Thymus and immune reconstitution after allogeneic hematopoietic stem cell transplantation in humans: never say never again

Assessment of the host immune status is becoming a key issue in allogeneic hematopoietic stem cell transplantation (allo-HSCT). In the long-term follow-up of these patients, severe post-transplant infections, relapse or secondary malignancies may be directly related to persistent immune defects. In allo-HSCT, T-cell differentiation of donor progenitors within the recipient thymus is required to generate naive recent T-cell emigrants (RTE). These cells account for a durable T-cell reconstitution, generating a diverse T-cell receptor (TCR) repertoire and robust response to infections. It is now possible to quantify the production of RTE by measuring thymic T-cell receptor excision circles or 'TREC' which are small circular DNA produced during the recombination of the genomic segments encoding the TCR alpha chain. Here we discuss the role of thymic function in allo-HSCT. The pre-transplant recipient thymic function correlates with clinical outcome in terms of survival and occurrence of severe infections. Post-transplant, TREC analysis showed that the thymus is a sensitive target to the allogeneic acute graft-versus-host disease (GvHD) reaction but is also prone to recovery in young adult patients. In all, thymus is a key player for the quality of immune reconstitution and clinical outcome after allo-HSCT. Thymic tissue is plastic and it is a future challenge to halt or reverse thymic GVHD therapeutically by acting at the level of T-cell progenitors generation, thymic homing and/or epithelial thymic tissue preservation.     

Clinical options after failure of allogeneic hematopoietic stem cell transplantation in patients with hematologic malignancies

Disease recurrence is the single most common cause of death after allogeneic or autologous hematopoietic stem cell transplantation (HSCT). Disease status and chemosensitivity at the time of transplantation, as well as the development of graft-versus-host disease (GVHD), are factors known to influence the risk of relapse post-HSCT. Both acute and chronic GVHD have been associated with decreased relapse rates; however, owing to toxicity, overall survival is not consistently improved in these patients. Furthermore, there is a transient period of immunodeficiency after HSCT, which may permit residual malignant cells to proliferate early in the post-transplant course, before the donor immune system can establish a graft-versus-tumor response. Patients who fail an initial HSCT have an extremely poor outcome; therefore, maneuvers to prevent, identify and treat recurrent disease as early as possible in these situations are necessary. Strategies to distinguish graft-versus-tumor from GVHD, to enhance both general and disease-specific immune reconstitution after transplantation, and to increase donor-mediated anti-host immune reactions are being investigated in clinical trials. Single agent nontoxic post-HSCT chemotherapy, cellular therapies and second allogeneic HSCT using reduced intensity regimens are among the modalities under investigation.     

T-cell receptor Vbeta repertoire after myeloablative and reduced intensity conditioning allogeneic haematopoietic stem cell transplantation

T cells play an important role in the adaptive immune system. After haematopoietic stem cell transplantation (HSCT), T-cell function is impaired. This is reflected by the emergence of opportunistic infections, infections that are often difficult to treat because of the patient's insufficient immune function. T-cell receptor reconstitution was studied using CDR3 spectratyping to analyze the diversity of the T-cell repertoire at 3, 6 and 12 months after myeloablative and reduced intensity conditioning (RIC) HSCT in 23 patients. Immune function in vitro was tested by lymphocyte stimulation at 3, 6 and 12 months after HSCT. Lower diversity in the CDR3 repertoire was demonstrated in CD4+ cells after RIC HSCT at 3 and 6 months and in CD8+ cells at 3 months compared with healthy donors. After myeloablative HSCT, lower diversity was seen at 3, 6 and 12 months in CD4+ cells and at 6 and 12 months in CD8+ cells after HSCT. Acute and chronic graft-versus-host-disease (GVHD) did not affect diversity. Responses to phytohaemagglutinin (PHA), Concanavalin A (Con A) and Staphylococcus aureus protein A were significantly lower compared with healthy donors during the first 6 months after RIC HSCT. After myeloablative HSCT, lymphocyte response to Con A was significantly lower at 3 months compared with healthy donors. Decreased responses to cytomegalovirus and varicella zoster virus antigens were seen in patients suffering from acute GVHD grade II or chronic GVHD. The T-cell repertoire is skewed under the first year after HSCT, and immune reconstitution after HSCT with myeloablative and RIC conditioning seems to be comparable. GVHD, infections and age are more important for immune reconstitution than type of conditioning.     

