Involvement of CD4⁺ Foxp3⁺ regulatory T cells in persistence of Leishmania donovani in the liver of alymphoplastic aly/aly mice

The nexus between Leishmania & HIV: Debilitating host immunity and Hastening Comorbid disease burden

The scintillating association between Leishmania and HIV has contributed exceptionally towards expansion of Visceral Leishmaniasis (VL) with Acquired Immunodeficiency Syndrome (AIDS). The co-infection poses a grievous threat to elimination of VL and containment of Human Immunodeficiency Virus (HIV). When coinfected, Leishmania and HIV complement each other's proliferation and survival by inducing immunesenescence, T cell fatigue and exhaustion. Antigen presentation is lost, co-stimulatory molecules are diminished whereas co-inhibitory molecules such as CTLA-4, TIGIT, LAG-3 etc. are upregulated to ensure a Th2-baised immune environment. As a consequence, Leishmania-HIV coinfection causes poor outcomes, inflates the spread of Leishmania parasites, enhances the severity of side-effects to drugs, as well as escalate the probability of treatment failure and mortality. What makes control extremely strenuous is that there are frequent episodes of VL relapse with no prognostic markers, no standard immunophenotype(s) and appearance of atypical clinical symptoms. Thus, a standard therapeutic regimen has been difficult to develop and treatment is majorly dependent upon a combination of liposomal Amphotericin B and Miltefosine, a therapy that is expensive and capable of causing drastic side-effects in recipients. As World Health Organization is committed to eliminate both VL and HIV in due course of future, the existing therapeutic interventions require advancements to grapple and overcome this hazardous co-infection. In this context, an overview of HIV-VL co-infection, immunopathology of HIV and Leishmania co-inhabitance, available therapeutic options and their limitations in the treatment of co-infection are discussed in-depth.

Inhibitory effect of Eimeria maxima IFN-γ inhibitory molecules on the immune function of T cell subsets in chickens

It has been reported that infection of chicken coccidian could inhibit the production of Th1 cytokine IFN-γ, thereby evading clearance by the host immune system. The present study aimed to have a further investigation into the effects of Eimeria maxima IFN-γ inhibitory molecules (EmHPSP-2 and EmHPSP-3) on the immune function of chicken peripheral blood mononuclear cells (PBMC) and various T cell subsets. First, separated PBMC or sorted T cell subsets were used for incubation with recombinant proteins of EmHPSP-2 (rEmHPSP-2) and EmHPSP-3 (rEmHPSP-3). Subsequently, the effects of rEmHPSP-2 and rEmHPSP-3 on proliferative capacity, nitric oxide (NO) release and mRNA levels of cytokines of the above cells were detected. The sorting purity of CD8+, CD4+ CD25-, CD4+, and CD4+ CD25+ T cells was 93.01, 88.88, 87.04, and 81.26%, respectively. The NO release of PBMC was significantly inhibited by rEmHPSP-2 and rEmHPSP-3. The proliferation of PBMC and CD4+ T cells was significantly inhibited by rEmHPSP-2 and rEmHPSP-3, whereas CD8+, CD4+ CD25-, and CD4+ CD25+ T cells was significantly promoted by the 2 proteins. The 2 proteins significantly downregulated interferon-gamma (IFN-γ) mRNA level, upregulated the transcriptional levels of interleukin-10 (IL-10) and transforming growth factor-beta1 (TGF-β1) in PBMC. IFN-γ and IL-2 transcriptional levels were markedly inhibited in CD8+ T cells. IFN-γ transcriptional level was significantly inhibited, but IL-4 was promoted by rEmHPSP-2 and rEmHPSP-3 in CD4+ CD25- T cells. Meanwhile, the inhibitory effects of rEmHPSP-2 and rEmHPSP-3 on the transcriptional levels of IFN-γ and IL-2 were more obvious in CD4+ T cells containing CD25+ cells compared with the CD25+ cells depletion group. It was found that IL-10, TGF-β1, and cytotoxic T lymphocyte-associated antigen-4 (CTLA-4) mRNA levels were significantly upregulated upon stimulation of chicken CD4+ CD25+ T cells by proteins. This study is not only of great significance to clarify the immune evasion mechanism of chicken coccidia, but also provides candidate antigen molecules for development of a novel vaccine against chicken coccidiosis.

