The overall goal of this procedure is to isolate single cell populations from different small intestinal compartments that can subsequently be used for flow cytometric analysis or an alternate means of characterization. This method can help answer key questions in mucosal immunology such as how the microbiota impacts the development of the intestinal immune system. The main advantages of this technique are that they are rapid, reproducible and don't require laborious percoll gradients.
To begin this procedure, place the mouse dorsal side down and spray the abdomen with 70 percent ethanol, perform a laparotomy by sequentially cutting the skin and then peritoneal fascia along the ventral midline from the pubic symphysis to the xiphoid process, thus exposing the peritoneal cavity. Next, separate the small intestine from the stomach by transecting the pyloric sphincter. Gently remove the small intestine from the peritoneum and tease away the mesenteric fat.
To fully remove the small intestine, make a second cut at the ileocecal junction. Place the isolated small intestine into four degree Celsius RPMI medium containing 10 percent FBS to maximize the cell viability. Gently remove large pieces of fat using curved forceps and use caution to avoid tearing the intestinal tissue itself during the process.
To remove the intestinal contents, cannulate the proximal end of the small intestine with an 18 gauge feeding needle affixed to a syringe and gently flush the intestine with 15 to 20 milliliters of cold PBS. Use scissors to excise Peyer's Patches which are located on the antimesenteric side of the small intestine and appear as a multilobulated white mass. Then, place them into cold RPMI medium containing five percent FBS.
Next, cut the small intestine into three to four inch segments, remove the residual fat by rolling each small intestinal segment on a paper towel moistened with RPMI and use a dull scalpel to tease the fat away from the tissue. Paying strict attention to the complete removal of mesenteric fat is critical to ensuring the viability of lymphocytes as even minute pieces of fat can cause cells to die. After that, turn the tissue inside out by cannulating the intestinal segments with curved forceps and grasping the distal end of the tissue, then use a pair of straight forceps to gently remove the tissue segment from the proximal end, resulting in the tissue being inverted.
Place the tissue segments in a cup containing 30 milliliters of extraction media and a stir bar. Secure the lid on the cap and stir for 15 minutes at 37 degrees Celsius. Stirring should be vigorous, but not turbulent.
After 15 minutes, use a steel strainer to separate the tissue pieces from the epithelium containing supernatant which should appear cloudy. Manually agitate the tissue pieces in RPMI medium to wash away the residual extraction media. Then, place the tissue on a dry paper towel, and flip it several times to facilitate the removal of residual mucous that was not liberated by the extraction medium.
Subsequently, place the tissue fragments in a one point five milliliter tube with 600 microliters of digestion medium. Mince the tissue until the pieces no longer stick to the scissors and the solution appears homogenous. This step is critical to ensure complete enzymatic digestion of the tissue.
Mincing the tissue until the pieces no longer stick to the scissors and the solution appears homogeneous are critical to ensuring cellular yield as well as ensuring reproducibility of results. Add the minced small intestine to a cup containing 25 milliliters of digestion media and stir for 30 minutes at 37 degrees Celsius. Halfway through the digestion, pipe it up and down with a serological pipette to help break down any large chunks of tissue.
Next, filter the digestive tissue through a 100 micrometer cell strainer into a 15 milliliter tube. Rinse the strainer with 20 milliliters of RPMI medium containing 10 percent FBS. Afterward, centrifuge the filtered solution for 10 minutes at four degrees Celsius.
Carefully decant the supernatant and re-suspend the pellet in one milliliter of RPMI medium containing 10 percent FBS. Filter the re-suspended cells through a 40 micrometer cell strainer into a 15 milliliter tube. Rinse the strainer with 20 milliliters of RPMI containing 10 percent FBS.
Subsequently, centrifuge the filtered solution for 10 minutes at four degrees Celsius. Carefully decant the supernatant and re-suspend the pellet in one milliliter of RPMI medium, containing two percent FBS. In this procedure, transfer the excised Peyer's Patches to a cup with 25 milliliters of digestion media and a stir bar.
Secure the lid and spin for 10 minutes at 37 degrees Celsius. Afterward, filter the digested Peyer's Patches through a 40 micrometer cell strainer into a 15 milliliter tube. If any clumps remain, press them through the strainer using the flat end of the plunger from a one milliliter syringe.
Then, rinse the strainer with 10 milliliters of RPMI medium, containing 10 percent FBS. Centrifuge the filtered solution for 10 minutes at four degrees Celsius. Then, carefully decant the supernatant and re-suspend the pellet in one milliliter of RPMI medium containing two percent FBS.
Flow cytometric analysis of single cell suspensions of small intestinal lymphocytes, should yield a discreet population of cells that have similar forward and side scatter characteristics as splenocytes. The lymphocytes may began to die if the tissue is not maintained at four degrees Celsius during the initial stages of the isolation, resulting in the lymphocyte population having a lower forward scatter and being more difficult to separate from other epithelial and dead cells. Moreover, if the mesenteric fat is not completely removed from the small intestinal tissue, there will be a virtually complete loss of lymphocytes.
Typically, an 80 percent viability for small intestinal LP lymphocytes is obtained. Once mastered, one can process four mice and have single cell suspensions ready for staining in less than four hours. During this procedure, it's important to completely remove all mesenteric fat and to mince the tissue well to ensure complete digestion.
Following this procedure, single cell suspensions can be used for purposes other than flow cytometry, such as en vitro experiments, gene expression analysis or adoptive transfer to further characterize the function of these cells. After its development, this technique paved the way for researchers in mucosal immunology to explore the ontogeny and functional implications of the intestinal immune system in mice. After watching this video, you should have a good understanding of how to isolate single cell suspensions from different small intestinal compartments, by cleaning the mesenteric fat, extracting the epithelial layer, and digesting the tissue with collagenase.
