Analyzing cellular internalization of nanoparticles and bacteria by multi-spectral imaging flow cytometry

Imaging Flow Cytometry: Development, Present Applications, and Future Challenges

Imaging flow cytometry (ImFC) represents a significant technological advancement in the field of cytometry, effectively merging the high-throughput capabilities of flow analysis with the detailed imaging characteristics of microscopy. In our comprehensive review, we adopt a historical perspective to chart the development of ImFC, highlighting its origins and current state of the art and forecasting potential future advancements. The genesis of ImFC stemmed from merging the hydraulic system of a flow cytometer with advanced camera technology. This synergistic coupling facilitates the morphological analysis of cell populations at a high-throughput scale, effectively evolving the landscape of cytometry. Nevertheless, ImFC's implementation has encountered hurdles, particularly in developing software capable of managing its sophisticated data acquisition and analysis needs. The scale and complexity of the data generated by ImFC necessitate the creation of novel analytical tools that can effectively manage and interpret these data, thus allowing us to unlock the full potential of ImFC. Notably, artificial intelligence (AI) algorithms have begun to be applied to ImFC, offering promise for enhancing its analytical capabilities. The adaptability and learning capacity of AI may prove to be essential in knowledge mining from the high-dimensional data produced by ImFC, potentially enabling more accurate analyses. Looking forward, we project that ImFC may become an indispensable tool, not only in research laboratories, but also in clinical settings. Given the unique combination of high-throughput cytometry and detailed imaging offered by ImFC, we foresee a critical role for this technology in the next generation of scientific research and diagnostics. As such, we encourage both current and future scientists to consider the integration of ImFC as an addition to their research toolkit and clinical diagnostic routine.

Detection and Quantification of Cryptococcus Uptake by Phagocytic Cells Using Imaging Flow Cytometry

Cryptococcus neoformans, the predominant etiological agent of cryptococcosis, is an encapsulated fungal pathogen found ubiquitously in the environment that causes pneumonia and life-threatening infections of the central nervous system. Following inhalation of yeasts or desiccated basidiospores into the lung alveoli, resident pulmonary phagocytic cells aid in the identification and eradication of Cryptococcus yeast through their arsenal of pattern recognition receptors (PRRs). PRRs recognize conserved pathogen-associated molecular patterns (PAMPs), such as branched mannans, β-glucans, and chitins that are the major components of the fungal cell wall. However, the key receptors/ligand interactions required for cryptococcal recognition and eventual fungal clearance have yet to be elucidated. Here we present an imaging flow cytometer (IFC) method that offers a novel quantitative cellular imaging and population statistics tool to accurately measure phagocytosis of fungal cells. It has the capacity to measure two distinct steps of phagocytosis: association/attachment and internalization in a high-throughput and quantitative manner that is difficult to achieve with other technologies. Results from these IFC studies allow for the potential to identify PRRs required for recognition, uptake, and subsequent activation of cytokine production, as well as other effector cell responses required for fungal clearance.

Microglial Phagocytosis/Cell Health High-Content Assay

We report a microglial phagocytosis/cell health high-content assay that has been used to test small molecule chemical probes and support our drug discovery projects targeting microglia for Alzheimer's disease therapy. The assay measures phagocytosis and cell health (cell count and nuclear intensity) simultaneously in 384-well plates processed with an automatic liquid handler. The mix-and-read live cell imaging assay is highly reproducible with capacity to meet drug discovery research needs. Assay procedures take 4 days including plating cells, treating cells, adding pHrodo-myelin/membrane debris to cells for phagocytosis, staining cell nuclei before performing high-content imaging, and analysis. Three selected parameters are measured from cells: 1) mean total fluorescence intensity per cell of pHrodo-myelin/membrane debris in phagocytosis vesicles to quantify phagocytosis; 2) cell counts per well (measuring compound effects on proliferation and cell death); and 3) average nuclear intensity (measuring compound induced apoptosis). The assay has been used on HMC3 cells (an immortalized human microglial cell line), BV2 cells (an immortalized mouse microglial cell line), and primary microglia isolated from mouse brains. Simultaneous measurements of phagocytosis and cell health allow for the distinction of compound effects on regulation of phagocytosis from cellular stress/toxicity related changes, a distinguishing feature of the assay. The combination of cell counts and nuclear intensity as indicators of cell health is also an effective way to measure cell stress and compound cytotoxicity, which may have broad applications as simultaneous profiling measurements for other phenotypic assays. © 2023 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol: Microglial phagocytosis/cell health high-content assay protocol Support Protocol: Procedures to isolate myelin/membrane debris from mouse brain and label with pHrodo.

Investigating nanoplastics toxicity using advanced stem cell-based intestinal and lung in vitro models

Plastic particles in the nanometer range-called nanoplastics-are environmental contaminants with growing public health concern. As plastic particles are present in water, soil, air and food, human exposure via intestine and lung is unavoidable, but possible health effects are still to be elucidated. To better understand the Mode of Action of plastic particles, it is key to use experimental models that best reflect human physiology. Novel assessment methods like advanced cell models and several alternative approaches are currently used and developed in the scientific community. So far, the use of cancer cell line-based models is the standard approach regarding in vitro nanotoxicology. However, among the many advantages of the use of cancer cell lines, there are also disadvantages that might favor other approaches. In this review, we compare cell line-based models with stem cell-based in vitro models of the human intestine and lung. In the context of nanoplastics research, we highlight the advantages that come with the use of stem cells. Further, the specific challenges of testing nanoplastics in vitro are discussed. Although the use of stem cell-based models can be demanding, we conclude that, depending on the research question, stem cells in combination with advanced exposure strategies might be a more suitable approach than cancer cell lines when it comes to toxicological investigation of nanoplastics.