Field-Deployable Treatments For Leishmaniasis: Intrinsic Challenges, Recent Developments and Next Steps

Leishmaniasis is a neglected tropical disease endemic primarily to low- and middle-income countries, for which there has been inadequate development of affordable, safe, and efficacious therapies. Clinical manifestations of leishmaniasis range from self-healing skin lesions to lethal visceral infection with chances of relapse. Although treatments are available, secondary effects limit their use outside the clinic and negatively impact the quality of life of patients in endemic areas. Other non-medicinal treatments, such as thermotherapies, are limited to use in patients with cutaneous leishmaniasis but not with visceral infection. Recent studies shed light to mechanisms through which Leishmania can persist by hiding in cellular safe havens, even after chemotherapies. This review focuses on exploring the cellular niches that Leishmania parasites may be leveraging to persist within the host. Also, the cellular, metabolic, and molecular implications of Leishmania infection and how those could be targeted for therapeutic purposes are discussed. Other therapies, such as those developed against cancer or for manipulation of the ferroptosis pathway, are proposed as possible treatments against leishmaniasis due to their mechanisms of action. In particular, treatments that target hematopoietic stem cells and monocytes, which have recently been found to be necessary components to sustain the infection and provide a safe niche for the parasites are discussed in this review as potential field-deployable treatments against leishmaniasis.

Detection of parasite-derived tRNA and rRNA fragments in the peripheral blood of mice experimentally infected with Leishmania donovani and Leishmania amazonensis using next-generation sequencing analysis

All prokaryotes and eukaryotes, including parasites, release extracellular vesicles or exosomes that contain selected proteins, lipids, nucleic acids, glycoconjugates, and metabolites. Leishmania exosomes are highly enriched in small RNAs derived from the rRNAs and tRNAs of the protozoan parasite species. Here, using plasma exosomes isolated by a kit and next-generation sequencing, we report the detection of fragments of parasite-derived rRNAs and tRNAs in the peripheral plasma samples of mice experimentally infected with Leishmania donovani and Leishmania amazonensis, the causative agents of Old World visceral leishmaniasis and New World disseminated cutaneous leishmaniasis, respectively. Detected RNA molecules of 28S rRNA, 5.8S rRNA, tRNA-Glu, and tRNA-Thr were common to both plasma samples of mice inoculated with L. donovani and L. amazonensis, whereas tRNA-Ile and tRNA-Trp were only detected in L. amazonensis-infected mice. The detected rRNAs and tRNA isotypes were matched with the exosomal components reported in a previous key study. Our preliminary results suggested that parasite-derived small RNAs were circulating in the blood of mice infected with Leishmania species, providing a better understanding of the roles of exosomal components in leishmaniasis and also new insights into exosome-based biomarkers for Leishmania infection.

Chronic Systemic Infection of Mice with Leishmania infantum Leads to Increased Bone Mass

Vector-borne infections of humans with the protozoan parasite Leishmania (L.) infantum can cause a systemic and potentially lethal disease termed visceral leishmaniasis. In the corresponding mouse model, an intravenous infection with L. infantum leads to the persistence of parasites in various organs, including bone marrow (BM). Considering the anatomical proximity between the BM and the cortical bone, we investigated whether a chronic infection with L. infantum affected bone homeostasis. Unexpectedly, chronic infection with L. infantum caused an increase in bone mass in mice. In vivo, an increased number of osteoblasts and osteocytes and a decreased maturation of osteoclasts characterized the phenotype. Confocal laser scanning fluorescence microscopy confirmed the infection of BM macrophages but also revealed the presence of parasites in osteoclasts. In vitro, mature osteoclasts took up L. infantum parasites. However, infection of osteoclast progenitors abolished their differentiation and function. In addition, secretory products of infected BM-derived macrophages inhibited the maturation of osteoclasts. Both in vitro and in vivo, infected macrophages and osteoclasts showed an enhanced expression of the anti-osteoclastogenic chemokine CCL5 (RANTES). Neutralization of CCL5 prevented the inhibition of osteoclast generation seen in the presence of culture supernatants from L. infantum-infected macrophages. Altogether, our study shows that chronic infection with Leishmania increases bone mass by inducing bone formation and impairing osteoclast differentiation and function. © 2022 American Society for Bone and Mineral Research (ASBMR).

An experimental challenge model for Leishmania donovani in beagle dogs, showing a similar pattern of parasite burden in the peripheral blood and liver

Leishmania donovani and Leishmania infantum are closely related species. However, the former is considered the causative agent for anthroponotic visceral leishmaniasis (AVL), while the latter is known to be responsible for zoonotic visceral leishmaniasis (ZVL) with dogs as the main reservoir host. Although molecular detection of L. donovani from naturally infected dogs has been reported in AVL endemic areas, the experimental infection of dogs with this species is very limited. Here, we constructed an experimental canine visceral leishmaniasis (CVL) model with L. donovani infection using beagle dogs. During an observation period of 8 months after parasite inoculation, few clinical symptoms were observed in the three inoculated dogs. The overall hematological and biochemical data of the dogs showed normal levels, and there were no remarkable changes in the peripheral CD4⁺, CD8⁺, CD25⁺, or FoxP3⁺ T cell populations. Liver biopsy sampling was conducted to monitor the parasite burden in the liver. A similar pattern of the amount of mitochondrial kinetoplast DNA was observed in the peripheral blood and liver by real-time PCR analysis. In addition, parasite antigens were detected from the liver biopsy sections by immunohistochemical analysis, further supporting the existence of parasites in the liver. These results showed a subclinical CVL model for L. donovani in beagle dogs with a similar kinetics of parasite burden in the peripheral blood and liver.

