In the world of biomedical research, the humble lab mouse is a superstar—it’s easy enough to rear in large quantities, shares a considerable amount of core biological processes with humans, and is experimentally tractable to an almost ridiculous degree. When it comes to using mice as a faithful model system of human biology, however, there are limits on how far the generalizations can stretch. Some of these limits are fairly obvious—for example, mice have tails, and can birth litters of up to 14 pups at a time—but there is one major biological factor which might significantly contribute to discrepancies between findings from mice and humans: genetic diversity. You see, scientists don’t use just any mice; the vast majority of research has been done using just a handful of mouse strains—populations of animals with a fixed genetic makeup which are heavily inbred to maintain this genetic homogeneity. Inbred mouse strains are essential for scientists seeking to perturb a single biological variable and understand the effects in controlled experimental environments. But clinicians seeking to translate findings obtained in mice to human patients might be hampered by this very same feature—humans are, after all, a genetically diverse bunch, and this genetic diversity can exert biologically meaningful effects that are wholly missed when studying inbred, genetically homogenous mice.
Members of the Lund lab in the Vaccine and Infectious Disease Division at Fred Hutch think a lot about pathogens and immunity, an arena where genetic diversity plays a key yet incompletely understood role. We know this to be true—you and a friend can get infected with the same variant of a pathogen (like SARS-CoV-2) but show wildly different responses (you get a fever for a day, while your friend loses their sense of taste for two weeks). But to what extent does genetic diversity influence the attributes of our immune systems, and is this variation alone enough to explain functional differences in response to infection? These are the questions that Dr. Jessica Graham, a staff scientist in the Lund lab, and colleagues addressed in a recent study published in iScience.
To model genetic diversity in a laboratory setting but retain some level of control over the system, Graham and colleagues utilized a resource called the Collaborative Cross (CC). In essence, the CC is a collection of mouse strains which were obtained by systematically breeding eight inbred lab mouse strains with each other to generate hundreds of genetically diverse (but still reasonably well-defined) populations—think of it as a sweet spot between using a single genetically-homogenous inbred mouse line and plucking a wild mouse from a field in rural Minnesota. To start, Graham and colleagues picked 63 of these ‘recombinant inbred’ mouse lines, harvested their spleens, and used multiparameter flow cytometry to profile the composition of their B cell, T cell, and innate immune cell populations. Where one might have predicted a relatively homogenous set of immune compositions (after all, many of these strains are virtually indistinguishable by eye), the team documented an astounding degree of diversity. CD8 T cells—the ‘killer’ T cells responsible for fighting intracellular pathogens and tumors—composed 14% of the lymphocyte population in some CC strains, while only comprising 2% of lymphocytes in other strains. The frequency of B cells (those cells responsible for producing antibodies to fight off infection) similarly varied from around 40% of lymphocytes in some strains to nearly 80% in others. Indeed, everywhere the team looked—dendritic cells, natural killer (NK) cells, and even different functional subclasses within the immune cell types—showed as high as 10-fold variation in abundance and frequency among different CC strains.
So, it appears that even the ‘partial’ genetic diversity of CC mouse strains is enough to produce wildly different immune compositions—but does this immunological diversity actually matter when it comes to mounting an immune response? To test this, Graham and colleagues challenged two subsets of their CC strains with two pathogens—a mouse-adapted strain of SARS-CoV-2 and human Herpes Simplex 2 (HSV-2) virus. Strikingly, this revealed a diverse range of responses to these viruses among CC strains which mirrored their diverse immune compositions. Among those strains receiving SARS-CoV-2, some cleared the virus from their lungs by 7 days post-infection, while others showed persistent viral loads; cross referencing these results with their in-depth immune composition data, the team found statistically-significant differences in certain subsets of B, T, and NK cells in strains that cleared the virus quickly, suggesting a role for these specific subpopulations in SARS-CoV-2 clearance and nominating them for further study. In the case of HSV-2, the differences were even more apparent: while vaginal HSV-2 infection was lethal in many of the CC strains, some strains showed survival rates as high as 60%! Again, the team was able to use their flow cytometry datasets to find specific immune signatures which correlated to survival following HSV-2 challenge.
“While it took us over ten years to source these strains, design the experiments, and collect all of the data, we’re thrilled at the quantity and quality of data that this effort produced, and we envision this study is a resource to those looking to select mouse strains best suited to their specific research questions,” notes Dr. Graham. “We also want to emphasize that some of these CC strains allow researchers to address questions that they previously couldn’t—for example, wild-type HSV-2 infection is generally lethal to C57BL/6J mice (these are one of the most common inbred mouse strains used by researchers), so CC strains which survive HSV-2 infection could be important tools to study immunological features which are important for response against HSV-2.” Overall, the team’s findings validate the Collaborative Cross as an important resource for immunology research and take an important step towards ‘bridging the gap’ between mouse models and human populations.
The spotlighted work was funded by the National Institutes of Health.
Graham, J. B., Swarts, J. L., Leist, S. R., Schäfer, A., Bell, T. A., Hock, P., Farrington, J., Shaw, G. D., Ferris, M. T., Pardo-Manuel De Villena, F., Baric, R. S., & Lund, J. M. 2024. Unique immune profiles in collaborative cross mice linked to survival and viral clearance upon infection. iScience. 27(3), 109103.
Science Spotlight writer David Sokolov is a graduate student in the Sullivan Lab at Fred Hutch. He studies how cancer cells modify their metabolism to facilitate rapid proliferation and tolerate tumor-driving mitochondrial defects. He's originally from the east coast and has bachelors' and masters' degrees from West Virginia University. Outside of the lab, you'll find him enjoying the outdoors, playing music, or raising composting worms in his front yard.