We describe a method to visualize GFP-labeled γδ IELs using intravital imaging of murine small intestine by inverted spinning disk confocal microscopy. This technique enables the tracking of live cells within the mucosa for up to 4 h and can be used to investigate a variety of intestinal immune-epithelial interactions.
Intraepithelial lymphocytes expressing γδ T cell receptor (γδ IEL) play a key role in immune surveillance of the intestinal epithelium. Due in part to the lack of a definitive ligand for the γδ T cell receptor, our understanding of the regulation of γδ IEL activation and their function in vivo remains limited. This necessitates the development of alternative strategies to interrogate signaling pathways involved in regulating γδ IEL function and the responsiveness of these cells to the local microenvironment. Although γδ IELs are widely understood to limit pathogen translocation, the use of intravital imaging has been critical to understanding the spatiotemporal dynamics of IEL/epithelial interactions at steady-state and in response to invasive pathogens. Herein, we present a protocol for visualizing IEL migratory behavior in the small intestinal mucosa of a GFP γδ T cell reporter mouse using inverted spinning disk confocal laser microscopy. Although the maximum imaging depth of this approach is limited relative to the use of two-photon laser-scanning microscopy, spinning disk confocal laser microscopy provides the advantage of high speed image acquisition with reduced photobleaching and photodamage. Using 4D image analysis software, T cell surveillance behavior and their interactions with neighboring cells can be analyzed following experimental manipulation to provide additional insight into IEL activation and function within the intestinal mucosa.
Intraepithelial lymphocytes (IEL) are located within the intestinal epithelium, and are found both along the basement membrane and between adjacent epithelial cells in the lateral intercellular space1. There is approximately one IEL for every 5-10 epithelial cells; these IELs serve as sentinels to provide immune surveillance of the large expanse of the intestinal epithelial barrier2. IELs expressing the γδ T cell receptor (TCR) comprise up to 60% of the total IEL population in the murine small intestine. Studies in γδ T-cell-deficient mice demonstrate a largely protective role of these cells in response to intestinal injury, inflammation and infection3,4,5. Despite the generation of the Tcrd knockout mouse6, our understanding of γδ IEL biology remains limited due in part to the fact that ligands recognized by the γδ TCR have yet to be identified7. As a result, the lack of tools to study this cell population has made it difficult to investigate the role of γδ TCR activation and function under physiological and pathological conditions. To fill this gap, we have developed live imaging techniques to visualize γδ IEL migratory behavior and interactions with neighboring enterocytes as a means to provide additional insight into γδ IEL function and responsiveness to external stimuli in vivo.
Over the last decade, intravital imaging has significantly expanded our understanding of the molecular events involved in multiple facets of intestinal biology, including epithelial cell shedding8, regulation of epithelial barrier function9,10, myeloid cell sampling of luminal contents11,12, and host-microbe interactions11,13,14,15,16. In the context of IEL biology, the use of intravital microscopy has shed light on the spatiotemporal dynamics of IEL motility and the factors mediating their surveillance behavior13,14,15,16. The development of TcrdH2BeGFP (TcrdEGFP) reporter mice, which labels γδ IELs by nuclear GFP expression17, revealed that γδ IELs are highly motile within the epithelium and exhibit a unique surveillance behavior that is responsive to microbial infection17,13,14. Recently, another γδ T cell reporter mouse was developed (Tcrd-GDL) which expresses GFP in the cytoplasm to allow visualization of the entire cell18. Similar methodology has been used to investigate the requirement of specific chemokine receptors, such as G protein-coupled receptor (GPCR)-18 and -55, on the dynamics of IEL motility19,20. In the absence of a cell-specific reporter, fluorescent conjugated antibodies against CD8α were used to visualize and track IEL motility in vivo19,20. Although two-photon laser scanning microscopy is commonly used for intravital imaging, the use of spinning disk confocal laser microscopy provides unique advantages to capture high speed and high-resolution multi-channel images with minimal background noise. This technology is ideal to elucidate the spatiotemporal dynamics of immune/epithelial interactions within the complex microenvironment of the intestinal mucosa. Moreover, through the use of various transgenic and/or knockout mouse models, these studies can provide insight into the molecular regulation of intestinal immune and/or epithelial cell function.
All studies were conducted in an Association of the Assessment and Accreditation of Laboratory Animal Care (AALAC)-accredited facility according to protocols approved by Rutgers New Jersey Medical School Comparative Medicine Resources.
1. Mouse Preparation
NOTE: The following procedure, including animal preparation and surgery, will take 30–40 min. Prior to the surgery, turn on the microscope and warm up the enclosed incubator on the microscope to 37 °C.
2. Mouse Surgery: Laparotomy to Expose Intestinal Mucosa
3. Image Acquisition by Spinning Disk Confocal Microscopy
4. 4D Analysis of Images
Using intravital imaging of TcrdEGFP reporter mice, we have previously shown that γδ IELs exhibit a dynamic surveillance behavior, in which they patrol the epithelium by migrating along the basement membrane and into the lateral intercellular space (LIS) at steady state (Figure 2, Movie 1).
