Translating immune cell cross-talk into a treatment opportunity for visceral leishmaniasis

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.

Meta-analysis of -308G > A polymorphism in TNFα gene and susceptibility to leishmaniasis

The clinical spectrum of leishmaniasis depends on several factors, including Leishmania species and immunogenetic factors. Tumor necrosis factor α (TNFα) plays a central role in immunity against intracellular infections. Many studies have reported that TNFα-308G > A polymorphism is associated with susceptibility to intracellular infections and influences TNFα production. Some studies on the implications of TNFα-308G > A polymorphism in the susceptibility to cutaneous leishmaniasis and visceral leishmaniasis showed controversial results. To draw an overall conclusion using accurate data analysis by increasing the number of cases studied, a meta-analysis was performed based on data from the studies included in the analysis. A total of 1264 patients and 2350 controls were enrolled in the meta-analysis. The results showed no significant association between allele G and allele A of -308G > A polymorphism and leishmaniasis by taking the two subgroups separately [OR_CL = 0.99 (0.84-1.18) and OR_VL = 1.19 (0.88-1.59)] or together [OR = 1.04 (0.90-1.20)]. This meta-analysis insinuates the absence of statistical evidence for an association between allele G and allele A of TNFα-308G > A polymorphism and Leishmania infection outcome. This suggests that TNFα, despite its crucial role in the immune response against Leishmania infection, is not the sole determinant factor. Other factors, such as gene-gene and gene-environment interactions, receptors, and signaling pathway efficiency, may influence TNFα function.

Systematic Review of Host-Mediated Activity of Miltefosine in Leishmaniasis through Immunomodulation

Host immune responses are pivotal for the successful treatment of the leishmaniases, a spectrum of infections caused by Leishmania parasites. Previous studies speculated that augmenting cytokines associated with a type 1 T-helper cell (Th1) response is necessary to combat severe forms of leishmaniasis, and it has been hypothesized that the antileishmanial drug miltefosine is capable of immunomodulation and induction of Th1 cytokines.

Host immune responses are pivotal for the successful treatment of the leishmaniases, a spectrum of infections caused by Leishmania parasites. Previous studies speculated that augmenting cytokines associated with a type 1 T-helper cell (Th1) response is necessary to combat severe forms of leishmaniasis, and it has been hypothesized that the antileishmanial drug miltefosine is capable of immunomodulation and induction of Th1 cytokines. A better understanding of the immunomodulatory effects of miltefosine is central to providing a rationale regarding synergistic mechanisms of activity to combine miltefosine optimally with other conventional and future antileishmanials that are currently under development. Therefore, a systematic literature search was performed to evaluate to what extent and how miltefosine influences the host Th1 response. Miltefosine’s effects observed in both a preclinical and a clinical context associated with immunomodulation in the treatment of leishmaniasis are evaluated in this review. A total of 27 studies were included in the analysis. Based on the current evidence, miltefosine is not only capable of inducing direct parasite killing but also of modulating the host immunity. Our findings suggest that miltefosine-induced activation of Th1 cytokines, particularly represented by increased gamma interferon (IFN-γ) and interleukin 12 (IL-12), is essential to prevail over the Leishmania-driven Th2 response. Differences in miltefosine-induced host-mediated effects between in vitro, ex vivo, animal model, and human studies are further discussed. All things considered, an effective treatment with miltefosine is acquired by enhanced functional Th1 cytokine responses and may further be enhanced in combination with immunostimulatory agents.

Nanotized Curcumin and Miltefosine, a Potential Combination for Treatment of Experimental Visceral Leishmaniasis

Leishmaniasis chemotherapy remains very challenging due to high cost of the drug and its associated toxicity and drug resistance, which develops over a period of time. Combination therapies (CT) are now in use to treat many diseases, such as cancer and malaria, since it is more effective and affordable than monotherapy. CT are believed to represent a new explorable strategy for leishmaniasis, a neglected tropical disease caused by the obligate intracellular parasite Leishmania In the present study, we investigated the effect of a combination of a traditional Indian medicine (ayurveda), a natural product curcumin and miltefosine, the only oral drug for visceral leishmaniasis (VL) using a Leishmania donovani-hamster model. We developed an oral nanoparticle-based formulation of curcumin. Nanoformulation of curcumin alone exhibited significant leishmanicidal activity both in vitro and in vivo In combination with miltefosine, it exhibited a synergistic effect on both promastigotes and amastigotes under in vitro conditions. The combination of these two agents also demonstrated increased in vivo leishmanicidal activity accompanied by increased production of toxic reactive oxygen/nitrogen metabolites and enhanced phagocytic activity. The combination also exhibited increased lymphocyte proliferation. The present study thus establishes the possible use of nanocurcumin as an adjunct to antileishmanial chemotherapy.

