The recent breakthrough of checkpoint inhibitors in oncology has demonstrated that the immune system is a critical player in fighting cancer, and that immunotherapies are a key new treatment class.
However, many questions around immunotherapy remain unanswered such as:
- why only a fraction of patients have a dramatic response to these agents
- how to identify patients that will benefit from a given immunotherapy
- how to improve patient response.
Fortunately, several types of preclinical I/O cancer model are available to support research and help in answering these questions. The main feature of these models is that they need some kind of functional immune system (in contrast to immunocompromised mice commonly used for anticancer agent preclinical studies).
With a variety of I/O models available, choosing which one is right for your studies can be challenging. We’ve reviewed the main model types, discussing potential applications and limitations of each, to help in making your choice.
Syngeneic Tumor Models
For syngeneic tumor models, murine tumor cell lines derived from an inbred strain are implanted into inbred mice of the same strain background. The main advantage of these preclinical models are that the mice have a competent immune system, and that the tumor cell lines are easily cultured and expanded, therefore providing a straightforward model system to work with.
Subcutaneous & Orthotopic Syngeneic Mice
“SubQ” syngeneics are useful for evaluating the potential of an immunotherapeutic compound, orthotopic models are also available which tend to be more clinically relevant and better predictors of drug efficacy. Moreover, orthotopic syngeneic models of metastasis including bioluminescent models, have been developed to study metastatic invasion, metastatic lesions in secondary organs, and the evaluation of agents to target this metastasis.
Limitations of Syngeneic Mouse Models
Some limitations with syngeneic models do exist. Due to the limited cell line availability of syngeneics, choice is restricted for some tumor types. There’s also species-specific functional differences between human and murine immune systems. As both tumor and immune cells are of murine origin in syngeneics, some drugs may not recognize the mouse ortholog of their human target (therefore a mouse surrogate antibody would need to be tested rather than a human-specific agent) and some targets may not even be expressed.
Genetically Engineered Mouse Models (GEMM)
In genetically engineered mice, one or several genes that are putatively involved in malignant transformation are deleted, mutated, or overexpressed resulting in spontaneous tumor development. These models provide a more physiologically relevant tumor microenvironment 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, adapted for I/O research.
The Limitation of GEMM
However, cancer GEMM often show delayed and/or highly variable incidence of tumor generation between individual mice, making consistent research difficult. To work around this problem, tumors from a cancer GEMM can be cultured and implanted into a cohort of mice of the same background strain, similar to a syngeneic model. This provides more consistent growth characteristics for preclinical study.
Another disadvantage of GEMM models is the low mutation burden, as tumors develop from limited numbers of mutated oncogene transgenes.
Mice with Human Immune Systems
To overcome the species specificity issues raised by syngeneics, a new and novel way to use GEMM has been developed – to “humanize” them. The models are engineered to express a specific human drug target e.g. mice are engineered to replace a murine target protein like PD-1 with its human counterpart.
In this way human-specific agents and antibodies can be tested. This provides a “chimeric” system with one specific humanized protein within a mouse immune system. Another way to overcome syngeneic and GEMM limitations, particularly regarding specific human immunological mechanisms, is to use humanized mice.
PBMC or HSC?
The two main common approaches use either human peripheral blood mononuclear cells (PBMC) or hematopoietic stem cell (HSC) engrafted in immunodeficient mice to create humanized animal models.
Human PBMCs
PBMC engraftment supports short-term studies, creating transient, partially-reconstituted human immunity. However, these models can still give significant insight into human tumor immunology and treatment efficacy, targeting T-cell function (BiTEs, immune checkpoint inhibitors/agonists, CAR-T) or NK function (ADCC, NK modulating agents).
However, the major drawback of this model is the human xenograft versus host disease (xGvHD) which develops a few weeks after hPBMC engraftment. The xGvHD is due to major histocompatibility complex (MHC) mismatch between human T cells and mouse cells.
Hematopoietic Stem Cell Engraftment
Alternatively, using human hematopoietic stem cells (HSCs) provides a longer term, stable model of human immune system engraftment. Humanized mice are generated through inoculating human hematopoietic cells (from cord blood stem cells) into immunocompromised mice (e.g. NOG, NSG). For immunotherapy targeting immune cells of myeloid lineages, transgenic immunocompromised mice expressing GM-CSF and IL-3 are more suitable models.
These models can provide information on the function of a target, as well as how the human immune response affects tumor growth. They can also be used to test different treatment regimens, including antibodies against specific tumor antigens, checkpoint inhibitors, and/or agents modulating human NK functions, including antibody-dependent cellular cytotoxicity (ADCC).
Hopefully this guide has given a brief introduction to I/O models of both murine and human immunity, and where you might start in choosing what you need for your studies.
