Novel murine immunocompetent models are needed to overcome issues with commonly used syngeneic tumor models and genetically-engineered animal models. Today we review tumor homograft models and the alternative ways they can be created.
Immunocompetent Preclinical Models
Immunocompetent mouse models served as a mainstay in preclinical cancer research for many decades. Today, syngeneic and genetically engineered mouse models (GEMM) are the workhorse in preclinical immuno-oncology studies.
They’re widely used to test cross reactive or surrogate immuno-oncology agents, in order to understand mechanism of action and predict response before moving to more complex humanized studies.
Syngeneic mouse tumor cell lines are the most widely-used model by far in preclinical immuno-oncology pharmacology studies. Syngeneic models are allografts immortalized from mouse cancer cell lines, developed from either spontaneously arising tumors in older mice or from carcinogen induction.
The models are engrafted back into the same inbred immunocompetent mouse strain. The identical host and cell line strain means that tumor rejection doesn’t occur, creating an immunocompetent model for immunotherapy assessment.
Limits of Syngeneic Models
There are a limited number of syngeneic cell lines available. This means that certain types of cancers are underrepresented, such as lung cancer, and that not all cancer types or subtypes are available.
In addition, as syngeneics are derived from immortalized cancer cell lines that have been passaged in vitro, you might see genetic drift from original disease. Specific or rare mutations and fusions seen in human disease are often not represented in these models.
As a result of their non-synonymous mutations, syngeneic models also often have a neo-antigen load significantly higher than found in most human cancers.
Genetically Engineered Mouse Models
In GEMM, one or several genes putatively involved in malignant transformation are deleted, mutated, or overexpressed. This results in spontaneous tumor development.
GEMM provide a more physiologically-relevant tumor microenvironment by recapitulating some of the oncogenesis steps and localizing tumor growth to a specific and appropriate site. Most importantly, they grow in a fully immuno-competent environment, and are therefore appropriate for immuno-oncology research.
Advantages of GEMM
There is a large collection of GEMM available covering many common cancer indications, such as lung, prostate, breast, colon, and pancreatic cancers. This provides more subtype availability than for limited syngeneic cell lines.
GEMM have many other advantages over syngeneic cell lines, including a wide variety of well-characterized models featuring tumors with a clear molecular pathogenesis of disease within an immunocompetent setting.
GEMM also demonstrate lower mutational burden than syngeneic lines, making them genetically more similar to human tumors. This is due to the use of strong oncogenic driver(s) and tumor suppressor loss to enable carcinogenesis.
Disadvantages of GEMM
However, GEMM do have significant drawbacks for in vivo pharmacology studies. They have long latency periods, mice develop disease at different stages, and 100% penetrance is not achievable. Therefore, trying to use GEMM for efficacy testing results in very long and challenging studies and a “rolling enrollment” approach.
Murine Tumor Homograft Models
To work around these problems, allograft fragments or dissociated tumor cells (DTC) of GEMM spontaneous tumors can be used to create tumor homograft models. The fragments/DTC are implanted into a cohort of mice of the same background strain, creating a system combining the strengths of GEMM with improved operational simplicity, consistency, and growth.
Generating tumor homograft models by either fragments or DTC each have distinct advantages and limitations.
Generation by Tumor Fragments
Tumor fragments better reproduce the histological nature of the primary tumor, with original 3D architecture and cell-to-cell contact. These factors are known to influence signaling, and therefore potentially the efficacy of the treatment.
Additionally, in an orthotopic setting, implantation of intact tumor fragments increases metastasis efficiency compared to cell suspensions, while avoiding the artificial diffusion of injections of cell suspensions.
However, implantation of tumor fragments with unknown cell number and regional heterogeneity within the tumor leads to higher inter- and intra-study variability. Also, generating multiple small similar-sized fragments limits the number of secondary recipients.
Generation by DTC
On the other hand, by implanting DTC, tumor heterogeneity is equally-distributed and engraftment and tumor growth are more efficient than for tumor fragments. Components of the tumor microenvironment are also homogeneously represented, including epithelial cells, fibroblasts, and tumor infiltrating lymphocytes.
DTC banks can be established with high percentages of viable cells both pre-freeze and post-thaw. DTC from an individual mouse tumor also provide sufficient viable cells that exceed the capacity of fragments obtained from an individual tumor.
Pooling cells from several tumors, or expanding individual tumors in secondary recipients, can generate a virtually unlimited, on-demand source of tumor cells for preclinical studies.
Syngeneics and murine tumor homografts both provide great model systems for testing the in vivo efficacy of immunotherapeutics. Syngeneics provide a robust, reproducible platform for performing large scale pharmacology studies. Murine tumor homografts share the advantages of GEMM but can be used in a manner similar to syngeneics.
Cohorts of these models can be engrafted in a highly reproducible and robust manner to allow larger scale evaluation of test immunotherapeutics.