Down-regulation of dendritic cell signaling pathways by Leishmania amazonensis amastigotes

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.

IFN-γ inhibitory molecules derived from Eimeria maxima inhibit IL-12 secretion by modulating MAPK pathways in chicken macrophages

IFN-γ plays a crucial role in resisting intracellular parasitic protozoa, such as Eimeria species. In our previous study, we identified 4 molecules derived from Eimeria maxima (E. maxima) that significantly inhibited IFN-γ production. However, the mechanism underlying this inhibitory effect remains unknown. In this study, we first investigated the effects of these 4 IFN-γ inhibitory molecules on the expression levels of chicken Toll-like receptors (chTLRs), IL-12, IL-10, TGF-β, and TNF-α in chicken macrophage HD11 and bone marrow-derived dendritic cells (BMDCs). The results demonstrated that these 4 inhibitory molecules significantly downregulated the mRNA levels of chTLR-2, chTLR-4, chTLR-21, and both mRNA and protein levels of IL-12. Subsequently, to clarify the effects of these 4 inhibitory molecules on the IL-12 secretion-related signaling pathways in chicken macrophages, qRT-PCR and Western blot were used to detect the changes of key molecules involved in the signaling pathways of IL-12 secretion (NF-κB, ERK1/2, p38, JNK, STAT3) following coincubation with these inhibitory molecules. Finally, RNAi was employed to verify the function of key molecules in the signaling pathway. The results revealed a significant upregulation in the expression of ERK1/2 phosphorylated protein induced by the 4 inhibitory molecules. Knockdown of the ERK1/2 gene significantly reduced the inhibitory effect of the 4 E. maxima inhibitory molecules on IL-12. These findings indicate that the 4 inhibitory molecules can inhibit the secretion of IL-12 by upregulating the expression of ERK1/2 phosphorylated protein, which is a key molecule in the ERK-MAPK pathway. Our study may contribute to elucidating the mechanisms underlying immune evasion during E. maxima infections, thereby providing new insights for the control of chicken coccidiosis.

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.

Malaria and leishmaniasis: Updates on co-infection

Malaria and leishmaniasis are endemic parasitic diseases in tropical and subtropical countries. Although the overlap of these diseases in the same host is frequently described, co-infection remains a neglected issue in the medical and scientific community. The complex relationship of concomitant infections with Plasmodium spp. and Leishmania spp. is highlighted in studies of natural and experimental co-infections, showing how this "dual" infection can exacerbate or suppress an effective immune response to these protozoa. Thus, a Plasmodium infection preceding or following Leishmania infection can impact the clinical course, accurate diagnosis, and management of leishmaniasis, and vice versa. The concept that in nature we are affected by concomitant infections reinforces the need to address the theme and ensure its due importance. In this review we explore and describe the studies available in the literature on Plasmodium spp. and Leishmania spp. co-infection, the scenarios, and the factors that may influence the course of these diseases.

Expression Profile of Genes Related to the Th17 Pathway in Macrophages Infected by Leishmania major and Leishmania amazonensis: The Use of Gene Regulatory Networks in Modeling This Pathway

Leishmania amazonensis and Leishmania major are the causative agents of cutaneous and mucocutaneous diseases. The infections‘ outcome depends on host–parasite interactions and Th1/Th2 response, and in cutaneous form, regulation of Th17 cytokines has been reported to maintain inflammation in lesions. Despite that, the Th17 regulatory scenario remains unclear. With the aim to gain a better understanding of the transcription factors (TFs) and genes involved in Th17 induction, in this study, the role of inducing factors of the Th17 pathway in Leishmania–macrophage infection was addressed through computational modeling of gene regulatory networks (GRNs). The Th17 GRN modeling integrated experimentally validated data available in the literature and gene expression data from a time-series RNA-seq experiment (4, 24, 48, and 72 h post-infection). The generated model comprises a total of 10 TFs, 22 coding genes, and 16 cytokines related to the Th17 immune modulation. Addressing the Th17 induction in infected and uninfected macrophages, an increase of 2- to 3-fold in 4–24 h was observed in the former. However, there was a decrease in basal levels at 48–72 h for both groups. In order to evaluate the possible outcomes triggered by GRN component modulation in the Th17 pathway. The generated GRN models promoted an integrative and dynamic view of Leishmania–macrophage interaction over time that extends beyond the analysis of single-gene expression.

Temporary Shutdown of ERK1/2 Phosphorylation Is Associated With Activation of Adaptive Immune Cell Responses and Disease Progression During Leishmania amazonensis Infection in BALB/c Mice

Leishmania spp. infection outcomes are dependent on both host and parasite factors. Manipulation of host signaling pathways involved in the generation of immune responses is thought to be one of the most common mechanisms used by parasites for persistence within the host. Considering the diversity of pathologies caused by different Leishmania spp., it is plausible that significant differences may exist in the mechanisms of host cell manipulation by each parasite species, which may have implications when developing new vaccine or treatment strategies. Here we show that in L. braziliensis-infection in BALB/c mice, a model of resistance, activation of ERK1/2 coincides with the peak of inflammatory responses and resolution of tissue parasitism. In contrast, in the susceptibility model of L. amazonensis-infection, an early silent phase of infection is observed, detected solely by quantification of parasite loads. At this early stage, only basal levels of P-ERK1/2 are observed. Later, after a brief shutdown of ERK1/2 phosphorylation, disease progression is observed and is associated with increased inflammation, lesion size and tissue parasitism. Moreover, the short-term down-regulation of ERK1/2 activation affected significantly downstream inflammatory pathways and adaptive T cell responses. Administration of U0126, a MEK/ERK inhibitor, confirmed this phenomenon, since bigger lesions and higher parasite loads were seen in infected mice that received U0126. To investigate how kinetics of ERK1/2 activation could affect the disease progression, U0126 was administered to L. amazonensis-infected animals earlier than the P-ERK1/2 switch off time-point. This intervention resulted in anticipation of the same effects on inflammatory responses and susceptibility phenotype seen in the natural course of infection. Additionally, in vitro inhibition of ERK1/2 affected the phagocytosis of L. amazonensis by BMDMs. Collectively, our findings reveal distinct temporal patterns of activation of inflammatory responses in L. braziliensis and L. amazonensis in the same animal background and a pivotal role for a brief and specific shutdown of ERK1/2 activation at late stages of L. amazonensis infection. Since activation of inflammatory responses is a crucial aspect for the control of infectious processes, these findings may be important for the search of new and specific strategies of vaccines and treatment for tegumentary leishmaniasis.

