Evaluation of nitric oxide production by Leishmania infantum-infected dog macrophages

MicroRNA-194 regulates parasitic load and IL-1β-dependent nitric oxide production in the peripheral blood mononuclear cells of dogs with leishmaniasis

Domestic dogs are the primary urban reservoirs of Leishmania infantum, the causative agent of visceral leishmaniasis. In Canine Leishmaniasis (CanL), modulation of the host's immune response may be associated with the expression of small non-coding RNAs called microRNA (miR). miR-194 expression increases in peripheral blood mononuclear cells (PBMCs) of dogs with leishmaniasis with a positive correlation with the parasite load and in silico analysis demonstrated that the TRAF6 gene is the target of miR-194 in PBMCs from diseased dogs. Here, we isolated PBMCs from 5 healthy dogs and 28 dogs with leishmaniasis, naturally infected with L. infantum. To confirm changes in miR-194 and TRAF6 expression, basal expression of miR-194 and gene expression of TRAF6 was measured using qPCR. PBMCs from healthy dogs and dogs with leishmaniasis were transfected with miR-194 scramble, mimic, and inhibitor and cultured at 37° C, 5% CO2 for 48 hours. The expression of possible targets was measured: iNOS, NO, T-bet, GATA3, and FoxP3 were measured using flow cytometry; the production of cytokines IL-1β, IL-4, IL-6, IL-10, TNF-α, IFN-γ, and TGF-β in cell culture supernatants was measured using capture enzyme-linked immunosorbent assays (ELISA). Parasite load was measured using cytometry and qPCR. Functional assays followed by miR-194 inhibitor and IL-1β blockade and assessment of NO production were also performed. Basal miR-194 expression was increased in PBMC from dogs with Leishmaniasis and was negatively correlated with TRAF6 expression. The mimic of miR-194 promoted an increase in parasite load. There were no significant changes in T-bet, GATA3, or FoxP3 expression with miR-194 enhancement or inhibition. Inhibition of miR-194 increased IL-1β and NO in PBMCs from diseased dogs, and blockade of IL-1β following miR-194 inhibition decreased NO levels. These findings suggest that miR-194 is upregulated in PBMCs from dogs with leishmaniasis and increases parasite load, possibly decreasing NO production via IL-1β. These results increase our understanding of the mechanisms of evasion of the immune response by the parasite and the identification of possible therapeutic targets.

CD4+ T-lymphocytes from asymptomatic dogs infected with Leishmania infantum are able to activate macrophages for higher leishmanicidal ability in an in vitro co-culture experiment

Dogs are the most common domestic reservoir of Leishmania infantum, making canine visceral leishmaniasis (CVL) a serious public health issue. Identifying new methodologies that can mimic lymphoid and myeloid competence in naturally infected dogs could lower costs and save time in preliminary screenings of potential immunotherapeutic agents and vaccines against CVL. For that, we established a cell-to-cell communication approach between lymphocytes and myeloid cells from healthy, asymptomatic (infected, without apparent clinical signs) and symptomatic (infected with apparent clinical signs) dogs. Peripheral blood mononuclear cells (PBMC) from these dogs were used as source of CD4⁺, CD8⁺ T lymphocytes and macrophages, that were posteriorly infected with L. infantum GFP⁺ promastigotes (green fluorescent protein). Macrophages co-cultured with purified lymphocytes were tested for the ability to control cellular parasitism, and their microbicidal function by producing nitric oxide (NO) and reactive oxygen species (ROS). The kind of T cell response within the co-culture was also evaluated, by assessing their ability to produce interferon-gamma (IFN-γ) and interleukin 4 (IL-4). The data suggests that T lymphocytes from symptomatic dogs are more prone to produce IL-4 than the ones from asymptomatic dogs. Macrophages from asymptomatic dogs also demonstrated a higher microbicidal potential, with increased levels of NO and ROS production, compared to symptomatic dogs, mainly in highly parasitized cells. Together, our results identify the ratio of IL-4/IFN-γ produced by CD4⁺ and CD8⁺ T cells, as well as, the ratio between parasite GFP signal/NO and ROS signal in macrophages as potential immunological biomarkers of failure and success of the screened agents. Our findings also propose a reliable methodology that can be used to follow the immune response in trials of potential drugs or vaccines targeting CVL.

