Leishmania promastigotes activate PI3K/Akt signalling to confer host cell resistance to apoptosis

Progress in antileishmanial drugs: Mechanisms, challenges, and prospects

Leishmaniasis, a neglected tropical disease caused by Leishmania parasites, continues to pose global health challenges. Current treatments face issues like resistance, safety, efficacy, and cost. This review covers the discovery, mechanisms of action, clinical applications, and limitations of key antileishmanial agents: pentavalent antimonials, amphotericin B, miltefosine, paromomycin, and pentamidine. Despite toxicity and resistance (antimonials), hospitalization needs and side effects (amphotericin B), regional efficacy variability (miltefosine), inconsistent outcomes (paromomycin), and severe side effects (pentamidine), these drugs are vital. Novel strategies to overcome the deficiencies of current therapies are highlighted, including combination regimens, advanced drug delivery systems, and immunomodulatory approaches. Comprehensive and cooperative efforts are crucial to fully realize the potential of advancements in antileishmanial pharmacotherapy and to reduce the unacceptable worldwide burden imposed by this neglected disease.

Mechanisms regulating host cell death during Leishmania infection

Parasites from the Leishmania genus are the causative agents of leishmaniasis and primarily reside within macrophages during mammalian infection. Their ability to establish intracellular infection provides a secure niche for proliferation while evading detection. However, successful multiplication within mammalian cells requires the orchestration of multiple mechanisms that control host cell viability. In contrast, innate immune cells, such as macrophages, can undergo different forms of cell death in response to pathogenic intracellular microbes. Thus, modulation of these different forms of host cell death is crucial for Leishmaniasis development. The regulation of host cell apoptosis, a form of programmed cell death, is crucial for sustaining parasites within viable host cells. Accordingly, several studies have demonstrated evasion of apoptosis induced by dermotropic and viscerotropic Leishmania species. Conversely, the prevention of pyroptosis, an inflammatory form of cell death, ensures the establishment of infection by silencing the release of mediators that could trigger massive proinflammatory responses. This manuscript explores how Leishmania regulates various host cell death pathways and overviews seminal studies on regulating host cell apoptosis by different Leishmania species.

Can letrozole be repurposed for the treatment of visceral leishmaniasis?

Visceral leishmaniasis, caused by Leishmania infantum in New World countries, is the most serious and potentially fatal form of leishmaniasis, if left untreated. There are currently no effective prophylactic measures, and therapeutic options are limited. Therefore, we investigated whether the aromatase inhibitor letrozole (LET), which is already used to treat breast cancer, has an antileishmanial activity and/or immunomodulatory potential and therefore may be used to treat L. infantum infection. LET was active against L. infantum promastigote and amastigote life cycle stages in an in vitro infection model using human THP-1 cell-derived macrophages. In human peripheral blood leukocytes ex vivo, LET reduced the internalized forms of L. infantum by classical monocytes and activated neutrophils. Concomitantly, LET stimulated the production of IL-12/TNF-α and decreased the production of IL-10/TGF-β by peripheral blood phagocytes, while in T and B cells, it promoted the production of TNF-α/IFN-γ and decreased that of IL-10. In a murine infection model, LET significantly reduced the parasite load in the liver after just 5 days and in the spleen after 15 days. During in vivo treatment with LET, the production of TNF-α/IFN-γ also increased. In addition, the proportion of developing granulomas decreased and that of mature granulomas increased in the liver, while there was no significant change in organ architecture in the spleen. Based on these data, repositioning of LET may be promising for the treatment of visceral leishmaniasis in humans.

Leishmania major surface components and DKK1 signalling via LRP6 promote migration and longevity of neutrophils in the infection site

Background

Host-related factors highly regulate the increased circulation of neutrophils during Leishmania infection. Platelet-derived Dickkopf-1 (DKK1) is established as a high-affinity ligand to LRP6. Recently, we demonstrated that DKK1 upregulates leukocyte-platelet aggregation, infiltration of neutrophils to the draining lymph node and Th2 differentiation during Leishmania infection, suggesting the potential involvement of the DKK1-LRP6 signalling pathway in neutrophil migration in infectious diseases.

Results

In this study, we further explored the potential role of DKK1-LRP6 signalling in the migration and longevity of activated neutrophils in the infection site using BALB/c mice with PMNs deficient in LRP6 (LRP6NKO) or BALB/c mice deficient in both PMN LRP6 and platelet DKK1 (LRP6NKO DKK1PKO). Relative to the infected wild-type BALB/c mice, reduced neutrophil activation at the infection site of LRP6NKO or LRP6NKO DKK1PKO mice was noted. The neutrophils obtained from either infected LRP6NKO or LRP6NKO DKK1PKO mice additionally showed a high level of apoptosis. Notably, the level of LRP6 expressing neutrophils was elevated in infected BALB/c mice. Relative to infected BALB/c mice, a significant reduction in parasite load was observed in both LRP6NKO and LRP6NKO DKK1PKO infected mice. Notably, DKK1 levels were comparable in the LRP6NKO and BALB/c mice in response to infection, indicating that PMN activation is the major pathway for DKK1 in promoting parasitemia. Parasite-specific components also play a crucial role in modulating neutrophil circulation in Leishmania disease. Thus, we further determine the contribution of Leishmania membrane components in the migration of neutrophils to the infection site using null mutants deficient in LPG synthesis (Δlpg1- ) or lacking all ether phospholipids (plasmalogens, LPG, and GIPLs) synthesis (Δads1- ). Relative to the WT controls, Δads1- parasite-infected mice showed a sustained decrease in neutrophils and neutrophil-platelet aggregates (for at least 14 days PI), while neutrophils returned to normal in Δlpg1- parasite-infected mice after day 3 PI.

Conclusion

Our results suggest that DKK1 signalling and Leishmania pathogen-associated molecular patterns appear to regulate the migration and sustenance of viable activated neutrophils in the infection site resulting in chronic type 2 cell-mediated inflammation.

Artemisinin: An Anti-Leishmania Drug that Targets the Leishmania Parasite and Activates Apoptosis of Infected Cells

Leishmaniasis is a relevant disease worldwide due to its presence in many countries and an estimated prevalence of 10 million people. The causative agent of this disease is the obligate intracellular parasite Leishmania which can infect different cell types. Part of its success depends on its ability to evade host defense mechanisms such as apoptosis. Apoptosis is a finely programmed process of cell death in which cells silently dismantle and actively participate in several processes such as immune response, differentiation, and cell growth. Leishmania has the ability to delay its initiation to persist in the cell. It has been well documented that different Leishmania species target different pathways that lead to apoptosis of cells such as macrophages, neutrophils, and dendritic cells. In many cases, the observed anti-apoptotic effect has been associated with a significant reduction in caspase-3 activity. Leishmania has also been shown to target several pathways involved in apoptosis such as MAPK, PI3K/Akt, and the antiapoptotic protein Bcl-xL. Understanding the strategies used by Leishmania to subvert the defense mechanisms of host cells, particularly apoptosis, is very relevant for the development of therapies and vaccines. In recent years, the drug artemisinin has been shown to be effective against several parasitic diseases. Its role against Leishmania may be promising. In this review, we provide important aspects of the disease, the strategies used by the parasite to suppress apoptosis, and the role of artemisinin in Leishmania infection.

Targeting and activation of macrophages in leishmaniasis. A focus on iron oxide nanoparticles

Macrophages play a pivotal role as host cells for Leishmania parasites, displaying a notable functional adaptability ranging from the proinflammatory, leishmanicidal M1 phenotype to the anti-inflammatory, parasite-permissive M2 phenotype. While macrophages can potentially eradicate amastigotes through appropriate activation, Leishmania employs diverse strategies to thwart this activation and redirect macrophages toward an M2 phenotype, facilitating its survival and replication. Additionally, a competition for iron between the two entities exits, as iron is vital for both and is also implicated in macrophage defensive oxidative mechanisms and modulation of their phenotype. This review explores the intricate interplay between macrophages, Leishmania, and iron. We focus the attention on the potential of iron oxide nanoparticles (IONPs) as a sort of immunotherapy to treat some leishmaniasis forms by reprogramming Leishmania-permissive M2 macrophages into antimicrobial M1 macrophages. Through the specific targeting of iron in macrophages, the use of IONPs emerges as a promising strategy to finely tune the parasite-host interaction, endowing macrophages with an augmented antimicrobial arsenal capable of efficiently eliminating these intrusive microbes.