Impact of fetal-maternal tolerance in hematopoietic stem cell transplantation

Allogeneic hematopoietic stem cell transplantation (HSCT) is known to cure various hematological disorders; however, its widespread use is limited due to a lack of histocompatible donors. Reciprocal cell traffic between the mother and fetus during pregnancy gives rise to postpartum fetal-maternal lymphohematopoietic microchimerism, which is frequently detected in the blood or tissue of healthy individuals. Studies in clinical and experimental transplantation provide evidence that exposure to non-inherited maternal antigens (NIMAs) during pregnancy may result in long-lasting fetomaternal microchimerism and tolerance induction. Studies of HLA-mismatched HSCT have suggested a relatively lower incidence of severe graft-versus-host disease (GVHD) after transplantation from a NIMA-mismatched donor. Studies using a mouse model have also demonstrated a “child-to-mother” bone marrow transplantation from an NIMA-exposed donor to reduce the morbidity and mortality of GVHD in an antigen-specific manner while preserving the graft-versus-leukemia effects and favoring the immune reconstitution, thus resulting in a marked improvement in outcome after HSCT. Prospective clinical studies are therefore warranted to confirm these beneficial effects of fetal-maternal tolerance in allogeneic HSCT.     

Reconstitution of Th17, Tc17 and Treg cells after paediatric haematopoietic stem cell transplantation: Impact of interleukin-7

Successful reconstitution of T lymphocytes after allogeneic haematopoietic stem cell transplantation (HSCT) is needed to establish the graft-versus-leukaemia effect and an effective anti-microbial defense, but the ratio between functionally different T-cell subsets needs to be balanced to avoid graft-versus-host disease (GVHD).IL-7 is essential for T-cell generation in the thymus and peripheral T-cell homeostasis. High IL-7 levels have been associated with impaired T-cell reconstitution, increased risk of acute GVHD and treatment-related mortality, but the underlying cellular mechanisms behind these associations have not been investigated previously. We hypothesized that increased levels of IL-7 post-transplant alters the balance between immune-regulatory T cell subsets during the post-transplant lymphocyte recovery towards a more pro-inflammatory profile.We quantified Th17 cells, Tc17 cells and Tregs in 29 children following HSCT. Th17 cell and Treg counts rose significantly from day +90 to +180 post-HSCT, and prior acute GVHD was associated with significant changes in the concentration of Tregs (9.4 × 10⁶/L vs. 1.3 × 10⁶/L, P = 0.0052) and the Th17/Treg ratio (1.5 vs. 4.2, P = 0.025). The plasma level of IL-7 at day +90 correlated inversely with Th17 cell counts (r_s = ?0.65, P = 0.0002) and the proportion of Tc17 cells (r_s = 0.64, P = 0.0005) at day +90, but not with Tregs. Furthermore, high IL-7 levels at day +7 were predictive of a less naïve T-cell phenotype at day +90.These findings add further evidence that IL-7 is a key regulatory factor that may tune the balance between functionally different T-cell subsets following HSCT.     

Reconstitution of the Ig heavy chain CDR3 repertoire after allogeneic haematopoietic stem cell transplantation with myeloablative or reduced-intensity conditioning regimens

The objective of this study was to investigate B-lymphocyte reconstitution in patients undergoing allogeneic haematopoietic stem cell transplantation (HSCT) after myeloablative conditioning (MAC) or reduced-intensity conditioning (RIC) regimens. B-lymphocyte reconstitution was studied by monitoring the CDR3 repertoire with spectratyping. We demonstrate a delay in the recovery of the B-lymphocyte repertoire, measured by variation in size distribution of the immunoglobulin H CDR3 in patients conditioned with RIC compared to MAC. We found no general explanation for this finding, but when clinical data for each patient were studied in detail, we could identify a cause for the oligoclonality of the B-lymphocyte repertoire after HSCT with RIC for each of the patients. Older patients and donors, low cell dose at transplantation, relapse, graft-versus-host disease (GVHD) and its treatment as well as cytomegalovirus infection and its treatment are all possible causes for the restriction of the B-lymphocyte repertoire observed in this study. Taken together, reconstitution of the B-lymphocyte repertoire after HSCT is a process dependent on multiple factors and differs between patients. The conditioning regimen may be of importance, but data from this study suggest that individual factors and the various complications occurring after HSCT are more likely to determine the development of the B-lymphocyte repertoire.     

