Bone marrow and splenic immune composition is unperturbed by short-term exposure to bedding from pet store mice

Mice with natural microbial exposures (NMEs) have been proposed to be superior to specific pathogen-free (SPF) laboratory mice for modeling adult human immune and inflammatory profiles. Research using NME mice has focused on the effects of sustained exposure on peripheral blood and lymphoid and nonlymphoid organs. The effect of short-term NME is not known, in particular the impact to bone marrow immune composition. Male and female C57Bl/6J mice were exposed to SPF conditions for 6 wk (age 4 to 10 wk) with or without exposure to dirty bedding from pet store mice for the first week. At sacrifice, substantial shifts in the fecal bacteriome, at the phylum, order, and genus levels were evident in male and female NME mice, confirming induction of a new exposome with NME. However, only female NME mice had pathogens in their feces detectable by polymerase chain reaction (2 of 35). Numerous sex-dependent differences in T cell and myeloid composition in bone marrow and spleens were observed. We report several novel immune composition findings by sex, including higher proportions of Ki67+ regulatory and CD8+ T cells in female bone marrow. We detected 2 differences resulting from NME in the spleens of females, including increased PD-1 in CD44+CD8+ T cells and higher mean Foxp3 expression in regulatory T cells. We conclude that short-term exposure to pet store bedding is insufficient to induce robust changes in immune composition in spleens or bone marrow. Future research with the NME model should focus on sustained exposure and investigate exposure timing.