Oral tolerance: mechanisms and therapy of autoimmune diseases

Oral tolerance is a state of immune hyporesponsiveness induced by the oral or mucosal exposure to antigens. This state is dependent on the dose of the oral antigen administered, with a low dose stimulating regulatory T cell development leading to an active immune suppression that is transferable via T cells. The active mechanism appears to be a cytokine mediated immune deviation with a predominant Th2 and Th3 response (TGF-beta). In contrast, high dose oral antigens lead to clonal deletion and anergy. The active suppression of low dose oral tolerance can also suppress an unrelated immune response (bystander suppression) paving the way for therapy of autoimmune diseases like rheumatoid arthritis. Oral tolerance has been effective in the treatment of autoimmune diseases like experimental autoimmune encephalomyelitis (EAE), collagen-induced arthritis (CIA) and insulin-dependent diabetes in animals. However, recent studies in human autoimmune diseases have not been as effective but the results are encouraging and more work is required to understand the mechanisms involved and other factors that may modulate the response.