Virulence loss and amastigote transformation failure determine host cell responses to Leishmania mexicana

Distinct neutrophil effector functions in response to different isolates of Leishmania aethiopica

BACKGROUND

In Ethiopia, cutaneous leishmaniasis is mainly caused by Leishmania (L.) aethiopica parasites and presents in three main clinical forms. It is still not clear if the host immune response plays a role in the development of these different presentations. Since neutrophils are likely to be one of the first immune cells present at the site of the sand fly bite, we set up an in vitro model of infection of neutrophils with L. aethiopica and assessed some of the main neutrophil effector functions: association with and internalisation of parasites, apoptosis and ROS production. We used three freshly isolated clinical isolates and one isolate that has been kept in culture for decades.

RESULTS

Our results showed by flow cytometry that all four L. aethiopica isolates had the ability to associate with neutrophils. The three clinical isolates of L. aethiopica associated more efficiently with neutrophils than the long-term cultured L. aethiopica. At 18 h, two distinct populations of neutrophils were identified that associated with L. aethiopica, CD15high and CD15low neutrophils. Confocal microscopy demonstrated that all isolates can be internalised. Our results also showed that all parasites induced apoptosis in L. aethiopica-associated neutrophils. Moreover, our results showed that after 2 h, L. aethiopica-associated neutrophils upregulated their production of ROS, but to a greater extent with the long-term cultured L. aethiopica. After 18 h of incubation, CD15lowparasite+ showed an impaired ability to produce ROS compared to CD15highparasite+.

CONCLUSIONS

Using this in vitro model, our results show that different L. aethiopica parasite isolates, most notably long-term cultured parasites, had differential effects on neutrophil effector functions.

Genomic and virulence analysis of in vitro cultured Cryptosporidium parvum

Recent advances in the in vitro cultivation of Cryptosporidium parvum using hollow fiber bioreactor technology (HFB) have permitted continuous growth of parasites that complete all life cycle stages. The method provides access to all stages of the parasite and provides a method for non-animal production of oocysts for use in clinical trials. Here we examined the effect of long-term (>20 months) in vitro culture on virulence-factors, genome conservation, and in vivo pathogenicity of the host by in vitro cultured parasites. We find low-level sequence variation that is consistent with that observed in calf-passaged parasites. Further using a calf model infection, oocysts obtained from the HFB caused diarrhea of the same volume, duration and oocyst shedding intensity as in vivo passaged parasites.

Transforming parasites into their own foes: parasitic extracellular vesicles as a vaccine platform

Parasitic diseases continue to afflict millions of people globally. However, traditional vaccine development strategies are often difficult to apply to parasites, leaving an immense unmet need for new effective vaccines for the prevention and control of parasitic infections. As parasites commonly use extracellular vesicles (EVs) to interact with, interfere with, or modulate the host immune response from a distance, parasite-derived EVs may provide promising vaccine agents that induce immunity against parasitic infections. We here present achievements to date and the challenges and limitations associated with using parasitic EVs in a clinical context. Despite the many difficulties that need to be overcome, we believe this direction could offer a new and reliable source of therapeutics for various neglected parasitic diseases.

