Inflammasomes and Leishmania: in good times or bad, in sickness or in health

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.

Immunotherapeutic Strategies as Potential Treatment Options for Cutaneous Leishmaniasis

Cutaneous leishmaniasis (CL), caused by protozoan parasites of the Leishmania genus, is prevalent in tropical and subtropical regions, with important morbidity, particularly in low- to middle-income countries. Current systemic treatments, including pentavalent antimonials and miltefosine, are associated with significant toxicity, reduced efficacy, and are frequently ineffective in cases of severe or chronic CL. Immunotherapies leverage the immune system to combat microbial infection and offer a promising adjunct or alternative approach to the current standard of care for CL. However, the heterogeneous clinical presentation of CL, which is dependent on parasite species and host immunity, may require informed clinical intervention with immunotherapies. This review explores the clinical and immunological characteristics of CL, emphasising the current landscape of immunotherapies in in vivo models and clinical studies. Such immune-based interventions aim to modulate immune responses against Leishmania, with additive therapeutic effects enabling the efficacy of lower drug doses and decreasing the associated toxicity. Understanding the mechanisms that underlie immunotherapy for CL provides critical insights into developing safer and more effective treatments for this neglected tropical disease. Identifying suitable therapeutic candidates and establishing their safety and efficacy are essential steps in this process. However, the feasibility and utility of these treatments in resource-limited settings must also be considered, taking into account factors such as cost of production, temperature stability, and overall patient access.

Canto F, Fraga-Junior VDS, Eustáquio Lopes M, Gomes Vaz L, Pessenda G, Paun A, Freitas-Mesquita AL, Meyer-Fernandes JR, Boroni M, Bellio M, Batista Menezes G, Brzostowski J, Mottram J, Sacks D, Lima APCA, Saraiva EM. Functional plasticity shapes neutrophil response to Leishmania major infection in susceptible and resistant strains of mice

Neutrophils rapidly infiltrate sites of infection and possess several microbicidal strategies, such as neutrophil extracellular traps release and phagocytosis. Enhanced neutrophil infiltration is associated with higher susceptibility to Leishmania infection, but neutrophil effector response contribution to this phenotype is uncertain. Here, we show that neutrophils from susceptible BALB/c mice (B/c) produce more NETs in response to Leishmania major than those from resistant C57BL/6 mice (B6), which are more phagocytic. The absence of neutrophil elastase contributes to phagocytosis regulation. Microarray analysis shows enrichment of genes involved in NET formation (mpo, pi3kcg, il1b) in B/c, while B6 shows upregulation of genes involved in phagocytosis and cell death (Arhgap12, casp9, mlkl, FasL). scRNA-seq in L. major-infected B6 showed heterogeneity in the pool of intralesional neutrophils, and we identified the N1 subset as the putative subpopulation involved with phagocytosis. In vivo, imaging validates NET formation in infected B/c ears where NETing neutrophils were mainly uninfected cells. NET digestion in vivo augmented parasite lymphatic drainage. Hence, a balance between NET formation and phagocytosis in neutrophils may contribute to the divergent phenotype observed in these mice.

Deciphering the intricate dynamics of inflammasome regulation in visceral and post-kala-azar dermal leishmaniasis: A meta-analysis of consistencies

Post Kala-azar dermal leishmaniasis (PKDL) arises as a significant dermal sequel following Visceral leishmaniasis (VL) caused by protozoan parasite Leishmania donovani (LD). PKDL acts as a significant constrain for VL elimination serving as a crucial reservoir for LD. PKDL patients exhibit depigmented macular and papular lesions on their skin, which results in social discrimination due to loss of natural skin color. Inflammatory reactions, prevalent in both VL and PKDL, potentially lead to tissue damage in areas harboring the parasite. Disruption of the immune-inflammasomal network not only facilitates LD persistence but also leads to the skin hypopigmentation seen in PKDL, impacting social well-being. Activation of inflammasomal markers like STAT1, NLRP1, NLRP3, AIM2, CASP11, and NLRP12 have been identified as a common host-defense mechanism across various Leishmania infections. Conversely, Leishmania modulates inflammasome activation to sustain its presence within the host. Nevertheless, in specific instances of Leishmania infection, inflammasome activation can worsen disease pathology by promoting parasite proliferation and persistence. This study encompasses recent transcriptomic analyses conducted between 2016 and 2023 on human and murine subjects afflicted with VL/PKDL, elucidating significant alterations in inflammasomal markers in both conditions. It offers a comprehensive understanding how these markers contribute in disease progression, drawing upon available literature for logical analysis. Furthermore, our analysis identifies validated miRNA network that could potentially disrupt this crucial immune-inflammasomal network, thereby offering a plausible explanation on how secreted LD-factors could enable membrane-bound LD, isolated from the host cytoplasm, to modulate cytoplasmic inflammasomal markers. Insights from this study could guide the development of host-directed therapeutics to impede transmission and address hypopigmentation, thereby mitigating the social stigma associated with PKDL.

