Syngeneic Tumor Model Health Standards

The growth in immuno-oncology research, in which the body's own immune system is recruited to fight a tumor, has prompted a shift in preclinical animal models used for oncology research. While traditional cell line xenografts in immunodeficient models are still widely used, they are generally not appropriate for immuno-oncology studies as they lack critical immune cell types required for immunotherapy efficacy.

Immuno-oncology researchers have turned to two model systems: syngeneic tumor models in immunocompetent inbred strains and mice engrafted both with a humanized immune system and a tumor. While humanized immune system mice are critical tools, syngeneic tumor modeling has become the workhorse. These experiments can be set up quickly and cost-effectively.

What is a Syngeneic Tumor Model?

Syngeneic studies use immunocompetent wild type mice or rats, typically inbred strains, for engraftment of a tumor cell from the same strain. Tumor growth and consistency may be affected by microbial health status. For that reason, understanding the microbial status and maintaining a consistent microbial status is critical.

Health Standards for Syngeneic Mouse Models

Facility managers and veterinarians involved in these types of studies must consider various factors. For syngeneic models, what health standard is acceptable or preferable?

Most common inbred strains are available from multiple vendors at either a specific pathogen free (SPF) or specific pathogen and opportunist free (SOPF) health status. These mouse models are not always interchangeable between vendors, however, as exactly which pathogens and opportunists are excluded to define those standards varies.

Maintaining consistent health standards through a set of experiments is critical. Inbred mice with higher health statuses are now readily available, and should be housed under strict barrier conditions rather than conventional rooms in order to maintain that health status.

Keep in mind that SOPF is a generic term without a specific definition. Each vendor may test for and exclude a different set of agents in their SOPF standard.

Specific Agents of Concern to Syngeneic Mice

There are numerous agents of concern for oncology studies. Agents such as mouse parvovirus (MPV), lactate dehydrogenase elevating virus (LDV), and Corynebacterium bovis have been shown to affect tumor growth kinetics in xenografts, creating a direct concern for those experimental outcomes.

C. bovis is a gram positive bacterium which can cause scaly skin disease, particularly in nude mice and other immunodeficients. C. bovis can be difficult to eradicate and exclude as it is highly infectious and can persist in the environment.

Immunocompetent animals can carry C. bovis. If immunocompetent animals will be housed with immunodeficient animals, we recommend that the immunocompetent mice be sourced from a location which excludes C. bovis. Look for SOPF animals, but be sure to ask the vendor if they test for this agent in their SOPF immunocompetent colonies.

Taconic Biosciences specifically tests for and excludes C. bovis at all health standards, including those which house only immunocompetent animals.

Variations in Microbiome

The microbiome — the collection of all microorganisms including viruses and fungi on/in an organism, such as a human or laboratory mouse — is increasingly recognized as a factor in studies of all types, including allografts and xenografts.

Sivan et al. demonstrated that B16.SIY melanoma allografts grew at different rates in C57BL/6 mice from Taconic and Jax, and that these differences could be eliminated through co-housing or administration of Bifidobacterium. Bifidobacterium was identified as enhancing tumor immunity.

There is no easy answer when it comes maintaining a consistent microbiome in animal models. The microbiome of animals will vary from vendor to vendor, and even between rooms in the same facility. It may also change over time! Current best practices focus on consistency: when possible, order from the same production location, and use the same animal room for sequential experiments.

Best Practices for Oncology Models

Best practices for husbandry of immunodeficient rodents include housing such animals in a dedicated, separate room. In some facilities, this may not be possible. Further, equipment such as imagers may be shared between immunocompetent animals used in syngeneic tumor studies and immunodeficients used in xenograft tumor studies. This can introduce risk to the immunodeficient animals.

How can these risks be minimized?

The growth of syngeneic tumor models for immuno-oncology research may require re-evaluation of housing, husbandry, and sourcing to support consistent health standards and experimental performance.