Biomarkers Associated With Leishmania infantum Exposure, Infection, and Disease in Dogs

Canine leishmaniosis (CanL) is a vector-borne disease caused by the protozoan Leishmania (Leishmania) infantum species [syn. L. (L.) infantum chagasi species in the Americas] which is transmitted by the bite of a female phlebotomine sand fly. This parasitosis is endemic and affect millions of dogs in Asia, the Americas and the Mediterranean basin. Domestic dogs are the main hosts and the main reservoir hosts for human zoonotic leishmaniosis. The outcome of infection is a consequence of intricate interactions between the protozoan and the immunological and genetic background of the host. Clinical manifestations can range from subclinical infection to very severe disease. Early detection of infected dogs, their close surveillance and treatment are essential to control the dissemination of the parasite among other dogs, being also a pivotal element for the control of human zoonotic leishmaniosis. Hence, the identification of biomarkers for the confirmation of Leishmania infection, disease and determination of an appropriate treatment would represent an important tool to assist clinicians in diagnosis, monitoring and in giving a realistic prognosis to subclinical infected and sick dogs. Here, we review the recent advances in the identification of Leishmania infantum biomarkers, focusing on those related to parasite exposure, susceptibility to infection and disease development. Markers related to the pathogenesis of the disease and to monitoring the evolution of leishmaniosis and treatment outcome are also summarized. Data emphasizes the complexity of parasite-host interactions and that a single biomarker cannot be used alone for CanL diagnosis or prognosis. Nevertheless, results are encouraging and future research to explore the potential clinical application of biomarkers is warranted.

Analysis using canine peripheral blood for establishing in vitro conditions for monocyte differentiation into macrophages for Leishmania chagasi infection and T-cell subset purification

Canine visceral leishmaniasis (CVL) is a parasitic disease endemic in many countries, and dogs present as the major natural reservoir of the parasite, Leishmania chagasi (syn. L. infantum). Biomarkers in the canine immune system is an important technique in the course of developing vaccines and treatment strategies against CVL. New methodologies for studying the immune response of dogs during Leishmania infection and after receiving vaccines and treatments against CVL would be useful. In this context, we used peripheral blood mononuclear cells (PBMCs) from healthy dogs to evaluate procedures related to (i) establishment of in vitro conditions of monocytes differentiated into macrophages infected with L. chagasi and (ii) purification procedures of T-cell subsets (CD4⁺ and CD8⁺) using microbeads. Our data demonstrated that after 5 days of differentiation, macrophages were able to induce significant phagocytic and microbicidal activity after L. chagasi infection and also showed increased frequency of parasitism and a higher parasite load. Although N-acetyl-β-d-glucosaminidase (NAG) levels presented similar levels of macrophage culture and L. chagasi infection, a progressive decrease in myeloperoxidase (MPO) levels was a hallmark over 5 days of culture. High purity levels (>90%) of CD4 and CD8 T cells were obtained on a magnetic separation column. We concluded that monocytes differentiated into macrophages at 5 days and displayed an intermediate frequency of parasitism and parasite load 72 h after L. chagasi infection. Furthermore, the purification system using canine T-lymphocyte subsets obtained after 5 days of monocyte differentiation proved efficient for CD4 or CD8 T-cell purification (≥90%). The in vitro analysis using L. chagasi-infected macrophages and purified T cells presented a prospective methodology that could be incorporated in CVL vaccine and treatment studies that aim to analyze the microbicidal potential induced by specific CD4⁺ and/or CD8⁺ T cells.