Parasite Genotype Is a Major Predictor of Mortality from Visceral Leishmaniasis

Visceral leishmaniasis (VL) is a potentially fatal disease caused mainly by Leishmania infantum in South America and Leishmania donovani in Asia and Africa. Disease outcomes have been associated with patient genotype, nutrition, age, sex, comorbidities, and coinfections. In this study, we examine the effects of parasite genetic variation on VL disease severity in Brazil. We collected and sequenced the genomes of 109 L. infantum isolates from patients in northeastern Brazil and retrieved matching patient clinical data from medical records, including mortality, sex, HIV coinfection, and laboratory data (creatinine, hemoglobin, and leukocyte and platelet counts). We identified genetic differences between parasite isolates, including single nucleotide polymorphisms (SNPs), small insertions/deletions (indels), and variations in genic, intergenic, and chromosome copy numbers (copy number variants [CNVs]). To describe associations between the parasite genotypes and clinical outcomes, we applied quantitative genetics methods of heritability and genome-wide association studies (GWAS), treating clinical outcomes as traits that may be influenced by parasite genotype. Multiple aspects of the genetic analysis indicate that parasite genotype affects clinical outcomes. We estimate that parasite genotype explains 83% chance of mortality (narrow-sense heritability [h²] = 0.83 ± 0.17) and has a significant relationship with patient sex (h² = 0.60 ± 0.27). Impacts of parasite genotype on other clinical traits are lower (h² ≤ 0.34). GWAS analysis identified multiple parasite genetic loci that were significantly associated with clinical outcomes; 17 CNVs were significantly associated with mortality, two with creatinine, and one with bacterial coinfection, jaundice, and HIV coinfection, and two SNPs/indels and six CNVs were associated with age, jaundice, HIV and bacterial coinfections, creatinine, and/or bleeding sites. Parasite genotype is an important factor in VL disease severity in Brazil. Our analysis indicates that specific genetic differences between parasites act as virulence factors, enhancing risks of severe disease and mortality. More detailed understanding of these virulence factors could be exploited for novel therapies. IMPORTANCE Multiple factors contribute to the risk of mortality from visceral leishmaniasis (VL), including, patient genotype, comorbidities, and nutrition. Many of these factors are influenced by socioeconomic biases. Our work suggests that the virulence of the infecting parasite is an important risk factor for mortality. We pinpoint some specific genomic markers that are associated with mortality, which can lead to a greater understanding of the molecular mechanisms that cause severe VL disease, to the identification of genetic markers for virulent parasites, and to the development of drug and vaccine therapies.

Three types of Leishmania mexicana amastigotes: Proteome comparison by quantitative proteomic analysis

Leishmania is the unicellular parasite transmitted by phlebotomine sand fly bite. It exists in two different forms; extracellular promastigotes, occurring in the gut of sand flies, and intracellular, round-shaped amastigotes residing mainly in vertebrate macrophages. As amastigotes originating from infected animals are often present in insufficient quality and quantity, two alternative types of amastigotes were introduced for laboratory experiments: axenic amastigotes and amastigotes from macrophages infected in vitro. Nevertheless, there is very little information about the degree of similarity/difference among these three types of amastigotes on proteomic level, whose comparison is crucial for assessing the suitability of using alternative types of amastigotes in experiments. In this study, L. mexicana amastigotes obtained from lesion of infected BALB/c mice were proteomically compared with alternatively cultivated amastigotes (axenic and macrophage-derived ones). Amastigotes of all three types were isolated, individually treated and analysed by LC-MS/MS proteomic analysis with quantification using TMT10-plex isobaric labeling. Significant differences were observed in the abundance of metabolic enzymes, virulence factors and proteins involved in translation and condensation of DNA. The most pronounced differences were observed between axenic amastigotes and lesion-derived amastigotes, macrophage-derived amastigotes were mostly intermediate between axenic and lesion-derived ones.

A Simple Bioluminescent Assay for the Screening of Cytotoxic Molecules Against the Intracellular Form of Leishmania infantum

This chapter describes a viability assay for the intracellular (amastigote) and clinically relevant form of Leishmania infantum that is based on the detection of bioluminescence (BL) signal. The assay uses a reporter cell line of L. infantum that expresses constitutively a redshifted luciferase from Photinus pyralis and murine macrophages (cell line J774.A1) as host cells for infection. The host cell line was selected because it is a differentiated cell line, easy to manipulate in vitro, and advantageous for ethical reasons. This chapter introduces an assay designed for the screening of bioactive compounds/molecules employing a 96-well microplate and a 24 h treatment. The assay setup shows excellent balance between simplicity (cell culture manipulation/infection and timing) and quality parameters, as well as potential to detect drug-like molecules acting in a fast and cytotoxic manner.

Label-Free Quantitative Proteomic Analysis of Three Strains of Viscerotropic Leishmania Isolated from Patients with Different Epidemiological Types of Visceral Leishmaniasis in China

BACKGROUND

There are three epidemiological types of visceral leishmaniasis in China, which are caused by Leishmania strains belonging to the L. donovani complex. The mechanisms underlying their differences in the population affected, disease latency, and animal host, etc., remain unclear. We investigated the protein abundance differences among Leishmania strains isolated from three types of visceral leishmaniasis endemic areas in China.

METHODS

Promastigotes of the three Leishmania strains were cultured to the log phase and harvested. The protein tryptic digests were analyzed with liquid chromatography-electrospray ionization-tandem mass spectrometry (LC-ESI-MS/MS), followed by label-free quantitative analysis. The MS experiment was performed on a Q Exactive mass spectrometer. Raw spectra were quantitatively analyzed with the MaxQuant software (ver 1.3.0.5) and matched with the reference database. Differentially expressed proteins were analyzed using the bioinformatics method. The MS analysis was repeated three times for each sample.