Allogeneic hematopoietic transplantation and natural killer cell recognition of missing self

Summary:  Although the optimal donor for allogeneic hematopoietic stem cell transplantation (HSCT) is a human leukocyte antigen-matched sibling, 75% of patients do not have a match, and alternatives are matched unrelated volunteers, unrelated umbilical cord blood units, and full-haplotype-mismatched family members. To cure leukemia, allogeneic HSCT relies on donor T cells in the allograft, which promote engraftment, eradicate malignant cells, and reconstitute immunity. Here, we focus on the open issues of rejection, graft-versus-host disease (GVHD), and infections and the benefits of natural killer (NK) cell alloreactivity and its underlying mechanisms. Donor-versus-recipient NK cell alloreactivity derives from a mismatch between inhibitory receptors for self-major histocompatibility complex (MHC) class I molecules on donor NK clones and the MHC class I ligands on recipient cells. These NK clones sense the missing expression of the self-MHC class I allele on the allogeneic targets and mediate alloreactions. HSCT from 'NK alloreactive' donors controls acute myeloid relapse without causing GVHD. We review the translation of NK cell recognition of missing self into the clinical practice of allogeneic hematopoietic transplantation and discuss how it has opened innovative perspectives in the cure of leukemia.     

A DL-4- and TNFα-based culture system to generate high numbers of nonmodified or genetically modified immunotherapeutic human T-lymphoid progenitors

Several obstacles to the production, expansion and genetic modification of immunotherapeutic T cells in vitro have restricted the widespread use of T-cell immunotherapy. In the context of HSCT, delayed naïve T-cell recovery contributes to poor outcomes. A novel approach to overcome the major limitations of both T-cell immunotherapy and HSCT would be to transplant human T-lymphoid progenitors (HTLPs), allowing reconstitution of a fully functional naïve T-cell pool in the patient thymus. However, it is challenging to produce HTLPs in the high numbers required to meet clinical needs. Here, we found that adding tumor necrosis factor alpha (TNFα) to a DL-4-based culture system led to the generation of a large number of nonmodified or genetically modified HTLPs possessing highly efficient in vitro and in vivo T-cell potential from either CB HSPCs or mPB HSPCs through accelerated T-cell differentiation and enhanced HTLP cell cycling and survival. This study provides a clinically suitable cell culture platform to generate high numbers of clinically potent nonmodified or genetically modified HTLPs for accelerating immune recovery after HSCT and for T-cell-based immunotherapy (including CAR T-cell therapy).     

Enrichment of IL-12–Producing Plasmacytoid Dendritic Cells in Donor Bone Marrow Grafts Enhances Graft-versus-Leukemia Activity in Allogeneic Hematopoietic Stem Cell Transplantation

A critical question in the field of allogeneic hematopoietic stem cell transplantation (HSCT) is how to enhance graft-versus-leukemia (GVL) activity while limiting graft-versus-host-disease (GVHD). We have previously reported that donor bone marrow (BM) precursors of plasmacytoid dendritic cells (pre-pDCs) can polarize donor T cells toward Th1 immunity and augment the GVL activity of donor T cells while attenuating their GVHD activity in a murine model of allogeneic HSCT. Clinical data on the role of donor pre-pDCs and conventional DCs (cDCs) on transplantation outcomes has been conflicting. To test the effect of increasing the proportion of pre-pDCs versus cDCs in a BM graft, we enriched CD11b⁻ pDCs by selectively depleting the CD11b⁺ myeloid DC (mDC) population from BM using FACS sorting in a murine model of allogeneic BM transplantation. Donor T cell expansion and GVL activity were greater in mice that received BM depleted of mDCs compared with mice that received undepleted BM. GVHD was not increased by depleting mDCs. To examine the mechanism through which mDC depletion enhances the GVL activity of donor T cells, we used BM and pDCs from IL-12p40KO mice, and found that the increased GVL activity of mDC-depleted BM was IL-12–dependent. This study indicates that a clinically translatable strategy of engineering the DC content of grafts can improve clinical outcomes in allogeneic HSCT through the regulation of donor T cell activation and GVL activity.     