Entry and elimination of marine mammal Brucella spp. by hooded seal (Cystophora cristata) alveolar macrophages in vitro

A high prevalence of Brucellapinnipedialis serology and bacteriology positive animals has been found in the Northeast Atlantic stock of hooded seal (Cystophoracristata); however no associated gross pathological changes have been identified. Marine mammal brucellae have previously displayed different infection patterns in human and murine macrophages. To investigate if marine mammal Brucella spp. are able to invade and multiply in cells originating from a presumed host species, we infected alveolar macrophages from hooded seal with a B. pinnipedialis hooded seal isolate. Hooded seal alveolar macrophages were also challenged with B. pinnipedialis reference strain (NCTC 12890) from harbor seal (Phocavitulina), B. ceti reference strain (NCTC 12891) from harbor porpoise (Phocoenaphocoena) and a B. ceti Atlantic white-sided dolphin (Lagenorhynchusacutus) isolate (M83/07/1), to evaluate possible species-specific differences. Brucella suis 1330 was included as a positive control. Alveolar macrophages were obtained by post mortem bronchoalveolar lavage of euthanized hooded seals. Phenotyping of cells in the lavage fluid was executed by flow cytometry using the surface markers CD14 and CD18. Cultured lavage cells were identified as alveolar macrophages based on morphology, expression of surface markers and phagocytic ability. Alveolar macrophages were challenged with Brucella spp. in a gentamicin protection assay. Following infection, cell lysates from different time points were plated and evaluated quantitatively for colony forming units. Intracellular presence of B. pinnipedialis hooded seal isolate was verified by immunocytochemistry. Our results show that the marine mammal brucellae were able to enter hooded seal alveolar macrophages; however, they did not multiply intracellularly and were eliminated within 48 hours, to the contrary of B. suis that showed the classical pattern of a pathogenic strain. In conclusion, none of the four marine mammal strains tested were able to establish a persistent infection in primary alveolar macrophages from hooded seal.

Canine leishmaniasis in Southern Italy: a role for nitric oxide released from activated macrophages in asymptomatic infection?

Background

Human and canine leishmaniasis (CanL) by Leishmania infantum is endemic in Italy, with a high percentage of infected asymptomatic animals. However, the immune response mechanisms underlying the clinical presentation of CanL have not been fully investigated. Among leishmanicidal molecules produced by activated macrophages, nitric oxide (NO) produced by an inducible NO synthase seems to play an important protective role, but no conclusive data are available. Therefore, NO released by cultured macrophages from dogs with natural Leishmania infection living in an endemic area for CanL was evaluated.

Methods

On the basis of one year's clinical and laboratory follow-up, 22 dogs infected by Leishmania infantum were identified and grouped as: asymptomatic dogs (n = 13) and dogs with symptoms of leishmaniasis (n = 9). Each animal was bled twice at 4-month intervals and macrophage and lymphocyte cultures were obtained from peripheral blood mononuclear cells. Supernatants of L. infantum-infected macrophage cultures, with or without addition of autologous lymphocytes, were assayed for NO production by Griess reaction for nitrites.

Results

In the first months of the infection the levels of NO in supernatants of Leishmania-infected macrophages were higher in symptomatic than in asymptomatic dogs, but they were significantly increased in the latter group eight months after the diagnosis of infection. Furthermore, NO release significantly decreased in the presence of autologous lymphocytes in both groups of animals.

Conclusion

These results suggest that NO may be involved in the long-term protection of dogs against natural Leishmania infection and in the clinical presentation of canine leishmaniasis in the Mediterranean area.

Peripheral blood mononuclear cell supernatants from asymptomatic dogs immunized and experimentally challenged with Leishmania chagasi can stimulate canine macrophages to reduce infection in vitro