NLRP1-dependent activation of Gasdermin D in neutrophils controls cutaneous leishmaniasis

Intracellular pathogens that replicate in host myeloid cells have devised ways to inhibit the cell's killing machinery. Pyroptosis is one of the host strategies used to reduce the pathogen replicating niche and thereby control its expansion. The intracellular Leishmania parasites can survive and use neutrophils as a silent entry niche, favoring subsequent parasite dissemination into the host. Here, we show that Leishmania mexicana induces NLRP1- and caspase-1-dependent Gasdermin D (GSDMD)-mediated pyroptosis in neutrophils, a process critical to control the parasite-induced pathology. In the absence of GSDMD, we observe an increased number of infected dermal neutrophils two days post-infection. Using adoptive neutrophil transfer in neutropenic mice, we show that pyroptosis contributes to the regulation of the neutrophil niche early after infection. The critical role of neutrophil pyroptosis and its positive influence on the regulation of the disease outcome was further demonstrated following infection of mice with neutrophil-specific deletion of GSDMD. Thus, our study establishes neutrophil pyroptosis as a critical regulator of leishmaniasis pathology.

Leishmania infantum infection modulates messenger RNA, microRNA and long non-coding RNA expression in human neutrophils in vitro

In the Americas, L. infantum (syn. chagasi) is the main cause of human visceral leishmaniasis. The role of neutrophils as part of the innate response to Leishmania spp. infection is dubious and varies according to the species causing the infection. Global expression of coding RNAs, microRNAs and long non-coding RNAs changes as part of the immune response against pathogens. Changes in mRNA and non-coding RNA expression resulting from infection by Leishmania spp. are widely studied in macrophages, but scarce in neutrophils, the first cell to encounter the trypanosomatid, especially following infection by L. infantum. Herein, we aimed to understand the expression patterns of coding and non-coding transcripts during acute in vitro infection of human neutrophils by L. infantum. We isolated neutrophils from whole blood of healthy male donors (n = 5) and split into groups: 1) infected with L. infantum (MOI = 5:1), and 2) uninfected controls. After 3 hours of exposure of infected group to promastigotes of L. infantum, followed by 17 hours of incubation, total RNA was extracted and total RNA-Seq and miRNA microarray were performed. A total of 212 genes were differentially expressed in neutrophils following RNA-Seq analysis (log2(FC)±0.58, FDR≤0.05). In vitro infection with L. infantum upregulated the expression of 197 and reduced the expression of 92 miRNAs in human neutrophils (FC±2, FDR≤0.01). Lastly, 5 downregulated genes were classified as lncRNA, and of the 10 upregulated genes, there was only 1 lncRNA. Further bioinformatic analysis indicated that changes in the transcriptome and microtranscriptome of neutrophils, following in vitro infection with L. infantum, may impair phagocytosis, apoptosis and decrease nitric oxide production. Our work sheds light on several mechanisms used by L. infantum to control neutrophil-mediated immune response and identifies several targets for future functional studies, aiming at the development of preventive or curative treatments for this prevalent zoonosis.

Transcriptomics analysis highlights potential ways in human pathogenesis in Leishmania braziliensis infected with the viral endosymbiont LRV1

The parasite Leishmania (Viannia) braziliensis is widely distributed in Brazil and is one of the main species associated with human cases of different forms of tegumentary leishmaniasis (TL) such as cutaneous leishmaniasis (CL) and mucosal leishmaniasis (ML). The mechanisms underlying the pathogenesis of TL are still not fully understood, but it is known that factors related to the host and the parasite act in a synergistic and relevant way to direct the response to the infection. In the host, macrophages have a central connection with the parasite and play a fundamental role in the defense of the organism due to their ability to destroy intracellular parasites and present antigens. In the parasite, some intrinsic factors related to the species or even the strain analyzed are fundamental for the outcome of the disease. One of them is the presence of Leishmania RNA Virus 1 (LRV1), an endosymbiont virus that parasitizes some species of Leishmania that triggers a cascade of signals leading to a more severe TL phenotype, such as ML. One of the strategies for understanding factors associated with the immune response generated after Leishmania/host interaction is through the analysis of molecular patterns after infection. Thus, the gene expression profile in human monocyte-derived macrophages obtained from healthy donors infected in vitro with L. braziliensis positive (LbLRV1+) and negative (LbLRV1-) for LRV1 was evaluated. For this, the microarray assay was used and 162 differentially expressed genes were identified in the comparison LbLRV1+ vs. LbLRV1-, 126 upregulated genes for the type I and II interferons (IFN) signaling pathway, oligoadenylate synthase OAS/RNAse L, non-genomic actions of vitamin D3 and RIG-I type receptors, and 36 down-regulated. The top 10 downregulated genes along with the top 10 upregulated genes were considered for analysis. Type I interferon (IFNI)- and OAS-related pathways results were validated by RT-qPCR and Th1/Th2/Th17 cytokines were analyzed by Cytometric Bead Array (CBA) and enzyme-linked immunosorbent assay (ELISA). The microarray results validated by RT-qPCR showed differential expression of genes related to IFNI-mediated pathways with overexpression of different genes in cells infected with LbLRV1+ compared to LbLRV1- and to the control. No significant differences were found in cytokine levels between LbLRV1+ vs. LbLRV1- and control. The data suggest the activation of gene signaling pathways associated with the presence of LRV1 has not yet been reported so far. This study demonstrates, for the first time, the activation of the OAS/RNase L signaling pathway and the non-genomic actions of vitamin D3 when comparing infections with LbLRV1+ versus LbLRV1- and the control. This finding emphasizes the role of LRV1 in directing the host's immune response after infection, underlining the importance of identifying LRV1 in patients with TL to assess disease progression.

