Leishmania Infection Induces Macrophage Vascular Endothelial Growth Factor A Production in an ARNT/HIF-Dependent Manner

Exploring the impact of epidemiological and clinical factors on the progression of canine leishmaniosis by statistical and whole genome analyses: from breed predisposition to comorbidities

Canine leishmaniosis (CanL), caused by Leishmania infantum, is a complex disease of growing importance in Europe. Clinical manifestations result from the down-modulation of the host immune response through multiple host-parasite interactions. Although several factors might influence CanL progression, this is the first known study evaluating risk factors for its different clinical stages in a large referral hospital population (n = 35.669) from an endemic area, over a 20 year period. Genome-wide scans for selection signatures were also conducted to explore the genomic component of clinical susceptibility to L. infantum infection. The prevalence of CanL was 3.2% (16.7% stage I; 43.6% stage II; 32.1% stage III; 7.6% stage IV). Dog breed (crossbreed), bodyweight (<10 kg), living conditions (indoors), regular deworming treatment, and being vaccinated against Leishmania significantly decreased the transmission risk and the risk for developing severe clinical forms. Conversely, the detection of comorbidities was associated with advanced clinical forms, particularly chronic kidney disease, neoplasia, cryptorchidism, infectious tracheobronchitis and urate urolithiasis, although those did not impact the clinical outcome. Significant associations between an increased risk of severe clinical stages and findings in the anamnesis (renal or skin-related manifestations) and physical examination (ocular findings) were also detected, highlighting their diagnostic value in referred cases of CanL. Sixteen breeds were found to be significantly more susceptible to developing severe stages of leishmaniosis (e.g. Great Dane, Rottweiler, English Springer Spaniel, Boxer, American Staffordshire Terrier, Golden Retriever), while 20 breeds displayed a clinical resistantance phenotype and, thus, are more likely to mount an efficient immune response against L. infantum (e.g. Pointer, Samoyed, Spanish Mastiff, Spanish Greyhound, English Setter, Siberian Husky). Genomic analyses of these breeds retrieved 12 regions under selection, 63 candidate genes and pinpointed multiple biological pathways such as the IRE1 branch of the unfolded protein response, which could play a critical role in clinical susceptibility to L. infantum infection.

Pathogenic CD8 T cell responses are driven by neutrophil-mediated hypoxia in cutaneous leishmaniasis

Cutaneous leishmaniasis caused by Leishmania parasites exhibits a wide range of clinical manifestations. Although parasites influence disease severity, cytolytic CD8 T cell responses mediate disease. While these responses originate in the lymph node, we find that expression of the cytolytic effector molecule granzyme B is restricted to lesional CD8 T cells in Leishmania - infected mice, suggesting that local cues within inflamed skin induce cytolytic function. Expression of Blimp-1 ( Prdm1 ), a transcription factor necessary for cytolytic CD8 T cell differentiation, is driven by hypoxia within the inflamed skin. Hypoxia is further enhanced by the recruitment of neutrophils that consume oxygen to produce reactive oxygen species, ultimately increasing granzyme B expression in CD8 T cells. Importantly, lesions from cutaneous leishmaniasis patients exhibit hypoxia transcription signatures that correlate with the presence of neutrophils. Thus, targeting hypoxia-driven signals that support local differentiation of cytolytic CD8 T cells may improve the prognosis for patients with cutaneous leishmaniasis, as well as other inflammatory skin diseases where cytolytic CD8 T cells contribute to pathogenesis.

Regulatory Functions of Hypoxia in Host-Parasite Interactions: A Focus on Enteric, Tissue, and Blood Protozoa

Body tissues are subjected to various oxygenic gradients and fluctuations and hence can become transiently hypoxic. Hypoxia-inducible factor (HIF) is the master transcriptional regulator of the cellular hypoxic response and is capable of modulating cellular metabolism, immune responses, epithelial barrier integrity, and local microbiota. Recent reports have characterized the hypoxic response to various infections. However, little is known about the role of HIF activation in the context of protozoan parasitic infections. Growing evidence suggests that tissue and blood protozoa can activate HIF and subsequent HIF target genes in the host, helping or hindering their pathogenicity. In the gut, enteric protozoa are adapted to steep longitudinal and radial oxygen gradients to complete their life cycle, yet the role of HIF during these protozoan infections remains unclear. This review focuses on the hypoxic response to protozoa and its role in the pathophysiology of parasitic infections. We also discuss how hypoxia modulates host immune responses in the context of protozoan infections.

