Uptake of Leishmania major amastigotes results in activation and interleukin 12 release from murine skin-derived dendritic cells: implications for the initiation of anti-Leishmania immunity

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

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

Exploiting Leishmania-Primed Dendritic Cells as Potential Immunomodulators of Canine Immune Response

Dendritic cells (DCs) capture pathogens and process antigens, playing a crucial role in activating naïve T cells, bridging the gap between innate and acquired immunity. However, little is known about DC activation when facing Leishmania parasites. Thus, this study investigates in vitro activity of canine peripheral blood-derived DCs (moDCs) exposed to L. infantum and L. amazonensis parasites and their extracellular vesicles (EVs). L. infantum increased toll-like receptor 4 gene expression in synergy with nuclear factor κB activation and the generation of pro-inflammatory cytokines. This parasite also induced the expression of class II molecules of major histocompatibility complex (MHC) and upregulated co-stimulatory molecule CD86, which, together with the release of chemokine CXCL16, can attract and help in T lymphocyte activation. In contrast, L. amazonensis induced moDCs to generate a mix of pro- and anti-inflammatory cytokines, indicating that this parasite can establish a different immune relationship with DCs. EVs promoted moDCs to express class I MHC associated with the upregulation of co-stimulatory molecules and the release of CXCL16, suggesting that EVs can modulate moDCs to attract cytotoxic CD8+ T cells. Thus, these parasites and their EVs can shape DC activation. A detailed understanding of DC activation may open new avenues for the development of advanced leishmaniasis control strategies.

Leishmania antigens activated CD4+ T cells expressing CD200R receptors are the prime IL-10 producing phenotype and an important determinant of visceral leishmaniasis pathogenesis

The excessive production of IL-10, an anti-inflammatory cytokine, by Leishmania antigen-activated T cells is supposed to be a key player in the onset and progression of visceral leishmaniasis (VL). The IL-10-producing sources in VL remain unidentified and uncharacterized. In this study, we reveal that antigen-activated CD4+ T cells, i.e., CD44+CD4+ T cells expressing CD200R receptors, are the prime IL-10-producing phenotypes in Leishmania donovani infection-induced pathogenesis. These phenotypes are separate from CD25+Foxp3+CD4+ T regulatory cells, which are classical IL-10-producing phenotypes. In order to ascertain the role of CD200R and CD25 receptors in IL-10 overexpression-associated VL pathogenesis, we abrogated CD200R and CD25 receptor-mediated signaling in the infected mice. The splenic load of parasites and the size of the liver and spleen were significantly reduced in CD200-blocked mice as compared to CD25-blocked mice. Further, the CD200 blocking polarized CD4+ T cells to pro-inflammatory cytokines-producing phenotypes, as we observed a higher frequency of IFN-γ, TNF-α, and IL-12 positive cells as compared to controls including the CD25 blocking. Our findings suggest that in L. donovani infection-induced pathogenesis the expression of CD200R on antigen-activated T cells helps them to acquire IL-10-producing abilities as part of its one of the survival strategies. However, more studies would be warranted to better understand CD200R receptors role in VL pathogenesis and to develop the next generation of therapeutic and prophylactic control measures.

Chemokine receptors on human regulatory T cells during cutaneous leishmaniasis

The aim of this work was to define the population of regulatory T cells (Tregs) which are circulating in the blood of Leishmania infected individuals clinically displaying a lesion (active disease-AD) and sub-clinical (SC) ones. We have individually collected blood samples, processed the PBMC and stained with fluorochrome-conjugated antibodies against CD3, CD4, Foxp3, CD25, CTLA-4, Ki-67, CCR4, CCR5, and CCR7. Cells were analyzed by flow cytometry. Our results suggest that CD25 and CTLA-4 are upregulated in Tregs of AD patients when compared to SC and uninfected (UN) controls. Moreover, Tregs proliferate upon infection based on Ki-67 nuclear antigen staining. Finally, we have observed that these Tregs of SC and AD patients upregulate CCR4, but not CCR5 and CCR7. There is an increase in the number of circulating Tregs in the blood of Leishmania infected individuals. These cells are potentially more suppressive based on the increased upregulation of CD25 and CTLA-4 during clinical infection (AD) when compared to SC infection. Tregs of both SC and AD cohorts are proliferating and express CCR4, which potentially guide them to the skin, but do not upregulate CCR5 and CCR7.

Role of antigen-presenting cells in non-ulcerated skin lesions caused by Leishmania (Leishmania) infantum chagasi

In Central America, infection by Leishmania (Leishmania) infantum chagasi causes visceral leishmaniasis and non-ulcerated cutaneous leishmaniasis (NUCL). This work aimed to evaluate the participation of subpopulations of antigen-presenting cells in skin lesions of patients affected by NUCL through double-staining immunohistochemistry using cellular and intracellular markers. Twenty-three skin biopsies from patients affected by NUCL were used. Histological sections stained by HE were used for histopathological study. Immunohistochemical studies were performed using primary antibodies against Langerhans cells, dermal dendritic cells, T lymphocytes, and the cytokines IL-12, IFN-γ, TNF-α, iNOS, and IL-10. The histopathological lesions were characterized by an inflammatory infiltrate, predominantly lymphohistiocytic, of variable intensity, with a diffuse arrangement associated with epithelioid granulomas and discreet parasitism. Double-staining immunohistochemistry showed higher participation of dendritic cells producing the proinflammatory cytokine IL-12 in relation to the other evaluated cytokines. Activation of the cellular immune response was marked by a higher density of CD8 Tc1-lymphocytes followed by CD4 Th1-lymphocytes producing mainly IFN-γ. The data obtained in the present study suggest that antigen-presenting cells play an important role in the in situ immune response through the production of proinflammatory cytokines, directing the cellular immune response preferentially to the Th1 and Tc1 types in NUCL caused by L. (L.) infantum chagasi.

Unrevealing the Mystery of Latent Leishmaniasis: What Cells Can Host Leishmania?

Leishmania spp. (Kinetoplastida) are unicellular parasites causing leishmaniases, neglected tropical diseases of medical and veterinary importance. In the vertebrate host, Leishmania parasites multiply intracellularly in professional phagocytes, such as monocytes and macrophages. However, their close relative with intracellular development-Trypanosoma cruzi-can unlock even non-professional phagocytes. Since Leishmania and T. cruzi have similar organelle equipment, is it possible that Leishmania can invade and even proliferate in cells other than the professional phagocytes? Additionally, could these cells play a role in the long-term persistence of Leishmania in the host, even in cured individuals? In this review, we provide (i) an overview of non-canonical Leishmania host cells and (ii) an insight into the strategies that Leishmania may use to enter them. Many studies point to fibroblasts as already established host cells that are important in latent leishmaniasis and disease epidemiology, as they support Leishmania transformation into amastigotes and even their multiplication. To invade them, Leishmania causes damage to their plasma membrane and exploits the subsequent repair mechanism via lysosome-triggered endocytosis. Unrevealing the interactions between Leishmania and its non-canonical host cells may shed light on the persistence of these parasites in vertebrate hosts, a way to control latent leishmaniasis.

