Bioluminescent imaging (BLI) is non-invasive method that allows for real-time imaging of cells or tissues in living organisms. Mice have become the primary model used for BLI, and the field of preclinical oncology has made great strides in basic research and drug screening with BLI-based murine tumor models. Here we highlight how BLI-based tumor models can be used to quantify tumor burden over time and how this application is particularly well-suited to drug and biologic screening.
Bioluminescent tumor cell lines have been engineered to express luciferase, an enzyme of animal or bacterial origin that can catalyze a reaction with its substrate luciferin and emit light. Cell lines for both solid tumor and hematologic malignancies have been developed to express luciferase using viruses like poxvirus or adenovirus-based vectors for gene transduction, . Vectors can be used to express luciferase under a constitutively active promoter that typically results in a linear relationship between bioluminescence and tumor burden, and this type of construct is useful for drug or biologic screening assuming the luciferase copy number does not change in a particular tumor cell line. Luciferase can also be expressed under a promoter driven by a tumor suppressor like p53 that allows for real-time visualization of p53 activity in tumors. These transduced cell lines can be infused or transplanted into mice, and tumor size or metastasis can be visualized by treating mice with luciferin substrate and using highly sensitive CCD (charge-coupled device) cameras for image acquisition. Light emission can be detected for several minutes after luciferin administration. A major advantage of BLI is that mice can be repeatedly imaged over days, weeks, or months through re-administration of luciferin as tumor cells will continue express luciferase.
BLI experiments typically need to be optimized for each cell line and mouse strain. Subcutaneous injection of tumors usually results in a tumor mass that can be readily visualized due to proximity to the cell surface, but intravenous delivery of solid tumor cells will result in different tissues, including specific target tissues, being seeded with different amounts of luciferase-expressing cells, which can result in dramatic light signal variation. Some tissues and organs are more likely to absorb emitted photons, thus reducing the light signal detected. Timing is key for BLI image acquisition. Emitted photons can typically be detected 5 to 20 minutes after luciferin delivery, but signal can often be detected up to 60 minutes later. Optimization imaging experiments are needed to determine if BLI signal acquisition should be done at a fixed time point or as a dynamic measurement over time (measured as area under the curve over a time range).
BLI methods are especially well-suited for screening candidate drugs or biologics, as changes in tumor volume or metastasis can be monitored over time under different treatment conditions. Cell lines that represent nearly every type of tumor or hematologic malignancy exist and have been or can be modified to express luciferase. Immunotherapeutics, like chimeric antigen receptor (CAR)-T cells, have also been modified to express luciferase for detection of CAR expression, which is useful for studying CAR expression in different immune cells.
BLI has become an indispensable tool for preclinical oncology research. The adaptability and sensitivity of BLI-based assays makes this a tool worth considering for drug screens or basic tumor biology studies.
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