An In Vitro Assay to Quantify the Intracellular Growth of Parasite in Macrophages
JoVE Encyclopedia of Experiments
Immunology
0 views • 3:29 min • July 8th, 2025
Take two microplates containing coverslips with bone marrow-derived macrophages or BMMs.
Add Leishmania metacyclic promastigotes — the elongated flagellated stage — into one plate and amastigotes — the oval non-flagellated stage — in another plate.
During incubation, the parasites bind to macrophage receptors, triggering receptor-mediated phagocytosis.
The phagosome fuses with the lysosomes and vesicles derived from the endoplasmic reticulum and Golgi apparatus, maturing into a parasitophorous vacuole.
The vacuole's decreased pH triggers promastigote differentiation into amastigote and its replication.
Wash to remove non-internalized parasites.
Fix the infected macrophages and add detergent to permeabilize the cells and parasites.
Introduce a fluorescent nucleic acid dye to stain the macrophage and parasite nuclei.
Mount the coverslips on slides and observe under a fluorescence microscope, quantifying the larger host nuclei and surrounding smaller amastigote nuclei.
Compare amastigote numbers to assess their multiplication within macrophages initially infected with undifferentiated promastigotes versus those infected with replicative amastigotes.
To infect the cells with L. Amazonensis, dilute the parasite suspension to the appropriate experimental multiplicity of infection, and add 50 to 100 microliters of parasites to each well of all 6-well plates, including all time points of the experiment. Incubate the bone marrow-derived macrophages for the appropriate infection period and temperature. Then, wash the wells three times with 2 milliliters of fresh 37 degrees Celsius PBS without calcium or magnesium per well, and fix the initial time point samples with 1.5 to 2 milliliters of 2% paraformaldehyde for 10 minutes.
For the plates corresponding to the later time points, add 2 milliliters of fresh bone marrow-derived macrophage medium to each well and return the plates to the appropriate incubator. Then, fix and wash the cells for each remaining time point as just demonstrated, and store the samples at 4 degrees Celsius in fresh PBS.
To stain the cells with DAPI, replace the supernatant with 1.5 milliliters of fresh PBS without calcium and magnesium, supplemented with 0.1% non-ionic detergent for 10 minutes at room temperature, followed by three 2-milliliter washes with PBS without calcium or magnesium. Next, stain the cells with 2 micrograms per milliliter of DAPI in 1 milliliter of PBS for 1 hour at room temperature.
After three washes, place the coverslips cell-side down onto individual glass microscope slides mounted with a commercially available anti-fade mounting reagent. Then, to quantify the number of infected cells, use the 100x immersion oil objective of a fluorescence microscope and Emanuel counter, to identify the number of smaller amastigote nuclei clustered around each large DAPI-stained macrophage nucleus.
Overview
This study investigates the interaction between Leishmania parasites and bone marrow-derived macrophages (BMMs). It details the processes of phagocytosis, vacuole maturation, and the differentiation of promastigotes into amastigotes within macrophages.
Key Study Components
Area of Science
- Microbiology
- Immunology
- Cell Biology
Background
- Leishmania is a genus of parasites that can infect macrophages.
- Understanding the lifecycle stages of Leishmania is crucial for developing treatments.
- Phagocytosis is a key mechanism by which macrophages respond to pathogens.
- Vacuole maturation is essential for the survival and replication of intracellular parasites.
Purpose of Study
- To analyze the infection process of Leishmania in BMMs.
- To compare the replication of amastigotes and promastigotes within macrophages.
- To quantify the number of infected cells using fluorescence microscopy.
Methods Used
- Infection of BMMs with Leishmania metacyclic promastigotes and amastigotes.
- Fluorescent staining of nuclei using DAPI.
- Microscopic observation to quantify infection levels.
- Experimental controls to ensure accurate comparisons between infection stages.
Main Results
- Successful differentiation of promastigotes into amastigotes within macrophages.
- Quantification of amastigote nuclei revealed differences in replication rates.
- Fluorescence microscopy effectively visualized the infection process.
- Infection dynamics were influenced by the initial stage of the parasite.
Conclusions
- The study enhances understanding of Leishmania's lifecycle in host cells.
- Findings may inform future therapeutic strategies against Leishmaniasis.
- Quantitative methods provide a robust framework for studying intracellular infections.
Frequently Asked Questions
Executive Industry Relevance
This assay enables quantitative assessment of intracellular parasite growth within physiologically relevant host cells, supporting target validation in antiparasitic drug discovery. By measuring amastigote replication in macrophages, it provides mechanistic de-risking for Leishmania virulence factors and host-pathogen interaction studies. The method delivers predictive confidence for lead identification campaigns targeting intracellular survival pathways.
Strategic Applications in Biopharma R&D
Early Discovery & Target Validation
- Scientific Value: Interrogates therapeutic hypotheses by quantifying parasite proliferation inside macrophages, clarifying virulence mechanisms.
- Operational Value: Enables functional target validation through direct measurement of intracellular growth dynamics.
- Predictive Value: Supports portfolio triage by differentiating compound effects on promastigote-to-amastigote differentiation and replication.
Screening & Assay Development
- Assay Readiness: Generates standardized, reproducible biological systems for downstream compound screening against intracellular parasites.
- Quantitative Output: Delivers numerical readouts of amastigote nuclei per macrophage, enabling dose-response and IC50 determinations.
- Scalability: Compatible with multi-well plate formats and fluorescence microscopy for medium-throughput screening applications.
Translational & Preclinical Research
- Disease Relevance: Uses bone marrow-derived macrophages, a disease-relevant system modeling the primary niche of Leishmania infection.
- Translational Continuity: Bridges discovery findings to preclinical validation by quantifying parasite burden in physiologically pertinent host cells.
- Risk-Adjusted Decisions: Informs go/no-go criteria based on reduction of intracellular parasite load, a key biomarker of efficacy.
Pipeline & Workflow Integration
The assay fits within the discovery continuum from target validation through lead identification to preclinical efficacy testing, providing quantitative infection metrics at each stage.
- Discovery Biology: Supports hypothesis testing of host-pathogen interactions and pathway clarification of parasite survival mechanisms.
- Screening: Delivers assay readiness with reproducible infection models and quantitative fluorescence-based readouts for compound evaluation.
- Analytics: Enables comparison of infection conditions through direct enumeration of amastigote nuclei, supporting structure-activity relationship analysis.
- Translational Research: Connects to preclinical work by using primary macrophages and quantifying clinically relevant parasite replication phenotypes.
- Enterprise Reuse: Establishes a reusable capability for intracellular pathogen screening across multiple Leishmania species and strains.
Operational & Enterprise Impact
- Scientific Value: Reduces mechanistic ambiguity in antiparasitic mechanism of action studies through direct visualization and quantification.
- Operational Value: Standardizes infection protocols and quantification methods, improving reproducibility across teams and sites.
- Strategic Value: Enhances capital efficiency by enabling early de-risking of compounds targeting intracellular survival pathways.
- Portfolio Impact: Supports risk-adjusted prioritization by identifying molecules that significantly reduce amastigote burden in macrophages.
Implementation Considerations
- Requires expertise in primary cell culture, parasite handling, and fluorescence microscopy.
- Needs access to fluorescence microscopes with 100x oil immersion objectives and nuclear dye staining capabilities.
- Demands standardization of infection timing, parasite purity, and macrophage differentiation states across experiments.
- Involves adaptation considerations when extending to other intracellular pathogens or alternative host cell models.
- Limited by the need for manual counting or image analysis automation to quantify small amastigote nuclei surrounding large macrophage nuclei.
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Last updated: 29 August 2026