Fig 1: Indoleamine 2,3-dioxygenase 1 (IDO) expression in the affected lymph nodes of patients with Hodgkin lymphoma (HL). (A) Immunostaining for IDO in the affected lymph node lesions from six individual HL patients. IDO expression can be seen in macrophages/dendritic cells, but not in HL tumor cells. Scale bar = 50 µm. (B) Patients’ serum kynurenine (Kyn)/tryptophan (Trp) ratios are plotted on the x-axis and IDO expression levels determined by immunohistochemistry (IDO-positive cells/0.25 mm2) on the y-axis. Spearman's rank correlation coefficient (Rs) between serum Kyn/Trp ratios and IDO expression levels; P-values are indicated in the panel. The dot plots labeled 1-6 in the panel correspond to photographs 1-6 in (A). (C) Cells were stimulated by ?-interferon and stained with anti-IDO mAb (blank histograms) or isotype control mAb (filled histogram)