Antigen and Lymphopenia-Driven Donor T Cells Are Differentially Diminished by Post-Transplantation Administration of Cyclophosphamide after Hematopoietic Cell Transplantation

Administration of cyclophosphamide after transplantation (post-transplantation cyclophosphamide, PTC) has shown promise in the clinic as a prophylactic agent against graft-versus-host disease (GVHD). An important issue with regard to recipient immune function and reconstitution after PTC is the extent to which, in addition to diminution of antihost allo-reactive donor T cells, the remainder of the nonhost allo-reactive donor T cell pool may be affected. To investigate PTC's effects on nonhost reactive donor CD8 T cells, ova-specific (OT-I) and gp100-specific Pmel-1 T cells were labeled with proliferation dyes and transplanted into syngeneic and allogeneic recipients. Notably, an intermediate dose (66 mg/kg) of PTC, which abrogated GVHD after allogeneic HSCT, did not significantly diminish these peptide-specific donor T cell populations. Analysis of the rate of proliferation after transplantation illustrated that lymphopenic-driven, donor nonhost reactive TCR Tg T cells in syngeneic recipients underwent slow division, resulting in significant sparing of these donor populations. In contrast, after exposure to specific antigens at the time of transplantation, these same T cells were significantly depleted by PTC, demonstrating the global susceptibility of rapidly dividing T cells after an encounter with cognate antigen. In total, our results, employing both syngeneic and allogeneic minor antigen-mismatched T cell replete models of transplantation, demonstrate a concentration of PTC that abrogates GVHD can preserve most cells that are dividing because of the accompanying lymphopenia after exposure. These findings have important implications with regard to immune function and reconstitution in recipients after allogeneic hematopoietic stem cell transplantation.     

The transplantation of hematopoietic stem cells after non-myeloablative conditioning: a cellular therapeutic approach to hematologic and genetic diseases

Originally, allogeneic hematopoietic stem cell transplantation (HSCT) was viewed as a form of rescue from the marrow lethal effects of high doses of chemo-radiotherapy used to both eradicate malignancy and to provide sufficient immunosuppression to ensure allogeneic engraftment. Clear evience of a therapeutic graft-versus-tumor (GVT) effect mediated by allogeneic affector cells (T cells) has prompted the exploration of HSCT regimens that rely solely upon host immunosuppression (non-myeloblative) to facilitate allogenic donor engraftment. The engrafted donor effector cells are then used to accomplish the task of eradicating host malignant cells. The non-myeloblative regimen developed in Seattle uses 2 Gy total body irradiation (TBI) before transplant followed by postgrafting cyclosporine (CSP) and mycophenolate mofetil (MMF). This regimen resulted in initial mixed donor-host chimerism in all patients with hematologic malignancies and genetic disorders who received HLA-matched sibling allografts. The 17% incidence of graft rejection was reduced to 3% with the addition of fludarabine, 30 mg/m²/day on d-4,-3, and-2. The non-myeloblative combination of fludarabine/TBI has also been successful at achieving high engraftment rates in recipients of 10 of 10 HLA antigen matched unrelated donor HSCTs in patients with hematologic malignancies. By reducing acute toxicities relative to conventional HSCT, most patients have received their pre- and post-HSCT therapy almost exclusively as outpatients. Acute and chronic GVHD occur after non-myeloablative HSCT, but the incidence and severity appear less compared to conventional HSCT. As in conventional transplants, immune dysregulation from GVHD and its treatment and delayed reconstitution of immune function continue to present risks to patients who have otherwise undergone successful non-myeloablative HSCT. Cellular therapeutic effects have been nobserved after non-myeloblative HSCT such as correction of inherited genetic disorders, and eradication of hematologic malignant diseases and renal cell carcinoma via GVT responses.     

Clinical Utility of Tetramer-Based Immune Monitoring in Allogeneic Stem Cell Transplantation