Quantitative Analysis of Latex Beads Phagocytosis by Human Macrophages Using Imaging Flow Cytometry with Extended Depth of Field (EDF)

The existing methods of quantitative analysis of phagocytosis are characterized by a number of limitations. The usual method of manually counting phagocytosed objects on photographs obtained by confocal microscopy is very labor-intensive and time-consuming. As well, the resolution of conventional flow cytometry does not allow the fluorescence detection of a large number of phagocytosis objects. Thus, there is a need to combine the rapid analysis by flow cytometry and the visualization capability by confocal microscopy. This is possible due to imaging flow cytometry. However, until now, no protocols have allowed one to quantify phagocytosis at its high intensity. The present paper presents the developed and tested algorithm for assessing the level of phagocytic activity using flow cytometry with visualization and IDEAS software.

Identification and characterization of extracellular vesicles from red cells infected with Babesia divergens and Babesia microti

Babesiosis is a zoonosis and an important blood-borne human parasitic infection that has gained attention because of its growing infection rate in humans by transfer from animal reservoirs. Babesia represents a potential threat to the blood supply because asymptomatic infections in man are common, and blood from such donors can cause severe disease in certain recipients. Extracellular vesicles (EVs) are vesicles released by cells that contain a complex mixture of proteins, lipids, glycans, and genetic information that have been shown to play important roles in disease pathogenesis and susceptibility, as well as cell–cell communication and immune responses. In this article, we report on the identification and characterization of EVs released from red blood cells (RBCs) infected by two major human Babesia species—Babesia divergens from in vitro culture and those from an in vivo B. microti mouse infection. Using nanoparticle tracking analysis, we show that there is a range of vesicle sizes from 30 to 1,000 nm, emanating from the Babesia-infected RBC. The study of these EVs in the context of hemoparasite infection is complicated by the fact that both the parasite and the host RBC make and release vesicles into the extracellular environment. However, the EV frequency is 2- to 10-fold higher in Babesia-infected RBCs than uninfected RBCs, depending on levels of parasitemia. Using parasite-specific markers, we were able to show that ~50%–60% of all EVs contained parasite-specific markers on their surface and thus may represent the specific proportion of EVs released by infected RBCs within the EV population. Western blot analysis on purified EVs from both in vivo and in vitro infections revealed several parasite proteins that were targets of the host immune response. In addition, microRNA analysis showed that infected RBC EVs have different microRNA signature from uninfected RBC EVs, indicating a potential role as disease biomarkers. Finally, EVs were internalized by other RBCs in culture, implicating a potential role for these vesicles in cellular communication. Overall, our study points to the multiple functional implications of EVs in Babesia–host interactions and support the potential that EVs have as agents in disease pathogenesis.

Identification of bacteria by poly-aromatic hydrocarbon biosensors

Human health is consistently threatened by different species of pathogenic bacteria. To fight the spread of diseases, it is important to develop rapid methods for bacterial identification. Over the years, different kinds of biosensors were developed for this cause. Another environmental risk is poly-aromatic hydrocarbons (PAHs) that may be emitted from industrial facilities and pollute environmental water and soil. One of the methods for their purification is conducted by the addition of bacteria that can degrade the PAHs, while the bacteria can be filtrated at the end of the process. Although many studies reported monitoring of the PAHs degradation by fluorescence, not much attention was dedicated to studying the influence of the PAHs on the intrinsic fluorescence of the degrading bacteria. In this work, we apply synchronous fluorescence (SF) measurements to study the ability of the 5 PAHs: 9-Antracene carboxylic acid (9ACA), Pyrene, Perylene, Pentacene, and Chrysene to interact with bacteria and change its fluorescence spectra. We show that upon incubation of each PAH with the bacterium E. coli, only the 2 PAHs 9ACA and Perylene cause an intensity decrease in the emission at λ = 300-375 nm, which derives from the emission of tyrosine and tryptophan (TT). Also, we show that upon incubation of 9ACA and Perylene with 5 different pathogenic bacteria, the intensity increase or decrease in the TT emission is unique to each bacterial species. Based on this observation, we suggest that the PAHs 9ACA and Perylene can be utilized as biosensors for bacterial identification.

Review on Optical Imaging Techniques for Multispectral Analysis of Nanomaterials

Biomedical imaging is an essential tool for investigating biological responses in vivo. Among the several imaging techniques, optical imaging systems with multispectral analysis of nanoparticles have been widely investigated due to their ability to distinguish the substances in biological tissues in vivo. This review article focus on multispectral optical imaging techniques that can provide molecular functional information. We summarize the basic principle of the spectral unmixing technique that enables the delineation of optical chromophores. Then, we explore the principle, typical system configuration, and biomedical applications of the representative optical imaging techniques, which are fluorescence imaging, two-photon microscopy, and photoacoustic imaging. The results in the recent studies show the great potential of the multispectral analysis techniques for monitoring responses of biological systems in vivo.

Radachlorin-Containing Microparticles for Photodynamic Therapy

Purpose: Reducing the undesirable systemic effect of photodynamic therapy (PDT) can be achieved by incorporating a photosensitizer in microparticles (MPs). This study is devoted to the preparation of biocompatible biodegradable MPs with the inclusion of the natural photosensitizer Radachlorin (RС) and an assessment of the possibility of their use for PDT. Methods: RC-containing MPs (RС MPs) with poly(lactic-co-glycolic acid) copolymer (PLGA) matrix were prepared by a double emulsion solvent evaporation methods. The size and morphology of RC MPs were surveyed using scanning electron microscopy, confocal laser scanning microscopy, and dynamic light scattering. The content of RC, its release from RC MPs, and singlet oxygen generation were evaluated by the optical spectroscopy. Cellular uptake and cytotoxic photodynamic effect of RC MPs were investigated with in vitro assays. Results: The average diameter of the prepared RC MPs was about 2-3 μm. The RC MPs prepared by the water/oil/oil method had a significantly higher inclusion of RC (1.74 μg/mg) then RC MPs prepared by the water/oil/water method (0.089 μg/mg). Exposure of the prepared RC MPs to PDT light radiation was accompanied by the singlet oxygen generation and a cytotoxic effect for tumor cells. The release of the RC from the RC MPs was prolonged and lasted at least two weeks. Conclusion: PLGA RC MPs were found to cause a photoactivated cytotoxic effect for tumor cells and can be used for local application in PDT of tumors.

