis

BACKGROUND

Leishmania parasites are transmitted to vertebrate hosts by phlebotomine sandflies and, in humans, may cause tegumentary or visceral leishmaniasis. The role of PKR (dsRNA activated kinase) and Toll-like receptor 3 (TLR3) activation in the control of Leishmania infection highlights the importance of the engagement of RNA sensors, which are usually involved in the antiviral cell response, in the fate of parasitism by Leishmania. We tested the hypothesis that Phlebovirus, a subgroup of the Bunyaviridae, transmitted by sandflies, would interfere with Leishmania infection.

METHODOLOGY/PRINCIPAL FINDINGS

We tested two Phlebovirus isolates, Icoaraci and Pacui, from the rodents Nectomys sp. and Oryzomys sp., respectively, both natural sylvatic reservoir of Leishmania (Leishmania) amazonensis from the Amazon region. Phlebovirus coinfection with L. (L.) amazonensis in murine macrophages led to increased intracellular growth of L. (L.) amazonensis. Further studies with Icoaraci coinfection revealed the requirement of the PKR/IFN1 axis on the exacerbation of the parasite infection. L. (L.) amazonensis and Phlebovirus coinfection potentiated PKR activation and synergistically induced the expression of IFNβ and IL-10. Importantly, in vivo coinfection of C57BL/6 mice corroborated the in vitro data. The exacerbation effect of RNA virus on parasite infection may be specific because coinfection with dengue virus (DENV2) exerted the opposite effect on parasite load.

CONCLUSIONS

Altogether, our data suggest that coinfections with specific RNA viruses shared by vectors or reservoirs of Leishmania may enhance and sustain the activation of host cellular RNA sensors, resulting in aggravation of the parasite infection. The present work highlights new perspectives for the investigation of antiviral pathways as important modulators of protozoan infections.

How to master the host immune system? Leishmania parasites have the solutions!

Infection by protozoan parasites of the genus Leishmania results in the development of leishmaniasis, an increasingly prevalent group of diseases affecting over 12 million people worldwide. Leishmaniasis can have very different outcomes ranging from cutaneous lesions, mucosal lesions to visceralization depending on the species of the infecting parasite and on the immune response developed by the host. As an obligate intracellular parasite, residing within macrophages, Leishmania evolved in strict contact with the host immune system, developing different mechanisms to evade or modulate the immune response. Various types of immune responses are observed during different Leishmania spp. infections, resulting in parasite clearance but also contributing to the pathogenesis, thus increasing the complexity of the course of the disease. Interestingly, depending on the type of leishmaniasis developed, opposite treatment strategies, which either boost or inhibit the inflammatory response, have shown efficacy. In this review, we summarize the contribution of different immune cell types to the development of the anti-leishmanial immune response and the parasite strategies to evade and modulate host immunity. Further, we discuss the involvement of co-infecting pathogens in the determination of the outcome of leishmaniasis and on the effectiveness of treatment and the implication of the immune response for treatment and vaccine development.

TLR2 Signaling in Skin Nonhematopoietic Cells Induces Early Neutrophil Recruitment in Response to Leishmania major Infection

Neutrophils are rapidly recruited to the mammalian skin in response to infection with the cutaneous Leishmania pathogen. The parasites use neutrophils to establish the disease; however, the signals driving early neutrophil recruitment are poorly known. Here, we identified the functional importance of TLR2 signaling in this process. Using bone marrow chimeras and immunohistology, we identified the TLR2-expressing cells involved in this early neutrophil recruitment to be of nonhematopoietic origin. Keratinocytes are damaged and briefly in contact with the parasites during infection. We show that TLR2 triggering by Leishmania major is required for their secretion of neutrophil-attracting chemokines. Furthermore, TLR2 triggering by L. major phosphoglycans is critical for neutrophil recruitment to negatively affect disease development, as shown by better control of lesion size and parasite load in Tlr2^-/- compared with wild-type infected mice. Conversely, restoring early neutrophil presence in Tlr2^-/- mice through injection of wild-type neutrophils or CXCL1 at the onset of infection resulted in delayed disease resolution comparable to that observed in wild-type mice. Taken together, our data show a crucial role for TLR2-expressing nonhematopoietic skin cells in the recruitment of the first wave of neutrophils after L. major infection, a process that delays disease control.

The criminal association of Leishmania parasites and viruses

In nature, humans infected with protozoan parasites can encounter viruses, which could alter their host immune response. The impact of viruses on human parasitic diseases remains largely unexplored due to the highly sterilized environment in experimental studies and the difficulty to draw a correlation between co-infection and pathology. Recent studies show that viral infections exacerbate pathology and promote dissemination of some Leishmania infections, based on a hyper-inflammatory reaction driven by type I interferons. Thus, not only the infecting parasite species, but also bystander viral infections could be a major determinant of the outcome of Leishmania infection. In this review, we focus on the contribution of viral co-infection to the exacerbation of leishmaniasis's pathology and its possible impact on treatment and vaccination strategies.