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.

Comparison of expression of CD1a and CD68 markers in skin leishmaniasis samples with positive and negative Leishman body

BACKGROUND

Leishmaniasis is one of the most important infectious illnesses around the world. Given the high commonness of this disease, specifically its skin type in Iran and due to the role of the Leishman bodies in diagnosis, the aim of present study was evaluating the expression of two CD1a and CD68 markers in cutaneous leishmaniasis lesions with and without Leishman bodies.

METHODS

In this case-control study, 70 skin samples of patients with cutaneous leishmaniasis (35 patients with Leishman body as case group and 35 patients without Leishman boy as control group) were investigated during 2018-2019. The expression of CD1a and CD68 markers and immunohistochemistry staining (IHC) were investigated in this study.

RESULTS

The expression of CD1a in the group with Leishman body was significantly higher than group without Leishman body (P=0.01), but there was no significant difference between groups as expression of CD68 (P=0.40). The frequency of hyperkeratosis, parakeratosis, exocytosis, acanthosis, spongiosis, hydropic degeneration of basal cell layer, lichenoid reaction, pseudoepitheliomatous hyperplasia, ulcer, thinning of the epidermis, mononuclear cells, and extension of inflammation into lower dermis in the group with Leishman body was higher than group without Leishman body (P<0.05).

CONCLUSION

The expression of CD1a and other morphological findings help to diagnose the difference between leishmaniasis with and without Leishman body.

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.

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.

Combination of Mycobacterium indicus pranii and Heat-Induced Promastigotes Cures Drug-Resistant Leishmania Infection: Critical Role of Interleukin-6-Producing Classical Dendritic Cells

The major issues in available therapeutic modalities against leishmaniasis are cost, toxicity, and the emergence of drug resistance. The aim of this work was to develop a successful therapeutic adjuvant against drug-resistant Leishmania donovani infection by means of combining Mycobacterium indicus pranii with heat-induced promastigotes (HIP). One-month postinfected BALB/c mice were administered subcutaneously with M. indicus pranii (10⁸ cells) and HIP (100 μg) for 5 days. Spleens were harvested for flow cytometric and reverse transcriptase PCR analysis. The antileishmanial effect of the combination strategy was associated with induction of a disease-resolving Th1 and Th17 response with simultaneous downregulation of CD4⁺ CD25⁺ Foxp3⁺ (nTreg) cells and CD4⁺ CD25^- Foxp3^- (Tr1) cells in the spleen. The significant expansion of CD4⁺ T_CM (CD4⁺ CD44^hi CD11a^hi CD62L^hi) cells was a further interesting outcome of this therapeutic strategy in the context of long-term protection of hosts against secondary infection. Toll-like receptor 2 (TLR2) was also found instrumental in this antiparasitic therapy. Induced interleukin-6 (IL-6) production from expanded CD11c⁺ CD8α⁺ (cDC1) and CD11c⁺ CD11b⁺ (cDC2) dendritic cells (DCs) but not from the CD11b⁺ Ly6c⁺ inflammatory monocytes (iMOs), was found critical in the protective expansion of Th17 as evidenced by an in vivo IL-6 neutralization assay. It also promoted the hematopoietic conversion toward DC progenitors (pre-DCs) from common dendritic cell progenitors (CDPs), the immediate precursors, in bone marrow. This novel combinational strategy demonstrated that expansion of Th17 by IL-6 released from CD11c⁺ classical DCs is crucial, together with the conventional Th1 response, to control drug-resistant infection.

CD4+ T Cell-Dependent Macrophage Activation Modulates Sustained PS Exposure on Intracellular Amastigotes of Leishmania amazonensis

Leishmania amazonensis amastigotes can make use of surface-exposed phosphatidylserine (PS) molecules to promote infection and non-classical activation of macrophages (MΦ), leading to uncontrolled intracellular proliferation of the parasites. This mechanism was quoted as apoptotic mimicry. Moreover, the amount of PS molecules exposed on the surface of amastigotes correlates with the susceptibility of the host. In this study, we tested whether host cellular responses influence PS expression on intracellular amastigotes. We found that the level of PS exposure on intracellular amastigotes was modulated by CD4⁺ T cell and MΦ activation status in vitro and in vivo. L. amazonensis infection generated a Th1/Th2-mixed cytokine profile, providing the optimal MΦ stimulation that favored PS exposure on intracellular amastigotes. Maintenance of PS exposed on the parasite was dependent on low, but sustained, levels of nitric oxide and polyamine production. Amastigotes obtained from lymphopenic nude mice did not expose PS on their surface, and adoptive transfer of CD4⁺ T cells reversed this phenotype. In addition, histopathological analysis of mice treated with anti-PS antibodies showed increased inflammation and similarities to nude mouse lesions. Collectively, our data confirm the role of pathogenic CD4⁺ T cells for disease progression and point to PS as a critical parasite strategy to subvert host immune responses.

The role of TLR9 on Leishmania amazonensis infection and its influence on intranasal LaAg vaccine efficacy

Leishmania (L.) amazonensis is one of the etiological agents of cutaneous leishmaniasis (CL) in Brazil. Currently, there is no vaccine approved for human use against leishmaniasis, although several vaccine preparations are in experimental stages. One of them is Leishvacin, or LaAg, a first-generation vaccine composed of total L. amazonensis antigens that has consistently shown an increase of mouse resistance against CL when administered intranasally (i.n.). Since Toll-like receptor 9 (TLR9) is highly expressed in the nasal mucosa and LaAg is composed of TLR9-binding DNA CpG motifs, in this study we proposed to investigate the role of TLR9 in both L. amazonensis infection and in LaAg vaccine efficacy in C57BL/6 (WT) mice and TLR9^-/- mice. First, we evaluated, the infection of macrophages by L. amazonensis in vitro, showing no significant difference between macrophages from WT and TLR9^-/- mice in terms of both infection percentage and total number of intracellular amastigotes, as well as NO production. In addition, neutrophils from WT and TLR9^-/- mice had similar capacity to produce neutrophil extracellular traps (NETs) in response to L. amazonensis. L. amazonensis did not activate dendritic cells from WT and TLR9^-/- mice, analysed by MHCII and CD86 expression. However, in vivo, TLR9^-/- mice were slightly more susceptible to L. amazonensis infection than WT mice, presenting a larger lesion and an increased parasite load at the peak of infection and in the chronic phase. The increased TLR9^-/- mice susceptibility was accompanied by an increased IgG and IgG1 production; a decrease of IFN-γ in infected tissue, but not IL-4 and IL-10; and a decreased number of IFN-γ producing CD8⁺ T cells, but not CD4⁺ T cells in the lesion-draining lymph nodes. Also, TLR9^-/- mice could not control parasite growth following i.n. LaAg vaccination unlike the WT mice. This protection failure was associated with a reduction of the hypersensitivity response induced by immunization. The TLR9^-/- vaccinated mice failed to respond to antigen stimulation and to produce IFN-γ by lymph node cells. Together, these results suggest that TLR9 contributes to C57BL/6 mouse resistance against L. amazonensis, and that the TLR9-binding LaAg comprising CpG motifs may be important for intranasal vaccine efficacy against CL.