Leishmania donovani mevalonate kinase regulates host actin for inducing phagocytosis

Despite the well-established role of macrophages in phagocytosing Leishmania, the contribution of the parasite to this process is not well understood. Present study provides insights into the mechanism underlying the MVK-induced entry of L. donovani and improve our knowledge of host-pathogen interactions. We have discussed Mevalonate kinase (MVK)-induced actin reorganization, modulation of signaling pathways and host cell functions. Our results show that LdMVK gains access to macrophage cytosol and induces actin assembly modulation through the activation of actin-related proteins: VASP, Src and ERM. We have also demonstrated that LdMVK induces Ca2+ signaling and Akt pathway in macrophages, which are critical components of Leishmania survival and proliferation. Interestingly, we found that antibodies against LdMVK can kill Leishmania-infected macrophages in culture by forming extracellular traps, highlighting the potential of LdMVK in inhibiting parasite death. Overall, LdMVK is a virulent factor in Leishmania that mediates parasite internalization and host modulation by targeting host proteins phosphorylation and calcium homeostasis having significant implications in disease progression.

Global Remodeling of Host Proteome in Response to Leishmania Infection

The protozoan parasite Leishmania possesses an intrinsic ability to modulate a multitude of pathways in the host, toward aiding its own proliferation. In response, the host reprograms its cellular, immunological, and metabolic machinery to evade the parasite's lethal impact. Besides inducing various antioxidant signaling pathways to counter the elevated stress response proteins like heme oxygenase-1 (HO-1), Leishmania also attempts to delay host cell apoptosis by promoting anti-apoptotic proteins like Bcl-2. The downstream modulation of apoptotic proteins is regulated by effector pathways, including the PI3K/Akt survival pathway, the mitogen-activated protein kinases (MAPKs) signaling pathway, and STAT phosphorylation. In addition, Leishmania assists in its infection in a time-dependent manner by modulating the level of various proteins of autophagic machinery. Immune effector cells, such as mast cells and neutrophils, entrap and kill the pathogen by secreting various granular proteins. In contrast, the host macrophages exert their leishmanicidal effect by secreting various cytokines, such as IL-2, IL-12, etc. An interplay of various signaling pathways occurs in an organized network that is highly specific to both pathogen and host species. This Review analyzes the modulation of expression of proteins, including the cytokines, providing a realistic approach toward understanding the pathophysiology of disease and predicting some prominent markers for disease intervention and vaccine support strategies.

Apoptosis and its pathways as targets for intracellular pathogens to persist in cells

Apoptosis is a finely programmed process of cell death in which cells silently dismantle and actively participate in several operations such as immune response, differentiation, and cell growth. It can be initiated by three main pathways: the extrinsic, the perforin granzyme, and the intrinsic that culminate in the activation of several proteins in charge of tearing down the cell. On the other hand, apoptosis represents an ordeal for pathogens that live inside cells and maintain a strong dependency with them; thus, they have evolved multiple strategies to manipulate host cell apoptosis on their behalf. It has been widely documented that diverse intracellular bacteria, fungi, and parasites can interfere with most steps of the host cell apoptotic machinery to inhibit or induce apoptosis. Indeed, the inhibition of apoptosis is considered a virulence property shared by many intracellular pathogens to ensure productive replication. Some pathogens intervene at an early stage by interfering with the sensing of extracellular signals or transduction pathways. Others sense cellular stress or target the apoptosis regulator proteins of the Bcl-2 family or caspases. In many cases, the exact molecular mechanisms leading to the interference with the host cell apoptotic cascade are still unknown. However, intense research has been conducted to elucidate the strategies employed by intracellular pathogens to modulate host cell death. In this review, we summarize the main routes of activation of apoptosis and present several processes used by different bacteria, fungi, and parasites to modulate the apoptosis of their host cells.

The PI3K-Akt pathway is a multifaceted regulator of the macrophage response to diverse group B Streptococcus isolates

Group B Streptococcus (GBS), also known as Streptococcus agalactiae, is a common member of the microbial flora in healthy individuals. However, problems may arise when GBS-colonized mothers become pregnant. GBS may be transferred from a colonized mother to her newborn or developing fetus, which may result in complications such as miscarriage, pre-term birth, meningitis, pneumonia, or sepsis. Macrophages play an especially important role in the fetal and newborn response to GBS due to the limited development of the adaptive immune system early in life. The goal of this study was to expand what is currently known about how GBS manipulates macrophage cell signaling to evade the immune system and cause disease. To this end, we investigated whether the PI3K-Akt pathway was involved in several key aspects of the macrophage response to GBS. We explored whether certain GBS strains, such as sequence type (ST)-17 strains, rely on this pathway for the more rapid macrophage uptake they induce compared to other GBS strains. Our findings suggest that this pathway is, indeed, important for macrophage uptake of GBS. Consistent with these findings, we used immunofluorescence microscopy to demonstrate that more virulent strains of GBS induce more actin projections in macrophages than less virulent strains. Additionally, we explored whether PI3K-Akt signaling impacted the ability of GBS to survive within macrophages after phagocytosis and whether this pathway influenced the survival rate of macrophages themselves following GBS infection. The PI3K-Akt pathway was found to promote the survival of both macrophages and intracellular GBS following infection. We also observed that inhibition of the PI3K-Akt pathway significantly reduced GBS-mediated activation of NFκB, which is a key regulator of cell survival and inflammatory responses. Overall, these insights into strain-dependent GBS-mediated manipulation of the PI3K-Akt pathway and its downstream targets in infected macrophages may provide new insights for the development of diagnostic and therapeutic tools to combat severe GBS disease.

Modulation of Macrophage Redox and Apoptotic Processes to Leishmania infantum during Coinfection with the Tick-Borne Bacteria Borrelia burgdorferi

Canine leishmaniosis (CanL) is a zoonotic disease caused by protozoan Leishmania infantum. Dogs with CanL are often coinfected with tick-borne bacterial pathogens, including Borrelia burgdorferi in the United States. These coinfections have been causally associated with hastened disease progression and mortality. However, the specific cellular mechanisms of how coinfections affect microbicidal responses against L. infantum are unknown. We hypothesized that B. burgdorferi coinfection impacts host macrophage effector functions, prompting L. infantum intracellular survival. In vitro experiments demonstrated that exposure to B. burgdorferi spirochetes significantly increased L. infantum parasite burden and pro-inflammatory responses in DH82 canine macrophage cells. Induction of cell death and generation of mitochondrial ROS were significantly decreased in coinfected DH82 cells compared to uninfected and L. infantum-infected cells. Ex vivo stimulation of PBMCs from L. infantum-seronegative and -seropositive subclinical dogs with spirochetes and/or total Leishmania antigens promoted limited induction of IFNγ. Coexposure significantly induced expression of pro-inflammatory cytokines and chemokines associated with Th17 differentiation and neutrophilic and monocytic recruitment in PBMCs from L. infantum-seropositive dogs. Excessive pro-inflammatory responses have previously been shown to cause CanL pathology. This work supports effective tick prevention and risk management of coinfections as critical strategies to prevent and control L. infantum progression in dogs.

Leishmania major drives host phagocyte death and cell-to-cell transfer depending on intracellular pathogen proliferation rate

The virulence of intracellular pathogens relies largely on the ability to survive and replicate within phagocytes but also on release and transfer into new host cells. Such cell-to-cell transfer could represent a target for counteracting microbial pathogenesis. However, our understanding of the underlying cellular and molecular processes remains woefully insufficient. Using intravital 2-photon microscopy of caspase-3 activation in the Leishmania major-infected (L. major-infected) live skin, we showed increased apoptosis in cells infected by the parasite. Also, transfer of the parasite to new host cells occurred directly without a detectable extracellular state and was associated with concomitant uptake of cellular material from the original host cell. These in vivo findings were fully recapitulated in infections of isolated human phagocytes. Furthermore, we observed that high pathogen proliferation increased cell death in infected cells, and long-term residency within an infected host cell was only possible for slowly proliferating parasites. Our results therefore suggest that L. major drives its own dissemination to new phagocytes by inducing host cell death in a proliferation-dependent manner.

BOTH THE INFECTION STATUS AND INFLAMMATORY MICROENVIRONMENT INDUCE TRANSCRIPTIONAL REMODELING IN MACROPHAGES IN MURINE LEISHMANIAL LESIONS

Cutaneous leishmaniasis is caused by infection with the protozoan parasite Leishmania, which resides intracellularly in dermal macrophages (Mø), producing lesions. The skin lesions are characterized by proinflammatory cytokines and growth factors as well as inflammatory hypoxia, creating a stressful microenvironment for Mø. Of importance, not all Mø in lesions harbor parasites. To distinguish the influence of the parasite from the inflammatory microenvironment after Leishmania major (LM) infection on the Mø, we performed single-cell RNA sequencing and compared Mø associated with LM transcripts (or 'infected' Mø) with Mø not associated with LM transcripts (or 'bystander' Mø) within the lesions. Our findings show coordinated lysosomal expression and regulation signaling with increased cathepsin and H+-ATPase transcripts are upregulated in infected compared with bystander Mø. Additionally, eukaryotic initiation factor 2 (EIF2) signaling is downregulated in infected compared with bystander Mø, which includes many small and large ribosomal subunit (Rps and Rpl) transcripts being decreased in Mø harboring parasites. Furthermore, we also find EIF2 signaling including EIF, Rps, and Rpl transcripts being downregulated in bystander Mø compared with Mø from naïve skin. These data suggest that both the parasite and the inflammatory host microenvironment affect the transcription of ribosomal machinery in lesional Mø, thereby potentially affecting the ability of these cells to perform translation, protein synthesis, and thus function. Altogether, these results suggest that both the parasite and host inflammatory microenvironment independently drive transcriptional remodeling in Mø during LM infection in vivo.