Leishmania amazonensis downregulates macrophage iNOS expression via Histone Deacetylase 1 (HDAC1): a novel parasite evasion mechanism

The induced expression of nitric oxide synthase (iNOS) controls the intracellular growth of Leishmania in infected macrophages. Histones deacetylases (HDACs) negatively regulate gene expression through the formation of complexes containing transcription factors such as NF-κB p50/50. Herein, we demonstrated the occupancy of p50/p50_HDAC1 to iNOS promoter associated with reduced levels of H3K9Ac. Remarkably, we found increased levels of HDAC1 in L. amazonensis-infected macrophages. HDAC1 upregulation was not found in L. major-infected macrophages. The parasite intracellular load was reduced in HDAC1 knocked-down macrophages, which presented increased nitric oxide levels. HDAC1 silencing led to the occupancy of CBP/p300 to iNOS promoter and the rise of H3K9Ac modification. Importantly, the immunostaining of skin samples from hiporeactive cutaneous leishmaniasis patients infected with L. amazonensis, revealed high levels of HDAC1. In brief, L. amazonensis induces HDAC1 in infected macrophages, which contribute to parasite survival and is associated to hiporeactive stage found in L. amazonensis infected patients.

Importance of polyphosphate in the Leishmania life cycle

Protozoan parasites contain negatively charged polymers of a few up to several hundreds of phosphate residues. In other organisms, these poly-phosphate (polyP) chains serve as an energy source and phosphate reservoir, and have been implicated in adaptation to stress and virulence of pathogenic organisms. In this study, we confirmed first that the polyP polymerase vacuolar transporter chaperone 4 (VTC4) is responsible for polyP synthesis in Leishmania parasites. During Leishmaniain vitro culture, polyP is accumulated in logarithmic growth phase and subsequently consumed once stationary phase is reached. However, polyP is not essential since VTC4-deficient (vtc4^-) Leishmania proliferated normally in culture and differentiated into infective metacyclic parasites and into intracellular and axenic amastigotes. In in vivo mouse infections, L. majorVTC4 knockout showed a delay in lesion formation but ultimately gave rise to strong pathology, although we were unable to restore virulence by complementation to confirm this phenotype. Knockdown of VTC4 did not alter the course of L. guyanensis infections in mice, suggesting that polyP was not required for infection, or that very low levels of it suffice for lesion development. At higher temperatures, Leishmania promastigotes highly consumed polyP, and both knockdown or deletion of VTC4 diminished parasite survival. Thus, although polyP was not essential in the life cycle of the parasite, our data suggests a role for polyP in increasing parasite survival at higher temperatures, a situation faced by the parasite when transmitted to humans.

Ribonuclease inhibitor 1 regulates erythropoiesis by controlling GATA1 translation

Ribosomal proteins (RP) regulate specific gene expression by selectively translating subsets of mRNAs. Indeed, in Diamond-Blackfan anemia and 5q- syndrome, mutations in RP genes lead to a specific defect in erythroid gene translation and cause anemia. Little is known about the molecular mechanisms of selective mRNA translation and involvement of ribosomal-associated factors in this process. Ribonuclease inhibitor 1 (RNH1) is a ubiquitously expressed protein that binds to and inhibits pancreatic-type ribonucleases. Here, we report that RNH1 binds to ribosomes and regulates erythropoiesis by controlling translation of the erythroid transcription factor GATA1. Rnh1-deficient mice die between embryonic days E8.5 and E10 due to impaired production of mature erythroid cells from progenitor cells. In Rnh1-deficient embryos, mRNA levels of Gata1 are normal, but GATA1 protein levels are decreased. At the molecular level, we found that RNH1 binds to the 40S subunit of ribosomes and facilitates polysome formation on Gata1 mRNA to confer transcript-specific translation. Further, RNH1 knockdown in human CD34+ progenitor cells decreased erythroid differentiation without affecting myelopoiesis. Our results reveal an unsuspected role for RNH1 in the control of GATA1 mRNA translation and erythropoiesis.

