Big insights from small volumes: deciphering complex leukocyte behaviors using microfluidics

What can we learn about fish neutrophil and macrophage response to immune challenge from studies in zebrafish

Fish rely, to a high degree, on the innate immune system to protect them against the constant exposure to potential pathogenic invasion from the surrounding water during homeostasis and injury. Zebrafish larvae have emerged as an outstanding model organism for immunity. The cellular component of zebrafish innate immunity is similar to the mammalian innate immune system and has a high degree of sophistication due to the needs of living in an aquatic environment from early embryonic stages of life. Innate immune cells (leukocytes), including neutrophils and macrophages, have major roles in protecting zebrafish against pathogens, as well as being essential for proper wound healing and regeneration. Zebrafish larvae are visually transparent, with unprecedented in vivo microscopy opportunities that, in combination with transgenic immune reporter lines, have permitted visualisation of the functions of these cells when zebrafish are exposed to bacterial, viral and parasitic infections, as well as during injury and healing. Recent findings indicate that leukocytes are even more complex than previously anticipated and are essential for inflammation, infection control, and subsequent wound healing and regeneration.

Use and application of organ-on-a-chip platforms in cancer research

Tumors are a major cause of death worldwide, and much effort has been made to develop appropriate anti-tumor therapies. Existing in vitro and in vivo tumor models cannot reflect the critical features of cancer. The development of organ-on-a-chip models has enabled the integration of organoids, microfluidics, tissue engineering, biomaterials research, and microfabrication, offering conditions that mimic tumor physiology. Three-dimensional in vitro human tumor models that have been established as organ-on-a-chip models contain multiple cell types and a structure that is similar to the primary tumor. These models can be applied to various foci of oncology research. Moreover, the high-throughput features of microfluidic organ-on-a-chip models offer new opportunities for achieving large-scale drug screening and developing more personalized treatments. In this review of the literature, we explore the development of organ-on-a-chip technology and discuss its use as an innovative tool in basic and clinical applications and summarize its advancement of cancer research.

Microfluidic Systems to Study Neutrophil Forward and Reverse Migration

During infection, neutrophils are the most abundantly recruited innate immune cells at sites of infection, playing critical roles in the elimination of local infection and healing of the injury. Neutrophils are considered to be short-lived effector cells that undergo cell death at infection sites and in damaged tissues. However, recent in vitro and in vivo evidence suggests that neutrophil behavior is more complex and that they can migrate away from the inflammatory site back into the vasculature following the resolution of inflammation. Microfluidic devices have contributed to an improved understanding of the interaction and behavior of neutrophils ex vivo in 2D and 3D microenvironments. The role of reverse migration and its contribution to the resolution of inflammation remains unclear. In this review, we will provide a summary of the current applications of microfluidic devices to investigate neutrophil behavior and interactions with other immune cells with a focus on forward and reverse migration in neutrophils.

Barotaxis: How cells live and move under pressure

Cell migration is an essential process that controls many physiological functions ranging from development to immunity. In vivo, cells are guided by a combination of physical and chemical cues. Chemokines have been the center of attention for years, but the role of physical properties of tissues has been under-investigated, despite the fact that these properties can be drastically modified in pathology. Here, we discuss the role of one important tissue physical property, hydraulic resistance, in cell guidance, a phenomenon referred to as barotaxis, and describe the underlying physical principles and molecular mechanisms. Finally, we speculate on the putative role of barotaxis in physiological processes involving immune and cancer cells.

Outer Membrane Structural Defects in Salmonella enterica Serovar Typhimurium Affect Neutrophil Chemokinesis but Not Chemotaxis

Neutrophils, the first line of defense against pathogens, are critical in the host response to acute and chronic infections. In Gram-negative pathogens, the bacterial outer membrane (OM) is a key mediator of pathogen detection; nonetheless, the effects of variations in its molecular structure on the neutrophil migratory response to bacteria remain largely unknown. Here, we developed a quantitative microfluidic assay that precludes physical contact between bacteria and neutrophils while maintaining chemical communication, thus allowing investigation of both transient and steady-state responses of neutrophils to a library of Salmonella enterica serovar Typhimurium OM-related mutants at single-cell resolution. Using single-cell quantitative metrics, we found that transient neutrophil chemokinesis is highly gradated based upon OM structure, while transient and steady-state chemotaxis responses differ little between mutants. Based on our finding of a lack of correlation between chemokinesis and chemotaxis, we define "stimulation score" as a metric that comprehensively describes the neutrophil response to pathogens. Complemented with a killing assay, our results provide insight into how OM modifications affect neutrophil recruitment and pathogen survival. Altogether, our platform enables the discovery of transient and steady-state migratory responses and provides a new path for quantitative interrogation of cell decision-making processes in a variety of host-pathogen interactions.IMPORTANCE Our findings provide insights into the previously unexplored effects of Salmonella envelope defects on fundamental innate immune cell behavior, which advance the knowledge in pathogen-host cell biology and potentially inspire the rational design of attenuated strains for vaccines or immunotherapeutic strains for cancer therapy. Furthermore, the microfluidic assay platform and analytical tools reported herein enable high-throughput, sensitive, and quantitative screening of microbial strains' immunogenicity in vitro This approach could be particularly beneficial for rapid in vitro screening of engineered microbial strains (e.g., vaccine candidates) as the quantitative ranking of the overall strength of the neutrophil response, reported by "stimulation score," agrees with in vivo cytokine response trends reported in the literature.

