Zhao et al. J Transl Med (2020) 18:168 https://doi.org/10.1186/s12967-020-02335-7 Journal of Translational Medicine REVIEW Open Access Microfuidic devices for neutrophil chemotaxis studies Wenjie Zhao1,2, Haiping Zhao3†, Mingxiao Li1* and Chengjun Huang1,2 Abstract Neutrophil chemotaxis plays a vital role in human immune system. Compared with traditional cell migration assays, the emergence of microfuidics provides a new research platform of cell chemotaxis study due to the advantages of visualization, precise control of chemical gradient, and small consumption of reagents. A series of microfuidic devices have been fabricated to study the behavior of neutrophils exposed on controlled, stable, and complex profles of chemical concentration gradients. In addition, microfuidic technology ofers a promising way to integrate the other functions, such as cell culture, separation and analysis into a single chip. Therefore, an overview of recent develop- ments in microfuidic-based neutrophil chemotaxis studies is presented. Meanwhile, the strength and drawbacks of these devices are compared. Keywords: Microfuidic, Neutrophil chemotaxis, Chemical gradient, Lab on a chip Introduction velocity is about 1–20 µm/min, which varies with difer- Chemotaxis, the direct movement of cells along the ent chemokines and diferent gradients. Te directional chemical gradient, is crucial in biologic process, such as persistence means the ratio of relative displacement along innate immunity and cancer metastasis [1, 2]. Neutro- the gradient to total path length, which is infuenced by phils, which are the most numerous and most important the concentration of chemokines [4]. Sensory G-protein- cellular component of the innate immune response, are coupled receptors on neutrophil membranes can detect considered as the frst protective barrier against purulent slight concentration change of chemokines. In certain infections [3]. Neutrophils can persist for hours to days concentration ranges, the neutrophils even response to within the vasculature until they reach senescence. Tey a concentration diference between across their dimen- accumulate rapidly and efciently within minutes to areas sions of ~ 1% [5]. Mesenchymal stem cells (MSCs) have of infammation, which depends on the extreme sensitiv- the potential to diferentiate into a wide variety of other ity of the inciting stimuli. For this efective response, they cell types. Te migration of MSCs was induced by hom- can detect extracellular chemical gradients and move ing signals released by cells at the site of injury and/or towards higher concentrations. Te migration of these infammation [6]. Chemokines, cytokines, and growth cells is mediated by chemotaxis, which acts as the attrac- factors (such as IL-6 (interleukin-6) and PDGF (plate- tive force to determine the direction in which neutrophils let derived growth factor)) released from tissue damage move. Velocity and directional persistence are usually or apoptosis mobilize and recruit stem cells to the dam- used to characterize neutrophil chemotaxis. Te typical aged site, where they proliferate and diferentiate, eventu- ally replacing the damaged tissues [6]. Te factors induce upregulation of selectins and activation of integrins on *Correspondence: [email protected] the stem cell surface, enabling cells to interact with the † Haiping Zhao—Co-frst author endothelium. Stem cells subsequently adhere and trans- 1 Institute of Microelectronics, Chinese Academy of Sciences, 3 Beitucheng West Road, Beijing 100029, China migrate across the endothelial layer into tissues. In addi- Full list of author information is available at the end of the article tion, they are also sensitive to the physical properties of © The Author(s) 2020. