Translating Observations From Leishmanization Into Non-Living Vaccines: The Potential of Dendritic Cell-Based Vaccination Strategies Against Leishmania

The role of helminths and their antigens in cancer therapy: insights from cell line models

BACKGROUND

Recent articles have explored the effect of worms on cancer cells. This review focused on various cell cultures employed to understand which cells are more commonly and less utilized.

METHODS

The present review analyzed studies published between 2013 and 2023 to obtain information about different cell cultures used in cancer studies involving helminths. Databases such as PubMed, Google Scholar, HINARI, and the Cochrane Library were searched.

RESULTS

This search yielded 130 records, but 97 papers were excluded because they were either irrelevant to the research topic (n = 72) or contradicted the research idea (n = 25).The remaining twenty-one articles focused on different types of worms, such as Echinococcus granulosus, Clonorchis sinensis, Opisthorchis felineus, Opisthorchis viverrini, Trichinella spiralis, Toxocara canis, and Heligmosomoides polygyrus.

CONCLUSION

Due to the presence of numerous antigens, parasites at different growth stages can impact various cells through unknown mechanisms. Given the high diversity of antigens and their effects, artificial intelligence can assist in predicting initial outcomes for future studies.

Live attenuated-nonpathogenic Leishmania and DNA structures as promising vaccine platforms against leishmaniasis: innovations can make waves

Leishmaniasis is a vector-borne disease caused by the protozoan parasite of Leishmania genus and is a complex disease affecting mostly tropical regions of the world. Unfortunately, despite the extensive effort made, there is no vaccine available for human use. Undoubtedly, a comprehensive understanding of the host-vector-parasite interaction is substantial for developing an effective prophylactic vaccine. Recently the role of sandfly saliva on disease progression has been uncovered which can make a substantial contribution in vaccine design. In this review we try to focus on the strategies that most probably meet the prerequisites of vaccine development (based on the current understandings) including live attenuated/non-pathogenic and subunit DNA vaccines. Innovative approaches such as reverse genetics, CRISP/R-Cas9 and antibiotic-free selection are now available to promisingly compensate for intrinsic drawbacks associated with these platforms. Our main goal is to call more attention toward the prerequisites of effective vaccine development while controlling the disease outspread is a substantial need.

High seroprevalence of Leishmania infantum is linked to immune activation in people with HIV: a two-stage cross-sectional study in Bahia, Brazil

Visceral leishmaniasis is an opportunistic disease in HIV-1 infected individuals, unrecognized as a determining factor for AIDS diagnosis. The growing geographical overlap of HIV-1 and Leishmania infections is an emerging challenge worldwide, as co-infection increases morbidity and mortality for both infections. Here, we determined the prevalence of people living with HIV (PWH) with a previous or ongoing infection by Leishmania infantum and investigated the virological and immunological factors associated with co-infection. We adopted a two-stage cross-sectional cohort (CSC) design (CSC-I, n = 5,346 and CSC-II, n = 317) of treatment-naïve HIV-1-infected individuals in Bahia, Brazil. In CSC-I, samples collected between 1998 and 2013 were used for serological screening for leishmaniasis by an in-house Enzyme-Linked Immunosorbent Assay (ELISA) with SLA (Soluble Leishmania infantum Antigen), resulting in a prevalence of previous or ongoing infection of 16.27%. Next, 317 PWH were prospectively recruited from July 2014 to December 2015 with the collection of sociodemographic and clinical data. Serological validation by two different immunoassays confirmed a prevalence of 15.46 and 8.20% by anti-SLA, and anti-HSP70 serology, respectively, whereas 4.73% were double-positive (DP). Stratification of these 317 individuals in DP and double-negative (DN) revealed a significant reduction of CD4+ counts and CD4+/CD8+ ratios and a tendency of increased viral load in the DP group, as compared to DN. No statistical differences in HIV-1 subtype distribution were observed between the two groups. However, we found a significant increase of CXCL10 (p = 0.0076) and a tendency of increased CXCL9 (p = 0.061) in individuals with DP serology, demonstrating intensified immune activation in this group. These findings were corroborated at the transcriptome level in independent Leishmania- and HIV-1-infected cohorts (Swiss HIV Cohort and Piaui Northeast Brazil Cohort), indicating that CXCL10 transcripts are shared by the IFN-dominated immune activation gene signatures of both pathogens and positively correlated to viral load in untreated PWH. This study demonstrated a high prevalence of PWH with L. infantum seropositivity in Bahia, Brazil, linked to IFN-mediated immune activation and a significant decrease in CD4+ levels. Our results highlight the urgent need to increase awareness and define public health strategies for the management and prevention of HIV-1 and L. infantum co-infection.

