Sequential array cytometry: multi-parameter imaging with a single fluorescent channel

Integrating Immunology and Microfluidics for Single Immune Cell Analysis

The field of immunoengineering aims to develop novel therapies and modern vaccines to manipulate and modulate the immune system and applies innovative technologies toward improved understanding of the immune system in health and disease. Microfluidics has proven to be an excellent technology for analytics in biology and chemistry. From simple microsystem chips to complex microfluidic designs, these platforms have witnessed an immense growth over the last decades with frequent emergence of new designs. Microfluidics provides a highly robust and precise tool which led to its widespread application in single-cell analysis of immune cells. Single-cell analysis allows scientists to account for the heterogeneous behavior of immune cells which often gets overshadowed when conventional bulk study methods are used. Application of single-cell analysis using microfluidics has facilitated the identification of several novel functional immune cell subsets, quantification of signaling molecules, and understanding of cellular communication and signaling pathways. Single-cell analysis research in combination with microfluidics has paved the way for the development of novel therapies, point-of-care diagnostics, and even more complex microfluidic platforms that aid in creating in vitro cellular microenvironments for applications in drug and toxicity screening. In this review, we provide a comprehensive overview on the integration of microsystems and microfluidics with immunology and focus on different designs developed to decode single immune cell behavior and cellular communication. We have categorized the microfluidic designs in three specific categories: microfluidic chips with cell traps, valve-based microfluidics, and droplet microfluidics that have facilitated the ongoing research in the field of immunology at single-cell level.

Capillarity Guided Patterning of Microliquids

Soft lithography and other techniques have been developed to investigate biological and chemical phenomena as an alternative to photolithography-based patterning methods that have compatibility problems. Here, a simple approach for nonlithographic patterning of liquids and gels inside microchannels is described. Using a design that incorporates strategically placed microstructures inside the channel, microliquids or gels can be spontaneously trapped and patterned when the channel is drained. The ability to form microscale patterns inside microfluidic channels using simple fluid drain motion offers many advantages. This method is geometrically analyzed based on hydrodynamics and verified with simulation and experiments. Various materials (i.e., water, hydrogels, and other liquids) are successfully patterned with complex shapes that are isolated from each other. Multiple cell types are patterned within the gels. Capillarity guided patterning (CGP) is fast, simple, and robust. It is not limited by pattern shape, size, cell type, and material. In a simple three-step process, a 3D cancer model that mimics cell-cell and cell-extracellular matrix interactions is engineered. The simplicity and robustness of the CGP will be attractive for developing novel in vitro models of organ-on-a-chip and other biological experimental platforms amenable to long-term observation of dynamic events using advanced imaging and analytical techniques.

A microfluidic device enabling high-efficiency single cell trapping

Single cell trapping increasingly serves as a key manipulation technique in single cell analysis for many cutting-edge cell studies. Due to their inherent advantages, microfluidic devices have been widely used to enable single cell immobilization. To further improve the single cell trapping efficiency, this paper reports on a passive hydrodynamic microfluidic device based on the "least flow resistance path" principle with geometry optimized in line with corresponding cell types. Different from serpentine structure, the core trapping structure of the micro-device consists of a series of concatenated T and inverse T junction pairs which function as bypassing channels and trapping constrictions. This new device enhances the single cell trapping efficiency from three aspects: (1) there is no need to deploy very long or complicated channels to adjust flow resistance, thus saving space for each trapping unit; (2) the trapping works in a "deterministic" manner, thus saving a great deal of cell samples; and (3) the compact configuration allows shorter flowing path of cells in multiple channels, thus increasing the speed and throughput of cell trapping. The mathematical model of the design was proposed and optimization of associated key geometric parameters was conducted based on computational fluid dynamics (CFD) simulation. As a proof demonstration, two types of PDMS microfluidic devices were fabricated to trap HeLa and HEK-293T cells with relatively significant differences in cell sizes. Experimental results showed 100% cell trapping and 90% single cell trapping over 4 × 100 trap sites for these two cell types, respectively. The space saving is estimated to be 2-fold and the cell trapping speed enhancement to be 3-fold compared to previously reported devices. This device can be used for trapping various types of cells and expanded to trap cells in the order of tens of thousands on 1-cm(2) scale area, as a promising tool to pattern large-scale single cells on specific substrates and facilitate on-chip cellular assay at the single cell level.

