The overall goal of this procedure is to effectively isolate and characterize macrophages from tissues commonly affected by diet-induced inflammation such as the liver. This method can help answer key questions in the fields of immunology and inflammation about how tissue-resident macrophage phenotype can dictate the progression of chronic inflammatory diseases. The main advantage of this is that the procedure optimizes the isolation steps needed for obtaining single-cell suspensions derived from tissues impacted by obesity-mediated low-grade chronic inflammation.
Demonstrating the procedure will be Joselyn Allen, a graduate student from my laboratory. To isolate the liver, first place an ethanol-soaked mouse in the supine position on a polystyrene foam dissecting board and secure the fore and hind paws. Next, use a medium point tip forceps to grasp the abdominal skin near the urethra opening and use sharp dissecting scissors to create a small incision in the raised skin.
Insert the lower blade of the scissors into the incision between the skin and the peritoneum and make a lateral incision from the groin to the chin. Gently pull back the skin to expose the intact peritoneal cavity and thoracic wall and make a lateral incision through the peritoneum. Lift the sternum and carefully incise the diaphragm.
Then move the gastrointestinal organs to the side and locate the subhepatic inferior vena cava or IVC. Using hemostatic forceps, clamp the suprahepatic IVC to maintain a localized perfusion and attach a syringe filled with pre-warmed perfusion buffer to a 23 gauge blood collection and infusion set. Gently depress the plunger until the tubing and needle are filled with perfusion buffer and insert the needle parallel to the subhepatic IVC.
Proper cannulation of the IVC ensures that the dissociation buffer is locally distributed throughout the liver and peripheral tissues optimizing the tissue digestion. Use a four centimeter hemostatic clamp to secure the needle in place and gently depress the plunger to begin the perfusion. If the needle is appropriately positioned, the color will rapidly flush from the liver and the lobes will become enlarged.
Promptly cut the portal vein to allow the perfusion buffer to flow freely through the liver, occasionally gently massaging the lobes between the thumb and forefinger to facilitate the removal of the blood. When only five milliliters of perfusion buffer are left, replace the perfusion buffer syringe with a syringe filled with pre-warmed dissociation buffer, continuing to gently massage the lobes and perfuse the liver until it is fully digested. Confirm a successful digestion by gently pressing the blunt edge of a pair of forceps into the left lateral lobe.
An indentation should appear on the surface that slowly fills once the forceps is removed. Then remove the needle and use a short straight blade from a pair of dissecting scissors to carefully cut through the connecting ligaments to excise the whole liver and the gallbladder. Transfer the liver to a 10 centimeter dish containing 10 milliliters of ice cold DMEM and use toothed forceps to grip the severed suprahepatic IVC.
Using medium point forceps, apply a rapid stroking motion to each lobe to release the cells from the Glisson's Capsule and pass the saturated DMEM through a 70 micrometer cell strainer in a 50 milliliter conical tube on ice. High yields of viable cells can only be achieved by properly dissociating the liver cells from the Glisson's Capsule. When all of the cells have been collected, invert the tube to gently mix the cell suspension and pellet the cells by centrifugation.
Transfer the supernatant to a new 50 milliliter conical tube for three more low-spin centrifugations. After the last centrifugation, transfer the supernatant to a new 50 milliliter tube and centrifuge the cells one more time at a higher speed. Then resuspend the non-parenchymal cell pellet in FACS buffer for counting and dilute the liver cells to the appropriate concentration according to their downstream application in labeled five milliliter round bottom polystyrene tubes.
Mice fed a high fat diet or a high fat high cholesterol diet exhibit an increased infiltration of classically activated M1 macrophages in affected tissues such as the aorta. RON receptor expressing aortic CD45 positive F4/80 positive macrophages which demonstrate an anti-inflammatory phenotype are decreased in high fat high cholesterol diet fed mice while pro-inflammatory CD45 positive F4/80 positive CD11c positive macrophages are found in increased numbers in aortas isolated from these animals. Further, sorted RON receptor expressing macrophages derived from digested aortas display an increased expression of Arginase 1 gene, a well-established M2 macrophage marker.
Indeed, the characterization of liver-resident macrophages reveals the prevailing phenotype of RON receptor expressing subpopulations with CD11c positive pro-inflammatory macrophage populations demonstrating a decreased expression of genes that are strongly associated with the ant-inflammatory M2 phenotype. Similar trends are observed in the macrophage populations isolated from digested white adipose tissue with pro-inflammatory phenotype macrophage populations exhibiting a decreased surface expression of the RON receptor. Once mastered, this technique can be completed in 20 to 30 minutes per animal if it is performed properly.
Following this procedure, other methods such as flow cytometry, fluorescent-activated cell sorting, and/or quantitative PCR can be performed to further characterize the phenotype of the isolated liver-derived tissue-resident macrophages. After watching this video, you should have a good understanding of how to properly isolate tissue-resident macrophages from diseased livers. Thanks for watching and good luck with your experiments.
