Infectious complications in patients undergoing unrelated donor bone marrow transplantation: experience from a single institution

Epidemiology of bloodstream infections and the impact of antimicrobial resistance in pediatric hematopoietic cell transplant

Bloodstream infections (BSI) pose a substantial threat to the well-being and survival of patients undergoing hematopoietic stem cell transplantation (HSCT). Risk factors for these infections vary across the different post-HSCT phases. In the pre-engraftment period, patients are particularly susceptible to infection due to prolonged neutropenia, mucosal damage, and extensive use of central venous line (CVL). In the post-engraftment phase, the emergence of graft versus host diseases further compounds the risk. The epidemiology of these infections has undergone notable changes over the years due to multifactorial reasons, including the evolution of protocols that intensify immunosuppression. In this context, the emergence of multi-drug resistance (MDR) microorganisms can be a challenge due to the elevated risk of mortality in these vulnerable patients. Unfortunately, there is a lack of comprehensive data on this topic, particularly in pediatrics. This article aims to provide a summary of the epidemiology of BSI in the different post-transplant phases and the impact of MDR pathogens. Having knowledge about the local epidemiology of BSI can be instrumental in tailoring targeted therapies, leading to improved survival rates in HSCT recipients.

Survey of HCMV in allogenic and autologous stem cell transplantation by real-time PCR in Kermanshah, west of Iran

INTRODUCTION

Human Cytomegalovirus (HCMV) is the most important viral pathogen in people undergoing bone marrow transplantation (BMT). HCMV detection in the early stages makes is possible to save the patients' lives through immediate and timely treatment. The aim of this study was to investigate the status of HCMV using the real-time PCR method in BMT patients in Kermanshah, west of Iran.

METHODS

HCMV monitoring was done in 120 patients who underwent BMT, 38 allogeneic cases and 82 autologous cases, using the ELISA serology test before transplantation. The participants were followed up 100 days after transplantation for HCMV detection in blood samples using real-time PCR. Preemptive therapy started with Ganciclovir and Foscarnet when the viral load was > 200 HCMV DNA copies/ml.

RESULTS

Despite preemptive therapy, infection recurred in less than 1 month. HCMV recurred more frequently in patients undergoing allogenic transplation versus those receiving autologous transplantation. Recurrence was seen in 5 patients receiving allogenic transplantation. HCMV recurrence occurred in five patients with allogeneic transplantation. Twelve patients undergoing allogeneic or autologous transplantation (83%) and a virus load of > 1000 copies/ml showed HCMV-related symptoms. Three patients died, two due to HCMV-related pneumonia and the other one due to a fungal infection.

CONCLUSION

Real-time PCR may be a useful method for quantification and monitoring of HCMV recurrence and may be helpful in choosing more efficient HCMV preemptive treatment in BMT recipients.

The D-index is not correlated with invasive fungal infection during the early-post engraftment phase among allogeneic hematopoietic stem cell transplant recipients

The D-index assesses neutropenia dynamics. Prolonged neutropenia is a major risk for invasive fungal infection (IFI); we hypothesized that D-index is predictive of IFI risk. We retrospectively reviewed 789 adults who underwent allogeneic hematopoietic transplant (HSCT) from 1/1/2005 to 9/30/2015. Medical records were reviewed from transplant (D0) through Day 100. The D-index was calculated as area over the neutrophil curve until engraftment. 714 patients were included for analysis. Sixteen (2%) developed probable (11) or proven (5) IFI. Median time to IFI was 40 days (range 8-98) after HSCT. Groups with and without IFI did not differ significantly in duration of mild or profound neutropenia. Median D-index of those with IFI was 4293 days neutrophil/µl compared to 3590 days neutrophil/µl for those without IFI (P = 0.17). Patients who were neutropenic on D0 showed higher rates of IFI than those who were not (10/123 [8%] vs 6/591 [1%]; P < 0.001). Only 2% developed IFI, likely due to mold-active antifungal prophylaxis. The D-index was not significantly higher in those with IFI. Duration of profound neutropenia and neutropenia at D0 may be better markers for IFI among HSCT recipients during the first 30 and 100 days after transplant.

Infection surveillance in pediatric hematopoietic stem cell transplantation recipients

OBJECTIVE

To describe the profile of reported healthcare-associated infections (HAIs) in pediatric patients submitted to hematopoietic stem cell transplantation (HSCT) at a reference center.