Leishmania chagasi is the causative agent of visceral leishmaniasis in both humans and dogs in the New World. The dog is the main domestic reservoir and its infection displays different clinical presentations, from asymptomatic to severe disease. Macrophages play an important role in the control of Leishmania infection. Although it is not an area of intense study, some data suggest a role for canine macrophages in parasite killing by a NO-dependent mechanism. It has been proposed that control of human disease could be possible with the development of an effective vaccine against canine visceral leishmaniasis. Development of a rapid in vitro test to predict animal responses to Leishmania infection or vaccination should be helpful. In this study, an in vitro model was established to test whether peripheral blood mononuclear cell (PBMC) supernatants from dogs immunized with promastigote lysates and infected with L. chagasi promastigotes could stimulate macrophages from healthy dogs in order to control parasite infection. PBMC from a majority of the immunized and experimentally infected dogs expressed IFN-gamma mRNA and secreted IFN-gamma when stimulated with soluble L. chagasi antigen (SLA) in vitro. Additionally, the supernatants from stimulated PBMC were able to reduce the percentage of infected donor macrophages. The results also indicate that parasite killing in this system is dependent on NO, since aminoguanidine (AMG) reversed this effect. This in vitro test appears to be useful for screening animal responses to parasite inoculation as well as studying the lymphocyte effector mechanisms involved in pathogen killing by canine macrophages.

Phenotypic, functional, and quantitative characterization of canine peripheral blood monocyte-derived macrophages

The yield as well as phenotypic and functional parameters of canine peripheral blood monocyte-derived macrophages were analyzed. The cells that remained adherent to Teflon after 10 days of culture had high phagocytic activity when inoculated with Leishmania chagasi. Flow cytometric analysis demonstrated that more than 80% of cultured cells were positive for the monocyte/macrophage marker CD14.

Lymphocytes of dogs immunised with purified excreted-secreted antigens of Leishmania infantum co-incubated with Leishmania infected macrophages produce IFN gamma resulting in nitric oxide-mediated amastigote apoptosis

The role of nitric oxide (NO) in the anti-leishmanial activity has been confirmed both in vitro and in vivo. Recently, we demonstrated that NO-mediated apoptosis-like amastigote death pathway is an important and highly regulated mechanism used for the clearance of Leishmania within infected murine macrophages stimulated to produce NO endogenously. To further characterize these important effector mechanisms in dog, a natural host-reservoir of L. infantum/L. chagasi, we have developed an ex vivo infection model of canine macrophages. Exposure of L. infantum-infected macrophages to autologous peripheral lymphocytes derived from dogs immunised with purified excreted-secreted antigens of L. infantum promastigotes (LiESAp) formulated with muramyl dipeptide (MDP) as adjuvant resulted in a significant leishmanicidal effect due to interferon (IFN)-gamma dependent macrophage activation. Concomitant accumulation of NO(3)(-)/NO(2)(-) in supernatants of co-cultured cells and in situ staining of parasites with terminal deoxynucleotidyltransferase-mediated dUTP nick-end labelling (TUNEL) and YOPRO-1 showed that NO-mediated apoptosis of intracellular L. infantum amastigotes is occurring in canine macrophages as previously observed in mouse models. Monitoring these parameters in dogs after immunisation and before experimental challenge can represent a useful and easy way to rapidly evaluate vaccine candidates against canine visceral leishmaniasis.

Inducible nitric oxide synthase expression in Leishmania-infected dog macrophages

Nitric oxide (NO) production by the inducible NO synthase (iNOS or NOS2) represents one of the main microbicidal mechanisms of murine macrophages, but its role in other animal models is poorly investigated. Therefore, the aim of this work was to evaluate NOS2 expression in dog macrophages infected with Leishmania infantum. Macrophages obtained from peripheral blood of healthy dogs were activated with recombinant human interferon (rhIFN)-gamma and bacterial lipopolysaccharide (LPS) and then infected with L. infantum promastigotes. zymodeme MONI. For the immunofluorescence assay fixed macrophages were incubated with polyclonal rabbit anti-NOS2 and then with rhodamine F(ab')2 goat anti-rabbit IgG. For immunoblotting, cell lysates were submitted to SDS-PAGE and blots were incubated with polyclonal rabbit anti-NOS2 and then with horseradish peroxidase-conjugated goat anti-rabbit IgG. Results demonstrated that L. infantum-infected cells, after stimulation with rhIFN-gamma and LPS, displayed high levels of fluorescence for the NOS2 in their cytoplasm, unlike unstimulated uninfected macrophages. In western blotting, polyclonal anti-NOS2 reacted specifically with a protein band corresponding to 130 kDa. The signal produced in Leishmania-infected cells stimulated with rhIFN-gamma and LPS was higher than that produced in Leishmania-infected unstimulated cells. No band was detected in cellular lysates from uninfected unstimulated cells. These results indicate that dog macrophages can express NOS2, and suggest a role for IFN-gamma and LPS in NOS2 induction also in this animal model.