Leishmania guyanensis parasites block the activation of the inflammasome by inhibiting maturation of IL-1β

The various symptomatic outcomes of cutaneous leishmaniasis relates to the type and potency of its underlying inflammatory responses. Presence of the cytoplasmic Leishmania RNA virus-1 (LRV1) within Leishmania guyanensis, worsens lesional inflammation and parasite burden, as the viral dsRNA genome acts as a potent innate immunogen stimulating Toll-Like-Receptor-3 (TLR3). Here we investigated other innate pattern recognition receptors capable of reacting to dsRNA and potentially contributing to LRV1-mediated inflammatory pathology. We included the cytoplasmic dsRNA sensors, namely, the RIG-like receptors (RLRs) and the inflammasome-dependent and -independent Nod-like-receptors (NLRs). Our study found no role for RLRs or inflammasome-dependent NLRs in the pathology of L. guyanensis infection irrespective of its LRV1-status. Further, neither LRV1-bearing L. guyanensis (LgyLRV1+) nor LRV1-negative L. guyanensis (LgyLRV1-) activated the inflammasome in vitro. Interestingly, similarly to L. donovani, L. guyanensis infection induced the up-regulation of the A20 protein, known to be involved in the evasion of inflammasome activation. Moreover, we observed that LgyLRV1+ promoted the transcription of inflammasome-independent NLRC2 (also called NOD2) and NLRC5. However, only NLRC2 showed some contribution to LRV1-dependent pathology. These data confirmed that the endosomal TLR3 pathway is the dominant route of LRV1-dependent signalling, thus excluding the cytosolic and inflammasome pathways. We postulate that avoidance of the inflammasome pathways is likely an important mechanism of virulence in Leishmania infection irrespective of the LRV1-status.

Guidelines and recommendations on yeast cell death nomenclature

Elucidating the biology of yeast in its full complexity has major implications for science, medicine and industry. One of the most critical processes determining yeast life and physiology is cel-lular demise. However, the investigation of yeast cell death is a relatively young field, and a widely accepted set of concepts and terms is still missing. Here, we propose unified criteria for the defi-nition of accidental, regulated, and programmed forms of cell death in yeast based on a series of morphological and biochemical criteria. Specifically, we provide consensus guidelines on the differ-ential definition of terms including apoptosis, regulated necrosis, and autophagic cell death, as we refer to additional cell death rou-tines that are relevant for the biology of (at least some species of) yeast. As this area of investigation advances rapidly, changes and extensions to this set of recommendations will be implemented in the years to come. Nonetheless, we strongly encourage the au-thors, reviewers and editors of scientific articles to adopt these collective standards in order to establish an accurate framework for yeast cell death research and, ultimately, to accelerate the pro-gress of this vibrant field of research.

Development of a semi-automated image-based high-throughput drug screening system

We previously reported that the innate sensing of the endosymbiont Leishmania RNA virus 1 (LRV1) within Leishmania (Viannia) guyanensis through Toll-like receptor 3, worsens the pathogenesis of parasite infection in mice. The presence of LRV1 has been associated with the failure of first-line treatment in patients infected with LRV1 containing -L. guyanensis and -L. braziliensis parasites. Here, we established a semi-automated image-based high-throughput drug screening (HTDS) protocol to measure parasiticidal activity of the Prestwick chemical library in primary murine macrophages infected with LRV1-containing L. guyanensis. The two-independent screens generated 14 hit compounds with over sixty-nine percent reduction in parasite growth compared to control, at a single dose in both screens. Our screening strategy offers great potential in the search for new drugs and accelerates the discovery rate in the field of drug repurposing against Leishmania. Moreover, this technique allows the concomitant assessment of the effect of drug toxicity on host cell number.

Systems Approach Reveals Nuclear Factor Erythroid 2-Related Factor 2/Protein Kinase R Crosstalk in Human Cutaneous Leishmaniasis

Leishmania parasites infect macrophages, causing a wide spectrum of human diseases, from cutaneous to visceral forms. In search of novel therapeutic targets, we performed comprehensive in vitro and ex vivo mapping of the signaling pathways upstream and downstream of antioxidant transcription factor [nuclear factor erythroid 2-related factor 2 (Nrf2)] in cutaneous leishmaniasis (CL), by combining functional assays in human and murine macrophages with a systems biology analysis of in situ (skin biopsies) CL patient samples. First, we show the PKR pathway controls the expression and activation of Nrf2 in Leishmania amazonensis infection in vitro. Nrf2 activation also required PI3K/Akt signaling and autophagy mechanisms. Nrf2- or PKR/Akt-deficient macrophages exhibited increased levels of ROS/RNS and reduced expression of Sod1 Nrf2-dependent gene and reduced parasite load. L. amazonensis counteracted the Nrf2 inhibitor Keap1 through the upregulation of p62 via PKR. This Nrf2/Keap1 observation was confirmed in situ in skin biopsies from Leishmania-infected patients. Next, we explored the ex vivo transcriptome in CL patients, as compared to healthy controls. We found the antioxidant response element/Nrf2 signaling pathway was significantly upregulated in CL, including downstream target p62. In silico enrichment analysis confirmed upstream signaling by interferon and PI3K/Akt, and validated our in vitro findings. Our integrated in vitro, ex vivo, and in silico approach establish Nrf2 as a central player in human cutaneous leishmaniasis and reveal Nrf2/PKR crosstalk and PI3K/Akt pathways as potential therapeutic targets.