Cellular mediators in human leishmaniasis: Critical determinants in parasite killing or disease progression

Data on cellular immunity mediators in the early phase of human leishmaniasis are still limited and controversial. In order to mimic the changes of humoral mediators during the early phase of human natural infection, some Th1, Th2, Treg, and Breg cytokines, MCP-1, and the nitric oxide (NO) from human PBMC, stimulated by Leishmania infantum, Leishmania major, Leishmania donovani and Leishmania tropica infective metacyclic promastigotes, were determined. After 4 h of L. major, L. donovani, and L. tropica challenge, TNFα, IL-1β, IL-6 levels were significantly higher than negative control cultures with saline (SF) instead of Leishmania promastigotes, unlike L. infantum-stimulated TNFα and L. major-stimulated IL-1β. We obtained higher levels of IL-4 and IL-10 cytokines after stimulation of human PBMCs by L. infantum and L. donovani, compared to those observed after the challenge of PBMCs by L. major and L. tropica. Regarding IL-35, such cytokine levels were significantly increased following infection with L. infantum and L. donovani, in contrast to L. major and L. tropica. Up to our knowledge, we are the first to study the effect of four different species of Leishmania on IL-35 levels in human cells. Our study highlights how several Leishmania species can up-regulate different groups of cytokines (Th1, Th2, Treg and Breg) and modulate NO release in a different way. This original aspect can be explained by different Leishmania cell products, such as LPG, obtained from different strains/species of live parasites. Our findings would contribute to the development of new therapeutics or vaccination strategies.

Immune dysregulation and inflammation causing hypopigmentation in post kala-azar dermal leishmaniasis: partners in crime?

Post kala-azar dermal leishmaniasis (PKDL), a heterogeneous dermal sequela of visceral leishmaniasis (VL), is challenging in terms of its etiopathogenesis. Hypopigmentation is a consistent clinical feature in PKDL, but mechanisms contributing to the loss of melanocytes remains poorly defined. Like other hypopigmentary dermatoses - for example, vitiligo, psoriasis, and leprosy - the destruction of melanocytes is likely a multifactorial phenomenon, key players being immune dysregulation and inflammation. This review focuses on immunological mechanisms responsible for the 'murder' of melanocytes, prime suspects at the lesional sites being CD8+ T cells and keratinocytes and their criminal tools being proinflammatory cytokines, for example, IFN-γ, IL-6, and TNF-α. Collectively, these may cause decreased secretion of melanocyte growth factors, loss/attenuation of cell adhesion molecules and inflammasome activation, culminating in melanocyte death.

The NLRP3 inflammasome recognizes alpha-2 and alpha-7.3 giardins and decreases the pathogenicity of Giardia duodenalis in mice

BACKGROUND

Giardia duodenalis is a parasitic organism that can cause giardiasis, an intestinal infection, particularly prevalent in young children, with clinical symptoms of diarrhea. We previously reported that extracellular G. duodenalis triggers intracellular nucleotide-binding oligomerization-like receptor 3 (NLRP3) inflammasome activation and regulates the host inflammatory response by secreting extracellular vesicles (EVs). However, the exact pathogen-associated molecular patterns in G. duodenalis EVs (GEVs) involved in this process and the role of the NLRP3 inflammasome in giardiasis remain to be elucidated.

METHODS

Recombinant eukaryotic expression plasmids of pcDNA3.1(+)-alpha-2 and alpha-7.3 giardins in GEVs were constructed, transfected into primary mouse peritoneal macrophages and screened by measuring the expression levels of the inflammasome target molecule caspase-1 p20. The preliminary identification of G. duodenalis alpha-2 and alpha-7.3 giardins was further verified by measuring the protein expression levels of key molecules of the NLRP3 inflammasome (NLRP3, pro-interleukin-1 beta [IL-1β], pro-caspase-1, and caspase-1 p20), the secretion levels of IL-1β, the level of apoptosis speck-like protein (ASC) oligomerization and the immunofluorescence localization of NLRP3 and ASC. The roles of the NLRP3 inflammasome in G. duodenalis pathogenicity were then evaluated using mice in which NLRP3 activation was blocked (NLRP3-blocked mice), and body weight, parasite burden in the duodenum and histopathological changes in the duodenum were monitored. In addition, we explored whether alpha-2 and alpha-7.3 giardins triggered IL-1β secretion in vivo through the NLRP3 inflammasome and determined the roles of these molecules in G. duodenalis pathogenicity in mice.

RESULTS

Alpha-2 and alpha-7.3 giardins triggered NLRP3 inflammasome activation in vitro. This led to caspase-1 p20 activation, upregulation of the protein expression levels of NLRP3, pro-IL-1β and pro-caspase-1, significant enhancement of IL-1β secretion, ASC speck formation in the cytoplasm and also induction of ASC oligomerization. Deletion of the NLRP3 inflammasome aggravated G. duodenalis pathogenicity in mice. Compared to wild-type mice gavaged with cysts, mice gavaged with cysts in NLRP3-blocked mice displayed increased trophozoite loads and severe duodenal villus damage, characterized by necrotic crypts with atrophy and branching. In vivo assays revealed that alpha-2 and alpha-7.3 giardins could induce IL-1β secretion through the NLRP3 inflammasome and that immunization with alpha-2 and alpha-7.3 giardins decreased G. duodenalis pathogenicity in mice.

CONCLUSIONS

Overall, the results of the present study revealed that alpha-2 and alpha-7.3 giardins trigger host NLRP3 inflammasome activation and decrease G. duodenalis infection ability in mice, which are promising targets for the prevention of giardiasis.