T Lymphocyte Exhaustion During Human and Experimental Visceral Leishmaniasis

A key point of immunity against protozoan Leishmania parasites is the development of an optimal T cell response, which includes a low apoptotic rate, high proliferative activity and polyfunctionality. During acute infection, antigen-specific T cells recognize the pathogen resulting in pathogen control but not elimination, promoting the development and the maintenance of a population of circulating effector cells that mount rapid response quickly after re-exposure to the parasite. However, in the case of visceral disease, the functionality of specific T cells is lost during chronic infection, resulting in inferior effector functions, poor response to specific restimulation, and suboptimal homeostatic proliferation, a term referred to as T cell exhaustion. Multiple factors, including parasite load, infection duration and host immunity, affect T lymphocyte exhaustion. These factors contribute to antigen persistence by promoting inhibitory receptor expression and sustained production of soluble mediators, influencing suppressive cell function and the release of endogenous molecules into chronically inflamed tissue. Together, these signals encourage several changes, reprogramming cells into a quiescent state, which reflects disease progression to more severe forms, and development of acquired resistance to conventional drugs to treat the disease. These points are discussed in this review.

A Historic Review of the Role of CD4+ T-Cell Subsets in Development of the Immune Responses against Cutaneous and Visceral Leishmaniases

The heterogeneity of CD4+ T cells has been investigated since the late 1970s, when their Th1 and Th2 subsets were coined. Later studies on the cutaneous form of the Leishmaniasis were focused on the experimental models of Leishmania major infection using the susceptible BALB/c and the resistant C57BL/6 mice. At the early 21st century, the regulatory T-cells subpopulation was introduced and its role in concomitant immunity, responsible for lifelong resistance of the host to the reinfection was proposed. Subsequent studies, mainly focused on the visceral form of the infection pointed to the role of IL-17, produced by Th17 subset of CD4+ T cells that along the neutrophils were shown to have important yet equivocal functions in protection against or exacerbation of the infection. Altogether, the current knowledge indicates that the above four subsets could orchestrate the immune, the regulatory and the inflammatory responses of the host against different forms of leishmaniases.

Liver- and Spleen-Specific Immune Responses in Experimental Leishmania martiniquensis Infection in BALB/c Mice

Leishmania martiniquensis is a neglected cause of an emerging leishmaniasis in many countries, including France, Germany, Switzerland, the United States of America, Myanmar, and Thailand, with different clinical manifestations ranging from asymptomatic, cutaneous (CL), visceral (VL), and atypically disseminated CL and VL. The persistence of parasites and the recurrence of the disease after treatment are challenges in controlling the disease. To explore efficient prophylaxis and therapy, this study aimed to investigate infection outcome and organ-specific immune responses after inoculation with L. martiniquensis (MHOM/TH/2011/PG; 5 x 10⁶ promastigotes) in BALB/c mice via intravenous and intraperitoneal routes. A quantitative PCR technique, targeting L. martiniquensis ITS1, was primarily established to estimate the parasite burden. We found that the infection in the liver resolved; however, persistent infection was observed in the spleen. Histopathology with Leishmania-specific immunostaining revealed efficient hepatic granuloma formation, while splenic disorganization with parasitized macrophages at different locations was demonstrated. The mRNA expression of Th1 cytokines (IFN-γ, TNF-α, IL-12p40) and iNOS in the liver and spleen was upregulated. In addition, high expression of IL-10 was observed in the spleen in the chronic phase, revealing a significant moderate correlation with the parasite persistence [r₍₁₂₎ = 0.72, P = 0.009]. Further clarification of the mechanisms of persistent infection and experimental infection in immunosuppressed murine models are warranted.

Host immune response against leishmaniasis and parasite persistence strategies: A review and assessment of recent research

Leishmaniasis, a neglected parasitic disease caused by a unicellular protozoan of the genus Leishmania, is transmitted through the bite of a female sandfly. The disease remains a major public health problem and is linked to tropical and subtropical regions, with an endemic picture in several regions, including East Africa, the Mediterranean basin and South America. The different causative species display a diversity of clinical presentations; therefore, the immunological data on leishmaniasis are both scarce and controversial for the different forms and infecting species of the parasite. The present review highlights the main immune parameters associated with leishmaniasis that might contribute to a better understanding of the pathogenicity of the parasite and the clinical outcomes of the disease. Our aim was to provide a concise overview of the immunobiology of the disease and the factors that influence it, as this knowledge may be helpful in developing novel chemotherapeutic and vaccine strategies.