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.

Hypoxia-Inducible Factor-1 Alpha Stabilization in Human Macrophages during Leishmania major Infection Is Impaired by Parasite Virulence

Hypoxia-inducible factor-1 alpha (HIF-1α) is one of the master regulators of immune and metabolic cellular functions. HIF-1α, a transcriptional factor whose activity is closely related to oxygen levels, is a target for understanding infectious disease control. Several studies have demonstrated that HIF-1α plays an important role during the infectious process, while its role in relation to parasite virulence has not been addressed. In this work, we studied the expression levels of HIF-1α and related angiogenic vascular endothelial growth factor A (VEGF-A) in human macrophages infected with promastigotes of hypo- or hyper-virulent Leishmania major human isolates. L. major parasites readily subverted host macrophage functions for their survival and induced local oxygen consumption at the site of infection. In contrast to hypo-virulent parasites that induce high HIF-1α expression levels, hyper-virulent L. major reduced HIF-1α expression in macrophages under normoxic or hypoxic conditions, and consequently impeded the expression of VEGF-A mRNA. HIF-1α may play a key role during control of disease chronicity, severity, or outcome.

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.

Low expression of hypoxia-inducible factor-1α and differential expression of immune mediators during experimental infection with Leishmania (Viannia) spp

Leishmania (Viannia) species are the major agents of cutaneous leishmaniasis (CL) in the Americas. Ulcerative stigmatizing skin lesions generally characterize CL. The microenvironment during Leishmania infection is rich in inflammatory cells and molecules, which contrasts with low oxygen levels. The hypoxia-inducible factor (HIF)-1α activates several genes in response to hypoxia and inflammatory reactions, but its role in the CL course is poorly understood. We investigated the expression pattern of the genes HIF-1α, arginase, inducible NO synthase (iNOS), interferon (IFN)-γ, interleukin (IL)-12, and IL-10 in skin lesions and lymph nodes of golden hamsters infected with L. braziliensis, L. lainsoni, and L. naiffi. The animals were infected and followed for 105 days, with paw volume measurements and photos taken weekly. Euthanasia was performed at 0, 15, 56, and 105 days post-infection. The parasite load of paw and lymph node tissues were measured through absolute quantification at real-time PCR (qPCR), and reverse transcription qPCR (RT-qPCR) was applied to demonstrate the relative mRNA expression of the target genes. Among groups, animals infected with L. braziliensis had the highest parasite load in paws and lymph nodes. HIF-1α mRNA was down-regulated during chronic Leishmania (Viannia) infection but demonstrated less inhibition in hamsters infected with L. lainsoni and L. naiffi. Arginase was the most detectable gene in animals infected by L. braziliensis; IFN-γ and IL-10 genes were the most detectable in L. lainsoni and L. naiffi-infected animals. HIF-1α gene transcription seemed to be down-modulated byL. (Viannia)infection and was less inhibited by L. lainsoni and L. naiffi when compared toL. braziliensis. Animals with L. lainsoni and L. naiffi showed better control of the disease. Further studies are necessary to evaluate the mechanism influencing HIF-1α expression and its role on CL protection; such research could elucidate potential use of HIF-1α as a therapeutic target.

HIF-α Activation Impacts Macrophage Function during Murine Leishmania major Infection