Progressive use of multispectral imaging flow cytometry in various research areas

Multi-spectral imaging flow cytometry (MIFC) has become one of the most powerful technologies for investigating general analytics, molecular and cell biology, biotechnology, medicine, and related fields. It combines the capabilities of the morphometric and photometric analysis of single cells and micrometer-sized particles in flux with regard to thousands of events. It has become the tool of choice for a wide range of research and clinical applications. By combining the features of flow cytometry and fluorescence microscopy, it offers researchers the ability to couple the spatial resolution of multicolour images of cells and organelles with the simultaneous analysis of a large number of events in a single system. This provides the opportunity to visually confirm findings and collect novel data that would otherwise be more difficult to obtain. This has led many researchers to design innovative assays to gain new insight into important research questions. To date, it has been successfully used to study cell morphology, surface and nuclear protein co-localization, protein-protein interactions, cell signaling, cell cycle, cell death, and cytotoxicity, intracellular calcium, drug uptake, pathogen internalization, and other applications. Herein we describe some of the recent advances in the field of multiparametric imaging flow cytometry methods in various research areas.

Imaging flow cytometry methods for quantitative analysis of label-free crystalline silica particle interactions with immune cells

Exposure to respirable fractions of crystalline silica quartz dust particles is associated with silicosis, cancer and the development of autoimmune conditions. Early cellular interactions are not well understood, partly due to a lack of suitable technological methods. Improved techniques are needed to better quantify and study high-level respirable crystalline silica exposure in human populations. Techniques that can be applied to complex biological matrices are pivotal to understanding particle-cell interactions and the impact of particles within real, biologically complex environments. In this study, we investigated whether imaging flow cytometry could be used to assess the interactions between cells and crystalline silica when present within complex biological matrices. Using the respirable-size fine quartz crystalline silica dust Min-u-sil® 5, we first validated previous reports that, whilst associating with cells, crystalline silica particles can be detected solely through their differential light scattering profile using conventional flow cytometry. This same property reliably identified crystalline silica in association with primary monocytic cells in vitro using an imaging flow cytometry assay, where darkfield intensity measurements were able to detect crystalline silica concentrations as low as 2.5 μg/mL. Finally, we ultilised fresh whole blood as an exemplary complex biological matrix to test the technique. Even after the increased sample processing required to analyse cells within whole blood, imaging flow cytometry was capable of detecting and assessing silica-association to cells. As expected, in fresh whole blood exposed to crystalline silica, neutrophils and cells of the monocyte/macrophage lineage phagocytosed the particles. In addition to the use of this technique in in vitro exposure models, this method has the potential to be applied directly to ex vivo diagnostic studies and research models, where the identification of crystalline silica association with cells in complex biological matrices such as bronchial lavage fluids, alongside additional functional and phenotypic cellular readouts, is required.

Low molecular weight chitosan nanoparticles for CpG oligodeoxynucleotides delivery: Impact of molecular weight, degree of deacetylation, and mannosylation on intracellular uptake and cytokine induction

Although synthetic CpG oligodeoxynucleotides (ODNs) have shown substantial potential as immunotherapeutic agents, their effective intracellular delivery remains challenging. In this work, nanoparticles prepared from low-molecular weight (LMW) chitosans were investigated as CpG ODN delivery systems. Chitosan samples with a molecular weight (M_w) of 5 and 15 kDa and degree of deacetylation (DDA) of 50 and 80% were prepared. Additionally, mannosylated chitosans with a substitution degree of 15% were synthesized. The impact of LMW chitosan M_w and DDA on nanoparticle physical properties and the associated immunostimulatory effect in RAW 264.7 cells was studied. Nanoparticles prepared with chitosan of higher DDA and larger M_w exhibited better CpG ODN binding ability and intracellular uptake. Nevertheless, the most efficient immunostimulatory effect was observed while using 50% acetylated and mannosylated samples. The decreased charge density on chitosan backbone resulted in the enhanced intracellular CpG ODN release, which promoted in vitro cytokine secretion. Moreover, mannose ligand grafting promoted nanoparticle uptake through receptor-mediated recognition. Overall, this research suggests that chitosan structural parameters can be modulated to prepare LMW chitosan nanoparticles that first efficiently encapsulate CpG ODN, and then release it in immune cells, thus may be used as an efficient vector for intracellular CpG ODN delivery.

Imaging Flow Cytometry Illuminates New Dimensions of Amyloid Peptide-Membrane Interactions

Membrane interactions of amyloidogenic proteins constitute central determinants both in protein aggregation as well as in amyloid cytotoxicity. Most reported studies of amyloid peptide-membrane interactions have employed model membrane systems combined with application of spectroscopy methods or microscopy analysis of individual binding events. Here, we applied for the first time, to our knowledge, imaging flow cytometry for investigating interactions of representative amyloidogenic peptides, namely, the 106-126 fragment of prion protein (PrP(106-126)) and the human islet amyloid polypeptide (hIAPP), with giant lipid vesicles. Imaging flow cytometry was also applied to examine the inhibition of PrP(106-126)-membrane interactions by epigallocatechin gallate, a known modulator of amyloid peptide aggregation. We show that imaging flow cytometry provided comprehensive population-based statistical information upon morphology changes of the vesicles induced by PrP(106-126) and hIAPP. Specifically, the experiments reveal that both PrP(106-126) and hIAPP induced dramatic transformations of the vesicles, specifically disruption of the spherical shapes, reduction of vesicle circularity, lobe formation, and modulation of vesicle compactness. Interesting differences, however, were apparent between the impact of the two peptides upon the model membranes. The morphology analysis also showed that epigallocatechin gallate ameliorated vesicle disruption by PrP(106-126). Overall, this study demonstrates that imaging flow cytometry provides powerful means for disclosing population-based morphological membrane transformations induced by amyloidogenic peptides and their inhibition by aggregation modulators.

