Bioluminescent Imaging Identifies Thymus, As Overlooked Colonized Organ, in a Chronic Model of Leishmania donovani Mouse Visceral Leishmaniasis

Animal models of neglected parasitic diseases: In vivo multimodal imaging of experimental trypanosomatid infections

African trypanosomiases and leishmaniases are significant neglected tropical diseases (NTDs) that affect millions globally, with severe health and socio-economic consequences, especially in endemic regions. Understanding the pathogenesis and dissemination of Trypanosoma brucei and Leishmania spp. parasites within their hosts is pivotal for the development of effective interventions. Whole-body bioluminescence and fluorescence imaging systems (BLI and FLI, respectively), are powerful tools to visualize and quantify the progression and distribution of these parasites in real-time within live animal models. By combining this technology with the engineering of stable T. brucei and Leishmania spp. strains expressing luciferase and/or fluorescent proteins, crucial aspects of the infection process including the parasites' homing, the infection dynamics, the tissue tropism, or the efficacy of experimental treatments and vaccines can be deeply investigated. This methodology allows for enhanced sensitivity and resolution, elucidating previously unrecognized infection niches and dynamics. Importantly, whole-body in vivo imaging is non-invasive, enabling for longitudinal studies during the course of an infection in the same animal, thereby aligning with the "3Rs" principle of animal research. Here, we detail a protocol for the generation of dual-reporter T. brucei and L. major, and their use to infect mice and follow the spatiotemporal dynamics of infection by in vivo imaging systems. Additionally, 3D micro-computed tomography (μCT) coupled to BLI in T. brucei-infected animals is applied to gain insights into the anatomical parasite distribution. This Chapter underscores the potential of these bioimaging modalities as indispensable tools in parasitology, paving the way for novel therapeutic strategies and deeper insights into host-parasite interactions.

Targeting Trypanothione Metabolism in Trypanosomatids

Infectious diseases caused by trypanosomatids, including African trypanosomiasis (sleeping sickness), Chagas disease, and different forms of leishmaniasis, are Neglected Tropical Diseases affecting millions of people worldwide, mainly in vulnerable territories of tropical and subtropical areas. In general, current treatments against these diseases are old-fashioned, showing adverse effects and loss of efficacy due to misuse or overuse, thus leading to the emergence of resistance. For these reasons, searching for new antitrypanosomatid drugs has become an urgent necessity, and different metabolic pathways have been studied as potential drug targets against these parasites. Considering that trypanosomatids possess a unique redox pathway based on the trypanothione molecule absent in the mammalian host, the key enzymes involved in trypanothione metabolism, trypanothione reductase and trypanothione synthetase, have been studied in detail as druggable targets. In this review, we summarize some of the recent findings on the molecules inhibiting these two essential enzymes for Trypanosoma and Leishmania viability.

The Potential Use of Peptides in the Fight against Chagas Disease and Leishmaniasis

Chagas disease and leishmaniasis are both neglected tropical diseases that affect millions of people around the world. Leishmaniasis is currently the second most widespread vector-borne parasitic disease after malaria. The World Health Organization records approximately 0.7-1 million newly diagnosed leishmaniasis cases each year, resulting in approximately 20,000-30,000 deaths. Also, 25 million people worldwide are at risk of Chagas disease and an estimated 6 million people are infected with Trypanosoma cruzi. Pentavalent antimonials, amphotericin B, miltefosine, paromomycin, and pentamidine are currently used to treat leishmaniasis. Also, nifurtimox and benznidazole are two drugs currently used to treat Chagas disease. These drugs are associated with toxicity problems such as nephrotoxicity and cardiotoxicity, in addition to resistance problems. As a result, the discovery of novel therapeutic agents has emerged as a top priority and a promising alternative. Overall, there is a need for new and effective treatments for Chagas disease and leishmaniasis, as the current drugs have significant limitations. Peptide-based drugs are attractive due to their high selectiveness, effectiveness, low toxicity, and ease of production. This paper reviews the potential use of peptides in the treatment of Chagas disease and leishmaniasis. Several studies have demonstrated that peptides are effective against Chagas disease and leishmaniasis, suggesting their use in drug therapy for these diseases. Overall, peptides have the potential to be effective therapeutic agents against Chagas disease and leishmaniasis, but more research is needed to fully investigate their potential.