Anti-leishmanial therapy: Caught between drugs and immune targets

Leishmaniasis is an enigmatic disease that has very restricted options for chemotherapy and none for prophylaxis. As a result, deriving therapeutic principles for curing the disease has been a major objective in Leishmania research for a long time. Leishmania is a protozoan parasite that lives within macrophages by subverting or switching cell signaling to the pathways that ensure its intracellular survival. Therefore, three groups of molecules aimed at blocking or eliminating the parasite, at least, in principle, include blockers of macrophage receptor- Leishmania ligand interaction, macrophage-activating small molecules, peptides and cytokines, and signaling inhibitors or activators. Macrophages also act as an antigen-presenting cell, presenting antigen to the antigen-specific T cells to induce activation and differentiation of the effector T cell subsets that either execute or suppress anti-leishmanial functions. Three groups of therapeutic principles targeting this sphere of Leishmania-macrophage interaction include antibodies that block pro-leishmanial response of T cells, ligands that activate anti-leishmanial T cells and the antigens for therapeutic vaccines. Besides these, prophylactic vaccines have been in clinical trials but none has succeeded so far. Herein, we have attempted to encompass all these principles and compose a comprehensive review to analyze the feasibility and adoptability of different therapeutics for leishmaniasis.

Centrin-deficient Leishmania mexicana confers protection against Old World visceral leishmaniasis

Leishmaniasis is one of the top neglected tropical diseases with significant morbidity and mortality in low and middle-income countries (LMIC). However, this disease is also spreading in the developed world. Currently, there is a lack of effective strategies to control this disease. Vaccination can be an effective measure to control leishmaniasis and has the potential to achieve disease elimination. Recently, we have generated centrin gene-deleted new world L. mexicana (LmexCen−/−) parasites using CRISPR/Cas9 and showed that they protect mice against a homologous L. mexicana infection that causes cutaneous disease. In this study, we tested whether LmexCen−/− parasites can also protect against visceral leishmaniasis caused by L. donovani in a hamster model. We showed that immunization with LmexCen−/− parasites is safe and does not cause lesions. Furthermore, such immunization conferred protection against visceral leishmaniasis caused by a needle-initiated L. donovani challenge, as indicated by a significant reduction in the parasite burdens in the spleen and liver as well as reduced mortality. Similar control of parasite burden was also observed against a sand fly mediated L. donovani challenge. Importantly, immunization with LmexCen−/− down-regulated the disease promoting cytokines IL-10 and IL-4 and increased pro-inflammatory cytokine IFN-γ resulting in higher IFN-γ/IL-10 and IFN-γ/IL4 ratios compared to non-immunized animals. LmexCen−/− immunization also resulted in long-lasting protection against L. donovani infection. Taken together, our study demonstrates that immunization with LmexCen−/− parasites is safe and efficacious against the Old World visceral leishmaniasis.

The History of Live Attenuated Centrin Gene-Deleted Leishmania Vaccine Candidates

Leishmaniasis, caused by an infection of the Leishmania protozoa, is a neglected tropical disease and a major health problem in tropical and subtropical regions of the world, with approximately 350 million people worldwide at risk and 2 million new cases occurring annually. Current treatments for leishmaniasis are not highly efficacious and are associated with high costs, especially in low- and middle-income endemic countries, and high toxicity. Due to a surge in the incidence of leishmaniases worldwide, the development of new strategies such as a prophylactic vaccine has become a high priority. However, the ability of Leishmania to undermine immune recognition has limited our efforts to design safe and efficacious vaccines against leishmaniasis. Numerous antileishmanial vaccine preparations based on DNA, subunit, and heat-killed parasites with or without adjuvants have been tried in several animal models but very few have progressed beyond the experimental stage. However, it is known that people who recover from Leishmania infection can be protected lifelong against future infection, suggesting that a successful vaccine requires a controlled infection to develop immunologic memory and subsequent long-term immunity. Live attenuated Leishmania parasites that are non-pathogenic and provide a complete range of antigens similarly to their wild-type counterparts could evoke such memory and, thus, would be effective vaccine candidates. Our laboratory has developed several live attenuated Leishmania vaccines by targeted centrin gene disruptions either by homologous recombination or, more recently, by using genome editing technologies involving CRISPR-Cas9. In this review, we focused on the sequential history of centrin gene-deleted Leishmania vaccine development, along with the characterization of its safety and efficacy. Further, we discussed other major considerations regarding the transition of dermotropic live attenuated centrin gene-deleted parasites from the laboratory to human clinical trials.

