Hematopoietic stem cell graft manipulation as a mechanism of immunotherapy

Dynamics of T cell repertoire renewal following autologous hematopoietic stem cell transplantation in multiple sclerosis

Autologous hematopoietic stem cell transplantation (aHSCT) is a highly effective treatment of multiple sclerosis (MS). It depletes autoreactive cells and subsequently renews adaptive immune cells. The possible proinflammatory potential of surviving T cells early after aHSCT has not been studied. Here, we examined the dynamics of new and surviving T cells in 27 patients after aHSCT by multidimensional flow cytometry, T cell receptor (TCR) sequencing, specificity testing, telomere length profiling, and HLA genotyping. Early after aHSCT, naïve T cells are barely detectable, whereas effector memory (EM) T cells quickly reconstitute to pre-aHSCT values. EM CD4+T cells early after aHSCT have shorter telomeres, have higher expression of senescence and exhaustion markers, and proliferate less than those before aHSCT. We find a median TCR repertoire overlap of 26% between the early post-aHSCT EM CD4+T cells and pre-aHSCT, indicating persistence of EM CD4+T cells early after transplantation. The EM CD4+TCR repertoire overlap declines to 15% at 12 months after aHSCT, whereas the naïve TCR repertoire entirely renews. HLA-DR–associated EM CD4+T cell reactivity toward MS-related antigens decreased after aHSCT, whereas reactivity toward EBV increased. Our data show substantial survival of pre-aHSCT EM CD4+T cells early after transplantation but complete renewal of the T cell repertoire by nascent T cells later.

Infused autograft lymphocyte to monocyte ratio predicts survival in classical Hodgkin lymphoma

The infused autograft lymphocyte to monocyte ratio (A-LMR) as a surrogate marker of host immunity (ie, absolute lymphocyte count) and CD14+ HLA-DR^low/neg immunosuppressive monocytes (ie, absolute monocyte count) is a prognostic factor for patients with diffuse large B-cell lymphoma after autologous peripheral hematopoietic stem cell transplantation (APHSCT). Thus, we set out to investigate if A-LMR can also affect survival post-APHSCT in classical Hodgkin lymphoma. From 1994 to 2012, 183 patients with classical Hodgkin lymphoma who underwent APHSCT were studied. The patients were randomly divided into a training set (n=122) and a validation set (n=61). The receiver operating characteristic and area under the curve identified an A-LMR ≥1 as the best cut-off value and validated by the k-fold cross-validation in the training set. Multivariate analysis showed A-LMR to be an independent prognostic factor for survival in the training set. Patients with an A-LMR ≥1.0 experienced a superior overall survival (OS) versus patients with an A-LMR <1.0 (median OS not reached versus 40.4 months, 5-year OS rates of 86% [95% CI 72–93] versus 43% [95% CI 28–58], P<0.0001, respectively) in the training set. In the validation set, an A-LMR ≥1 showed a median OS of not reached versus 41.4 months for an A-LMR <1, 5-year OS rates of 90% (95% CI 73–97) versus 48% (95% CI 28–68; P<0.0001). A-LMR provides a platform to engineer an autograft versus tumor effect to improve clinical outcomes in patients with classical Hodgkin lymphoma undergoing APHSCT.

Infused autograft lymphocyte to monocyte ratio and survival in diffuse large B cell lymphoma

