Altered immunity of laboratory mice in the natural environment is associated with fungal colonization

Abstract

32The immune systems of free-living mammals such as humans and wild mice display a 33 heightened degree of activation compared with laboratory mice maintained under artificial 34 conditions. Here, we demonstrate that releasing inbred laboratory mice into an outdoor enclosure 35to mimic life in a natural environment alters the state of immunity. In addition to enhancing the 36 differentiation of T cell populations previously associated with pathogen exposure, we found that 37 outdoor release of mice led to an increase in circulating granulocytes. However, rewilded mice 38 were not infected by pathogens previously implicated in immune activation. Rather, changes to 39 the immune system were associated with an altered composition of the microbiota, and fungi 40 isolated from rewilded mice were sufficient to increase circulating granulocytes. These findings 41 establish an experimental procedure to investigate the impact of the natural environment on 42 immune development and identify a role for sustained fungal exposure in determining 43 granulocyte numbers. 44 45 46 48 research and has enabled fundamental advances in basic immunology. Yet, this ubiquitous model 49 fails to recreate certain aspects of human immunity. Inbred laboratory mice and adult humans 50 differ in the proportion of leukocyte subsets, transcriptional responses to microbial challenges, 51 and other immune parameters (Masopust et al., 2017; Tao and Reese, 2017). Such differences 52 may limit the predictive value of experiments with mice when studying complex inflammatory 53 and infectious diseases, resulting in significant shortcomings in translating laboratory 54 observations to humans. 55Recent findings suggest that this shortcoming of the rodent model may be due to the 56 specific pathogen free (SPF) environment in which they are maintained. Wild mice and pet store 57 mice, both of which are exposed to a litany of pathogens that are typically excluded from SPF 58 facilities, display an abundance of differentiated memory T cells that more closely resembles the 59 state of immunity in adult humans (Abolins et al., 2017; Beura et al., 2016; Choi et al., 2019). 60Similarly, transferring embryos from lab mice into wild mice generates commensal-and 61 pathogen-exposed offspring (wildlings) that more faithfully recreate human immunity than 62 standard SPF mice, including the unresponsiveness to immunotherapies that failed in clinical 63 trials (Rosshart et al., 2019). Sequentially infecting SPF mice with 3 viruses and a helminth shifts 64 the gene expression profile of peripheral blood mononuclear cells (PBMCs) towards that of pet 65 store mice and adult humans (Reese et al., 2016), further highlighting the role for pathogen 66 experience in normalizing the immune system. SPF mice are also distinguished from free-living 67 mammals by the lack of exposure to potentially immuno-stimulatory members of the microbiota 68 that are absent in a laboratory animal facility. For example, the offspring of germ-free mice 69 inoculated with ileocecal contents from ...

Genomic, microbial and environmental standardization in animal experimentation limiting immunological discovery

“…One intriguing protocol that creates mice that are colonized with natural/wild microbiota is called "rewilding". Graham and coworkers recently reported that when lab mice are transferred to outdoor enclosures where they are exposed to the weather and microbiome that inhabits the soil and vegetation, they display maturation and differentiation of different T cell subsets, increased numbers of circulating granulocytes and changes in intestinal microbiota that are similar to those described above in feral and pet store mice [95][96][97]. When taken together, it is becoming clear that colonization of mice with diverse populations of naturally occurring microorganisms protects them against an environment that contains potentially lethal infectious microbes, inflammogens and carcinogens.…”

Staphylococcus aureus Host Tropism and Its Implications for Murine Infection Models

“…In the dirty mouse approach, conventional laboratory mice are exposed to microorganisms that usually do not appear in breeding facilities. This exposure can either be very defined by infecting mice directly with known pathogens [ 229 ] or undefined by either co-housing them with “dirty” non-laboratory mice, such as pet-store mice [ 230 ], or exposing them to a natural environment [ 231 , 232 ]. The latter type of mouse is sometimes referred to as “rewilded”.…”

“…Moreover, their guts were also colonized by different fungal communities than those of laboratory mice [ 231 ]. Compared to laboratory mice, dirty mice also present a more mature phenotype of the immune system, as shown for instance by a higher amount of differentiated effector and memory T cells, more serum antibodies and class-switched B cells, a more “adult human being-like” transcription profile, and an increase in mucosally distributed T cells [ 230 , 231 , 232 ].…”

“…Reese et al observed a reduced antibody response after vaccination against Yellow Fever Virus [ 229 ]; Beura et al described an increased resistance to infection with Listeria monocytogenes and Plasmodium berghei ANKA. Furthermore, the differentiation of CD8+ T cells after Lymphocytic choriomeningitis virus infection was shifted from memory precursor to short-lived effector cells when compared to laboratory mice [ 230 ]; Leung et al observed an increased gut nematode susceptibility in rewilded mice, which might be the result of a decreased type 2 and increased type 1 immune response in these mice [ 231 ]; Yeung et al observed an increased cytokine production of mesenteric lymph node cells after ex vivo stimulation with various bacterial stimuli [ 232 ]. These examples clearly demonstrate that the exposure of laboratory mice to environmental microorganisms drastically changes their immunological phenotype, which in turn influences future experiments.…”

“…Looking at S. aureus, our research group has discovered that laboratory mice are frequently colonized with, and therefore pre-exposed to, S. aureus and hence mount a systemic immune response against it [ 30 ]. Most of the rewilded mice in Yeung et al’s study were also positive for seemingly animal-adapted S. aureus , since they were lacking IEC-encoding prophages, but the authors could not exclude that S. aureus was already present in the laboratory mice before [ 232 ]. This has serious implications for S. aureus research, because such animals are no longer naïve towards S. aureus , and an uneven distribution of pre-exposed animals between control and experimental groups may largely distort results.…”