DC-SIGN receptor is expressed by cells from cutaneous leishmaniasis lesions and differentially binds to Leishmania (Viannia) braziliensis and L. (Leishmania) amazonensis promastigotes

BACKGROUND

Dendritic cells (DCs) specific intercellular adhesion molecule (ICAM)-3-grabbing non integrin receptor (DC-SIGN) binds to subgenera Leishmania promastigotes mediating its interaction with DC and neutrophils, potentially influencing the infection outcome.

OBJECTIVES

In this work, we investigated whether DC-SIGN receptor is expressed in cells from cutaneous leishmaniasis (CL) lesions as well as the in vitro binding pattern of Leishmania (Viannia) braziliensis (Lb) and L. (L.) amazonensis (La) promastigotes.

METHODS

DC-SIGN receptor was labeled by immunohistochemistry in cryopreserved CL tissue fragments. In vitro binding assay with CFSE-labeled Lb or La promastigotes and RAJI-transfecting cells expressing DC-SIGN (DC-SIGN^POS) or mock-transfected (DC-SIGN^NEG) were monitored by flow cytometry at 2 h, 24 h and 48 h in co-culture.

RESULTS

In CL lesion infiltrate, DC-SIGN^POS cells were present in the dermis and near the epidermis. Both Lb and La bind to DC-SIGN^POS cells, while binding to DC-SIGN^NEG was low. La showed precocious and higher affinity to DC-SIGN^hi population than to DC-SIGN^low, while Lb binding was similar in these populations.

CONCLUSION

Our results demonstrate that DC-SIGN receptor is present in L. braziliensis CL lesions and interact with Lb promastigotes. Moreover, the differences in the binding pattern to Lb and La suggest DC-SIGN can influence in a difference way the intake of the parasites at the first hours after Leishmania infection. These results raise the hypothesis that DC-SIGN receptor could participate in the immunopathogenesis of American tegumentary leishmaniasis accounting for the differences in the outcome of the Leishmania spp. infection.

Leishmania intercepts IFN-γR signaling at multiple levels in macrophages

IFN-γ, a type 2 interferon and a cytokine, is critical for both innate and adaptive immunity. IFN-γ binds to the IFN-γRs on the cell membrane of macrophages, signals through JAK1-STAT-1 pathway and induces IFN-γ-stimulated genes (ISGs). As Leishmania amastigotes reside and replicate within macrophages, IFN-γ mediated macrophage activation eventuate in Leishmania elimination. As befits the principle of parasitism, the impaired IFN-γ responsiveness in macrophages ensures Leishmania survival. IFN-γ responsiveness is a function of integrated molecular events at multiple levels in the cells that express IFN-γ receptors. In Leishmania-infected macrophages, reduced IFN-γRα expression, impaired IFN-γRα and IFN-γRβ hetero-dimerization due to altered membrane lipid composition, reduced JAK-1 and STAT-1 phosphorylation but increased STAT-1 degradation and impaired ISGs induction collectively determine the IFN-γ responsiveness and the efficacy of IFN-γ induced antileishmanial function of macrophages. Therefore, parasite load is not only decided by the levels of IFN-γ produced but also by the IFN-γ responsiveness. Indeed, in Leishmania-infected patients, IFN-γ is produced but IFN-γ signalling is downregulated. However, the molecular mechanisms of IFN-γ responsiveness remain unclear. Therefore, we review the current understanding of IFN-γ responsiveness of Leishmania-infected macrophages.

Regulatory role of Transcription factor-EB (TFEB) in parasite control through alteration of antigen presentation in visceral leishmaniasis

Leishmania donovani, an obligate intracellular parasite, the causative agent of visceral leishmaniasis is known to subvert the host immune system for its own survival. Although the precise mechanism is still unknown, emerging evidences indicate that L. donovani efficiently suppress MHC I mediated antigen presentation, rendering inadequate CD8⁺T cell activation and weakening host defense against parasite. The role of transcription factor EB (TFEB) was recognized in modulating antigen presentation besides its role in lysosomal biogenesis and function. Here, we investigated the regulatory role of TFEB in the modulation of presentation of Leishmania antigen in host tissue. Our results showed an increased expression of TFEB after Leishmania infection both in vitro and in vivo and there was a decrease in the expression of Th-1 cytokine IFNγ along with MHC class I and CD8⁺T cells indicating attenuation of cell mediated immunity and possibly MHC I restricted antigen presentation. Silencing of TFEB resulted in increased expression of IFNγ and MHC I along with increased CD8⁺T cells population without any significant change in CD4⁺T cell number. We also observed a decreased parasite burden in TFEB silenced condition which indicates enhanced parasite clearance by alteration of immunological response possibly through induction of presentation of Leishmania antigen through MHC I. The present study explains the role of TFEB silencing in parasite clearance through regulating the antigen presentation of Leishmania antigen thereby promises to formulate a potential therapeutic strategy against visceral leishmaniasis.

Immune Responses in Leishmaniases: An Overview

Leishmaniasis is a parasitic, widespread, and neglected disease that affects more than 90 countries in the world. More than 20 Leishmania species cause different forms of leishmaniasis that range in severity from cutaneous lesions to systemic infection. The diversity of leishmaniasis forms is due to the species of parasite, vector, environmental and social factors, genetic background, nutritional status, as well as immunocompetence of the host. Here, we discuss the role of the immune system, its molecules, and responses in the establishment, development, and outcome of Leishmaniasis, focusing on innate immune cells and Leishmania major interactions.

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.

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.

The Paradox of a Phagosomal Lifestyle: How Innate Host Cell-Leishmania amazonensis Interactions Lead to a Progressive Chronic Disease

Intracellular phagosomal pathogens represent a formidable challenge for innate immune cells, as, paradoxically, these phagocytic cells can act as both host cells that support pathogen replication and, when properly activated, are the critical cells that mediate pathogen elimination. Infection by parasites of the Leishmania genus provides an excellent model organism to investigate this complex host-pathogen interaction. In this review we focus on the dynamics of Leishmania amazonensis infection and the host innate immune response, including the impact of the adaptive immune response on phagocytic host cell recruitment and activation. L. amazonensis infection represents an important public health problem in South America where, distinct from other Leishmania parasites, it has been associated with all three clinical forms of leishmaniasis in humans: cutaneous, muco-cutaneous and visceral. Experimental observations demonstrate that most experimental mouse strains are susceptible to L. amazonensis infection, including the C57BL/6 mouse, which is resistant to other species such as Leishmania major, Leishmania braziliensis and Leishmania infantum. In general, the CD4⁺ T helper (Th)1/Th2 paradigm does not sufficiently explain the progressive chronic disease established by L. amazonensis, as strong cell-mediated Th1 immunity, or a lack of Th2 immunity, does not provide protection as would be predicted. Recent findings in which the balance between Th1/Th2 immunity was found to influence permissive host cell availability via recruitment of inflammatory monocytes has also added to the complexity of the Th1/Th2 paradigm. In this review we discuss the roles played by innate cells starting from parasite recognition through to priming of the adaptive immune response. We highlight the relative importance of neutrophils, monocytes, dendritic cells and resident macrophages for the establishment and progressive nature of disease following L. amazonensis infection.

