We hypothesize that: (1) L -tryptophan (Trp) is greatly utilized and not depleted in pregnancy; (2) fetal tolerance is achieved in part through immunosuppressive kynurenine (Kyn) metabolites produced by the flux of plasma free (non-albumin-bound) Trp down the Kyn pathway; (3) the role of indoleamine 2,3-dioxygenase (IDO) in infection is not related to limitation of an essential amino acid, but is rather associated with stress responses and the production of Kyn metabolites that regulate the activities of antigen presenting cells and T-cells, as well as increased NAD + synthesis in IDO-expressing cells; (4) Trp depletion is not a host defence mechanism, but is a consequence of Trp utilization. We recommend that future studies in normal and abnormal pregnancies and in patients with infections or cancer should include measurements of plasma free Trp, determinants of Trp binding (albumin and non-esterified fatty acids), total Trp, determinants of activities of the Trp-degrading enzymes Trp 2,3-dioxygenase (TDO) (cortisol) and IDO (cytokines) and levels of Kyn metabolites. We also hypothesize that abnormal pregnancies and failure to combat infections or cancer may be associated with excessive Trp metabolism that can lead to pathological immunosuppression by excessive production of Kyn metabolites. Mounting evidence from many laboratories indicates that Trp metabolites are key regulators of immune cell behaviour, whereas Trp depletion is an indicator of extensive utilization of this key amino acid.

Secondary infections due to post-sepsis immunosuppression are a major cause of death in patients with sepsis. Repetitive inoculation of increasing doses of lipopolysaccharide (LPS) into mice mimics the immunosuppression associated with sepsis. Myeloid-derived suppressor cells (MDSCs, Gr-1 + CD11b + ) are considered a major component of the immunosuppressive network, interfering with T-cell responses in many pathological conditions. We used LPS-immunosuppressed (IS) mice to address whether MDSCs acquired their suppressive ability in the bone marrow (BM) and whether they could migrate to lymph nodes (LNs) to exert their suppressive function. Our results showed that Gr-1 + CD11b + cells of IS mice already had the potential to inhibit T-cell proliferation in the BM. Moreover, soluble factors present in the BM from IS mice were responsible for inducing this inhibitory ability in control BM cells. In addition, migration of Gr-1 + CD11b + to LNs in vivo was maximal when cells obtained from the BM of IS mice were inoculated into an IS context. In this regard, we found chemoattractant activity in cell-free LN extracts (LNEs) from IS mice and an increased expression of the LN-homing chemokine receptor C–C chemokine receptor type 7 (CCR7) in IS BM Gr-1 + CD11b + cells. These results indicate that Gr-1 + CD11b + cells found in BM from IS mice acquire their suppressive activity in the same niche where they are generated, and migrate to LNs to exert their inhibitory role. A better understanding of MDSC generation and/or regulation of factors able to induce their inhibitory function may provide new and more effective tools for the treatment of sepsis-associated immunosuppression.

After traumatic skeletal muscle injury, natural killer (NK) cells are recruited to the draining lymph node in a TLR4 (toll-like receptor 4)–myeloid differentiation factor 88 (MyD88)-dependent manner and impair the development of specific T-cell responses. The suppressed T-cell function might increase the susceptibility of severely injured patients to nosocomial infections.

Secondary infections due to post-sepsis immunosuppression are a major cause of death in patients with sepsis. Strategies aimed at restoring immune functions offer a new perspective in the treatment of sepsis. In the present study, we used LPS (lipopolysaccharide)-immunosuppressed mice to analyse the effects of ATRA (all- trans retinoic acid) on different immune parameters. The IS (immunocompromised) group had decreased lymphocyte and increased MDSC (myeloid-derived suppressor cell) counts in lymph nodes. They also had an impaired in vitro T-cell proliferation, mediated by MDSCs. ATRA administration restored T-cell proliferation, which was associated with a decreased number of live MDSCs. The IS group treated with ATRA had an increased number of CD4 + and CD8 + T-cells. ATRA partially improved the primary humoral immune response, even when immunosuppression was established first and ATRA was administered subsequently. Our results demonstrate that ATRA restores immunocompetence by modulating the number of leucocytes and the survival of MDSCs, and thus represents an additional potential strategy in the treatment of the immunosuppressive state of sepsis.