Leishmaniasis is a major neglected tropical disease, being responsible for more than 20 million deaths per year. The high mortality rate highlights the difficulties and ineffectiveness of the current prophylactic approaches and treatments currently available. Therefore, the development of an effective vaccine would be highly advantageous to circumvent these problems. Despite the many vaccines preparations that have been studied in the last few years, none have shown satisfactory efficacy to be approved for human use. Immune receptors, including the TLR family, are known to be important for host defense during parasitic infections, such as leishmaniasis, and also for vaccine efficacy. In this work, we investigate the role of TLR9 during Leishmania amazonensis infection in vaccinated and non-vaccinated mice. We used a C57BL/6 TLR9-/- mouse model and a first-generation vaccine preparation (LaAg) composed of a total lysate of L. amazonensis. We demonstrate that TLR9 is important for controlling leishmaniasis infection caused by L. amazonensis and is involved in the efficacy of the LaAg vaccine. These findings will certainly help in the development of a better vaccine against leishmaniasis.

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.

Leishmania amazonensis-Induced cAMP Triggered by Adenosine A2B Receptor Is Important to Inhibit Dendritic Cell Activation and Evade Immune Response in Infected Mice

Differently from others Leishmania species, infection by the protozoan parasite L. amazonensis is associated with a lack of antigen-specific T-cell responses. Dendritic cells (DC) are essential for the innate immune response and for directing the differentiation of T-helper lymphocytes. Previously, we showed that L. amazonensis infection impairs DC activation through the activation of adenosine A_2B receptor, and here, we evaluated the intracellular events triggered by this receptor in infected cells. To this aim, bone marrow-derived DC from C57BL/6J mice were infected with metacyclic promastigotes of L. amazonensis. Our results show, for the first time, that L. amazonensis increases the production of cAMP and the phosphorylation of extracellular signal-regulated protein kinases 1/2 (ERK1/2) in infected DC by a mechanism dependent on the A_2B receptor. Furthermore, L. amazonensis impairs CD40 expression and IL-12 production by DC, and the inhibition of adenylate cyclase, phosphoinositide 3-kinase (PI3K), and ERK1/2 prevent these effects. The increase of ERK1/2 phosphorylation and the inhibition of DC activation by L. amazonensis are independent of protein kinase A (PKA). In addition, C57BL/6J mice were inoculated in the ears with metacyclic promastigotes, in the presence of PSB1115, an A_2B receptor antagonist. PSB1115 treatment increases the percentage of CD40⁺ DC on ears and draining lymph nodes. Furthermore, this treatment reduces lesion size and tissue parasitism. Lymph node cells from treated mice produce higher levels of IFN-γ than control mice, without altering the production of IL-10. In conclusion, we suggest a new pathway used by the parasite (A_2B receptor → cAMP → PI3K → ERK1/2) to suppress DC activation, which may contribute to the decrease of IFN-γ production following by the deficiency in immune response characteristic of L. amazonensis infection.

Retinoic Acid Regulates Immune Responses by Promoting IL-22 and Modulating S100 Proteins in Viral Hepatitis

Although large amounts of vitamin A and its metabolite all-trans retinoic acid (RA) are stored in the liver, how RA regulates liver immune responses during viral infection remains unclear. In this study, we demonstrated that IL-22, mainly produced by hepatic γδ T cells, attenuated liver injury in adenovirus-infected mice. RA can promote γδ T cells to produce mTORC1-dependent IL-22 in the liver, but inhibits IFN-γ and IL-17. RA also affected the aptitude of T cell responses by modulating dendritic cell (DC) migration and costimulatory molecule expression. These results suggested that RA plays an immunomodulatory role in viral infection. Proteomics data revealed that RA downregulated S100 family protein expression in DCs, as well as NF-κB/ERK pathway activation in these cells. Furthermore, adoptive transfer of S100A4-repressed, virus-pulsed DCs into the hind foot of naive mice failed to prime T cell responses in draining lymph nodes. Our study has demonstrated a crucial role for RA in promoting IL-22 production and tempering DC function through downregulating S100 family proteins during viral hepatitis.

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.

Purinergic signaling and infection by Leishmania: A new approach to evasion of the immune response

Infection by protozoan parasites is part of the most common Tropical Neglected Diseases. In the case of leishmaniasis, several millions of people are at risk of contracting the disease. In spite of innumerous studies that elucidated the immune response capable of killing the parasite, the understanding of the evasion mechanisms utilized by the parasite to survive within the very cell responsible for its destruction is still incomplete. In this review, we offer a new approach to the control of the immune response against the parasite. The ability of the parasite to modulate the levels of extracellular ATP and adenosine either by directly acting on the levels of these molecules or by inducing the expression of CD39 and CD73 on the infected cell may influence the magnitude of the immune response against the parasite contributing to its growth and survival.

