Cancer Research
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Labeling of Breast Cancer Patient-derived Xenografts with Traceable Reporters for Tumor Growth and Metastasis Studies
Chapters
Summary November 30th, 2016
We describe a method for stable labeling of patient-derived xenografts (PDXs) with lentiviral particles expressing green-fluorescent protein and luciferase reporters. This method allows for tracking the growth of PDXs at the primary site, as well as detecting spontaneous and experimental metastases using in vivo imaging systems.
Transcript
The overall goal of this transduction protocol is to stably introduce reporter genes in patient-derived xenografts, or PDXs, to facilitate their use in experimental and spontaneous metastasis models. This method can help answer key questions in cancer research, such as the extent of excutaneous metastasis from patient-derived xenografts. The main advantage of this technique is that labeled PDX can be track, using in vivo imaging, facilitating the assessment of organized, specific metastasis without the extensive histological analysis post-mortem.
Demonstrating this protocol will be Jessica Finlay-Schultz, a post-doc, who routinely applies this protocol for the label of breast cancer patient-derived xenografts. Using aseptic technique, begin by using a scalpel to cut the skin around the tumor. Then, pull the skin with forceps and use a clean scalpel to separate the skin from the tumor.
When the tumor is completely exposed, use a clean set of forceps and scalpel to transfer the tissue into five milliliters of wash medium on ice. Next, discard the medium and wash the tumor two times with 10 milliliters of HBSS, supplemented with HEPES. Place the rinsed tumor in a sterile, pre-weighed 60 centimeter tissue culture plate, and weigh the plate to estimate the tumor mass.
Using a scalpel, cut the tumor in half. Then cut a three millimeter disc from one of the halves, and place the disc in 10%formalin for 24 hours. Add 200 microliters of HBSS with HEPES to the rest of the tumor, and use forceps and a clean scalpel to mince the tissue into the smallest possible pieces.
Transfer the fragments into a steril, 50 milliliter conical tube, and add at least one milliliter of digestion buffer, supplemented with antimicotics and antibiotics per 100 milligrams of tumor. After three hours at 30 degrees Celsius, and 125 to 200 rotation per minute, add 35 milliliters of wash medium to stop the digestion, and filter the resulting tissue slurry through a 70 micron nylon mesh into a new 50 milliliter conical tube, to remove any undigested tissue. Collect the cells by centrifugation, and re-suspend the pellet in one milliliter of red blood cell lysis buffer, for five minutes at room temperature.
At the end of the lysis, restore the tonicity with 10 milliliters of wash buffer, and strain the cells through a 40 micron filter to remove any clumps. Centrifuge the cells again, and re-suspend the pellet in five milliliters of wash buffer on ice. Then count the number of viable cells by trypan blue exclusion.
To enrich for the human epithelial cancer cells using lineage plus Mel cell depletion, transfer up to one times ten to the seventh cells into a new five milliliter polypropylene tube, and add two milliliters of epithelial enrichment buffer to the cells. Collect the cells by centrifugation, and use a pipette to completely remove the supernatant. Next, re-suspend the pellet in 40 microliters of ice cold epithelial enrichment buffer, and add 10 microliters of biotin antibody cocktail per one times ten to the seventh cells.
Mix by gentle pipetting, and incubate the cell solution for 10 minutes at four degrees Celsius. At the end of incubation, mix the cells with 30 microliters of epithelial enrichment buffer per one times ten to the seventh cells. Followed by the addition of 20 microliters of antibiotin microbeads with gentle mixing.
After 15 minutes at four degrees Celsius, wash the cells in three milliliters of cold epithelial enrichment buffer, and carefully aspirate the supernatant. Re-suspend the pellet in 500 microliters of epithelial enrichment buffer on ice, and place a column in a magnetic separator. Rinse the column with 0.5 milliliters of epithelial enrichment buffer.
And place a new polypropylene collection tube under the column. Then slowly add the labeled cells over the column. Collecting the enriched, unlabeled, lineage negative, tumor cell fraction affluent, followed by three column rinses with 500 microliters of fresh epithelial enrichment buffer.
To transduce the tumor cells, centrifuge the isolated cells for re-suspension in two milliliters of mammosphere medium. After counting, centrifuge the cells again, and re-suspend the pellet in 2 milliliters of mammosphere medium, supplemented with 20 microliters of polybrene. Next, plate two times ten to the fifth viable tumor cells per well in an ultra low adherence six well tissue culture plate, and add lentiviral particles to the cells at a multiplicity of infection of 10.
Swirl the plate to mix the virus with the cells. And incubate the co-cultures at 37 degrees Celsius and 5%carbon dioxide for up to 96 hours. Adding 500 microliters of fresh mammosphere medium after the first 24 hours.
When at least 10%of the cells are GFP positive, transfer the transduce cells from at least one well into a 15 milliliter conical tube on ice, and wash the well with one milliliter of mammosphere medium. Pool the wash with the cells, and centrifuge the tumor cells in 10 milliliters of HBSS plus HEPES. Finally, re-suspend the cells in 50 microliters of basement matrix extract on ice.
And load the embedded cells into a 0.5 milliliter insulin syringe for re-implantation. Some high titer vial vectors express a fluorescent marker that allows estimation of the transduction efficiency in vitro, as early as 24 hours after infection. For most PDXs, the GFP expression will be delayed up to 72 hours after infection.
At which point, the formation of cell aggregates is commonly observed. After extracellular matrix suspension, and re-implantation, the labeled tumor cells regenerate tumors that can be tracked using bioluminescence or fluorescence. A successfully labeled PDX will be nearly 100%GFP positive at the first generation post transduction, and will remain so in subsequent passages.
However, some PDX's will exhibit a patchy distribution of GFP positive cells, indicating a suboptimal in vitro transduction. Immunohistochemical tumor tissue staining reveals that the expression of critical markers, such as epidermal growth factor receptor, and pan cytokeratins, are conserved even after several passages and re-implantations. Experimental models of metastasis provide an alternative to studying organ tropism and organ colonization steps in the metastatic cascade.
Indeed, a transduced PDX injected at 2.5 times ten to the fifth cells per mouse, forms metastasis in the liver that are easily identified with luciferase imaging in vivo. And in the lungs that can be visualized by fluorescence microscopy ex vivo. Once mastered, the dissociation and labeling of the PDXs takes about 4 hours.
The actual expansion of the tumor can take 8 to 12 weeks depending on the tumor, and system used. When attempting this procedure it's important to use a high titer virus with a reporter gene, so you can monitor both transfection efficiency and tumor growth in vivo. Following this procedure, other methods such as in vivo metastasis studies can be performed to answer other questions on tumor progression, metastasis, and response to treatments.
This technique paves the way for breast cancer researchers to explore multiple aspects of tumor progression, using novel PDX models that better represent tumor heterogeneity. After watching this video, you should have a good understanding of how to label dissociated tumor cells with high titer lentiviral particles. Which will allow you to both trace tumor cells in vivo as well as potentially manipulate the expression of genes of interest.
Don't forget that working with lentiviral particles can be extremely hazardous, and you should always take proper precautions when performing this procedure.
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