Enrichment and Characterization of the Tumor Immune and Non-immune Microenvironments in Established Subcutaneous Murine Tumors

The overall goal of this technique is to provide a method for simultaneous profiling of both the immune and cancer cell fractions contributing to the immune microenvironment of solid tumors. This method can help answer key questions in the immune oncology fields, such as what immune and non-immune serial changes occur within the tumor following a given treatment. The main advantage of this technique is that it facilitates the effective enrichment of tumor immune and non-immune components, allowing a thorough profiling of the tumor immunologic state.

The overall goal of this method is to efficiently profile the tumor immune microenvironment and identify treatment induced changes. After spraying the mouse with 70%ethanol to prevent hair contamination, surgically remove the subcutaneous tumors, taking care that the tumors are free of any contaminating tissue. Then, place each tumor in 1.8 milliliters of base RPMI 1640 medium without fetal bovine serum in individual wells of a 24-well plate on ice.

When all of the tumors have been collected, use scissors to cut the samples into less than one cubic millimeter pieces and add 200 microliters of freshly prepared 10X dissociation cocktail containing collagenase-1, collagenase-4, and DNase one to each well. After one hour at 37 degrees Celsius, in an orbital plate shaker at 60 to 100 RPM, neutralize the reaction with one milliliter of RPMI 1640 medium supplemented with 5%FBS and two millimolar EDTA, and use a modified one milliliter pipette tip to transfer each tumor slurry sample into individual 50 milliliter conical tubes topped with individual 40 micrometer strainers. Using a 10 milliliter syringe plunger, mechanically disaggregate the tumor pieces through the strainers, periodically rinsing each strainer with two milliliters of RPMI 1640 medium plus 5%FBS until the strainers are clear of tumor.

When all of the tumors have been dissociated, add medium to each tube as necessary to bring all of the samples up to the same volume and pellet the cells by centrifugation. Then, use a five milliliter serological pipette to resuspend the pellets in two milliliters of FBS supplemented medium per tube. To separate the immune and tumor cell fractions, add three milliliters of density gradient medium to the bottom of one 15 milliliter conical tube per sample and slowly layer two milliliters of tumor cell suspension down the side of each tube onto each gradient.

Take care to ensure that the tumor cell suspensions are mixed well before layering, that the cell suspensions are slowly pipette down the side of the tubes, and that the tubes are handled carefully to prevent mixing of the layers. Separate the cells by density gradient centrifugation and carefully collect the top medium and tumor infiltrating leukocyte layers into one new 15 milliliter tube per sample. Discard the remaining density gradient media and resuspend the tumor pellets in two milliliters of medium plus FBS per tube.

Then, centrifuge all of the samples and resuspend the tumor infiltrating leukocytes in 200 microliters and the tumor cells in two milliliters of medium plus FBS per tube on ice. To plate the cells, label one 96-well U-bottom plate for each immune staining panel plus an additional plate for cell counts and aliquot the single cell suspensions into each of the staining panel plates and the calc plate. Wash the cells two times with 200 microliters of Dulbecco's PBS per sample and block the cells with 50 microliters of FC block per well for 20 minutes at four degrees Celsius.

At the end of the blocking incubation, add 50 microliters of 2X concentrated extracellular targeting antibody and fixable viability stain mixture to each well for a 30 minute incubation in the dark. At the end of the staining incubation, wash the samples two times with FACS buffer and resuspend the pellets in 200 microliters of fixation and permeabilization solution overnight at four degrees Celsius, protected from light. The next day, collect the cells by centrifugation, followed by a wash with 200 microliters of permeabilization buffer per well.

Label the cells with 100 microliters of intracellular antibody staining panel in permeabilization buffer for a 30 minute incubation in the dark, followed by another permeabilization buffer wash. Then, wash the cells again with FACS buffer and resuspend the samples in 300 microliters of fresh FACS buffer for their analysis by flow cytometry. Tumor digestion, as demonstrated, with or without density gradient medium enrichment, results in an approximately 70 to 90%viable whole tumor cell population as assessed by flow cytometry.

Density gradient medium enrichment promotes a greater than two-fold increase in the CD45 positive tumor infiltrating leukocyte fraction, compared to non-enriched tumor digestions, as well as numerous immune cell subsets commonly observed in immune microenvironment studies. The CD45 negative tumor cell fraction can also be profiled for numerous tumor features, such as overall tumor viability or apoptosis, proliferation capacity, and the expression of various immunosuppressive markers. To determine how myeloid and lymphocyte immunologic subsets vary throughout a single solid tumor, the tumor can be divided into four equal parts in such a way that each section contains approximately equal intratumoral regions, as well as equal dermal and peritoneal facing portions.

Each piece can then be digested separately and analyzed, as demonstrated, for the presence of various immune cell populations. Once mastered, this technique can be completed in four to 8 hours depending on how many samples you have. While attempting this procedure, it's important to work quickly but efficiently to preserve cell viability.

Visual demonstration of this method is critical, as the dissociation and separation steps involve techniques that are difficult to learn by written text alone. Following this procedure, flow cytometry can be performed to answer additional questions about how cellular populations change within a tumor, both in terms of abundance and phenotype. After watching this video, you should have a good understanding of how to separate and analyze both the immune and non-immune components of an established subcutaneous murine tumor using the demonstrated digestion and density gradient separation techniques.