A New Microfluidic Platform for Studying Natural Killer Cell and Dendritic Cell Interactions

The importance of the bi-directional natural killer–dendritic cell crosstalk in coordinating anti-tumour and anti-microbial responses in vivo has been well established. However, physical parameters associated with natural killer–dendritic cell interactions have not been fully elucidated. We have previously used a simple “Y” shaped microfluidic device to study natural killer cell-migratory responses toward chemical gradients from a conditioned medium of dendritic cells. There are, however, limitations of the Y-shaped microfluidic devices that could not support higher throughput analyses and studies of cell–cell interactions. Here, we report two novel microfluidic devices (D³-Chip, T²-Chip) we applied in advanced studies of natural killer-cell migrations and their interactions with dendritic cells in vitro. The D³-Chip is an improved version of the previously published Y-shaped device that supports high-throughput analyses and docking of the cells of interest in the migration assay before they are exposed to a chemical gradient. The T²-Chip is created to support analyses of natural killer–dendritic cell cell–cell interactions without the requirement of promoting a natural killer cell to migrate long distances to find a loaded dendritic cell in the device. Using these two microfluidic platforms, we observe quantitative differences in the abilities of the immature and lipopolysaccharide-activated mature dendritic cells to interact with activated natural killer cells. The contact time between the activated natural killer cells and immature dendritic cells is significantly longer than that of the mature dendritic cells. There is a significantly higher frequency of an immature dendritic cell coming into contact with multiple natural killer cells and/or making multiple simultaneous contacts with multiple natural killer cells. To contrast, an activated natural killer cell has a significantly higher frequency of coming into contact with the mature dendritic cells than immature dendritic cells. Collectively, these differences in natural killer–dendritic cell interactions may underlie the differential maturation of immature dendritic cells by activated natural killer cells. Further applications of these microfluidic devices in studying natural killer–dendritic cell crosstalk under defined microenvironments shall enrich our understanding of the functional regulations of natural killer cells and dendritic cells in the natural killer–dendritic cell crosstalk.

Microfluidic arenas for war games between neutrophils and microbes

Measurements of neutrophil activities such as cell migration and phagocytosis are generally performed using low-content bulk assays, which provide little detail activity at the single cell level, or flow cytometry methods, which have the single cell resolution but lack perspective on the kinetics of the process. Here, we present a microfluidic assay for measuring the essential functions that contribute to the antimicrobial activity of neutrophils: migration towards the target, and killing of microbes. The assay interrogates the interactions between isolated human neutrophils and populations of live, proliferating microbes. The outcome is measured in a binary mode that is reflective of in vivo infections, which are either cleared or endure the host response. The outcome of the interactions is also characterized at single cell resolution for both the neutrophils and the microbes. We applied the assay to test the response of neutrophils from intensive care patients to live Staphylococcus aureus, and observed alterations of antimicrobial neutrophil activity in patients, including those with sepsis. By directly measuring neutrophil activity against live targets at high spatial and temporal resolution, this assay provides unique insights into the life-or-death contest shaping the outcome of interactions between populations of neutrophils and microbes.

Analysis of Leukocyte Behaviors on Microfluidic Chips

The orchestration of massive leukocytes in the immune system protects humans from invading pathogens and abnormal cells in the body. So far, researches focusing on leukocyte behaviors are performed based on both in vivo and in vitro models. The in vivo animal models are usually less controllable due to their extreme complexity and nonignorable species issue. Therefore, many researchers turn to in vitro models. With the advances in micro/nanofabrication, the microfluidic chip has emerged as a novel platform for model construction in multiple biomedical research fields. Specifically, the microfluidic chip is used to study leukocyte behaviors, due to its incomparable advantages in high throughput, precise control, and flexible integration. Moreover, the small size of the microstructures on the microfluidic chip can better mimic the native microenvironment of leukocytes, which contributes to a more reliable recapitulation. Herein are reviewed the recent advances in microfluidic chip-based leukocyte behavior analysis to provide an overview of this field. Detailed discussions are specifically focused on host defense against pathogens, immunodiagnosis, and immunotherapy studies on microfluidic chips. Finally, the current technical challenges are discussed, as well as possible innovations in this field to improve the related applications.