Role of Cytokines in Experimental and Human Visceral Leishmaniasis

Visceral Leishmaniasis (VL) is the most fatal form of disease leishmaniasis. To date, there are no effective prophylactic measures and therapeutics available against VL. Recently, new immunotherapy-based approaches have been established for the management of VL. Cytokines, which are predominantly produced by helper T cells (Th) and macrophages, have received great attention that could be an effective immunotherapeutic approach for the treatment of human VL. Cytokines play a key role in forming the host immune response and in managing the formation of protective and non-protective immunities during infection. Furthermore, immune response mediated through different cytokines varies from different host or animal models. Various cytokines viz. IFN-γ, IL-2, IL-12, and TNF-α play an important role during protection, while some other cytokines viz. IL-10, IL-6, IL-17, TGF-β, and others are associated with disease progression. Therefore, comprehensive knowledge of cytokine response and their interaction with various immune cells is very crucial to determine appropriate immunotherapies for VL. Here, we have discussed the role of cytokines involved in VL disease progression or host protection in different animal models and humans that will determine the clinical outcome of VL and open the path for the development of rapid and accurate diagnostic tools as well as therapeutic interventions against VL.

The potential therapeutic effect of adipose-derived mesenchymal stem cells in the treatment of cutaneous leishmaniasis caused by L. major in BALB/c mice

Leishmaniasis is one of the most neglected tropical infectious diseases in the world. The emergence of drug resistance and toxicity and the high cost of the available drugs with a lack of new anti-leishmanial drugs highlight the need to search for newer therapies with anti-leishmanial activities. Due to the mesenchymal stem cell (MSC) immunomodulatory capacity, they have been applied in a wide variety of disorders. In this study, the potential effects of adipose-derived MSC (AD-MSCs) therapy and its combination with glucantime were evaluated in a murine model of cutaneous leishmaniasis induced by L. major. The results showed that AD-MSCs improved wound healing and decreased parasite burden. The real-time PCR results obtained from mice treated with AD-MSCs showed that IL-12 and TNF-α genes were upregulated. IL-10, arginase, and FOXP3 genes were downregulated whereas no differences in expression of the IL-4 gene were found. Overall, it seems that AD-MSCs therapy enhances Th1 immune response in L. major infected BALB/c mice. Unexpectedly, our results showed that the association of glucantime to AD-MSCs treatments did not lead to an increment in the anti-leishmanial activity.

Sensing Host Arginine Is Essential for Leishmania Parasites' Intracellular Development

In this study, we report that the ability of the human pathogen Leishmania to sense and monitor the lack of arginine in the phagolysosome of the host macrophage is essential for disease development. Phagolysosomes of macrophages are the niche where Leishmania resides and causes human leishmaniasis. During infection, the arginine concentration in the phagolysosome decreases as part of the host innate immune response. An arginine sensor on the Leishmania cell surface activates an arginine deprivation response pathway that upregulates the expression of a parasite arginine transporter (AAP3). Here, we use CRISPR/Cas9-mediated disruption of the AAP3 locus to show that this response enables Leishmania parasites to successfully compete with the host macrophage in the “hunger games” for arginine.

Arginine homeostasis in lysosomes is critical for the growth and metabolism of mammalian cells. Phagolysosomes of macrophages are the niche where the parasitic protozoan Leishmania resides and causes human leishmaniasis. During infection, parasites encounter arginine deprivation, which is monitored by a sensor on the parasite cell surface. The sensor promptly activates a mitogen-activated protein kinase 2 (MAPK2)-mediated arginine deprivation response (ADR) pathway, resulting in upregulating the abundance and activity of the Leishmania arginine transporter (AAP3). Significantly, the ADR is also activated during macrophage infection, implying that arginine levels within the host phagolysosome are limiting for growth. We hypothesize that ADR-mediated upregulation of AAP3 activity is necessary to withstand arginine starvation, suggesting that the ADR is essential for parasite intracellular development. CRISPR/Cas9-mediated disruption of the AAP3 locus yielded mutants that retain a basal level of arginine transport but lack the ability to respond to arginine starvation. While these mutants grow normally in culture, they were impaired in their ability to develop inside THP-1 macrophages and were ∼70 to 80% less infective in BALB/c mice. Hence, inside the host macrophage, Leishmania must overcome the arginine “hunger games” by upregulating the transport of arginine via the ADR. We show that the ability to monitor and respond to changes in host metabolite levels is essential for pathogenesis.