An alphavirus-derived self-amplifying mRNA encoding PpSP15-LmSTI1 fusion protein for the design of a vaccine against leishmaniasis

The main aims of the present study were to design a fusion protein of Leishmania major stress-inducible protein 1 (LmSTI1) and Phlebotomus papatasi SP15 (PpSP15), and to express it in the form of alphavirus packaged Self-amplifying mRNA (SAM). Two combinations, PpSP15-LmSTI1 and LmSTI1-PpSP15 fusion forms, were analyzed for folding and minimum free energies of the mRNA. Conformational studies on 3D modeled fusion and native forms were performed, and the Root-Mean-Square-distance (RMSD) of the Cα atomic coordinates were calculated. Antigenicity and stability were predicted using bioinformatics tools. The coding sequences of PpSP15-LmSTI1 fusion, PpSP15, and LmSTI1 were cloned into an alphavirus-based vector and used to produce the SAM constructs. All the subcloned constructs were then subjected to packaging in the form of viral replicon particles (VRPs),and were evaluated for their ability to infect BHK-21 cells and express the recombinant fusion proteins. The in-silico analysis indicated that the PpSP15-LmSTI1 combination could be a promising candidate based on lower folding ΔG of mRNA, higher protein antigenicity and lower instability indexes, and less conformational changes compared to the native proteins and the LmSTI1-PpSP15 fusion form. Packaged SAM encoding fusion and native antigens are used for infection of mammalian cells and for recombinant protein expression. This is the first study on in silico designing and successful packaging of an alphavirus-derived SAM in the form of the VRPs to target leishmaniasis.

Host–Pathogen Interaction in Leishmaniasis: Immune Response and Vaccination Strategies

Leishmaniasis is a zoonotic and vector-borne infectious disease that is caused by the genus Leishmania belonging to the trypanosomatid family. The protozoan parasite has a digenetic life cycle involving a mammalian host and an insect vector. Leishmaniasisis is a worldwide public health problem falling under the neglected tropical disease category, with over 90 endemic countries, and approximately 1 million new cases and 20,000 deaths annually. Leishmania infection can progress toward the development of species–specific pathologic disorders, ranging in severity from self-healing cutaneous lesions to disseminating muco-cutaneous and fatal visceral manifestations. The severity and the outcome of leishmaniasis is determined by the parasite’s antigenic epitope characteristics, the vector physiology, and most importantly, the immune response and immune status of the host. This review examines the nature of host–pathogen interaction in leishmaniasis, innate and adaptive immune responses, and various strategies that have been employed for vaccine development.

Interleukin-4 Responsive Dendritic Cells Are Dispensable to Host Resistance Against Leishmania mexicana Infection

