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This protocol demonstrates methods to enable extended in vitro culture of patient-derived xenografts (PDX). One step enhances overall viability of multicellular cluster cultures in 3D hydrogels, through straightforward removal of non-viable single cells. A secondary step demonstrates best practices for PDX culture in a perfused microfluidic platform.
Patient-derived xenografts (PDX), generated when resected patient tumor tissue is engrafted directly into immunocompromised mice, remain biologically stable, thereby preserving molecular, genetic, and histological features, as well as heterogeneity of the original tumor. However, using these models to perform a multitude of experiments, including drug screening, is prohibitive both in terms of cost and time. Three-dimensional (3D) culture systems are widely viewed as platforms in which cancer cells retain their biological integrity through biochemical interactions, morphology, and architecture. Our team has extensive experience culturing PDX cells in vitro using 3D matrices composed of hyaluronic acid (HA). In order to separate mouse fibroblast stromal cells associated with PDXs, we use rotation culture, where stromal cells adhere to the surface of tissue culture-treated plates while dissociated PDX tumor cells float and self-associate into multicellular clusters. Also floating in the supernatant are single, often dead cells, which present a challenge in collecting viable PDX clusters for downstream encapsulation into hydrogels for 3D cell culture. In order to separate these single cells from live cell clusters, we have employed density step gradient centrifugation. The protocol described here allows for the depletion of non-viable single cells from the healthy population of cell clusters that will be used for further in vitro experimentation. In our studies, we incorporate the 3D cultures in microfluidic plates which allow for media perfusion during culture. After assessing the resultant cultures using a fluorescent image-based viability assay of purified versus non-purified cells, our results show that this additional separation step substantially reduced the number of non-viable cells from our cultures.
Over the past decade, the field of cancer research has demonstrated renewed enthusiasm for patient-derived xenografts (PDXs) as a tool for assessing cancer cell pathway reliance and drug susceptibility1. The most common PDX models are established by subcutaneous or orthotopic implantation of human tumor cells—a tumor fragment, a cluster of dissociated tumor-derived cells, or a sample of isolated circulating tumor cells (CTCs)—into a rodent host. If the tumor “take” is successful, the xenograft cells will proliferate, vascularize, and otherwise interact with the host tissue to create a tumor, which can be harvested at an optimal size, subdivided, and re-implanted into other hosts. Among their many advantages as a model system, PDXs typically retain a substantial portion of the native tumor cell population’s heterogeneity and enable the assessment of human-specific pathways and cell responses2,3. The in vivo context enables tumor interaction with vasculature and other adjacent stroma and recapitulates tissue characteristics such as drug diffusion dynamics, oxygen tension, and extracellular matrix influence that biologically and mechanically impact tumor progression. A negative aspect of PDXs is their reliance on a rodent host, both for tumor expansion and ultimately for hypothesis testing. Because many PDXs cannot adapt to traditional two-dimensional (2D) culture on tissue culture polystyrene without losing many of their desirable characteristics, there has been minimal middle ground for researchers between this relatively controlled in vitro method, and the significant increase in expense, facilities, and time requirements for in vivo PDX use.
We have described multiple in vitro models that implement 3D cell culture within a supportive matrix, and recently expanded that work to demonstrate the ex vivo culture of multiple prostate cancer (PCa)-derived PDXs, both alone and in co-culture with bone marrow-derived fibroblasts4,5. Hyaluronic acid (HA)-based hydrogel matrices provide customizable and biologically-relevant support for both cell types, with facile control over hydrogel characteristics and optical clarity for imaging through the hydrogel depth6.
Mature PDX tumor tissues comprise a variable mixture of heterogeneous human cancer cells and mouse stroma (fibroblasts, endothelial cells, etc.). To study cell-type specific contributions to tumor progression in vitro, it can be advantageous to dissociate tumors, separate the cell populations, and experimentally incorporate them in an organized manner to dissect pathways of intercellular communication. The mixed cell populations within tissue digestates have differential compatibility with specific culture conditions. For example, tumor-associated fibroblast viability necessitates either surface adherence or 3D matrices functionalized with integrin ligands, while epithelial-derived PDX cells do not typically have these requirements, instead favoring cell-cell interactions. These differences can be exploited to achieve effective separation of PDX cells from contaminating mouse stromal cells. Rotation culture of tissue digestates allows stromal cell adherence to the tissue culture surface while cell-cell adhesions drive PDX cells floating above the rotating culture surface to form multicellular clusters in the supernatant in 24−48 h. The specific characteristics of these clusters vary with the PDX (e.g., large, tight, highly spherical clusters or smaller, looser aggregates resembling bunches of grapes), but are typically of biologically relevant sizes (50−250 µm diameter), sufficient for assessing cellular interactions that rely on intercellular contacts.
Tumor retrieval and processing inevitably results in some degree of collateral cell death, either due to short-term damage from mechanical/enzymatic disruption, or long-term incompatibility of subpopulations with the chosen culture conditions. Despite the utility of rotation culture as an initial bulk separation, dead or dying cells are inevitably transferred with the PDX clusters and can influence the resultant culture. These dead cells are often individual PDX cells that were not integrated into a cluster, mouse stromal fibroblasts that cannot survive in selected culture conditions, or particularly fragile endothelial cells. Such dying cells can influence experimental results from “survivors” and can substantially impact quantification, e.g., via fluorescent image-based viability screening assays. To improve the selection of live PDX cells from this method, we adapted centrifugation methods with density steps to easily remove individual dead/dying cells from PDX mixtures and retain predominantly live multicellular clusters.