Gasdermin-D activation promotes NLRP3 activation and host resistance to Leishmania infection

Intracellular parasites from the Leishmania genus cause Leishmaniasis, a disease affecting millions of people worldwide. NLRP3 inflammasome is key for disease outcome, but the molecular mechanisms upstream of the inflammasome activation are still unclear. Here, we demonstrate that despite the absence of pyroptosis, Gasdermin-D (GSDMD) is active at the early stages of Leishmania infection in macrophages, allowing transient cell permeabilization, potassium efflux, and NLRP3 inflammasome activation. Further, GSDMD is processed into a non-canonical 25 kDa fragment. Gsdmd-/- macrophages and mice exhibit less NLRP3 inflammasome activation and are highly susceptible to infection by several Leishmania species, confirming the role of GSDMD for inflammasome-mediated host resistance. Active NLRP3 inflammasome and GSDMD are present in skin biopsies of patients, demonstrating activation of this pathway in human leishmaniasis. Altogether, our findings reveal that Leishmania subverts the normal functions of GSDMD, an important molecule to promote inflammasome activation and immunity in Leishmaniasis.

Emerging strategies and challenges of molecular therapeutics in antileishmanial drug development

Molecular therapy refers to targeted therapies based on molecules which have been intelligently directed towards specific biomolecular structures and include small molecule drugs, monoclonal antibodies, proteins and peptides, DNA or RNA-based strategies, targeted chemotherapy and nanomedicines. Molecular therapy is emerging as the most effective strategy to combat the present challenges of life-threatening visceral leishmaniasis, where the successful human vaccine is currently unavailable. Moreover, current chemotherapy-based strategies are associated with the issues of ineffective targeting, unavoidable toxicities, invasive therapies, prolonged treatment, high treatment costs and the development of drug-resistant strains. Thus, the rational approach to antileishmanial drug development primarily demands critical exploration and exploitation of biochemical differences between host and parasite biology, immunocharacteristics of parasite homing, and host-parasite interactions at the molecular/cellular level. Following this, the novel technology-based designing and development of host and/or parasite-targeted therapeutics having leishmanicidal and immunomodulatory activity is utmost essential to improve treatment efficacy. Thus, the present review is focused on immunological and molecular checkpoint targets in host-pathogen interaction, and molecular therapeutic prospects for Leishmania intervention, and the challenges ahead.

Genomic conservation and putative downstream functionality of the phosphatidylinositol signalling pathway in the cnidarian-dinoflagellate symbiosis

The mutualistic cnidarian-dinoflagellate symbiosis underpins the evolutionary success of stony corals and the persistence of coral reefs. However, a molecular understanding of the signalling events that lead to the successful establishment and maintenance of this symbiosis remains unresolved. For example, the phosphatidylinositol (PI) signalling pathway has been implicated during the establishment of multiple mutualistic and parasitic interactions across the kingdoms of life, yet its role within the cnidarian-dinoflagellate symbiosis remains unexplored. Here, we aimed to confirm the presence and assess the specific enzymatic composition of the PI signalling pathway across cnidaria and dinoflagellates by compiling 21 symbiotic anthozoan (corals and sea anemones) and 28 symbiotic dinoflagellate (Symbiodiniaceae) transcriptomic and genomic datasets and querying genes related to this pathway. Presence or absence of PI-kinase and PI-phosphatase orthologs were also compared between a broad sampling of taxonomically related symbiotic and non-symbiotic species. Across the symbiotic anthozoans analysed, there was a complete and highly conserved PI pathway, analogous to the pathway found in model eukaryotes. The Symbiodiniaceae pathway showed similarities to its sister taxon, the Apicomplexa, with the absence of PI 4-phosphatases. However, conversely to Apicomplexa, there was also an expansion of homologs present in the PI5-phosphatase and PI5-kinase groups, with unique Symbiodiniaceae proteins identified that are unknown from non-symbiotic unicellular organisms. Additionally, we aimed to unravel the putative functionalities of the PI signalling pathway in this symbiosis by analysing phosphoinositide (PIP)-binding proteins. Analysis of phosphoinositide (PIP)-binding proteins showed that, on average, 2.23 and 1.29% of the total assemblies of anthozoan and Symbiodiniaceae, respectively, have the potential to bind to PIPs. Enrichment of Gene Ontology (GO) terms associated with predicted PIP-binding proteins within each taxon revealed a broad range of functions, including compelling links to processes putatively involved in symbiosis regulation. This analysis establishes a baseline for current understanding of the PI pathway across anthozoans and Symbiodiniaceae, and thus a framework to target future research.

Systems biology of autophagy in leishmanial infection and its diverse role in precision medicine

Autophagy is a contentious issue in leishmaniasis and is emerging as a promising therapeutic regimen. Published research on the impact of autophagic regulation on Leishmania survival is inconclusive, despite numerous pieces of evidence that Leishmania spp. triggers autophagy in a variety of cell types. The mechanistic approach is poorly understood in the Leishmania parasite as autophagy is significant in both Leishmania and the host. Herein, this review discusses the autophagy proteins that are being investigated as potential therapeutic targets, the connection between autophagy and lipid metabolism, and microRNAs that regulate autophagy and lipid metabolism. It also highlights the use of systems biology to develop novel autophagy-dependent therapeutics for leishmaniasis by utilizing artificial intelligence (AI), machine learning (ML), mathematical modeling, network analysis, and other computational methods. Additionally, we have shown many databases for autophagy and metabolism in Leishmania parasites that suggest potential therapeutic targets for intricate signaling in the autophagy system. In a nutshell, the detailed understanding of the dynamics of autophagy in conjunction with lipids and miRNAs unfolds larger dimensions for future research.

miRNAs in the regulation of mTOR signaling and host immune responses: The case of Leishmania infections

Micro RNAs (miRNAs), as regulators of gene expression at the post-transcriptional level, can respond to/or interact with cell signaling and affect the pathogenesis of different diseases/infections. The interaction/crosstalk of miRNAs with various cellular signaling networks including mTOR (as a master regulator of signaling relevant to different cellular mechanisms) might lead to the initiation, progression or restriction of certain disease processes. There are numerous studies that have identified the crosstalk between regulatory miRNA expression and the mTOR pathway (or mTOR signaling regulated by miRNAs) in different diseases which has a dual function in pathogenesis. However, the corresponding information in parasitic infections remains scarce. miRNAs have been suggested as specific targets for therapeutic strategies in several disorders such as parasitic infections. Thus, the targeting of miRNAs (as the modulators/regulators of mTOR) by small molecules and RNA-based therapeutics and consequently managing and modulating mTOR signaling and the downstream/related cell signaling/pathways might shed some light on the design of new therapeutic strategies against parasitic diseases, including Leishmaniasis. Accordingly, the present study attempts to highlight the importance of the crosstalk between regulatory miRNAs and mTOR signaling, and to review the relevant insights into parasitic infections by focusing specifically on Leishmania.

Host-directed antileishmanial interventions: Harvesting unripe fruits to reach fruition

Leishmaniasis is an exemplary paradigm of immune evasion, fraught with the perils of limited clinical assistance, escalating costs of treatment and made worse with the lack of suitable vaccine. While drugs remain central to large-scale disease control, the growing emergence of parasite resistance necessitates the need for combination therapy involving host-directed immunological agents. Also, since prolonged disease progression is associated with strong immune suppression of the host, augmentation of host immunity via restoration of the immunoregulatory circuit involving antigen-presenting cells and T-cells, activation of macrophage function and/or CD4⁺ T helper 1 cell differentiation may serve as an ideal approach to resolve severe cases of leishmaniasis. As such, therapies that embody a synergistic approach that involve direct killing of the parasite in addition to elevating host immunity are likely to pave the way for widespread elimination of leishmaniasis in the future. With this review, we aim to recapitulate the various immunotherapeutic agents found to hold promise in antileishmanial treatment both in vitro and in vivo. These include parasite-specific antigens, dendritic cell-targeted therapy, recombinant inhibitors of various components intrinsic to immune cell signaling and agonists or antagonists to immune cells and cytokines. We also summarize their abilities to direct therapeutic skewing of the host cell-immune response and review their potential to combat the disease either alone, or as adjunct modalities.