Melittin as a promising anti-protozoan peptide: current knowledge and future prospects

Protozoan diseases such as malaria, leishmaniasis, Chagas disease, and sleeping sickness still levy a heavy toll on human lives. Deplorably, only few classes of anti-protozoan drugs have thus far been developed. The problem is further compounded by their intrinsic toxicity, emergence of drug resistance, and the lack of licensed vaccines. Thus, there is a genuine exigency to develop novel anti-protozoan medications. Over the past years, melittin, the major constituent in the venom of European honeybee Apis mellifera, has gathered the attention of researchers due to its potential therapeutic applications. Insofar as we are aware, there has been no review pertinent to anti-protozoan properties of melittin. The present review outlines the current knowledge about anti-protozoan effects of melittin and its underlying mechanisms. The peptide has proven to be efficacious in killing different protozoan parasites such as Leishmania, Plasmodium, Toxoplasma, and Trypanosoma in vitro. Apart from direct membrane-disruptive activity, melittin is capable of destabilizing calcium homeostasis, reducing mitochondrial membrane potential, disorganizing kinetoplast DNA, instigating apoptotic cell death, and induction of autophagy in protozoan pathogens. Emerging evidence suggests that melittin is a promising candidate for future vaccine adjuvants. Transmission-blocking activity of melittin against vector-borne pathogens underscores its potential utility for both transgenic and paratransgenic manipulations. Nevertheless, future research should focus upon investigating anti-microbial activities of melittin, alone or in combination with the current anti-protozoan medications, against a far broader spectrum of protozoan parasites as well as pre-clinical testing of the peptide in animal models.

Potential biomarkers of immune protection in human leishmaniasis

Leishmaniasis is a vector-borne neglected tropical disease endemic in over 100 countries around the world. Available control measures are not always successful, therapeutic options are limited, and there is no vaccine available against human leishmaniasis, although several candidate antigens have been evaluated over the last decades. Plenty of studies have aimed to evaluate the immune response development and a diverse range of host immune factors have been described to be associated with protection or disease progression in leishmaniasis; however, to date, no comprehensive biomarker(s) have been identified as surrogate marker of protection or exacerbation, and lack of enough information remains a barrier for vaccine development. Most of the current understanding of the role of different markers of immune response in leishmaniasis has been collected from experimental animal models. Although the data generated from the animal models are crucial, it might not always be extrapolated to humans. Here, we briefly review the events during Leishmania invasion of host cells and the immune responses induced against Leishmania in animal models and humans and their potential role as a biomarker of protection against human leishmaniasis.

Unraveling the Role of Immune Checkpoints in Leishmaniasis

Leishmaniasis are Neglected Tropical Diseases affecting millions of people every year in at least 98 countries and is one of the major unsolved world health issues. Leishmania is a parasitic protozoa which are transmitted by infected sandflies and in the host they mainly infect macrophages. Immunity elicited against those parasites is complex and immune checkpoints play a key role regulating its function. T cell receptors and their respective ligands, such as PD-1, CTLA-4, CD200, CD40, OX40, HVEM, LIGHT, 2B4 and TIM-3 have been characterized for their role in regulating adaptive immunity against different pathogens. However, the exact role those receptors perform during Leishmania infections remains to be better determined. This article addresses the key role immune checkpoints play during Leishmania infections, the limiting factors and translational implications.

Cytokine saga in visceral leishmaniasis

In humans, infection with Leishmania manifests into a spectrum of diseases. The manifestation of the diseases depend on the resultant evasion of the parasite to immune responses namely by macrophages, which is an exclusive host of Leishmania. The B cells valiantly mount antibody responses, however, to no avail as the Leishmania parasites occupy the intracellular niches of the macrophages and subvert the immune response. Extensive studies have been documented on the role of cell-mediated immunity (CMI) in protection and counter survival strategies of the parasites leading to downregulation of CMI. The present review attempts to discuss the cytokines in progression or resolution of visceral form of leishmaniasis or kala-azar, predominantly affecting the Indian subcontinent. The components/cytokine(s) responsible for the regulation of the critical balance of T helper cells and their subsets have been discussed in the perspective. Therefore, any strategy involving the treatment of visceral leishmania (VL) needs to consider the balance and regulation of T cell function.

Programmed Cell Death Ligand (PD-L)-1 Contributes to the Regulation of CD4+ T Effector and Regulatory T Cells in Cutaneous Leishmaniasis

Cutaneous Leishmaniasis (CL) affects up to one million people every year and treatments are costly and toxic. The regulation of the host immune response is complex and the knowledge of how CD4⁺ T cells are activated and maintained during Leishmania infection is still limited. Current therapies aim to target programmed cell death (PD)-1 and programmed cell death ligand (PD-L)-1 in order to boost T cell activity. However, the role of the PD-1/PD-L1 axis during Leishmania infection is still unclear. In this study, we found that patients with active and post-treatment CL displayed different subsets of CD4⁺PD-1⁺ T cells. Accordingly, L. major-infected mice upregulated PD-1 on activated CD4⁺ T effector cells and PD-L1 on resident macrophages and infiltrating monocytes at the site of infection. L. major-infected Pdl1^−/− mice expressed lower levels of MHCII and higher levels of CD206 on macrophages and monocytes and, more importantly, the lack of PD-L1 contributed to a reduced frequency of CD4⁺Ly6C^hi T effector cells and an increase of CD4⁺Foxp3⁺ regulatory T cells at the site of infection and in draining lymph nodes. Additionally, the lack of PD-L1 was associated with lower production of IL-27 by infiltrating monocytes and lower levels of the Th1 cytokines IFN-γ and TNF-α produced by CD4⁺ T effector cells. Pdl1^−/− mice initially exhibited larger lesions despite having a similar parasite load. Our results describe for the first time how the interruption of the PD-1/PD-L1 axis influences the immune response against CL and suggests that this axis regulates the balance between CD4⁺Ly6C^hi T effector cells and CD4⁺Foxp3⁺ regulatory T cells.

To the Skin and Beyond: The Immune Response to African Trypanosomes as They Enter and Exit the Vertebrate Host

African trypanosomes are single-celled extracellular protozoan parasites transmitted by tsetse fly vectors across sub-Saharan Africa, causing serious disease in both humans and animals. Mammalian infections begin when the tsetse fly penetrates the skin in order to take a blood meal, depositing trypanosomes into the dermal layer. Similarly, onward transmission occurs when differentiated and insect pre-adapted forms are ingested by the fly during a blood meal. Between these transmission steps, trypanosomes access the systemic circulation of the vertebrate host via the skin-draining lymph nodes, disseminating into multiple tissues and organs, and establishing chronic, and long-lasting infections. However, most studies of the immunobiology of African trypanosomes have been conducted under experimental conditions that bypass the skin as a route for systemic dissemination (typically via intraperitoneal or intravenous routes). Therefore, the importance of these initial interactions between trypanosomes and the skin at the site of initial infection, and the implications for these processes in infection establishment, have largely been overlooked. Recent studies have also demonstrated active and complex interactions between the mammalian host and trypanosomes in the skin during initial infection and revealed the skin as an overlooked anatomical reservoir for transmission. This highlights the importance of this organ when investigating the biology of trypanosome infections and the associated immune responses at the initial site of infection. Here, we review the mechanisms involved in establishing African trypanosome infections and potential of the skin as a reservoir, the role of innate immune cells in the skin during initial infection, and the subsequent immune interactions as the parasites migrate from the skin. We suggest that a thorough identification of the mechanisms involved in establishing African trypanosome infections in the skin and their progression through the host is essential for the development of novel approaches to interrupt disease transmission and control these important diseases.