METHODS

Retrospective cohort of pediatric patients who were submitted to HSCT from 2008 to 2016. The criteria for HAI were based on those established by the National Healthcare Safety Network. Data were collected by active surveillance performed daily by professionals. This study was approved by the institutional research ethics committee.

RESULTS

A total of 86 HSCTs were performed in 81 patients younger than 18 years of age (median, 10 years). Of these, 69 (85%) were males. Aplastic anemia and leukemia were the main diagnoses. A total of 140 HAIs were diagnosed with an incidence density of 28.2 infections/1000 patient-days. The most common HAI was laboratory-confirmed bloodstream infection (46), the majority of which was reported to be central venous catheter-associated (43). Gram-negative bacteria were the most prevalent microorganisms (58.5%). Almost all the infections occurred until 30 days after transplantation, and 17 deaths were observed within 180 days after the procedure.

CONCLUSION

Active surveillance of HAIs in HSCT children allowed the evaluation of the incidence and profile of HAIs, which is essential for the health care of these patients.

Parasitic Infections in Hematopoietic Stem Cell Transplantation

Parasitic infections are rarely documented in hematopoietic stem cell transplant recipients. However they may be responsible for fatal complications that are only diagnosed at autopsy. Increased awareness of the possibility of parasitic diseases both in autologous and allogeneic stem cell transplant patients is relevant not only for implementing preventive measures but also for performing an early diagnosis and starting appropriate therapy for these unrecognized but fatal infectious complications in hematopoietic transplant recipients. In this review, we will focus on parasitic diseases occurring in this population especially those with major clinical relevance including toxoplasmosis, American trypanosomiasis, leishmaniasis, malaria, and strongyloidiasis, among others, highlighting the diagnosis and management in hematopoietic transplant recipients.

Cytomegalovirus infection in the bone marrow transplant patient

Cytomegalovirus (CMV) infection is an important contributor to the morbidity and mortality associated with bone marrow transplantation (BMT). Infection may lead to CMV disease involving multiple organs such as pneumonia, gastroenteritis, retinitis, central nervus system involvement and others. CMV seropositivity is an important risk factor and approximately half of BMT recipients will develop clinically significant infection most commonly in the first 100 d post-transplant. The commonly used tests to diagnose CMV infection in these patients include the pp65 antigenemia test and the CMV DNA polymerase chain reaction (PCR) assay. Because of its greater sensitivity and lesser turnaround time, the CMV PCR is nowadays the preferred test and serves as a main guide for pre-emptive therapy. Methods of CMV prevention include use of blood products from seronegative donors or leukodepleted products. Prophylaxis or pre-emptive therapy strategies for CMV prevention may be used post-transplant with the latter becoming more common. The commonly used antivirals for pre-emptive therapy and CMV disease management include intravenous gancyclovir and foscarnet. The role of intravenous immunoglobulin, although used commonly in CMV pneumonia is not clear.

Fungal and viral infections after allogeneic hematopoietic transplantation from unrelated donors in adults: improving outcomes over time

Umbilical cord blood (CB) is increasingly used as an alternative source of stem cells in adult unrelated transplantation. Although registry studies report similar overall outcomes in comparison with BM/PB, comparative studies focusing on severe infections and infection-RM (IRM) with a long follow-up are scarce. A total of 434 consecutive unrelated transplants (1997-2009) were retrospectively analyzed to compare overall outcomes, incidence and risk factors of severe viral and invasive fungal infections in CB (n=65) vs BM/PB recipients (n=369). The 5-year OS was 38 vs 43%, respectively (P=0.2). CB transplantation (CBT) was associated with a higher risk of invasive aspergillosis (100-days-cumulative incidence 16 vs 6%, P=0.04) and CMV infection without differences in RM. No statistically significant differences were found regarding NRM (NRM of 38% in CB vs 37% in BM/PB at 1 year) nor IRM (30% in CB vs 27% in BM/PB at 1 year). In the overall population, NRM and IRM improved in more recent years. In adults who receive a single CBT, the risk of severe infections is increased when compared with unrelated BM/PB recipients, but mortality from infections is similar, leading to similar NRM and survival.

Incidence of hospital-acquired pneumonia, bacteraemia and urinary tract infections in patients with haematological malignancies, 2004-2010: a surveillance-based study

Objective

This study charted incidence trends of hospital-acquired (HA) pneumonia, bacteraemia and urinary tract infections (UTI) in a haematology department.

