Commensal Escherichia coli Induces Systemic Antigen-specific Tolerance

Objective: While gut commensal bacteria are locally tolerated, it remains unknown whether they can induce systemic antigen-specific tolerance as it has been described for nutritional antigens. Here we analyse whether commensal-derived antigens mediate systemic immunological tolerance and whether localisation of the antigen to different bacterial compartments influences this process.

Methods: A 85aa ovalbumin (Ova) fragment, complete Ova or a control protein were expressed in different compartments of E. coli K12 (EC-Ova^OM, 85aa membrane-bound; EC-Ova^SECR, 85aa secretable; EC-Ova^CYTO1, 85aa cytoplasmatic; EC-Ova ^CYTO2, complete cytoplasmatic; EC-contr, control protein). Western blot analysis confirmed comparable expression levels. Mice were colonised for 20 days with the strains, sensitised with Ova+Alum and exposed to inhalative Ova-challenges.

Results: Mice colonised with EC-Ova^OM and EC-Ova^SECR displayed reduced eosinophil counts in bronchoalveolar lavages, decreased pulmonary mRNA expression of Th2-type cytokines and reduced inflammatory tissue damage compared to mice colonised with EC-contr. Colonisation with EC-Ova^CYTO1 and EC-Ova^CYTO2 did not efficiently mediate tolerance. Surprisingly, EC-Ova^OM and EC-Ova^SECR, but not EC-Ova^CYTO1/2 strongly induced proliferation and cytokine production of Ova-specific TCR-transgenic OTII CD4 T cells co-cultured in vitro with antigen-presenting cells.

Conclusion: A model antigen introduced into commensal E. coli triggers systemic oral tolerance upon intestinal colonisation. However, tolerance was only induced if the expressed antigen was present at the outer membrane or in a secretable form, but not in the cytoplasm, which correlated with the capacity to present the antigen to T cells. The unexpected differences in presentation of bacterial antigens located in distinct bacterial compartments require further investigation