Bacteria Associated with Immunoregulatory Cells in Mice

This study examined bacteria-immune interactions in a mouse model possessing microbiota-dependent immune regulatory features similar to those occurring in human atopy, colitis, and immune regulation. Associations between the abundance of several bacterial phylotypes and immunoregulatory target cell types were identified, suggesting that they may play a role in these phenotypes.Bacteria are involved in critical aspects of immune system development and regulation (5, 23, 26, 29). Mice raised under germfree conditions exhibit a variety of abnormalities, including hypoplastic Peyer''s patches, reduced numbers of IgA-producing cells, relatively unstructured spleen and lymph nodes, and hypogammaglobulinemic serum (23). Remarkably, after several weeks of exposure to standard intestinal microbiota, normal immune structure and function are restored. Mechanistic details underlying microbe-immune interactions have been recently elucidated for two common intestinal bacteria. Bacteroides thetaiotaomicron was shown to induce the angiogenin Ang4, a component of innate immunity possessing microbicidal activity against a wide range of intestinal microbes, including both bacterial and fungal pathogens (16). In addition, studies of the Bacteroides fragilis zwitterionic capsular polysaccharide A have established it as a cognate antigen of certain CD4⁺ T cells, which programs immune effector polarization (24) and protection of mice from infection by Helicobacter hepaticus through several immune-mediated mechanisms (25). Resident microbiota also modify the interaction of dendritic cells with regulatory T-cell populations, with resultant susceptibility to chronic inflammatory disease, like colitis (15, 28).Recent work by Braun and colleagues has characterized a mouse model with unique immunologic features linking resident microbiota with levels of regulatory CD8⁺ T cells (13, 17, 39). This model is comprised of two physically isolated colonies of isogenic mice harboring distinct microbial communities: conventional floras (CF) and restricted floras (RF). CF refers to C57BL/6 mice housed in a standard specific-pathogen-free facility, while RF refers to C57BL/6 mice containing a different complement of intestinal microorganisms (13, 30), originally created by transferring several nonpathogenic anaerobic bacteria into antibiotic-treated mice (13). RF mice differ from CF mice in several immunologic phenotypes, including selective reduction of marginal zone (MZ) B cells (39), plasmacytoid dendritic cells (pDC) (13), and invariant natural killer (iNK) T cells (38a), as well as naïve CD4⁺ and CD8⁺ T cells (17). In addition, RF mice were shown to be resistant to colitis under genetic or adoptive transfer conditions that permit disease activity in CF mice (2). RF mice also cleared experimental infections by Campylobacter jejuni more slowly than did their CF counterparts (6). The resulting concept is that certain resident microbiota, which may be more abundant in RF mice than in CF mice, induce invariant Qa-1 T cells, with resultant changes in host immunoregulation and microbial surveillance (2).An important issue raised by the foregoing observations is the identity of resident microbiota responsible for this host immunoregulatory response. The objective of this study was to develop a methodology, based on bacteria-immune interactions in the RF/CF mouse model, to identify candidate microbiota. In this study, we employed a series of experiments examining associations between the population densities of bacterial rRNA genes and several immunologic features that differ between CF and RF mice.