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Background: The resident flora plays a critical role in initiation and perpetuation of intestinal inflammation, as demonstrated in experimental models of colitis where animals fail to develop disease under germ free conditions. However, the importance of exposure to commensal bacteria before the onset of colitis is unclear. Our aim was to investigate the influence of previous exposure of donor animals to bacterial antigens on colitis development using a transfer model. Methods: Clinical course and histology were evaluated after transfer of CD4+CD62L+ lymphocytes from germ free and conventionally housed donor mice into SCID recipients. Cotransfer of CD4+CD62L+ cells with CD4+CD62L- lymphocytes from both groups of mice was initiated. Lymphocytes were analysed by FACS, polarisation potential of cells determined, and cytokines measured within the supernatant by enzyme linked immunosorbent assay. Results: Animals that received cells from germ free donors developed an earlier onset of colitis compared with mice reconstituted with lymphocytes from conventionally housed animals. Additionally, CD4+CD62L- cells from germ free mice were not able to abrogate colitis induced by cotransfer with CD4+CD62L+ lymphocytes whereas CD4+CD62L- T cells from normal mice ameliorated disease. The higher percentage of CD4+GITR+ expressing lymphocytes and the production of interleukin 10 after priming by dendritic cells suggests the presence of Treg cells within the CD4+CD62L+ lymphocyte subset derived from conventional housed mice and assumes a lack of Treg cells within germ free mice. Conclusion: The results indicate that bacterial antigens are crucial for the generation and/or expansion of Treg cells in a healthy individual. Therefore, bacterial colonisation is of great importance in maintaining the immunological balance. [ABSTRACT FROM AUTHOR]

Background and methods: Cytosin-guanosin dinucleotide (CpG) motifs of bacterial DNA are known to be potent activators of innate immunity. We have shown previously that administration of CpG containing oligodeoxynucleotide (CpG-ODN) to mice before the onset of dextran sodium sulphate induced colitis ameliorated colitis and inhibited induction of proinflammatory cytokines. To investigate the possible involvement of CD4+ T cells in the prophylactic CpG-ODN effects, we used the SCID transfer model of colitis. Results: CD4+D62L7+ T cells from CpG-ODN treated donors did not induce significant intestinal inflammation in SCID recipients, in contrast with control cells. Additionally, cotransfer of these cells with CD4+CD62L+ cells from normal mice protected recipient animals from colitis, indicating regulatory activity. Also, CD4+CD62L+ cells from toll-like receptor 9 deficient animals induced a significantly more severe colitis in SCID recipients than cells from wild-type littermate controls, suggesting a similar protective role of "endogenous" bacterial DNA leading to a less "aggressive" phenotype of these cells. There was no detectable difference in regulatory T cell surface markers between aggressive and attenuated cell pools but attenuated cell pools showed reduced proliferation in vitro and in vivo and produced less interferon y, interleukin (IL)-5, and IL-6 after anti-CD3 stimulation. Conclusions: Collectively, our data support the concept that both endogenous bacterial DNA and exogenously supplied CpG motifs of bacterial DNA induce regulatory properties in CD4+ T cells. Therefore, bacterial DNA derived from the normal gut flora may contribute essentially to the homeostasis between effector and regulatory immune mechanisms in healthy individuals to protect them from chronic intestinal inflammation. [ABSTRACT FROM AUTHOR]