Influence of Intestinal Microflora on Murine Bone Marrow and Spleen Macrophage Precursors

To investigate the adjuvant effect of intestinal flora on macrophage–colony‐stimulating factor‐responsive macrophage progenitors from spleen and bone marrow, we compared progenitor numbers and phenotypic characteristics of in vitro matured macrophages in germ‐free and flora‐associated mice (conventional, Escherichia coli‐monoassociated and conventionalized mice). The data obtained show that the flora affected differentially bone marrow and spleen progenitors. It increased the numbers of progenitors in the spleen but not in the bone marrow. It did not modify the expression of F4/80, Mac‐1 and major histocompatibility complex (MHC) class II on bone‐marrow‐derived macrophages (BMDM), while it clearly up‐regulated MHC class II expression on spleen‐derived macrophages (SDM). This effect was more pronounced in flora‐associated ex germ‐free mice than in conventional mice and it was greatly enhanced in the absence of M‐CSF. In vitro stimulation by lipopolysaccharide had no effect on marker expression of BMDM, while it decreased F4/80 and enhanced MHC class II molecules on SDM from germ‐free and flora‐associated mice. However, the expression of MHC class II remained lower in germ‐free mice. Enhancement of MHC class II molecule expression on SDM may contribute to the protective role of flora, because successful immune responses are dependent on the expression of these molecules.