Leishmania amazonensis Subverts the Transcription Factor Landscape in Dendritic Cells to Avoid Inflammasome Activation and Stall Maturation

Leishmania parasites are the causative agents of human leishmaniases. They infect professional phagocytes of their mammalian hosts, including dendritic cells (DCs) that are essential for the initiation of adaptive immune responses. These immune functions strictly depend on the DC's capacity to differentiate from immature, antigen-capturing cells to mature, antigen-presenting cells—a process accompanied by profound changes in cellular phenotype and expression profile. Only little is known on how intracellular Leishmania affects this important process and DC transcriptional regulation. Here, we investigate these important open questions analyzing phenotypic, cytokine profile and transcriptomic changes in murine, immature bone marrow-derived DCs (iBMDCs) infected with antibody-opsonized and non-opsonized Leishmania amazonensis (L.am) amastigotes. DCs infected by non-opsonized amastigotes remained phenotypically immature whereas those infected by opsonized parasites displayed a semi-mature phenotype. The low frequency of infected DCs in culture led us to use DsRed2-transgenic parasites allowing for the enrichment of infected BMDCs by FACS. Sorted infected DCs were then subjected to transcriptomic analyses using Affymetrix GeneChip technology. Independent of parasite opsonization, Leishmania infection induced expression of genes related to key DC processes involved in MHC Class I-restricted antigen presentation and alternative NF-κB activation. DCs infected by non-opsonized parasites maintained an immature phenotype and showed a small but significant down-regulation of gene expression related to pro-inflammatory TLR signaling, the canonical NF-kB pathway and the NLRP3 inflammasome. This transcriptomic profile was further enhanced in DCs infected with opsonized parasites that displayed a semi-mature phenotype despite absence of inflammasome activation. This paradoxical DC phenotype represents a Leishmania-specific signature, which to our knowledge has not been observed with other opsonized infectious agents. In conclusion, systems-analyses of our transcriptomics data uncovered important and previously unappreciated changes in the DC transcription factor landscape, thus revealing a novel Leishmania immune subversion strategy directly acting on transcriptional control of gene expression. Our data raise important questions on the dynamic and reciprocal interplay between trans-acting and epigenetic regulators in establishing permissive conditions for intracellular Leishmania infection and polarization of the immune response.

Immune Responses in Post Kala-azar Dermal Leishmaniasis

Kala-azar, commonly known as visceral leishmaniasis (VL), is a neglected tropical disease that has been targeted in South Asia for elimination by 2020. Presently, the Kala-azar Elimination Programme is aimed at identifying new low-endemic foci by active case detection, consolidating vector control measures, and decreasing potential reservoirs, of which Post Kala-azar Dermal Leishmaniasis (PKDL) is considered as the most important. PKDL is a skin condition that occurs after apparently successful treatment of VL and is characterized by hypopigmented patches (macular) or a mixture of papules, nodules, and/or macules (polymorphic). To achieve this goal of elimination, it is important to delineate the pathophysiology so that informed decisions can be made regarding the most appropriate and cost-effective approach. We reviewed the literature with regard to PKDL in Asia and Africa and interpreted the findings in establishing a potential correlation between the immune responses and pathophysiology. The overall histopathology indicated the presence of a dense, inflammatory cellular infiltrate, characterized by increased expression of alternatively activated CD68+ macrophages, CD8+ T cells showing features of exhaustion, CD20+ B cells, along with decreased CD1a+ dendritic cells. Accordingly, this review is an update on the overall immunopathology of PKDL, so as to provide a better understanding of host-parasite interactions and the immune responses generated which could translate into availability of markers that can be harnessed for assessment of disease progression and improvement of existing treatment modalities.

Interleukin (IL)-21 in Inflammation and Immunity During Parasitic Diseases

Parasitic diseases cause significant morbidity and mortality in the developing and underdeveloped countries. No efficacious vaccines are available against most parasitic diseases and there is a critical need for developing novel vaccine strategies for care. IL-21 is a pleiotropic cytokine whose functions in protection and immunopathology during parasitic diseases have been explored in limited ways. IL-21 and its cognate receptor, IL-21R, are highly expressed in parasitized organs of infected humans as well in murine models of the human parasitic diseases. Prior studies have indicated the ability of the IL-21/IL-21R signaling axis to regulate the effector functions (e.g., cytokine production) of T cell subsets by enhancing the expression of T-bet and STAT4 in human T cells, resulting in an augmented production of IFN-γ. Mice deficient for either IL-21 (Il21^−/−) or IL-21R (Il21r^−/−) showed significantly reduced inflammatory responses following parasitic infections as compared with their WT counterparts. Targeting the IL-21/IL-21R signaling axis may provide a novel approach for the development of new therapeutic agents for the prevention of parasite-induced immunopathology and tissue destruction.

Glycans in the roles of parasitological diagnosis and host-parasite interplay

The investigation of the glycan repertoire of several organisms has revealed a wide variation in terms of structures and abundance of glycan moieties. Among the parasites, it is possible to observe different sets of glycoconjugates across taxa and developmental stages within a species. The presence of distinct glycoconjugates throughout the life cycle of a parasite could relate to the ability of that organism to adapt and survive in different hosts and environments. Carbohydrates on the surface, and in excretory-secretory products of parasites, play essential roles in host-parasite interactions. Carbohydrate portions of complex molecules of parasites stimulate and modulate host immune responses, mainly through interactions with specific receptors on the surface of dendritic cells, leading to the generation of a pattern of response that may benefit parasite survival. Available data reviewed here also show the frequent aspect of parasite immunomodulation of mammalian responses through specific glycan interactions, which ultimately makes these molecules promising in the fields of diagnostics and vaccinology.

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.

Molecular Aspects of Dendritic Cell Activation in Leishmaniasis: An Immunobiological View

Dendritic cells (DC) are a diverse group of leukocytes responsible for bridging innate and adaptive immunity. Despite their functional versatility, DCs exist primarily in two basic functional states: immature and mature. A large body of evidence suggests that upon interactions with pathogens, DCs undergo intricate cellular processes that culminate in their activation, which is paramount to the orchestration of effective immune responses against Leishmania parasites. Herein we offer a concise review of the emerging hallmarks of DCs activation in leishmaniasis as well as a comprehensive discussion of the following underlying molecular events: DC-Leishmania interaction, antigen uptake, costimulatory molecule expression, parasite ability to affect DC migration, antigen presentation, metabolic reprogramming, and epigenetic alterations.

