This article provides the detailed method of performing a rapid neutrophil chemotaxis assay by integrating the on-chip neutrophil isolation from whole blood and the chemotaxis test on a single microfluidic chip.
Neutrophil migration and chemotaxis are critical for our body’s immune system. Microfluidic devices are increasingly used for investigating neutrophil migration and chemotaxis owing to their advantages in real-time visualization, precise control of chemical concentration gradient generation, and reduced reagent and sample consumption. Recently, a growing effort has been made by the microfluidic researchers toward developing integrated and easily operated microfluidic chemotaxis analysis systems, directly from whole blood. In this direction, the first all-on-chip method was developed for integrating the magnetic negative purification of neutrophils and the chemotaxis assay from small blood volume samples. This new method permits a rapid sample-to-result neutrophil chemotaxis test in 25 min. In this paper, we provide detailed construction, operation and data analysis method for this all-on-chip chemotaxis assay with a discussion on troubleshooting strategies, limitations and future directions. Representative results of the neutrophil chemotaxis assay testing a defined chemoattractant, N-Formyl-Met-Leu-Phe (fMLP), and sputum from a chronic obstructive pulmonary disease (COPD) patient, using this all-on-chip method are shown. This method is applicable to many cell migration-related investigations and clinical applications.
Chemotaxis, a process of directed cell migration to soluble chemical concentration gradient, is critically involved in many biological processes including immune response1,2,3, tissue development4 and cancer metastasis5. Neutrophils are the most abundant white blood cell subset and play crucial roles in enabling the body's innate host defense functions, as well as in mediating adaptive immune responses6,7. Neutrophils are equipped with highly-regulated chemotactic machinery allowing these motile immune cells to respond to both pathogen-derived chemoattractants (e.g. fMLP) and host-derived chemoattractants (e.g. interleukin-8) throughchemotaxis8. Neutrophil migration and chemotaxis mediate various physiological problems and diseases such as inflammation and cancers1,9. Thus, the accurate assessment of neutrophil chemotaxis provides an important functional readout for studying the neutrophil biology and the associated diseases.
Compared to the widely-used conventional chemotaxis assays (e.g. transwell assay10), the microfluidic devices show great promise for quantitative evaluation of cell migration and chemotaxis owing to the precisely controlled chemical gradient generation and miniaturization11,12,13. Over the last two decades or so, various microfluidic devices have been developed to study the chemotaxis of different biological cell types, especially neutrophils11. Significant effort was devoted to characterizing neutrophil migration in spatiotemporally complex chemical gradients that were configured in the microfluidic devices14,15. Interesting strategies were also developed to study directional decision making by neutrophils using the microfluidic devices16.Aside from biologically-oriented research, the applications of microfluidic devices have been extended to test clinical samples for disease evaluation17,18,19. However, the use of many microfluidic devices is limited to specialized research laboratories and requires lengthy neutrophil isolation from large volume of blood samples. Therefore, there has been a growing trend of developing integrated microfluidic devices for rapid neutrophil chemotaxis analysis directly from a drop of whole blood20,21,22,23,24.
Toward this direction, an all-on-chip method was developed that integrates the magnetic negative neutrophil purification and the subsequent chemotaxis assay on a single microfluidic device25. This all-on-chip method has the following novel features: 1) in contrast to previous on-chip strategies that isolate neutrophils from the blood by adhesion-based cell capture or cell size-based filtering20,22, this new method permits high purity, on-chip magnetic separation of the neutrophils from small volumes of whole blood, as well as chemotaxis measurement upon chemoattractant stimulation; 2) the cell docking structure helps align the initial positions of the neutrophils close to the chemical gradient channel and permits simple chemotaxis analysis without single cell tracking; 3) the integration of the neutrophil isolation and chemotaxis assay on a single microfluidic device permits rapid sample-to-result chemotaxis analysis in 25 min when there is no interruption between experimental steps.
This paper provides a detailed protocol for the construction, operation and data analysis method of this all-on-chip chemotaxis assay. The paper demonstrates the effective use of the developed method for performing neutrophil chemotaxis by testing a known recombinant chemoattractant and complex chemotactic samples from patients, followed by a discussion on troubleshooting strategies, limitations and future directions.
All human sample collection protocols were approved by the Joint-Faculty Research Ethics Board at the University of Manitoba, Winnipeg.
1. Microfluidic Device Fabrication (Figure 1A)
2. Microfluidic Cell Migration Assay Preparation
3. All-on-chip Chemotaxis Assay Operation
4. Cell Migration Data Analysis (Figure 1C)
Neutrophils are negatively selected from a drop of whole blood directly in the microfluidic device. The purity of the isolated neutrophils was verified by on-chip Giemsa staining and the results showed the typical ring-shaped and lobe-shaped nuclei of neutrophils (Figure 2A)25. This indicates an effective on-chip neutrophil isolation at high purity from a small volume of whole blood. Furthermore, the docking structure can effectively align cells next to the gradient channel before applying the chemical gradient (Figure 2B)25.