Leishmania Parasites Differently Regulate Antioxidant Genes in Macrophages Derived From Resistant and Susceptible Mice

Macrophage-Leishmania interactions are central to parasite growth and disease outcome. Macrophages have developed various strategies to fight invaders, including oxidative burst. While some microorganisms seem to survive and even thrive in an oxidative environment, others are susceptible and get killed. To counter oxidative stress, macrophages switch the expressions of cytoprotective and detoxifying enzymes, which are downstream targets of the nuclear factor erythroid 2-related factor 2 (Nrf2), to enhance cell survival. We have explored the transcription of NRF2 and of its target genes and compared the effect of the parasite on their transcription in bone marrow-derived macrophages (BMdMs) from Leishmania-resistant and Leishmania-susceptible mice. While heme oxygenase 1 (HO-1) transcription is independent of the genetic background, the transcription of glutathione reductase (Gsr) and of cysteine/glutamate exchange transporter (Slc7a11), involved in glutathione accumulation, was differentially regulated in BMdMs from both mouse strains. We also show that, except for HO-1, known to favor the survival of the parasite, the transcription of the selected genes, including Gsr, CD36, and catalase (CAT), was actively repressed, if not at all time points at least at the later ones, by the parasite, especially in Balb/c BMdMs. Consistent with these results, we found that the silencing of NRF2 in this study increases the survival and multiplication of the parasite.

Management of cell death in parasitic infections

For a long time, host cell death during parasitic infection has been considered a reflection of tissue damage, and often associated with disease pathogenesis. However, during their evolution, protozoan and helminth parasites have developed strategies to interfere with cell death so as to spread and survive in the infected host, thereby ascribing a more intriguing role to infection-associated cell death. In this review, we examine the mechanisms used by intracellular and extracellular parasites to respectively inhibit or trigger programmed cell death. We further dissect the role of the prototypical “eat-me signal” phosphatidylserine (PtdSer) which, by being exposed on the cell surface of damaged host cells as well as on some viable parasites via a process of apoptotic mimicry, leads to their recognition and up-take by the neighboring phagocytes. Although barely dissected so far, the engagement of different PtdSer receptors on macrophages, by shaping the host immune response, affects the overall infection outcome in models of both protozoan and helminth infections. In this scenario, further understanding of the molecular and cellular regulation of the PtdSer exposing cell-macrophage interaction might allow the identification of new therapeutic targets for the management of parasitic infection.

Effects of Visceralising Leishmania on the Spleen, Liver, and Bone Marrow: A Pathophysiological Perspective

The leishmaniases constitute a group of parasitic diseases caused by species of the protozoan genus Leishmania. In humans it can present different clinical manifestations and are usually classified as cutaneous, mucocutaneous, and visceral (VL). Although the full range of parasite—host interactions remains unclear, recent advances are improving our comprehension of VL pathophysiology. In this review we explore the differences in VL immunobiology between the liver and the spleen, leading to contrasting infection outcomes in the two organs, specifically clearance of the parasite in the liver and failure of the spleen to contain the infection. Based on parasite biology and the mammalian immune response, we describe how hypoxia-inducible factor 1 (HIF1) and the PI3K/Akt pathway function as major determinants of the observed immune failure. We also summarize existing knowledge on pancytopenia in VL, as a direct effect of the parasite on bone marrow health and regenerative capacity. Finally, we speculate on the possible effect that manipulation by the parasite of the PI3K/Akt/HIF1 axis may have on the myelodysplastic (MDS) features observed in VL.

Jagged-Notch-mediated divergence of immune cell crosstalk maintains the anti-inflammatory response in visceral leishmaniasis

Notch signaling governs crucial aspects of intercellular communication spanning antigen-presenting cells and T-cells. In this study, we investigate how Leishmania donovani takes advantage of this pathway to quell host immune responses. We report induction of the Notch ligand Jagged1 in L. donovani-infected bone marrow macrophages (BMMϕs) and subsequent activation of RBPJκ (also known as RBPJ) in T cells, which in turn upregulates the transcription factor GATA3. Activated RBPJκ also associates with the histone acetyltransferase p300 (also known as EP300), which binds with the Bcl2l12 promoter and enhances its expression. Interaction of Bcl2L12 with GATA3 in CD4⁺ T cells facilitates its binding to the interleukin (IL)-10 and IL-4 promoters, thereby increasing the secretion of these cytokines. Silencing Jagged1 hindered these events in a BMMϕ-T cell co-culture system. Upon further scrutiny, we found that parasite lipophosphoglycan (LPG) induces the host phosphoinositide 3-kinase (PI3K)/Akt pathway, which activates β-catenin and Egr1, the two transcription factors responsible for driving Jagged1 expression. In v ivo morpholino-silencing of Jagged1 suppresses anti-inflammatory cytokine responses and reduces organ parasite burden in L. donovani-infected Balb/c mice, suggesting that L. donovani-induced host Jagged1-Notch signaling skews macrophage-T cell crosstalk into disease-promoting Th2 mode in experimental visceral leishmaniasis.This article has an associated First Person interview with the first author of the paper.

Leishmania: manipulation of signaling pathways to inhibit host cell apoptosis

The maintenance of homeostasis in living systems requires the elimination of unwanted cells which is performed, among other mechanisms, by type I cell death or apoptosis. This type of programmed cell death involves several morphological changes such as cytoplasm shrinkage, chromatin condensation (pyknosis), nuclear fragmentation (karyorrhexis), and plasma membrane blebbing that culminate with the formation of apoptotic bodies. In addition to the maintenance of homeostasis, apoptosis also represents an important defense mechanism for cells against intracellular microorganisms. In counterpart, diverse intracellular pathogens have developed a wide array of strategies to evade apoptosis and persist inside cells. These strategies include the manipulation of signaling pathways involved in the inhibition of apoptosis where mitogen-activated protein kinase (MAPK) and phosphoinositide 3-kinase (PI3K) play a key role. Leishmania is an intracellular protozoan parasite that causes a wide spectrum of diseases known as leishmaniasis. This parasite displays different strategies, including apoptosis inhibition, to down-regulate host cell defense mechanisms in order to perpetuate infection.

The Molecular Language of the Cnidarian-Dinoflagellate Symbiosis

The cnidarian-dinoflagellate symbiosis is of huge importance as it underpins the success of coral reefs, yet we know very little about how the host cnidarian and its dinoflagellate endosymbionts communicate with each other to form a functionally integrated unit. Here, we review the current knowledge of interpartner molecular signaling in this symbiosis, with an emphasis on lipids, glycans, reactive species, biogenic volatiles, and noncoding RNA. We draw upon evidence of these compounds from recent omics-based studies of cnidarian-dinoflagellate symbiosis and discuss the signaling roles that they play in other, better-studied symbioses. We then consider how improved knowledge of interpartner signaling might be used to develop solutions to the coral reef crisis by, for example, engineering more thermally resistant corals.

Control of myeloid cell density in barrier tissues

The interface between the mammalian host and its environment is formed by barrier tissues, for example, of the skin, and the respiratory and the intestinal tracts. On the one hand, barrier tissues are colonized by site-adapted microbial communities, and on the other hand, they contain specific myeloid cell networks comprising macrophages, dendritic cells, and granulocytes. These immune cells are tightly regulated in function and cell number, indicating important roles in maintaining tissue homeostasis and immune balance in the presence of commensal microorganisms. The regulation of myeloid cell density and activation involves cell-autonomous 'single-loop circuits' including autocrine mechanisms. However, an array of microenvironmental factors originating from nonimmune cells and the microbiota, as well as the microanatomical structure, impose additional layers of regulation onto resident myeloid cells. This review discusses models integrating these factors into cell-specific programs to instruct differentiation and proliferation best suited for the maintenance and renewal of immune homeostasis in the tissue-specific environment.

Leishmanicidal Potential of Hardwickiic Acid Isolated From Croton sylvaticus

Leishmania is a parasitic protozoon responsible for the neglected tropical disease Leishmaniasis. Approximately, 350 million people are susceptible and close to 70,000 death cases globally are reported annually. The lack of effective leishmanicides, the emergence of drug resistance and toxicity concerns necessitate the pursuit for effective antileishmanial drugs. Natural compounds serve as reservoirs for discovering new drugs due to their chemical diversity. Hardwickiic acid (HA) isolated from the stembark of Croton sylvaticus was evaluated for its leishmanicidal potential against Leishmania donovani and L. major promastigotes. The susceptibility of the promastigotes to HA was determined using the 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide/phenazine methosulfate colorimetric assay with Amphotericin B serving as positive control. HA showed a significant antileishmanial activity on L. donovani promastigotes with an IC₅₀ value of 31.57± 0.06 µM with respect to the control drug, amphotericin B with IC₅₀ of 3.35 ± 0.14 µM). The cytotoxic activity was observed to be CC₅₀ = 247.83 ± 6.32 µM against 29.99 ± 2.82 µM for curcumin, the control, resulting in a selectivity index of SI = 7.85. Molecular modeling, docking and dynamics simulations of selected drug targets corroborated the observed antileishmanial activity of HA. Novel insights into the mechanisms of binding were obtained for trypanothione reductase (TR), pteridine reductase 1 (PTR1), and glutamate cysteine ligase (GCL). The binding affinity of HA to the drug targets LmGCL, LmPTR1, LdTR, LmTR, LdGCL, and LdPTR1 were obtained as -8.0, -7.8, -7.6, -7.5, -7.4 and -7.1 kcal/mol, respectively. The role of Lys16, Ser111, and Arg17 as critical residues required for binding to LdPTR1 was reinforced. HA was predicted as a Caspase-3 stimulant and Caspase-8 stimulant, implying a possible role in apoptosis, which was shown experimentally that HA induced parasite death by loss of membrane integrity. HA was also predicted as antileishmanial molecule corroborating the experimental activity. Therefore, HA is a promising antileishmanial molecule worthy of further development as a biotherapeutic agent.