Leishmania infantum and Leishmania braziliensis: Differences and Similarities to Evade the Innate Immune System

Visceral leishmaniasis is a severe form of the disease, caused by Leishmania infantum in the New World. Patients present an anergic immune response that favors parasite establishment and spreading through tissues like bone marrow and liver. On the other hand, Leishmania braziliensis causes localized cutaneous lesions, which can be self-healing in some individuals. Interactions between host and parasite are essential to understand disease pathogenesis and progression. In this context, dendritic cells (DCs) act as essential bridges that connect innate and adaptive immune responses. In this way, the aim of this study was to compare the effects of these two Leishmania species, in some aspects of human DCs' biology for better understanding of the evasion mechanisms of Leishmania from host innate immune response. To do so, DCs were obtained from monocytes from whole peripheral blood of healthy volunteer donors and from those infected with L. infantum or L. braziliensis for 24 h. We observed similar rates of infection (around 40%) as well as parasite burden for both Leishmania species. Concerning surface molecules, we observed that both parasites induced CD86 expression when DCs were infected for 24 h. On the other hand, we detected a lower surface expression of CD209 in the presence of both L. braziliensis and L. infantum, but only the last one promoted the survival of DCs after 24 h. Therefore, DCs infected by both Leishmania species showed a higher expression of CD86 and a decrease of CD209 expression, suggesting that both enter DCs through CD209 molecule. However, only L. infantum had the ability to inhibit DC apoptotic death, as an evasion mechanism that enables its spreading to organs like bone marrow and liver. Lastly, L. braziliensis was more silent parasite, once it did not inhibit DC apoptosis in our in vitro model.

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.

Ultradeformable Archaeosomes for Needle Free Nanovaccination with Leishmania braziliensis Antigens

Total antigens from Leishmania braziliensis promastigotes, solubilized with sodium cholate (dsLp), were formulated within ultradeformable nanovesicles (dsLp-ultradeformable archaeosomes, (dsLp-UDA), and dsLp-ultradeformable liposomes (dsLp-UDL)) and topically administered to Balb/c mice. Ultradeformable nanovesicles can penetrate the intact stratum corneum up to the viable epidermis, with no aid of classical permeation enhancers that can damage the barrier function of the skin. Briefly, 100 nm unilamellar dsLp-UDA (soybean phosphatidylcholine: Halorubrum tebenquichense total polar lipids (TPL): sodium cholate, 3:3:1 w:w) of -31.45 mV Z potential, containing 4.84 ± 0.53% w/w protein/lipid dsLp, 235 KPa Young modulus were prepared. In vitro, dsLp-UDA was extensively taken up by J774A1 and bone marrow derive cells, and the only that induced an immediate secretion of IL-6, IL-12p40 and TNF-α, followed by IL-1β, by J774A1 cells. Such extensive uptake is a key feature of UDA ascribed to the highly negatively charged archaeolipids of the TPL, which are recognized by a receptor specialized in uptake and not involved in downstream signaling. Despite dsLp alone was also immunostimulatory on J774A1 cells, applied twice a week on consecutive days along 7 weeks on Balb/c mice, it raised no measurable response unless associated to UDL or UDA. The highest systemic response, IgGa2 mediated, 1 log lower than im dsLp Al₂O₃, was elicited by dsLp-UDA. Such findings suggest that in vivo, UDL and UDA acted as penetration enhancers for dsLp, but only dsLp-UDA, owed to its pronounced uptake by APC, succeeded as topical adjuvants. The actual TPL composition, fully made of sn2,3 ether linked saturated archaeolipids, gives the UDA bilayer resistance against chemical, physical and enzymatic attacks that destroy ordinary phospholipids bilayers. Together, these properties make UDA a promising platform for topical drug targeted delivery and vaccination, that may be of help for countries with a deficient healthcare system.

The Mitogen-Activated Protein Kinase (MAPK) Pathway: Role in Immune Evasion by Trypanosomatids

Leishmania spp. and Trypanosoma cruzi are the causative agents of leishmaniasis and Chagas disease, respectively, two neglected tropical diseases that affect about 25 million people worldwide. These parasites belong to the family Trypanosomatidae, and are both obligate intracellular parasites that manipulate host signaling pathways and the innate immune system to establish infection. Mitogen-activated protein kinases (MAPKs) are serine and threonine protein kinases that are highly conserved in eukaryotes, and are involved in signal transduction pathways that modulate physiological and pathophysiological cell responses. This mini-review highlights existing knowledge concerning the mechanisms that Leishmania spp. and T. cruzi have evolved to target the host’s MAPK signaling pathways and highjack the immune response, and, in this manner, promote parasite maintenance in the host.

Encephalitozoon intestinalis Inhibits Dendritic Cell Differentiation through an IL-6-Dependent Mechanism

Microsporidia are a group of intracellular pathogens causing self-limited and severe diseases in immunocompetent and immunocompromised individuals, respectively. A cellular type 1 adaptive response, mediated by IL-12, IFNγ, CD4+, and CD8+ T cells has been shown to be essential for host resistance, and dendritic cells (DC) play a key role at eliciting anti-microsporidial immunity. We investigated the in vitro response of DC and DC precursors/progenitors to infection with Encephalitozoon intestinalis (Ei), a common agent of human microsporidosis. Ei-exposed DC cultures up-regulated the surface expression of MHC class II and the costimulatory molecules CD86 and CD40, only when high loads of spores were used. A vigorous secretion of IL-6 but not of IL-1β or IL-12p70 was also observed in these cultures. Ei-exposed DC cultures consisted of immature infected and mature bystander DC, as assessed by MHC class II and costimulatory molecules expression, suggesting that intracellular Ei spores deliver inhibitory signals in DC. Moreover, Ei selectively inhibited the secretion of IL-12p70 in LPS-stimulated DC. Whereas Ei-exposed DC promoted allogeneic naïve T cell proliferation and IL-2 and IFNγ secretion in DC-CD4+ T cell co-cultures, separated co-cultures with bystander or infected DCs showed stimulation or inhibition of IFNγ secretion, respectively. When DC precursors/progenitors were exposed to Ei spores, a significant inhibition of DC differentiation was observed without shifting the development toward cells phenotypically or functionally compatible with myeloid-derived suppressor cells. Neutralization experiments demonstrated that this inhibitory effect is IL-6-dependent. Altogether this investigation reveals a novel potential mechanism of immune escape of microsporidian parasites through the modulation of DC differentiation and maturation.

The Mitogen-Activated Protein Kinase (MAPK) Pathway: Role in Immune Evasion by Trypanosomatids

Leishmania spp. and Trypanosoma cruzi are the causative agents of leishmaniasis and Chagas disease, respectively, two neglected tropical diseases that affect about 25 million people worldwide. These parasites belong to the family Trypanosomatidae, and are both obligate intracellular parasites that manipulate host signaling pathways and the innate immune system to establish infection. Mitogen-activated protein kinases (MAPKs) are serine and threonine protein kinases that are highly conserved in eukaryotes, and are involved in signal transduction pathways that modulate physiological and pathophysiological cell responses. This mini-review highlights existing knowledge concerning the mechanisms that Leishmania spp. and T. cruzi have evolved to target the host's MAPK signaling pathways and highjack the immune response, and, in this manner, promote parasite maintenance in the host.