IL-4 and IL-13 cytokines have been associated with a non-healing phenotype in murine leishmaniasis in L. mexicana -infected BALB/c mice as demonstrated in IL-4−/−, IL-13−/− and IL-4Rα-/- global knockout mouse studies. However, it is unclear from the studies which cell-type-specific IL-4/IL-13 signaling mediates protection to L. mexicana. Previous studies have ruled out a role for IL-4-mediated protection on CD4+ T cells during L. mexicana infections. A candidate for this role may be non-lymphocyte cells, particularly DCs, as was previously shown in L. major infections, where IL-4 production drives dendritic cell-IL-12 production thereby mediating a type 1 immune response. However, it is unclear if this IL-4-instruction of type 1 immunity also occurs in CL caused by L. mexicana, since the outcome of cutaneous leishmaniasis often depends on the infecting Leishmania species. Thus, BALB/c mice with cell-specific deletion of the IL-4Rα on CD11c+ DCs (CD11ccreIL-4Rα-/lox) were infected with L. mexicana promastigotes in the footpad and the clinical phenotype, humoral and cellular immune responses were investigated, compared to the littermate control. Our results show that CL disease progression in BALB/c mice is independent of IL-4Rα signaling on DCs as CD11ccreIL-4Rα-/lox mice had similar footpad lesion progression, parasite loads, humoral responses (IgE, IgG1, IgG 2a/b), and IFN-γ cytokine secretion in comparison to littermate controls. Despite this comparable phenotype, surprisingly, IL-4 production in CD11ccreIL-4Rα-/lox mice was significantly increased with an increasing trend of IL-13 when compared to littermate controls. Moreover, the absence of IL-4Rα signaling did not significantly alter the frequency of CD4 and CD8 lymphocytes nor their activation, or memory phenotype compared to littermate controls. However, these populations were significantly increased in CD11ccreIL-4Rα-/lox mice due to greater total cell infiltration into the lymph node. A similar trend was observed for B cells whereas the recruitment of myeloid populations (macrophages, DCs, neutrophils, and Mo-DCs) into LN was comparable to littermate IL-4Rα-/lox mice. Interestingly, IL-4Rα-deficient bone marrow-derived dendritic cells (BMDCs), stimulated with LPS or L. mexicana promastigotes in presence of IL-4, showed similar levels of IL-12p70 and IL-10 to littermate controls highlighting that IL-4-mediated DC instruction was not impaired in response to L. mexicana. Similarly, IL-4 stimulation did not affect the maturation or activation of IL-4Rα-deficient BMDCs during L. mexicana infection nor their effector functions in production of nitrite and arginine-derived metabolite (urea). Together, this study suggests that IL-4 Rα signaling on DCs is not key in the regulation of immune-mediated protection in mice against L. mexicana infection.

From infection to vaccination: reviewing the global burden, history of vaccine development, and recurring challenges in global leishmaniasis protection

INTRODUCTION

Leishmaniasis is a major public health problem and the second most lethal parasitic disease in the world due to the lack of effective treatments and vaccines. Even when not lethal, leishmaniasis significantly affects individuals and communities through life-long disabilities, psycho-sociological trauma, poverty, and gender disparity in treatment.

AREAS COVERED

This review discusses the most relevant and recent research available on Pubmed and GoogleScholar highlighting leishmaniasis' global impact, pathogenesis, treatment options, and lack of effective control strategies. An effective vaccine is necessary to prevent morbidity and mortality, lower health care costs, and reduce the economic burden of leishmaniasis for endemic low- and middle-income countries. Since there are several forms of leishmaniasis, a pan-Leishmania vaccine without geographical restrictions is needed. This review also focuses on recent advances and common challenges in developing prophylactic strategies against leishmaniasis.

EXPERT OPINION

Despite advances in pre-clinical vaccine research, approval of a human leishmaniasis vaccine still faces major challenges - including manufacturing of candidate vaccines under Good Manufacturing Practices, developing well-designed clinical trials suitable in endemic countries, and defined correlates of protection. In addition, there is a need to explore Challenge Human Infection Model to avoid large trials because of fluctuating incidence and prevalence of leishmanasis.

Preclinical validation of a live attenuated dermotropic Leishmania vaccine against vector transmitted fatal visceral leishmaniasis

Visceral Leishmaniasis (VL), a potentially fatal disease is caused by Leishmania donovani parasites with no vaccine available. Here we produced a dermotropic live attenuated centrin gene deleted Leishmania major (LmCen^−/−) vaccine under Good Laboratory Practices and demonstrated that a single intradermal injection confers robust and durable protection against lethal VL transmitted naturally via bites of L. donovani-infected sand flies and prevents mortality. Surprisingly, immunogenicity characteristics of LmCen^−/− parasites revealed activation of common immune pathways like L. major wild type parasites. Spleen cells from LmCen^−/− immunized and L. donovani challenged hamsters produced significantly higher Th1-associated cytokines including IFN-γ, TNF-α, and reduced expression of the anti-inflammatory cytokines like IL-10, IL-21, compared to non-immunized challenged animals. PBMCs, isolated from healthy people from non-endemic region, upon LmCen^−/− infection also induced more IFN-γ compared to IL-10, consistent with our immunogenicity data in LmCen^−/− immunized hamsters. This study demonstrates that the LmCen^−/− parasites are safe and efficacious against VL and is a strong candidate vaccine to be tested in a human clinical trial.

Karmakar et al produced a dermotropic, live attenuated centrin gene-deleted Leishmania major (LmCen^−/−) vaccine against Visceral Leishmaniasis (VL). They demonstrated in hamsters that a single intradermal injection confers robust and durable protection against lethal VL that is transmitted naturally via bites of L. donovani-infected sand flies.

