Microbiota-Independent Ameliorative Effects of Antibiotics on Spontaneous Th2-Associated Pathology of the Small Intestine

We have previously generated a mouse model of spontaneous Th2-associated disease of the small intestine called TRAF6 Delta DC, in which dendritic cell (DC)-intrinsic expression of the signaling mediator TRAF6 is ablated. Interestingly, broad-spectrum antibiotic treatment ameliorates TRAF6 Delta DC disease, implying a role for commensal microbiota in disease development. However, the relationship between the drug effects and commensal microbiota status remains to be formally demonstrated. To directly assess this relationship, we have now generated TRAF6 Delta DC bone marrow chimera mice under germ-free (GF) conditions lacking commensal microbiota, and found, unexpectedly, that Th2-associated disease is actually exacerbated in GF TRAF6 Delta DC mice compared to specific pathogen-free (SPF) TRAF6 Delta DC mice. At the same time, broad-spectrum antibiotic treatment of GF TRAF6 Delta DC mice has an ameliorative effect similar to that observed in antibiotics-treated SPF TRAF6 Delta DC mice, implying a commensal microbiota-independent effect of broad-spectrum antibiotic treatment. We further found that treatment of GF TRAF6 Delta DC mice with broadspectrum antibiotics increases Foxp3(+) Treg populations in lymphoid organs and the small intestine, pointing to a possible mechanism by which treatment may directly exert an immunomodulatory effect. To investigate links between the exacerbated phenotype of the small intestines of GF TRAF6 Delta DC mice and local microbiota, we performed microbiotic profiling of the luminal contents specifically within the small intestines of diseased TRAF6 Delta DC mice, and, when compared to co-housed control mice, found significantly increased total bacterial content characterized by specific increases in Firmicutes Lactobacillus species. These data suggest a protective effect of Firmicutes Lactobacillus against the spontaneous Th2-related inflammation of the small intestine of the TRAF6 Delta DC model, and may represent a potential mechanism for related disease phenotypes.