Changes in the function and phenotype of resident peritoneal macrophages after housing in an enriched environment

Exposure to an enriched environment (EE) affects not only brain functions but also immune responses upon viral or bacterial infections. In this study, we examined changes in the phagocytic response and chemokine production of resident peritoneal macrophages after mice had been housed under EE conditions for 6 or 8 weeks, and then explored the possibility that EE could cause a change in the macrophage phenotype by means of flow cytometry as well as quantitative RT-PCR. The percentages of EE macrophages phagocytosing S. aureus and apoptotic neutrophils were significantly larger than those of standard environment (SE) macrophages. After coculturing with S. aureus, EE macrophages tended to produce greater amounts of chemokines such as MIP-2, KC and MCP-1 than SE ones, although the increases for MIP-2 and KC were not statistically significant. As compared with SE macrophages, EE macrophages included more CD40-positive cells (M1 marker), and expressed more mRNAs of IL-6 (M1 marker) and IRF4 (M2 marker), and less mRNA of CD38 (M1 marker), suggesting either the possibility that EE macrophages are a mixed population of M1 and M2 macrophages or the possibility that they are a unique population with a mixed M1 and M2 macrophage phenotype.