CD4 + T reg -cells (regulatory T-cells) probably contribute to the impaired virus-specific T-cell responses in chronic HCV (hepatitis C virus) infection; however, their antigen-specificity has remained elusive. In the present study, we analysed peripheral blood CD4 + T reg -cells in patients with chronic hepatitis C and subjects with self-limited HCV infection and characterized individual T reg -cell clones obtained from both groups at the phenotypic and functional level. Foxp3 (forkhead box p3) + CD25 + CD4 + T reg -cells were detected more frequently in patients with chronic hepatitis C than self-limited HCV infection, which responded to HCV core stimulation and inhibited proliferation of reporter cells. Cloning under limiting dilution conditions resulted in 14 and six hypoproliferative Foxp3 + CD25 + CD127 − CD4 + T-cell clones from patients with chronic hepatitis C and subjects with self-limited HCV infection respectively. All clones expressed T reg -cell markers and produced IL (interleukin)-10 upon mitogen stimulation. However, exclusively T reg -cell clones from chronic hepatitis C produced IL-10 in response to HCV core and inhibited proliferation of reporter T-cells. These core-specific T reg -cell clones recognized epitopes in two regions of HCV core (amino acids 1–44 and 79–113). Co-culture inhibition assays demonstrated T reg -cells to inhibit reporter T-cells via secretion of IL-10 and IL-35 rather than cell-contact-dependent mechanisms. Finally, the HCV-specific T reg -cell clones lost their functional capacity, along with Foxp3 expression, if kept in culture without HCV core exposure. In conclusion, we identified functionally active HCV core-specific T reg -cells in patients with chronic hepatitis C, which share their epitopes with conventional T-cells and require the continued presence of antigen to maintain their functional differentiation. Thus HCV core-specific T reg -cells may contribute to the immunoregulatory balance in chronic hepatitis C.

DM (diabetes mellitus) is a metabolic disorder of either absolute or relative insulin deficiency. Optimized insulin injections remain the mainstay life-sustaining therapy for patients with T1DM (Type I DM) in 2006; however, a small subset of patients with T1DM (approx. 10%) are exquisitely sensitive to insulin and lack counter-regulatory measures, putting them at higher risk of neuroglycopenia. One alternative strategy to injected insulin therapy is pancreatic islet transplantation. Islet transplantation came of age when Paul E. Lacy successfully reversed chemical diabetes in rodent models in 1972. In a landmark study published in 2000, Shapiro et al. [A. M. Shapiro, J. R. Lakey, E. A. Ryan, G. S. Korbutt, E. Toth, G. L. Warnock, N. M. Kneteman and R. V. Rajotte (2000) N. Engl. J. Med. 343 , 230–238] reported seven consecutive patients treated with islet transplants under the Edmonton protocol, all of whom maintained insulin independence out to 1 year. Substantial progress has occurred in aspects of pancreas procurement, transportation (using the oxygenated two-layer method) and in islet isolation (with controlled enzymatic perfusion and subsequent digestion in the Ricordi chamber). Clinical protocols to optimize islet survival and function post-transplantation improved dramatically with the introduction of the Edmonton protocol, but it is clear that this approach still has potential limitations. Newer pharmacotherapies and interventions designed to promote islet survival, prevent apoptosis, to promote islet growth and to protect islets in the long run from immunological injury are rapidly approaching clinical trials, and it seems likely that clinical outcomes of islet transplantation will continue to improve at the current exponential pace.

The immunosuppressant MMF (mycophenolate mofetil) has increasingly replaced AZA (azathioprine) in renal transplantation. MMF is a prodrug of MPA (mycophenolic acid), which inhibits lymphocyte IMPDH (inosine monophosphate dehydrogenase), thereby drastically decreasing GTP concentrations essential to lymphocyte proliferation in vitro and in vivo . Erythrocyte GTP concentrations are commonly elevated in severe renal disease, but normalize following successful engraftment. Consequently, elevated GTP in renal transplant recipients might signal impending loss of immunosuppression and graft failure. In the present study, we compared erythrocyte nucleotides and plasma metabolites in two groups of 25 patients after renal transplantation, both receiving prednisolone and cyclosporin A, but one group receiving MMF and the other AZA. No patients had recent allograft biopsy evidence of rejection. Erythrocyte GTP concentrations at MMF commencement were 50.4±23.4 μmol/l. An increase occurred during the first 3 months after transplant when MMF was used de novo , stabilizing at 146.7±62.9 μmol/l after 4 months. This was significantly higher ( P =2.5×10 −6 ) than erythrocyte GTP (40.4±15.9 μmol/l) in the AZA group, which was essentially unchanged from values immediately after successful transplantation. The effect of MMF on erythrocyte GTP levels was reversible, since GTP levels fell when MMF therapy was terminated. The results demonstrate paradoxically high GTP concentrations in erythrocytes of renal transplant patients receiving MMF. MPA may stabilize reticulocyte IMPDH, allowing the protein to persist during erythropoiesis. This behaviour is in marked contrast with the decrease in GTP levels seen in white blood cells of patients on chronic MMF therapy.