Leishmanial skin lesions are characterized by inflammatory hypoxia alongside the activation of hypoxia-inducible factors, HIF-1α and HIF-2α, and subsequent expression of the HIF-α target VEGF-A during Leishmania major infection. However, the factors responsible for HIF-α activation are not known. We hypothesize that hypoxia and proinflammatory stimuli contribute to HIF-α activation during infection. RNA-Seq of leishmanial lesions revealed that transcripts associated with HIF-1α signaling were induced. To determine whether hypoxia contributes to HIF-α activation, we followed the fate of myeloid cells infiltrating from the blood and into hypoxic lesions. Recruited myeloid cells experienced hypoxia when they entered inflamed lesions, and the length of time in lesions increased their hypoxic signature. To determine whether proinflammatory stimuli in the inflamed tissue can also influence HIF-α activation, we subjected macrophages to various proinflammatory stimuli and measured VEGF-A. While parasites alone did not induce VEGF-A, and proinflammatory stimuli only modestly induced VEGF-A, HIF-α stabilization increased VEGF-A during infection. HIF-α stabilization did not impact parasite entry, growth, or killing. Conversely, the absence of ARNT/HIF-α signaling enhanced parasite internalization. Altogether, these findings suggest that HIF-α is active during infection, and while macrophage HIF-α activation promotes lymphatic remodeling through VEGF-A production, HIF-α activation does not impact parasite internalization or control.

Network-Based Approaches Reveal Potential Therapeutic Targets for Host-Directed Antileishmanial Therapy Driving Drug Repurposing

Leishmania parasites are the causal agent of leishmaniasis, an endemic disease in more than 90 countries worldwide. Over the years, traditional approaches focused on the parasite when developing treatments against leishmaniasis. Despite numerous attempts, there is not yet a universal treatment, and those available have allowed for the appearance of resistance. Here, we propose and follow a host-directed approach that aims to overcome the current lack of treatment. Our approach identifies potential therapeutic targets in the host cell and proposes known drug interactions aiming to improve the immune response and to block the host machinery necessary for the survival of the parasite. We started analyzing transcription factor regulatory networks of macrophages infected with Leishmania major. Next, based on the regulatory dynamics of the infection and available gene expression profiles, we selected potential therapeutic target proteins. The function of these proteins was then analyzed following a multilayered network scheme in which we combined information on metabolic pathways with known drugs that have a direct connection with the activity carried out by these proteins. Using our approach, we were able to identify five host protein-coding gene products that are potential therapeutic targets for treating leishmaniasis. Moreover, from the 11 drugs known to interact with the function performed by these proteins, 3 have already been tested against this parasite, verifying in this way our novel methodology. More importantly, the remaining eight drugs previously employed to treat other diseases, remain as promising yet-untested antileishmanial therapies. IMPORTANCE This work opens a new path to fight parasites by targeting host molecular functions by repurposing available and approved drugs. We created a novel approach to identify key proteins involved in any biological process by combining gene regulatory networks and expression profiles. Once proteins have been selected, our approach employs a multilayered network methodology that relates proteins to functions to drugs that alter these functions. By applying our novel approach to macrophages during the Leishmania infection process, we both validated our work and found eight drugs already approved for use in humans that to the best of our knowledge were never employed to treat leishmaniasis, rendering our work as a new tool in the box available to the scientific community fighting parasites.

Hypoxia-Inducible Factor Signaling in Macrophages Promotes Lymphangiogenesis in Leishmania major Infection

Vascular remodeling is a phenomenon seen in the cutaneous lesions formed during infection with Leishmania parasites. Within the lesion, Leishmania major infection leads to the infiltration of inflammatory cells, including macrophages, and is associated with hypoxic conditions and lymphangiogenesis in the local site. This low-oxygen environment is concomitant with the expression of hypoxic inducible factors (HIFs), which initiate the expression of vascular endothelial growth factor-A (VEGF-A) in macrophages during the infection. Here, we found that macrophage hypoxia is elevated in the skin, and the HIF target Vegfa is preferentially expressed at the site of infection. Further, transcripts indicative of both HIF-1α and HIF-2α activation were increased at the site of infection. Given that HIF mediates VEGF-A and that VEGF-A/VEGFR-2 signaling induces lymphangiogenesis, we wanted to investigate the link between myeloid HIF activation and lymphangiogenesis during L. major infection. We show that myeloid aryl hydrocarbon receptor nuclear translocator (ARNT)/HIF/VEGF-A signaling promotes lymphangiogenesis (the generation of newly formed vessels within the local lymphatic network), which helps resolve the lesion by draining away inflammatory cells and fluid. Concomitant with impaired lymphangiogenesis, we find the deletion of myeloid ARNT/HIF signaling leads to an exacerbated inflammatory response associated with a heightened CD4⁺ Th1 immune response following L. major infection. Altogether, our data suggest that VEGF-A-mediated lymphangiogenesis occurs through myeloid ARNT/HIF activation following Leishmania major infection and this process is critical in limiting immunopathology.