To enhance the study of resultant PDX-derived clusters in 3D culture, we utilized a microfluidics-based perfusion culture platform, the OrganoPlate (Figure 1), which is a high-throughput organ-on-a-chip platform that allows for simultaneous culture of up to 96 individual perfused, 3D cultures on a 384-well microtiter plate-base (Figure 1A)7,8. In the 2-lane microfluidic plate, a single tissue chip is connected by two microfluidic channels (Figure 1B, gel channel: red, perfusion channel: blue) which span four wells in a row. The two microfluidic channels are separated by a short plastic ridge called a Phaseguide which prevents overflow of one channel into its adjacent neighbor channel, and simultaneously allows for a membrane-free interface between the contents of the gel and perfusion channel9. Because the bottom of the microfluidic plate is composed of microscope-grade glass, the cultures can be viewed in the observation window through the bottom of the plate with a standard or automated microscope. Perfusion is established in the microfluidic plate with a programmable rocker, using gravity to drive media through the microfluidic channels, between reservoir wells (Figure 1C). The perfusion flow-mimic more closely recapitulates the tumor microenvironment than static culture, allowing for the incorporation of shear stress and enhanced distribution of gases and nutrients. The benefits of maintaining a perfused cancer cell culture in the microfluidic plate have previously been described as perfused breast cancer cultures exhibited optimal viability as compared to a static 3D culture of the same cells7.
The present report describes an adapted density gradient centrifugation method for isolating live multicellular PDX clusters and demonstrates its utility in establishing 3D PDX cultures within perfusable microfluidic plates. Because an increasing number of research laboratories are seeking methods to facilitate PDX use, we anticipate that the protocols presented here will be of immediate utility.
Tumor tissue was obtained with patient consent and according to an approved Institutional Review Board (IRB) protocol. Xenografts were implanted, grown, and harvested according to an accepted Institutional Animal Care and Use Committee (IACUC) protocol.
NOTE: All work is to be performed in a sterile biological safety cabinet to maintain sterility. All steps should be conducted at room temperature unless otherwise specified.
1. Preparation of materials for PDX processing
2. PDX dissociation and initial purification of stromal component
3. Density gradient centrifugation-based separation of PDX-derived clusters from single cells
4. Hydrogel preparation and microfluidic plate seeding
5. Cell staining, imaging, and image quantification
A programmable perfusion rocker was prepared in a standard water-jacked cell culture incubator, and two-lane microfluidic plates were prepared in a standard biosafety cabinet for loading (Figure 1). An MDA-PCA-118b PDX tumor was expanded in vivo, harvested when it had reached a maximum size, and dissociated as described in protocol section 2 to create a slurry suspension of cells, at approximately a single-cell state (Figure 2A). The slurry was dispensed into 6-...
Here we describe a method for processing and culturing viable PDX-derived tumor cells in a high-throughput, perfused 3D culture system. While this protocol utilizes PCa PDX tissue, it is equally effective for other epithelial-derived tumors. Tumor characteristics vary between individual PDX lines even within the same tissue of origin (prostate, breast, etc.). Some PCa PDX lines are more fibrotic and difficult to isolate viable cells from while others are more cellular. The tumor size noted here can be varied within IACUC...
The authors have nothing to disclose.
This work was supported by National Institutes of Health National Cancer Institute SBIR Phase I (HHSN26120700015C) and P01CA098912.
Name | Company | Catalog Number | Comments |
1N NaOH | any suitable tissue culture grade | ||
60 mm round tissue culture dishes | any suitable | ||
6-well tissue culture plates | any suitable | ||
70 µm cell strainers | Corning | 431751 | or equivalent |
Centrifuge | Eppendorf | 5810R with suitable rotor and buckets for 15/50 mL conical centrifuge tubes | or equivalent |
Density gradient centrifugation solution | Millipore Sigma | P1644 | Percoll |
Dimethylsulfoxide | any suitable tissue culture grade | ||
Dissociation enzyme solution | StemCell Technologies | 07921 | ACCUMAX |
DMEM-F12 | ThermoFisher Scientific | 11039021 | or equivalent |
Forceps | any suitable | ||
HA hydrogel kit | ESI BIO | GS311 | HyStem (Hyaluronic acid-SH and PEGDA) |
Hanks Balanced Salt Solution | Lonza | 10-527F | or equivalent |
Heat-inactivated fetal bovine serum | Atlanta Biologicals | S11150 | |
Hemocytometer | Fisher Scientific | 02-671-51B Hausser BrightLine | or equivalent |
Hoechst 33342 | ThermoFisher Scientific | H1398 | or equivalent |
Image processing software | Oxford Instruments | Imaris 9.3 | or equivalent |
LIVE/DEAD Cell Viability/Cytotoxicity Kit (Calcein-AM/Ethidium Homodimer-1) | ThermoFisher Scientific | L3224 | or equivalent |
Microfluidic culture plate | Mimetas | 9603-400-B | 2-lane OrganoPlate |
Microscope | Nikon | A1R | or equivalent |
Multichannel pipette | Eppendorf | 3125000036 | or equivalent |
PDX-derived tumor tissue | obtained under IRB approval for human tissue and IACUC approval for animal host | ||
Penicillin-streptomycin | ThermoFisher Scientific | 15140-122 | or equivalent |
Perfusion rocker | Mimetas | OrganoPlate Perfusion Rocker Mini | |
pH strips (pH 5-9) | any suitable | ||
Phosphate-buffered saline solution | Lonza | 17-516F | or equivalent |
Razor blades | any suitable | ||
Rotating xy-shaker | VWR | Advanced 3500 Orbital Shaker | or equivalent |
Scalpel handle | any suitable | ||
Single channel repeating pipette | Eppendorf | 22260201 | |
Sterile, 15mL conical centrifuge tubes | any suitable | ||
Sterile, 50mL conical centrifuge tubes | any suitable |
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