Immune suppression plays an important role in the pathogenesis of acute pancreatitis. Monocyte expression of HLA (human leucocyte antigen)-DR, a cellular marker of immune suppression, was determined in relation to the development of organ dysfunction in patients with acute pancreatitis. A total of 310 consecutive patients with acute pancreatitis, admitted to a university hospital within 72 h of pain onset, were studied; 194 (63%) had mild disease (group I), 87 (28%) had severe disease without organ dysfunction (group II), and 29 (9%) had severe disease with organ dysfunction (group III). HLA-DR expression, defined both as the proportion of monocytes that were HLA-DR-positive and as monocyte HLA-DR fluorescence intensity, was determined at admission, using whole-blood flow cytometry. Of the patients in group III, 13 (45%) developed organ dysfunction within 24 h of admission. The proportion of HLA-DR-positive monocytes and monocyte HLA-DR density were both related to the severity of pancreatitis ( P <0.001 for linear trend). In predicting organ dysfunction, the sensitivity, specificity and positive-likelihood ratio for the proportion of HLA-DR-positive monocytes were 83% [95% CI (confidence interval) 64–94%], 72% (67–77%) and 3.0 respectively, and for monocyte HLA-DR density the respective values were 69% (49–85%), 84% (79–88%) and 4.3. In conclusion, monocyte HLA-DR expression predicts the development of organ dysfunction that occurs early in patients with acute pancreatitis.

Plasma endothelin (ET)-1 concentrations have been shown to be elevated in patients receiving calcineurin-inhibitors (CI). We investigated urinary and plasma ET-1 (uET-1, pET-1), BigET-1 (uBigET-1, pBigET-1) concentrations, and plasma soluble endothelin-converting enzyme (ECE) concentrations in 381 adult caucasian kidney allograft recipients with graft survival of more than 2 years from the outpatients department of our clinic. Blood and urine probes were always drawn immediately before morning dosage of immunosuppressants. Mean of urinary protein excretion (meanProt) and mean of serum creatinine (meanCrea) were calculated from all available measurements in the most recent year. uET-1 and uBigET-1 were adjusted for urinary protein excretion by calculating uET-1/meanProt and uBigET-1/meanProt. Patients ( n = 310) were on a cyclosporine A or FK506 (CI-group) based immunosuppression protocol, and 71 patients were immunosuppressed without use of CI (nonCI group). Time since transplantation was similar in both groups (mean±S.D.; CI-group: 7.55±2.50; nonCI-group: 7.74±3.06 years, P = 0.240) as well as meanCrea (mean±S.D.; CI-group: 1.97±1.34; nonCI-group: 1.77±1.29mg/dl, P = 0.326). pET-1 was significantly higher in the CI-group, compared with nonCI (mean±S.D.; 0.87±1.4 versus 0.56±0.76fmol/ml, P = 0.011). pBigET-1 was similar (mean±S.D.; 0.85±1.41 versus 0.70±1.21fmol/ml, P = 0.33). ECE concentrations were higher in the CI group (mean±S.D.; 14.30±18.02 versus 9.23±7.42ng/ml, P = 0.001). uET-1/meanProt and uBigET-1/meanProt concentration were similar in the CI-group compared with the nonCI-group (mean±S.D.; uET-1/meanProt: 15±24 versus 21±40pmol/g, P = 0.139; uBigET-1/meanProt: 34±55 versus 19±23pmol/g, P = 0.248). pET-1 elevation in patients receiving CI might be more likely to be due to elevated conversion of pBig-ET-1 by more ECE, and not to higher concentrations of pBigET-1.

Recurrent infections are common features in patients affected by propionic acidaemia (McKusick 232000) and methylmalonic acidaemia (McKusick 251000). Since these disorders are biochemically characterized by tissue accumulation of propionic acid and methylmalonic acid respectively, it is possible that these compounds may act as immunosuppressants. We therefore investigated the effect of propionate and methylmalonate on cellular growth of human peripheral lymphocytes stimulated in vitro by phytohaemagglutinin, concanavalin A and pokeweed mitogen, a recognized test of cellular immunocompetence. Lymphocytes were cultured in flat-bottomed 96-well microplates at 37 ;°C for 96 ;h (phytohaemagglutinin and concanavalin A) or 144 ;h (pokeweed mitogen) in the presence of one mitogen at different concentrations and of one acid added at doses of 1.0, 2.5 or 5.0 ;mM. Cell blastogenesis was measured by the incorporation of tritiated thymidine into cellular DNA and compared with that of identical cultures with no acid added (controls). A consistent and progressive inhibitory effect of propionic acid with increasing concentrations in culture was identified with all mitogens and was more pronounced with pokeweed mitogen. Lymphocyte blastogenesis was not altered in the presence of methylmalonic acid. The effect of propionate was observed only when the drug was added at the beginning (phytohaemagglutinin-activated) or until 24 ;h (concanavalin A- and pokeweed mitogen-activated) of culture. The viability of lymphocytes after treatment with the drug, as assessed by the Trypan Blue exclusion test, revealed no change when compared with the same untreated lymphocytes, indicating no lymphocytotoxic activity. In conclusion, propionic acid, which accumulates in tissues of patients with propionic acidaemia, causes ‘ in vitro ’ immunosuppression, which may be related to the recurrent infections characteristic of these patients.