Miltefosine and Nifuratel Combination: A Promising Therapy for the Treatment of Leishmania donovani Visceral Leishmaniasis

Visceral leishmaniasis is a neglected vector-borne tropical disease caused by Leishmania donovani and Leishmania infantum that is endemic not only in East African countries, but also in Asia, regions of South America and the Mediterranean Basin. For the pharmacological control of this disease, there is a limited number of old and, in general, poorly adherent drugs, with a multitude of adverse effects and low oral bioavailability, which favor the emergence of resistant pathogens. Pentavalent antimonials are the first-line drugs, but due to their misuse, resistant Leishmania strains have emerged worldwide. Although these drugs have saved many lives, it is recommended to reduce their use as much as possible and replace them with novel and more friendly drugs. From a commercial collection of anti-infective drugs, we have recently identified nifuratel-a nitrofurantoin used against vaginal infections-as a promising repurposing drug against a mouse model of visceral leishmaniasis. In the present work, we have tested combinations of miltefosine-the only oral drug currently used against leishmaniasis-with nifuratel in different proportions, both in axenic amastigotes from bone marrow and in intracellular amastigotes from infected Balb/c mouse spleen macrophages, finding a potent synergy in both cases. In vivo evaluation of oral miltefosine/nifuratel combinations using a bioimaging platform has revealed the potential of these combinations for the treatment of this disease.

Anti-trypanosomatid drug discovery: progress and challenges

Leishmaniasis (visceral and cutaneous), Chagas disease and human African trypanosomiasis cause substantial death and morbidity, particularly in low- and middle-income countries. Although the situation has improved for human African trypanosomiasis, there remains an urgent need for new medicines to treat leishmaniasis and Chagas disease; the clinical development pipeline is particularly sparse for Chagas disease. In this Review, we describe recent advances in our understanding of the biology of the causative pathogens, particularly from the drug discovery perspective, and we explore the progress that has been made in the development of new drug candidates and the identification of promising molecular targets. We also explore the challenges in developing new clinical candidates and discuss potential solutions to overcome such hurdles.

Comparison of Bioluminescent Substrates in Natural Infection Models of Neglected Parasitic Diseases

The application of in vivo bioluminescent imaging in infectious disease research has significantly increased over the past years. The detection of transgenic parasites expressing wildtype firefly luciferase is however hampered by a relatively low and heterogeneous tissue penetrating capacity of emitted light. Solutions are sought by using codon-optimized red-shifted luciferases that yield higher expression levels and produce relatively more red or near-infrared light, or by using modified bioluminescent substrates with enhanced cell permeability and improved luminogenic or pharmacokinetic properties. In this study, the in vitro and in vivo efficacy of two modified bioluminescent substrates, CycLuc1 and AkaLumine-HCl, were compared with that of D-luciferin as a gold standard. Comparisons were made in experimental and insect-transmitted animal models of leishmaniasis (caused by intracellular Leishmania species) and African trypanosomiasis (caused by extracellular Trypanosoma species), using parasite strains expressing the red-shifted firefly luciferase PpyRE9. Although the luminogenic properties of AkaLumine-HCl and D-luciferin for in vitro parasite detection were comparable at equal substrate concentrations, AkaLumine-HCl proved to be unsuitable for in vivo infection follow-up due to high background signals in the liver. CycLuc1 presented a higher in vitro luminescence compared to the other substrates and proved to be highly efficacious in vivo, even at a 20-fold lower dose than D-luciferin. This efficacy was consistent across infections with the herein included intracellular and extracellular parasitic organisms. It can be concluded that CycLuc1 is an excellent and broadly applicable alternative for D-luciferin, requiring significantly lower doses for in vivo bioluminescent imaging in rodent models of leishmaniasis and African trypanosomiasis.