Modification of sperm quality after sexual abstinence in Seba's short-tailed bat, Carollia perspicillata

In polygynous mating systems, few males have stable access to sexual mates. With an expected higher copulation rate, harem males may deplete seminal fluids or increase epididymal sperm maturation, generating poor sperm quality. In a first study, we reported a higher sperm quality in sneaker males of Carollia perspicillata To test whether the lower sperm quality observed in harem males was generated by an elevated copulation rate, we temporarily removed males of both social statuses from the colony. We thus assessed status-related changes of sperm quality resulting from sexual abstinence. Moreover, released from territory and female guarding, harem males were expected to show a reduction in somatic costs. On the basis of sperm competition models, we predicted a higher resource investment in the ejaculate with the reduction of pre-copulatory efforts. In line with our predictions, sperm quality of harem males improved significantly in contrast to sneaker males, whose sperm quality did not change. Without an increase in ejaculate lipid peroxidation, our results also provide evidence that the duration of sexual abstinence was not sufficient to generate sperm oxidative damage through senescence. Harem males did not show a reduction in blood lipid peroxidation or in the ratio of oxidized to reduced glutathione. In line with the maintenance of these somatic costs, harem males did not invest more superoxide dismutase to the ejaculate to maintain sperm quality. Our results suggest that a difference in copulation rate rather than an adaptation to sperm competition provides sneaker males with higher sperm quality in C. perspicillata.

Macrophage Survival Assay Using High Content Microscopy

Macrophages maintain tissue homoeostasis by regulating inflammation and tissue repair mechanisms. Thus, the fate of macrophages has an impact on the state of the tissue. The aim of this protocol is to quantify macrophage survival using high content microscopy and image processing software. Here, we describe a high-content image based protocol to assess the effect of diverse stimuli in combination with pharmacological treatments on macrophage survival in a quantitative, unbiased and high-throughput manner.

Exacerbated Leishmaniasis Caused by a Viral Endosymbiont can be Prevented by Immunization with Its Viral Capsid

Recent studies have shown that a cytoplasmic virus called Leishmaniavirus (LRV) is present in some Leishmania species and acts as a potent innate immunogen, aggravating lesional inflammation and development in mice. In humans, the presence of LRV in Leishmania guyanensis and in L. braziliensis was significantly correlated with poor treatment response and symptomatic relapse. So far, no clinical effort has used LRV for prophylactic purposes. In this context, we designed an original vaccine strategy that targeted LRV nested in Leishmania parasites to prevent virus-related complications. To this end, C57BL/6 mice were immunized with a recombinant LRV1 Leishmania guyanensis viral capsid polypeptide formulated with a T helper 1-polarizing adjuvant. LRV1-vaccinated mice had significant reduction in lesion size and parasite load when subsequently challenged with LRV1+ Leishmania guyanensis parasites. The protection conferred by this immunization could be reproduced in naïve mice via T-cell transfer from vaccinated mice but not by serum transfer. The induction of LRV1 specific T cells secreting IFN-γ was confirmed in vaccinated mice and provided strong evidence that LRV1-specific protection arose via a cell mediated immune response against the LRV1 capsid. Our studies suggest that immunization with LRV1 capsid could be of a preventive benefit in mitigating the elevated pathology associated with LRV1 bearing Leishmania infections and possibly avoiding symptomatic relapses after an initial treatment. This novel anti-endosymbiotic vaccine strategy could be exploited to control other infectious diseases, as similar viral infections are largely prevalent across pathogenic pathogens and could consequently open new vaccine opportunities.

About 80% of leishmaniasis infections result in cutaneous manifestations with a broad symptomatic spectrum, ranging from self-healing localized to disseminated lesions. The mechanism behind these latter aggravated forms of leishmaniasis is still poorly understood. One possible factor is an endosymbiotic RNA virus identified in the cytoplasm of several Leishmania species (Leishmania RNA virus 1). LRV1 acts as a virulence factor, causing a destructive hyper-inflammatory response. In this study, we tested the prophylactic potential of a vaccine formulated with a recombinant LRV1 capsid and a T helper 1-polarizing adjuvant. Our approach conferred significant protection against LRV1+ Leishmania guyanensis infection, decreasing lesional inflammation and parasite burden. Further analysis demonstrated that this vaccine induced a potent T helper 1 response. Consequently, we propose that the LRV1-capsid is a promising vaccine component in order to reduce clinical complications (e.g. symptomatic relapses) in areas endemic to LRV1 co-infected Leishmania species. Taken together, we present an original strategy, whereby targeting the pervasive intracellular viruses within pathogens may reduce pathologic inflammation and offer an extra-genetic candidate that may circumvent escape mutations or poor response to drug treatment.