Methods

Prospective surveillance of hospital-acquired infections (HAI) was undertaken in a 42-bed haematology department of a university hospital. All patients hospitalized ≥48 hours between 1^st January 2004 and 31^st December 2010 were included. Definitions of HAI were based on a standardized protocol. The incidence was the number of events per 1000 patient-days at risk; only the first HAI was counted. Multivariate Poisson regression was fitted to assess temporal trends.

Results

Among 3 355 patients (58 063 patient-days at risk) included, 1 055 (31%) had HAI. The incidence of HA pneumonia, HA bacteraemia and HA UTI was respectively 3.3, 12.0 and 2.9 per 1000 patient-days at risk. HA bacteraemia incidence increased by 11% (95% confidence interval: +6%, +15%, P<0.001) per year, independently of neutropenia, central venous catheterization (CVC) and haematological disease. The incidences of HA pneumonia and HA UTI were stable. The most frequently isolated pathogens were Aspergillus spp. (59.2%) for pneumonia, coagulase-negative Staphylococcus (44.2%) for bacteraemia and enterobacteria (60%) for UTI.

Conclusion

The incidence of bacteraemia increased, indicating that factors other than CVC exposure, including chemotherapy with its impact on the immune system, could explain this trend. Further analytic studies are needed to explore the factors that could explain this trend.

Effects of prophylactic hematopoietic colony stimulating factors on stem cell transplantations: meta-analysis

Hematopoietic growth factors are often given for prevention of febrile neutropenia (FN), infections, and other complications by hastening neutrophil recovery in the treatment of malignancies after high dose chemotherapy (HDCT). Although several meta-analyses have already demonstrated beneficial effects of prophylactic granulocyte colony-stimulating factors (G-CSF) administration, the effects of G-CSF have not been confirmed in cancer patients receiving stem cell transplantation (SCT) after HDCT. Therefore, we performed a statistical combination of controlled clinical trials to investigate the efficacy of prophylactic use of G-CSF in preventing the neutropenic complications associated with SCT following HDCT in cancer patients. We searched PubMed to identify potentially relevant references and finally selected seven randomized controlled trials that met all of the eligibility criteria. Our meta-analysis demonstrated that prophylactic G-CSF reduced the risk of documented infections and time to hematologic recovery manifested by days to absolute neutrophil count (ANC) ≥ 0.5 × 10(9)/L, days to ANC ≥ 1.0 × 10(9)/L, and days to platelets ≥ 20 × 10(9)/L in SCT patients with cancer following HDCT. The G-CSF treated group also showed a decrease in the length of hospital stay. However, there was no difference between G-CSF treatment group and placebo group in regard to all-cause mortality, infection-related mortality, grade 2∼4 acute graft-versus-host-disease, and episode of fever.

Effect of blood decrease on micafungin disposition in rats

Micafungin (MCFG) is a novel echinocandin-class antifungal agent that extensively undergoes metabolic removal in the liver. In the present study, the influence of decreased blood volume on pharmacokinetic disposition of MCFG was examined using a rat model prepared by phlebotomy. In phlebotomized rats, hematocrit level and plasma albumin concentration were decreased by 50 and 15%, respectively. Regarding the pharmacokinetic parameters of MCFG, there were no significant differences in the total body clearance (CL(tot)) and elimination rate constant (k (e)) between control and phlebotomized rat groups. A slight increase was observed in the apparent volume of distribution at steady-state (Vd(ss)), but the degree of change was minimal. These findings demonstrate that the elimination capacity for MCFG is only slightly affected by severe anemia and moderate hypoalbuminemia, and provide experimental evidence for the preceding clinical studies suggesting that neither hematocrit level nor serum albumin concentration is a contributory factor for the metabolic clearance of MCFG.

Early infections in patients undergoing high-dose treatment with stem cell support: a comparison of patients with non-Hodgkin lymphoma and multiple myeloma

BACKGROUND

Infections are life-threatening complications in patients undergoing high-dose chemotherapy with stem cell support (HDT). Knowledge of the infectious pathogens is essential to make a safe outpatient setting.

METHODS

We conducted a retrospective study of 208 patients treated with HDT. The population included non-Hodgkin lymphoma (NHL) and multiple myeloma (MM) patients. No patients received prophylactic antibacterial treatment.