The construction and use of Class I human leucocyte antigen (HLA) tetramers has, for the first time, allowed the direct enumeration of CD8(+) T lymphocytes specific for the antigen of interest. Tetramer staining can be combined with functional assays of antigen-specific T cells measuring their production of intracellular cytokines after short-term stimulation with antigen. The advantages of flow cytometric tetramer-based assays are their short turn-around time and their amenability to standardisation. Currently, their main limitation is that only a limited number of Class I HLA tetramers are available. This situation may bias the information derived from such studies. Nevertheless, clinically useful information has been obtained in tetramer-based studies of the regeneration of cytomegalovirus (CMV) and Epstein-Barr virus (EBV)-specific CD8(+) T cells after allogeneic stem cell transplantation (SCT). Following myeloablative cytoreductive therapy and SCT, a period of deep cellular immunodeficiency follows until the donor-derived immune system has sufficiently regenerated. As a result of the lack of immunosurveillance, endogenous viruses such as CMV and EBV may reactivate and cause disease. In order to prevent these complications, pre-emptive therapeutic interventions are made, in which antiviral treatment is administered based on frequent viral load measurements. In this way, overtreatment associated with prophylactic strategies is reduced; however, a subgroup of patients developing recurrent reactivations and/or disease remains. Interventions aimed at the prevention of graft versus host disease (GVHD) and the reduction of its severity greatly reduce the rate of (virus-specific) T-cell reconstitution and hence, increase the frequency and severity of viral reactivations. Specifically, T-cell depletion of SCT and the use of antithymocyte globulin as part of the conditioning regimen reduces the (virus-specific) T lymphocytes transferred with the graft, which otherwise would have contributed to the first phase of T-cell regeneration post-SCT. Consequently, patients whose CMV- and EBV-specific CD8(+) T cells fail to regenerate to levels detectable by tetramer-based flow cytometry during the first 3-6 months post-SCT were at highly increased risk for CMV disease and EBV(+) B-lymphoproliferative disease, respectively. Furthermore, delayed reconstitution of the CD4(+) T-cell compartment results in the lack of adequate help for the generation of sufficient antiviral CD8(+) T cells. Conversely, adoptive immunotherapy using CMV- or EBV-specific T cells results in the swift restoration of virus-specific T-cell immunity and the reduction of viral load, unless corticosteroids have to be administered to treat concurrent GVHD. Studies have shown the clinical utility of tetramer-based immune monitoring in the setting of allogeneic SCT and indicate that such assays, extended by functional studies of the virus-specific T cells, may constitute a valuable extension of current viral load monitoring strategies.     

Ex vivo fludarabine exposure inhibits graft-versus-host activity of allogeneic T cells while preserving graft-versus-leukemia effects.

Allogeneic donor T cells in bone marrow transplantation (BMT) can contribute to beneficial graft-versus-leukemia (GVL) effects but can also cause detrimental graft-versus-host disease (GVHD). A successful method for the ex vivo treatment of donor T cells to limit their GVHD potential while retaining GVL activity would have broad clinical applications for patients undergoing allogeneic hematopoietic cell transplantation for malignant diseases. We hypothesized that donor lymphocyte infusions treated with fludarabine, an immunosuppressive nucleoside analog, would have reduced GVHD potential in a fully major histocompatibility complex-mismatched C57BL/6 --> B10.BR mouse BMT model. Recipients of fludarabine-treated donor lymphocyte infusions (F-DLI) had significantly reduced GVHD mortality, reduced histopathologic evidence of GVHD, and lower inflammatory serum cytokine levels than recipients of untreated DLI. Combined comparisons of GVHD incidence and donor-derived hematopoietic chimerism indicated that F-DLI had a therapeutic index superior to that of untreated DLI. Furthermore, adoptive immunotherapy of lymphoblastic lymphoma using F-DLI in the C57BL/6 --> B10.BR model demonstrated a broad therapeutic index with markedly reduced GVHD activity and preservation of GVL activity compared with untreated allogeneic T cells. Fludarabine exposure markedly reduced the CD4+CD44(low)-naive donor T-cell population within 48 hours of transplantation and altered the relative representation of cytokine-producing CD4+ T cells, consistent with T-helper type 2 polarization. However, proliferation of fludarabine-treated T cells in allogeneic recipient spleens was equivalent to that of untreated T cells. The results suggest that fludarabine reduces the GVHD potential of donor lymphocytes through effects on a CD4+CD44(low) T-cell population, with less effect on alloreactive T cells and CD4+CD44(high) memory T cells that are able to mediate GVL effects. Thus, F-DLI represents a novel method of immune modulation that may be useful to enhance immune reconstitution among allograft recipients with reduced risk of GVHD while retaining beneficial GVL effects.     