Imaging Flow Cytometry Protocols for Examining Phagocytosis of Microplastics and Bioparticles by Immune Cells of Aquatic Animals

Imaging flow cytometry (IFC) is a powerful tool which combines flow cytometry with digital microscopy to generate quantitative high-throughput imaging data. Despite various advantages of IFC over standard flow cytometry, widespread adoption of this technology for studies in aquatic sciences is limited, probably due to the relatively high equipment cost, complexity of image analysis-based data interpretation and lack of core facilities with trained personnel. Here, we describe the application of IFC to examine phagocytosis of particles including microplastics by cells from aquatic animals. For this purpose, we studied (1) live/dead cell assays and identification of cell types, (2) phagocytosis of degradable and non-degradable particles by Atlantic salmon head kidney cells and (3) the effect of incubation temperature on phagocytosis of degradable particles in three aquatic animals–Atlantic salmon, Nile tilapia, and blue mussel. The usefulness of the developed method was assessed by evaluating the effect of incubation temperature on phagocytosis. Our studies demonstrate that IFC provides significant benefits over standard flow cytometry in phagocytosis measurement by allowing integration of morphometric parameters, especially while identifying cell populations and distinguishing between different types of fluorescent particles and detecting their localization.

Using Imaging Flow Cytometry to Quantify Neutrophil Phagocytosis

Neutrophils are professional phagocytes that are important for innate host defenses against pathogens and resolution of inflammation. Traditionally, the phagocytic capacity of neutrophils was quantified by enumeration of cells containing either internalized or bound bacteria or other cargo from a series of microscopic images. Here we describe an imaging flow cytometry-based protocol and analysis method for quantifying the binding and uptake of Neisseria gonorrhoeae by primary adherent human neutrophils. Imaging flow cytometry combines the capacity for quantitative, high-throughput analysis of tens of thousands of cells per condition, with the imaging power of fluorescence microscopy. Here, all bacteria are labeled with Tag-it Violet™ and bound bacteria are differentially stained with a DyLight™ 650-conjugated antibody. Images are analyzed using spot count and other algorithms. Outputs include the percent of neutrophils associated with bacteria, the percent of neutrophils with internalized bacteria, and the percent of internalized bacteria. This basic protocol can be adapted to a variety of particle types and can be used for multiplex analysis in combination with staining for different neutrophil surface and intracellular markers.

Internalization Characterization of Si Nanorod with Camouflaged Cell Membrane Proteins Reveals ATXN2 as a Negative Regulator

The fabrication of shape-controlled nanocarriers is critical for efficient delivery of biomolecules across the cell membrane. Surface coating of the nanocarrier can improve internalization efficiency. Here, we developed a facile method of silicon nanorod fabrication leading to a controlled size and shape. We then systematically evaluated five surface modifications with membrane proteins from different cancer cell lines including MCF7, MD231, Hela, Panc-PDX, and Panc-1. We demonstrated that silicon nanorods coated with either a homolytic or heterolytic membrane protein coating have significantly improved internalization efficiency as compared with uncoated Si nanorods. To elucidate the molecular mechanism of the improved efficiency associated with a modified coating, we analyzed the coating membrane proteins derived from five cell lines with proteomics and identified 601 proteins shared by different cell sources. These proteins may function as cell-substrate adhesion molecules that contribute to the enhanced internalization. We also tested the internalization efficiency of nanorods with different coatings in each of the five cell lines to determine the influencing factors from target cells. We found that the internalization efficiency varied among different target cells, and the ranking of the average efficiency was as follows: Hela > Panc-PDX > MD231 > MCF7 > Panc-1. The bioinformatics analysis suggested that the low internalization efficiency in Panc-1 cells might be associated with the upregulation of ATXN2, which is a negative regulator of endocytosis. We further demonstrated that ATXN2 knockdown with specific siRNA significantly improved nanorod internalization efficiency in Panc-1 cells suggesting that ATXN2 can be a reference for efficiency prediction of nanoparticle delivery to tumor cells. Thus, we studied the effect of different cancer cell membrane proteins on nanorod uptake efficiencies. These results can improve nanorod internalization to cancer cells, including a fundamental understanding of the internalization efficiency of cancer cells.

Quantification of Efferocytosis by Single-cell Fluorescence Microscopy

Studying the regulation of efferocytosis requires methods that are able to accurately quantify the uptake of apoptotic cells and to probe the signaling and cellular processes that control efferocytosis. This quantification can be difficult to perform as apoptotic cells are often efferocytosed piecemeal, thus necessitating methods which can accurately delineate between the efferocytosed portion of an apoptotic target versus residual unengulfed cellular fragments. The approach outlined herein utilizes dual-labeling approaches to accurately quantify the dynamics of efferocytosis and efferocytic capacity of efferocytes such as macrophages. The cytosol of the apoptotic cell is labeled with a cell-tracking dye to enable monitoring of all apoptotic cell-derived materials, while surface biotinylation of the apoptotic cell allows for differentiation between internalized and non-internalized apoptotic cell fractions. The efferocytic capacity of efferocytes is determined by taking fluorescent images of live or fixed cells and quantifying the amount of bound versus internalized targets, as differentiated by streptavidin staining. This approach offers several advantages over methods such as flow cytometry, namely the accurate delineation of non-efferocytosed versus efferocytosed apoptotic cell fractions, the ability to measure efferocytic dynamics by live-cell microscopy, and the capacity to perform studies of cellular signaling in cells expressing fluorescently-labeled transgenes. Combined, the methods outlined in this protocol serve as the basis for a flexible experimental approach that can be used to accurately quantify efferocytic activity and interrogate cellular signaling pathways active during efferocytosis.