RESULTS

Pathogens were isolated from 44% of all patients. MM patients more frequently had multiple pathogens in blood cultures (38% versus 25%). Transplantation related mortality was similar between the groups.

CONCLUSION

The frequency of isolated pathogens, positive blood cultures, and the diversity of pathogens were higher in MM patients as compared to NHL patients. However, this did not translate into higher transplantation-related mortality, probably because broad-spectrum antibiotic treatment could be initiated immediately. A safe outpatient setting with prophylactic antibiotic treatment is dependent on continuous collection and registration of microbiological findings.

Infectious complications after allogeneic stem cell transplantation: incidence in matched-related and matched-unrelated transplant settings

BACKGROUND

Bacterial, viral, and fungal pathogens frequently cause severe, life-threatening infections in immunocompromised patients after allogeneic hematopoietic stem cell transplantation (SCT).

OBJECTIVE

To compare the frequency of infections in patients with matched-related (Group A) or with human leukocyte antigen (HLA)-matched-unrelated donors (Group B).

PATIENTS AND METHODS

Patients treated at our transplantation unit between April 2004 and April 2005 were enrolled into this analysis. Documentation comprised demographic data, conditioning treatment, stem cell source, clinical course, as well as microbiological and clinical data and mortality.

RESULTS

We analyzed 59 patients, 22 in Group A and 37 in Group B. Both groups were well balanced regarding demographic data. Diagnoses were acute myeloid leukemia (30 of 59 patients, 50.8%), multiple myeloma (15.2%), acute lymphoblastic leukemia (11.9%), and chronic myeloid leukemia (10.2%). Patients in Group A developed infections in 95.5% of the cases compared with 97.3% in patients in Group B. Most frequently detected pathogens were Staphylococcus species, human herpesvirus-6, and Epstein-Barr virus. Three proven fungal infections were detected in Group A compared with 9 proven fungal infections in Group B. Lung infiltrations were observed in equivalent incidence in both groups. Two years after transplantation, 55.9% of patients were alive (Group A: 68.2%; Group B: 48.6%, not significant).

CONCLUSION

Allogeneic SCT from HLA-matched-unrelated donors does not have a higher infection risk than patients transplanted from matched-related donors.

Umbilical cord blood transplantation: basic biology and clinical challenges to immune reconstitution

Allogeneic stem cell transplantation has continued to evolve as a common procedure for the treatment of hematological malignancies and bone marrow failure. Donor bone marrow and mobilized peripheral stem cells are routinely employed for the reconstitution of immune function in leukemia and lymphoma patients following radiation and/or chemotherapy. Unfortunately, only 30% of patients have an HLA-identical sibling donor and the identification of matched unrelated donors, particularly for minorities, can present an exceptional challenge. The transplantation of umbilical cord blood (UCB) represents the most recent strategy to expand the potential donor pool while maintaining an acceptable level of treatment-related complications. First utilized in children, UCB transplantation permits a higher degree of HLA disparity while demonstrating a reduction in the incidence and severity of graft-versus-host disease (GvHD) compared to previous transplantation modalities. Despite the apparent decrease in GvHD, relapse rates remain comparable to transplantation with bone marrow or mobilized peripheral blood suggesting a strong graft-versus-leukemia/lymphoma (GvL) effect. However, several issues complicate the use of UCB transplantation and its extension to the treatment of adults. Many infections that afflict transplant patients are particularly frequent and more severe in the context of UCB transplantation. UCB T-cells are naive and therefore display less proliferation and IFN-gamma production in response to cognate antigen and also appear to demonstrate defects in signal transduction mechanisms. In addition, UCB contains T regulatory cells (Treg) with more potent suppressor function than adult Treg. Furthermore, adult patients often require more total cells and CD34+ progenitors for transplantation than a single UCB unit can provide. Thus, strategies to expand selected subpopulations from UCB and the use of multi-unit transplantation are areas of active research. This review will provide a condensed summary of the clinical history of UCB transplantation and emphasize the advantages and disadvantages of this approach to hematological malignancies in comparison to other methods of hematopoietic stem cell transplantation. Subsequently, it will mainly focus on the current challenges to immune reconstitution presented by UCB transplantation, recent research into their cellular and molecular mechanisms, and experimental approaches to overcome them.