Comparative transcriptomic analysis of long noncoding RNAs in Leishmania-infected human macrophages

It is well established that infection with Leishmania alters the host cell's transcriptome. Since mammalian cells have multiple mechanisms to control gene expression, different molecules, such as noncoding RNAs, can be involved in this process. MicroRNAs have been extensively studied upon Leishmania infection, but whether long noncoding RNAs (lncRNAs) are also altered in macrophages is still unexplored. We performed RNA-seq from THP-1-derived macrophages infected with Leishmania amazonensis (La), L. braziliensis (Lb), and L. infantum (Li), investigating a previously unappreciated fraction of macrophage transcriptome. We found that more than 24% of the total annotated transcripts and 30% of differentially expressed (DE) RNAs in Leishmania-infected macrophage correspond to lncRNAs. LncRNAs and protein coding RNAs with altered expression are similar among macrophages infected with the Leishmania species. Still, some species-specific alterations could occur due to distinct pathophysiology in which Li infection led to a more significant number of exclusively DE RNAs. The most represented classes among DE lncRNAs were intergenic and antisense lncRNAs. We also found enrichment for immune response-related pathways in the DE protein coding RNAs, as well as putative targets of the lncRNAs. We performed a coexpression analysis to explore potential cis regulation of coding and antisense noncoding transcripts. We identified that antisense lncRNAs are similarly regulated as its neighbor protein coding genes, such as the BAALC/BAALC-AS1, BAALC/BAALC-AS2, HIF1A/HIF1A-AS1, HIF1A/HIF1A-AS3 and IRF1/IRF1-AS1 pairs, which can occur as a species-specific modulation. These findings are a novelty in the field because, to date, no study has focused on analyzing lncRNAs in Leishmania-infected macrophage. Our results suggest that lncRNAs may account for a novel mechanism by which Leishmania can control macrophage function. Further research must validate putative lncRNA targets and provide additional prospects in lncRNA function during Leishmania infection.

Distinct plasma chemokines and cytokines signatures in Leishmania guyanensis-infected patients with cutaneous leishmaniasis

The immunopathology associated with Leishmaniasis is a consequence of inflammation. Upon infection with Leishmania, the type of host-immune response is determinant for the clinical manifestations that can lead to either self-healing or chronic disease. Multiple pathways may determine disease severity. A comparison of systemic immune profiles in patients with cutaneous leishmaniasis caused by L. guyanensis and healthy individuals with the same socio-epidemiological characteristics coming from the same endemic areas as the patients is performed to identify particular immune profile and pathways associated with the progression of disease development. Twenty-seven plasma soluble circulating factors were evaluated between the groups by univariate and multivariate analysis. The following biomarkers pairs IL-17/IL-9 (ρ=0,829), IL-17/IL-12 (ρ=0,786), IL-6/IL-1ra (ρ=0,785), IL-6/IL-12 (ρ=0,780), IL-1β/G-CSF (ρ=0,758) and IL-17/MIP-1β (ρ=0,754) showed the highest correlation mean among the patient while only INF-γ/IL-4 (ρ=0.740), 17/MIP-1β (ρ=0,712) and IL-17/IL-9 (ρ=0,707) exhibited positive correlation among the control group. The cytokine IL-17 and IL1β presented the greater number of positive pair correlation among the patients. The linear combinations of biomarkers displayed IP-10, IL-2 and RANTES as the variables with the higher discriminatory activity in the patient group compared to PDGF, IL-1ra and eotaxin among the control subjects. IP-10, IL-2, IL-1β, RANTES and IL-17 seem to be predictive value of progression to the development of disease among the Lg-infected individuals.

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

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

Eimeria falciformis secretes extracellular vesicles to modulate proinflammatory response during interaction with mouse intestinal epithelial cells

BACKGROUND

Protozoan parasite secretions can be triggered by various modified media and diverse physicochemical stressors. Equally, host-parasite interactions are known to co-opt the exchange and secretion of soluble biochemical components. Analysis of Eimeria falciformis sporozoite secretions in response to interaction with mouse intestinal epithelial cells (MIECs) may reveal parasite secretory motifs, protein composition and inflammatory activities of E. falciformis extracellular vesicles (EVs).

METHODS

Eimeria falciformis sporozoites were allowed to interact with inactivated MIECs. Parasite secretions were separated into EV and vesicle-free (VF) fractions by discontinuous centrifugation and ultracentrifugation. Secreted EVs were purified in an iodixanol density gradient medium and the protein composition of both EV and VF fractions were analyzed by liquid chromatoraphy-tandem mass spectroscopy. The inflammatory activities of E. falciformis sporozoite EV on MIECs were then investigated.

RESULTS

During the interaction of E. falciformis sporozoites with inactivated MIECs, the parasite secreted VF and vesicle-bound molecules. Eimeria falciformis vesicles are typical pathogenic protozoan EVs with a mean diameter of 264 ± 2 nm, and enclosed heat shock protein (Hsp) 70 as classical EV marker. Refractile body-associated aspartyl proteinase (or eimepsin), GAP45 and aminopeptidase were the main components of E. falciformis sporozoite EVs, while VF proteins include Hsp90, actin, Vps54 and kinases, among others. Proteomic data revealed that E. falciformis EV and VF proteins are aggregates of bioactive, antigenic and immunogenic molecules which act in concert for E. falciformis sporozoite motility, pathogenesis and survival. Moreover, in MIECs, E. falciformis EVs induced upregulation of gene expression and secretion of IL-1β, IL-6, IL-17, IL-18, MCP1 as well as pyroptosis-dependent caspase 11 and NLRP6 inflammasomes with the concomitant secretion of lactate dehydrogenase.