Thymus, undernutrition, and infection: Approaching cellular and molecular interactions

Undernutrition remains a major issue in global health. Low protein-energy consumption, results in stunting, wasting and/or underweight, three deleterious forms of malnutrition that affect roughly 200 million children under the age of five years. Undernutrition compromises the immune system with the generation of various degrees of immunodeficiency, which in turn, renders undernourished individuals more sensitive to acute infections. The severity of various infectious diseases including visceral leishmaniasis (VL), influenza, and tuberculosis is associated with undernutrition. Immunosuppression resulting from protein-energy undernutrition severely impacts primary and secondary lymphoid organs involved in the response to related pathogens. The thymus—a primary lymphoid organ responsible for the generation of T lymphocytes—is particularly compromised by both undernutrition and infectious diseases. In this respect, we will discuss herein various intrathymic cellular and molecular interactions seen in undernutrition alone or in combination with acute infections. Many examples illustrated in studies on humans and experimental animals clearly revealed that protein-related undernutrition causes thymic atrophy, with cortical thymocyte depletion. Moreover, the non-lymphoid microenvironmental compartment of the organ undergoes important changes in thymic epithelial cells, including their secretory products such as hormones and extracellular matrix proteins. Of note, deficiencies in vitamins and trace elements also induce thymic atrophy. Interestingly, among the molecular interactions involved in the control of undernutrition-induced thymic atrophy is a hormonal imbalance with a rise in glucocorticoids and a decrease in leptin serum levels. Undernutrition also yields a negative impact of acute infections upon the thymus, frequently with the intrathymic detection of pathogens or their antigens. For instance, undernourished mice infected with Leishmania infantum (that causes VL) undergo drastic thymic atrophy, with significant reduction in thymocyte numbers, and decreased levels of intrathymic chemokines and cytokines, indicating that both lymphoid and microenvironmental compartments of the organ are affected. Lastly, recent data revealed that some probiotic bacteria or probiotic fermented milks improve the thymus status in a model of malnutrition, thus raising a new field for investigation, namely the thymus-gut connection, indicating that probiotics can be envisioned as a further adjuvant therapy in the control of thymic changes in undernutrition accompanied or not by infection.

Design, synthesis and evaluation of novel phenanthridine triazole analogs as potential antileishmanial agents

Aim: To synthesize and screen phenanthridine and 1,2,3-triazole derivatives for antileishmanial activity. Methodology: Synthesized analogs were tested for antileishmanial activity against transgenic strain of Leishmania infantum promastigotes and ex vivo infections. Results: Compounds T01, T08 and T11 revealed significant activity with EC₅₀ <30 μm and lacked toxicity in mouse spleen and HepG2 cells. T01 with EC₅₀ 3.07 μm is four-fold more potent than the drug miltefosine (EC₅₀ 12.6 μM) against L. infantum promastigotes. In silico studies indicate that the analogs are nontoxic. A molecular docking analysis was also carried out on the T01 and T08 to investigate the binding pattern at the active site of the chosen target trypanothione reductase. Conclusion: The results of this study reveal that phenanthridine triazoles exhibit antileishmanial activity.

Ex Vivo Phenotypic Screening of Two Small Repurposing Drug Collections Identifies Nifuratel as a Potential New Treatment against Visceral and Cutaneous Leishmaniasis

Leishmaniases are vector-borne neglected diseases caused by single-celled parasites. The search for new antileishmanial drugs has experienced a strong boost thanks to the application of bioimaging to phenotypic screenings based on intracellular amastigotes. Mouse splenic explants infected with fluorescent strains of Leishmania are proven tools of drug discovery, where hits can be easily transferred to preclinical in vivo models. We have developed a two-staged platform for antileishmanial drugs. First, we screened two commercial collections of repurposing drugs with a total of 1769 compounds in ex vivo mouse splenocytes infected with an infrared emitting Leishmania infantum strain. The most active and safest compounds were scaled-up to in vivo models of chronic Leishmania donovani visceral leishmaniasis and Leishmania major cutaneous leishmaniasis. From the total of 1769 compounds, 12 hits with selective indices >35 were identified, and 4 of them were tested in vivo in a model of L. donovani visceral leishmaniasis. Nifuratel, a repurposed synthetic nitrofuran, when administered orally at 50 mg/kg bw once or twice a day for 10 days, caused >80% reduction in the parasitic load. Furthermore, the intralesional administration of nifuratel in a model of cutaneous leishmaniasis by L. major produced the parasitological cure. From the previous results we have deduced the great capacity of mouse splenic explants to identify new hits, a model which could be easily transferred to in vivo models, as well as the potential use of nifuratel as an alternative to the current treatment of cutaneous leishmaniasis.