Severe infections after unrelated donor allogeneic hematopoietic stem cell transplantation in adults: comparison of cord blood transplantation with peripheral blood and bone marrow transplantation

We evaluated the occurrence of severe infections in 192 consecutive adult recipients of volunteer unrelated donor allogeneic hematopoietic stem cell transplants, with a detailed analysis of severe infections after receipt of cord blood transplants (CBTs; n = 48) or bone marrow transplants (BMTs)/peripheral blood stem cell transplants (PBSCTs; n = 144). At a 3-year median follow-up, CBT recipients had a higher risk of developing any severe infection (85% versus 69% in BMT/PBSCT recipients, P < .01). CBT recipients had a higher incidence of severe bacterial infections before day +100, but at 3 years the risks of these and other infections were similar in the CBT and BMT/PBSCT groups. In addition, the 100-day and 3-year incidences of infection-related mortality (IRM) did not differ between groups (P = .2 and .5, respectively). In multivariate analysis, the most significant risk factor for IRM in all 192 patients was monocytopenia (.2 x 10(9)/L). In CBT recipients, only neutropenia (.2 x 10(9)/L) on day +30 and low nucleated cell dose infusion (< 2 x 10(7)/kg) showed a trend for increased IRM (P = .05 in both cases). Stem cell source had no effect on day +100 or 3-year nonrelapse mortality (NRM), cytomegalovirus infection, cytomegalovirus disease (7% versus 6%), or overall survival (36% versus 39%, respectively). The number of mismatches in HLA (A, B, and DRB1) had no effect on any outcome in CBT recipients. In contrast, in the BMT/PBSCT group, the presence of any mismatch by low or high-resolution HLA typing (A, B, C, and DRB1) increased NRM and decreased overall survival (P < .01). IRM was the primary or secondary cause of death in 61% and 59% of CBT and BMT/PBSCT recipients who died, respectively. Our results confirm the relevance of severe infectious complications as source of severe morbidity and NRM after volunteer unrelated donor hematopoietic stem cell transplantation in adults, but suggest that CBT recipients have a similar risk of dying from an infection if an accurate selection of a cord blood unit is done.

Morphology of the bone marrow after stem cell transplantation

In many haematological conditions the only curative option is stem cell (SCT) or bone marrow (BM) transplantation. Little information exists about BM morphology following non-ablative engraftment. During the pretransplantation period and depending on the kind of pretreatment, there may be hypoplasia, residual disease and varying degrees of fibrosis. In the post-transplantation period, after 1-3 weeks of transfusion-dependent pancytopenia, the first signs of successful engraftment are indicated by the recurrence of neutrophils, monocytes and erythrocytes in the peripheral blood. In the BM there is slow regeneration of erythropoiesis, followed by the other lineages of haematopoiesis and increase in reticulin fibres or even a resolution of fibrosis. Diagnostic problems arise when neoplastic lympho- or haematopoiesis are maintained following transplantation. Moreover, there may be a significant graft versus tumour response reaction or an already relapsing disease needing aggressive treatment. On the other hand, a conspicuous dyshaematopoiesis should not be mistaken as representing a myelodysplastic syndrome. The presence of granulomas being treatment-related or a manifestation of intercurrent granulomatous disease has to be considered. More advanced knowledge of the histological features of regenerating BM will certainly aid the recognition of relapsing disease and is needed for the adequate reporting of post-transplant alterations associated with a successful or failing engraftment.

Prophylaxis of early bacterial infections after autologous peripheral blood stem cell transplantation (PBSCT): a matched-pair study comparing oral fluoroquinolones and intravenous piperacillin-tazobactam

The safety and efficacy of early bacterial prophylaxis with piperacillin-tazobactam were prospectively evaluated in 51 autologous peripheral blood stem cell transplantation (PBSCT) recipients. The results were compared with those obtained in 51 control patients receiving oral fluoroquinolones in a retrospective matched-pair control study. Overall, 76% of the study group and 98% of the control group developed at least one febrile episode during neutropenia (P=0.002). Time from neutropenia to the first febrile episode (FFE) was significantly longer in the study group than in the control group (P=0.04). Once a febrile episode appeared, the duration of fever was significantly longer in cases than in controls (median of 5 and 2 days respectively, P<0.001), and led to a more frequent use of empirical amphotericin B (AmB), not statistically significant (P=0.13). However, the total time of antibiotic administration was significantly greater in the control than in the study group (P=0.05). The duration of AmB treatment shows a trend toward a longer duration in the control than in study group (P=0.2). Overall, 86% of the Gram-positive bacteremia and 85% of the Gram-negative bacteria were susceptible to the tested antibiotics. Our study suggests that a subgroup of patients could benefit from prophylaxis with piperacillin-tazobactam without increasing toxicity or bacterial resistance.

