Inducing Ischemia-reperfusion Injury in the Mouse Ear Skin for Intravital Multiphoton Imaging of Immune Responses

This protocol describes the induction of an ischemia-reperfusion (IR) model on mouse ear skin using magnet clamping. Using a custom-built intravital imaging model, we study in vivo inflammatory responses post-reperfusion. The rationale behind the development of this technique is to extend the understanding of how leukocytes respond to skin IR injury.

The overall goal of this experiment is to understand how immune cells respond to ischemia reperfusion through intravital multiphoton imaging of the mouse ear skin. This method can help to address some of the very important questions in the field of Immunology and Dermatology to understand the dynamic behavior of immune cell. The main advantage of this technique is that it provide a robust imaging model for studying cellular activities in the skin under inflamed or a resting state.

Though this method may provide insight into ischemic reperfusion injuries, this can also be applied to other systems such as hypersensitivity reactions as well as infections. To begin this procedure, place the mouse on a heating pad to maintain its body temperature at 37 degrees Celsius throughout the preparation procedure. Confirm the depth of anesthesia by the absence of toe pinch reflex.

Next, apply ophthalmic lubricant on the animal's eyes to prevent dryness while under anesthesia. Afterward, carefully apply depilatory cream to the upper two thirds of the dorsal ear and wait for two to three minutes. Then, remove the cream using wet cotton tip applicators in a thorough but gentle manner.

Now, induce ischemia in the mouse ear skin using the neodymium magnets. To do so, first slot the magnets into their individual plastic guides. Position one magnet such that only the edge will contact with a second ventral magnet, which should be placed below the ventral side of the ear with the ear laying flat on the magnet.

Once ready, carefully let the magnets come together. After 1.5 hours of ischemia, remove the magnets by twisting them away from each other using the plastic guides, allowing reperfusion to take place. Immediately after magnet removal, administer Evans Blue or another blood vessel labeling agent of choice intravenously.

In this step, cut two pieces of masking tape. Allow the adhesive sides to stick together whilst leaving about one millimeter of adhesive along its width. Then, cut the masking tape in two to accommodate its placement within the slit on the ear platform.

Insert this masking tape about halfway through the slit such that the adhesive side faces up. After that, position the mouse on the heating pad such that the ear to be imaged is next to the masking tape strip. By using two PBS moistened cotton tip applicators, gently press the ear against the adhesive strip.

Using the strip as a guide, bring the mouse ear through the slit while simultaneously adjusting the mouse closer towards the stage. As the ear skin is very fragile, this step must be done very carefully to avoid any preparation-induced injuries. To remove the sticky tape, first add a drop of PBS to reduce the adhesiveness of the tape.

Then, separate the mouse ear from the masking tape as gently as possible using a fine paint brush. Flatten the ear against the ear platform by gently rolling a moist cotton tip applicator over the ear. Subsequently, put a drop of PBS underneath the coverslip and gently place it over the ear.

Afterward, insert the rectal temperature probe and connect the wires to the heating system according to manufacturer's instructions. To acquire images, place the animal under the multiphoton microscope. Start with an imaging region that is in close proximity to the edge of the ischemia characterized by massive Evans Blue leakage.

The schematic diagram here shows the positioning of the magnets. These two photos show a pre-ischemic and a post-ischemic ear after magnet clamping. In this schematic of the ear indicates the region imaged with respect to where the magnet had been placed.

This time lapse sequence of maximum projection, shows the neutrophil infiltration in response to reperfusion in LysM-eGFPM albino mouse ear skin after 1.5 hours of ischemia. At earlier time points, the lack of Evans Blue signal indicates temporary vessel occlusion after ischemia. Once mastered, preparations for the ischemic reperfused ear for imaging can be done in less than 30 minutes if it is performed properly.

While performing this procedure, it is important to constantly check the depth of anesthesia and the temperature of the mouse. Following this procedure, other method like whole mouse staining of the mouse ear skin can be performed to have a global view of the cellular localizations. After the developments of this protocol, it paved the way for researcher in the field of Immunology and Dermatology to explore innate immunity in the mouse ear skin.

After watching these videos, you should have a good understanding of how to establish a mouse ear skin imaging model to study ischemic reperfusion injury. Don't forget that working with multiphoto microscope can be hazardous. And precautions such as laser safety training should be undertaken.