Leishmania donovani infection drives the priming of human monocyte-derived dendritic cells during Plasmodium falciparum co-infections

Functional impairment of dendritic cells (DCs) is part of a survival strategy evolved by Leishmania and Plasmodium parasites to evade host immune responses. Here, the effects of co-exposing human monocyte-derived DCs to Leishmania donovani promastigotes and Plasmodium falciparum-infected erythrocytes were investigated. Co-stimulation resulted in a dual, dose-dependent effect on DC differentiation which ranged from semi-mature cells, secreting low interleukin(-12p70 levels to a complete lack of phenotypic maturation in the presence of high parasite amounts. The effect was mainly triggered by the Leishmania parasites, as illustrated by their ability to induce semi-mature, interleukin-10-producing DCs, that poorly responded to lipopolysaccharide stimulation. Conversely, P. falciparum blood-stage forms failed to activate DCs and only slightly interfered with lipopolysaccharide effects. Stimulation with high L. donovani concentrations triggered phosphatidylserine translocation, whose onset presented after initiating the maturation impairment process. When added in combination, the two parasites could co-localize in the same DCs, confirming that the leading effects of Leishmania over Plasmodium may not be due to mutual exclusion. Altogether, these results suggest that in the presence of visceral leishmaniasis-malaria co-infections, Leishmania-driven effects may overrule the more silent response elicited by P. falciparum, shaping host immunity towards a regulatory pattern and possibly delaying disease resolution.

Innate immunity against Leishmania infections

Leishmaniasis is a major health problem that affects more than 300 million people throughout the world. The morbidity associated with the disease causes serious economic burden in Leishmania endemic regions. Despite the morbidity and economic burden associated with Leishmaniasis, this disease rarely gets noticed and is still categorized under neglected tropical diseases. The lack of research combined with the ability of Leishmania to evade immune recognition has rendered our efforts to design therapeutic treatments or vaccines challenging. Herein, we review the literature on Leishmania from innate immune perspective and discuss potential problems as well as solutions and future directions that could aid in identifying novel therapeutic targets to eliminate this parasite.

Interactions between Neutrophils and Leishmania braziliensis Amastigotes Facilitate Cell Activation and Parasite Clearance

Leishmania braziliensis and Leishmania amazonensis are both causative agents of cutaneous leishmaniasis in South America. However, patient prognosis and the host immune response differ considerably depending on the infecting parasite species. The mechanisms underlying these differences appear to be multifactorial, with both host and parasite components contributing to disease outcome. As neutrophils are a prominent component of the inflammatory infiltrate in chronic cutaneous, diffuse cutaneous and mucocutaneous lesions, we examined neutrophil activation and microbicidal activity against amastigotes of L. amazonensis and L. braziliensis. We found that murine neutrophils internalized L. braziliensis amastigotes with greater efficiency than did L. amazonensis amastigotes. Additionally, L. braziliensis infection was a potent trigger for neutrophil activation, oxidative burst, degranulation and the production of interleukin (IL)-22 and IL-10, while L. amazonensis amastigotes poorly induced these responses. Finally, neutrophils were able to kill L. braziliensis amastigotes, especially when cells were activated with phorbol myristate acetate. L. amazonensis amastigotes, however, were highly resistant to neutrophil microbicidal mechanisms. This study reveals, for the first time, differential neutrophil responsiveness to distinct species of Leishmania amastigotes and highlights the complexity of neutrophil-amastigote interactions during chronic leishmaniasis.

Deception and manipulation: the arms of leishmania, a successful parasite

Leishmania spp. are intracellular parasitic protozoa responsible for a group of neglected tropical diseases, endemic in 98 countries around the world, called leishmaniasis. These parasites have a complex digenetic life cycle requiring a susceptible vertebrate host and a permissive insect vector, which allow their transmission. The clinical manifestations associated with leishmaniasis depend on complex interactions between the parasite and the host immune system. Consequently, leishmaniasis can be manifested as a self-healing cutaneous affliction or a visceral pathology, being the last one fatal in 85–90% of untreated cases. As a result of a long host–parasite co-evolutionary process, Leishmania spp. developed different immunomodulatory strategies that are essential for the establishment of infection. Only through deception and manipulation of the immune system, Leishmania spp. can complete its life cycle and survive. The understanding of the mechanisms associated with immune evasion and disease progression is essential for the development of novel therapies and vaccine approaches. Here, we revise how the parasite manipulates cell death and immune responses to survive and thrive in the shadow of the immune system.

Impact of Leishmania mexicana infection on dendritic cell signaling and functions

Leishmania parasites have the ability to modify macrophage signaling pathways in order to survive and multiply within its mammalian host. They are also known to invade other cells including neutrophils, fibroblasts and dendritic cells (DCs). DCs have an important role in immunity as the link between innate and adaptive immunity, necessary for the development of an effective response; however, the impact of Leishmania mexicana infection on DCs has been poorly studied. Herein, we report that Leishmania infection rapidly induced DC protein tyrosine phosphatases activity, leading to MAP kinases inactivation. In line with this, L. mexicana was found to decrease the nuclear translocation of transcription factors such as AP-1 and NF-κB. Concomitantly, L. mexicana-infected DCs showed reduced expression of several surface antigen-presenting and co-stimulatory molecules upon LPS stimulation. Leishmania-induced interference on DC maturation was further reflected by their reduced capacity to present OVA antigen to OVA-specific T cells, as shown by abrogation of IL-2 production by the T cells. Collectively, our data revealed that DC infection by L. mexicana appears to affect the cellular and immunological mechanisms necessary for the development of an effective and protective immune response, therefore favouring the survival and propagation of the parasite within its host.

Parasites of the Leishmania genus have developed many strategies to survive inside their host. Initially, they were only considered capable of infecting macrophages; however, it has been observed that Leishmania is able to infect other cell types, such as fibroblast, neutrophils and dendritic cells (DCs). DCs are well known for their antigen-presentation capabilities, and they are considered as the fundamental bridge between the innate and adaptive immune responses. In this study, we attempted to elucidate the effect of L. mexicana promastigotes on DCs. Our results showed that L. mexicana inactivates signaling cascades responsible for the expression of immune effector molecules, such as cytokines, concomitantly with the activation of protein phosphatases in the host. Furthermore, we observed that promastigote-infected cells had lower expression of MHC and co-stimulatory molecules on their surface, as well as decreased antigen-presentation capacity. In conclusion, our study showed that Leishmania parasites are able to inactivate the immunological mechanisms of DCs, as they do in macrophages, in order to survive inside its host.