This approach can also be used to evaluate how the inhibition of specific cell signaling pathways and/or receptors influences γδ IEL migratory behavior. For example, interleukin (IL)-15 is a pleiotropic cytokine that is essential for γδ IEL homeostasis23,24. To determine whether IL-15 signaling through IL-2Rβ receptor contributes to the kinetics of γδ IEL motility at steady state, TcrdEGFP reporter mice were imaged 2 h following administration of anti-IL-2Rβ antibody (TM-β1) or isotype IgG2b control. The colored tracks indicate the migratory paths of individual γδ IELs over the course of 30 min (Figure 3A). Although the frequency of γδ IELs in the LIS was increased in mice treated with TM-β1 (Figure 3A,B), more than 30% of these γδ IELs exhibited an idling behavior (Figure 3A,C). Idle γδ IELs were defined by imposing upper limits on track straightness and track displacement length. This idling phenotype was confirmed by a significant reduction in both the instantaneous speed and confinement ratio of γδ IELs following IL-2Rβ blockade relative to control (Figure 3D,E). Further, the idle γδ IELs have longer dwell times and were more frequently localized within the LIS compared to motile γδ IELs (Figure 3F).
Figure 1: Using forceps to create a hole in the mesentery. Place forceps on either side of the membrane to create a hole for the sutures. Please click here to view a larger version of this figure.
Figure 2: Representative 3D image of γδ IELs in the jejunum. A single time point was selected from a time-lapse video of taken of the jejunal mucosa of a TcrdEGFP reporter mouse at steady-state. γδ IELs are shown in green, nuclei in white and the lumen in red. Scale bar = 30 μm. Please click here to view a larger version of this figure.
Figure 3: Inhibition of IL-2Rβ induces γδ IEL idling in the lateral intercellular space. This figure has been adapted from “Epithelial IL-15 Is a Critical Regulator of γδ Intraepithelial Lymphocyte Motility within the Intestinal Mucosa.” by Hu, M. D. et al. 2018, J Immunol, 201 (2), p. 747-756.15 Copyright 2018 by the American Association of Immunologists, Inc. (A) Time-lapse images showing migrating GFP γδ IELs within the jejunal villous epithelium following 2 h treatment with 0.45 mg of IgG2b or TM-β1. The motility of individual γδ IELs (green) over the course of 30 min are shown with colored tracks. Nuclei are shown in white, and the lumen is shown in red. Scale bars = 20 µm. (B) Frequency of γδ IELs in the LIS (n = 3 mice per treatment, n = 6-7 videos). (C) Percentage of γδ IELs that were idle in IgG2b- or TM-β1-treated mice. (D) Instantaneous speed (n = 13,299 and 9,600 time points). (E) Track confinement ratios (n = 350 and 278 tracks) are shown. (F) Distance of idle and motile γδ IELs from the lumen. Mean + SEM is shown (+). *p < 0.05, **p < 0.01, #p < 0.0001. Please click here to view a larger version of this figure.
IEL | lumen | |
Surface detail (μm) | 1.2 | 1.5 |
Background subtraction (μm) | 1.67 | 2 |
Seed points diameter (μm) | 7.5 | N/A |
Filter: number of voxels | 250 | 6,500 |
Tracking: max distance (μm) | 10 | 10 |
Tracking: max gap | 2 | 2 |
Table 1: Representative settings for generating γδ IEL and luminal “Surfaces” in Imaris. These settings can be used as a starting point when generating and tracking Surfaces of the 488 nm channel (IEL) and 640 nm channel (lumen). Depending on experimental conditions, these settings may need to be modified.
Movie 1: Intravital imaging of GFP γδ IEL migratory behavior in mouse jejunum. Time-lapse intravital microscopy of γδ IEL motility in jejunum from a TcrdEGFP mouse treated with 0.45 mg of IgG2b. γδ IELs are shown in green, nuclei in blue, and the lumen in red. Scale bar, 20 μm. Frames were collected every 30 s for approximately 30 min. Movie is adapted from Hu, M. D. et al. 201815. Please click here to view this video. (Right-click to download.)
The development of intravital microscopy techniques has provided an unprecedented opportunity to observe the reorganization of subcellular structures8,9,22, cell-cell interactions12,25 and cell migratory behavior13,14,15,16,26 in otherwise inaccessible tissues. There has been a general lack of tools to study the regulation and function of γδ IELs in vivo18,27, and as a result, the use of intravital imaging has opened new avenues of investigation regarding the molecular regulation and functional characteristics of IEL populations13,14,15,16,20. γδ IELs were previously presumed to be stationary within the intestinal mucosa28. However, further analysis demonstrated that these cells are highly dynamic, but migrate more slowly than lymphocytes in secondary lymphoid organs due to the tight confines of the epithelial compartment13. Moreover, the use of in vivo imaging has provided additional insight into the spatiotemporal dynamics of γδ IEL surveillance behavior, including the crosstalk between IELs and epithelial cells that drives γδ IEL motility and function13,14,15,16. Analysis of this migratory behavior has defined new metrics for IEL cellular responses that would not have been accurately quantifiable using ex vivo or in vitro experiments.