Leishmaniavirus-Dependent Metastatic Leishmaniasis Is Prevented by Blocking IL-17A

Cutaneous leishmaniasis has various outcomes, ranging from self-healing reddened papules to extensive open ulcerations that metastasise to secondary sites and are often resistant to standard therapies. In the case of L. guyanensis (L.g), about 5-10% of all infections result in metastatic complications. We recently showed that a cytoplasmic virus within L.g parasites (LRV1) is able to act as a potent innate immunogen, worsening disease outcome in a murine model. In this study, we investigated the immunophenotype of human patients infected by L.g and found a significant association between the inflammatory cytokine IL-17A, the presence of LRV1 and disease chronicity. Further, IL-17A was inversely correlated to the protective cytokine IFN-γ. These findings were experimentally corroborated in our murine model, where IL-17A produced in LRV1+ L.g infection contributed to parasite virulence and dissemination in the absence of IFN-γ. Additionally, IL-17A inhibition in mice using digoxin or SR1001, showed therapeutic promise in limiting parasite virulence. Thus, this murine model of LRV1-dependent infectious metastasis validated markers of disease chronicity in humans and elucidated the immunologic mechanism for the dissemination of Leishmania parasites to secondary sites. Moreover, it confirms the prognostic value of LRV1 and IL-17A detection to prevent metastatic leishmaniasis in human patients.

Unveiling the Role of the Integrated Endoplasmic Reticulum Stress Response in Leishmania Infection - Future Perspectives

The integrated endoplasmic reticulum stress response (IERSR) is an evolutionarily conserved adaptive mechanism that ensures endoplasmic reticulum (ER) homeostasis and cellular survival in the presence of stress including nutrient deprivation, hypoxia, and imbalance of Ca(+) homeostasis, toxins, and microbial infection. Three transmembrane proteins regulate integrated signaling pathways that comprise the IERSR, namely, IRE-1 that activates XBP-1, the pancreatic ER kinase (PERK) that phosphorylates the eukaryotic translation initiation factor 2 and transcription factor 6 (ATF6). The roles of IRE-1, PERK, and ATF4 in viral and some bacterial infections are well characterized. The role of IERSR in infections by intracellular parasites is still poorly understood, although one could anticipate that IERSR may play an important role on the host's cell response. Recently, our group reported the important aspects of XBP-1 activation in Leishmania amazonensis infection. It is, however, necessary to address the relevance of the other IERSR branches, together with the possible role of IERSR in infections by other Leishmania species, and furthermore, to pursue the possible implications in the pathogenesis and control of parasite replication in macrophages.

Severe Cutaneous Leishmaniasis in a Human Immunodeficiency Virus Patient Coinfected with Leishmania braziliensis and Its Endosymbiotic Virus

Leishmania parasites cause a broad range of disease, with cutaneous afflictions being, by far, the most prevalent. Variations in disease severity and symptomatic spectrum are mostly associated to parasite species. One risk factor for the severity and emergence of leishmaniasis is immunosuppression, usually arising by coinfection of the patient with human immunodeficiency virus (HIV). Interestingly, several species of Leishmania have been shown to bear an endogenous cytoplasmic dsRNA virus (LRV) of the Totiviridae family, and recently we correlated the presence of LRV1 within Leishmania parasites to an exacerbation murine leishmaniasis and with an elevated frequency of drug treatment failures in humans. This raises the possibility of further exacerbation of leishmaniasis in the presence of both viruses, and here we report a case of cutaneous leishmaniasis caused by Leishmania braziliensis bearing LRV1 with aggressive pathogenesis in an HIV patient. LRV1 was isolated and partially sequenced from skin and nasal lesions. Genetic identity of both sequences reinforced the assumption that nasal parasites originate from primary skin lesions. Surprisingly, combined antiretroviral therapy did not impact the devolution of Leishmania infection. The Leishmania infection was successfully treated through administration of liposomal amphotericin B.

A functional BH3 domain in an aquaporin from Leishmania infantum

Despite the absence of sequences showing significant similarity to any of the members of the Bcl-2 family of proteins in protozoa, experiments carried out in yeast or trypanosomatids have demonstrated that ectopic expression of some of these members alters their response to different death stimuli. Because the BH3 domain is the smallest common signature in all the proteins of this family of apoptosis regulators and also because they are essential for molecular interactions between antagonistic members, we looked for sequences with significant similarity to the BH3 motif in the Leishmania infantum genome. Among the top scoring ones, we found the MYLALQNLGDEV amino-acid stretch at the C terminus of a previously described aquaporin, now renamed as Li-BH3AQP. This motif is highly conserved in homologous proteins from other species of the Leishmania genus. The association of Li-BH3AQP with human Bcl-XL was demonstrated by both co-immunoprecipitation and yeast two-hybrid experiments. Ectopic expression of Li-BH3AQP reduced viability of HeLa cells and this deleterious effect was abrogated by the simultaneous overexpression of Bcl-XL. Although we were not able to demonstrate a reduction in parasite viability when the protein was overexpressed in Leishmania promastigotes, a prodeath effect could be observed when the parasites overexpressing Li-BH3AQP were treated with staurosporine or antimycin A. Surprisingly, these parasites were more resistant, compared with wild-type parasites, to hypotonic stress or nutrient deprivation. The prodeath activity was abolished upon replacement of two highly conserved amino acids in this BH3 domain. Taken together, these results point to Li-BH3AQP as the first non-enzymatic protein ever described in trypanosomatids that is involved in cell death.