Thank you to our reviewers

The role of dendritic cells (DCs) and macrophages in allogeneic haematopoietic stem cell transplant (HSCT) is critical in determining the extent of graft‐versus‐host response. The goal of this study was to analyse slanDCs, a subset of human proinflammatory DCs, in haematopoietic stem cell (HSC) sources, as well as to evaluate their 1‐year kinetics of reconstitution, origin and functional capacities in peripheral blood (PB) and bone marrow (BM) of patients who have undergone HSCT, and their presence in graft‐versus‐host disease (GVHD) tissue specimens. slanDCs were also compared to myeloid (m)DCs, plasmacytoid (p)DCs and monocytes in HSC sources and in patients' PB and BM throughout reconstitution. slanDCs accounted for all HSC sources. In patients' PB and BM, slanDCs were identified from day +21, showing median frequencies comparable to healthy donors, donor origin and kinetics of recovery similar to mDCs, pDCs, and monocytes. Under cyclosporin treatment, slanDCs displayed a normal pattern of maturation, and maintained an efficient chemotactic activity and capacity of releasing tumour necrosis factor (TNF)‐α upon lipopolysaccharide (LPS) stimulation. None the less, they were almost undetectable in GVHD tissue specimens, being present only in intestinal acute GVHD samples. slanDCs reconstitute early, being donor‐derived and functionally competent. The absence of slanDCs from most of the GVHD‐targeted tissue specimens seems to rule out the direct participation of these cells in the majority of the local reactions characterizing GVHD.     

Selective elimination of alloreactive donor T cells attenuates graft-versus-host disease and enhances T-cell reconstitution.

Impaired T-cell immune reconstitution is a major complication after allogeneic bone marrow transplantation (BMT) and is particularly exacerbated in the setting of graft-versus-host disease (GVHD). Conventional approaches to reduce GVHD, such as T-cell depletion or pharmacologic immunosuppression, typically fail to enhance T-cell immunity and often further exacerbate this problem. An alternative strategy to mitigate GVHD severity is the selective elimination of graft-versus-host-reactive donor T cells by using an incorporated thymidine kinase suicide gene. This approach has been shown to effectively reduce GVHD, although the effect of this strategy on T-cell reconstitution is unresolved. We addressed this question in a murine BMT model (C57BL/6 [H-2(b)] --> AKR/J [H-2(k)]) in which donor and recipient differ at major and minor histocompatibility antigens. Lethally irradiated AKR recipients transplanted with T cell-depleted bone marrow plus thymidine kinase-positive T cells followed by post-BMT ganciclovir (GCV) administration had more prompt and complete normalization of the T-cell repertoire than phosphate-buffered saline-treated GVHD control animals. By 60 days after transplantation, mice administered GCV had T-cell repertoires that were virtually indistinguishable from those of mice that underwent transplantation with T cell-depleted bone marrow alone (no GVHD controls) when assayed by T-cell receptor (TCR) spectratyping. In contrast, phosphate-buffered saline-treated animals had persistent skewing in most Vbeta families. T cells obtained from GCV-treated mice also had significantly higher in vitro proliferative responses after posttransplantation inoculation with ovalbumin than GVHD animals, indicating that CD4(+) T-cell responses against a nominal antigen were better preserved in these chimeras. Finally, GCV-treated mice had augmented immune reconstitution in response to exogenous interleukin-7 administration, as evidenced by increased overall spleen cellularity and absolute numbers of T and B cells. This was in contrast to GVHD control animals, which had a blunted response to interleukin-7 administration. These data indicate that GVHD severity can be significantly reduced by selective elimination of alloreactive donor T cells without compromise of T-cell immunity. Moreover, in light of previous studies demonstrating that this strategy can reduce GVHD without loss of alloengraftment and antileukemia reactivity, further examination of this approach in humans seems warranted.     

Bone marrow B cell precursor number after allogeneic stem cell transplantation and GVHD development

Patients without chronic graft-versus-host disease (cGVHD) have robust B cell reconstitution and are able to maintain B cell homeostasis after allogeneic hematopoietic stem cell transplantation (HSCT). To determine whether B lymphopoiesis differs before cGVHD develops, we examined bone marrow (BM) biopsies for terminal deoxynucleotidyl transferase (TdT) and PAX5 immunostaining early post-HSCT at day 30 when all patients have been shown to have high B cell activating factor (BAFF) levels. We found significantly greater numbers of BM B cell precursors in patients who did not develop cGVHD compared with those who developed cGVHD (median = 44 vs 2 cells/high powered field [hpf]; respectively; P < .001). Importantly, a significant increase in precursor B cells was maintained when patients receiving high-dose steroid therapy were excluded (median = 49 vs 20 cells/hpf; P = .017). Thus, we demonstrate the association of BM B cell production capacity in human GVHD development. Increased BM precursor B cell number may serve to predict good clinical outcome after HSCT.     