Leishmania aethiopica cell-to-cell spreading involves caspase-3, AkT, and NF-κB but not PKC-δ activation and involves uptake of LAMP-1-positive bodies containing parasites

Development of human leishmaniasis is dependent on the ability of intracellular Leishmania parasites to spread and enter macrophages. The mechanism through which free promastigotes and amastigotes bind and enter host macrophages has been previously investigated; however, little is known about intracellular trafficking and cell-to-cell spreading. In this study, the mechanism involved in the spreading of Leishmania aethiopica and Leishmania mexicana was investigated. A significant increase in phosphatidylserine (PS) exhibition, cytochrome C release, and active caspase-3 expression was detected (P < 0.05) during L. aethiopica, but not L. mexicana spreading. A decrease (P < 0.05) of protein kinase B (Akt) protein and BCL2-associated agonist of cell death (BAD) phosphorylation was also observed. The nuclear factor kappa-light-chain enhancer of activated B cells (NF-kB) signaling pathway and pro-apoptotic protein protein kinase C delta (PKC-δ) were downregulated while inhibition of caspase-3 activation prevented L. aethiopica spreading. Overall suggesting that L. aethiopica induces host cell's apoptosis during spreading in a caspase-3-dependent manner. The trafficking of amastigotes within macrophages following cell-to-cell spreading differed from that of axenic parasites and involved co-localization with lysosomal-associated membrane protein 1 (LAMP-1) within 10 min postinfection. Interestingly, following infection with axenic amastigotes and promastigotes, co-localization of parasites with LAMP-1-positive structures took place at 1 and 4 h, respectively, suggesting that the membrane coat and LAMP-1 protein were derived from the donor cell. Collectively, these findings indicate that host cell apoptosis, demonstrated by PS exhibition, caspase-3 activation, cytochrome C release, downregulation of Akt, BAD phosphorylation, NF-kB activation, and independent of PKC-δ expression, is involved in L. aethiopica spreading. Moreover, L. aethiopica parasites associate with LAMP-rich structures when taken up by neighboring macrophages.

The Potential Role of Nrf2 Signaling in Leishmania Infection Outcomes

Nrf2 [nuclear factor erythroid 2-related factor 2 (Nrf2)] regulates the expression of a plethora of genes involved in the response to oxidative stress due to inflammation, aging, and tissue damage, among other pathological conditions. Deregulation of this cytoprotective system may also interfere with innate and adaptive immune responses. Oxidative burst, one of the main microbicidal mechanisms, could be impaired during initial phagocytosis of parasites, which could lead to the successful establishment of infection and promote susceptibility to diseases. A wide diversity of infections, mainly those caused by intracellular pathogens such as viruses, bacteria, and protozoan parasites, modulate the activation of Nrf2 by interfering with post-translational modifications, interactions between different protein complexes and the immune response. Nrf2 may be induced by pathogens via distinct pathways such as those involving the engagement of Toll-like receptors, the activation of PI3K/Akt, and endoplasmic reticulum stress. Recent studies have revealed the importance of Nrf2 on leishmaniasis. This mini-review discusses relevant findings that reveal the connection between Leishmania-induced modifications of the host pathways and their relevance to the modulation of the Nrf2-dependent antioxidative response to the infection.

Activation of autophagy and IL-10 production are regulated by Jun N-terminal kinase 1 and 2 and p38 mitogen activated protein kinase signaling pathways during Talaromyces marneffei infection within dendritic cells

Previous study have shown that Talaromyces marneffei (T. marneffei) induced activation of autophagy. Therefore, we explore signaling pathway that regulates activation of autophagy by intracellular signaling mechanisms during T. marneffei infection. Further, we examine c-Jun N-terminal kinase 1 and 2 (JNK1/2) and p38 signaling pathways that regulate IL-1β and IL-10 production and activation of autophagy during T. marneffei infection in human dendritic cells (DCs). We found that T. marneffei induced activation of JNK1/2 and p38 in human DCs. Furthermore, the inhibition of JNK1/2 and p38 increased activation of autophagy and decreased the replication of T. marneffei in T. marneffei-infected human DCs. Moreover, IL-1β secretion in T. marneffei-infected human DCs was dependent on JNK1/2 and autophagy pathways, whereas IL-10 secretion was dependent on JNK1/2, p38 and autophagy pathways. These data suggest that JNK1/2 and p38 pathways play critical roles in activation of autophagy, the multiplication of T. marneffei and subsequent cytokine production during T. marneffei infection.

Involvement of apoptosis in the dialogue between the parasite Bonamia ostreae and the flat oyster Ostrea edulis

The protozoan parasite Bonamia ostreae has been associated with the decline of flat oyster Ostrea edulis populations in some European countries. Control of shellfish diseases mostly relies on prevention measures including transfer restrictions and stock management measures such as breeding programmes. These prevention and mitigation measures require a better understanding of interactions between host and pathogens. Previous in vitro studies allowed identifying apoptosis as a mechanism activated by the flat oyster in response to B. ostreae. However, these experiments also suggested that the parasite is able to regulate apoptosis in order to survive and multiply within hemocytes. By simplifying the conditions of infection, in vitro studies allow identifying most distinct features of the response of the host. In order to appreciate the relative importance of apoptosis in this response at the oyster scale, in vivo trials were carried out by injecting with parasites oysters from two French locations, Quiberon Bay (Brittany) and Diana Lagoon (Corsica). Apoptosis was investigated on pools of hemolymph from oysters collected at early and later times after injection using previously developed tools. Apoptotic cellular activities including intracytoplasmic calcium concentration, mitochondrial membrane potential and phosphatidyl serine externalization were analysed using flow cytometry. Moreover, the expression of flat oyster genes involved in both extrinsic and intrinsic pathways was measured using real time quantitative PCR.

Evaluation of synergy between host and pathogen-directed therapies against intracellular Leishmania donovani

Visceral leishmaniasis (VL) is associated with treatment complications due to the continued growth of resistant parasites toward currently available pathogen-directed therapeutics. To limit the emergence and combat resistant parasites there is a need to develop new anti-leishmanial drugs and alternative treatment approaches, such as host-directed therapeutics (HDTs). Discovery of new anti-leishmanial drugs including HDTs requires suitable in vitro assay systems. Herein, we modified and evaluated a series of resazurin assays against different life-stages of the VL causing parasite, Leishmania donovani to identify novel HDTs. We further analyzed the synergy of combinatorial interactions between traditionally used pathogen-directed drugs and HDTs for clearance of intracellular L. donovani. The inhibitory concentration at 50% (IC₅₀) of the five evaluated therapies [amphotericin B (AMB), miltefosine, paromomycin, DNER-4, and AR-12 (OSU-03012)] was determined against promastigotes, extracellular amastigotes, and intracellular amastigotes of L. donovani via a resazurin-based assay and compared to image-based microscopy. Using the resazurin-based assay, all evaluated therapies showed reproducible anti-leishmanial activity against the parasite's different life-stages. These results were consistent to the traditional image-based technique. The gold standard of therapy, AMB, showed the highest potency against intracellular L. donovani, and was further evaluated for combinatorial effects with the HDTs. Among the combinations analyzed, pathogen-directed AMB and host-directed AR-12 showed a synergistic reduction of intracellular L. donovani compared to individual treatments. The modified resazurin assay used in this study demonstrated a useful technique to measure new anti-leishmanial drugs against both intracellular and extracellular parasites. The synergistic interactions between pathogen-directed AMB and host-directed AR-12 showed a great promise to combat VL, with the potential to reduce the emergence of drug-resistant strains.

