Microbiota-dependent proteolysis of gluten subverts diet-mediated protection against type 1 diabetes.

Diet and commensals can affect the development of autoimmune diseases like type 1 diabetes (T1D). However, whether dietary interventions are microbe-mediated was unclear. We found that a diet based on hydrolyzed casein (HC) as a protein source protects non-obese diabetic (NOD) mice in conventional and germ-free (GF) conditions via improvement in the physiology of insulin-producing cells to reduce autoimmune activation. The addition of gluten (a cereal protein complex associated with celiac disease) facilitates autoimmunity dependent on microbial proteolysis of gluten: T1D develops in GF animals monocolonized with Enterococcus faecalis harboring secreted gluten-digesting proteases but not in mice colonized with protease deficient bacteria. Gluten digestion by E. faecalis generates T cell-activating peptides and promotes innate immunity by enhancing macrophage reactivity to lipopolysaccharide (LPS). Gnotobiotic NOD Toll4-negative mice monocolonized with E. faecalis on an HC + gluten diet are resistant to T1D. These findings provide insights into strategies to develop dietary interventions to help protect humans against autoimmunity.

Cutting Edge: Commensal Microbiota Has Disparate Effects on Manifestations of Polyglandular Autoimmune Inflammation.

Polyglandular autoimmune inflammation accompanies type 1 diabetes (T1D) in NOD mice, affecting organs like thyroid and salivary glands. Although commensals are not required for T1D progression, germ-free (GF) mice had a very low degree of sialitis, which was restored by colonization with select microbial lineages. Moreover, unlike T1D, which is blocked in mice lacking MyD88 signaling adaptor under conventional, but not GF, housing conditions, sialitis did not develop in MyD88(-/-) GF mice. Thus, microbes and MyD88-dependent signaling are critical for sialitis development. The severity of sialitis did not correlate with the degree of insulitis in the same animal and was less sensitive to a T1D-reducing diet, but it was similar to T1D with regard to microbiota-dependent sexual dimorphism. The unexpected distinction in requirements for the microbiota for different autoimmune pathologies within the same organism is crucial for understanding the nature of microbial involvement in complex autoimmune disorders, including human autoimmune polyglandular syndromes.

Microbiota and autoimmunity: exploring new avenues.

Given the recognized role of the commensal microbiota in regulating host immunity to pathogens, it is not surprising that microbiota are also capable of regulating autoimmune responses. The underlying mechanisms of autoimmune regulation by the microbiota are just beginning to emerge. Here, we discuss possible pressure points toward the development of autoimmune diseases that can be influenced by the microbiota. Besides acting on the adaptive and innate arms of the immune response, the microbiota can affect the targets of autoimmunity directly, even during development in utero, and be involved in regulation of autoimmunity via interactions with hormones.