Deletion of Interleukin-4 Receptor Alpha-Responsive Keratinocytes in BALB/c Mice Does Not Alter Susceptibility to Cutaneous Leishmaniasis

The skin microenvironment at the site of infection plays a role in the early events that determine protective T helper 1/type 1 immune responses during cutaneous leishmaniasis (CL) infection. During CL in nonhealing BALB/c mice, early interleukin-4 (IL-4) can instruct dendritic cells for protective Th1 immunity.

The skin microenvironment at the site of infection plays a role in the early events that determine protective T helper 1/type 1 immune responses during cutaneous leishmaniasis (CL) infection. During CL in nonhealing BALB/c mice, early interleukin-4 (IL-4) can instruct dendritic cells for protective Th1 immunity. Additionally, keratinocytes, which are the principal cell type in the skin epidermis, have been shown to secrete IL-4 early after Leishmania major infection. Here, we investigated whether IL-4/IL-13 signaling via the common IL-4 receptor alpha chain (IL-4Rα) on keratinocytes contributes to susceptibility during experimental CL. To address this, keratinocyte-specific IL-4Rα-deficient (KRT14^cre IL-4Rα^−/lox) mice on a BALB/c genetic background were generated by gene targeting and site-specific recombination (Cre/loxP) under the control of the keratinocyte-specific krt14 locus. Following high-dose infection with L. major IL-81 and LV39 promastigotes subcutaneously in the footpad, footpad swelling, parasite burden, IFN-γ/IL-4/IL-13 cytokine production, and type 1 and type 2 antibody responses were similar between KRT14^cre IL-4Rα^−/lox and littermate control IL-4Rα^−/lox BALB/c mice. An intradermal infection with low-dose L. major IL-81 and LV39 promastigotes in the ear showed results in infected KRT14^cre IL-4Rα^−/lox BALB/c mice similar to those of littermate control IL-4Rα^−/lox BALB/c mice, with the exception of a significant decrease observed in parasite burden only at the site of LV39 infection in the ear. Collectively, our results show that autocrine and paracrine signaling of IL-4/IL-13 through the IL-4Rα chain on keratinocytes does not influence the establishment of a nonhealing Th2 immune response in BALB/c mice during L. major infection.

Leishmania Hijacks Myeloid Cells for Immune Escape

Protozoan parasites of the Leishmania genus are the causative agents of leishmaniasis, a group of neglected tropical diseases whose clinical manifestations vary depending on the infectious Leishmania species but also on host factors. Recognition of the parasite by host myeloid immune cells is a key to trigger an effective Leishmania-specific immunity. However, the parasite is able to persist in host myeloid cells by evading, delaying and manipulating host immunity in order to escape host resistance and ensure its transmission. Neutrophils are first in infiltrating infection sites and could act either favoring or protecting against infection, depending on factors such as the genetic background of the host or the parasite species. Macrophages are the main host cells where the parasites grow and divide. However, macrophages are also the main effector population involved in parasite clearance. Parasite elimination by macrophages requires the priming and development of an effector Th1 adaptive immunity driven by specific subtypes of dendritic cells. Herein, we will provide a comprehensive outline of how myeloid cells regulate innate and adaptive immunity against Leishmania, and the mechanisms used by the parasites to promote their evasion and sabotage. Understanding the interactions between Leishmania and the host myeloid cells may lead to the development of new therapeutic approaches and improved vaccination to leishmaniases, an important worldwide health problem in which current therapeutic or preventive approaches are limited.

CD8+ T Cells Lack Local Signals To Produce IFN-γ in the Skin during Leishmania Infection

Resolution of leishmaniasis depends upon parasite control and limiting inflammation. CD4⁺ Th1 cells are required to control parasites, whereas CD8⁺ T cells play a dual role: they promote Th1 cell differentiation but can also increase inflammation at the site of infection as a consequence of cytolysis. Although CD8⁺ T cells taken from leishmanial lesions are cytolytic, in this study, we showed that only a few CD8⁺ T cells produced IFN-γ. Correspondingly, only low levels of IL-12 and/or IL-12 mRNA were present in lesions from infected mice, as well as patients. Addition of IL-12 increased IFN-γ production by CD8⁺ T cells isolated from leishmanial lesions, suggesting that a lack of IL-12 at the site of infection limits IFN-γ production by CD8⁺ T cells. To determine whether CD8⁺ T cells could promote resistance in vivo if IL-12 was present, we administered IL-12 to Leishmania-infected RAG mice reconstituted with CD8⁺ T cells. IL-12 treatment increased the ability of CD8⁺ T cells to make IFN-γ, but CD8⁺ T cells still failed to control the parasites. Furthermore, despite the ability of CD8⁺ T cells to promote immunity to secondary infections, we also found that CD8⁺ T cells from immune mice were unable to control Leishmania in RAG mice. Taken together, these results indicate that lesional CD8⁺ T cells fail to make IFN-γ because of a deficit in IL-12 but that, even with IL-12, CD8⁺ T cells are unable to control Leishmania in the absence of CD4⁺ T cells.

Cutaneous leishmaniasis: Distinct functions of dendritic cells and macrophages in the interaction of the host immune system with Leishmania major

Leishmaniasis is transmitted by sand flies leading to parasite inoculation into skin. In the mammalian host, the parasite primarily resides in skin macrophages (MΦ) and dendritic cells (DC). MΦ are silently invaded by the parasite eliciting a stress response, whereas DC become activated, release IL-12, and prime antigen-specific T cells. Here we review the basics of the immune response against this human pathogen and elucidate the role and function DC and MΦ for establishment of protective immunity against leishmaniasis. We focus on cell type-specific differences in parasite uptake, phagocyte activation and processing of parasite antigens to facilitate an understanding how their respective function may be modulated e.g. under therapeutic considerations.

Differential phagocytosis of Leishmania mexicana promastigotes and amastigotes by monocyte-derived dendritic cells

Leishmania species are dimorphic protozoan parasites that live and replicate in the gut of sand flies as promastigotes or in mammalian hosts as amastigotes. Different immune cells, including DCs, and receptors differ in their involvement in phagocytosis of promastigotes and amastigotes and in recognition of different Leishmania species. In the case of L. mexicana, differences in phagocytosis of promastigotes and amastigotes by DCs and participation of C-type lectin receptors (CLRs) have not been established. In the present study, flow cytometry and confocal microscopy were used to investigate the phagocytosis by monocyte-derived dendritic cells (moDCs) of L. mexicana promastigotes and amastigotes in the presence or absence of immune serum during various periods of time. Blocking antibodies against mannose receptors and DC-SIGN were used to explore the participation of these receptors in the phagocytosis of L. mexicana by moDC. The major differences in interactions of L. mexicana promastigotes and amastigotes with moDC were found to occur within the first 3 hr, during which phagocytosis of promastigotes predominated as compared with opsonization of promastigotes and amastigotes. However, after 6 hr of incubation, opsonized promastigotes were preferentially phagocytosed as compared with unopsonized promastigotes and amastigotes and after 24 hr of incubation there were no differences in the phagocytosis of promastigotes and amastigotes. Finally, after 3 hr incubation, DC-SIGN was involved in the phagocytosis of promastigotes, but not of amastigotes.