Immunoprotective effect of lentinan in combination with miltefosine on Leishmania-infected J-774A.1 macrophages

Rejuvenation of deteriorated host immune functions is imperative for successful annihilation of Leishmania parasites. The use of immunomodulatory agents may have several advantages as they conquer immunosuppression and, when given in combination, improve current therapeutic regimens. We herein investigated the immunostimulatory potency of a β-glucan, lentinan either alone or in combination with short dose of standard drug, miltefosine on Leishmania-infected J-774A.1 macrophages. Our study shows that infected macrophages when stimulated with 2.5 μg/mL and above concentrations of lentinan secreted significant amount of host-protective molecules. The in vitro interaction between lentinan and miltefosine showed some synergy (mean sum of fractional inhibitory concentration [mean ∑FIC] 0.87) at IC50 level. Lentinan (2.5 μg/mL) plus low-dose miltefosine (2 μM) displayed heightened level of pro-inflammatory cytokines, IL-12 (13.6-fold) and TNF-α (6.8-fold) along with nitric oxide (7.2-fold higher) when compared with infected control. In combination group, we also observed remarkably (P<.001) suppressed levels of anti-inflammatory cytokines, IL-10 and TGF-β, than that of untreated macrophages. Additionally, in comparison with infected group, we observed significant induction in phagocytic activity of macrophages in combination with treated group. Collectively, these findings emphasize the immunostimulatory effect of lentinan alone and in combination with low dose of miltefosine against Leishmania donovani.

Leishmania donovani Utilize Sialic Acids for Binding and Phagocytosis in the Macrophages through Selective Utilization of Siglecs and Impair the Innate Immune Arm

Background

Leishmania donovani, belonging to a unicellular protozoan parasite, display the differential level of linkage-specific sialic acids on their surface. Sialic acids binding immunoglobulin-like lectins (siglecs) are a class of membrane-bound receptors present in the haematopoetic cell lineages interact with the linkage-specific sialic acids. Here we aimed to explore the utilization of sialic acids by Leishmania donovani for siglec-mediated binding, phagocytosis, modulation of innate immune response and signaling pathways for establishment of successful infection in the host.

Methodology/Principle Findings

We have found enhanced binding of high sialic acids containing virulent strains (AG83^+Sias) with siglec-1 and siglec-5 present on macrophages compared to sialidase treated AG83^+Sias (AG83^-Sias) and low sialic acids-containing avirulent strain (UR6) by flow cytometry. This specific receptor-ligand interaction between sialic acids and siglecs were further confirmed by confocal microscopy. Sialic acids-siglec-1-mediated interaction of AG83^+Sias with macrophages induced enhanced phagocytosis. Additionally, sialic acids-siglec-5 interaction demonstrated reduced ROS, NO generation and Th2 dominant cytokine response upon infection with AG83^+Sias in contrast to AG83^-Sias and UR6. Sialic acids-siglecs binding also facilitated multiplication of intracellular amastigotes. Moreover, AG83^+Sias induced sialic acids-siglec-5-mediated upregulation of host phosphatase SHP-1. Such sialic acids-siglec interaction was responsible for further downregulation of MAPKs (p38, ERK and JNK) and PI3K/Akt pathways followed by the reduced translocation of p65 subunit of NF-κβ to the nucleus from cytosol in the downstream signaling pathways. This sequence of events was reversed in AG83^-Sias and UR6-infected macrophages. Besides, siglec-knockdown macrophages also showed the reversal of AG83^+Sias infection-induced effector functions and downstream signaling events.

Conclusions/Significances

Taken together, this study demonstrated that virulent parasite (AG83^+Sias) establish a unique sialic acids-mediated binding and subsequent phagocytosis in the host cell through the selective exploitation of siglec-1. Additionally, sialic acids-siglec-5 interaction altered the downstream signaling pathways which contributed impairment of immune effector functions of macrophages. To the best of our knowledge, this is a comprehensive report describing sialic acids-siglec interactions and their role in facilitating uptake of the virulent parasite within the host.

Sialic acids are nine carbon sugars present on terminal cell surface glycoproteins and glycolipids. Siglec is a membrane receptor that belongs to an immunoglobulin super family present in almost all the haematopoetic cell lineages. There are 14 different types of siglecs present on human immune cells that take an active part in balancing the magnitude of immunological reactions. In general, these siglecs bind with sialic acids and negatively regulate the immune response. Leishmania contains sialic acids on its surface. Virulent parasites utilize this sugar to bind with macrophages through siglec-1 and siglec-5 compared to low sialic acids containing avirulent parasites. Such sialic acids-siglec-mediated interactions exhibited a suppressed host immune response which helped them to establish successful infection compared to desialylated virulent and avirulent parasites, as well as, siglec-depleted macrophages. Interestingly, interaction between sialic acids and siglec-1 induced enhanced phagocytosis, while sialic acids-siglec-5 interaction upregulated the phosphatase SHP-1. This interaction with the virulent strain exhibited deactivation of various downstream signaling pathways and ultimately controlled translocation of a functional component of transcription factor NF-κβ for regulation of cytokines and other effector molecules in infected macrophages. Thus, the interaction between the parasite and the host cells through sialic acids-siglec binding is clearly a newly identified mechanism by which parasites can establish successful infection by subverting the host’s innate immune response.