The goal of this protocol is to show how to use Lattice Light-Sheet Microscopy to four-dimensionally visualize surface receptor dynamics in live cells. Here T cell receptors on CD4+ primary T cells are shown.
The signaling and function of a cell are dictated by the dynamic structures and interactions of its surface receptors. To truly understand the structure-function relationship of these receptors in situ, we need to visualize and track them on the live cell surface with enough spatiotemporal resolution. Here we show how to use recently developed Lattice Light-Sheet Microscopy (LLSM) to image T-cell receptors (TCRs) four-dimensionally (4D, space and time) at the live cell membrane. T cells are one of the main effector cells of the adaptive immune system, and here we used T cells as an example to show that the signaling and function of these cells are driven by the dynamics and interactions of the TCRs. LLSM allows for 4D imaging with unprecedented spatiotemporal resolution. This microscopy technique therefore can be generally applied to a wide array of surface or intracellular molecules of different cells in biology.
The precise dynamics of molecules trafficking and diffusing on the three-dimensional cell surface in real time have been an enigma to solve. Microscopy has always been a balance of speed, sensitivity, and resolution; if any one or two are maximized, the third is minimized. Therefore, due to the small size and immense speed with which surface receptors move, tracking their dynamics has remained a major technological challenge to the field of cell biology. For example, many studies have been conducted using total internal reflection fluorescence (TIRF) microscopy1,2,3, which has high temporal resolution, but can only image a very thin slice of the T-cell membrane (~100 nm), and therefore misses events happening farther away in the cell. These TIRF images also only showing a two-dimensional section of the cell. By contrast, super-resolution techniques, such as stochastic optical reconstruction microscopy (STORM)4, photoactivated localization microscopy (PALM)5, and stimulated emission depletion microscopy (STED)6, can overcome the Abbe diffraction limit of light. These techniques have high spatial resolution (~20 nm resolution)4,5,6,7, but they often take many minutes to acquire a full two-dimensional (2D) or three-dimensional (3D) image, and therefore the temporal resolution is lost. In addition, techniques such as STORM and PALM that rely on blinking signals may have inaccuracies in counting8,9. Electron microscopy has by far the highest resolution (up to 50 pm resolution)10; it can even be conducted three-dimensionally with focused ion beam scanning electron microscopy (FIB-SEM), resulting in up to 3 nm XY and 500 nm Z resolution11. However, any form of electron microscopy requires harsh sample preparation and can only be conducted with fixed cells or tissues, eliminating the possibility of imaging live samples over time.
Techniques to obtain the high spatiotemporal resolution required to identify the dynamics of surface and intracellular molecules in live cells in their true physiological 3D nature is only being recently developed. One of these techniques is Lattice Light-Sheet Microscopy (LLSM)12, which utilizes a structured light sheet to drastically lower photobleaching. Developed in 2014 by Nobel Laureate Eric Betzig, the high axial resolution, low photobleaching and background noise, and ability to simultaneously image hundreds of planes per field of view make LLS microscopes superior to widefield, TIRF and confocal microscopes12,13,14,15,16,17,18,19. This four-dimensional (x, y, z and time) imaging technique, while still diffraction limited (~200 nm XYZ resolution), has incredible temporal resolution (we have achieved a frame rate of about 100 fps, resulting in a 3D reconstructed cell image with 0.85 seconds per frame) for 3D spatial acquisition.
LLSM can be generally used to track real-time dynamics of any molecules within any cell at the single-molecule and single-cell level, particularly those in highly motile cells such as immune cells. For example, we show here how to use LLSM to visualize T-cell receptor (TCR) dynamics. T cells are the effector cells of the adaptive immune system. TCRs are responsible for recognizing peptide-MHC (pMHC) ligands displayed on the surface of antigen-presenting cells (APC), which determines the selection, development, differentiation, fate, and function of a T cell. This recognition occurs at the interface between T cells and APCs, resulting in localized receptor clustering to form what is called the immunological synapse. While it is known that TCRs at the immunological synapse are imperative for T-cell effector function, still unknown are the underlying mechanisms of real-time TCR trafficking to the synapse. LLSM has allowed us to visualize in real time the dynamics of TCRs before and after trafficking to the synapse with the resultant pMHC-TCR interaction (Figure 1). LLSM can therefore be used to solve current questions of the formative dynamics of TCRs and provide insights to understand how a cell distinguishes between self and foreign antigens.
5C.C7 TCR-transgenic RAG2 knockout mice in B10.A background were used in this study according to a protocol approved by the Institutional Animal Care and Use Committee of the University of Chicago.
1. Harvest and Activate T Cells
NOTE: This part of the protocol is based on previous protocols. See citations for further detail20,21.
2. Prepare Cells
3. Conducting LLSM Daily Alignment
NOTE: (Important) This alignment protocol is based on the LLSM instrument used (see the Table of Materials). Each LLSM may be different and require different alignment strategies, especially those that are home-built. Carry out the appropriate routine alignment and continue to section 4.