Possible Interrelationship of Inflammatory Cells in Dry Type Cutaneous Leishmaniasis

BACKGROUND & OBJECTIVE

There is a complicated interaction between leishmaniasis and the host immune cells, and also between the host immune cells. These interactions have fundamental effects on the outcome of the disease.The current study aimed at characterizing the number, distribution, co-localization, and interrelation of 4 types of inflammatory cells in different clinical forms of dry-type cutaneous leishmaniasis (CL).

METHODS

Thirty-nine cases of CL were studied. The cases were classified clinically as 14 cases of acute leishmaniasis with indurated papules, nodules, and plaques with central crust formation < 2 years, 7 cases of chronic type with non-healing lesions > 2 years, and 12 cases of lupoid leishmaniasis with characteristic papules around previous scars of CL > 2 years. Paraffin-embedded blocks were stained with hematoxylin and eosin (H&E) and also stained immunohistochemically for CD4, CD8, CD68, and CD1a.

RESULTS

In acute CL, there was a significant correlation between CD68+ macrophages and CD1a+ epidermal dendritic cells (DCs); the population of CD68+ macrophages and CD1a+ epidermal DCs increased in parallel.In lupoid CL, there was a significant correlation between CD1a+ epidermal DCs, and CD1a+ dermal DCs and population of CD1a+ epidermal DCs; the number of CD1a+ dermal DCs increased in parallel.

CONCLUSIONS

The result of the current study could be used as a baseline to design and study the new targeted therapy of synergistic effects of macrophages and DCs to phagocytizing leishmania bodies; and/or suggestion planning of individualizing setup of vaccine by autologous interaction of macrophages and DC in CL.

Parasites Dampen Dendritic Cell Activation to Ensure Their Survival

Dendritic cells (DCs) are critical for induction of protective immunity against Leishmania major. However, DC activation occurs only several weeks after parasite transmission. Parasites synthesize a macrophage-inducible C-type lectin (Mincle) ligand. Engagement of Mincle by the parasite ligand dampens DC activation, thus delaying induction of interferon-γ-producing T cells responsible for parasite eradication.

Immune responses against protozoan parasites: a focus on the emerging role of Nod-like receptors

Nod-like receptors (NLRs) have gained attention in recent years because of the ability of some family members to assemble into a multimeric protein complex known as the inflammasome. The role of NLRs and the inflammasome in regulating innate immunity against bacterial pathogens has been well studied. However, recent studies show that NLRs and inflammasomes also play a role during infections caused by protozoan parasites, which pose a significant global health burden. Herein, we review the diseases caused by the most common protozoan parasites in the world and discuss the roles of NLRs and inflammasomes in host immunity against these parasites.

Cutaneous leishmaniasis: immune responses in protection and pathogenesis

Cutaneous leishmaniasis is a major public health problem and causes a range of diseases from self-healing infections to chronic disfiguring disease. Currently, there is no vaccine for leishmaniasis, and drug therapy is often ineffective. Since the discovery of CD4(+) T helper 1 (TH1) cells and TH2 cells 30 years ago, studies of cutaneous leishmaniasis in mice have answered basic immunological questions concerning the development and maintenance of CD4(+) T cell subsets. However, new strategies for controlling the human disease have not been forthcoming. Nevertheless, advances in our knowledge of the cells that participate in protection against Leishmania infection and the cells that mediate increased pathology have highlighted new approaches for vaccine development and immunotherapy. In this Review, we discuss the early events associated with infection, the CD4(+) T cells that mediate protective immunity and the pathological role that CD8(+) T cells can have in cutaneous leishmaniasis.

Dendritic Cells Pulsed with Peptides of GP63 Induce Differential Protection against Experimental Cutaneous Leishmaniasis

The need for a vaccine against Leishmania spp., a major cause of worldwide morbidity and mortality, is urgent. We tested the efficacy of an experimental vaccination in murine models of cutaneous leishmaniasis, using dendritic cells (DCs) pulsed with synthetic or native parasite antigens. DCs pulsed with peptide 154–169aa of gp63 or soluble promastigote lysate (SPL) triggered antigen-specific immune responses and efficiently reduced lesion formation and parasite load of genetically susceptible BALB/c mice infected with Leishmania major. This effect was accompanied by a modulation of the cellular immune response towards a Th1 profile. Vaccination of genetically resistant CBA mice with DCs pulsed with peptide 154–169aa or SPL did not affect the course of the disease, whereas pulsing with the epitope 467–482aa of gp63 resulted in disease exacerbation, accompanied by a switch to a Th2 profile. In view of our continuously growing knowledge about the immunobiology of DCs, these findings suggest that vaccination with DCs pulsed with defined peptides could be a strategy against infectious diseases. Peptide selection is a prerequisite as they can differentially regulate the type of immune response in susceptible or resistant hosts.

Escaping Deleterious Immune Response in Their Hosts: Lessons from Trypanosomatids

The Trypanosomatidae family includes the genera Trypanosoma and Leishmania, protozoan parasites displaying complex digenetic life cycles requiring a vertebrate host and an insect vector. Trypanosoma brucei gambiense, Trypanosoma cruzi, and Leishmania spp. are important human pathogens causing human African trypanosomiasis (HAT or sleeping sickness), Chagas' disease, and various clinical forms of Leishmaniasis, respectively. They are transmitted to humans by tsetse flies, triatomine bugs, or sandflies, and affect millions of people worldwide. In humans, extracellular African trypanosomes (T. brucei) evade the hosts' immune defenses, allowing their transmission to the next host, via the tsetse vector. By contrast, T. cruzi and Leishmania sp. have developed a complex intracellular lifestyle, also preventing several mechanisms to circumvent the host's immune response. This review seeks to set out the immune evasion strategies developed by the different trypanosomatids resulting from parasite-host interactions and will focus on: clinical and epidemiological importance of diseases; life cycles: parasites-hosts-vectors; innate immunity: key steps for trypanosomatids in invading hosts; deregulation of antigen-presenting cells; disruption of efficient specific immunity; and the immune responses used for parasite proliferation.

