Laboratory mice are a mainstay of biomedical research and have been instrumental for many important discoveries in the field of immunology. However, there are also major limitations, including conflicting results rooted in divergent microbiota among research facilities and the limited ability to predict the complex immune responses of humans. Recent studies have shown that conventional laboratory mice are too far removed from natural environmental conditions to faithfully mirror the physiology of free-living mammals such as humans. Mammals and their immune systems evolved to survive and thrive in a microbial world and behave differently in a sanitized environment.

To generate a mouse model that more closely resembles the natural mammalian metaorganism with coevolved microbes and pathogens, we transferred C57BL/6 embryos into wild mice. This resulted in a colony of C57BL/6 mice, which we call “wildlings.”

Wildlings resembled wild mice and differed substantially from conventional laboratory mice with regard to their bacterial microbiome at important epithelial barrier sites (gut, skin, and vagina), their gut mycobiome and virome, and their level of pathogen exposure. The natural microbiota of wildlings were stable over multiple generations and resilient against antibiotic, dietary, and microbial challenges.

Next, we delineated the immune landscape of wildlings, wild mice, and laboratory mice at immunologically important barrier sites (gut, skin, and vagina), a central nonlymphoid organ (liver), and a central lymphoid organ (spleen) by mass cytometry. Additionally, we characterized the blood immune cell profile by RNA sequencing. The differential contribution of microbial and host genomes in shaping the immune phenotype varied among tissues. Wildlings closely mirrored the wild mouse immune phenotype in the spleen and blood.

Finally, we tested the translational research value of wildlings in a retrospective bench-to-bedside approach. This required well-documented, rodent-based studies that had failed upon transitioning to clinical trials in humans. We chose the CD28-superagonist (CD28SA) trial as representative for treatments targeting adaptive immune responses. Although CD28SA expanded anti-inflammatory regulatory T cells (T_regs) in laboratory mice and showed therapeutic effects in multiple models of autoimmune and inflammatory diseases, the first phase 1 clinical trial resulted in life-threatening activation of inflammatory T cells and cytokine storms. Similarly, the CD28SA treatment of wildlings, but not laboratory mice, resulted in an inflammatory cytokine response and lack of T_reg expansion. As a representative for trials targeting innate immune responses, we chose anti–tumor necrosis factor–alpha (TNF-α) treatment (anti-TNF-α or TNF-receptor:Fc fusion protein) during septic shock, which was successful in animal models, but failed in humans. Anti–TNF-α treatment during lethal endotoxemia rescued laboratory mice, but not wildlings. Thus, wildlings better phenocopied human immune responses than did conventional laboratory mice in the two models studied.

The wildling model combines resilient natural microbiota and pathogens at all body sites and the tractable genetics of C57BL/6. Given the wide-ranging effects of microbiota on host physiology, natural microbiota-based models may benefit different research fields (e.g., metabolism and neurodegenerative diseases) and may also be applicable to other animals. Such models may enhance the validity and reproducibility of biomedical studies among research institutes, facilitate the discovery of disease …