Sequential glycan profiling at single cell level with the microfluidic lab-in-a-trench platform: a new era in experimental cell biology

It is now widely recognised that the earliest changes that occur on a cell when it is stressed or becoming diseased are alterations in its surface glycosylation. Current state-of-the-art technologies in glycoanalysis include mass spectrometry, protein microarray formats, techniques in cytometry and more recently, glyco-quantitative polymerase chain reaction (Glyco-qPCR). Techniques for the glycoprofiling of the surfaces of single cells are either limited to the analysis of large cell populations or are unable to handle multiple and/or sequential probing. Here, we report a novel approach of single live cell glycoprofiling enabled by the microfluidic "Lab-in-a-Trench" (LiaT) platform for performing capture and retention of cells, along with shear-free reagent loading and washing. The significant technical improvement on state-of-the-art is the demonstration of consecutive, spatio-temporally profiling of glycans on a single cell by sequential elution of the previous lectin probe using their corresponding free sugar. We have qualitatively analysed glycan density on the surface of individual cells. This has allowed us to qualitatively co-localise the observed glycans. This approach enables exhaustive glycoprofiling and glycan mapping on the surface of individual live cells with multiple lectins. The possibility of sequentially profiling glycans on cells will be a powerful new tool to add to current glycoanalytical techniques. The LiaT platform will enable cell biologists to perform many high sensitivity assays and also will also make a significant impact on biomarker research.

On-chip investigation of cell-drug interactions

Investigation of cell-drug interaction is of great importance in drug discovery but continues to pose significant challenges to develop robust, fast and high-throughput methods for pharmacologically profiling of potential drugs. Recently, cell chips have emerged as a promising technology for drug discovery/delivery, and their miniaturization and flow-through operation significantly reduce sample consumption while dramatically improving the throughput, reliability, resolution and sensitivity. Herein we review various types of miniaturized cell chips used in investigation of cell-drug interactions. The design and fabrication of cell chips including material selection, surface modification, cell trapping/patterning, concentration gradient generation and mimicking of in vivo environment are presented. Recent advances of on-chip investigations of cell-drug interactions, in particular the high-throughput screening, cell sorting, cytotoxicity testing, drug resistance analysis and pharmacological profiling are examined and discussed. It is expected that this survey can provide thoughtful basics and important applications of on-chip investigations of cell-drug interactions, thus greatly promoting research and development interests in this area.

Hydrodynamic trapping of Tetrahymena thermophila for the long-term monitoring of cell behaviors

Microfluidic trapping technology has been widely applied for single-cell observation in order to reveal characteristic cell behaviors. However, this strategy has yet to be tested for monitoring highly motile cells, which are often biologically important. In this paper, we seek the conditions that enable effective and long-term trapping of a prominent model ciliate Tetrahymena thermophila within a hydrodynamic microfluidic device. Although motility and flexibility of T. thermophila make it difficult to avoid escaping from the trap, we show that tuning some key parameters in the hydrodynamic circuit was effective to achieve approximately 40 h cell retention, which is long enough to monitor cell behaviors over several generations. Here, we demonstrate the real-time observation of cell division and phagocytic digestion, revealing interesting phenomena such as a wide distribution in doubling time in a poor synthetic medium and heterogeneous time courses in digestion processes. Our results present a strategy for trapping highly motile ciliate cells in order to study the dynamic behaviors of single cells.

Microfluidically-unified cell culture, sample preparation, imaging and flow cytometry for measurement of cell signaling pathways with single cell resolution

We have developed a microfluidic platform that enables, in one experiment, monitoring of signaling events spanning multiple time-scales and cellular locations through seamless integration of cell culture, stimulation and preparation with downstream analysis. A combination of two single-cell resolution techniques-on-chip multi-color flow cytometry and fluorescence imaging provides multiplexed and orthogonal data on cellular events. Automated, microfluidic operation allows quantitatively- and temporally-precise dosing leading to fine time-resolution and improved reproducibility of measurements. The platform was used to profile the toll-like receptor (TLR4) pathway in macrophages challenged with lipopolysaccharide (LPS)-beginning with TLR4 receptor activation by LPS, through intracellular MAPK signaling, RelA/p65 translocation in real time, to TNF-α cytokine production, all in one small macrophage population (< 5000 cells) while using minute reagent volume (540 nL/condition). The platform is easily adaptable to many cell types including primary cells and provides a generic platform for profiling signaling pathways.

Introduction: why analyze single cells?

Powerful methods in molecular biology are abundant; however, in many fields including hematology, stem cell biology, tissue engineering, and cancer biology, data from tools and assays that analyze the average signals from many cells may not yield the desired result because the cells of interest may be in the minority-their behavior masked by the majority-or because the dynamics of the populations of interest are offset in time. Accurate characterization of samples with high cellular heterogeneity may only be achieved by analyzing single cells. In this chapter, we discuss the rationale for performing analyses on individual cells in more depth, cover the fields of study in which single-cell behavior is yielding new insights into biological and clinical questions, and speculate on how single-cell analysis will be critical in the future.