Leishmania molecules that mediate intracellular pathogenesis

Parasites of the Leishmania genus are the causative agents of a complex disease called leishmaniasis. Many activities of infected cells including their responses to a range of stimuli are modulated by Leishmania parasites. This review will profile some of the parasite molecules that target host cell processes for which there has been recent progress.

Immune responses during cutaneous and visceral leishmaniasis

SUMMARY Leishmania are protozoan parasites spread by a sandfly insect vector and causing a spectrum of diseases collectively known as leishmaniasis. The disease is a significant health problem in many parts of the world, resulting in an estimated 1·3 million new cases and 30 000 deaths annually. Current treatment is based on chemotherapy, which is difficult to administer, expensive and becoming ineffective in several endemic regions. To date there is no vaccine against leishmaniasis, although extensive evidence from studies in animal models indicates that solid protection can be achieved upon immunization. This review focuses on immune responses to Leishmania in both cutaneous and visceral forms of the disease, pointing to the complexity of the immune response and to a range of evasive mechanisms utilized by the parasite to bypass those responses. The amalgam of innate and acquired immunity combined with the paucity of data on the human immune response is one of the major problems currently hampering vaccine development and implementation.

Leishmania amazonensis amastigotes trigger neutrophil activation but resist neutrophil microbicidal mechanisms

Neutrophils are the first cells to infiltrate to the site of Leishmania promastigote infection, and these cells help to reduce parasite burden shortly after infection is initiated. Several clinical reports indicate that neutrophil recruitment is sustained over the course of leishmaniasis, and amastigote-laden neutrophils have been isolated from chronically infected patients and experimentally infected animals. The goal of this study was to compare how thioglycolate-elicited murine neutrophils respond to L. amazonensis metacyclic promastigotes and amastigotes derived from axenic cultures or from the lesions of infected mice. Neutrophils efficiently internalized both amastigote and promastigote forms of the parasite, and phagocytosis was enhanced in lipopolysaccharide (LPS)-activated neutrophils or when parasites were opsonized in serum from infected mice. Parasite uptake resulted in neutrophil activation, oxidative burst, and accelerated neutrophil death. While promastigotes triggered the release of tumor necrosis factor alpha (TNF-α), uptake of amastigotes preferentially resulted in the secretion of interleukin-10 (IL-10) from neutrophils. Finally, the majority of promastigotes were killed by neutrophils, while axenic culture- and lesion-derived amastigotes were highly resistant to neutrophil microbicidal mechanisms. This study indicates that neutrophils exhibit distinct responses to promastigote and amastigote infection. Our findings have important implications for determining the impact of sustained neutrophil recruitment and amastigote-neutrophil interactions during the late phase of cutaneous leishmaniasis.

Phosphatidylserine exposure on the surface of Leishmania amazonensis amastigotes modulates in vivo infection and dendritic cell function

Leishmania amazonensis parasites can cause diverse forms of leishmaniasis in humans and persistent lesions in most inbred strains of mice. In both cases, the infection is characterized by a marked immunosuppression of the host. We previously showed that amastigote forms of the parasite make use of surface-exposed phosphatidylserine (PS) molecules to infect host cells and promote alternative macrophage activation, leading to uncontrolled intracellular proliferation of the parasites. In this study, we demonstrated that treatment of infected mice with a PS-targeting monoclonal antibody ameliorated parasite loads and lesion development, which correlated with increased proliferative responses by lymphocytes. In addition, we observed an enhanced dendritic cell (DC) activation and antigen presentation in vitro. Our data imply that the recognition of PS exposed on the surface of amastigotes plays a role in down-modulating DC functions, in a matter similar to that of apoptotic cell clearance. This study provides new information regarding the mechanism of immune suppression in Leishmania infection.

Role of Toll-like receptor 9 signaling in experimental Leishmania braziliensis infection

Infection with Leishmania braziliensis causes cutaneous or mucocutaneous leishmaniasis in humans. Toll-like receptor 9 (TLR9) expression has been found in granulomas of lesions in L. braziliensis-infected individuals. L. braziliensis inoculation in mice induces very small lesions that are self-healing, whereas deficiency in the TLR adaptor molecule, MyD88, renders mice susceptible to infection. The TLR involved has not been identified, prompting us to investigate if TLR9 triggering by the parasite contributes to the strong resistance to infection observed in L. braziliensis-inoculated mice. The parasites activated wild-type (WT) dendritic cells (DCs) in vitro but not DCs derived from TLR9(-/-) mice. TLR9(-/-) mice inoculated with L. braziliensis exhibited a transient susceptibility characterized by increased lesion size and parasite burden compared to those of WT mice. Surprisingly, elevated levels of gamma interferon (IFN-γ) were measured at the site of infection and in draining lymph node T cells of TLR9(-/-) mice at the peak of susceptibility, suggesting that unlike observations in vitro, the parasite could induce DC activation leading to the development of Th1 cells in the absence of TLR9 expression. Taken together, these data show that TLR9 signaling is important for the early control of lesion development and parasite burden but is dispensable for the differentiation of Th1 cells secreting IFN-γ, and the high levels of this cytokine are not sufficient to control early parasite replication following L. braziliensis infection.

Leishmania (V.) braziliensis and L. (L.) amazonensis promote differential expression of dendritic cells and cellular immune response in murine model

The expression of Langerhans cell (LC) and dermal dendritic cell (dDC) as well as T CD4(+) and CD8(+) immune responses was evaluated in the skin of BALB/c mice experimentally infected by L. (L.) amazonensis (La) and L. (V.) braziliensis (Lb). At 4th and 8th weeks post infection (PI), skin biopsies were collected to determine the parasite load and CD207(+), CD11c(+), CD4(+), CD8(+), iNOS(+) cellular densities. Cytokine (IFN-γ, IL-4 and IL-10) profiles were also analysed in draining lymph node. At 4th week, the densities of CD207(+) and CD11c(+) were higher in the La infection, while in the Lb infection, these markers revealed a significant increase at 8th week. At 4th week, CD4(+) and CD8(+) were higher in the La infection, but at 8th week, there was a substantial increase in both markers in the Lb infection. iNOS(+) was higher in the Lb infection at 4th and 8th weeks. In contrast, the parasite load was higher in the La infection at 4th and 8th weeks. The concentration of IFN-γ was higher in the Lb infection, but IL-4 and IL-10 were higher in the La infection at 4th and 8th weeks. These results confirm the role of the Leishmania species in the BALB/c mice disease characterized by differences in the expression of dendritic cells and cellular immune response.