Immunophenotyping of Peripheral Blood, Lymph Node, and Bone Marrow T Lymphocytes During Canine Leishmaniosis and the Impact of Antileishmanial Chemotherapy

Dogs are a major reservoir of Leishmania infantum, etiological agent of canine leishmaniosis (CanL) a zoonotic visceral disease of worldwide concern. Therapeutic protocols based on antileishmanial drugs are commonly used to treat sick dogs and improve their clinical condition. To better understand the impact of Leishmania infection and antileishmanial drugs on the dog's immune response, this study investigates the profile of CD4⁺ and CD8⁺ T cell subsets in peripheral blood, lymph node, and bone marrow of sick dogs and after two different CanL treatments. Two CanL groups of six dogs each were treated with either miltefosine or meglumine antimoniate combined with allopurinol. Another group of 10 clinically healthy dogs was used as control. Upon diagnosis and during the following 3 months of treatment, peripheral blood, popliteal lymph node, and bone marrow mononuclear cells were collected, labeled for surface markers CD45, CD3, CD4, CD8, CD25, and intracellular nuclear factor FoxP3, and T lymphocyte subpopulations were immunophenotyped by flow cytometry. CanL dogs presented an overall increased frequency of CD8⁺ and CD4⁺CD8⁺ double-positive T cells in all tissues and a decreased frequency of CD4⁺ T cells in the blood. Furthermore, there was a higher frequency of CD8⁺ T cells expressing CD25⁺FoxP3⁺ in the blood and bone marrow. During treatment, these subsets recovered to levels similar to those of healthy dogs. Nevertheless, antileishmanial therapy caused an increase of CD4⁺CD25⁺FoxP3⁺ T cells in all tissues, associated with the decrease of CD8⁺CD25⁻FoxP3⁻ T cell percentages. These findings may support previous studies that indicate that L. infantum manipulates the dog's immune system to avoid the development of a protective response, ensuring the parasite's survival and the conditions that allow the completion of Leishmania life cycle. Both treatments used appear to have an effect on the dog's immune response, proving to be effective in promoting the normalization of T cell subsets.

IL-4 Mediated Resistance of BALB/c Mice to Visceral Leishmaniasis Is Independent of IL-4Rα Signaling via T Cells

Previous studies infecting global IL-4Rα^−/−, IL-4^−/−, and IL-13^−/−mice on a BALB/c background with the visceralizing parasite Leishmania donovani have shown that the T helper 2 cytokines, IL-4, and IL-13, play influential but not completely overlapping roles in controlling primary infection. Subsequently, using macrophage/neutrophil-specific IL-4Rα deficient BALB/c mice, we demonstrated that macrophage/neutrophil unresponsiveness to IL-4 and IL-13 did not have a detrimental effect during L. donovani infection. Here we expand on these findings and show that CD4⁺ T cell-(Lck^cre), as well as pan T cell-(iLck^cre) specific IL-4Rα deficient mice, on a BALB/c background, unlike global IL-4Rα deficient mice, are also not adversely affected in terms of resistance to primary infection with L. donovani. Our analysis suggested only a transient and tissue specific impact on disease course due to lack of IL-4Rα on T cells, limited to a reduced hepatic parasite burden at day 30 post-infection. Consequently, the protective role(s) demonstrated for IL-4 and IL-13 during L. donovani infection are mediated by IL-4Rα-responsive cell(s) other than macrophages, neutrophils and T cells.

The Contribution of Immune Evasive Mechanisms to Parasite Persistence in Visceral Leishmaniasis

Leishmania is a genus of protozoan parasites that give rise to a range of diseases called Leishmaniasis that affects annually an estimated 1.3 million people from 88 countries. Leishmania donovani and Leishmania (L.) infantum chagasi are responsible to cause the visceral leishmaniasis. The parasite can use assorted strategies to interfere with the host homeostasis to establish persistent infections that without treatment can be lethal. In this review, we highlight the mechanisms involved in the parasite subversion of the host protective immune response and how alterations of host tissue physiology and vascular remodeling during VL could affect the organ-specific immunity against Leishmania parasites.