Infused autograft absolute lymphocyte count is a prognostic factor for survival after autologous peripheral hematopoietic stem cell transplantation (APHSCT) for diffuse large B cell lymphoma (DLBCL). CD14(+) HLA-DR(low/neg) immunosuppressive monocytes affect tumor growth by suppressing host antitumor immunity. Thus, we set out to investigate if the infused autograft lymphocyte to monocyte ratio (A-LMR), as a biomarker of host immunity (ie, lymphocytes) and immunosuppression (ie, monocytes), affects survival after APHSCT. From 1994 to 2012, 379 DLBCL patients who underwent APHSCT were studied. The 379 patients were randomly divided into a training set (n = 253) and a validation set (n = 126). Receiver operating characteristic and area under the curve identified an A-LMR ≥1 as the best cut-off value, which was validated by the k-fold cross-validation in the training set. Multivariate analysis showed A-LMR to be an independent prognostic factor for survival in the training set. Patients with an A-LMR ≥ 1.0 experienced superior overall survival (OS) compared with patients with an A-LMR <1.0 (median OS: 167.2 versus 17.6 months; 5-year OS: 73% [95% confidence interval (CI), 63% to 80%] versus 30% [95% CI, 2% to 38%], P < .0001, respectively) in the training set. In the validation set, an A-LMR ≥ 1 showed a median OS of 181.2 months versus 19.5 months for an A-LMR <1, and 5-year OS rates of 67% (95% CI, 52% to 79%) versus 35% (95% CI, 25% to 47%), P < .0001, respectively. The A-LMR provides a platform to engineer immunocompetent autograft to improve clinical outcomes in DLBCL patients undergoing APHSCT.

Pre-induced adult human peripheral blood mononuclear cells migrate widely into the degenerative retinas of rd1 mice

BACKGROUND AIMS

Recent advances in stem cell research have raised the possibility of stem cells repairing or replacing retinal photoreceptor cells that are either dysfunctional or lost in many retinal diseases. Various types of stem cells have been used to replace retinal photoreceptor cells. Recently, peripheral blood stem cells, a small proportion of pluripotent stem cells, have been reported to mainly exist in the peripheral blood mononuclear cells (PBMCs).

METHODS

In this study, the effects of pre-induced adult human PBMCs (hPBMCs) on the degenerative retinas of rd1 mice were investigated. Freshly isolated adult hPBMCs were pre-induced with the use of the conditioned medium of rat retinas for 4 days and were then labeled with chloromethyl-benzamidodialkylcarbocyanine (CM-DiI) and then transplanted into the subretinal space of the right eye of rd1 mice through a trans-scleral approach. The right eyes were collected 30 days after transplantation. The survival and migration of the transplanted cells in host retinas were investigated by whole-mount retinas, retinal frozen sections and immunofluorescent staining.

RESULTS

After subretinal transplantation, pre-induced hPBMCs were able to survive and widely migrate into the retinas of rd1 mice. A few CM-DiI-labeled cells migrated into the inner nuclear layer and the retinal ganglion cell layer. Some transplanted cells in the subretinal space of rd1 host mice expressed the human photoreceptor-specific marker rhodopsin.

CONCLUSIONS

This study suggests that pre-induced hPBMCs may be a potential cell source of cell replacement therapy for retinal degenerative diseases.

Donor-derived CD4(+)/CCR7(+) T-cell partial selective depletion does not alter acquired anti-infective immunity

In previous studies, we observed that a high proportion of donor-derived CD4(+) T cells expressing the chemokine receptor 7 (CCR7) was a major determinant of acute GVHD, without interfering with the incidence of other post-transplant outcomes, especially relapse and nonrelapse mortality rates. Here, we investigated in vitro the impact of partially selective CD4(+)/CCR7(+) T lymphocytes on acquired anti-infective immune response in 10 donors who underwent G-CSF-primed PBSC collection. Similar quantitative and functional proliferative reactions were observed in lymphocyte cultures in the presence of adenovirus and pp65 Ags with unmanipulated and partially depleted donor samples. No responses were observed in the presence of human T-cell lymphotropic virus type 1 used as a negative control. These results complete the proof of concept needed to build a clinical trial investigating partially selective CD4(+)/CCR7(+) T cell-depleted allo-SCT.

Characterization of various blood and graft sources: a prospective series

BACKGROUND

Studies comparing cell components of blood and graft sources are very scarce. We present here a thorough study examining the cellular content of various sources of blood and cell therapy products.

STUDY DESIGN AND METHODS

We have prospectively compared by fluorescence-activated cell sorting analyses the cellular composition of three blood sources on the one hand--peripheral blood (PB; n = 10) versus granulocyte-colony-stimulating factor (G-CSF)-mobilized PB (GCSF-PB, n = 10) versus cord blood (CB, n = 10)--and of three graft sources on the other hand--unmanipulated bone marrow (uBM, n = 5) versus leukapheresis product (LP, n = 10) versus thawed CB graft (n = 7).

