Indoleamine 2,3‐Dioxygenase Expression in Monocytes of Healthy Nonpregnant Women After Induction with Human Choriongonadotropine

Dear Sir The interferon-g (IFN-g)-induced enzyme indoleamine 2,3-dioxygenase (IDO) is discussed to play an important role in the induction of allogeneic immune tolerance in pregnant women. IDO is expressed in human placenta and in certain macrophages that affects the suppression of T-cell activity by catabolizing the essential amino acid L-tryptophan [1]. Indeed, during pregnancy, the systemic plasma tryptophan concentration falls significantly [2]. Moreover, T-cell proliferation could be inhibited by macrophage tryptophan catabolism as demonstrated by Munn et al. [3] But it is still not clear how allogeneic T-cell tolerance in pregnancy may be achieved by this mechanism. In a previous study, we have shown a spontaneous IDO expression in monocytes of healthy pregnant women by real-time reverse transcriptase-polymerase chain reaction (RT-PCR) in contrast to nonpregnant healthy volunteers [4]. Up to date, it is not clear why pregnant women have this spontaneous IDO expression. An increased expression of IFN-g does not seem to induce IDO, because IFN-g is one of the cytokines that are significantly decreased in the decidua of healthy pregnant women [5]. Here we analysed the most upregulated hormones in pregnancy for their ability to induce IDO. The IDO expression induced by the hormones human choriongonadotropine (b-HCG), oestrogen and progesterone in monocytes derived of the peripheral blood of six healthy nonpregnant women was assessed by a quantitative real-time RT-PCR and compared to the spontaneous and IFN-g-induced IDO expression. The median age of the examined healthy nonpregnant women was at 35 years (range 24–43 years), two of the six women had previous pregnancies. The monocytes isolated from whole blood were incubated for 24 h (37 C, 5% CO2) with 1500 IU b-HCG, 1.4-mg oestrogen and 20-mg progesterone. Real-time RT-PCR was performed using the Lightcycler (Roche Diagnostics, Mannheim, Germany) with the forward primer 50-ACAGACCACAAGTCACAGCG and the reverse primer 50-AACTGAGCAGCATGTCCTCC and the hybridization probes GCAGTGCAGGCCAAAGCAGCGTCTTTCAGTGCTTT-X and LC Red640ACGTCCTTGCTGGGCATCCA. The expression of IDO transcript was determined and normalized using the glyceraldehyde-3-phosphate dehydrogenase (GAPDH)housekeeping gene as an endogenous reference with the primers 50-TTCACCACCATGGAGAAGGCT and 50-ATGGCATGGACTGTGGTCATG and the probes ATCATCAGCAATGCCTCCTGCACCACCAACTGCTX and LC Red640-AGCACCCCTGGCCAAGGTCATCCAT-ph. All analyses were performed in duplicate. Here we describe for the first time that IDO was expressed in the monocytes of all the six nonpregnant healthy volunteers after the induction of the above hormones. Interestingly, the b-HCG-induced IDO expression was the highest with 117% at median (range 27.9–609.7%), which was quit comparable to the IFN-g-induced IDO expression with a median at 109.7% (range 35.7–301.4%). Further, we also found an IDO expression after stimulation with oestrogen and progesterone, which was lower as the IFN-gand b-HCG-induced IDO expression. The median of the oestrogen-induced IDO expression was at 18.4% (range 5.5–228.7%) and the median of the progesterone-induced IDO expression was at 29.4% (range 2.9–101.4%). Like in our previous study, none of the six healthy nonpregnant volunteers had a detectable spontaneous IDO expression. The data presented here underline our previous data that the induction of allogeneic immune tolerance in pregnant women could be caused by a higher IDO expression during pregnancy. Up to date, it was not clear how IDO was expressed in pregnancy. However, in our earlier report, we had demonstrated an IFN-g-independent way of IDO expression in pregnancy. Now, we found that the upregulated hormones during pregnancy could cause this IDO expression. Especially, we showed that the b-HCGinduced IDO expression was comparable to the IFN-ginduced IDO expression. These data suggest that the induction of allogeneic immune tolerance in pregnant women might be caused by the upregulation of IDO by hormones like b-HCG, oestrogen and progesterone during pregnancy. Further studies that evaluate the effect of IDO expression on the induction of immune tolerance are strongly recommended. N. K. Steckel M. Koldehoff D. W. Beelen A. H. Elmaagacli Department of Bone Marrow Transplantation University Hospital of Essen Essen, Germany E-mail: nina.steckel@uni-essen.de LETTER TO THE EDITOR ..................................................................................................................................................................................................