Leishmania amazonensis impairs DC function by inhibiting CD40 expression via A2B adenosine receptor activation

Dendritic cells (DCs) play an essential role in the modulation of immune responses and several studies have evaluated the interactions between Leishmania parasites and DCs. While extracellular ATP exhibits proinflammatory properties, adenosine is an important anti-inflammatory mediator. Here we investigated the effects of Leishmania infection on DC responses and the participation of purinergic signalling in this process. Bone marrow-derived dendritic cells (BMDCs) from C57BL/6J mice infected with Leishmania amazonensis, Leishmania braziliensis or Leishmania major metacyclic promastigotes showed decreased major histocompatibility complex (MHC) class II and CD86 expression and increased ectonucleotidase expression as compared with uninfected cells. In addition, L. amazonensis-infected DCs, which had lower CD40 expression, exhibited a decreased ability to induce T-cell proliferation. The presence of MRS1754, a highly selective A(2B) adenosine receptor antagonist at the time of infection increased MHC class II, CD86 and CD40 expression in L. amazonensis-infected DCs and restored the ability of the infected DCs to induce T-cell proliferation. Similar results were obtained through the inhibition of extracellular ATP hydrolysis using suramin. In conclusion, we propose that A(2B) receptor activation may be used by L. amazonensis to inhibit DC function and evade the immune response.

The early interaction of Leishmania with macrophages and dendritic cells and its influence on the host immune response

The complicated interactions between Leishmania and the host antigen-presenting cells (APCs) have fundamental effects on the final outcome of the disease. Two major APCs, macrophages and dendritic cells (DCs), play critical roles in mediating resistance and susceptibility during Leishmania infection. Macrophages are the primary resident cell for Leishmania: they phagocytose and permit parasite proliferation. However, these cells are also the major effector cells to eliminate infection. The effective clearance of parasites by macrophages depends on activation of appropriate immune response, which is usually initiated by DCs. Here, we review the early interaction of APCs with Leishmania parasites and how these interactions profoundly impact on the ensuing adaptive immune response. We also discuss how the current knowledge will allow further refinement of our understanding of the interplay between Leishmania and its hosts that leads to resistance or susceptibility.

Exposure of phosphatidylserine on Leishmania amazonensis isolates is associated with diffuse cutaneous leishmaniasis and parasite infectivity

Diffuse cutaneous leishmaniasis (DCL) is a rare clinical manifestation of leishmaniasis, characterized by an inefficient parasite-specific cellular response and heavily parasitized macrophages. In Brazil, Leishmania (Leishmania) amazonensis is the main species involved in DCL cases. In the experimental model, recognition of phosphatidylserine (PS) molecules exposed on the surface of amastigotes forms of L. amazonensis inhibits the inflammatory response of infected macrophages as a strategy to evade the host immune surveillance. In this study, we examined whether PS exposure on L. amazonensis isolates from DCL patients operated as a parasite pathogenic factor and as a putative suppression mechanism of immune response during the infection. Peritoneal macrophages from F1 mice (BALB/c×C57BL/6) were infected with different L. amazonensis isolates from patients with localized cutaneous leishmaniasis (LCL) or DCL. DCL isolates showed higher PS exposure than their counterparts from LCL patients. In addition, PS exposure was positively correlated with clinical parameters of the human infection (number of lesions and time of disease) and with characteristics of the experimental infection (macrophage infection and anti-inflammatory cytokine induction). Furthermore, parasites isolated from DCL patients displayed an increased area in parasitophorous vacuoles (PV) when compared to those isolated from LCL patients. Thus, this study shows for the first time that a parasite factor (exposed PS) might be associated with parasite survival/persistence in macrophages and lesion exacerbation during the course of DCL, providing new insights regarding pathogenic mechanism in this rare chronic disease.

Subversion of Immunity by Leishmania amazonensis Parasites: Possible Role of Phosphatidylserine as a Main Regulator

Leishmania amazonensis parasites cause progressive disease in most inbred mouse strains and are associated with the development of diffuse cutaneous leishmaniasis in humans. The poor activation of an effective cellular response is correlated with the ability of these parasites to infect mononuclear phagocytic cells without triggering their activation or actively suppressing innate responses of these cells. Here we discuss the possible role of phosphatidylserine exposure by these parasites as a main regulator of the mechanism underlying subversion of the immune system at different steps during the infection.

Mechanisms underlying the induction of regulatory T cells and its relevance in the adaptive immune response in parasitic infections

To fulfill its function, the immune system must detect and interpret a wide variety of signals and adjust the magnitude, duration, and specific traits of each response during the complex host-parasite relationships in parasitic infections. Inflammation must be tightly regulated since uncontrolled inflammation may be as destructive as the triggering stimulus and leads to immune-mediated tissue injury.

During recent years, increasing evidence points to regulatory T cells (Tregs) as key anti-inflammatory cells, critically involved in limiting the inflammatory response. Herein, we review the published information on the induction of Tregs and summarize the most recent findings on Treg generation in parasitic diseases.

Leishmania promotes its own virulence by inducing expression of the host immune inhibitory ligand CD200

Leishmania parasites infect macrophages, cells normally involved in innate defense against pathogens. Leishmania amazonensis and Leishmania major cause severe or mild disease, respectively, consistent with each parasite's ability to survive within activated macrophages. The mechanisms underlying increased virulence of L. amazonensis are mostly unknown. We show that L. amazonensis promotes its own survival by inducing expression of CD200, an immunoregulatory molecule that inhibits macrophage activation. L. amazonensis does not form typical nonhealing lesions in CD200(-/-) mice and cannot replicate in CD200(-/-) macrophages, an effect reversed by exogenous administration of soluble CD200-Fc. The less virulent L. major does not induce CD200 expression and forms small, self-healing lesions in both wild-type and CD200(-/-) mice. Notably, CD200-Fc injection transforms the course of L. major infection to one resembling L. amazonensis, with large, nonhealing lesions. CD200-dependent iNOS inhibition allows parasite growth in macrophages, identifying a mechanism for the increased virulence of L. amazonensis.