Cathepsin B-Deficient Mice Resolve Leishmania major Inflammation Faster in a T Cell-Dependent Manner

A critical role for intracellular TLR9 has been described in recognition and host resistance to Leishmania parasites. As TLR9 requires endolysosomal proteolytic cleavage to achieve signaling functionality, we investigated the contribution of different proteases like asparagine endopeptidase (AEP) or cysteine protease cathepsins B (CatB), L (CatL) and S (CatS) to host resistance during Leishmania major (L. major) infection in C57BL/6 (WT) mice and whether they would impact on TLR9 signaling. Unlike TLR9-/-, which are more susceptible to infection, AEP-/-, CatL-/- and CatS-/- mice are as resistant to L. major infection as WT mice, suggesting that these proteases are not individually involved in TLR9 processing. Interestingly, we observed that CatB-/- mice resolve L. major lesions significantly faster than WT mice, however we did not find evidence for an involvement of CatB on either TLR9-dependent or independent cytokine responses of dendritic cells and macrophages or in the innate immune response to L. major infection. We also found no difference in antigen presenting capacity. We observed a more precocious development of T helper 1 responses accompanied by a faster decline of inflammation, resulting in resolution of footpad inflammation, reduced IFNγ levels and decreased parasite burden. Adoptive transfer experiments into alymphoid RAG2-/-γc-/- mice allowed us to identify CD3+ T cells as responsible for the immune advantage of CatB-/- mice towards L. major. In vitro data confirmed the T cell intrinsic differences between CatB-/- mice and WT. Our study brings forth a yet unappreciated role for CatB in regulating T cell responses during L. major infection.

Modulation of Innate Immune Mechanisms to Enhance Leishmania Vaccine-Induced Immunity: Role of Coinhibitory Molecules

No licensed human vaccines are currently available against any parasitic disease including leishmaniasis. Several antileishmanial vaccine formulations have been tested in various animal models, including genetically modified live-attenuated parasite vaccines. Experimental infection studies have shown that Leishmania parasites utilize a broad range of strategies to undermine effector properties of host phagocytic cells, i.e., dendritic cells (DCs) and macrophages (MΦ). Furthermore, Leishmania parasites have evolved strategies to actively inhibit T_H1 polarizing functions of DCs and to condition the infected MΦ toward anti-inflammatory/alternative/M2 phenotype. The altered phenotype of phagocytic cells is characterized by decreased production of antimicrobial reactive oxygen, nitrogen molecules, and pro-inflammatory cytokines, such as IFN-γ, IL-12, and TNF-α. These early events limit the activation of T_H1-effector cells and set the stage for pathogenesis. Furthermore, this early control of innate immunity by the virulent parasites results in substantial alteration in the adaptive immunity characterized by reduced proliferation of CD4⁺ and CD8⁺ T cells and T_H2-biased immunity that results in production of anti-inflammatory cytokines, such as TGF-β, and IL-10. More recent studies have also documented the induction of coinhibitory ligands, such as CTLA-4, PD-L1, CD200, and Tim-3, that induce exhaustion and/or non-proliferation in antigen-experienced T cells. Most of these studies focus on viral infections in chronic phase, thus limiting the direct application of these results to parasitic infections and much less to parasitic vaccines. However, these studies suggest that vaccine-induced protective immunity can be modulated using strategies that enhance the costimulation that might reduce the threshold necessary for T cell activation and conversely by strategies that reduce or block inhibitory molecules, such as PD-L1 and CD200. In this review, we will focus on the polarization of antigen-presenting cells and subsequent role of costimulatory and coinhibitory molecules in mediating vaccine-induced immunity using live-attenuated Leishmania parasites as specific examples.

Dendritic Cells and Leishmania Infection: Adding Layers of Complexity to a Complex Disease

Leishmaniasis is a group of neglected diseases whose clinical manifestations depend on factors from the host and the pathogen. It is an important public health problem worldwide caused by the protozoan parasite from the Leishmania genus. Cutaneous Leishmaniasis (CL) is the most frequent form of this disease transmitted by the bite of an infected sandfly into the host skin. The parasites can be uptook and/or recognized by macrophages, neutrophils, and/or dendritic cells (DCs). Initially, DCs were described to play a protective role in activating the immune response against Leishmania parasites. However, several reports showed a dichotomic role of DCs in modulating the host immune response to susceptibility or resistance in CL. In this review, we discuss (1) the interactions between DCs and parasites from different species of Leishmania and (2) the crosstalk of DCs and other cells during CL infection. The complexity of these interactions profoundly affects the adaptive immune response and, consequently, the disease outcome, especially from Leishmania species of the New World.

Decreased presence of Langerhans cells is a critical determinant for Indian Post kala-azar dermal leishmaniasis

Post kala-azar dermal leishmaniasis (PKDL) is the dermal sequel of visceral leishmaniasis (VL) and occurs after apparent cure or alongside with VL. It is confined to South Asia (India, Nepal and Bangladesh) and East Africa (mainly Sudan), the incidence being 5-10% and 50-60% respectively. In South Asia, as the transmission of VL is anthroponotic, PKDL patients are the proposed disease reservoir, thus assuming epidemiological significance, its eradication being linked to the control of leishmaniasis. In the absence of an animal model and its low incidence, factors contributing towards the immunopathogenesis of PKDL remain an open-ended, yet pertinent question. This study delineated the lesional immunopathology in terms of granuloma formation, Langerhans cells, tissue macrophages along with mRNA expression of IL-12p40 and IL-10. Our study in Indian PKDL for the first time identified that the number of CD1a(+) /CD207(+) Langerhans cells are decreased and CD68(+) macrophages are increased along with the absence of an epitheloid granuloma. Importantly, this decrease in Langerhans cells was associated with decreased mRNA expression of IL-12p40 and increased IL-10. This was reverted with treatment allowing for elimination of parasites and disease resolution along with an increase in Langerhans cells and decrease in macrophages. Thus, in Indian PKDL, absence of a granuloma formation along with a decrease in Langerhans cells collectively caused immune inactivation essential for parasite persistence and disease sustenance.

Differential Impact of LPG-and PG-Deficient Leishmania major Mutants on the Immune Response of Human Dendritic Cells

Background

Leishmania major infection induces robust interleukin-12 (IL12) production in human dendritic cells (hDC), ultimately resulting in Th1-mediated immunity and clinical resolution. The surface of Leishmania parasites is covered in a dense glycocalyx consisting of primarily lipophosphoglycan (LPG) and other phosphoglycan-containing molecules (PGs), making these glycoconjugates the likely pathogen-associated molecular patterns (PAMPS) responsible for IL12 induction.

Methodology/Principal Findings

Here we explored the role of parasite glycoconjugates on the hDC IL12 response by generating L. major Friedlin V1 mutants defective in LPG alone, (FV1 lpg1-), or generally deficient for all PGs, (FV1 lpg2-). Infection with metacyclic, infective stage, L. major or purified LPG induced high levels of IL12B subunit gene transcripts in hDCs, which was abrogated with FV1 lpg1- infections. In contrast, hDC infections with FV1 lpg2- displayed increased IL12B expression, suggesting other PG-related/LPG2 dependent molecules may act to dampen the immune response. Global transcriptional profiling comparing WT, FV1 lpg1-, FV1 lpg2- infections revealed that FV1 lpg1- mutants entered hDCs in a silent fashion as indicated by repression of gene expression. Transcription factor binding site analysis suggests that LPG recognition by hDCs induces IL-12 in a signaling cascade resulting in Nuclear Factor κ B (NFκB) and Interferon Regulatory Factor (IRF) mediated transcription.