RESULTS

All median absolute numbers of cell subsets were found significantly higher in GCSF-PB and LP, except for monocytoid dendritic cells (mDCs) in CB and uBM. The most impressive results were the median quantities of memory T and B lymphocytes but also of plasmacytoid DCs (pDCs) contained in LP compared to thawed CB graft, with ratios of 375, 318, and 247, respectively. The proportions of naive and CD4+/CD8- T cells, transitional B cells, and CD5+ and naive B lymphocytes were found significantly higher in CB samples while the proportions of mDCs and pDCs were found significantly lower.

CONCLUSION

Our study shows strong differences in terms of quantitative and qualitative cellular composition between several blood or graft sources, possibly explaining the differences observed in terms of outcomes after transplant.

Successful transplantation of ethnically mismatched cord blood in a boy with atypical chronic myeloid leukemia

In the present study, we describe unrelated umbilical cord blood transplantation (CBT) in a 7-year-old Taiwanese boy with atypical chronic myeloid leukemia (BCR-ABL 1 negative). Physical examination was notable for splenomegaly. Cytogenetic analyses from the bone marrow revealed a t(3;5)(p21;q31) translocation. The patient then underwent CBT from an HLA-mismatched (two loci by serotype, three loci by genotype) unrelated donor of Caucasian origin. Times to neutrophil and platelet engraftment were 21 and 62 days post-transplant, respectively. Acute graft-versus-host disease following transplantation was minimal. The patient remains in continuing hematological remission with full donor chimerism 28 months after transplantation.

Renal thrombotic microangiopathy associated with chronic graft-versus-host disease after allogeneic hematopoietic stem cell transplantation

Thrombotic microangiopathy (TMA) is a major complication after hematopoietic stem cell transplantation (HSCT). In this study, we examined the clinical and pathologic features of 2 patients and 5 autopsy cases with HSCT-associated renal TMA to clarify the association between graft-versus-host disease (GVHD) and renal TMA. The median interval between HSCT and renal biopsy or autopsy was 7 months (range 3-42 months). Clinically, acute and chronic GVHD occurred in 7 and 4 patients, respectively. Clinical evidence for TMA was detected in 2 patients, while chronic kidney disease developed in all patients. The main histopathological findings were diffuse endothelial injury in glomeruli, peritubular capillaries (PTCs), and small arteries. In addition, all cases showed glomerulitis, renal tubulitis, and peritubular capillaritis with infiltration of CD3+ T cells and TIA-1+ cytotoxic cells, suggesting that GVHD occurred during the development of TMA. Diffuse and patchy C4d deposition was noted in glomerular capillaries and PTCs, respectively, in 2 biopsy and 2 autopsy cases, suggesting the involvement of antibody-mediated renal endothelial injury in more than 50% of renal TMA cases. In conclusion, the kidney is a potential target of chronic GVHD that may induce the development of HSCT-associated TMA. Importantly, some cases are associated with chronic humoral GVHD.

Dendritic cells in hepatitis C infection: can they (help) win the battle?

Infection with hepatitis C virus (HCV) is a public health problem; it establishes a chronic course in ~85% of infected patients and increases their risk for developing liver cirrhosis, hepatocellular carcinoma, and significant extrahepatic manifestations. The mechanisms of HCV persistence remain elusive and are largely related to inefficient clearance of the virus by the host immune system. Dendritic cells (DCs) are the most efficient inducers of immune responses; they are capable of triggering productive immunity and maintaining the state of tolerance to self- and non-self antigens. During the past decade, multiple research groups have focused on DCs, in hopes of unraveling an HCV-specific DC signature or DC-dependent mechanisms of antiviral immunity which would lead to a successful HCV elimination strategy. This review incorporates the latest update in the current status of knowledge on the role of DCs in anti-HCV immunity as it relates to several challenging questions: (a) the phenotype and function of diverse DC subsets in HCV-infected patients; (b) the characteristics of non-human HCV infection models from the DCs' point of view; (c) how can in vitro systems, ranging from HCV protein- or peptide-exposed DC to HCV protein-expressing DCs, and in vivo systems, ranging from HCV protein-expressing transgenic mice to HCV-infected non-human primates, be employed to dissect the role of DCs in triggering/maintaining a robust antiviral response; and (d) the prospect of DC-based strategy for managing and finding a cure for HCV infection.