Coinfection with Leishmania major and Staphylococcus aureus enhances the pathologic responses to both microbes through a pathway involving IL-17A

Cutaneous leishmaniasis (CL) is a parasitic disease causing chronic, ulcerating skin lesions. Most humans infected with the causative Leishmania protozoa are asymptomatic. Leishmania spp. are usually introduced by sand flies into the dermis of mammalian hosts in the presence of bacteria from either the host skin, sand fly gut or both. We hypothesized that bacteria at the dermal inoculation site of Leishmania major will influence the severity of infection that ensues. A C57BL/6 mouse ear model of single or coinfection with Leishmania major, Staphylococcus aureus, or both showed that single pathogen infections caused localized lesions that peaked after 2–3 days for S. aureus and 3 weeks for L. major infection, but that coinfection produced lesions that were two-fold larger than single infection throughout 4 weeks after coinfection. Coinfection increased S. aureus burdens over 7 days, whereas L. major burdens (3, 7, 28 days) were the same in singly and coinfected ears. Inflammatory lesions throughout the first 4 weeks of coinfection had more neutrophils than did singly infected lesions, and the recruited neutrophils from early (day 1) lesions had similar phagocytic and NADPH oxidase capacities. However, most neutrophils were apoptotic, and transcription of immunomodulatory genes that promote efferocytosis was not upregulated, suggesting that the increased numbers of neutrophils may, in part, reflect defective clearance and resolution of the inflammatory response. In addition, the presence of more IL-17A-producing γδ and non-γδ T cells in early lesions (1–7 days), and L. major antigen-responsive Th17 cells after 28 days of coinfection, with a corresponding increase in IL-1β, may recruit more naïve neutrophils into the inflammatory site. Neutralization studies suggest that IL-17A contributed to an enhanced inflammatory response, whereas IL-1β has an important role in controlling bacterial replication. Taken together, these data suggest that coinfection of L. major infection with S. aureus exacerbates disease, both by promoting more inflammation and neutrophil recruitment and by increasing neutrophil apoptosis and delaying resolution of the inflammatory response. These data illustrate the profound impact that coinfecting microorganisms can exert on inflammatory lesion pathology and host adaptive immune responses.

Cutaneous leishmaniasis (CL) is a vector-borne ulcerating skin disease affecting several million people worldwide. The causative Leishmania spp. protozoa are transmitted by infected phlebotomine sand flies. During a sand fly bite, bacteria can be coincidentally inoculated into the dermis with the parasite. Staphylococcus aureus is the most common bacterium in CL skin lesions. Symptomatic CL is characterized by papulonodular skin lesions that ulcerate and resolve with scarring, although most cutaneous Leishmania infections are asymptomatic. We sought to explore factors that determine whether infection with a cutaneous Leishmania species would result in symptomatic CL rather than asymptomatic infection. We hypothesized that local bacteria promote the development of symptomatic CL lesions during infection with Leishmania major. We discovered that cutaneous lesions were significantly larger in mice inoculated simultaneously with S. aureus and L. major than in mice infected with either organism alone. Coinfection led to increased S. aureus growth in skin lesions, whereas L. major parasite numbers were unchanged by coinfection. The size of the exacerbated lesion correlated with early increased numbers of neutrophils and elevated levels of proinflammatory cytokines IL-1β and IL-17A during the first 7 days, and with sustained increases in IL-17A through 28 days of coinfection. Neutralizing antibody experiments suggested IL-17A was partially responsible for lesion exacerbation during coinfection, whereas IL-1β was important for both control of early lesion exacerbation and promotion of IL-17A production. These data suggest that treatment of symptomatic CL targeting the parasite, local commensal bacteria, and host proinflammatory IL-17A immune responses might improve the outcome of CL.

Crosstalk of PD-1 signaling with the SIRT1/FOXO-1 axis during the progression of visceral leishmaniasis

Previously, we documented the role of the programmed death-1 (PD-1, also known as PDCD1) pathway in macrophage apoptosis and the downregulation of this signaling during infection by the intra-macrophage parasite Leishmania donovani However, we also found that, during the late phase of infection, PD-1 expression was significantly increased without activating host cell apoptosis; here we show that inhibition of PD-1 led to markedly decreased parasite survival, along with increased production of TNFα, IL-12, reactive oxygen species (ROS) and nitric oxide (NO). Increased PD-1 led to inactivation of AKT proteins resulting in nuclear sequestration of FOXO-1. Transfecting infected cells with constitutively active FOXO-1 (CA-FOXO) led to increased cell death, thereby suggesting that nuclear FOXO-1 might be inactivated. Infection significantly induced the expression of SIRT1, which inactivated FOXO-1 through deacetylation, and its knockdown led to increased apoptosis. SIRT1 knockdown also significantly decreased parasite survival along with increased production of TNFα, ROS and NO. Administration of the SIRT1 inhibitor sirtinol (10 mg/kg body weight) in infected mice decreased spleen parasite burden and a synergistic effect was found with PD-1 inhibitor. Collectively, our study shows that Leishmania utilizes the SIRT1/FOXO-1 axis for differentially regulating PD-1 signaling and, although they are interconnected, both pathways independently contribute to intracellular parasite survival.This article has an associated First Person interview with the first author of the paper.

Detection and characterization of an albumin-like protein in Leishmania donovani

The protozoan parasite, Leishmania donovani, undergoes several molecular adaptations and secretes many effector molecules for host cell manipulation and successful parasitism. The current study identifies an albumin-like secretory protein, expressed in its extracellular promastigote forms. A leishmanial complementary DNA sequence of a partial gene has been cloned, and the encoded peptide (14 kD) is used for the production of polyclonal antibody. This targeted antibody identifies a large native protein (66.421 kD), expressed stage-specifically in promastigotes. Through electron microscopic studies, the native protein is found to be localized in the flagellar pocket and flagella and at the surface of the promastigotes. This native protein is purified with the same customized antibody for future characterization and sequencing. The sequence analysis reveals its homology with the mammalian serum albumin. It is evidenced from in silico studies that this albumin-like protein remains associated with long-chain fatty acids while in vitro studies indicate its close association with membrane cholesterol. Since antibody-mediated blocking compromises the parasite infectivity, these leishmanial albumin-like molecules are hereby proposed to play an instrumental role in the infectivity of L. donovani to peripheral blood monocyte cells. Thus, identification and characterization of an albumin-like protein in L. donovani promastigotes may be interpreted as a molecular adaptation candidate. It may be hypothesized that the parasite mimics the mammalian system for importing fatty acids into the intracellular amastigotes, facilitating its host cell infectivity.

Encephalitozoon cuniculi and Vittaforma corneae (Phylum Microsporidia) inhibit staurosporine-induced apoptosis in human THP-1 macrophages in vitro

Obligately intracellular microsporidia regulate their host cell life cycles, including apoptosis, but this has not been evaluated in phagocytic host cells such as macrophages that can facilitate infection but also can be activated to kill microsporidia. We examined two biologically dissimilar human-infecting microsporidia species, Encephalitozoon cuniculi and Vittaforma corneae, for their effects on staurosporine-induced apoptosis in the human macrophage-differentiated cell line, THP1. Apoptosis was measured after exposure of THP-1 cells to live and dead mature organisms via direct fluorometric measurement of Caspase 3, colorimetric and fluorometric TUNEL assays, and mRNA gene expression profiles using Apoptosis RT2 Profiler PCR Array. Both species of microsporidia modulated the intrinsic apoptosis pathway. In particular, live E. cuniculi spores inhibited staurosporine-induced apoptosis as well as suppressed pro-apoptosis genes and upregulated anti-apoptosis genes more broadly than V. corneae. Exposure to dead spores induced an opposite effect. Vittaforma corneae, however, also induced inflammasome activation via Caspases 1 and 4. Of the 84 apoptosis-related genes assayed, 42 (i.e. 23 pro-apoptosis, nine anti-apoptosis, and 10 regulatory) genes were more affected including those encoding members of the Bcl2 family, caspases and their regulators, and members of the tumour necrosis factor (TNF)/TNF receptor R superfamily.

Miransertib (ARQ 092), an orally-available, selective Akt inhibitor is effective against Leishmania

Leishmaniasis is amongst the most important neglected diseases, afflicting more than 12 million people in 88 countries. There is an urgent need for safe orally bioavailable and cost-effective drugs for the treatment of leishmaniasis. It has recently been shown that Leishmania activates host macrophage serine/threonine kinase Akt, to promote survival of both parasites and infected cells. Here, we sought to evaluate a compound, Miransertib (ARQ 092), an orally bioavailable and selective allosteric Akt inhibitor currently in clinical trials for patients with PI3K/Akt-driven tumors or Proteus syndrome. Miransertib was tested against Leishmania donovani and Leishmania amazonensis, causative agents of visceral and cutaneous leishmaniasis, respectively. Cultured promastigotes were susceptible to Miransertib. In addition, Miransertib was markedly effective against intracellular amastigotes of L. donovani or L. amazonensis-infected macrophages. Miransertib also enhanced mTOR dependent autophagy in Leishmania-infected macrophages, which may represent one mechanism of Miransertib-mediated killing of intracellular Leishmania. Whereas parasite clearance in the spleen of mice infected with L. donovani and treated with Miransertib was comparable to that when treated with miltefosine, Miransertib caused a greater reduction in the parasite load in the liver. In the cutaneous leishmaniasis infection model, lesions were reduced by 40% as compared to mock treated mice. Together, these results provide direct evidence to support the conclusion that Miransertib is an excellent lead compound for the development of a new oral drug therapy for visceral and cutaneous leishmaniasis.