CONCLUSIONS

Eimeria falciformis sporozoite interaction with MIECs triggered the secretion of immunogenic and antigenic proteins. In addition, E. falciformis sporozoite EVs constitute parasite-associated molecular pattern that induced inflammatory response and cell death. This study offers additional insight in the secretion and protein composition of E. falciformis secretomes as well as the proinflammatory functions of E. falciformis sporozoite EVs.

A Mouse Model of Ulcerative Cutaneous Leishmaniasis by Leishmania (Viannia) panamensis to Investigate Infection, Pathogenesis, Immunity, and Therapeutics

A mouse model of cutaneous leishmaniasis (CL) by Leishmania (Viannia) panamensis (L(V)p) that reproduces the characteristics of the human disease remains elusive. Here we report the development of a CL model that uses a mouse-adapted L(V)p isolate to reproducibly induce a dermal disease with a remarkable similarity to human CL. BALB/c mice infected intradermally in the ear with 105 stationary UA-946 L(V)p promastigotes develop a progressive cutaneous disease that exhibits the typical ulcerated lesions with indurated borders observed in CL patients. Although most of parasites in the inoculum die within the first week of infection, the survivors vigorously multiply at the infection site during the following weeks, paralleling disease appearance and aggravation. Regional lymphadenopathy as well as lymphatic dissemination of parasites to draining lymph nodes (dLN) was evidenced early after infection. Viable parasites were also isolated from spleen at later timepoints indicating systemic parasitic dissemination, but, strikingly, no signs of systemic disease were observed. Increasing numbers of myeloid cells and T lymphocytes producing IFNγ and IL-4 were observed in the dLN as disease progressed. A mixed adaptive L(V)p-specific T cell-mediated response was induced, since ex vivo recall experiments using dLN cells and splenocytes revealed the production of type 1 (IFNγ, IL-2), type 2 (IL-4, IL-13), regulatory (IL-10), and inflammatory (GM-CSF, IL-3) cytokines. Humoral adaptive response was characterized by early production of IgG1- followed by IgG2a-type of L(V)p-specific antibodies. IFNγ/IL-4 and IgG2a/IgG1 ratios indicated that the initial non-protective Th2 response was redirected toward a protective Th1 response. In situ studies revealed a profuse recruitment of myeloid cells and of IFNγ- and IL-4-producing T lymphocytes to the site of infection, and the typical histopathological changes induced by dermotropic Leishmania species. Evidence that this model is suitable to investigate pharmacological and immunomodulatory interventions, as well as for antigen discovery and vaccine development, is also presented. Altogether, these results support the validity and utility of this novel mouse model to study the pathogenesis, immunity, and therapeutics of L(V)p infections.

Epigenetic paradigms/exemplars of the macrophage: inflammasome axis in Leishmaniasis

The infectious paradigms have recently led to the recognition interplay of complex phenomenon underpinning disease diagnosis and prognosis. Evidently, parasitic infection studies are depicting converging trends of the epigenetic, environmental, and microbiome contributions, assisting pathogen-directed modulations of host biological system. The molecular details of epigenetic variations and memory, along with the multi-omics data at the interface of the host-pathogen level becomes strong indicator of immune cell plasticity, differentiation, and pathogen survival. Despite being one of the most important aspects of the disease's etiopathology, the epigenetic regulation of host-pathogen interactions and evolutionary epigenetics have received little attention thus far. Recent evidence has focused on the growing need to link epigenetic and microbiome modulations on parasite phenotypic plasticity and pathogen-induced host phenotypic plasticity for designing futuristic therapeutic regimes. Leishmaniasis is a neglected tropical illness with varying degrees of disease severity that is linked to a trans-species and epigenetic heredity process, including the pathogen-induced host and strain-specific modulations. The review configures research findings aligning to the epigenetic epidemiology niche, involving co-evolutionary epigenetic inheritance and plasticity disease models. The epigenetic exemplars focus on the host-pathogen interactome expanse at the macrophage-inflammasome axis.

A T-Cell Epitope-Based Multi-Epitope Vaccine Designed Using Human HLA Specific T Cell Epitopes Induces a Near-Sterile Immunity against Experimental Visceral Leishmaniasis in Hamsters

Visceral leishmaniasis is a neglected tropical disease affecting 12 million people annually. Even in the second decade of the 21st century, it has remained without an effective vaccine for human use. In the current study, we designed three multiepitope vaccine candidates by the selection of multiple IFN-γ inducing MHC-I and MHC-II binder T-cell specific epitopes from three previously identified antigen genes of Leishmania donovani from our lab by an immuno-informatic approach using IFNepitope, the Immune Epitope Database (IEDB) T cell epitope identification tools, NET-MHC-1, and NET MHC-2 webservers. We tested the protective potential of these three multiepitope proteins as a vaccine in a hamster model of visceral leishmaniasis. The immunization data revealed that the vaccine candidates induced a very high level of Th1 biased protective immune response in-vivo in a hamster model of experimental visceral leishmaniasis, with one of the candidates inducing a near-sterile immunity. The vaccinated animals displayed highly activated monocyte macrophages with the capability of clearing intracellular parasites due to increased respiratory burst. Additionally, these proteins induced activation of polyfunctional T cells secreting INF-γ, TNF-α, and IL-2 in an ex-vivo stimulation of human peripheral blood mononuclear cells, further supporting the protective nature of the designed candidates.