Nitric oxide production by macrophages of dogs vaccinated with killed Leishmania infantum promastigotes

Human visceral leishmaniosis is endemic in Southern Italy, where the dog is the main reservoir of viscerotropic strains of Leishmania infantum. The release of nitric oxide (NO) by interferon (IFN)-gamma-activated macrophages is an important leishmanicidal mechanism in several animal species. In this work NO production, phagocytosis and killing capacity of monocyte-derived dog macrophages were evaluated in vitro before and after administration of a vaccine composed of killed Leishmania infantum promastigotes. Moreover, IFN-gamma content was measured in concanavalin A-activated dog peripheral blood mononuclear cell (PBMC) supernatants employed for macrophage stimulation. Phagocytosis, killing capacity and NO production by canine macrophages increased significantly 1 month after vaccine administration, and the increase also persisted 5 months later. In addition, the amount of IFN-gamma in PBMC supernatants was significantly higher after vaccination. Overall, our results suggest the usefulness of evaluating the in vivo protective role of this promastigote preparation in dogs.

Infection of a canine macrophage cell line with leishmania infantum: determination of nitric oxide production and anti-leishmanial activity

We have previously shown that resistance to Leishmania infantum in dogs is associated with a Th1 type of immune response. In this study, we use a canine macrophage cell line (030-D) that can readily be infected with this protozoan parasite. Our aim is to further characterize the effector mechanisms involved in killing of Leishmania parasite in dogs. We observed that activation of 030-D cells by incubation with a supernatant derived from a Leishmania-specific T cell line containing IFN-gamma, TNF-alpha and interleukin-2 (IL-2) resulted in enhanced nitric oxide (NO) production by these cells. In addition, we observed enhanced anti-leishmanial activity of infected 030-cells after activation. Both, NO production and anti-leishmanial activity were abrogated by addition of L-N(G)-nitroargininemethyl ester (L-NAME), an analogue of L-arginine. Thus, NO play an important role in the anti-leishmanial activity of these canine macrophages. We propose the infection of the 030-D cell line as a good in vitro model to further investigate parasite-host cell interactions in dogs, a natural host of Leishmania parasites.

Inducible nitric oxide synthase and nitric oxide production in Leishmania infantum-infected human macrophages stimulated with interferon-gamma and bacterial lipopolysaccharide

Nitric oxide produced by an inducible nitric oxide synthase constitutes one of the main microbicidal mechanisms of murine macrophages and its importance is now being recognized for human macrophages. In this study we evaluated inducible nitric oxide synthase expression, nitric oxide release, and parasitocidal ability of Leishmania infantum-infected monocyte-derived human macrophages. The inducible nitric oxide synthase was detected by immunofluorescence and western blotting and nitric oxide production was measured by the Griess reaction for nitrites. Parasite killing was microscopically evaluated by fluorescent dyes. Experiments were performed on macrophages with or without previous stimulation with recombinant human interferon-gamma and bacterial lipopolysaccharide. Inducible nitric oxide synthase expression and nitric oxide release were higher in Leishmania-infected stimulated macrophages than in uninfected cells or infected cells without previous stimulation. Nitric oxide production and parasitocidal activity against Leishmania infantum were reduced in macrophages treated with the nitric oxide synthase inhibitor L-N(G) monomethylarginine. These results suggest a microbicidal role for nitric oxide in human leishmaniasis, with the possible practical application of immunological or pharmacological regulation of nitric oxide synthesis in the treatment of this infection.