Summary Pregnancy is a natural model of successful tolerance induction against allogeneic tissues. Recent studies pointed to a role of indoleamine 2,3‐dioxygenase (IDO), a tryptophan‐degrading enzyme expressed in the placenta, in mediation of T‐cell suppression. We want to apply to organ transplantation what nature has developed for suppression of fetal rejection during pregnancy. Here we analyze whether IDO‐induced tryptophan metabolites are able to suppress the allogeneic T‐cell response and allograft rejection in rats. Rat lymphocytes were stimulated with allogeneic dendritic cells in vitro in the presence of increasing amounts of tryptophan metabolites (kynurenine, 3‐hydroxykynurenine, anthranilic acid, 3‐hydroxyanthranilic acid and quinolinic acid) and T‐cell proliferation was determined. The findings showed that kynurenine, 3‐hydroxykynurenine and 3‐hydroxyanthranilic acid strongly suppress the T‐cell response, whereas anthranilic and quinolinic acid are noneffective. Vital staining of cells with subsequent fluorescence‐activated cell sorter analyses demonstrated that suppression is mediated by T‐cell death. Thereafter, the action of metabolites was analyzed in a skin allograft model (BN→LEW). Lewis recipients received daily s.c. injections of tryptophan metabolite mixture (kynurenine + 3‐hydroxyanthranilic acid), cyclosporin A (positive control), or no treatment (negative control). The metabolites induced a significant prolongation (P = 0.0018) of graft survival. We conclude that IDO‐induced tryptophan metabolites suppress the T‐cell response and prolong allograft survival in rats.

Microsurgery is a precise surgical skill that requires an extensive training period and the supervision of expert instructors. The classical training schemes in microsurgery have started with multiday experimental courses on the rat model. These courses have offered a low threat supervised high fidelity laboratory setting in which students can steadily and rapidly progress. This simulated environment allows students to make and recognise mistakes in microsurgery techniques and thus shifts any related risks of the early training period from the operating room to the lab. To achieve a high level of skill acquisition before beginning clinical practice, students are trained on a comprehensive set of exercises the rat model can uniquely provide, with progressive complexity as competency improves. This paper presents the utility of the classical rat model in three of the earliest microsurgery training centres and the new prospects that this versatile and expansive training model offers.

Although dendritic cells (DCs) strongly stimulate the immune response, they can also induce unresponsiveness. Recently, a human monocyte-derived DC subpopulation was described that constitutively expresses indoleamine 2,3-dioxygenase (IDO). These DCs were defined as nonadherent CD123 ؉ /CC chemokine receptor 6 ؉ (CCR6 ؉ ) cells that suppress the allogeneic T-cell response. In the present study, we generated nonadherent, mature DCs from human blood monocytes. As expected, in addition to the classic markers, these cells expressed CD123 and CCR6. IntroductionSince 1998, when a series of brilliant mouse experiments 1 showed that a placental enzyme called indoleamine 2,3-dioxygenase (IDO) was able to prevent rejection of the fetus during pregnancy, the scientific community has been intrigued by a novel, basic immunoregulatory mechanism whose main player is IDO. Munn et al 1 presented a convincing experiment: they implanted time-release capsules containing the IDO inhibitor 1-methyl-tryptophan (1-MT) into pregnant mice bred to genetically different fathers. This treatment induced fetal rejection. The experimental design relied on the observation that, under certain circumstances, macrophages inhibit the T-cell response, apparently because they produce IDO, 2,3 an enzyme that is also manufactured in the placenta by the fetus-derived syncytiotrophoblast. 4 Based on their experimental findings, Munn et al forwarded the hypothesis that once the embryo implants and begins establishing connections with the mother's blood supply, fetal-derived cells located in the placenta begin making IDO. By destroying tryptophan-so went the speculation-IDO suppresses maternal T cells that otherwise would make their way through the placenta and attack the fetus.Subsequent studies addressed the mechanism by which tryptophan degradation affects the T-cell response and came to the conclusion that certain metabolites have a strong T-cell inhibitory action. 5,6 Among the tryptophan metabolites, 3-OH-kynurenine and 3-OH-anthranilic acid were shown to be strongly inhibitory, whereas kynurenine had a significant but weaker effect. 5 The mechanism that is able to efficiently regulate the immune reaction during pregnancy, a phenomenon of outstanding importance for the perpetuation of species, can be expected to be "used" by nature for controlling other unwanted immune reactions. A series of interesting studies emerged, shedding light on the mechanism of IDO up-regulation and its hypothetical role in immunoregulation. Most notable is the study of Grohmann et al 7 showing that cytotoxic T-lymphocyte antigen-4-immunoglobulin (CTLA4-Ig) up-regulates IDO in murine dendritic cells (DCs) by ligation to B7 molecules via induction of interferon-␥ (IFN-␥) synthesis. DCs from CTLA4-Ig-treated mice showed an increased rate of IDO production, suggesting that this mechanism also works in vivo. If murine IDO-producing DCs inhibit T-cell responses, as shown by some studies, 8,9 administration of the IDO inhibitor 1-MT would be expected to reverse CTLA4-Ig-induced ...

Our previous studies in rats showed that incubation of monocytic dendritic cells (DCs) with the chemotherapeutic drug mitomycin C (MMC) renders the cells immunosuppressive. Donor-derived MMC-DCs injected into the recipient prior to transplantation prolonged heart allograft survival. Although the generation of DCs is labour-intensive and time-consuming, peripheral blood mononuclear cells (PBMCs) can be easily harvested. In the present study, we analyse under which conditions DCs can be replaced by PBMCs and examine their mode of action. When injected into rats, MMC-incubated donor PBMCs (MICs) strongly prolonged heart allograft survival. Removal of monocytes from PBMCs completely abrogated their suppressive effect, indicating that monocytes are the active cell population. Suppression of rejection was donor-specific. The injected MICs migrated into peripheral lymphoid organs and led to an increased number of regulatory T-cells (Tregs) expressing cluster of differentiation (CD) markers CD4 and CD25 and forkhead box protein 3 (FoxP3). Tolerance could be transferred to syngeneic recipients with blood or spleen cells. Depletion of Tregs from tolerogenic cells abrogated their suppressive effect, arguing for mediation of immunosuppression by CD4⁺CD25⁺FoxP3⁺ Tregs. Donor-derived MICs also prolonged kidney allograft survival in pigs. MICs generated from donor monocytes were applied for the first time in humans in a patient suffering from therapy-resistant rejection of a haploidentical stem cell transplant. We describe, in the present paper, a simple method for in vitro generation of suppressor blood cells for potential use in clinical organ transplantation. Although the case report does not allow us to draw any conclusion about their therapeutic effectiveness, it shows that MICs can be easily generated and applied in humans.

MMC treatment converts rat DC into tolerogenic cells. This mechanism is mediated by decrease of ICAM-1, CD80, and CD86.