Towards the development of human immune-system-on-a-chip platforms

Organ-on-a-chip (OoCs) platforms could revolutionize drug discovery and might ultimately become essential tools for precision therapy. Although many single-organ and interconnected systems have been described, the immune system has been comparatively neglected, despite its pervasive role in the body and the trend towards newer therapeutic products (i.e., complex biologics, nanoparticles, immune checkpoint inhibitors, and engineered T cells) that often cause, or are based on, immune reactions. In this review, we recapitulate some distinctive features of the immune system before reviewing microfluidic devices that mimic lymphoid organs or other organs and/or tissues with an integrated immune system component.

Microfluidic Platform to Quantify Neutrophil Migratory Decision-Making

Neutrophils are the most abundant leukocytes in blood, serving as the first line of host defense in tissue damage and infections. Upon activation by chemokines released from pathogens or injured tissues, neutrophils migrate through complex tissue microenvironments toward sites of infections along the chemokine gradients, in a process named chemotaxis. However, current methods for measuring neutrophil chemotaxis require large volumes of blood and are often bulk, endpoint measurements. To address the need for rapid and robust assays, we engineered a novel dual gradient microfluidic platform that precisely quantifies neutrophil migratory decision-making with high temporal resolution. Here, we present a protocol to measure neutrophil migratory phenotypes (velocity, directionality) with single-cell resolution.

The Neutrophil Nucleus: An Important Influence on Neutrophil Migration and Function

Neutrophil nuclear morphology has historically been used in haematology for neutrophil identification and characterisation, but its exact role in neutrophil function has remained enigmatic. During maturation, segmentation of the neutrophil nucleus into its mature, multi-lobulated shape is accompanied by distinct changes in nuclear envelope composition, resulting in a unique nucleus that is believed to be imbued with extraordinary nuclear flexibility. As a rate-limiting factor for cell migration, nuclear morphology and biomechanics are particularly important in the context of neutrophil migration during immune responses. Being an extremely plastic and fast migrating cell type, it is to be expected that neutrophils have an especially deformable nucleus. However, many questions still surround the dynamic capacities of the neutrophil nucleus, and which nuclear and cytoskeletal elements determine these dynamics. The biomechanics of the neutrophil nucleus should also be considered for their influences on the production of neutrophil extracellular traps (NETs), given this process sees the release of chromatin "nets" from nucleoplasm to extracellular space. Although past studies have investigated neutrophil nuclear composition and shape, in a new era of more sophisticated biomechanical and genetic techniques, 3D migration studies, and higher resolution microscopy we now have the ability to further investigate and understand neutrophil nuclear plasticity at an unprecedented level. This review addresses what is currently understood about neutrophil nuclear structure and its role in migration and the release of NETs, whilst highlighting open questions surrounding neutrophil nuclear dynamics.

Progressive mechanical confinement of chemotactic neutrophils induces arrest, oscillations, and retrotaxis

Neutrophils reach the sites of inflammation and infection in a timely manner by navigating efficiently through mechanically complex interstitial spaces, following the guidance of chemical gradients. However, our understanding of how neutrophils that follow chemical cues overcome mechanical obstacles in their path is restricted by the limitations of current experimental systems. Observations in vivo provide limited insights due to the complexity of the tissue environment. Here, we developed microfluidic devices to study the effect of progressive mechanical confinement on the migration patterns of human neutrophils toward chemical attractants. Using these devices, we identified four migration patterns: arrest, oscillation, retrotaxis, and persistent migration. The proportion of these migration patterns is different in patients receiving immunosuppressant treatments after kidney transplant, patients in critical care, and neonatal patients with infections and is distinct from that in healthy donors. The occurrence of these migration patterns is independent of the nuclear lobe number of the neutrophils and depends on the integrity of their cytoskeletal components. Our study highlights the important role of mechanical cues in moving neutrophils and suggests the mechanical constriction-induced migration patterns as potential markers for infection and inflammation.

Convergent and Divergent Migratory Patterns of Human Neutrophils inside Microfluidic Mazes

Neutrophils are key cellular components of the innate immune response and characteristically migrate from the blood towards and throughout tissues. Their migratory process is complex, guided by multiple chemoattractants released from injured tissues and microbes. How neutrophils integrate the various signals in the tissue microenvironment and mount effective responses is not fully understood. Here, we employed microfluidic mazes that replicate features of interstitial spaces and chemoattractant gradients within tissues to analyze the migration patterns of human neutrophils. We find that neutrophils respond to LTB4 and fMLF gradients with highly directional migration patterns and converge towards the source of chemoattractant. We named this directed migration pattern convergent. Moreover, neutrophils respond to gradients of C5a and IL-8 with a low-directionality migration pattern and disperse within mazes. We named this alternative migration pattern divergent. Inhibitors of MAP kinase and PI-3 kinase signaling pathways do not alter either convergent or divergent migration patterns, but reduce the number of responding neutrophils. Overlapping gradients of chemoattractants conserve the convergent and divergent migration patterns corresponding to each chemoattractant and have additive effects on the number of neutrophils migrating. These results suggest that convergent and divergent neutrophil migration-patterns are the result of simultaneous activation of multiple signaling pathways.