An experimental challenge model of visceral leishmaniasis by Leishmania donovani promastigotes in mice

Although visceral leishmaniasis is a fatal disease in humans and dogs, the use of mouse models is important for obtaining a better understanding of the pathogenesis, immunity, and host-parasite interactions of this disease. Such models are also useful for the evaluation of vaccines and chemotherapies for treatment of visceral leishmaniasis. Here, we present our method of experimental inoculation of mice with Leishmania donovani promastigotes. Nutrient-enriched undefined media may be beneficial for laboratory maintenance of promastigotes for maintaining their virulence or infectivity in mice. With this method, we could preserve the infectivity of promastigote lines recovered from inoculated animals and use these lines for further in vivo experiments. Furthermore, the use of cryopreserved stabilates is highly recommended for the reproducibility of experiments. To assess a newly developed method for determination of parasite burden in infected animal tissues, initial comparison of parasite burden in the liver obtained in the classic Leishman-Donovan units (LDU) with values obtained from the new method is recommended. As an example, the association between parasite burden determined by LDU and real-time PCR assay targeting the leishmanial gp63 gene in the liver of mice is presented.

Hepatic stellate cells regulate liver immunity to visceral leishmaniasis through P110δ-dependent induction and expansion of regulatory T cells in mice

Visceral leishmaniasis (VL) is associated with severe immune dysfunction and if untreated leads to death. Because the liver is one of the primary target organs in VL, unraveling the mechanisms governing the local hepatic immune response is important for understanding the immunopathogenesis of VL. We previously reported that mice with inactivating knockin mutation in the p110δ gene (p110δ(D910A) ) are resistant to VL, due in part to impaired regulatory T-cell (Treg) expansion. In this study, we investigated the mechanism of this resistance by focusing on hepatic stellate cells (HSCs), which are known to regulate Treg induction and expansion. We show that HSCs are infected with Leishmania donovani in vivo and in vitro and that this infection leads to the production of interleukin-2, interleukin-6, and transforming growth factor-β, cytokines known to induce Tregs. We further demonstrate that L. donovani infection leads to expansion of HSCs in a p110δ-dependent manner and that this correlated with proliferation of hepatic Tregs in vivo. In vitro studies clearly show that L. donovani-infected HSCs induce CD4(+) T cells to become Tregs and expand Tregs in a p110δ-dependent manner. Targeted depletion of HSCs during infection caused a dramatic reduction in liver Treg numbers and proliferation, which was associated with a decrease in interleukin-10 production by hepatic T cells and a more efficient parasite control.

CONCLUSION

These results demonstrate the critical role of HSCs in the pathogenesis of VL and suggest that the enhanced resistance of p110δ(D910A) mice to L. donovani infection is due in part to impaired expansion and inability of their HSCs to induce and expand Tregs in the liver.

Lessons from other diseases: granulomatous inflammation in leishmaniasis

The Leishmania granuloma shares some, though not all, properties with that formed following mycobacterial infection. As a simplified, noncaseating granuloma composed of relatively few and largely mononuclear cell populations, it provides a tractable model system to investigate intra-granuloma cellular dynamics, immune regulation, and antimicrobial resistance. Here, the occurrence of granulomatous pathology across the spectrum of leishmaniasis, in humans and animal reservoir hosts, is first described. However, this review focuses on the process of hepatic granuloma formation as studied in rodent models of visceral leishmaniasis, starting from the initial infection of Kupffer cells to the involution of the granuloma after pathogen clearance. It describes how the application of intravital imaging and the use of computational modeling have changed some of our thoughts on granuloma function, and illustrates how host-directed therapies have been used to manipulate granuloma form and function for therapeutic benefit. Where appropriate, lessons that may be equally applicable across the spectrum of granulomatous diseases are highlighted.

Transcription of Toll-Like Receptors 2, 3, 4 and 9, FoxP3 and Th17 Cytokines in a Susceptible Experimental Model of Canine Leishmania infantum Infection

Canine leishmaniosis (CanL) due to Leishmania infantum is a chronic zoonotic systemic disease resulting from complex interactions between protozoa and the canine immune system. Toll-like receptors (TLRs) are essential components of the innate immune system and facilitate the early detection of many infections. However, the role of TLRs in CanL remains unknown and information describing TLR transcription during infection is extremely scarce. The aim of this research project was to investigate the impact of L. infantum infection on canine TLR transcription using a susceptible model. The objectives of this study were to evaluate transcription of TLRs 2, 3, 4 and 9 by means of quantitative reverse transcription polymerase chain reaction (qRT-PCR) in skin, spleen, lymph node and liver in the presence or absence of experimental L. infantum infection in Beagle dogs. These findings were compared with clinical and serological data, parasite densities in infected tissues and transcription of IL-17, IL-22 and FoxP3 in different tissues in non-infected dogs (n = 10), and at six months (n = 24) and 15 months (n = 7) post infection. Results revealed significant down regulation of transcription with disease progression in lymph node samples for TLR3, TLR4, TLR9, IL-17, IL-22 and FoxP3. In spleen samples, significant down regulation of transcription was seen in TLR4 and IL-22 when both infected groups were compared with controls. In liver samples, down regulation of transcription was evident with disease progression for IL-22. In the skin, upregulation was seen only for TLR9 and FoxP3 in the early stages of infection. Subtle changes or down regulation in TLR transcription, Th17 cytokines and FoxP3 are indicative of the silent establishment of infection that Leishmania is renowned for. These observations provide new insights about TLR transcription, Th17 cytokines and Foxp3 in the liver, spleen, lymph node and skin in CanL and highlight possible markers of disease susceptibility in this model.