Purinergic signaling: a new front-line determinant of resistance and susceptibility in leishmaniasis

Leishmaniasis is a neglected tropical disease that causes several clinical manifestations. Parasites of the genus Leishmania cause this disease. Spread across five continents, leishmaniasis is a particular public health problem in developing countries. Leishmania infects phagocytic cells such as macrophages, where it induces adenosine triphosphate (ATP) release at the time of infection. ATP activates purinergic receptors in the cell membranes of infected cells and promotes parasite control by inducing leukotriene B4 release and NLRP3 inflammasome activation. Moreover, uridine triphosphate induces ATP release, exacerbating the immune response. However, ATP may also undergo catalysis by ectonucleotidases present in the parasite membrane, generating adenosine, which activates P1 receptors and induces the production of anti-inflammatory molecules such as prostaglandin E2 and IL-10. These mechanisms culminate in Leishmania's survival. Thus, how Leishmania handles extracellular nucleotides and the activation of purinergic receptors determines the control or the dissemination of the disease.

Endosymbiotic RNA virus inhibits Leishmania-induced caspase-11 activation

New World species of the intracellular protozoan parasites of the Leishmania genus can cause mucocutaneous leishmaniases. The presence of an endosymbiotic Leishmania RNA virus (LRV) in Leishmania guyanensis (L.g.) promotes disease exacerbation and the development of mucocutaneous disease. It was previously reported that LRV blocks the NLRP3 inflammasome, but additional mechanisms remain unclear. Here, we investigated whether LRV interferes with the inflammasome via caspase-11, which induces non-canonical NLRP3 activation and was reported to be activated by Leishmania. By using macrophages and mice, we found that LRV inhibits caspase-11 activation and IL-1β release by L.g. in a TLR3- and ATG5-dependent manner. Moreover, LRV exacerbates disease in C57BL/6 mice but not in Casp11 ^-/- , Nlrp3 ^-/- , and 129 mice, a mouse strain that is naturally mutant for caspase-11. These results demonstrate that LRV interferes with caspase-11 activation by Leishmania, expanding our understanding about the mechanisms by which LRV promotes disease exacerbation.

Insight Into Inflammasome Signaling: Implications for Toxoplasma gondii Infection

Inflammasomes are multimeric protein complexes regulating the innate immune response to invading pathogens or stress stimuli. Recent studies have reported that nucleotide-binding leucine-rich repeat-containing (NLRs) proteins and DNA sensor absent in melanoma 2 (AIM2) serve as inflammasome sentinels, whose stimulation leads to the proteolytic activation of caspase-1, proinflammatory cytokine secretion, and pyroptotic cell death. Toxoplasma gondii, an obligate intracellular parasite of phylum Apicomplexans, is reportedly involved in NLRP1, NLRP3 and AIM2 inflammasomes activation; however, mechanistic evidence regarding the activation of these complexes is preliminary. This review describes the current understanding of inflammasome signaling in rodent and human models of T. gondii infection.

Multi-omic Analyses of Plasma Cytokines, Lipidomics, and Transcriptomics Distinguish Treatment Outcomes in Cutaneous Leishmaniasis

Leishmania braziliensis infection frequently results in cutaneous leishmaniasis (CL). An increase in incidence of drug-resistant CL leading to treatment failure has been reported. Identification of reliable predictors of treatment outcomes is necessary to optimize patient care. Here, we performed a prospective case-control study in which plasma levels of cytokines and lipid mediators were assessed at different time points during antileishmanial therapy in patients with CL from Brazil. Multidimensional analyses were employed to describe a combination of biomarkers able to predict and characterize treatment failure. We found a biosignature influenced mainly by plasma levels of lipid mediators that accurately predicted treatment failure. Furthermore, transcriptomic analysis of a publicly available data set revealed that expression levels of genes related to lipid metabolism measured in skin lesions could distinguish treatment outcomes in CL. Thus, activation of pathways linked to lipid biosynthesis predicts treatment failure in CL. The biomarkers identified may be further explored as therapeutic targets.

Involvement of the Inflammasome and Th17 Cells in Skin Lesions of Human Cutaneous Leishmaniasis Caused by Leishmania (Viannia) panamensis

Localized cutaneous leishmaniasis (LCL) caused by Leishmania (Viannia) panamensis is an endemic disease in Panama. This condition causes ulcerated skin lesions characterized by a mixed Th1/Th2 immune response that is responsible for disease pathology. However, the maintenance of the in situ inflammatory process involves other elements, such as Th17 and inflammasome responses. Although these processes are associated with parasite elimination, their role in the increase in disease pathology cannot be discarded. Thus, the role in Leishmania infection is still unclear. In this sense, the present study aimed at characterizing the Th17 and inflammasome responses in the skin lesions of patients with LCL caused by L. (V.) panamensis to help elucidate the pathogenesis of this disease in Panama. Th17 and inflammasome responses were evaluated by immunohistochemistry (IHQ) in 46 skin biopsies from patients with LCL caused by L. (V.) panamensis. The Th17 immune response was assessed using CD3, CD4, RoRγt, IL-17, IL-6, IL-23, and TGF-β1 antibodies, and the inflammasome response was assessed by IL-1β, IL-18, and caspase-1 antibodies. The presence of the Th17 and inflammasome responses was evidenced by a positive reaction for all immunological markers in the skin lesions. An inverse correlation between the density of amastigotes and the density of RoRγt⁺, IL-17⁺, IL-1β⁺, and caspase-1⁺ cells was observed, but no correlation between Th17 and the inflammasome response with evolutionary disease pathology was reported. These data showed the participation of Th17 cells and the inflammasome in the inflammatory response of the skin lesions of LCL caused by L. (V.) panamensis infection. These results suggest a role in the control of tissue parasitism of IL-17 and the activation of the NLRP3 inflammasome dependent on IL-1β but cannot exclude their role in the development of disease pathology.

