Human multipotent mesenchymal stromal cells (MSCs) exhibit multilineage differentiation potential, support hematopoiesis, and inhibit proliferation and effector function of various immune cells. On the basis of these properties, MSC are currently under clinical investigation in a range of therapeutic applications including tissue repair and immune-mediated disorders such as graft-versus-host-disease refractory to pharmacological immunosuppression. Although initial clinical results appear promising, there are significant concerns that application of MSC might inadvertently suppress antimicrobial immunity with an increased risk of infection. We demonstrate here that on stimulation with inflammatory cytokines human MSC exhibit broad-spectrum antimicrobial effector function directed against a range of clinically relevant bacteria, protozoal parasites and viruses. Moreover, we identify the tryptophan catabolizing enzyme indoleamine 2,3-dioxygenase (IDO) as the underlying molecular mechanism. We furthermore delineate significant differences between human and murine MSC in that murine MSC fail to express IDO and inhibit bacterial growth. Conversely, only murine but not human MSC express inducible nitric oxide synthase on cytokine stimulation thus challenging the validity of murine in vivo models for the preclinical evaluation of human MSC. Collectively, our data identify human MSC as a cellular immunosuppressant that concurrently exhibits potent antimicrobial effector function thus encouraging their further evaluation in clinical trials.

Tryptophan metabolism occurs via the protohemoprotein enzymes tryptophan 2,3-dioxygenase (TDO) and indoleamine 2,3-dioxygenase (IDO), the latter action of which has a number of effects in the body including both antimicrobial defence and immune regulation. Whilst the antimicrobial action of IDO is largely due to depletion of the essential amino acid tryptophan, the immune regulatory function of IDO is still unclear and controversial. The list of pathogens that are "sensitive" to IDO-mediated tryptophan degradation covers intra-cellular parasites such as toxoplasma and possibly plasmodia, viruses (herpes viruses) to intra-cellular bacteria (chlamydia and rickettsia) and extra-cellular bacteria such as streptococci, enterococci and staphylococci. Immune regulation may be a consequence of tryptophan depletion, the accumulation of immune-active or toxic metabolites or due to other signalling events. This review covers the latest data and controversy pertaining to the antimicrobial and immune regulatory effects of tryptophan metabolism.

In mammals, the regulation of local tryptophan concentrations by the IFN-c-i inducible enzyme IDO is a prominent antimicrobial and immunoregulatory effector mechanism. Here, we show for the first time that another tryptophan-degrading enzyme, the liverspecific tryptophan 2,3-dioxygenase (TDO), is also capable of mediating antimicrobial and immunoregulatory effects. Using a tetracycline inducible eukaryotic system, we were able to express recombinant TDO protein, which exhibits functional properties of native TDO. We found that HeLa cells expressing recombinant TDO were capable of inhibiting the growth of bacteria (Staphylococcus aureus), parasites (Toxoplasma gondii) and viruses (herpes simplex virus). These TDO-mediated antimicrobial effects could be blocked by the addition of tryptophan. In addition, we observed that, similar to IDO-positive cells, TDOpositive cells were capable of inhibiting anti CD3-driven T-cell proliferation and IFN-c production. Furthermore, TDO-positive cells also restricted alloantigen-induced T-cell activation. Here, we describe for the first time that TDO mediates antimicrobial and immunoregulatory effects and suggest that TDO-dependent inhibition of T-cell growth might be involved in the immunotolerance observed in vivo during allogeneic liver transplantation.Key words: Kynurenine . T cells . Tolerance . Tryptophan . Tryptophan 2,3-dioxygenase Introduction L-tryptophan (L-trp) is an essential amino acid that is not only required for the synthesis of proteins, but also for the biosynthesis of neurotransmitters such as serotonin and melatonin. Nevertheless, most of the dietary L-trp is catabolised via the kynurenine pathway to kynurenines and these are eliminated in the urine (hence their names). A small amount of the dietary tryptophan is used to produce the physiological relevant NAD [1,2].In mammals, the first and rate-limiting step of the kynurenine pathway, namely the oxidation of tryptophan to N-formyl kynurenine, is catalysed by the hepatic tryptophan 2,3-dioxygenase (TDO, EC 1.13.11.11) and the extra-hepatic IDO (EC 1.13.11.52). Recently, a third tryptophan-degrading enzyme, IDO2, was described, however, the in vivo function of this enzyme remains speculative [3,4].The function of IDO has been most intensively analysed and shown to be involved in several essential processes. Being an immunoregulated enzyme with antimicrobial and immunoregulatory function, IDO regulates T-cell responses and induces maternal tolerance towards the allogeneic foetus [5]. Interestingly, IDO also seems to play a role in cancer progression as the magnitude of its expression, for example, correlates with the overall survival of serous-type ovarian cancer patients, specifying IDO as a marker for a poor prognosis [6].IDO mediates its activity locally, in inflamed tissue or lymph nodes. In contrast, TDO activity is mainly expressed in the liver and is not regulated by the immune system but does have à These authors contributed equally to this work. systemic effects by controlling the tryptophan levels in ...

Multipotent mesenchymal stromal cells (MSCs) are bone marrow-derived cells of nonhematopoietic origin with immunoregulatory properties. Although some functions of MSCs have been identified, there are still features that are not explained thus far. The aim of the present study was to identify novel factors involved in MSC-mediated inhibition of T-cell proliferation. We here demonstrate that the surface molecule CD39 is coexpressed in concert with CD73 on murine MSCs catalyzing the generation of adenosine, which can directly act on activated T cells via the adenosine A2A receptor. Blocking of the adenosine pathway either by the A2A receptor antagonist SCH58261 or the specific CD39 inhibitor polyoxotungstate 1 (POM-1) blocked MSC-mediated suppression of T-cell proliferation almost completely. We conclude that CD39/CD73 coexpression is a novel important component of the immunoregulatory functions of murine MSCs. Our findings may both be important to improve our understanding of MSC function and for the development of immunomodulatory cellular therapies.

The interferon (IFN)–γ-inducible tryptophan degrading enzyme indoleamine 2,3-dioxygenase (IDO) has not only been recognized as a potent antimicrobial effector molecule for the last 25 years but was recently found also to have potent immunoregulatory properties. In this study, we provide evidence that both tryptophan starvation and production of toxic tryptophan metabolites are involved in the immunoregulation mediated by IDO, whereas tryptophan starvation seems to be the only antibacterial effector mechanism. A long-studied controversy in the IDO research field is the seemingly contradictory effect of IDO in the defence against infectious diseases. On the one hand, IFN-γ-induced IDO activity mediates an antimicrobial effect, while at the same time IDO inhibits T-cell proliferation and IFN–γ production. Here, we suggest that both effects, dependent on the threshold for tryptophan, cooperate in a reasonable coherence. We found that the minimum concentration of tryptophan required for bacterial growth is 10-40-fold higher than the minimum concentration necessary for T-cell activation. Therefore, we suggest that during the first phase of infection the IDO-mediated tryptophan depletion has a predominantly antimicrobial effect whereas in the next stage, and with ongoing tryptophan degradation, the minimum threshold concentration of tryptophan for T-cell activation is undercut, resulting in an inhibition of T-cell growth and subsequent IDO activation.