Neutrophil Chemotaxis in One Droplet of Blood Using Microfluidic Assays

Neutrophils are the most abundant leukocytes in blood serving as the first line of host defense in tissue damage and infections. Upon activation by chemokines released from pathogens or injured tissues, neutrophils migrate through tissues toward sites of infections along the chemokine gradients, in a process named chemotaxis. Studying neutrophil chemotaxis using conventional tools, such as a transwell assay, often requires isolation of neutrophils from whole blood. This process requires milliliters of blood, trained personnel, and can easily alter the ability of chemotaxis. Microfluidics is an enabling technology for studying chemotaxis of neutrophils in vitro with high temporal and spatial resolution. In this chapter, we describe a procedure for probing human neutrophil chemotaxis directly in one droplet of whole blood, without neutrophil isolation, using microfluidic devices. The same devices can be applied to the study the chemotaxis of neutrophils from small animals, e.g., mice and rats.

Rapid antibiotic sensitivity testing in microwell arrays

The widespread bacterial resistance to a broad range of antibiotics necessitates rapid antibiotic susceptibility testing before effective treatment could start in the clinic. Among resistant bacteria, Staphylococcus aureus is one of the most important, and Methicillin-resistant (MRSA) strains are a common cause of life threatening infections. However, standard susceptibility testing for S. aureus is time consuming and thus the start of effective antibiotic treatment is often delayed. To circumvent the limitations of current susceptibility testing systems, we designed an assay that enables measurements of bacterial growth with higher spatial and temporal resolution than standard techniques. The assay consists of arrays of microwells that confine small number of bacteria in small spaces, where their growth is monitored with high precision. These devices enabled us to investigate the effect of different antibiotics on S. aureus growth. We measured the Minimal Inhibitory Concentration (MIC) in less than 3 hours. In addition to being significantly faster than the 48 hours needed for traditional microbiological methods, the assay is also capable of differentiating the specific effects of different antibiotic classes on S. aureus growth. Overall, this assay has the potential to become a rapid, sensitive, and robust tool for use in hospitals and laboratories to assess antibiotic sensitivity.

Neutrophil Interactions Stimulate Evasive Hyphal Branching by Aspergillus fumigatus

Invasive aspergillosis (IA), primarily caused by Aspergillus fumigatus, is an opportunistic fungal infection predominantly affecting immunocompromised and neutropenic patients that is difficult to treat and results in high mortality. Investigations of neutrophil-hypha interaction in vitro and in animal models of IA are limited by lack of temporal and spatial control over interactions. This study presents a new approach for studying neutrophil-hypha interaction at single cell resolution over time, which revealed an evasive fungal behavior triggered by interaction with neutrophils: Interacting hyphae performed de novo tip formation to generate new hyphal branches, allowing the fungi to avoid the interaction point and continue invasive growth. Induction of this mechanism was independent of neutrophil NADPH oxidase activity and neutrophil extracellular trap (NET) formation, but could be phenocopied by iron chelation and mechanical or physiological stalling of hyphal tip extension. The consequence of branch induction upon interaction outcome depends on the number and activity of neutrophils available: In the presence of sufficient neutrophils branching makes hyphae more vulnerable to destruction, while in the presence of limited neutrophils the interaction increases the number of hyphal tips, potentially making the infection more aggressive. This has direct implications for infections in neutrophil-deficient patients and opens new avenues for treatments targeting fungal branching.

Microdroplet chain array for cell migration assays

Establishing cell migration assays in multiple different microenvironments is important in the study of tissue repair and regeneration, cancer progression, atherosclerosis, and arthritis. In this work, we developed a miniaturized and massive parallel microfluidic platform for multiple cell migration assays combining the traditional membrane-based cell migration technique and the droplet-based microfluidic technique. Nanoliter-scale droplets are flexibly assembled as building blocks based on a porous membrane to form microdroplet chains with diverse configurations for different assay modes. Multiple operations including in-droplet 2D/3D cell culture, cell co-culture and cell migration induced by a chemoattractant concentration gradient in droplet chains could be flexibly performed with reagent consumption in the nanoliter range for each assay and an assay scale-up to 81 assays in parallel in one microchip. We have applied the present platform to multiple modes of cell migration assays including the accurate cell migration assay, competitive cell migration assay, biomimetic chemotaxis assay, and multifactor cell migration assay based on the organ-on-a-chip concept, for demonstrating its versatility, applicability, and potential in cell migration-related research.