P2Y2 Receptor Induces L. amazonensis Infection Control in a Mechanism Dependent on Caspase-1 Activation and IL-1β Secretion

Leishmaniasis is a neglected tropical disease caused by an intracellular parasite of the genus Leishmania. Damage-associated molecular patterns (DAMPs) such as UTP and ATP are released from infected cells and, once in the extracellular medium, activate P2 purinergic receptors. P2Y₂ and P2X7 receptors cooperate to control Leishmania amazonensis infection. NLRP3 inflammasome activation and IL-1β release resulting from P2X7 activation are important for outcomes of L. amazonensis infection. The cytokine IL-1β is required for the control of intracellular parasites. In the present study, we investigated the involvement of the P2Y₂ receptor in the activation of NLRP3 inflammasome elements (caspase-1 and 11) and IL-1β secretion during L. amazonensis infection in peritoneal macrophages as well as in a murine model of cutaneous leishmaniasis. We found that 2-thio-UTP (a selective P2Y₂ agonist) reduced parasite load in L. amazonensis-infected murine macrophages and in the footpads and lymph nodes of infected mice. The antiparasitic effects triggered by P2Y₂ activation were not observed when cells were pretreated with a caspase-1 inhibitor (Z-YVAD-FMK) or in macrophages from caspase-1/11 knockout mice (CASP-1,11^−/−). We also found that UTP treatment induced IL-1β secretion in wild-type (WT) infected macrophages but not in cells from CASP-1,11^−/− mice, suggesting that caspase-1 activation by UTP triggers IL-1β secretion in L. amazonensis-infected macrophages. Infected cells pretreated with IL-1R antagonist did not show reduced parasitic load after UTP and ATP treatment. Our in vivo experiments also showed that intralesional UTP treatment reduced both parasite load (in the footpads and popliteal lymph nodes) and lesion size in wild-type (WT) and CASP-11^−/− but not in CASP-1,11^−/− mice. Taken together, our findings suggest that P2Y₂R activation induces CASP-1 activation and IL-1β secretion during L. amazonensis infection. IL-1β/IL-1R signaling is crucial for P2Y₂R-mediated protective immune response in an experimental model of cutaneous leishmaniasis.

Leishmania Viannia guyanensis, LRV1 virus and extracellular vesicles: a dangerous trio influencing the faith of immune response during muco-cutaneous leishmaniasis

Parasites of Leishmania genus have developed various strategies to overcome host immune response favoring its infection and development toward leishmaniasis. With an array of virulence factors, those parasites modify host macrophage signaling and functions. Depending of the species involved, visceral or cutaneous leishmaniasis will develop. Several years ago, Leishmania Viannia guyanensis that is naturally infected with the endosymbiotic virus Leishmania RNA Virus 1 was found to cause a particularly aggressive form of South-American mucocutaneous leishmaniasis. This virus, when co-transmitted with the parasite was shown to strongly modulate RNA sensors and NLRP3 inflammasome network that could explain in part the exacerbated skin pathology caused by this particular parasite. In this review, we will be discussing how this endosymbiotic virus-infected Leishmania in conjunction with Leishmania exosomes partner together to manipulate host immune response in their favor.

Schistosoma antigens as activators of inflammasome pathway: from an unexpected stimulus to an intriguing role

Parasites of the genus Schistosoma are organisms capable of living for decades within the definitive host. They interfere with the immune response by interacting with host's receptors. In this review, we discuss from the first reports to the most recent discoveries regarding the ability of Schistosoma antigens in triggering intracellular receptors and inducing inflammasome activation.

The different faces of the NLRP3 inflammasome in cutaneous Leishmaniasis: A review

Cutaneous leishmaniasis (CL) is a vector-borne parasitic disease caused by Protozoa of the genus Leishmania. Clinical manifestations of this disease are the result of a complex interplay of diverse factors, including the genetic background and the immune status of the host. Understanding the impact of these factors on the CL pathology may provide new targets to manage the infection and improve clinical outcome. The NLRP3 inflammasome, an innate immune complex of several cell types, seems to be involved in the CL physiopathology. Current studies of its role show contradictory effects of this complex on the evolution of Leishmania infection in mice and humans. In this review, we discuss the data regarding different roles of the NLRP3 inflammasome in murine and human CL.