Immunity to visceral leishmaniasis: implications for immunotherapy

Visceral leishmaniasis, caused by Leishmania donovani, L. infantum (syn. Leishmania chagasi), is a globally widespread disease with a burden of about 400,000 new infections reported annually. It is the most dangerous form of human leishmaniasis in terms of mortality and morbidity and is spreading to several nonendemic areas because of migration, global traveling and military conflicts. The emergence of Leishmania-HIV co-infection and increased prevalence of drug-resistant strains have worsened the impact of the disease. The traditional low-cost drugs are often toxic with several adverse effects, highlighting the need for development of new therapeutic and prophylactic strategies. Therefore, a detailed understanding of mechanisms of protective immunity is extremely important in order to develop new therapeutics in the form of vaccines or immunotherapies. This review gives an overview of visceral leishmaniasis, with particular emphasis on the innate and adaptive immune responses, vaccine and vaccination strategies and their potentials for immunotherapy against the disease.

Expression of regulatory T cells in jejunum, colon, and cervical and mesenteric lymph nodes of dogs naturally infected with Leishmania infantum

Using flow cytometry, we evaluated the frequencies of CD4(+) and CD8(+) T cells and Foxp3(+) regulatory T cells (Tregs) in mononuclear cells in the jejunum, colon, and cervical and mesenteric lymph nodes of dogs naturally infected with Leishmania infantum and in uninfected controls. All infected dogs showed chronic lymphadenitis and enteritis. Despite persistent parasite loads, no erosion or ulcers were evident in the epithelial mucosa. The colon harbored more parasites than the jejunum. Frequencies of total CD4(+), total Foxp3, and CD4(+) Foxp3(+) cells were higher in the jejunum than in the colon. Despite negative enzyme-linked immunosorbent assay (ELISA) serum results for cytokines, levels of interleukin-10 (IL-10), gamma interferon (IFN-γ), transforming growth factor beta (TGF-β), and tumor necrosis factor alpha (TNF-α) were higher in the jejunum than in the colon for infected dogs. However, IL-4 levels were higher in the colon than in the jejunum for infected dogs. There was no observed correlation between clinical signs and histopathological changes or immunological and parasitological findings in the gastrointestinal tract (GIT) of canines with visceral leishmaniasis. However, distinct segments of the GIT presented different immunological and parasitological responses. The jejunum showed a lower parasite load, with increased frequencies and expression of CD4, Foxp3, and CD8 receptors and IL-10, TGF-β, IFN-γ, and TNF-α cytokines. The colon showed a higher parasite load, with increasing expression of IL-4. Leishmania infantum infection increased expression of CD4, Foxp3, IL-10, TGF-β, IFN-γ, and TNF-α and reduced CD8 and IL-4 expression in both the jejunum and the colon.

Peripheral whole blood FOXP3 TSDR methylation: a potential marker in severity assessment of autoimmune diseases and chronic infections

Immune dysregulation is a cardinal feature of autoimmune diseases and chronic microbial infections. In particular, regulatory T cells are downregulated in autoimmune diseases while upregulated in chronic microbial infections. FOXP3 is the master regulator of Treg development. Treg-specific demethylated region (TSDR) is a highly conserved locus on the FOXP3 gene that is fully demethylated in natural Tregs but methylated in effector T cells. In our study, we used high resolution melt-polymerase chain reaction (HRM-PCR) to determine the FOXP3 TSDR methylation status in autoimmune diseases and chronic microbial infections. We found that FOXP3 TSDR to have the highest mean melting temperature (highly methylated) in active SLE patients compared to all the other groups (p < 0.001). The psoriasis group also had a significantly high mean melting temperature (78.62 ± 0.20) when compared with the inactive SLE group (78.49 ± 0.29, p < 0.05) and control group (78.44 ± 0.25, p < 0.01). There was no significant difference in melting temperature between inactive SLE and healthy controls. Disease activity in SLE was directly associated with methylation of the FOXP3 TSDR. On the other hand, patients with chronic microbial infections had significantly lower FOXP3 TSDR mean melting temperature (demethylated) when compared with healthy controls (78.28 ± 0.21 vs 78.44 ± 0.25, p < 0.05). Our results suggest that the use of HRM-PCR to detect FOXP3 TSDR methylation status is a reliable and easy method to predict natural regulatory T cell levels in peripheral blood in different disease conditions. Determining FOXP3 TSDR methylation status can be a useful tool in diagnosis, and monitoring the severity of autoimmune diseases and chronic microbial infections.