The immunological, environmental, and phylogenetic perpetrators of metastatic leishmaniasis

Cutaneous leishmaniases have persisted for centuries as chronically disfiguring parasitic infections affecting millions of people across the subtropics. Symptoms range from the more prevalent single, self-healing cutaneous lesion to a persistent, metastatic disease, where ulcerations and granulomatous nodules can affect multiple secondary sites of the skin and delicate facial mucosa, even sometimes diffusing throughout the cutaneous system as a papular rash. The basis for such diverse pathologies is multifactorial, ranging from parasite phylogeny to host immunocompetence and various environmental factors. Although complex, these pathologies often prey on weaknesses in the innate immune system and its pattern recognition receptors. This review explores the observed and potential associations among the multifactorial perpetrators of infectious metastasis and components of the innate immune system.

MyD88 and TLR9 dependent immune responses mediate resistance to Leishmania guyanensis infections, irrespective of Leishmania RNA virus burden

Infections with Leishmania parasites of the Leishmania Viannia subgenus give rise to both localized cutaneous (CL), and metastatic leishmaniasis. Metastasizing disease forms including disseminated (DCL) and mutocutaneous (MCL) leishmaniasis result from parasitic dissemination and lesion formation at sites distal to infection and have increased inflammatory responses. The presence of Leishmania RNA virus (LRV) in L. guyanensis parasites contributes to the exacerbation of disease and impacts inflammatory responses via activation of TLR3 by the viral dsRNA. In this study we investigated other innate immune response adaptor protein modulators and demonstrated that both MyD88 and TLR9 played a crucial role in the development of Th1-dependent healing responses against L. guyanensis parasites regardless of their LRV status. The absence of MyD88- or TLR9-dependent signaling pathways resulted in increased Th2 associated cytokines (IL-4 and IL-13), which was correlated with low transcript levels of IL-12p40. The reliance of IL-12 was further confirmed in IL12AB^−/− mice, which were completely susceptible to infection. Protection to L. guyanensis infection driven by MyD88- and TLR9-dependent immune responses arises independently to those induced due to high LRV burden within the parasites.

Cross-presenting dendritic cells are required for control of Leishmania major infection

Leishmania major infection induces self-healing cutaneous lesions in C57BL/6 mice. Both IL-12 and IFN-γ are essential for the control of infection. We infected Jun dimerization protein p21SNFT (Batf3(-/-) ) mice (C57BL/6 background) that lack the major IL-12 producing and cross-presenting CD8α(+) and CD103(+) DC subsets. Batf3(-/-) mice displayed enhanced susceptibility with larger lesions and higher parasite burden. Additionally, cells from draining lymph nodes of infected Batf3(-/-) mice secreted less IFN-γ, but more Th2- and Th17-type cytokines, mirrored by increased serum IgE and Leishmania-specific immunoglobulin 1 (Th2 indicating). Importantly, CD8α(+) DCs isolated from lymph nodes of L. major-infected mice induced significantly more IFN-γ secretion by L. major-stimulated immune T cells than CD103(+) DCs. We next developed CD11c-diptheria toxin receptor: Batf3(-/-) mixed bone marrow chimeras to determine when the DCs are important for the control of infection. Mice depleted of Batf-3-dependent DCs from day 17 or wild-type mice depleted of cross-presenting DCs from 17-19 days after infection maintained significantly larger lesions similar to mice whose Batf-3-dependent DCs were depleted from the onset of infection. Thus, we have identified a crucial role for Batf-3-dependent DCs in protection against L. major.