Revival of Leishmanization and Leishmanin

Leishmaniasis includes a spectrum of diseases ranging from debilitating cutaneous to fatal visceral infections. This disease is caused by the parasitic protozoa of the genus Leishmania that is transmitted by infected sandflies. Over 1 billion people are at risk of leishmaniasis with an annual incidence of over 2 million cases throughout tropical and subtropical regions in close to 100 countries. Leishmaniasis is the only human parasitic disease where vaccination has been successful through a procedure known as leishmanization that has been widely used for decades in the Middle East. Leishmanization involved intradermal inoculation of live Leishmania major parasites resulting in a skin lesion that following natural healing provided protective immunity to re-infection. Leishmanization is however no longer practiced due to safety and ethical concerns that the lesions at the site of inoculation that can last for months in some people. New genome editing technologies involving CRISPR has now made it possible to engineer safer attenuated strains of Leishmania, which induce protective immunity making way for a second generation leishmanization that can enter into human trials. A major consideration will be how the test the efficacy of a vaccine in the midst of the visceral leishmaniasis elimination program. One solution will be to use the leishmanin skin test (LST) that was also used for decades to determine exposure and immunity to Leishmania. The LST involves injection of antigen from Leishmania in the skin dermis resulting in a delayed type hypersensitivity (DTH) immune reaction associated with a Th1 immune response and protection against visceral leishmaniasis. Reintroduction of novel approaches for leishmanization and the leishmanin skin test can play a major role in eliminating leishmaniasis.

Evaluation of protection induced by in vitro maturated BMDCs presenting CD8+ T cell stimulating peptides after a heterologous vaccination regimen in BALB/c model against Leishmania major

Leishmaniasis is a complex vector-borne disease mediated by Leishmania parasite and a strong and long-lasting CD4⁺ Th1 and CD8⁺-T cell immunity is required to control the infection. Thus far multivalent subunit vaccines have met this requirement more promisingly. However several full protein sequences cannot be easily arranged in one construct. Instead, new emerging immune-informatics based epitope formulations surpass this restriction. Herein, we aimed to examine the protective potential of a dendritic cell based vaccine presenting epitopes to CD8⁺ and CD4⁺-T cells in combination with DNA vaccine encoding the same epitopes against murine cutaneous leishmaniasis. Immature DCs were loaded with epitopes (selected from parasite proteome) in vitro with or without CpG oligonucleotides and were used to immunize BALB/c mice. Peptide coding DNA was used to boost the system and immunological responses were evaluated after Leishmania (L.) major infectious challenge. The pre-challenge response to included epitopes was Th1 polarized which potentially lowered the infection at early time points post-challenge but not at later weeks. Collectively, DC prime-DNA boost was found to be a promising approach for Th1 polarization however the constituent epitopes undoubtedly make a significant contribution in the protection outcome of the vaccine.

The In Silico Identification of Potential Members of the Ded1/DDX3 Subfamily of DEAD-Box RNA Helicases from the Protozoan Parasite Leishmania infantum and Their Analyses in Yeast

DEAD-box RNA helicases are ubiquitous proteins found in all kingdoms of life and that are associated with all processes involving RNA. Their central roles in biology make these proteins potential targets for therapeutic or prophylactic drugs. The Ded1/DDX3 subfamily of DEAD-box proteins is of particular interest because of their important role(s) in translation. In this paper, we identified and aligned the protein sequences of 28 different DEAD-box proteins from the kinetoplast-protozoan parasite Leishmania infantum, which is the cause of the visceral form of leishmaniasis that is often lethal if left untreated, and compared them with the consensus sequence derived from DEAD-box proteins in general, and from the Ded1/DDX3 subfamily in particular, from a wide variety of other organisms. We identified three potential homologs of the Ded1/DDX3 subfamily and the equivalent proteins from the related protozoan parasite Trypanosoma brucei, which is the causative agent of sleeping sickness. We subsequently tested these proteins for their ability to complement a yeast strain deleted for the essential DED1 gene. We found that the DEAD-box proteins from Trypanosomatids are highly divergent from other eukaryotes, and consequently they are suitable targets for protein-specific drugs.