Early Suppression of Macrophage Gene Expression by Leishmania braziliensis

Leishmania braziliensis is an intracellular parasite that resides mostly in macrophages. Both the parasite genome and the clinical disease manifestations show considerable polymorphism. Clinical syndromes caused by L. braziliensis include localized cutaneous (CL), mucosal (ML), and disseminated leishmaniasis (DL). Our prior studies showed that genetically distinct L. braziliensis clades associate with different clinical types. Herein, we hypothesized that: (1) L. braziliensis induces changes in macrophage gene expression that facilitates infection; (2) infection of macrophages with strains associated with CL (clade B), ML (clade C), or DL (clade A) will differentially affect host cell gene expression, reflecting their different pathogenic mechanisms; and (3) differences between the strains will be reflected by differences in macrophage gene expression after initial exposure to the parasite. Human monocyte derived macrophages were infected with L. braziliensis isolates from clades A, B, or C. Patterns of gene expression were compared using Affymetrix DNA microarrays. Many transcripts were significantly decreased by infection with all isolates. The most dramatically decreased transcripts encoded proteins involved in signaling pathways, apoptosis, or mitochondrial oxidative phosphorylation. Some transcripts encoding stress response proteins were up-regulated. Differences between L. braziliensis clades were observed in the magnitude of change, rather than the identity of transcripts. Isolates from subjects with metastatic disease (ML and DL) induced a greater magnitude of change than isolates from CL. We conclude that L. braziliensis enhances its intracellular survival by inhibiting macrophage pathways leading to microbicidal activity. Parasite strains destined for dissemination may exert a more profound suppression than less invasive L. braziliensis strains that remain near the cutaneous site of inoculation.

Molecular and cellular characterization of apoptosis in flat oyster a key mechanisms at the heart of host-parasite interactions

Bonamia ostreae has been associated with the decline of flat oyster Ostrea edulis populations in some European countries. This obligatory intracellular parasite persists and multiplies into hemocytes. Previous in vitro experiments showed that apoptosis is activated in hemocytes between 1 h and 4 h of contact with the parasite. The flat oyster uses the apoptosis pathway to defend against B. ostreae. However, the parasite might be also able to modulate this response in order to survive in its host. In order to investigate this hypothesis the apoptotic response of the host was evaluated using flow cytometry, transmission electron microscopy and by measuring the response of genes involved in the apoptotic pathway after 4 h. In parallel, the parasite response was investigated by measuring the expression of B. ostreae genes involved in different biological functions including cell cycle and cell death. Obtained results allow describing molecular apoptotic pathways in O. edulis and confirm that apoptosis is early activated in hemocytes after a contact with B. ostreae. Interestingly, at cellular and molecular levels this process appeared downregulated after 44 h of contact. Concurrently, parasite gene expression appeared reduced suggesting that the parasite could inhibit its own metabolism to escape the immune response.

Leishmania parasitophorous vacuole membranes display phosphoinositides that create conditions for continuous Akt activation and a target for miltefosine in Leishmania infections

Miltefosine is an important drug for the treatment of leishmaniasis; however, its mechanism of action is still poorly understood. In these studies, we tested the hypothesis that like in cancer cells, miltefosine's efficacy in leishmaniasis is due to its inhibition of Akt activation in host cells. We show using pharmacologic agents that block Akt activation by different mechanisms and also using an inducible knockdown approach that miltefosine loses its efficacy when its access to Akt1 is limited. Interestingly, limitation of Akt activation results in clearance of established Leishmania infections. We then show, using fluorophore-tagged probes that bind to phosphoinositides, that Leishmania parasitophorous vacuole membranes (LPVMs) display the relevant phosphoinositides to which Akt can be recruited and activated continuously. Taken together, we propose that the acquisition of PI(4) P and the display of PI (3,4)P2 on LPVMs initiate the machinery that supports continuous Akt activation and sensitivity to miltefosine.

Leishmania donovani induced Unfolded Protein Response delays host cell apoptosis in PERK dependent manner

Background

Endoplasmic reticulum (ER) stress generated unfolded stress response (UPR) is a basic survival mechanism which protects cell under unfavourable conditions. Leishmania parasite modulates host macrophages in various ways to ensure its survival. Modulation of PI3K-Akt pathway in delayed apoptotic induction of host; enables parasite to stabilize the infection for further propagation.

Methodology

Infected RAW macrophages were exposed to campothecin or thagsigargin and phosphorylation status of PERK, Akt, BAD and Cyt-C was determined through western blotting using phospho specific antibody. Expression at transcriptional level for cIAP1 &2, ATF4, CHOP, ATF3, HO-1 and sXBP1 was determined using real time PCR. For inhibition studies, RAW macrophages were pre-treated with PERK inhibitor GSK2606414 before infection.

Findings

Our studies in RAW macrophages showed that induction of host UPR against L.donovani infection activates Akt mediated pathway which delays apoptotic induction of the host. Moreover, Leishmania infection results in phosphorylation and activation of host PERK enzyme and increased transcription of genes of inhibitor of apoptosis gene family (cIAP) mRNA. In our inhibition studies, we found that inhibition of infection induced PERK phosphorylation under apoptotic inducers reduces the Akt phosphorylation and fails to activate further downstream molecules involved in protection against apoptosis. Also, inhibition of PERK phosphorylation under oxidative exposure leads to increased Nitric Oxide production. Simultaneously, decreased transcription of cIAP mRNA upon PERK phosphorylation fates the host cell towards apoptosis hence decreased infection rate.

Conclusion

Overall the findings from the study suggests that Leishmania modulated host UPR and PERK phosphorylation delays apoptotic induction in host macrophage, hence supports parasite invasion at early stages of infection.

Visceral Leishmaniasis or Kala-azar is one of the severe tropical neglected parasitic diseases caused by Leishmania donovani in Indian subcontinent. Modulation of host in terms of delayed apoptotic induction is one of the aspects which favours disease establishment; however the mechanism is not clearly understood yet. In the present study, we tried to explore the connection between L.donovani infection induced UPR in host with delayed onset of apoptosis. We found that L.donovani infection phosphorylates the PERK and Akt molecule in host along with delayed apoptosis. Simultaneously, the levels of cellular IAP (cIAP1 & 2) genes were also up-regulated in infected macrophages. To assess the involvement of PERK in delayed apoptosis of host, we inhibited the phosphorylation of PERK under the exposure to apoptotic inducers. We found that PERK inhibition decreased the Akt phosphorylation and fails to activate other associated downstream molecules involved in delayed apoptosis of host. Also, a significant reduction in cIAP levels was observed. Under oxidative exposure, inhibition of PERK phosphorylation debilitates infected RAW cell’s ability to maintain redox homeostasis leading to higher nitric oxide production. Altogether, L.donovani infection modulates host apoptosis in a PERK dependent manner and favours infection.

Apoptosis: Activation and Inhibition in Health and Disease

There are many types of cell death, each involving multiple and complex molecular events. Cell death can occur accidentally when exposed to extreme physical, chemical, or mechanical conditions, or it can also be regulated, which involves a genetically coded complex machinery to carry out the process. Apoptosis is an example of the latter. Apoptotic cell death can be triggered through different intracellular signalling pathways that lead to morphological changes and eventually cell death. This is a normal and biological process carried out during maturation, remodelling, growth, and development in tissues. To maintain tissue homeostasis, regulatory, and inhibitory mechanisms must control apoptosis. Paradoxically, these same pathways are utilized during infection by distinct intracellular microorganisms to evade recognition by the immune system and therefore survive, reproduce and develop. In cancer, neoplastic cells inhibit apoptosis, thus allowing their survival and increasing their capability to invade different tissues and organs. The purpose of this work is to review the generalities of the molecular mechanisms and signalling pathways involved in apoptosis induction and inhibition. Additionally, we compile the current evidence of apoptosis modulation during cancer and Leishmania infection as a model of apoptosis regulation by an intracellular microorganism.

Resistance to apoptosis and autophagy leads to enhanced survival in Sertoli cells

STUDY QUESTION

What is the underlying mechanism of Sertoli cell (SC) resistance to cell death?

SUMMARY ANSWER

High expression of prosurvival B-cell lymphoma-2 (BCL2) proteins and inhibition of apoptosis and autophagy prolongs SC survival upon exposure to stress stimuli.