Imaging flow cytometry assays for quantifying pigment grade titanium dioxide particle internalization and interactions with immune cells in whole blood

Pigment grade titanium dioxide is composed of sub‐micron sized particles, including a nanofraction, and is widely utilized in food, cosmetic, pharmaceutical, and biomedical industries. Oral exposure to pigment grade titanium dioxide results in at least some material entering the circulation in humans, although subsequent interactions with blood immune cells are unknown. Pigment grade titanium dioxide is employed for its strong light scattering properties, and this work exploited that attribute to determine whether single cell–particle associations could be determined in immune cells of human whole blood at “real life” concentrations. In vitro assays, initially using isolated peripheral blood mononuclear cells, identified titanium dioxide associated with the surface of, and within, immune cells by darkfield reflectance in imaging flow cytometry. This was confirmed at the population level by side scatter measurements using conventional flow cytometry. Next, it was demonstrated that imaging flow cytometry could quantify titanium dioxide particle‐bearing cells, within the immune cell populations of fresh whole blood, down to titanium dioxide levels of 10 parts per billion, which is in the range anticipated for human blood following titanium dioxide ingestion. Moreover, surface association and internal localization of titanium dioxide particles could be discriminated in the assays. Overall, results showed that in addition to the anticipated activity of blood monocytes internalizing titanium dioxide particles, neutrophil internalization and cell membrane adhesion also occurred, the latter for both phagocytic and nonphagocytic cell types. What happens in vivo and whether this contributes to activation of one or more of these different cells types in blood merits further attention. © 2017 The Authors. Cytometry Part A Published by Wiley Periodicals, Inc. on behalf of ISAC.

High-Throughput Particle Uptake Analysis by Imaging Flow Cytometry

Quantifying the efficiency of particle uptake by host cells is important in the fields of infectious diseases, autoimmunity, cancer, developmental biology, and drug delivery. Here we present a protocol for high-throughput analysis of particle uptake by imaging flow cytometry, using the bacterium Neisseria gonorrhoeae attached to and internalized by neutrophils as an example. Cells are exposed to fluorescently labeled bacteria, fixed, and stained with a bacteria-specific antibody of a different fluorophore. Thus, in the absence of a permeabilizing agent, extracellular bacteria are double-labeled with two fluorophores while intracellular bacteria remain single-labeled. A spot count algorithm is used to determine the number of single- and double-labeled bacteria in individual cells, to calculate the percent of cells associated with bacteria, percent of cells with internalized bacteria, and percent of cell-associated bacteria that are internalized. These analyses quantify bacterial association and internalization across thousands of cells and can be applied to diverse experimental systems. © 2017 by John Wiley & Sons, Inc.

Connected component masking accurately identifies the ratio of phagocytosed and cell-bound particles in individual cells by imaging flow cytometry

Innate immune cell-mediated recognition, capture, and engulfment of large particulate targets such as bacteria is known as phagocytosis. This highly dynamic cellular process involves a series of steps including receptor-mediated target binding, phagocytic cup formation, pseudopod extension, and phagosome closure, which depend on distinct actin polymerization events. Using flow cytometry, precise determination of target locations relative to cell membranes (i.e., surface-bound vs. fully engulfed/internalized) during the phagocytic process is difficult to quantify. Here, we describe the application of new analysis features within the IDEAS® software to distinguish internalized and surface-bound particles on individual cells with a high degree of accuracy and reproducibility. Through the use of connected component masks, the accurate discrimination of surface-bound beads versus those internalized is clearly demonstrated. In addition, we were able to further analyze the ratio of beads that had been surface-bound or internalized within individual cells. This novel method of analyzing the phagocytic process provides more accurate determination of target-cell interactions that will assist in examination of the signalling events that occur during the various stages of phagocytosis. © 2017 International Society for Advancement of Cytometry.

Imaging flow cytometry analysis of intracellular pathogens

Imaging flow cytometry has been applied to address questions in infection biology, in particular, infections induced by intracellular pathogens. This methodology, which utilizes specialized analytic software makes it possible to analyze hundreds of quantified features for hundreds of thousands of individual cellular or subcellular events in a single experiment. Imaging flow cytometry analysis of host cell-pathogen interaction can thus quantitatively addresses a variety of biological questions related to intracellular infection, including cell counting, internalization score, and subcellular patterns of co-localization. Here, we provide an overview of recent achievements in the use of fluorescently labeled prokaryotic or eukaryotic pathogens in human cellular infections in analysis of host-pathogen interactions. Specifically, we give examples of Imagestream-based analysis of cell lines infected with Toxoplasma gondii or Mycobacterium tuberculosis. Furthermore, we illustrate the capabilities of imaging flow cytometry using a combination of standard IDEAS™ software and the more recently developed Feature Finder algorithm, which is capable of identifying statistically significant differences between researcher-defined image galleries. We argue that the combination of imaging flow cytometry with these software platforms provides a powerful new approach to understanding host control of intracellular pathogens.