Leishmania mexicana promastigotes inhibit macrophage IL-12 production via TLR-4 dependent COX-2, iNOS and arginase-1 expression

The effects of Leishmania mexicana metacyclic promastigotes upon MAP kinase signalling in mouse bone marrow macrophages and subsequent expression of the disease regulatory proteins iNOS and COX-2 were studied. At a ratio of 5:1, promastigotes caused a marked increase in phosphorylation of the three major MAP kinases, ERK, p38 and JNK. MAP kinase signalling was substantially reduced in TLR-4^−/− but not TLR-2^−/− deficient macrophages and completely abolished in double TLR-2/4^−/− macrophages. A similar outcome was observed using cysteine peptidase B deficient amastigotes. Furthermore, whilst promastigotes had no independent effect on iNOS or COX-2 expression, they prolonged the induction of these proteins stimulated by LPS and enhanced PGE₂ and NO production. Induction of COX-2 and iNOS was also TLR-4 dependent. Blockade of either PGE₂ or NO production with indomethacin or l-NAME reversed promastigote inhibition of LPS induced IL-12 production. Promastigotes also increased macrophage arginase-1 expression and enhanced arginase activity, both of which were substantially reduced in TLR-4 but not TLR-2 deficient macrophages. Surprisingly, arginase inhibition by Nor-NOHA also caused a reversal of promastigote mediated inhibition of macrophage IL-12 production. These data demonstrate for the first time the role of TLR-4 in mediating the effects of L. mexicana promastigotes on MAP kinase activation, up-regulation of COX-2, iNOS as well as arginase-1 expression in macrophages and further shows that PGE₂, NO and arginase activity all contribute substantially to the inhibition of host cell IL-12 production.

Interferon regulatory factor 7 contributes to the control of Leishmania donovani in the mouse liver

Optimal hepatic resistance to Leishmania donovani in mice requires the coordinated effort of a variety of leukocyte populations that together induce activation of local macrophages to a leishmanicidal state. Although nitric oxide and reactive oxygen intermediates are potent leishmanicidal effector molecules operating in the acquired phase of immunity, there have long been suggestions that other mechanisms of leishmanicidal activity exist. We recently discovered that Irf-7 regulates a novel innate leishmanicidal response in resident splenic macrophages that line the marginal zone. Here, we tested whether this mechanism also operates in Kupffer cells, the resident macrophage population of the liver and the major target for hepatic infection by L. donovani. Comparing the Kupffer cell responses in situ in B6 and B6.Irf-7(-/-) mice, we found no evidence that Irf-7 affected amastigote uptake or early survival. However, we did find that Irf-7-deficient mice had impaired acquired resistance to hepatic L. donovani infection. This phenotype was attributable to a reduction in the capacity of hepatic CD4(+) T cells, NK cells, and NKT cells to produce gamma interferon (IFN-γ) and also to defective induction of NOS2 in infected Kupffer cells. Our data therefore add interferon regulatory factor 7 (IRF-7) to the growing list of interferon regulatory factors that have effects on downstream events in the acquired cellular immune response to nonviral pathogens.

Activation of phosphatidylinositol 3-kinase/Akt and impairment of nuclear factor-kappaB: molecular mechanisms behind the arrested maturation/activation state of Leishmania infantum-infected dendritic cells

Understanding the complex interactions between Leishmania and dendritic cells (DCs) is central to the modulation of the outcome of this infection, given that an effective immune response against Leishmania is dependent on the successful activation and maturation of DCs. We report here that Leishmania infantum promastigotes successfully infect mouse bone marrow-derived DCs without triggering maturation, as shown by a failure in the up-regulation of CD40 and CD86 expression, and that parasites strongly counteract the lipopolysaccharide-triggered maturation of DCs. A small increase in interleukin (IL)-12 and IL-10 transcription and secretion and a decrease in IL-6 were observed in infected cells. This arrested DC maturation state is actively promoted by parasites because heat-killed or fixed parasites increased cytokine and costimulatory molecule expression. At a molecular level, L. infantum rapidly induced activation of phosphatidylinositol 3-kinase/Akt and extracellular signal-regulated kinase 1/2, whereas no effect was observed in the c-Jun N-terminal kinase and p38 mitogen-activated protein kinase proinflammatory pathways. Moreover, parasites actively promoted cleavage of the nuclear factor-κB p65(RelA) subunit, causing its impairment. The blockade of phosphatidylinositol 3-kinase/Akt by either treatment of bone marrow-derived DCs with wortmannin or transfection with an Akt dominant-negative mutant resulted in a strong decrease in infection rates, revealing for the first time a crucial role of this pathway on Leishmania engulfment by DCs. Overall, our data indicate that activation of Akt and impairment of nuclear factor-κB are responsible for immunogenicity subversion of L. infantum-infected DCs.

Regulation of the expression of nitric oxide synthase by Leishmania mexicana amastigotes in murine dendritic cells

In mammalian hosts, Leishmania parasites are obligatory intracellular organisms that invade macrophages (M phi) and dendritic cells (DC). In M phi, the production of nitric oxide (NO) catalyzed by the inducible nitric oxide synthase (iNOS) has been implicated as a major defense against Leishmania infection. The modulation of this microbicidal mechanism by different species of Leishmania has been well studied in M phi. Although DC are permissive for infection with Leishmania both in vivo and in vitro, the effect of this parasite in the expression of iNOS and NO production in these cells has not been established. To address this issue, we analyzed the regulation of iNOS by Leishmania mexicana amastigotes in murine bone marrow-derived dendritic cells (BMDC) stimulated with LPS and IFN-gamma. We show that the infection of BMDC with amastigotes down regulated NO production and diminished iNOS protein levels in cells stimulated with LPS alone or in combination with IFN-gamma. The reduction in iNOS protein levels and NO production did not correlate with a decrease in iNOS mRNA expression, suggesting that the parasite affects post-transcriptional events of NO synthesis. Although amastigotes were able to reduce NO production in BMDC, the interference with this cytotoxic mechanism was not sufficient to permit the survival of L. mexicana. At 48 h post-infection, BMDC stimulated with LPS+IFN-gamma were able to eliminate the parasites. These results are the first to identify the regulation of iNOS by L. mexicana amastigotes in DC.