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creat iveco mmons .org/publi cdoma in/ zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Zhao et al. J Transl Med (2020) 18:168 Page 2 of 19 the extracellular matrix (ECM) including stifness, topog- receptor-mediated uptake into degradatory vesicles raphy, and dimensionality [7]. Neutrophils are terminally (growth factor breakdown), dummy or atypical recep- diferentiated cells and cannot be genetically manipu- tors which are used to scavenge ligand or extracellular lated. A model system of neutrophil is the promyelocytic enzymes secreted or bound to the outer leafet of the leukemia cells line (HL-60) which can be induced to dif- cell membrane. In addition, chemoattractant breakdown ferentiate into neutrophil-like cells by using dimethyl sul- is important for sharpening gradients and bringing the foxide (DMSO) or dibutyryl-cAMP [8]. Te capacity of attractant levels down below the receptors’ dissociation diferentiated HL-60 cells to responds with chemokine- constant (Kd) so they can precisely perceived [15]. Even sis and chemotaxis to stimuli is diferent to neutrophils. in common assays, where cells are directly exposed to For example, polymorphonuclear cells (PMN)-like HL-60 gradients, chemoattractant breakdown makes chemot- cells respond diferently to thrombospondin (TSP) than axis markedly more efcient. human peripheral blood PMN. HL-60 cells were needed Since the concept of chemotaxis was proposed in to be diferentiated with dimethyl sulfoxide (DMSO), the 1960s, cell chemotaxis studies have been greatly retinoic acid (RA), vitamin D, or l-ascorbic acid (l-AA) improved. Traditional techniques in cell chemotaxis before chemokinesis and chemotaxis of HL-cells were researches based on cell culture in vitro are very mature assayed [9]. Undiferentiated HL-60 cells did not adhere now. One of the most widely used chemotaxis equipment and were not motile in response to TSP. With diferen- is “Boyden Chamber” [16] developed by Boyden in 1962, tiation, a maximal response was obtained with 100 to which is usually used to detect the chemotaxis of leuko- 300 nM TSP, tenfold lower than required for maximal cytes and macrophages. However, the Boyden Chamber PMN chemotaxis [10]. Tese diferences may refect (1) was unsuitable for observing single cell responses with an aberration in HL-60 diferentiation refecting their the unstable chemical gradient profles, which is also leukemic phenotype (2) diferentiation of HL-60 cells to hard to distinguish between chemotaxis and enhanced a cell type characteristic of “activated” PMN [10]. Te motility of cells. To observe cell migration in real time, biological mechanism of neutrophil chemotaxis is highly researchers developed “Zigmond Chamber” [5] based complex, including four steps [11]: neutrophil protec- on “Boyden Chamber”, which is considered as a proto- tion, adhesion to endothelial cells, endothelial cell wall type device based on microfuidic technology. Zigmond penetration, and migration to infected tissue. Incorrectly Chamber was the frst device to allow direct visualiza- signaled neutrophil chemotaxis can cause series cellular tion of cell behavior in the presence of the biomolecule malfunctions such as autoimmune diseases and fatal dis- gradient. Te main limitation of Zigmond Chamber was orders [12]. Terefore, the study of neutrophil chemot- that the characteristics of the gradient was determined by axis is of great signifcance in clinical medicine. the geometry of the device and the difusion coefcient Te classical chemotaxis model is defned as some of the biomolecule. Te generated gradient had short life source releases attractant (or repellent) into the envi- spans (~ 1 h) and was extremely sensitive to evaporation. ronment, while a distant sink absorbs it, forming a dif- Ten, “Dunn Chamber” [17] and “Insall Chamber” [18] fusive gradient that can direct cell migration. Te signals were designed to increase the throughput and achieve of chemoattractant were transmitted through interaction long-term observation of cell migration. Te gradient with heptahelical G protein-coupled receptors (GPCRs) generated in the Dunn Chamber was less susceptible expressed on cell surfaces. Once the chemoattractant to evaporation because source and sink chambers were interacts with its receptor on the neutrophil surface, they sealed. However, the gradient cannot be modifed once undergo cytoskeleton rearrangement, cell shape change solutions were loaded and the coverslip was secured, like and polarize. A “leading edge/pseudopod” is showed at Zigmond Chamber. Te Insall Chamber was designed the front, which pushes the cell forward and a “trailing for compatibility with thin coverslips for optimal optical end/uropod” is showed at the rear, which enables them properties and to allow use of high numerical aperture oil to migrate along a concentration gradient [13]. Tweedy