RESULTS

A total of 5012 proteins were identified across the KS-2, JIASHI-5 and SC6 strains in at least 2 of the three samples replicate. Of them, 1758 were identified to be differentially expressed at least between 2 strains, including 349 with known names. These differentially expressed proteins with known names are involved in biological functions such as energy and lipid metabolic process, nucleotide acid metabolic process, amino acid metabolic process, response to stress, cell membrane/cytoskeleton, cell cycle and proliferation, biological adhesion and proteolysis, localization and transport, regulation of the biological process, and signal transduction.

CONCLUSION

The differentially expressed proteins and their related biological functions may shed light on the pathogenicity of Leishmania and targets for the development of vaccines and medicines.

Expression of MicroRNA of Macrophages Infected with Attenuated Leishmania major Parasite

Objective Leishmaniasis has been proposed as one of the neglected vector-borne diseases due to an obligate intracellular parasite of the genus Leishmania. MicroRNAs (miRNAs) with a length of 22-nucleotide are known as the noncoding small RNAs. MiRNAs contribute to many biological and cellular approaches. Therefore, the present study evaluated expressing mmu-miR-721, mmu-miR-294–3p, mmu-miR-155–3p, and mmu-miR-30a in murine macrophages infected with attenuated Leishmania major parasites on 3 days after infection.

Methods Attenuated promastigotes have been achieved after 20 passages of Leishmania major parasites. Cell line J774A.1 (murine macrophage) has been used for in vitro experiments. The stationary phase of attenuated L. major promastigotes has been chosen to infect the cells, and then their incubation has been performed with 5% CO2 at 37°C for 3 days. The real-time polymerase chain reaction (PCR) has also been performed with SYBR Green master-mix Kit for measuring the level of mmu-miR-721, mmu-miR-294–3p, mmu-miR-30a, and mmu-miR-155-3p expression. Uninfected macrophages have been considered as a control group.

Results Real-time PCR demonstrated overexpression of mmu-miR-155-3p, mmu-miR-294–3p, and, mmu-miR-721 in the infected cells with Leishmania parasites after 3 days. Results showed no statistically significant difference in the mmu-miR-30a expression between infected macrophages and the uninfected control group.

Conclusion Our findings suggested the significant contribution of the alterations in the miRNA levels to the regulation of macrophage functions following the creation of intracellular parasites like Leishmania. These data could help to understand better the genes' expression in the host cells in the course of leishmaniasis.

Workflow of Lotmaria passim isolation: Experimental infection with a low-passage strain causes higher honeybee mortality rates than the PRA-403 reference strain

The impact of trypanosomatid parasites on honeybee health may represent a major threat to bee colonies worldwide. However, few axenic isolates have been generated to date and with no details on cell culture passages, a parameter that could influence parasite virulence. To address this question, a trypanosomatid isolation protocol was developed and a new strain was obtained, named L. passim C1. Using experimental infection of worker honeybees, we compared the virulence and mortality rates of the ATCC PRA-403 reference strain and C1 strain, the latter showing higher virulence from 10 days post-infection onward. This study highlights the impact of cell culture passages on the pathogenicity of L. passim in honeybees, providing new evidence of its negative effects on honeybee health.

TLR4 abrogates the Th1 immune response through IRF1 and IFN-β to prevent immunopathology during L. infantum infection

A striking feature of human visceral leishmaniasis (VL) is chronic inflammation in the spleen and liver, and VL patients present increased production levels of multiple inflammatory mediators, which contribute to tissue damage and disease severity. Here, we combined an experimental model with the transcriptional profile of human VL to demonstrate that the TLR4-IFN-β pathway regulates the chronic inflammatory process and is associated with the asymptomatic form of the disease. Tlr4-deficient mice harbored fewer parasites in their spleen and liver than wild-type mice. TLR4 deficiency enhanced the Th1 immune response against the parasite, which was correlated with an increased activation of dendritic cells (DCs). Gene expression analyses demonstrated that IRF1 and IFN-β were expressed downstream of TLR4 after infection. Accordingly, IRF1- and IFNAR-deficient mice harbored fewer parasites in the target organs than wild-type mice due to having an increased Th1 immune response. However, the absence of TLR4 or IFNAR increased the serum transaminase levels in infected mice, indicating the presence of liver damage in these animals. In addition, IFN-β limits IFN-γ production by acting directly on Th1 cells. Using RNA sequencing analysis of human samples, we demonstrated that the transcriptional signature for the TLR4 and type I IFN (IFN-I) pathways was positively modulated in asymptomatic subjects compared with VL patients and thus provide direct evidence demonstrating that the TLR4-IFN-I pathway is related to the nondevelopment of the disease. In conclusion, our results demonstrate that the TLR4-IRF1 pathway culminates in IFN-β production as a mechanism for dampening the chronic inflammatory process and preventing immunopathology development.

