A distinct non-enzymatic role for Indoleamine-2,3-dioxygenase (IDO)2 vs. IDO1 in autoimmune arthritis

Abstract

Abstract IDO1 and IDO2 are closely related immune modulating enzymes encoded by linked genes. Whereas IDO1 is best known for its immunoregulatory role in tumor immune evasion, IDO2 acts as a proinflammatory effector of B cell-mediated autoimmunity. Given their opposing roles in inflammatory responses, interpretation of results obtained using IDO1 or IDO2 single knockout (ko) mice have been complicated by the expression of the other enzyme. Using single and double knockout (dko) mice and the KRN arthritis model, we distinguished differential roles for IDO1 and IDO2 in autoreactive B vs. T cell responses driving arthritis. Both autoreactive T and B cell responses and severity of arthritis were decreased in IDO2, but not IDO1, ko mice. Despite a similar attenuation of disease, only autoreactive B cell responses were reduced in dko mice, demonstrating that IDO2 directly mediates autoreactive B cell responses, while autoreactive T cell responses are indirectly affected by IDO1. Like IDO1, IDO2 was originally identified as a tryptophan catabolizing enzyme. However, we recently showed that the enzymatic activity of IDO2 is not required to drive arthritis. To define the non-enzymatic mechanism mediating IDO2 function, we identified IDO2-interacting proteins using a yeast two-hybrid screen. Specific interaction between IDO2, but not IDO1, and several candidate proteins was confirmed using overexpressing cell lines and co-IP/Western. Using site-directed mutagenesis and an in vitro IDO2-expression model, we identified potential regions outside of the catalytic site on IDO2 that mediate these interactions, providing the first evidence for the non-enzymatic mechanism by which IDO2 mediates proinflammatory B cell responses.