Poly(γ-glutamic acid) and poly(γ-glutamic acid)-based nanocomplexes enhance type II collagen production in intervertebral disc

Intervertebral disc (IVD) degeneration often leads to low back pain, which is one of the major causes of disability worldwide, affecting more than 80% of the population. Although available treatments for degenerated IVD decrease symptoms' progression, they fail to address the underlying causes and to restore native IVD properties. Poly(γ-glutamic acid) (γ-PGA) has recently been shown to support the production of chondrogenic matrix by mesenchymal stem/stromal cells. γ-PGA/chitosan (Ch) nanocomplexes (NCs) have been proposed for several biomedical applications, showing advantages compared with either polymer alone. Hence, this study explores the potential of γ-PGA and γ-PGA/Ch NCs for IVD regeneration. Nucleotomised bovine IVDs were cultured ex vivo upon injection of γ-PGA (pH 7.4) and γ-PGA/Ch NCs (pH 5.0 and pH 7.4). Tissue metabolic activity and nucleus pulposus DNA content were significantly reduced when NCs were injected in acidic-buffered solution (pH 5.0). However, at pH 7.4, both γ-PGA and NCs promoted sulphated glycosaminoglycan production and significant type II collagen synthesis, as determined at the protein level. This study is a first proof of concept that γ-PGA and γ-PGA/Ch NCs promote recovery of IVD native matrix, opening new perspectives on the development of alternative therapeutic approaches for IVD degeneration.

Antibody-Mediated Internalization of Infectious HIV-1 Virions Differs among Antibody Isotypes and Subclasses

Emerging data support a role for antibody Fc-mediated antiviral activity in vaccine efficacy and in the control of HIV-1 replication by broadly neutralizing antibodies. Antibody-mediated virus internalization is an Fc-mediated function that may act at the portal of entry whereby effector cells may be triggered by pre-existing antibodies to prevent HIV-1 acquisition. Understanding the capacity of HIV-1 antibodies in mediating internalization of HIV-1 virions by primary monocytes is critical to understanding their full antiviral potency. Antibody isotypes/subclasses differ in functional profile, with consequences for their antiviral activity. For instance, in the RV144 vaccine trial that achieved partial efficacy, Env IgA correlated with increased risk of HIV-1 infection (i.e. decreased vaccine efficacy), whereas V1-V2 IgG3 correlated with decreased risk of HIV-1 infection (i.e. increased vaccine efficacy). Thus, understanding the different functional attributes of HIV-1 specific IgG1, IgG3 and IgA antibodies will help define the mechanisms of immune protection. Here, we utilized an in vitro flow cytometric method utilizing primary monocytes as phagocytes and infectious HIV-1 virions as targets to determine the capacity of Env IgA (IgA1, IgA2), IgG1 and IgG3 antibodies to mediate HIV-1 infectious virion internalization. Importantly, both broadly neutralizing antibodies (i.e. PG9, 2G12, CH31, VRC01 IgG) and non-broadly neutralizing antibodies (i.e. 7B2 mAb, mucosal HIV-1+ IgG) mediated internalization of HIV-1 virions. Furthermore, we found that Env IgG3 of multiple specificities (i.e. CD4bs, V1-V2 and gp41) mediated increased infectious virion internalization over Env IgG1 of the same specificity, while Env IgA mediated decreased infectious virion internalization compared to IgG1. These data demonstrate that antibody-mediated internalization of HIV-1 virions depends on antibody specificity and isotype. Evaluation of the phagocytic potency of vaccine-induced antibodies and therapeutic antibodies will enable a better understanding of their capacity to prevent and/or control HIV-1 infection in vivo.

Nanostructured lipid carriers loaded with resveratrol modulate human dendritic cells

Dendritic cells (DCs) are promising targets for drug delivery, as they can induce immunity or tolerance. The current study aims to examine the potential of using nanostructured lipid carriers (NLC) as delivery systems for human DC by evaluating nanoparticle internalization, cell labeling, and drug activity. NLC were formulated incorporating the fluorochrome fluorescein isothiocyanate (FITC-NLC) or the natural anti-inflammatory molecule resveratrol (rsv-NLC). Primary human DCs were differentiated from peripheral blood monocytes, and the innovative imaging flow cytometry technique was used to examine FITC-NLC internalization. The capacity of rsv-NLC to inhibit DC activation in response to proinflammatory cytokine tumor necrosis factor-α (TNF- α) was investigated by conventional flow cytometry. A combination of imaging and conventional flow cytometry was used to assess NLC cytotoxicity. The results obtained indicate that both NLC formulations were stable over time, with mean diameter <200 nm and highly negative zeta potential (about -30 mV). When DCs were placed in contact with NLC, imaging flow cytometry clearly showed that DCs efficiently internalized FITC-NLC, with nearly 100% of cells internalizing nanoparticles upon 1 hour of incubation. Both immature and mature DCs internalized NLC to high and comparable levels, and without cytotoxicity. Stimulating DC with TNF-α in the presence of rsv-NLC revealed that, using these nanoparticles, very small concentrations of rsv were sufficient to significantly decrease surface expression of activation marker CD83 (5 µM) and major histocompatibility complex-class II molecule human leukocyte antigen - antigen D related (10 µM), both upregulated in response to TNF-α stimulation. Rsv-NLC were compared with free rsv; at 5 µM, rsv-NLC were able to inhibit nuclear factor κ beta phosphorylation and significantly decrease the level of interleukin-12/23, both upregulated in response to TNF-α, while 10 µM free rsv were needed to promote a similar effect. Taken together, the results presented show that NLC are suitable carriers of fluorescent labels or bioactive molecules for human DCs, leading to inflammation modulation.

Semi-automated quantification of living cells with internalized nanostructures

BACKGROUND

Nanostructures fabricated by different methods have become increasingly important for various applications in biology and medicine, such as agents for medical imaging or cancer therapy. In order to understand their interaction with living cells and their internalization kinetics, several attempts have been made in tagging them. Although methods have been developed to measure the number of nanostructures internalized by the cells, there are only few approaches aimed to measure the number of cells that internalize the nanostructures, and they are usually limited to fixed-cell studies. Flow cytometry can be used for live-cell assays on large populations of cells, however it is a single time point measurement, and does not include any information about cell morphology. To date many of the observations made on internalization events are limited to few time points and cells.