4. Setting Up Cells with LLSM
5. Track Surface Dynamics
Here, we describe the isolation, preparation, and imaging of primary mouse 5C.C7 T cells using a lattice light-sheet microscope. During section 3, it is imperative to align the microscope correctly, and to collect PSF daily with which to deconvolve the data after collection. In Figure 2, we show the correct alignment images that will be seen when aligning the microscope. Figure 2A and Figure 2B show the correct beam path and beam alignment, respectively, when imaged in fluorescein. The objective scan should show a large X shape in the XZ and YZ projections that is as symmetrical as possible; this should also be adjusted to be as small of an X as possible (Figure 2C). This is mainly achieved by adjusting the emission objective collar. The z galvo scan should show an oval in XZ and XY with a single dot on either side (above and below for XZ and to the left and right for YZ) (Figure 2D). This is mainly achieved by adjusting the galvo mirror tilt, either manually or with the motorized adjustment in the software. Finally, both the z+objective scan and the sample scan should show dots that look as round as possible (Figure 2E and Figure 2F, respectively). These may have a small X, but this should be as dimished as possible. These should be well aligned if the objective scan and z galvo were set well, but if adjustments are necessary, they will be mostly conducted with the galvo mirror. It is important to note that during this alignment, any time the collar and galvo are adjusted, the focus (micrometer above the emission objective) will need to be adjusted as well.
Using this protocol, we can see the four-dimensional dynamics of the TCRs on a T-cell surface (Figure 1, Movie 1). The main advantage of this microscope lies in the ability to track the visualized surface units of the TCRs, and to obtain quantified data from their size, motion, signal intensity, etc. (see Table of Materials). Movie 2 shows an example of the tracks obtained.
Figure 1: 4-dimensional imaging of T cell-APC Synapse. (A) A representative example 3D time-lapse LLSM images showing a T cell interacting with an APC. Shown are the TCR (green, labeled by anti-TCR-AF488) dynamics in recognizing antigens presented on the surface of an APC (red, cytosolic mCherry). Scale bar = 5 μm. Also see Movie 1. (B) Orthogonal XY slice of (A). Inset is a reference frame of a whole cell. Scale bar = 5 μm. (C) Orthogonal YZ slice of (A). Inset is a reference frame of a whole cell. Scale bar = 5 μm. (D) Dual orthogonal slice of (A). Inset is reference frame of whole cell. Scale bar = 5 μm. Also see Movie 3. Please click here to view a larger version of this figure.
Figure 2: LLSM alignment. (A) Desired beam pattern for LLSM imaging experiment. (B) Screenshot of the beam alignment process; on the left is the focus window showing the narrowed, focused beam; at the top right is a graph showing that the beam is centered within the window; at the bottom right is the finder camera, which should also be a thin, focused beam. (C) Maximum intensity projections (MIPs) of a bead by objective scan. (D) Maximum intensity projections (MIPs) of a bead by z-galvo scan. (E) Maximum intensity projections (MIPs) of a bead by z+objective scan. (F) Maximum intensity projections (MIPs) of a bead by sample scan. Please click here to view a larger version of this figure.
Movie 1: T cell synapse formation. Two-color volume rendering of the interaction of a T cell labeled with αTCR-AF488 (green) with a target APC expressing cytosolic mCherry (red) over 70 time points at 1.54 s interval. Please click here to view this video. (Right-click to download.)
Movie 2: Tracking TCR dynamic motion. Compare with Movie 1. Clusters corresponding to visible TCR structures were tracked in 3D over time with imaging software. Dragon tails showing cluster positions over the previous four frames are color-coded by displacement length. Please click here to view this video. (Right-click to download.)
Movie 3: Orthogonal slices of T cell synapse. Compare with Movie 1 and Figure 1B-D. Dual orthogonal slice of Movie 1, showing that αTCRβ-AF488 Fab is indeed labeling the cell surface TCRs. Inset is a reference frame of whole cell. Scale bar = 5 μm. Please click here to view this video. (Right-click to download.)
The presented protocol was optimized for the usage of CD4+ T cells isolated from 5C.C7 transgenic mice on the LLSM instrument used, and therefore other cell systems and LLSMs may need to be optimized differently. However, this protocol shows the power of 4D imaging, as it can be used to quantify the dynamics of a surface receptor on an entire cell with the least distortion in physiological conditions. Therefore, there are many possible future applications of this technique.
A critical step is allowing the cells to settle at an appropriate concentration. If too many APCs settle on the coverslip and become too dense, it is hard to find a T cell that is interacting with only a single APC. When a T cell has multiple synapses, tracking and interpretation of data can become very complicated. Similarly, if too few APCs are present, finding a T cell forming a synapse is also difficult. In our hands, allowing 50,000 cells to settle for 10 min achieves an optimal density. However, this problem can be avoided if using a system with adherent cells. Cells can be grown in the incubator with the coverslips to a desired confluency.