Conclusions/Significance

These data suggest that L. major LPG is a major PAMP recognized by hDC to induce IL12-mediated protective immunity and that there is a complex interplay between PG-baring Leishmania surface glycoconjugates that result in modulation of host cellular IL12.

Leishmaniasis is a group of parasitic diseases caused by intracellular protozoa belonging to the genus Leishmania, pathological manifestations ranging from self-healing cutaneous forms to severe visceral infections that result in death. These clinical outcomes are dictated by the Leishmania species initiating the infection and are influenced by early responses of host immune cells, which ultimately initiate an IL12 mediated immune response in resolving infections. Like the diseases themselves, the magnitude of IL12 induction in hDCs is Leishmania-species and strain specific, where species that elicit visceral disease do not induce IL12, while most cutaneous disease-causing L. major strains induce robust IL12 responses and confer life-long immunity. The molecular mechanisms that mediate the ability of these innate immune cells to discriminate between pathogens remain elusive and have been primarily investigated in murine model systems. Here we identified L. major LPG as a major PAMP that induces IL12 in hDCs. Elucidation of this critical component of human immunity to L. major has ramifications for leishmaniasis vaccine development.

Tolerogenic Dendritic Cells for Regulatory T Cell Induction in Man

Dendritic cells (DCs) are highly specialized professional antigen-presenting cells that regulate immune responses, maintaining the balance between tolerance and immunity. Mechanisms via which they can promote central and peripheral tolerance include clonal deletion, the inhibition of memory T cell responses, T cell anergy, and induction of regulatory T cells (Tregs). These properties have led to the analysis of human tolerogenic DCs as a therapeutic strategy for the induction or re-establishment of tolerance. In recent years, numerous protocols for the generation of human tolerogenic DCs have been developed and their tolerogenic mechanisms, including induction of Tregs, are relatively well understood. Phase I trials have been conducted in autoimmune disease, with results that emphasize the feasibility and safety of treatments with tolerogenic DCs. Therefore, the scientific rationale for the use of tolerogenic DCs therapy in the fields of transplantation medicine and allergic and autoimmune diseases is strong. This review will give an overview on efforts and protocols to generate human tolerogenic DCs with focus on IL-10-modulated DCs as inducers of Tregs and discuss their clinical applications and challenges faced in further developing this form of immunotherapy.

Toll-like receptor 9 signaling in dendritic cells regulates neutrophil recruitment to inflammatory foci following Leishmania infantum infection

Leishmania infantum is a protozoan parasite that causes visceral leishmaniasis (VL). This infection triggers dendritic cell (DC) activation through the recognition of microbial products by Toll-like receptors (TLRs). Among the TLRs, TLR9 is required for DC activation by different Leishmania species. We demonstrated that TLR9 is upregulated in vitro and in vivo during infection. We show that C57BL/6 mice deficient in TLR9 expression (TLR9(-/-) mice) are more susceptible to infection and display higher parasite numbers in the spleen and liver. The increased susceptibility of TLR9(-/-) mice was due to the impaired recruitment of neutrophils to the infection foci associated with reduced levels of neutrophil chemoattractants released by DCs in the target organs. Moreover, both Th1 and Th17 cells were also committed in TLR9(-/-) mice. TLR9-dependent neutrophil recruitment is mediated via the MyD88 signaling pathway but is TIR domain-containing adapter-inducing interferon beta (TRIF) independent. Furthermore, L. infantum failed to activate both plasmacytoid and myeloid DCs from TLR9(-/-) mice, which presented reduced surface costimulatory molecule expression and chemokine release. Interestingly, neutrophil chemotaxis was affected both in vitro and in vivo when DCs were derived from TLR9(-/-) mice. Our results suggest that TLR9 plays a critical role in neutrophil recruitment during the protective response against L. infantum infection that could be associated with DC activation.

Deficiency of prolactin-inducible protein leads to impaired Th1 immune response and susceptibility to Leishmania major in mice

Although the strategic production of prolactin-inducible protein (PIP) at several ports of pathogen entry into the body suggests it might play a role in host defense, no study has directly implicated it in immunity against any infectious agent. Here, we show for the first time that PIP deficiency is associated with reduced numbers of CD4(+) T cells in peripheral lymphoid tissues and impaired CD4(+) Th1-cell differentiation in vitro. In vivo, CD4(+) T cells from OVA-immunized, PIP-deficient mice showed significantly impaired proliferation and IFN-γ production following in vitro restimulation. Furthermore, PIP-deficient mice were highly susceptible to Leishmani major infection and failed to control lesion progression and parasite proliferation. This susceptibility was associated with impaired NO production and leishmanicidal activity of PIP KO macrophages following IFN-γ and LPS stimulation. Collectively, our findings implicate PIP as an important regulator of CD4(+) Th1-cell-mediated immunity.

Blocking junctional adhesion molecule C enhances dendritic cell migration and boosts the immune responses against Leishmania major

The recruitment of dendritic cells to sites of infections and their migration to lymph nodes is fundamental for antigen processing and presentation to T cells. In the present study, we showed that antibody blockade of junctional adhesion molecule C (JAM-C) on endothelial cells removed JAM-C away from junctions and increased vascular permeability after L. major infection. This has multiple consequences on the output of the immune response. In resistant C57BL/6 and susceptible BALB/c mice, we found higher numbers of innate immune cells migrating from blood to the site of infection. The subsequent migration of dendritic cells (DCs) from the skin to the draining lymph node was also improved, thereby boosting the induction of the adaptive immune response. In C57BL/6 mice, JAM-C blockade after L. major injection led to an enhanced IFN-γ dominated T helper 1 (Th1) response with reduced skin lesions and parasite burden. Conversely, anti JAM-C treatment increased the IL-4-driven T helper 2 (Th2) response in BALB/c mice with disease exacerbation. Overall, our results show that JAM-C blockade can finely-tune the innate cell migration and accelerate the consequent immune response to L. major without changing the type of the T helper cell response.

Leishmaniasis is a parasitic disease transmitted to humans through sand fly bites. Clinical symptoms vary from self-healing cutaneous lesions to death. Cutaneous leishmaniasis is particularly studied in mice inoculated with Leishmania major. In this model, some strains (e.g. C57BL/6) are resistant due to a Th1 immune response promoting parasite killing. Conversely, other strains (e.g. BALB/c) are susceptible due to a nonprotective Th2 response. DCs are professional antigen-presenting cells that educate antigen-specific T cells. Improving the migration of DCs from the site of infection to the lymph nodes, where T cells reside, may improve the T cell response. JAM-C is a vascular adhesion molecule implicated in leukocyte migration in different inflammatory models. We found that JAM-C blockade with antibodies increases vascular permeability and consequently improves the migration of DCs to sites of infection and draining lymph nodes. This increased leukocyte migration boosted the induction of the Th1 response in resistant mice, while in susceptible mice the Th2 response was augmented. This led to disease improvement or exacerbation, respectively. Our results illustrate the key role of a vascular adhesion molecule in controlling leukocyte migration and the subsequent immune events in response to pathogen infections.