Deficiency of p110δ isoform of the phosphoinositide 3 kinase leads to enhanced resistance to Leishmania donovani

Background

Visceral leishmaniasis is the most clinically relevant and dangerous form of human leishmaniasis. Most traditional drugs for treatment of leishmaniasis are toxic, possess many adverse reactions and drug resistance is emerging. Therefore, there is urgent need for identification of new therapeutic targets. Recently, we found that mice with an inactivating knock-in mutation in the p110δ isoform of pi3k, (p110δ^d910a) are hyper resistant to L. major, develop minimal cutaneous lesion and rapidly clear their parasite. Here, we investigated whether pi3k signaling also regulates resistance to L. donovani, one of the causative agents of visceral leishmaniasis.

Methodology/Principal Findings

WT and p110δ^D910A mice (on a BALB/c background) were infected with L. donovani. At different time points, parasite burden and granuloma formation were assessed. T and B cell responses in the liver and spleen were determined. In addition, Tregs were expanded in vivo and its impact on resistance was assessed. We found that p110δ^D910A mice had significantly reduced splenomegaly and hepatomegaly and these organs harbored significantly fewer parasites than those of WT mice. Interestingly, infected p110δ^D910A mice liver contains fewer and less organized granulomas than their infected WT counterparts. Cells from p110δ^D910A mice were significantly impaired in their ability to produce cytokines compared to WT mice. The percentage and absolute numbers of Tregs in infected p110δ^D910A mice were lower than those in WT mice throughout the course of infection. In vivo expansion of Tregs in infected p110δ^D910A mice abolished their enhanced resistance to L. donovani infection.

Conclusions/Significance

Our results indicate that the enhanced resistance of p110δ^D910A mice to L. donovani infection is due to impaired activities of Tregs. They further show that resistance to Leishmania in the absence of p110δ signaling is independent of parasite species, suggesting that targeting the PI3K signaling pathway may be useful for treatment of both visceral and cutaneous leishmaniasis.

Visceral leishmaniasis (VL) is the most dangerous form of human leishmaniasis in terms of mortality and morbidity and is spreading to several non-endemic areas because of global traveling and military conflicts. The emergence of Leishmania-HIV coinfection and increased prevalence of drug resistant strains have compounded an already bad situation. In addition, the drugs available are toxic, expensive and have several side effects. Therefore, a detailed understanding of protective immune response is extremely important in order to identify new therapeutic targets. The phosphoinositide 3 kinase (PI3K) family of enzymes mediate several important immunologic and physiologic cellular process including proliferation, differentiation, growth and host defense. We previously showed that genetic inactivation of the p110δ isoform of PI3K results in resistant to L. major (the causative agent of cutaneous leishmaniasis (CL)). Here, we investigate the role of PI3K in immunity to VL and the mechanisms underlying its protective effect. Collectively, our results demonstrate that signaling via the p110δ also regulates immunity to L. donovani, an effect that is dependent on the impact of p110δ signaling on expansion and function of regulatory T cells in vivo. Thus, our studies suggest that targeting the p110δ pathway may be a novel therapeutic strategy for controlling VL and CL.

Macrophage cytokines: involvement in immunity and infectious diseases

The evolution of macrophages has made them primordial for both development and immunity. Their functions range from the shaping of body plans to the ingestion and elimination of apoptotic cells and pathogens. Cytokines are small soluble proteins that confer instructions and mediate communication among immune and non-immune cells. A portfolio of cytokines is central to the role of macrophages as sentries of the innate immune system that mediate the transition from innate to adaptive immunity. In concert with other mediators, cytokines bias the fate of macrophages into a spectrum of inflammation-promoting "classically activated," to anti-inflammatory or "alternatively activated" macrophages. Deregulated cytokine secretion is implicated in several disease states ranging from chronic inflammation to allergy. Macrophages release cytokines via a series of beautifully orchestrated pathways that are spatiotemporally regulated. At the molecular level, these exocytic cytokine secretion pathways are coordinated by multi-protein complexes that guide cytokines from their point of synthesis to their ports of exit into the extracellular milieu. These trafficking proteins, many of which were discovered in yeast and commemorated in the 2013 Nobel Prize in Physiology or Medicine, coordinate the organelle fusion steps that are responsible for cytokine release. This review discusses the functions of cytokines secreted by macrophages, and summarizes what is known about their release mechanisms. This information will be used to delve into how selected pathogens subvert cytokine release for their own survival.