RESULTS

In this study, we present a method for quantifying cells with internalized magnetic nanowires (NWs). A machine learning-based computational framework, CellCognition, is adapted and used to classify cells with internalized and no internalized NWs, labeled with the fluorogenic pH-dependent dye pHrodo™ Red, and subsequently to determine the percentage of cells with internalized NWs at different time points. In a "proof-of-concept", we performed a study on human colon carcinoma HCT 116 cells and human epithelial cervical cancer HeLa cells interacting with iron (Fe) and nickel (Ni) NWs.

CONCLUSIONS

This study reports a novel method for the quantification of cells that internalize a specific type of nanostructures. This approach is suitable for high-throughput and real-time data analysis and has the potential to be used to study the interaction of different types of nanostructures in live-cell assays.

Quantification of heterotypic granule fusion in human neutrophils by imaging flow cytometry

Human neutrophils are filled with intracellular storage organelles, called granules and secretory vesicles, which differ in their content of soluble matrix proteins and membrane-bound molecules. To date, at least four distinct granule/vesicle subsets have been identified. These organelles may secrete their content extracellularly following mobilization to and fusion with the plasma membrane, but some of them may also fuse with internal membrane-enclosed organelles, typically a plasma membrane-derived phagosome. There are also instances where different granules appear to fuse with one another, a process that would enable mixing of their matrix and membrane components. Such granule fusion enables e.g., myeloperoxidase-processing of intragranular oxygen radicals, a key event in the formation of neutrophil extracellular traps (Björnsdottir et al., 2015) [1]. Described herein are data that show the quantification of such heterotypic granule–granule fusion by the use of imaging flow cytometry, a technique that combines flow cytometry with microscopy. The analysis described is based on immunofluorescent staining of established granule markers (lactoferrin and/or NGAL for one granule subset; the specific granules, and CD63 for another granule subset, the azurophil granules) and calculation of a colocalization score for resting and PMA-stimulated neutrophils.

Phagocyte interactions with Mycobacterium tuberculosis--Simultaneous analysis of phagocytosis, phagosome maturation and intracellular replication by imaging flow cytometry

Utilization of compounds that enhance the innate immune response against Mycobacterium tuberculosis is an attractive strategy for combating tuberculosis in the post-antibiotic era. Thus, it is crucial to develop methods that can be used to screen for such compounds and to investigate their mechanisms of action. Here, we used imaging flow cytometry (ImageStreamX Mk II),which enables rapid quantification of microscopic images in flow, to study the interaction between phagocytes and M. tuberculosis. Macrophage-differentiated THP-1 cells were infected with GFP-expressing M. tuberculosis H37Ra, and methods for rapidly assessing phagocytosis, phagosome maturation, and bacterial replication inside the cells were developed and evaluated. These aspects of innate immunity are essential in determining the outcome of mycobacterial infection of phagocytes. The technique was found effective for monitoring phagocytosis of mycobacteria, phagosomal acidification and phagolysosomal fusion, as well as for measuring mycobacterial replication inside the cells. Several of these aspects could be analyzed simultaneously in the same sample, providing a great deal of information about the phagocyte–mycobacterial interaction at once. Thus, this method has great potential to be useful both for basic research questions and for evaluating compounds that enhance the innate immune response against M. tuberculosis.

Biodistribution and Toxicity Studies of PRINT Hydrogel Nanoparticles in Mosquito Larvae and Cells

Mosquito-borne diseases continue to remain major threats to human and animal health and impediments to socioeconomic development. Increasing mosquito resistance to chemical insecticides is a great public health concern, and new strategies/technologies are necessary to develop the next-generation of vector control tools. We propose to develop a novel method for mosquito control that employs nanoparticles (NPs) as a platform for delivery of mosquitocidal dsRNA molecules to silence mosquito genes and cause vector lethality. Identifying optimal NP chemistry and morphology is imperative for efficient mosquitocide delivery. Toward this end, fluorescently labeled polyethylene glycol NPs of specific sizes, shapes (80 nm x 320 nm, 80 nm x 5000 nm, 200 nm x 200 nm, and 1000 nm x 1000 nm) and charges (negative and positive) were fabricated by Particle Replication in Non-Wetting Templates (PRINT) technology. Biodistribution, persistence, and toxicity of PRINT NPs were evaluated in vitro in mosquito cell culture and in vivo in Anopheles gambiae larvae following parenteral and oral challenge. Following parenteral challenge, the biodistribution of the positively and negatively charged NPs of each size and shape was similar; intense fluorescence was observed in thoracic and abdominal regions of the larval body. Positively charged NPs were more associated with the gastric caeca in the gastrointestinal tract. Negatively charged NPs persisted through metamorphosis and were observed in head, body and ovaries of adults. Following oral challenge, NPs were detected in the larval mid- and hindgut. Positively charged NPs were more efficiently internalized in vitro than negatively charged NPs. Positively charged NPs trafficked to the cytosol, but negatively charged NPs co-localized with lysosomes. Following in vitro and in vivo challenge, none of the NPs tested induced any cytotoxic effects.

