Cytomegalovirus infection impairs immunosuppressive and antimicrobial effector functions of human multipotent mesenchymal stromal cells.

Human mesenchymal stromal cells (MSC) possess immunosuppressive and antimicrobial effects that are partly mediated by the tryptophan-catabolizing enzyme indoleamine-2,3-dioxygenase (IDO). Therefore MSC represent a promising novel cellular immunosuppressant which has the potential to control steroid-refractory acute graft versus host disease (GvHD). In addition, MSC are capable of reducing the risk of infection in patients after haematopoietic stem cell transplantation (HST). Recent data indicate that signals from the microenvironment including those from microbes may modulate MSC effector functions. As Cytomegalovirus (CMV) represents a prominent pathogen in immunocompromised hosts, especially in patients following HST, we investigated the impact of CMV infection on MSC-mediated effects on the immune system. We demonstrate that CMV-infected MSC lose their cytokine-induced immunosuppressive capacity and are no longer able to restrict microbial growth. IDO expression is substantially impaired following CMV infection of MSC and this interaction critically depends on intact virus and the number of MSC as well as the viral load. Since overt CMV infection may undermine the clinical efficacy of MSC in the treatment of GvHD in transplant patients, we recommend that patients scheduled for MSC therapy should undergo thorough evaluation for an active CMV infection and receive CMV-directed antiviral therapy prior to the administration of MSC.

Antimicrobial and immunoregulatory properties of human tryptophan 2,3-dioxygenase.

In mammals, the regulation of local tryptophan concentrations by the IFN-gamma-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 liver-specific 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, TDO-positive cells were capable of inhibiting anti CD3-driven T-cell proliferation and IFN-gamma 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.

The missing link between indoleamine 2,3-dioxygenase mediated antibacterial and immunoregulatory effects.

The interferon (IFN)-gamma-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-gamma-induced IDO activity mediates an antimicrobial effect, while at the same time IDO inhibits T-cell proliferation and IFN-gamma 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.

Indoleamine 2,3-dioxygenase is involved in defense against Neospora caninum in human and bovine cells.

Neospora caninum is an apicomplexan parasite closely related to Toxoplasma gondii. In nature this parasite is found especially in dogs and cattle, but it may also infect other livestock. The growth of N. caninum, which is an obligate intracellular parasite, is controlled mainly by the cell-mediated immune response. During infection the cytokine gamma interferon (IFN-gamma) plays a prominent role in regulating the growth of N. caninum in natural and experimental disease. The present study showed that induction of the tryptophan-degrading enzyme indoleamine 2,3-dioxygenase (IDO) is responsible for the inhibition of parasite growth that is mediated by IFN-gamma-activated bovine fibroblasts and endothelial cells. This antiparasite effect could be abrogated by addition of tryptophan, as well as by the IDO-specific inhibitor 1-L-methyltryptophan. In conclusion, our data show that human and bovine cells use the same effector mechanism to control the growth of N. caninum.

Antimicrobial and immunoregulatory effects mediated by human lung cells: role of IFN-gamma-induced tryptophan degradation.

Pneumonia caused by bacterial, viral and parasitic pathogens is one of the most common clinical problems facing primary and secondary care physicians. Staphylococcus aureus is a common cause of lung abscesses in humans and, in immunocompromised patients, herpes simplex virus type I and Toxoplasma gondii can cause severe life-threatening pneumonia. The authors focused their interest in the antimicrobial effects mediated by human lung cells against these pathogens. It was found that IFN-gamma-stimulated lung cells are capable of inhibiting T cell proliferation and restrict the replication of microorganisms such as T. gondii, S. aureus and herpes simplex virus. This immunoregulatory and antimicrobial effect was enhanced in the presence of IL-1 or tumor necrosis factor-alpha (TNF-alpha). Furthermore, the IFN-gamma-dependent antimicrobial effects of HBE4-E6/E7 (human lung bronchus epithelial cells) and A549 (human type II alveolar cells) correlated with the activation of the tryptophan-degrading enzyme indoleamine 2,3-dioxygenase (IDO). It was found that both the abrogation of IDO activity by the specific IDO-inhibitor 1-L-methyltryptophan and the supplementation of cultures with tryptophan result in an inhibition of IFN-gamma-induced antimicrobial effects mediated by lung cells. Therefore it is suggested that tryptophan depletion via IFN-gamma-mediated IDO induction is a major antibacterial, antiparasitic, antiviral and immunoregulatory mechanism in human lung cells.

Cytomegalovirus impairs the induction of indoleamine 2,3-dioxygenase mediated antimicrobial and immunoregulatory effects in human fibroblasts.

Human fibroblasts provide immunosuppressive functions that are partly mediated by the tryptophan-catabolizing enzyme indoleamine-2,3-dioxygenase (IDO). Moreover, upon stimulation with inflammatory cytokines human fibroblasts exhibit broad-spectrum antimicrobial effector functions directed against various clinically relevant pathogens and these effects are also IDO-dependent. Therefore human fibroblasts are suggested to be involved in the control of immune reactions during infectious diseases. As human cytomegalovirus (HCMV) represents a pathogen frequently found in immunocompromised hosts and IDO is involved in the control of HCMV growth, we here investigated the impact of HCMV infection on IDO-mediated antimicrobial and immunoregulatory effects. We show that infection with HCMV substantially impairs IFN-γ-induced IDO-activity in human fibroblasts in a dose and time dependent fashion. Consequently, these cells are no longer able to restrict bacterial and parasitic growth and, furthermore, loose their IDO-mediated immunosuppressive capacity. Our results may have significant implications for the course of HCMV infection during solid organ transplantation: we suggest that loss of IDO-mediated antimicrobial and immunoregulatory functions during a HCMV infection might at least in part explain the enhanced risk of organ rejection and infections observed in patients with HCMV reactivation after solid organ transplantation.

Inhibition of T-cell proliferation by murine multipotent mesenchymal stromal cells is mediated by CD39 expression and adenosine generation.

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.

Antimicrobial and immunoregulatory effector mechanisms in human endothelial cells. Indoleamine 2,3-dioxygenase versus inducible nitric oxide synthase.

In infectious diseases, interferon-gamma (IFN-gamma) is generally accepted as one of the most important inducers of antimicrobial and immunoregulatory effects, and both seemingly contradictory effects, can be mediated by the same effector molecules. In detail, several IFN-gamma induced enzymes such as the inducible nitric oxide synthase (iNOS) as well as the indoleamine 2,3-dioxygenase (IDO) also exert this double function. In this review we focus on antimicrobial and immunoregulatory properties of both enzymes expressed by human endothelial cells, which are prominent players in infectious diseases, tumour immunology and transplant medicine.