Visceral leishmaniasis (VL) is one of the most lethal neglected tropical diseases and is caused by Leishmania parasites. Most subjects infected with Leishmania present subclinical VL symptoms, and their immune response is mediated by Th1 cells and immunoregulatory mechanisms. However, when infection progresses to disease, VL patients present increased levels of inflammatory mediators in the serum which are related to the severity of disease. During infection, Toll-like receptors (TLRs) interact with Leishmania parasites and contribute to the outcome of the disease. Herein, we report that TLR4 signaling hampers the chronic immune response during VL to prevent immunopathology. TLR4 triggers the activation of IRF1 and thus induces the transcription of IFN-β, which in turn acts directly on Th1 cells to limit the production of IFN-γ. In addition, a transcription analysis of human VL samples provides direct evidence demonstrating that the TLR4-IFN-I pathway is related to the asymptomatic form of the disease. Collectively, our findings reveal that TLR4 hampers the Th1 immune response through IRF1 and IFN-β to prevent immunopathology during VL.

In Vitro Infections of Macrophage-Like Cell Lines with Leishmania infantum for Drug Screening

The study of in vitro infections is essential to evaluate distinct aspects of Leishmania biology and also invaluable for more meaningful in vitro screening of promising chemical entities. Macrophage-like cells lines from different origins are amenable to Leishmania infection. Cell lines due to their stability and standardization potential are highly valued for their capacity to support reproducible infections and consistent data. In fact, these cells have been a mainstay of leishmaniasis research for more than 40 years. In this context, the human monocytic THP-1 cell line is commonly used as it can be differentiated with phorbol-12myristate-13-acetate (PMA) into macrophages that are susceptible to Leishmania infection. In this section, we will describe generalities concerning the use of cell lines for in vitro Leishmania infection using THP-1 derived macrophages and Leishmania infantum axenic amastigotes expressing luciferase associated to preclinical drug screening as example.

Genome Plasticity in Cultured Leishmania donovani: Comparison of Early and Late Passages

Leishmania donovani possesses a complex heteroxenic life cycle where infective metacyclic promastigotes are pre-adapted to infect their host and cope up with intracellular stress. Exploiting the similarities between cultured and sandfly derived promastigotes, we used early and late passage cultured promastigotes to show specific changes at genome level which compromise pathogen fitness reflected in gene expression and infection studies. The pathogen loses virulence mostly via transcriptional and translational regulations and long-time cultivation makes them struggle to convert to virulent metacyclics. At the genomic level very subtle plasticity was observed between the early and the late passages mostly in defense-related, nutrient acquisition and signal transduction genes. Chromosome Copy number variation is seen in the early and late passages involving several genes that may be playing a role in pathogenicity. Our study highlights the importance of ABC transporters and calpain like cysteine proteases in parasite virulence in cultured promastigotes. Interestingly, these proteins are emerging as important patho-adaptive factors in clinical isolates of Leishmania. We found that the currently available genome of Leishmania in the NCBI database are from late passages. Our early passage genome can act as a reference for future studies on virulent isolates of Leishmania. The annotated leads from this study can be used for virulence surveillance and therapeutic studies in the Indian subcontinent.

