In this respect, activation of the KYN pathway has been reported in patients with autoimmune diseases, such as rheumatoid arthritis, systemic lupus erythematosus and systemic sclerosis, likely as a result of IDO1 expression by non-hematopoietic cell types, such as fibroblasts, epithelial cells and vascular endothelial cells [47,48]. Here, IDO1 is linked to systemic sclerosis.