Th1 concomitant immune response mediated by IFN-γ protects against sand fly delivered Leishmania infection: Implications for vaccine design

Leishmaniasis is an unresolved global health problem with a high socio-economic impact. Data generated in mouse models has revealed that the Th1 response, with IL-12, IFN-γ, TNF-α, and IL-2 as prominent cytokines, predominantly controls the disease progression. Premised on these findings, all examined vaccine formulations have been aimed at generating a long-lived memory Th1 response. However, all vaccine formulations with the exception of live Leishmania inoculation (leishmanization) have failed to sufficiently protect against sand fly delivered infection. It has been recently unraveled that sand fly dependent factors may compromise pre-existing Th1 memory. Further scrutinizing the immune response after leishmanization has uncovered the prominent role of early (within hours) and robust IFN-γ production (Th1 concomitant immunity) in controlling the sand fly delivered secondary infection. The response is dependent upon parasite persistence and subclinical ongoing primary infection. The immune correlates of concomitant immunity (Resident Memory T cells and Effector T subsets) mitigate the early effects of sand fly delivered infection and help to control the disease. In this review, we have described the early events after sand fly challenge and the role of Th1 concomitant immunity in the protective immune response in leishmanized resistant mouse model, although leishmanization is under debate for human use. Undoubtedly, the lessons we learn from leishmanization must be further implemented in alternative vaccine approaches.

Isolation and Functions of Extracellular Vesicles Derived from Parasites: The Promise of a New Era in Immunotherapy, Vaccination, and Diagnosis

Experimental and epidemiological evidence shows that parasites, particularly helminths, play a central role in balancing the host immunity. It was demonstrated that parasites can modulate immune responses via their excretory/secretory (ES) and some specific proteins. Extracellular vesicles (EVs) are nano-scale particles that are released from eukaryotic and prokaryotic cells. EVs in parasitological studies have been mostly employed for immunotherapy of autoimmune diseases, vaccination, and diagnosis. EVs can carry virulence factors and play a central role in the development of parasites in host cells. These molecules can manipulate the immune responses through transcriptional changes. Moreover, EVs derived from helminths modulate the immune system via provoking anti-inflammatory cytokines. On the other hand, EVs from parasite protozoa can induce efficient immunity, that makes them useful for probable next-generation vaccines. In addition, it seems that EVs from parasites may provide new diagnostic approaches for parasitic infections. In the current study, we reviewed isolation methods, functions, and applications of parasite's EVs in immunotherapy, vaccination, and diagnosis.

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.

Vaccines for leishmaniasis and the implications of their development for American tegumentary leishmaniasis

The leishmaniases are a collection of vector-borne parasitic diseases caused by a number of different Leishmania species that are distributed worldwide. Clinical and laboratory research have together revealed several important immune components that control Leishmania infection and indicate the potential of immunization to prevent leishmaniasis. In this review we introduce previous and ongoing experimental research efforts to develop vaccines against Leishmania species. First, second and third generation vaccine strategies that have been proposed to counter cutaneous and visceral leishmaniasis (CL and VL, respectively) are summarized. One of the major bottlenecks in development is the transition from results in animal model studies to humans, and we highlight that although American tegumentary leishmaniasis (ATL; New World CL) can progress to destructive and disfiguring mucosal lesions, most research has been conducted using mouse models and Old World Leishmania species. We conclude that assessment of vaccine candidates in ATL settings therefore appears merited.

Pathogen Evasion of Chemokine Response Through Suppression of CXCL10

Clearance of intracellular pathogens, such as Leishmania (L.) major, depends on an immune response with well-regulated cytokine signaling. Here we describe a pathogen-mediated mechanism of evading CXCL10, a chemokine with diverse antimicrobial functions, including T cell recruitment. Infection with L. major in a human monocyte cell line induced robust CXCL10 transcription without increasing extracellular CXCL10 protein concentrations. We found that this transcriptionally independent suppression of CXCL10 is mediated by the virulence factor and protease, glycoprotein-63 (gp63). Specifically, GP63 cleaves CXCL10 after amino acid A81 at the base of a C-terminal alpha-helix. Cytokine cleavage by GP63 demonstrated specificity, as GP63 cleaved CXCL10 and its homologs, which all bind the CXCR3 receptor, but not distantly related chemokines, such as CXCL8 and CCL22. Further characterization demonstrated that CXCL10 cleavage activity by GP63 was produced by both extracellular promastigotes and intracellular amastigotes. Crucially, CXCL10 cleavage impaired T cell chemotaxis in vitro, indicating that cleaved CXCL10 cannot signal through CXCR3. Ultimately, we propose CXCL10 suppression is a convergent mechanism of immune evasion, as Salmonella enterica and Chlamydia trachomatis also suppress CXCL10. This commonality suggests that counteracting CXCL10 suppression may provide a generalizable therapeutic strategy against intracellular pathogens.