Detection of Leishmania RNA virus in Leishmania parasites

Background

Patients suffering from cutaneous leishmaniasis (CL) caused by New World Leishmania (Viannia) species are at high risk of developing mucosal (ML) or disseminated cutaneous leishmaniasis (DCL). After the formation of a primary skin lesion at the site of the bite by a Leishmania-infected sand fly, the infection can disseminate to form secondary lesions. This metastatic phenotype causes significant morbidity and is often associated with a hyper-inflammatory immune response leading to the destruction of nasopharyngeal tissues in ML, and appearance of nodules or numerous ulcerated skin lesions in DCL. Recently, we connected this aggressive phenotype to the presence of Leishmania RNA virus (LRV) in strains of L. guyanensis, showing that LRV is responsible for elevated parasitaemia, destructive hyper-inflammation and an overall exacerbation of the disease. Further studies of this relationship and the distribution of LRVs in other Leishmania strains and species would benefit from improved methods of viral detection and quantitation, especially ones not dependent on prior knowledge of the viral sequence as LRVs show significant evolutionary divergence.

Methodology/Principal Findings

This study reports various techniques, among which, the use of an anti-dsRNA monoclonal antibody (J2) stands out for its specific and quantitative recognition of dsRNA in a sequence-independent fashion. Applications of J2 include immunofluorescence, ELISA and dot blot: techniques complementing an arsenal of other detection tools, such as nucleic acid purification and quantitative real-time-PCR. We evaluate each method as well as demonstrate a successful LRV detection by the J2 antibody in several parasite strains, a freshly isolated patient sample and lesion biopsies of infected mice.

Conclusions/Significance

We propose that refinements of these methods could be transferred to the field for use as a diagnostic tool in detecting the presence of LRV, and potentially assessing the LRV-related risk of complications in cutaneous leishmaniasis.

The endosymbiosis of viruses in microbes is a well-described and prevalent environmental partnership, where viruses offer their cellular host incentives of fitness in exchange for the use of their metabolic machinery. We have recently exposed this as an important factor in certain metastatic leishmaniases of South America, where the nucleic acid of a virus residing within some Leishmania parasites acts as a potent innate immunogen causing a destructive inflammatory response, which worsens disease. Leishmania RNA Virus (LRV) exists within many species of Leishmania as a stable infection; these LRV positive strains have been found throughout South America in cutaneous leishmaniases that are often complicated by the occurrence of infectious metastasis with an underlying hyperinflammatory response. In this report, we describe the use of an anti-dsRNA monoclonal antibody (J2), which specifically recognizes dsRNA in a quantitative and sequence-independent fashion. Refined versions of these methods could be transferred to the field as diagnostic tools for detecting the presence of LRV (or other dsRNA viruses), and potentially assessing the LRV-related risk of complicated cutaneous leishmaniasis.

Leishmania RNA virus: when the host pays the toll

The presence of an RNA virus in a South American subgenus of the Leishmania parasite, L. (Viannia), was detected several decades ago but its role in leishmanial virulence and metastasis was only recently described. In Leishmania guyanensis, the nucleic acid of Leishmania RNA virus (LRV1) acts as a potent innate immunogen, eliciting a hyper-inflammatory immune response through toll-like receptor 3 (TLR3). The resultant inflammatory cascade has been shown to increase disease severity, parasite persistence, and perhaps even resistance to anti-leishmanial drugs. Curiously, LRVs were found mostly in clinical isolates prone to infectious metastasis in both their human source and experimental animal model, suggesting an association between the viral hyperpathogen and metastatic complications such as mucocutaneous leishmaniasis (MCL). MCL presents as chronic secondary lesions in the mucosa of the mouth and nose, debilitatingly inflamed and notoriously refractory to treatment. Immunologically, this outcome has many of the same hallmarks associated with the reaction to LRV: production of type 1 interferons, bias toward a chronic Th1 inflammatory state and an impaired ability of host cells to eliminate parasites through oxidative stress. More intriguing, is that the risk of developing MCL is found almost exclusively in infections of the L. (Viannia) subtype, further indication that leishmanial metastasis is caused, at least in part, by a parasitic component. LRV present in this subgenus may contribute to the destructive inflammation of metastatic disease either by acting in concert with other intrinsic "metastatic factors" or by independently preying on host TLR3 hypersensitivity. Because LRV amplifies parasite virulence, its presence may provide a unique target for diagnostic and clinical intervention of metastatic leishmaniasis. Taking examples from other members of the Totiviridae virus family, this paper reviews the benefits and costs of endosymbiosis, specifically for the maintenance of LRV infection in Leishmania parasites, which is often at the expense of its human host.

Muco-cutaneous leishmaniasis in the New World: the ultimate subversion

Infection by the human protozoan parasite Leishmania can lead, depending primarily on the parasite species, to either cutaneous or mucocutaneous lesions, or fatal generalized visceral infection. In the New World, Leishmania (Viannia) species can cause mucocutaneous leishmaniasis (MCL). Clinical MCL involves a strong hyper-inflammatory response and parasitic dissemination (metastasis) from a primary lesion to distant sites, leading to destructive metastatic secondary lesions especially in the nasopharyngal areas. Recently, we reported that metastasizing, but not non-metastatic strains of Leishmania (Viannia) guyanensis, have high burden of a non-segmented dsRNA virus, Leishmania RNA Virus (LRV). Viral dsRNA is sensed by the host Toll-like Receptor 3 (TLR3) thereby inducing a pro-inflammatory response and exacerbating the disease. The presence of LRV in Leishmania opens new perspectives not only in basic understanding of the intimate relation between the parasite and LRV, but also in understanding the importance of the inflammatory response in MCL patients.

