Here, we describe a non-invasive two-photon (2P) microscopy approach to study leukocyte homing in the mouse footpad. We discuss the technical aspects of our tissue imaging preparation and walk the reader through a typical experiment from initial set up to execution and data collection.
Two-photon (2P) microscopy is a high resolution imaging technique that has been broadly adapted by biologists. The value of 2P microscopy is that it provides rich spatiotemporal information regarding cell behaviors within intact tissues and in live mice. Leukocyte recruitment plays a significant role in host defense against infection and when unchecked, can contribute to inflammatory and autoimmune diseases. Studying leukocyte recruitment in vivo is technically challenging since cells are moving rapidly within vessels located deep within light scattering tissues. To date, most intravital imaging studies require surgical preparation to expose the blood vessels and tissues. To avoid the tissue damage and inflammation induced by surgery itself, here, we describe a non-invasive single-cell imaging approach that can be used to study leukocyte trafficking in the mouse footpad and phalanges. We discuss the technical aspects of our 2P imaging preparation and walk the reader through a typical experiment from initial set up to execution and data collection.
1. Animal preparation:
This protocol uses adult female adult LysM-eGFP mice, in which neutrophils and macrophages are fluorescently labeled by the expression of eGFP 1.
2. Imaging stage setup:
The imaging platform consists of an aluminum plate with a circular hole cut through the center. A large cover glass is glued to the bottom of the plate and a rubber O-ring is glued on the top to create a fluid-tight recessed chamber to accommodate the rear paw. The aluminum plate is warmed to 36°C via two heating elements attached to the top of the plate.
Mice can be imaged for up to 4 hours without comprising the animal’s viability. The foot can be gently released with small amount of acetone and mice revived for longitudinal imaging studies. However, in this case, we recommend the imaging period to < 2 hours and waiting 24-48 h between sessions to minimize the risk of anesthesia overdose or dehydration.
3. Imaging Acquisition
The two-photon microscope (2P microscope) used in this protocol is a custom-built dual-laser video-rate system consisting an upright microscope. The system is equipped with two Ti: Sapphire lasers, four head-on Bi-alkali PMTs for simultaneous 3 and 4 channel acquisitions and a 20x water immersion objective (Olympus, 20x/0.95 NA).
4. Representative Results
1.Real-time (30f/sec) imagining of neutrophil rolling along the vessels at 200x. Time lapse imaging shows neutrophils, in green, rolling along the endothelium of blood vessels about 10mins after infection with Listeria monocytogenes, shown in red. Collagen fibers in the connective tissue appear in blue. Please click here to see the video.
2.Time-lapse 3D imaging of neutrophil recruitment dynamics at 200x. Typical neutrophil recruitment from circulation and migration through inflamed tissue is shown. Please click here to see the video.
In this video protocol we demonstrate the procedures for non-invasive 2P imaging of leukocyte recruitment in response to inflammation.
The footpad is a classic physiological site for studying inflammation such as allergy, infection and autoimmune disease 4-7. 2P imaging provides a detailed picture of distinct steps in leukocyte trafficking pathways, from rolling and adhesion in the blood vessels to chemotaxis to sites of effector function. We have found that the leukocyte recruitment varies in response to different types of inflammatory stimuli. The dose of the stimulus should be chosen carefully and every attempt made to recapitulate a physiological insult. If the dose is too high or delivered to a non-physiological site, the stimulus might overwhelm the host or lead to experimental artifacts and produce abnormal phenomena. We recommend titrated doses of stimuli for pilot experiments paired with a suitable control and using a bent needle to increase the precision of both the volume delivered and the depth of injection. It is also crucial that the site of injection can be easily located, especially for serial imaging experiments. This can be accomplished by injecting at a specific location (i.e., second digit on third toe of right hind paw) or by tattooing the skin and injecting nearby.
2P imaging provides a unique opportunity to visualize leukocyte trafficking and effector function in vivo. The majority of intravital imaging approaches used to study leukocyte recruitment require surgery to expose tissues and blood vessels in anesthetized mice 8-10. The surgery itself induces inflammation that will impact leukocyte behavior. However, 2P imaging allows leukocytes to be studied non-invasively and at single-cell resolution deep within the native 3D tissue environment. Furthermore, since the imaging procedure itself does not harm the mouse, it is an excellent choice for performing longitudinal studies for models of cancer and arthritis. 2P imaging has the potential to provide and unprecedented view of a disease process from disease initiation and progression to resolution, all within an individual animal subject. This approach is a valuable tool for obtaining insight into the cellular and molecular mechanisms that underlie cellular immunity to infection and how these same responses, if improperly regulated, may lead to inflammatory and autoimmune diseases. Ultimately, a detailed picture of leukocyte behavior in vivo will be aid in the development of new therapeutics for treating inflammatory and infectious diseases.
The authors have nothing to disclose.
This work was supported by NIH grant R01-3155-53502 and Washington University-Pfizer Biomedical Research Agreement.
Material Name | Tipo | Company | Catalogue Number | Comment |
---|---|---|---|---|
Isoflurane, USP | Butler Animal Health Supply | 029405 | ||
655nm non-targeted quantum dots | Invitrogen | Q21021MP | ||
Tetramathylrhodamine conjugated E. coli BioParticles |
Invitrogen | E2862 | ||
Listeria monocytogenes (Lm) | Reference 2 | |||
Quick gel | Duro | |||
Puralube Vet Ointment | Pharmaderm Animal Health | |||
Insulin syringes | BD | 08290-3284-38 | ||
PBS | Thermo Scirntific | SH30256.02 |