Gradient generation is based on the continuous laminar flow chemical mixing, and the flows are driven by the pressure difference from the different levels of the inlet and outlet solutions. No external pumps are required. The chemical gradient is established within a few minutes in the microfluidic device, which is characterized by the fluorescence intensity profile of FITC-Dextran across the gradient channel. The gradient is stable for at least 1 h, which is enough time for the current neutrophil chemotaxis experiment (Figure 1C).
To demonstrate the use of the all-on-chip method for cell migration research, the neutrophil chemotaxis in medium alone or in a fMLP gradient were compared. The test results showed that few cells crawled through the barrier channel in the medium control experiment. By contrast, many neutrophils rapidly moved through the barrier channel and migrated toward the 100 nM fMLP gradient (Figure 2B)25. The cell migration test is quantitatively measured by the migration distance, which is significantly higher for the fMLP gradient than the medium control (Figure 2C)25.
Furthermore, the all-on-chip method was demonstrated for potential clinical applications by comparing the neutrophil migration in medium alone to a gradient of sputum sample from COPD patients. The results showed a strong cell migration to the COPD sputum gradient, which is quantitatively indicated by the significantly higher migration distance compared to the medium control (Figure 2B–C)25.
Figure 1: Illustration of the all-on-chip method for neutrophil chemotaxis analysis. (A) Illustration of the microfluidic device. The device includes two layers. The first layer (4 µm high) defines the cell docking barrier channel to trap the cells beside the gradient channel. The second layer (60 µm high) defines the gradient generating channel, the port and channel for cell loading, the chemical inlet reservoirs and the waste outlet. Alignment marks are designed for the two layers. For the second layer, the length and width of the upstream serpentine input channel is 60 mm and 200 µm, respectively; the length and width of the downstream serpentine input channel is 6 mm and 280 µm, respectively; (B) Illustration of the all-on-chip cell isolation method; (C) Illustration of the chemotaxis test. Please click here to view a larger version of this figure.
Figure 2: Representative results of the all-on-chip neutrophil chemotaxis analysis25. (A) Giemsa staining image (using a 60X objective) of the all-on-chip isolated cells in the microfluidic channel; (B) Comparison of the cell distribution in the medium control, a 100 nM fMLP gradient and a COPD sputum gradient; (C) The averaged cell migration distance in the gradient channel in the medium control, a fMLP gradient and a COPD sputum gradient. The error bars indicate the standard error of the mean (SEM). *indicates p <0.05 from the Student's t-test.The figures were adapted from reference25 with permission from World Scientific Publishing. Please click here to view a larger version of this figure.
In this paper, a detailed protocol to directly isolate neutrophils from whole blood followed by the chemotaxis test, all on a single microfluidic chip, was described. This method offers useful features in its easy operation, negative selection of high purity neutrophils, rapid sample-to-result chemotaxis test, reduced reagents and sample consumption, and accurate cell migration data analysis. As a rough estimate, at least 25% of the neutrophils from the input whole blood sample effectively entered the docking structure in the device and we found the neutrophil purity is high by on-chip Giemsa staining.
This developed all-on-chip chemotaxis analysis method has great potential in various cell migration research and clinical applications. A research application of this method was demonstrated by comparing neutrophil chemotaxis in medium alone to a fMLP gradient. Similarly, this method can be used to test neutrophil chemotaxis in COPD sputum as an example of developing a cell functional biomarker for clinical diagnosis. With this method, a researcher can easily test neutrophil chemotaxis to different chemoattractants individually or in combinations. Researchers can also test neutrophil chemotaxis to complex chemotactic factors from patients or test neutrophils from diseased patients for the potentially altered chemotaxis response using this method. This integrated all-on-chip method is particularly useful for performing the test in research or clinical labs that do not have specialized cell culture and live cell imaging facilities. The test can be easily operated by researchers or clinicians following this protocol. For more advanced research applications, this method allows time-lapse microscopy to track individual cell movement.