Strategies to accelerate immune recovery after allogeneic hematopoietic stem cell transplantation

The interplay existing between immune reconstitution and patient outcome has been extensively demonstrated in allogeneic hematopoietic stem cell transplantation. One of the leading causes of infection-related mortality is the slow recovery of T-cell immunity due to the conditioning regimen and/or age-related thymus damage, poor naïve T-cell output, and restricted T-cell receptor (TCR) repertoires. With the aim of improving posttransplantation immune reconstitution, several immunotherapy approaches have been explored. Donor leukocyte infusions are widely used to accelerate immune recovery, but they carry the risk of provoking graft-versus-host disease. This review will focus on sophisticated strategies of thymus function-recovery, adoptive infusion of donor-derived, allodepleted T cells, T-cell lines/clones specific for life-threatening pathogens, regulatory T cells, and of T cells transduced with suicide genes.     

Separating antiviral and GVHD activities of donor T cells prior to bone marrow transplantation

Approaches to speed immune reconstitution following bone marrow transplantation (BMT) or peripheral-blood hematopoietic stem-cell transplantation (HSCT) could markedly reduce morbidity and mortality, particularly following partially major histocompatibility complex (MHC)-matched related donor (PMRD) transplants. However, it is critical to simultaneously eliminate the subpopulation of donor T cells that are alloreactive with the recipient and may produce graft-vs-host disease (GVHD). In this article, we discuss a number of promising cellular engineering approaches that could be applied to this problem, including the use of veto cells, regulatory T-cell subsets, and psoralen-treated donor lymphocytes. Emphasis is placed on whether these approaches can simultaneously transfer broad-spectrum immunity to the recipient without producing GVHD.     

Longitudinal Analysis of T-Cell Receptor Variable β Chain Repertoire in Patients with Acute Graft-versus-Host Disease after Allogeneic Stem Cell Transplantation

T-cell receptor variable β chain (TCRBV) repertoire spectratyping involves the estimation of CDR3 length distributions for monitoring T-cell receptor diversity and has proven useful for analyses of immune reconstitution and T-cell clonal expansions in graft-versus-host disease (GVHD) and graft-versus-leukemia after allogeneic stem cell transplantation. We performed a longitudinal spectratype analysis of 23 TCRBV families in 28 patients who underwent allogeneic T cell–depleted peripheral blood stem cell transplantation. Sixteen patients subsequently developed acute GVHD. We recently developed statistical methods that bring increased power and flexibility to spectratype analysis and allow us to analyze TCRBV repertoire development under appropriately complex statistical models. Applying these methods, we found that patients with acute GVHD demonstrated TCRBV repertoire development statistically distinct from that repertoire development in patients without GVHD. Specifically, GVHD patients showed spectratypes indicative of lower diversity and greater deviation from the spectratypes expected in healthy individuals at intermediate times. Most individual TCRBV subfamilies had spectratypes statistically distinguishable between GVHD and non-GVHD patients at 6 months after transplantation. These results suggest that the T-cell receptor repertoire perturbations associated with acute GVHD are widely spread throughout the TCRBV families.     

Lymphoid Reconstruction and Vaccines

Allogeneic HSCT is the most reliable, robust, and effective cell-based biotherapy currently available to pediatric and adult patients with hematologic malignancies. The central role of donor-derived lymphocytes in mediating an effective antitumor effect, preventing and controlling opportunistic infections, and causing GVHD is well documented in animal experiments and human trials. The profound lymphopenia after conditioning regimens coupled with molecular tools to distinguish host versus donor cells provides investigators a window into immune recovery after allogeneic HSCT. Serial analyses of T cell subsets linking immunophenotype with function have revealed the kinetics of donor-derived T cell recovery after allografting and provided insights into ways the immune system can be manipulated to augment the graft-versus-tumor (GVT) effect without inducing GVHD. As this review demonstrates, investigators are not limited to being passive observers of this immune reconstitution; rather, we have an opportunity to shape the allografted T cells repertoire to selectively augment immune function.