Throughout the entire procedure, it is essential to continuously monitor the sedation level of the mouse. Continuous administration of gas anesthesia (isoflurane) can be used as an alternative to ketamine/xylazine. When performing the laparotomy, it is critical to keep the neurovascular supply intact to provide an accurate evaluation of the biology. Tying off mesenteric blood vessels can lead to hypoxia and gradual necrosis of the tissue. In contrast, severing blood vessels will result in bleeding into the exposed mucosa, which can reduce the fluorescence intensity of some channels during imaging. If the IELs develop a rounded morphology and stop moving during image acquisition, this may be due to a slight decrease in the body temperature of the mouse. Placing temperature sensor close to the microscope stage ensures that the temperature at the site of the exposed mucosa is maintained at 37 °C. Raising the temperature to 38–39 °C may be necessary depending on the temperature stability of the incubation chamber.
Once the mouse has been positioned on the microscope stage, it can be difficult to identify fields of villi that lack excessive movement due to peristalsis or contraction of adjacent blood vessels. If this occurs, reposition the mouse within the dish or manipulate the mouse’s flanks externally in an effort to tightly pack villi against the coverslip. Alternatively, the imaging field may become more stable a few minutes after placing the mouse on the stage. If there is gradual XY drift during the acquisition, this can be corrected using drift correction in the 4D rendering software by selecting 4–5 nuclei that are present in different locations of the image throughout the time course and tracking a Spot for each nucleus. Once the Spots have been generated, select Correct Drift under the Edit Track menu to correct translational drift. It is important that the drift correction be done prior to any other analysis, since this will alter the coordinates used to track other objects in the image.
Although two-photon laser scanning microscopy allows for a wide depth of imaging that is useful for penetrating deep into tissues29, spinning disk laser confocal microscopy has its own unique advantages for intravital imaging applications. Two-photon laser scanning microscopy relies on photomultiplier tubes (PMT) that can detect only one pixel at a time with a quantum efficiency of 40–50%. In contrast, spinning disk confocal laser microscopy uses an electron multiplication (EM)CCD camera to directly detect up to a million pixels simultaneously, thus increasing the quantum efficiency to 95%. As a result, the potential for photobleaching and photodamage is significantly reduced, while the increased acquisition speed maximizes temporal resolution when imaging cell migration in more superficial tissues. Regardless of the imaging modality, this surgical procedure is ideal for exposing the intestinal mucosa for imaging on an inverted microscope.
With the increasing availability of mouse strains expressing fluorescent reporters, fluorescently-labeled probes or fluorescent strains of microorganisms that have been generated in a multitude of wavelengths, the combinations to evaluate in vivo intestinal responses in real-time are endless. Not only can intravital imaging provide novel insight into cell-cell and host-microbe interactions, but it can also highlight dynamic molecular and cellular processes in both epithelial and immune cells under homeostatic or pathological conditions.
The authors have nothing to disclose.
This work is supported by NIH R21 AI143892, New Jersey Health Foundation Grant, Busch Biomedical Grant (KLE). We thank Madeleine Hu for her assistance in editing the manuscript and providing the data shown in the representative results.
35mm dish, No. 1.5 Coverslip | MatTek | P35G-1.5-14-C | |
Alexa Fluor 633 Hydrazide | Invitrogen | A30634 | |
BD PrecisionGlide Hypodermic needles – 27g | Thermo Fisher Scientific | 14-826-48 | |
BD Slip Tip Sterile Syringe – 1 ml | Thermo Fisher Scientific | 14-823-434 | |
BD Tuberculin Syringe | Thermo Fisher Scientific | 14-829-9 | |
Dissecting scissors | Thermo Fisher Scientific | 08-940 | |
Electrocautery | Thermo Fisher Scientific | 50822501 | |
Enclosed incubation chamber | OKOLAB | Microscope | |
Eye Needles, Size #3; 1/2 Circle, Taper Point, 12 mm Chord Length | Roboz | RS-7983-3 | |
Hank's Balanced Salt Solution | Sigma-Aldrich | 55037C | |
Hoechst 33342 | Invitrogen | H3570 | |
Imaris (v. 9.2.1) with Start, Track, XT modules | Bitplane | Software | |
Inverted DMi8 | Leica | Microscope | |
IQ3 (v. 3.6.3) | Andor | Software | |
Ketamine | Putney | Anesthesia | |
Kimwipes | VWR | 21905-026 | |
McPherson-Vannas scissors 3” (7.5 cm) Long 5X0.15mm Straight Sharp | Roboz | RS-5600 | |
Non-absorbable surgical suture, Silk Spool, Black Braided | Fisher Scientific | NC0798934 | |
Nugent Forceps 4.25” (11 cm) Long Angled Smooth 1.2mm Tip | Roboz | RS-5228 | |
Puralube Vet Ointment | Dechra | Lubricating Eye Ointment | |
Spinning disk Yokogawa CSU-W1 with a 63x 1.3 N.A. HC PLAN APO glycerol immersion objective, iXon Life 888 EMCCD camera, 405 nm diode laser, 488 nm DPSS laser, 640 nm diode laser | Andor | Confocal system | |
Xylazine | Akorn | Anesthesia |