Alloantigen specific deletion of primary human T cells by Fas ligand (CD95L)-transduced monocyte-derived killer-dendritic cells

Numerous studies have been performed in vitro and in various animal models to modulate the interaction of dendritic cells (DC) and T cells by Fas (CD95/Apo-1) signalling to delete activated T cells via induction of activation-induced cell death (AICD). Previously, we could demonstrate that Fas ligand (FasL/CD95L)-expressing 'killer-antigen-presenting cells' can be generated from human monocyte-derived mature DC (mDC) using adenoviral gene transfer. To evaluate whether these FasL-expressing mDC (mDC-FasL) could eliminate alloreactive primary human T cells in vitro, co-culture experiments were performed. Proliferation of human T cells was markedly reduced in primary co-cultures with allogeneic mDC-FasL, whereas a strong proliferative T-cell response could be observed in co-cultures with enhanced green fluorescent protein-transduced mDC. Inhibition of T-cell proliferation was related to the transduction efficiency, and the numbers of mDC-FasL present in co-cultures. In addition, proliferation of pre-activated alloreactive CD4(+) and CD8(+) T cells could be almost completely inhibited in secondary co-cultures using mDC-FasL as stimulatory cells, which was the result of induction of apoptosis in the majority of preactivated T cells. The specific deletion of alloreactive T cells by mDC-FasL was confirmed by an unaffected proliferative response of surviving T cells towards allogeneic 'third-party' peripheral blood mononuclear cells in a third stimulation, or upon unspecific stimulation with anti-CD3/CD28 beads. The results of this study demonstrate that allospecifically activated T cells are efficiently eliminated by mDC-FasL, supporting further investigations to apply FasL-expressing 'killer-DC' as a novel strategy for the treatment of allograft rejection.

Antileukemia and antitumor effects of the graft-versus-host disease: a new immunovirological approach

In leukemic mice, the native host's explicit and well-defined immune reactions to the leukemia virus (a strong exogenous antigen) and to leukemia cells (pretending in their native hosts to be protected "self" elements) are extinguished and replaced in GvHD (graft-versus-host disease) by those of the immunocompetent donor cells. In many cases, the GvHD-inducer donors display genetically encoded resistance to the leukemia virus. In human patients only antileukemia and anti-tumor cell immune reactions are mobilized; thus, patients are deprived of immune reactions to a strong exogenous antigen (the elusive human leukemia-sarcoma retroviruses). The innate and adaptive immune systems of mice have to sustain the immunosuppressive effects of leukemia-inducing retroviruses. Human patients due to the lack of leukemiainducing retroviral pathogens (if they exist, they have not as yet been discovered), escape such immunological downgrading. After studying leukemogenic retroviruses in murine and feline (and other mammalian) hosts, it is very difficult to dismiss retroviral etiology for human leukemias and sarcomas. Since no characterized and thus recognized leukemogenic-sarcomagenic retroviral agents are being isolated from the vast majority of human leukemias-sarcomas, the treatment for these conditions in mice and in human patients vastly differ. It is immunological and biological modalities (alpha interferons; vaccines; adoptive lymphocyte therapy) that dominate the treatment of murine leukemias, whereas combination chemotherapy remains the main remission-inducing agent in human leukemias-lymphomas and sarcomas (as humanized monoclonal antibodies and immunotoxins move in). Yet, in this apparently different backgrounds in Mus and Homo, GvHD, as a treatment modality, appears to work well in both hosts, by replacing the hosts' anti-leukemia and anti-tumor immune faculties with those of the donor. The clinical application of GvHD in the treatment of human leukemias-lymphomas and malignant solid tumors remains a force worthy of pursuit, refinement and strengthening. Graft engineering and modifications of the inner immunological environment of the recipient host by the activation or administration of tumor memory T cells, selected Treg cells and natural killer (NKT) cell classes and cytokines, and the improved pharmacotherapy of GvHD without reducing its antitumor efficacy, will raise the value of GvHD to the higher ranks of the effective antitumor immunotherapeutical measures. Clinical interventions of HCT/HSCT (hematopoietic cell/stem cell transplants) are now applicable to an extended spectrum of malignant diseases in human patients, being available to elderly patients, who receive non-myeloablative conditioning, are re-enforced by post-transplant donor lymphocyte (NK cell and immune T cell) infusions and post-transplant vaccinations, and the donor cells may derive from engineered grafts, or from cord blood with reduced GvHD, but increased GvL/GvT-inducing capabilities (graft-versus leukemia/tumor). Post-transplant T cell transfusions are possible only if selected leukemia antigen-specific T cell clones are available. In verbatim quotation: "Ultimately, advances in separation of GvT from GvHD will further enhance the potential of allogeneic HCT as a curative treatment for hematological malignancies" (Rezvani, A.R. and Storb, R.F., Journal of Autoimmunity 30:172-179, 2008 (see in the text)). It may be added: for cure, a combination of the GvL/T effects with new targeted therapeutic modalities, as elaborated on in this article, will be necessary.