An improved method for differentiating cell-bound from internalized particles by imaging flow cytometry

Recognition, binding, internalization, and elimination of pathogens and cell debris are important functions of professional as well as non-professional phagocytes. However, high-throughput methods for quantifying cell-associated particles and discriminating bound from internalized particles have been lacking. Here we describe a protocol for using imaging flow cytometry to quantify the attached and phagocytosed particles that are associated with a population of cells. Cells were exposed to fluorescent particles, fixed, and exposed to an antibody of a different fluorophore that recognizes the particles. The antibody is added without cell permeabilization, such that the antibody only binds extracellular particles. Cells with and without associated particles were identified by imaging flow cytometry. For each cell with associated particles, a spot count algorithm was employed to quantify the number of extracellular (double fluorescent) and intracellular (single fluorescent) particles per cell, from which the percent particle internalization was determined. The spot count algorithm was empirically validated by examining the fluorescence and phase contrast images acquired by the flow cytometer. We used this protocol to measure binding and internalization of the bacterium Neisseria gonorrhoeae by primary human neutrophils, using different bacterial variants and under different cellular conditions. The results acquired using imaging flow cytometry agreed with findings that were previously obtained using conventional immunofluorescence microscopy. This protocol provides a rapid, powerful method for measuring the association and internalization of any particle by any cell type.

An in vitro model of antibody-enhanced killing of the intracellular parasite Leishmania amazonensis

Footpad infection of C3HeB/FeJ mice with Leishmania amazonensis leads to chronic lesions accompanied by large parasite loads. Co-infecting these animals with L. major leads to induction of an effective Th1 immune response that can resolve these lesions. This cross-protection can be recapitulated in vitro by using immune cells from L. major-infected animals to effectively activate L. amazonensis-infected macrophages to kill the parasite. We have shown previously that the B cell population and their IgG2a antibodies are required for effective cross-protection. Here we demonstrate that, in contrast to L. major, killing L. amazonensis parasites is dependent upon FcRγ common-chain and NADPH oxidase-generated superoxide from infected macrophages. Superoxide production coincided with killing of L. amazonensis at five days post-activation, suggesting that opsonization of the parasites was not a likely mechanism of the antibody response. Therefore we tested the hypothesis that non-specific immune complexes could provide a mechanism of FcRγ common-chain/NADPH oxidase dependent parasite killing. Macrophage activation in response to soluble IgG2a immune complexes, IFN-γ and parasite antigen was effective in significantly reducing the percentage of macrophages infected with L. amazonensis. These results define a host protection mechanism effective during Leishmania infection and demonstrate for the first time a novel means by which IgG antibodies can enhance killing of an intracellular pathogen.

Carbohydrate-functionalized nanovaccines preserve HIV-1 antigen stability and activate antigen presenting cells

The functionalization of polymeric nanoparticles with ligands that target specific receptors on immune cells offers the opportunity to tailor adjuvant properties by conferring pathogen mimicking attributes to the particles. Polyanhydride nanoparticles are promising vaccine adjuvants with desirable characteristics such as immunomodulation, sustained antigen release, activation of antigen presenting cells (APCs), and stabilization of protein antigens. These capabilities can be exploited to design nanovaccines against viral pathogens, such as HIV-1, due to the important role of dendritic cells (DCs) and macrophages in viral spread. In this work, an optimized process was developed for carbohydrate functionalization of HIV-1 antigen-loaded polyanhydride nanoparticles. The carbohydrate-functionalized nanoparticles preserved antigenic properties upon release and also enabled sustained antigen release kinetics. Particle internalization was observed to be chemistry-dependent with positively charged nanoparticles being taken up more efficiently by DCs. Up-regulation of the activation makers CD40 and CD206 was demonstrated with carboxymethyl-α-d-mannopyranosyl-(1,2)-d-mannopyranoside functionalized nanoparticles. The secretion of the cytokines IL-6 and TNF-α was shown to be chemistry-dependent upon stimulation with carbohydrate-functionalized nanoparticles. These results offer important new insights upon the interactions between carbohydrate-functionalized nanoparticles and APCs and provide foundational information for the rational design of targeted nanovaccines against HIV-1.

Polyreactive antibodies plus complement enhance the phagocytosis of cells made apoptotic by UV-light or HIV

Polyreactive antibodies are a major component of the natural antibody repertoire and are capable of binding a variety of structurally unrelated antigens. Many of the properties attributed to natural antibodies, in fact, are turning out to be due to polyreactive antibodies. In humans, each day, billions of cells undergo apoptosis. In the present experiments, we show by ImageStream technology that although polyreactive antibodies do not bind to live T cells they bind to both the plasma membrane and cytoplasm of late apoptotic cells, fix complement, generate the anaphylatoxin C5a and increase by as much as 5 fold complement-mediated phagocytosis by macrophages. Of particular importance, T cells undergoing apoptosis following infection with HIV also bind polyreactive antibodies and are phagocytosed. We conclude that the polyreactive antibodies in the natural antibody repertoire contribute in a major way to the clearance of cells made apoptotic by a variety of natural and infectious processes.

Localization and relative quantification of carbon nanotubes in cells with multispectral imaging flow cytometry

Carbon-based nanomaterials, like carbon nanotubes (CNTs), belong to this type of nanoparticles which are very difficult to discriminate from carbon-rich cell structures and de facto there is still no quantitative method to assess their distribution at cell and tissue levels. What we propose here is an innovative method allowing the detection and quantification of CNTs in cells using a multispectral imaging flow cytometer (ImageStream, Amnis). This newly developed device integrates both a high-throughput of cells and high resolution imaging, providing thus images for each cell directly in flow and therefore statistically relevant image analysis. Each cell image is acquired on bright-field (BF), dark-field (DF), and fluorescent channels, giving access respectively to the level and the distribution of light absorption, light scattered and fluorescence for each cell. The analysis consists then in a pixel-by-pixel comparison of each image, of the 7,000-10,000 cells acquired for each condition of the experiment. Localization and quantification of CNTs is made possible thanks to some particular intrinsic properties of CNTs: strong light absorbance and scattering; indeed CNTs appear as strongly absorbed dark spots on BF and bright spots on DF with a precise colocalization. This methodology could have a considerable impact on studies about interactions between nanomaterials and cells given that this protocol is applicable for a large range of nanomaterials, insofar as they are capable of absorbing (and/or scattering) strongly enough the light.