Similarly, the number of T cells dropped into the system is dependent upon the size of the bath and the distance they can disperse. In the LLSM system used here, there is a 12 mL bath and a 2.5 mL bath, as opposed to the previous version of the system which only had a 10 mL bath available. We use the 12 mL bath for the original fluorescein imaging then switch to the 2.5 mL bath for imaging the cells. This allows for less thorough washing of the bath following the beam visualization step, and also lowers the number of T cells required for each imaging session. In turn, this would allow users to utilize fewer cells.
Finding cells at the correct point of interaction is also a challenge. In our hands, T cells take about 2 min to settle down to the APCs on the coverslip, so it is important to begin searching the coverslip for dynamic T cells close to APCs. A major improvement of this has been the recent addition of the LED light in the finder camera. If using a home-built system, we highly recommend including this feature in the design.
Finally, fluorescent labeling strategies are another important consideration. Each fluorescent protein or dye has a different quantum yield and rate of photobleaching. Fluorescent dyes are typically brighter, but if the cells have been stably transduced with a fluorescent protein labeled molecule, the label is replenished as the cell continues to produce the molecule. Therefore, labeling strategy is an important factor to consider when designing experiments.
We would like to conclude with a discussion on future directions for the technology. LLSM is also capable of structured illumination microscopy (SIM), which results in 150 nm XY resolution and 280 nm Z resolution, and is at least 10 times faster than widefield SIM12. Therefore, while LLSM provides unprecedented speed of 4D imaging, it cannot achieve the spatial resolution of current super-resolution techniques4,5,6. However, this resolution could be improved if a STED LLSM could be created. Light sheet STED and stimulated emission depletion with selective plane illumination microscopy (STED-SPIM) have been utilized, but lack the temporal resolution of LLSM21,22,23,24. If STED-SPIM were adapted to incorporate a lattice, we could potentially obtain 50 nm axial resolution with far less photobleaching, and image faster than currently available techniques. Nevertheless, with current available technologies, LLSM gives us the fastest temporal resolution with high axial resolution.
The authors have nothing to disclose.
We would like to acknowledge the advice and guidance from Dr. Vytas Bindokas at the University of Chicago. We thank the Integrated Light Microscopy Core Facility at the University of Chicago for supporting and maintaining the lattice light-sheet microscope. This work was supported by NIH New Innovator Award 1DP2AI144245 and NSF Career Award 1653782 (To J.H.). J.R. is supported by the NSF Graduate Research Fellowships Program.
1 mL Syringe | BD | 309659 | For T cell harvest |
2-Mercaptoethanol | Sigma-Aldrich | M3148-25ML | For T cell culture |
5 mm round coverslips | World Precision Instruments | 502040 | For Imaging |
70um Sterile Cell Strainer | Corning | 7201431 | For T cell harvest |
Alexa Fluor 488 anti-mouse TCR β chain Antibody | BioLegend | 109215 | For Imaging |
Fetal Bovine Serum (FBS) | X&Y Cell Culture | FBS-500 | For T cell culture |
Ficoll | GE Healthcare | 17-1440-02 | Denisty gradient reagent for T cell harvest |
Fluorescein sodium salt | Sigma-Aldrich | F6377 | For microscope alignment |
FluoSpheres Carboxylate-Modified Microspheres | Thermo Fisher Scientific | F8810 | For microscope alignment |
Imaris | Bitplane | N/A | Tracking Software; Other options for tracking software include Amira or Trackmate (Fiji). |
Lattice Light-Sheet Microscope | 3i | N/A | Microscope Used |
Leibovitz's L-15 Medium, no phenol red | Thermo Fisher Scientific | 21083027 | For Imaging |
L-Glutamine | Thermo Fisher Scientific | 25030-081 | For T cell culture |
LLSpy | Janelia Research Campus | N/A | LLSpy was used under license from Howard Hughes Medical Institute, Janelia Research Campus. Contact innovation@janelia.hhmi.org for access. Other deconvolution and deksewing methods are available in image processing softwares such as Fiji, Slidebook, Amira, and others. https://llspy.readthedocs.io/en/latest/ |
Moth Cytochrome C (MCC), sequence ANERADLIAYLKQATK | Elimbio | Custom Synthesis | For T cell harvest |
Penacillin/Streptamycin | Life Technologies | 15140122_3683884612 | For T cell culture |
Poly-L-Lysine | Phenix Research Products | P8920-100ML | For Imaging |
RBC Lysis Buffer | eBioscience | 00-4300-54 | For T cell harvest |
Recombinant mouse IL-2 | Sigma-Aldrich | I0523 | For T cell culture |
RPMI 1640 Medium | Corning | MT10040CV | For T cell culture |
Slidebook | 3i | N/A | LLSM imaging software |
Surgical Dissection Tools | Nova-Tech International | DSET10 | For T cell harvest |
T-25 Flasks | Eppendorf | 2231710126 | For T cell culture |
Thermo Scientific Pierce Fab Micro Preparation Kits | Thermo Fisher Scientific | 44685 | For preparing Fab |