Peripheral blood progenitor cell product contains Th1-biased noninvariant CD1d-reactive natural killer T cells: implications for posttransplant survival

OBJECTIVE

Bone marrow (BM) Th1 populations can contribute to graft-vs-leukemia responses. Granulocyte/granulocyte macrophage colony-stimulating factor (CSF)-mobilized peripheral blood progenitor cells (PBPC) have become widely accepted alternatives to BM transplantation. T cells coexpressing natural killer cell proteins (NKT) include a CD1d-reactive subset that influences immunity by rapidly producing large amounts of Th1 and/or Th2 cytokines dependent upon microenvironment and disease. There are two types of CD1d-reactive NKT. iNKT express a semi-invariant T-cell receptor-alpha. Other noninvariant CD1d-reactive NKT from BM and liver produce large amounts of interleukin-4 or interferon-gamma, respectively, and within the intestine can be biased in either direction. Recent data suggests that NKT might contribute to clinical benefits of PBPC.

MATERIALS AND METHODS

To address these issues, we phenotypically and functionally studied PBPC NKT.

RESULTS

Similarly to BM, NKT-like cells were common in allogeneic and autologous PBPC, there were relatively few classical iNKT, but high CD1d-reactivity concentrated in NKT fractions. Significantly, PBPC CD1d-reactive cells were relatively Th1-biased and their presence was associated with better prognosis. Granulocyte CSF treatment of BM to yield PBPC in vivo as well as in vitro Th2-polarizes conventional T cells and iNKT. However, granulocyte CSF treatment of BM in vitro produced Th1-biased NKT, providing a mechanism for opposite polarization of NKT from BM vs PBPC.

CONCLUSIONS

These results suggest distinct Th1 CD1d-reactive NKT cells could stimulate anti-tumor responses from those previously described, which can suppress graft-vs-host disease.

Immune reconstitution after autologous hematopoietic transplantation with Lin-, CD34+, Thy-1lo selected or intact stem cell products