Evidence of Subpopulations with Distinct Biological Features Within a Leishmania (Viannia) braziliensis Strain

The present study demonstrates that the Leishmania (Viannia) braziliensis strain MCAN/BR/1998/R619 is composed of multiple subpopulations with measurable distinctions. Single parasites were separated from a culture of promastigotes in stationary phase by cell sorting and then cultivated as subpopulations. Subsequently, these subpopulations were evaluated for features of in vitro growth, infectivity to murine macrophages and proteinase gene expression. The first evidence of distinct characteristics was observed during the in vitro cultivation of isolated subpopulations, as distinct clusters of patterns were formed among the cultures, indicating the existence of quantifiable fluctuations in metrics. Further, when infecting murine macrophages, the subpopulations induced distinct patterns of production of immune response mediators. While some subpopulations mainly induced the production of IL-1β, IL-6 and TNF-α, others induced the production of IL-12p70 and nitric oxide. Finally, amastigotes of these subpopulations had higher expression of proteinase genes than promastigotes. Additionally, cysteine proteinase, serine proteinase, metalloproteinase and aspartic proteinases were differentially expressed in promastigote and amastigote forms. These data suggest the existence of distinct profiles for the L. (V.) braziliensis MCAN/BR/1998/R619 strain and subpopulations that could drive the success of parasite adaptation to the environments that they inhabit.

New insights into experimental visceral leishmaniasis: Real-time in vivo imaging of Leishmania donovani virulence

Visceral leishmaniasis is an insidious neglected disease with worldwide distribution. It is caused by parasites from the Leishmania donovani complex, which are able to be transmitted by different species of phlebotomine sand flies and to infect numerous mammal hosts. Despite the high number of people infected or at risk, and the remarkable quantity of studies focusing on this disease, a proper experimental model to efficiently decipher the infectious process of visceral leishmaniasis taking into account the nuances of parasite’s virulence and the duration of the infection is still lacking. Therefore, using golden Syrian hamsters and BALB/c mice, state-of-the-art genetic manipulation applied on a fully virulent L. donovani strain and in vivo imaging approaches, we describe herein three benefits for experimental visceral leishmaniasis: (i) the development of a double transfected bioluminescent (firefly luciferase) and fluorescent (E2-crimson) virulent strain of L. donovani (Ld1S_luci_E2-crimson), favoring a wide range of both in vivo and in vitro investigations, (ii) the establishment of a non-invasive mouse model to evaluate the infectious process during visceral leishmaniasis and the parasite’s virulence in real time, allowing longitudinal studies with the same animals, and (iii) the elaboration of a suitable method to reinstate (and verify anew) the virulence in a population of attenuated parasites, by recovering persistent parasites from chronic infected mice. Consequently, these results open up new perspectives on the study of visceral leishmaniasis, especially in the fields of therapeutics and vaccinology, since the model described herein renders now possible long-lasting follow up studies, with easy and accurate day-by-day verifications of the infection status along with a reduced number of laboratory animals.

TRIAL REGISTRATION

ClinicalTrials.gov 2013-0047.

A vaccine composed of a hypothetical protein and the eukaryotic initiation factor 5a from Leishmania braziliensis cross-protection against Leishmania amazonensis infection

In the present study, two proteins cloned from Leishmania braziliensis species, a hypothetical protein (LbHyp) and the eukaryotic initiation factor 5a (EiF5a), were evaluated to protect BALB/c mice against L. amazonensis infection. The animals were immunized with the antigens, either separately or in combination, using saponin as an immune adjuvant in both cases. Spleen cells from vaccinated and later infected mice produced significantly higher levels of protein and parasite-specific IFN-γ, IL-12, and GM-CSF, in addition to low levels of IL-4 and IL-10. Evaluating the parasite load by means of a limiting dilution technique and quantitative Real-Time PCR, vaccinated animals presented significant reductions in the parasite load in both infected tissues and organs, as well as lower footpad swelling, when compared to the control (saline and saponin) groups. The best results regarding the protection of the animals were achieved when the combined vaccine was administered into the animals. Protection was associated with an IFN-γ production against parasite antigens, which was mediated by both CD4+ and CD8+ T cells and correlated with antileishmanial nitrite production. In conclusion, data from the present study show that this polyprotein vaccine, which combines two L. braziliensis proteins, can induce protection against L. amazonensis infection.

Leishmania, the phagosome, and host responses: The journey of a parasite

Leishmania is the eukaryotic parasite responsible for leishmaniases, a spectrum of diseases that puts at risk roughly 350millions of people in 98 countries according to the Drugs for Neglected Diseases initiative (DNDi). This parasite has a complex life cycle composed of two distinct stages, the promastigote form found in the female sand-fly vector and the amastigote form that replicates in the mammalian host (Teixeira et al., 2013) [1]. To survive, the parasite interacts with its host immune system at multiple levels. In this review, we discuss the nature of those interactions, how they affect the host immune system, and how they affect parasite survival from the very beginning of the life cycle in the vector to its dissemination within the mammalian host.