Leishmania infantum infection reduces the amyloid β42-stimulated NLRP3 inflammasome activation

Activation of the NLRP3 inflammasome has been shown to play a major role in the neuroinflammation that accompanies Alzheimer's disease (AD); interventions that down regulate the NLRP3 inflammasome could thus be beneficial in AD. Parasite infections were recently shown to be associated with improved cognitive functions in Apolipoprotein E4 (ApoE4)-expressing members of an Amazonian tribe. We verified in an in vitro model whether Leishmania infantum infection could reduce NLRP3. Results obtained in an initial experimental model in which PBMC were LPS primed and nigericin-stimulated showed that L. infantum infection significantly reduced ASC-speck formation (i.e. intracellular inflammasome proteins assembly), as well as the production of activated caspase 5 and IL-1β, but increased that of activated caspase 1 and IL-18. Moreover, L. infantum infection induced the generation of an anti-inflammatory milieu by suppressing the production of TNFα and increasing that of IL-10. These results were replicated when cells that had been LPS-primed were stimulated with Aβ₄₂ and infected with L. infantum. Results herein indicate that Leishmania infection favors an anti-inflammatory milieu, which includes the down-regulation of NLRP3 inflammasome activation, possibly to facilitate its survival inside host cells. A side effect of Leishmaniasis would be the hampering of neuroinflammation; this could play a protective role against AD development.

Efficacy of histamine H1 receptor antagonists azelastine and fexofenadine against cutaneous Leishmania major infection

Current drug therapies for cutaneous leishmaniasis are often difficult to administer and treatment failure is an increasingly common occurrence. The efficacy of anti-leishmanial therapy relies on a combination of anti-parasite activity of drugs and the patient's immune response. Previous studies have reported in vitro antimicrobial activity of histamine 1-receptor antagonists (H1RAs) against different pathogens. We used an ex vivo explant culture of lymph nodes from mice infected with Leishmania major to screen H1RAs compounds. Azelastine (AZ) and Fexofenadine (FX) showed remarkable ex vivo efficacy (EC50 = 0.05 and 1.50 μM respectively) and low in vitro cytotoxicity yielding a high therapeutic index. AZ significantly decreased the expression of H1R and the proinflammatory cytokine IL-1ẞ in the ex vivo system, which were shown to be augmented by histamine addition. The anti-leishmanial efficacy of AZ was enhanced in the presence of T cells from infected mice suggesting an immune-modulatory mechanism of parasite suppression. L. major infected BALB/c mice treated per os with FX or intralesionally with AZ showed a significant reduction of lesion size (FX = 69%; AZ = 52%). Furthermore, there was significant parasite suppression in the lesion (FX = 82%; AZ = 87%) and lymph nodes (FX = 81%; AZ = 36%) with no observable side effects. AZ and FX and potentially other H1RAs are good candidates for assessing efficacy in larger studies as monotherapies or in combination with current anti-leishmanial drugs to treat cutaneous leishmaniasis.

Dynamic imaging reveals surface exposure of virulent Leishmania amastigotes during pyroptosis of infected macrophages

Leishmania spp. are obligate intracellular parasites that infect phagocytes, notably macrophages. No information is available on how Leishmania parasites respond to pyroptosis of their host cell, which is known to limit microbial infection. Here, we analyzed the pyroptotic process and the fate of intracellular amastigotes at the single-cell level using high-content real-time imaging. Bone marrow-derived macrophages were infected with virulent Leishmania amazonensis amastigotes and sequentially treated with lipopolysaccharide and ATP to induce pyroptosis. Real-time monitoring identified distinct pyroptotic phases, including rapid decay of the parasitophorous vacuole (PV), progressive cell death and translocation of the luminal PV membrane to the cell surface in 40% of macrophages, resulting in the extracellular exposure of amastigotes that remained anchored to PV membranes. Electron microscopy analyses revealed an exclusive polarized orientation of parasites, with the anterior pole exposed toward the extracellular milieu, and the parasite posterior pole attached to the PV membrane. Exposed parasites retained their full infectivity towards naïve macrophages suggesting that host cell pyroptosis may contribute to parasite dissemination.

Inflammasome Activation in Response to Intracellular Protozoan Parasites

Inflammasomes are cytosolic complexes that assemble in response to cellular stress or upon sensing microbial molecules, culminating in cytokine processing and an inflammatory form of cell death called pyroptosis. Inflammasomes are usually composed of a sensor molecule, an adaptor protein, and an inflammatory caspase, such as Caspase-1, which cleaves and activates multiple substrates, including Gasdermin-D, pro-IL-1β, and pro-IL-18. Ultimately, inflammasome activation promotes inflammation and restriction of the microbial infection. In recent years, many studies have addressed the role of inflammasomes during fungal, bacterial, viral, and parasitic diseases, revealing sophisticated aspects of the host-pathogen interaction. In this review, we summarize recent advances on inflammasome activation in response to intracellular parasites, including Leishmania spp., Plasmodium spp., Trypanosoma cruzi, and Toxoplasma gondii.

Protective or Detrimental? Understanding the Role of Host Immunity in Leishmaniasis

The intracellular protozoan parasites of the genus Leishmania are the causative agents of leishmaniasis, a vector-borne disease of major public health concern, estimated to affect 12 million people worldwide. The clinical manifestations of leishmaniasis are highly variable and can range from self-healing localized cutaneous lesions to life-threatening disseminated visceral disease. Once introduced into the skin by infected sandflies, Leishmania parasites interact with a variety of immune cells, such as neutrophils, monocytes, dendritic cells (DCs), and macrophages. The resolution of infection requires a finely tuned interplay between innate and adaptive immune cells, culminating with the activation of microbicidal functions and parasite clearance within host cells. However, several factors derived from the host, insect vector, and Leishmania spp., including the presence of a double-stranded RNA virus (LRV), can modulate the host immunity and influence the disease outcome. In this review, we discuss the immune mechanisms underlying the main forms of leishmaniasis, some of the factors involved with the establishment of infection and disease severity, and potential approaches for vaccine and drug development focused on host immunity.