WHAT IS KNOWN ALREADY

In human and in experimental models of orchitis, tolerogenic SC survive stress conditions, while germ cells undergo massive apoptosis. In general, non-dividing highly differentiated cells tend to resist stress conditions for a longer time by favoring activation of prosurvival mechanisms and inhibition of cell death pathways.

STUDY DESIGN, SIZE, DURATION

In this cross sectional study, conditions stimulating apoptosis and autophagy were used to induce cell death in primary rat SC. Primary rat peritubular cells (PTC) and immortalized rat 93RS2 SC were used as controls. Each cell isolation was counted as one experiment (n = 1), and each experiment was repeated three to six times.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Testis biopsy samples from infertile or subfertile patients and testis samples from rats with experimental autoimmune orchitis were used for immunohistological analysis. Primary SC were isolated from 19-day-old male Wistar rats. To maintain cell purity, cells were cultured in serum-free medium for apoptosis experiments and in medium supplemented with 1% serum for autophagy analyses. To induce apoptosis, cells were stimulated with staurosporine, borrelidin, cisplatin and etoposide for 4 or 24 h. Caspase three activation was examined by immunoblotting and enzymatic activity assay. Mitochondrial membrane potential was measured using tetramethylrhodamine methyl ester followed by flow cytometric analysis. Cytochrome c release was monitored by immunofluorescence. Cell viability was determined using the methylthiazole tetrazolium assay. To monitor autophagy flux, cells were deprived of nutrients using Hank's balanced salt solution for 1, 2 and 3 h. Formation of autophagosomes was analyzed by using immunoblotting, immunofluorescence labeling and ultrastructural analyses. Relative mRNA levels of genes involved in the regulation of apoptosis and autophagy were evaluated. Extracellular high mobility group box protein one was measured as a marker of necrosis using ELISA.

MAIN RESULTS AND THE ROLE OF CHANCE

SC survive the inflammatory conditions in vivo in human testis and in experimental autoimmune orchitis. Treatment with apoptosis inducing chemotherapeutics did not cause caspase three activation in isolated rat SC. Moreover, mitochondrial membrane potential and mitochondrial localization of cytochrome c were not changed by treatment with staurosporine, suggesting a premitochondrial blockade of apoptosis in SC. Expression levels of prosurvival BCL2 family members were significantly higher in SC compared to PTC at both mRNA and protein levels. Furthermore, after nutrient starvation, autophagy signaling was initiated in SC as observed by decreased levels of phosphorylated UNC- 51-like kinase -1 (ULK1). However, levels of light chain 3 II (LC3 II) and sequestosome1 (SQSTM1) remained unchanged, indicating blockade of the autophagy flux. Lysosomal activity was intact in SC as shown by accumulation of LC3 II following administration of lysosomal protease inhibitors, indicating that inhibition of autophagy flux occurs at a preceding stage.

LARGE SCALE DATA

N/A.

LIMITATIONS, REASONS FOR CAUTION

In this study, we have used primary SC from prepubertal rats. Caution should be taken when translating our results to adult animals, where crosstalk with other testicular cells and hormonal factors may also play a role in regulating survival of SC.

WIDER IMPLICATIONS OF THE FINDINGS

Our results suggest that inhibition of autophagy and apoptosis following exposure to extrinsic stress stimuli promotes SC survival, and is a possible mechanism to explain the robustness of SC in response to stress. Cell death resistance in SC is crucial for the recovery of spermatogenesis after chemotherapy treatment in cancer patients. Additionally, understanding the molecular mechanisms of SC survival unravels valuable target proteins, such as BCL2, that may be manipulated therapeutically to control cell viability depending on the context of the disease.

STUDY FUNDING AND COMPETING INTEREST(S)

This study was funded by the Deutsche Forschungsgemeinschaft (DFG) Grant BH93/1-1, and by the International Research Training Group between Justus Liebig University of Giessen and Monash University, Melbourne (GRK 1871/1) funded by the DFG and Monash University. The support of the Medical Faculty of Justus-Liebig University of Giessen is gratefully acknowledged. The authors declare no conflict of interest.

Role of glutathione, ROS, and Bcl-xL in the inhibition of apoptosis of monocyte-derived dendritic cells by Leishmania mexicana promastigotes

Dendritic cells (DCs) are one of the principal host cells of the obligate intracellular parasite Leishmania that can survive and reproduce within cells due to the ability to regulate different cellular events, including apoptosis. Inhibition of host cell apoptosis is a strategy employed by multiple pathogens to ensure their survival in the infected cell. We have previously reported that Leishmania mexicana promastigotes and amastigotes inhibit camptothecin-induced apoptosis of monocyte-derived dendritic cells (moDCs) through the downregulation of p38 and JNK phosphorylation. The upregulation of glutathione (GSH), the most important regulator of reactive oxygen species (ROS) concentration, has proven to protect cells from apoptosis through the inhibition of JNK1. Another mechanism employed by cells for the protection of apoptosis is the expression of anti-apoptotic proteins of the Bcl-2 family. The aim of this study was to determine if GSH, ROS, and Bcl-xL participate in the inhibition of camptothecin-induced apoptosis of moDC by L. mexicana promastigotes. GSH quantification assays showed that camptothecin and BSO (an inhibitor of glutathione synthesis) strongly decreased intracellular GSH concentration in moDC, while infection with L. mexicana promastigotes had no effect in the level of GSH. On the other hand, infection with L. mexicana promastigotes of BSO- and camptothecin-treated moDC diminished the concentration of ROS and induced the expression of the anti-apoptotic protein Bcl-xL. Our findings suggest that inhibition of camptothecin-induced apoptosis of moDC by L. mexicana promastigotes is preferentially regulated by the expression of anti-apoptotic proteins of the Bcl-2 family rather than by the redox status of the cell.

Ammonium trichloro [1,2-ethanediolato-O,O']-tellurate cures experimental visceral leishmaniasis by redox modulation of Leishmania donovani trypanothione reductase and inhibiting host integrin linked PI3K/Akt pathway

In an endeavor to search for affordable and safer therapeutics against debilitating visceral leishmaniasis, we examined antileishmanial potential of ammonium trichloro [1,2-ethanediolato-O,O']-tellurate (AS101); a tellurium based non toxic immunomodulator. AS101 showed significant in vitro efficacy against both Leishmania donovani promastigotes and amastigotes at sub-micromolar concentrations. AS101 could also completely eliminate organ parasite load from L. donovani infected Balb/c mice along with significant efficacy against infected hamsters (˃93% inhibition). Analyzing mechanistic details revealed that the double edged AS101 could directly induce apoptosis in promastigotes along with indirectly activating host by reversing T-cell anergy to protective Th1 mode, increased ROS generation and anti-leishmanial IgG production. AS101 could inhibit IL-10/STAT3 pathway in L. donovani infected macrophages via blocking α4β7 integrin dependent PI3K/Akt signaling and activate host MAPKs and NF-κB for Th1 response. In silico docking and biochemical assays revealed AS101's affinity to form thiol bond with cysteine residues of trypanothione reductase in Leishmania promastigotes leading to its inactivation and inducing ROS-mediated apoptosis of the parasite via increased Ca2+ level, loss of ATP and mitochondrial membrane potential along with metacaspase activation. Our findings provide the first evidence for the mechanism of action of AS101 with excellent safety profile and suggest its promising therapeutic potential against experimental visceral leishmaniasis.

Countervailing, time-dependent effects on host autophagy promotes intracellular survival of Leishmania

Autophagy is essential for cell survival under stress and has also been implicated in host defense. Here, we investigated the interactions between Leishmania donovani, the main etiological agent of visceral leishmaniasis, and the autophagic machinery of human macrophages. Our results revealed that during early infection-and via activation of the Akt pathway-Leishmania actively inhibits the induction of autophagy. However, by 24 h, Leishmania switched from being an inhibitor to an overall inducer of autophagy. These findings of a dynamic, biphasic response were based on the accumulation of lipidated light chain 3 (LC3), an autophagosome marker, by Western blotting and confocal fluorescence microscopy. We also present evidence that Leishmania induces delayed host cell autophagy via a mechanism independent of reduced activity of the mechanistic target of rapamycin (mTOR). Notably, Leishmania actively inhibited mTOR-regulated autophagy even at later stages of infection, whereas there was a clear induction of autophagy via some other mechanism. In this context, we examined host inositol monophosphatase (IMPase), reduced levels of which have been implicated in mTOR-independent autophagy, and we found that IMPase activity is significantly decreased in infected cells. These findings indicate that Leishmania uses an alternative pathway to mTOR to induce autophagy in host macrophages. Finally, RNAi-mediated down-regulation of host autophagy protein 5 (ATG5) or autophagy protein 9A (ATG9A) decreased parasite loads, demonstrating that autophagy is essential for Leishmania survival. We conclude that Leishmania uses an alternative pathway to induce host autophagy while simultaneously inhibiting mTOR-regulated autophagy to fine-tune the timing and magnitude of this process and to optimize parasite survival.