In sequential studies, we compared immune reconstitution following high-dose chemotherapy (HDT) and stem cell transplantation (SCT) using intact mobilized peripheral blood stem cell (PSC) in intermediate grade non-Hodgkin's lymphoma (NHL) patients and CD34(+), lineage-negative (Lin(-)), Thy-1(lo) (CD34(+)Lin(-)Thy-1(lo)) stem cells in low-grade NHL patients. Cytokine expression and cellular phenotype and function were used as the basis of comparison. Despite differences in cellular composition of the stem cell grafts, immune reconstitution in both groups was similar. Significantly higher levels of type 1- and 2-associated cytokine messenger ribonucleic acid (mRNA) were observed both prior to and following transplant in the peripheral blood (PB) of both cohorts as compared to normal individuals. Similar levels of interleukin (IL)-4, IL-10, interferon-gamma (IFN-gamma), and tumor necrosis factor-alpha (TNF-alpha) messenger ribonucleic acid (mRNA) were seen in PB mononuclear cells following transplant with either product. In contrast, patients receiving isolated CD34(+)Lin(-)Thy-1(lo) cells expressed significantly higher IL-2 levels at all times examined post-transplant. Despite the high levels of cytokine gene expression and rapid restoration to pretransplant levels of CD3 cell number by day 30, T cell function and CD4:CD8 and CD4(+)CD45RA:CD4(+)CD45RO(+) ratios were significantly depressed in both cohorts compared to normal donors, and significantly lower in patients transplanted with CD34(+)Lin(-)Thy-1(lo) compared to patients receiving an intact PSC product. These data suggest that the peripheral tolerance in patients receiving HDT and an autologous SCT occurs independent of graft composition, although immune function and CD4 recovery are better facilitated by transplantation of an intact product.

Evaluation of CD34+ - and Lin- -selected cells from peripheral blood stem cell grafts of patients with lymphoma during differentiation in culture ex vivo using a cDNA microarray technique

OBJECTIVE

Hematopoietic stem cells (enriched in fraction of CD34+ cells) have the ability to regenerate hematopoiesis in all of its lineages, and this potential is clinically used in transplanting bone marrow or peripheral blood stem cells. Our objective was to assemble a suitable method for evaluating gene expression in enriched populations of hematopoietic stem cells. We compared biologic properties of cells cultured ex vivo obtained using two different ways of immunomagnetic separation (positive selection of CD34+ cells and negative selection of Lin- cells) by means of a cDNA microarray technique.

METHODS

CD34+ and Lin- cells were enriched from peripheral blood stem cell (PBSCs) grafts of patients with non-Hodgkin's lymphoma. Isolated cells were in the presence of cytokine PBSCs, Flt-3 ligand, interleukin-3, interleukin-6, and granulocyte colony-stimulating factor. At days 0, 4, 6, 8, 10, 12, and 14 cells were harvested and analyzed by cDNA microarrays. Total cell expansion, CD34+, colony-forming unit for granulocyte-macrophage and megakaryocytes expansion, vitality, and phenotype of cells were also analyzed.

RESULTS

cDNA microarray analysis of cultured hematopoietic cells proved equivalence of the two enrichment methods for PBSC samples and helped us characterize differentiating cells cultured ex vivo.

CONCLUSION

Our methodologic approach is helpful in characterizing cultured hematopoietic cells cultured ex vivo, but it is also suitable for more general purposes. Equivalence of CD34+ and Lin- selection methods from PBSC samples proved by cDNA microarray may have an implication for graft manipulation in an experimental setting of hematopoietic transplantation. Total cell expansion and colony formation and phenotype from CD34+ selected and from Lin- samples were comparable.

Peritransplantation vaccination with chaperone-rich cell lysate induces antileukemia immunity

We have reported that chaperone-rich cell lysate (CRCL) is an effective anticancer vaccine in immunocompetent mice. In this study, we explored the therapeutic applicability of CRCL in the context of syngeneic hematopoietic cell transplantation (HCT) to treat preexisting leukemia. Our results demonstrate that tumor growth is significantly delayed in mice receiving syngeneic HCT from 12B1 tumor CRCL-immunized donors compared with animals receiving HCT from nonimmunized donors. CRCL immunization after immune HCT further hindered tumor growth when compared with immune HCT without posttransplantation vaccination. The magnitude of the immune response was consistent with the antitumor effects observed in vivo. Rechallenge of surviving mice with 12B1 or A20 cells in opposite groins confirmed that mice had developed long-term tumor-specific immunity against 12B1 tumor cells. In addition, we documented that both T cells and natural killer cells contributed to the antitumor effect of CRCL vaccination, because depletion of either subset hampered tumor growth delay. Thus, our results indicate that CRCL is a promising vaccine capable of generating specific immune responses. This antitumor immunity can be effectively transferred to a host via HCT and further enhanced after HCT with additional tumor CRCL immunizations.

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.