Current concepts in granulomatous immune responses

Persistent irritants that are resistant to innate and cognate immunity induce granulomas. These macrophage-dominated lesions that partially isolate the healthy tissue from the irritant and the irritant induced inflammation. Particles, toxins, autoantigens and infectious agents can induce granulomas. The corresponding lesions can be protective for the host but they can also cause damage and such damage has been associated with the pathology of more than a hundred human diseases. Recently, multiple molecular mechanisms underlying how normal macrophages transform into granuloma-inducing macrophages have been discovered and new information has been gathered, indicating how these lesions are initiated, spread and regulated. In this review, differences between the innate and cognate granuloma pathways are discussed by summarizing how the dendritic cell - T cell axis changes granulomatous immunity. Granuloma lesions are highly dynamic and depend on continuous cell replacement. This feature provides new therapeutic approaches to treat granulomatous diseases.

Macrophages as host, effector and immunoregulatory cells in leishmaniasis: Impact of tissue micro-environment and metabolism

Leishmania are protozoan parasites that predominantly reside in myeloid cells within their mammalian hosts. Monocytes and macrophages play a central role in the pathogenesis of all forms of leishmaniasis, including cutaneous and visceral leishmaniasis. The present review will highlight the diverse roles of macrophages in leishmaniasis as initial replicative niche, antimicrobial effectors, immunoregulators and as safe hideaway for parasites persisting after clinical cure. These multiplex activities are either ascribed to defined subpopulations of macrophages (e.g., Ly6C^highCCR2⁺ inflammatory monocytes/monocyte-derived dendritic cells) or result from different activation statuses of tissue macrophages (e.g., macrophages carrying markers of of classical [M1] or alternative activation [M2]). The latter are shaped by immune- and stromal cell-derived cytokines (e.g., IFN-γ, IL-4, IL-10, TGF-β), micro milieu factors (e.g., hypoxia, tonicity, amino acid availability), host cell-derived enzymes, secretory products and metabolites (e.g., heme oxygenase-1, arginase 1, indoleamine 2,3-dioxygenase, NOS2/NO, NOX2/ROS, lipids) as well as by parasite products (e.g., leishmanolysin/gp63, lipophosphoglycan). Exciting avenues of current research address the transcriptional, epigenetic and translational reprogramming of macrophages in a Leishmania species- and tissue context-dependent manner.

The role of vascular endothelium and exosomes in human protozoan parasitic diseases

The vascular endothelium is a vital component in maintaining the structure and function of blood vessels. The endothelial cells (ECs) mediate vital regulatory functions such as the proliferation of cells, permeability of various tissue membranes, and exchange of gases, thrombolysis, blood flow, and homeostasis. The vascular endothelium also regulates inflammation and immune cell trafficking, and ECs serve as a replicative niche for many bacterial, viral, and protozoan infectious diseases. Endothelial dysfunction can lead to vasodilation and pro-inflammation, which are the hallmarks of many severe diseases. Exosomes are nanoscale membrane-bound vesicles that emerge from cells and serve as important extracellular components, which facilitate communication between cells and maintain homeostasis during normal and pathophysiological states. Exosomes are also involved in gene transfer, inflammation and antigen presentation, and mediation of the immune response during pathogenic states. Protozoa are a diverse group of unicellular organisms that cause many infectious diseases in humans. In this regard, it is becoming increasingly evident that many protozoan parasites (such as Plasmodium, Trypanosoma, Leishmania, and Toxoplasma) utilize exosomes for the transfer of their virulence factors and effector molecules into the host cells, which manipulate the host gene expression, immune responses, and other biological activities to establish and modulate infection. In this review, we discuss the role of the vascular endothelium and exosomes in and their contribution to pathogenesis in malaria, African sleeping sickness, Chagas disease, and leishmaniasis and toxoplasmosis with an emphasis on their actions on the innate and adaptive immune mechanisms of resistance.