Plasmodium falciparum metacaspase PfMCA-1 triggers a z-VAD-fmk inhibitable protease to promote cell death

Activation of proteolytic cell death pathways may circumvent drug resistance in deadly protozoan parasites such as Plasmodium falciparum and Leishmania. To this end, it is important to define the cell death pathway(s) in parasites and thus characterize proteases such as metacaspases (MCA), which have been reported to induce cell death in plants and Leishmania parasites. We, therefore, investigated whether the cell death function of MCA is conserved in different protozoan parasite species such as Plasmodium falciparum and Leishmania major, focusing on the substrate specificity and functional role in cell survival as compared to Saccharomyces cerevisae. Our results show that, similarly to Leishmania, Plasmodium MCA exhibits a calcium-dependent, arginine-specific protease activity and its expression in yeast induced growth inhibition as well as an 82% increase in cell death under oxidative stress, a situation encountered by parasites during the host or when exposed to drugs such as artemisins. Furthermore, we show that MCA cell death pathways in both Plasmodium and Leishmania, involve a z-VAD-fmk inhibitable protease. Our data provide evidence that MCA from both Leishmania and Plasmodium falciparum is able to induce cell death in stress conditions, where it specifically activates a downstream enzyme as part of a cell death pathway. This enzymatic activity is also induced by the antimalarial drug chloroquine in erythrocytic stages of Plasmodium falciparum. Interestingly, we found that blocking parasite cell death influences their drug sensitivity, a result which could be used to create therapeutic strategies that by-pass drug resistance mechanisms by acting directly on the innate pathways of protozoan cell death.

Type I interferon drives dendritic cell apoptosis via multiple BH3-only proteins following activation by PolyIC in vivo

Background

DC are activated by pathogen-associated molecular patterns (PAMPs), and this is pivotal for the induction of adaptive immune responses. Thereafter, the clearance of activated DC is crucial to prevent immune pathology. While PAMPs are of major interest for vaccine science due to their adjuvant potential, it is unclear whether and how PAMPs may affect DC viability. We aimed to elucidate the possible apoptotic mechanisms that control activated DC lifespan in response to PAMPs, particularly in vivo.

Methodology/Principal Findings

We report that polyinosinic:polycytidylic acid (PolyIC, synthetic analogue of dsRNA) induces dramatic apoptosis of mouse splenic conventional DC (cDC) in vivo, predominantly affecting the CD8α subset, as shown by flow cytometry-based analysis of splenic DC subsets. Importantly, while Bim deficiency conferred only minor protection, cDC depletion was prevented in mice lacking Bim plus one of three other BH3-only proteins, either Puma, Noxa or Bid. Furthermore, we show that Type I Interferon (IFN) is necessary and sufficient for DC death both in vitro and in vivo, and that TLR3 and MAVS co-operate in IFNß production in vivo to induce DC death in response to PolyIC.

Conclusions/Significance

These results demonstrate for the first time in vivo that apoptosis restricts DC lifespan following activation by PolyIC, particularly affecting the CD8α cDC subset. Such DC apoptosis is mediated by the overlapping action of pro-apoptotic BH3-only proteins, including but not solely involving Bim, and is driven by Type I IFN. While Type I IFNs are important anti-viral factors, CD8α cDC are major cross-presenting cells and critical inducers of CTL. We discuss such paradoxical finding on DC death with PolyIC/Type I IFN. These results could contribute to understand immunosuppression associated with chronic infection, and to the optimization of DC-based therapies and the clinical use of PAMPs and Type I IFNs.

Leishmania RNA virus controls the severity of mucocutaneous leishmaniasis

Mucocutaneous leishmaniasis is caused by infections with intracellular parasites of the Leishmania Viannia subgenus, including Leishmania guyanensis. The pathology develops after parasite dissemination to nasopharyngeal tissues, where destructive metastatic lesions form with chronic inflammation. Currently, the mechanisms involved in lesion development are poorly understood. Here we show that metastasizing parasites have a high Leishmania RNA virus-1 (LRV1) burden that is recognized by the host Toll-like receptor 3 (TLR3) to induce proinflammatory cytokines and chemokines. Paradoxically, these TLR3-mediated immune responses rendered mice more susceptible to infection, and the animals developed an increased footpad swelling and parasitemia. Thus, LRV1 in the metastasizing parasites subverted the host immune response to Leishmania and promoted parasite persistence.