Attenuation and Production of the Amphotericin B-Resistant Leishmania tropica Strain

Background

Infections caused by Leishmania are becoming major public health problems on a global scale. Many species of Leishmania around the world are obtaining resistance levels of up to 15 folds, as estimated by the World Health Organization. Leishmania showing resistance is relatively difficult to observe and maintain in laboratory settings.

Objectives

The current study deals with the generation of Leishmania tropica strains that are resistant to amphotericin B (amp B).

Materials and Methods

The L. tropica strain was attenuated using continuous passaging 20 times. The infectivity of L. tropica was confirmed in BALB/c mice. The L. tropica resistant strain was produced in vitro using a continuous increase in drug pressure. The cross resistance of L. tropica to other drugs was also investigated.

Results

After 20 continuous passages, the BALB/c mice tested negative in the development of leishmaniasis. At a concentration of 0.1 µg/mL, L. tropica showed resistance to amp B. The newly developed promastigotes were 16 times more resistant compared to the resistance of the wild type promastigotes. The resistant L. tropica strain showed cross resistance to itraconazole and had a resistance index that was greater than five. The resistant strain displayed maximum stability for more than three months in the drug-free medium.

Conclusions

The resistant strain of L. tropica can be produced in laboratories using continuous drug pressure. The attenuated resistant strain has significant implications (both medically and academically) in the ability to overcome resistance.

Comparative Fitness of a Parent Leishmania donovani Clinical Isolate and Its Experimentally Derived Paromomycin-Resistant Strain

Paromomycin has recently been introduced for the treatment of visceral leishmaniasis and emergence of drug resistance can only be appropriately judged upon its long term routine use in the field. Understanding alterations in parasite behavior linked to paromomycin-resistance may be essential to assess the propensity for emergence and spread of resistant strains. A standardized and integrated laboratory approach was adopted to define and assess parasite fitness of both promastigotes and amastigotes using an experimentally induced paromomycin-resistant Leishmania donovani strain and its paromomycin-susceptible parent wild-type clinical isolate. Primary focus was placed on parasite growth and virulence, two major components of parasite fitness. The combination of in vitro and in vivo approaches enabled detailed comparison of wild-type and resistant strains for which no differences could be demonstrated with regard to promastigote growth, metacyclogenesis, in vitro infectivity, multiplication in primary peritoneal mouse macrophages and infectivity for Balb/c mice upon infection with 2 x 10⁷ metacyclic promastigotes. Monitoring of in vitro intracellular amastigote multiplication revealed a consistent decrease in parasite burden over time for both wild-type and resistant parasites, an observation that was subsequently also confirmed in a larger set of L. donovani clinical isolates. Though the impact of these findings should be further explored, the study results suggest that the epidemiological implications of acquired paromomycin-resistance may remain minimal other than the loss of one of the last remaining drugs effective against visceral leishmaniasis.

High-content assay for measuring intracellular growth of Leishmania in human macrophages

Leishmania species are sandfly-transmitted protozoan parasites that cause a spectrum of diseases, ranging from localized skin lesions to fatal visceral disease, in more than 12 million people worldwide. These parasites primarily target macrophages in their mammalian hosts and proliferate as non-motile amastigotes in the phagolysosomal compartment of these cells. High-throughput screens for measuring Leishmania growth within this intracellular niche are needed to identify host and parasite factors that are required for virulence and to identify new drug candidates. Here we describe the development of a new high-content imaging method for quantifying the intracellular growth of Leishmania mexicana parasites in THP-1 macrophages. Wild-type parasites were pre-stained with the fluorescent dye CellTracker(™) Orange CMRA and used to infect THP-1 macrophages in 384-well plates. Infected and uninfected macrophages were subsequently stained with CellTracker Green CMFDA, allowing accurate quantitation of the number of parasites per macrophage using separate detector channels. We validated this method for use in high-content drug screening by examining the dose dependence of known anti-leishmanial drugs on intracellular growth. Unlike previous protocols, this method does not require the generation of transgenic fluorescent or bioluminescent parasite lines and can be readily adapted for screening different Leishmania species, strains, or mutant lines in a wide range of phagocytic host cell types.