Importance

Leishmaniasis, an infectious disease that annually affects over one million people, is caused by intracellular parasites that have evolved to evade the host's attempts to eliminate the parasite. Cutaneous leishmaniasis results in disfiguring skin lesions if the host immune system does not appropriately respond to infection. A family of molecules called chemokines coordinate recruitment of the immune cells required to eliminate infection. Here, we demonstrate a novel mechanism that Leishmania (L.) spp. employ to suppress host chemokines: a Leishmania-encoded protease cleaves chemokines known to recruit T cells that fight off infection. We observe that other common human intracellular pathogens, including Chlamydia trachomatis and Salmonella enterica, reduce levels of the same chemokines, suggesting a strong selective pressure to avoid this component of the immune response. Our study provides new insights into how intracellular pathogens interact with the host immune response to enhance pathogen survival.

Infection of Human Neutrophils With Leishmania infantum or Leishmania major Strains Triggers Activation and Differential Cytokines Release

Leishmaniases are neglected diseases, caused by intracellular protozoan parasites of the Leishmania (L.) genus. Although the principal host cells of the parasites are macrophages, neutrophils are the first cells rapidly recruited to the site of parasites inoculation, where they play an important role in the early recognition and elimination of the parasites. The nature of early interactions between neutrophils and Leishmania could influence the outcome of infection. Herein we aimed to evaluate whether different Leishmania strains, responsible for distinct clinical manifestations, could influence ex vivo functional activity of neutrophils. Human polymorphonuclear leukocytes were isolated from 14 healthy volunteers and the ex vivo infection of these cells was done with two L. infantum and one L. major strains. Infection parameters were determined and neutrophils activation was assessed by oxidative burst, degranulation, DNA release and apoptosis; cytokine production was measured by a multiplex flow cytometry analysis. Intracellular amastigotes were rescued to determine Leishmania strains survival. The results showed that L. infantum and L. major promastigotes similarly infected the neutrophils. Oxidative burst, neutrophil elastase, myeloperoxidase activity and apoptosis were significantly increased in infected neutrophils but with no differences between strains. The L. infantum-infected neutrophils induced more DNA release than those infected by L. major. Furthermore, Leishmania strains induced high amounts of IL-8 and stimulated the production of IL-1β, TNF-α, and TGF-β by human neutrophils. We observed that only one strain promoted IL-6 release by these neutrophils. The production of TNF-α was also differently induced by the parasites strains. All these results demonstrate that L. infantum and L. major strains were able to induce globally a similar ex vivo activation and apoptosis of neutrophils; however, they differentially triggered cytokines release from these cells. In addition, rescue of intracellular parasites indicated different survival rates further emphasizing on the influence of parasite strains within a species on the fate of infection.

Leishmania infantum LeIF and its recombinant polypeptides induce the maturation of dendritic cells in vitro: An insight for dendritic cells based vaccine

We previously showed that recombinant Leishmania infantum eukaryotic initiation factor (LieIF) was able to induce the secretion of cytokines IL-12, IL-10 and TNF-α by human monocytes. In this study, we explored in vitro the potential of LieIF to induce phenotypic maturation and functional differentiation of murine bone-marrow derived dendritic cells (BM-DCs). Moreover, in order to identify potential immunnomodulatory regions of LieIF, eight recombinant overlapping protein fragments covering the whole amino acid sequence of protein, were constructed and assessed in vitro for their ability to induce maturation of BM-DCs. Our data showed that LieIF and some of its recombinant polypeptides were able to induce elevated expression of CD40, CD80 and CD86 co-stimulatory molecules with concurrent IL-12 production. Moreover, we used an in vivo experimental model of cutaneous leishmaniasis consisted of susceptible Leishmania major-infected BALB/c mice and we demonstrated that LieIF-pulsed-BM-DCs adoptively transferred in mice were capable to confer protection against a high dose parasite challenge. This study further describes the immunomodulatory properties of LieIF and its polypeptides bringing relevant information for their exploitation as candidate molecules for vaccine development against leishmaniasis.