Pulmonary complications following hematopoietic stem cell transplantation: diagnostic approaches

Pulmonary complications are a significant cause of morbidity and mortality in hematopoietic stem cell transplant recipients. Pulmonary infiltrates in such patients pose a major challenge for clinicians because of the wide differential diagnosis of infectious and noninfectious conditions. It is rare for the diagnosis to be made by chest radiograph, and commonly these patients will need further invasive and noninvasive studies to confirm the etiology of the pulmonary infiltrates. This review describes the role of the different diagnostic tools available to reach a diagnosis in a timely manner in this patient population.

Risk factors for cytomegalovirus, hepatitis B and C virus reactivation after bone marrow transplantation

To derive guidelines for a safer bone marrow transplantation (BMT) policy, we have to study pre-BMT risk factors that may be associated with an increased post-BMT death. Among those factors, the importance of pre-BMT viral hepatitis markers in BMT donors and recipients remains unsettled. In the present study, we have determined the effect of prior donor and recipient cytomegalovirus (CMV), hepatitis B virus (HBV), and hepatitis C virus (HCV) exposure on the incidence of those viral infections after bone marrow transplantation (BMT). The study included 63 patients presented to the BMT unit; 28 of them underwent transplantation and 35 were not transplanted. All serum markers of CMV, HBV, and HCV infections were monitored using ELISA technique, as well as PCR-DNA for CMV, HBV and HCV RT-PCR techniques for HCV. The incidence of active CMV and HCV was 11/28 (39%) and 6/28 (21%) in post-BMT recipients compared to 2/35 (6%) and 2/35 (6%) in the 35 untransplanted patients (P=0.00003 and P=0.05). Whereas active HBV infection was non significantly (P=0.13) higher 3/28 (11%) in the BMT patients in comparison to 1/35 (3%) in untransplanted patients. Ten out of the 19 (53%) of the CMV-seropositive recipients developed CMV reactivation compared to 1/9 (11%) of the CMV-seronegative recipients who developed CMV seroconversion. In addition, 3/8 (38%) of the HBV-seropositive recipients developed HBV reactivation in comparison to 0/20 of the HBV-seronegative recipients. Moreover, 5/13 (39%) of the HCV-seropositive recipients developed HCV reactivation in comparison to 1/16 (6%) of the HCV-seronegative recipients who developed HCV seroconversion. In conclusion, previous exposure to CMV, HBV, and HCV infections in the recipients of BMT patients were found to influence the risk of developing those viral infections.

Monocyte-mediated T-cell suppression and augmented monocyte tryptophan catabolism after human hematopoietic stem-cell transplantation

T-cell dysfunction after human hematopoietic stem-cell transplantation (HSCT) is generally attributed to intrinsic T-cell defects. Here we show that the characteristic impaired proliferative responses to polyclonal stimulation of post-HSCT peripheral blood mononuclear cells (PB-MCs) were markedly (4-fold) improved by T-cell enrichment. Conversely, addback of post-HSCT monocytes to these enriched T cells dampened their proliferative responses, suggesting that post-HSCT monocytes effectively mediate T-cell suppression. As a mechanism possibly contributing to monocyte-mediated T-cell suppression, we investigated monocyte tryptophan catabolism by indoleamine 2,3-dioxygenase into kynurenine, which has been implicated in regulating T-cell responses. Compared with controls, all post-HSCT monocyte-containing cell cultures (total PBMCs, monocytes, and monocyte/T-cell cocultures), but not monocyte-depleted populations, secreted elevated amounts of kynurenine. Blockade of tryptophan catabolism improved the proliferative responses. The slightly increased kynurenine release and substantial release of neopterin by unstimulated post-HSCT monocytes suggests that they were in a state of continuous activation. Superimposed on this state, stimulation of these cells caused a striking, additional increase (10-fold) in kynurenine release, and they triggered marked apoptosis of autologous post-HSCT T cells. We conclude that the amplified kynurenine release by post-HSCT monocytes, particularly induced upon stimulation, may underlie their suppressor activity, which in turn may contribute to the depressed T-cell immune responses after HSCT.