IDO attenuates AI progression in several mouse models of type I diabetes, multiple sclerosis (MS), rheumatoid arthritis (RA), systemic lupus erythematosus, and graft-versus-host disease since IDO1 gene ablation potentiated disease onset and severity mediated by T cells, while enhancing IDO1 gene expression attenuated disease progression and severity (2). Here, IDO1 is linked to graft versus host disease.