Indoleamine 2,3-dioxygenase expression in patients with acute graft-versus-host disease after allogeneic stem cell transplantation and in pregnant women: association with the induction of allogeneic immune tolerance?

Indoleamine 2,3-dioxygenase (IDO) is an interferon-gamma (IFN-gamma)-induced enzyme, which is suggested to play an important role in the prevention of allogeneic fetal rejection. IDO effects the suppression of T-cell activity by catabolizing the essential amino acid l-tryptophan. We studied IDO expression by reverse transcription polymerase chain reaction (RT-PCR) in dendritic cells and by real-time RT-PCR in monocytes of patients undergoing allogeneic transplantation for leukaemia, who developed acute graft-versus-host disease (aGvHD), and compared the IDO expression with that of pregnant women and healthy volunteers. A spontaneous IDO expression was detected in the monocytes of 20 pregnant women with an IDO/glyceraldehyde-3-phosphate dehydrogenase (GAPDH) ratio at a median of 1.0%, whereas none of 15 healthy volunteers or patients after allogeneic transplant had any detectable spontaneous IDO expression. The IDO expression increased by in vitro IFN-gamma stimulation in pregnant women (median 116%), healthy volunteers (median 11.7%) and patients with a low-grade aGvHD (grades 0-II) 28 days after transplant (median 433%) but not in patients with a severe aGvHD (grades III-IV) (median 0%), which was highly significant (P < 0.01). IDO expression was also measured in dendritic cells by qualitative RT-PCR, where a spontaneous IDO expression was detected in 16 of 31 (52%) pregnant women versus none of 17 healthy volunteers and none of 62 studied patients after transplant. IFN-gamma-induced IDO expression was detected in all pregnant women, all volunteers and 47 of 49 (96%) patients with a low-grade aGvHD (grades 0-II) after transplant, whereas only in two of 13 (16%) patients with aGvHD grade III-IV was IFN-gamma-induced IDO expression observed. These data suggest that IDO expression might be involved in the development of allogeneic immune tolerance.