In general, this all-on-chip method is easy to operate and the result is robust. Several technical reminders will further ensure a successful experiment. First, the PDMS replica should be gently pressed onto the class substrate during plasma bonding to avoid damaging the very thin barrier channel. Second, the evaporation of the medium in the cell loading port can disturb the chemical gradient. It is recommended to cover the cell loading port with a sealing tab during the chemotaxis test. Third, the blood sample should be gently loaded onto the device to avoid high pressure that can push the cells over the barrier channel before the chemotaxis experiment. Fourth, in the current setting, we recommend keeping the magnets attached to the cell loading port during the chemotaxis assay to prevent unwanted cells from entering the channel. Alternatively, a separate piece of PDMS with a through-hole and the magnetic disks attached can be aligned to the cell loading port of the device. In this case, the top PDMS part with the magnetic disks and the trapped cells can be removed from the device after cell isolation.
This all-on-chip method can be further developed to overcome its current limitations and to improve and expand its functionalities. First, the current device only allows a single assay at a time thus limiting the throughput. Further development of the device with multiple parallel test units will improve the experimental throughput requirement. Second, the current flow-based chemical gradient generator limits gradient generation in 1D. Further development of 2D or 3D flow-free gradient generators will better mimic the physiological gradient conditions. Third, in addition to neutrophils, this all-on-chip method in principle can be used to test other white blood cell types such as T cells, B cells and NK cells using similar magnetic cell isolation kits. It will be important to study if this method can be effectively used to test blood cell populations at lower frequency and those cells that require on-chip activation and culture before the chemotaxis test. Then the all-on-chip cell isolation method can be further extended for some other applications. Different barrier channel thickness were tested and the results showed that 3-4 µm is most suitable for the neutrophil migration experiment; that is, it sufficiently trapped the un-stimulated cells and allowed the cells to crawl through the barrier channel upon stimulation. The barrier channel dimension should be optimized for different cell types. Finally, this integrated and rapid chemotaxis test will allow researchers to explore relevant clinical applications. To enable practical testing in clinics, a portable system has been developed that integrates the microfluidic device, temperature and stage control, as well as smartphone-based optical imaging and data analysis modules. In addition to the COPD-related study as demonstrated in this paper, the cell migration for other relevant diseases such as chronic kidney disease is being tested using this all-on-chip method.
The authors have nothing to disclose.
This work is in part supported by grants from the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Canadian Institutes of Health Research (CIHR). We thank the Clinical Institute of Applied Research and Education at the Victoria General Hospital in Winnipeg and Seven Oaks General Hospital in Winnipeg for managing clinical samples from human subjects. We thank Dr. Hagit Peretz-Soroka for helpful discussion about the assay operation strategies. We thank Professor Carolyn Ren and Dr. Xiaoming (Cody) Chen from the University of Waterloo for their generous support in the filming process.
Device fabrication | |||
Mask aligner | ABM | N/A | |
Spinner | Solitec | 5000 | |
Hotplate | VWR | 11301-022 | |
Plasma cleaner | Harrick Plasma | PDC-001 | |
Vacuum dessicator | Fisher Scientific | 08-594-15A | |
Digital scale | Ohaus | CS200 | |
SU-8 2000 thinner | Microchem | SU-8 2000 | |
SU-8 2025 photoresist | Microchem | SU-8 2025 | |
SU-8 developer | Microchem | SU-8 developer | |
Si wafer | Silicon, Inc | LG2065 | |
isopropyl alcohol | Fisher Scientific | A416-4 | |
(tridecafluoro-1,1,2,2-tetrahydrooctyl) trichlorosilane | Gelest | 78560-45-9 | |
Polydimethylsiloxane (PDMS) |
Ellsworth Adhesives | 2065622 | |
Petri Dish | Fisher Scientific | FB0875714 | |
Glass slides | Fisher Scientific | 12-544-4 | |
Cutting pad | N/A | N/A | Custom-made |
Punchers | N/A | N/A | Custom-made |
Name | Source | Catalog Number | Comments |
On-chip cell isolation and chemotaxis assay | |||
RPMI 1640 | Fisher Scientific | SH3025502 | |
DPBS | Fisher Scientific | SH3002802 | |
Bovine serum albumin (BSA) |
Sigma-Aldrich | SH3057402 | |
Fibronectin | VWR | CACB356008 | |
fMLP | Sigma-Aldrich | F3506-10MG | |
Magnetic disks | Indigo Instruments | 44202-1 | 5 mm in diameter, 1 mm thick |
FITC-Dextran | Sigma-Aldrich | FD10S | |
Rhodamine | Sigma-Aldrich |
R4127-5G | |
Giemsa stain solution | Rowley Biochemical Inc. | G-472-1-8OZ | |
EasySep Direct Human Neutrophil Isolation Kit |
STEMCELL Technologies Inc |
19666 | |
Dithiothreitol | Sigma-Aldrich | D0632 | |
Nikon Ti-U inverted fluorescent microscope | Nikon | Ti-U | |
Microscope environmental chamber. | InVivo Scientific | N/A | |
CCD camera | Nikon | DS-Fi1 |