CD3+/CD19+-depleted grafts in HLA-matched allogeneic peripheral blood stem cell transplantation lead to early NK cell cytolytic responses and reduced inhibitory activity of NKG2A

Post-transplant cyclophosphamide or cell selection in haploidentical allogeneic hematopoietic cell transplantation?

BACKGROUND

One major limitation for broader applicability of allogeneic hematopoietic cell transplantation (allo-HCT) in the past was the lack of HLA-matched histocompatible donors. Preclinical mouse studies using T-cell depleted haploidentical grafts led to an increased interest in the use of ex vivo T-cell depleted (TCD) haploidentical allo-HCT. TCD grafts through negative (T-cell depletion) or positive (CD34+ cell selection) techniques have been investigated to reduce the risk of graft-versus-host disease (GVHD) given the known implications of alloreactive T cells. A more practical approach to deplete alloreactive T cells in vivo using high doses of cyclophosphamide after allografting has proved to be feasible in overcoming the HLA barrier. Such approach has extended allo-HCT feasibility to patients for whom donors could not be found in the past. Nowadays, haploidentical donors represent a common donor source for patients in need of an allo-HCT. The broad application of haploidentical donors became possible by understanding the importance of depleting alloreactive donor T cells to facilitate engraftment and reduce incidence and severity of GVHD. These techniques involve ex vivo graft manipulation or in vivo utilization of pharmacologic agents, notably post-transplant cyclophosphamide (PTCy).

DISCUSSION

While acknowledging that no randomized controlled prospective studies have been yet conducted comparing TCD versus PTCy in haploidentical allo-HCT recipients, there are two advantages that would favor the PTCy, namely ease of application and lower cost. However, emerging data on adverse events associated with PTCy including, but not limited to cardiac associated toxicities or increased incidence of post-allograft infections, and others, are important to recognize.

Early recovery of natural killer cells post T-cell depleted allogeneic stem cell transplantation using alemtuzumab "in the bag"

BACKGROUND

Allogeneic stem cell transplantation (SCT) is a critical therapy for haematological malignancy but may lead to acute and chronic graft versus host disease (GvHD). T-cell depletion with alemtuzumab, either in vivo or ex vivo, reduces the incidence of GvHD but is a risk factor for disease relapse and poor immune reconstitution. Natural killer (NK) cells are the first lymphocytes to recover. Classical NK cells make up >90% of the normal circulating population and can directly kill neoplastic or virally infected cells while the regulatory subset makes up <10%, secretes cytokines and is not cytotoxic. The recovery and balance of these subsets post SCT remains controversial, with most studies analysing patients who received unmanipulated grafts and in vivo immunosuppression.

OBJECTIVE

The aim was to assess the early recovery of NK cells in 18 consecutive patients receiving ex vivo T-cell depleted SCT and to compare the results to 25 individuals receiving haploidentical non-T cell depleted grafts.

METHODS

All patients received myeloablative conditioning. After stem cell collection, the stem cells of the T cell depleted group were treated "in the bag" with alemtuzumab (CAMPATH 1H) at a concentration of 1mg/108 mononuclear cells and thereafter immediately infused. For those receiving non-T cell depleted grafts, GvHD prophylaxis was with post infusion therapeutic doses of cyclophosphamide. Blood samples were collected at days 21, 28 and 90. Complete blood counts were performed on an automated analyser while lymphocyte and NK subsets were examined using multiparameter flowcytometry. NK cells were defined as lymphocytes which were CD3-/CD56+. The classical subset was recognised as CD56dim/CD16+ while the regulatory population as CD56bright/CD16-. The results for both transplant types were compared at all time points using SPSS v8 statistical software.

RESULTS

The recovery of lymphocytes was slow in both groups. Those receiving non-T cell depleted grafts had significantly higher T cell counts at day 21 and 28 when compared to the T cell depleted group (P < 0.05). In contrast, NK cells in the ex vivo T-cell depleted patients recovered rapidly and by day 21 was no different to normal (p > 0.05), while the non-T cell depleted group had significantly decreased numbers (p < 0.001), only recovering at day 90. Both groups had abnormal NK cell subset ratios with significantly elevated percentages of regulatory cells (p < 0.05). However, significant differences were observed between the two groups with those receiving T cell depleted grafts having lower percentages of regulatory cells as well as higher numbers of classical NK cells at day 21 and 28 (p < 0.01).

CONCLUSION

This study of ex vivo T-cell depleted SCT's demonstrates that NK cells recover quicker when compared to those receiving unfractionated grafts. These results may have implications for GvHD and the GvL effect which warrants further study.

Impact of Early Natural Killer Cell Reconstitution on the Outcomes of T Cell-Replete Allogeneic Hematopoietic Stem Cell Transplantation

Background

Early immune reconstitution is crucial to successful outcomes after allogeneic stem cell transplantation (allo-HSCT). However, in T cell-replete HSCT, the impact of natural killer (NK) cells on transplantation outcome and the factors influencing early NK cell reconstitution remain unclear.

Methods

In this retrospective study, we analyzed 128 patients with hematological malignancies who received the first T cell-replete allo-HSCT between May 2019 and September 2021. After application of a conditioning regimen, prophylaxis for graft versus host disease (GVHD), and engraftment, the patients received prevention and treatment procedures for cytomegalovirus (CMV) reactivation. NK cells, T lymphocytes and B lymphocytes in peripheral blood were collected and analyzed at 30, 60, 90, 135 and 180 days after transplantation to observe immune cell reconstitution. Overall survival (OS), relapse-free survival (RFS), minimal residual disease (MRD), relapse, and non-relapse mortality (NRM) were evaluated. SPSS 25.0 and R version 4.2.1 were used for statistical analysis.

Results

In patients with rapid NK recovery (NK cell count at 30 days post-HSCT [NK30] >165/μL and 60 days post-HSCT [NK60] >265/μL), we observed lower rates of NRM, CMV reactivation and acute GVHD (aGVHD). Multivariate analysis indicated that a lower NK30 (≤165/μL) was an independent factor associated with inferior OS and RFS. The NK30 and NK60 in patients with CMV reactivation and aGVHD after transplantation were significantly lower than those in patients without these complications. In addition, CD107a expression in NK cells was also significantly lower in patients who experienced aGVHD. Correlation analysis did not find an inhibitory effect of T-lymphocyte subset reconstitution on NK cells in the early stage after transplantation.

Conclusion

Rapid NK cell reconstitution early after allo-HSCT had protective effects on NRM and survival. Promoting early NK cell reconstitution represents a new approach to improving the outcomes of allo-HSCT.

Donor chimerism and immune reconstitution following haploidentical transplantation in sickle cell disease

Introduction

We previously reported the initial results of a phase II multicenter transplant trial using haploidentical parental donors for children and aolescents with high-risk sickle cell disease achieving excellent survival with exceptionally low rates of graft-versus-host disease and resolution of sickle cell disease symptoms. To investigate human leukocyte antigen (HLA) sensitization, graft characteristics, donor chimerism, and immune reconstitution in these recipients.

Methods

CD34 cells were enriched using the CliniMACS^® system with a target dose of 10 x 10⁶ CD34⁺ cells/kg with a peripheral blood mononuclear cell (PBMNC) addback dose of 2x10⁵ CD3/kg in the final product. Pre-transplant HLA antibodies were characterized. Donor chimerism was monitored 1-24 months post-transplant. Comprehensive assessment of immune reconstitution included lymphocyte subsets, plasma cytokines, complement levels, anti-viral T-cell responses, activation markers, and cytokine production. Infections were monitored.

Results

HLA antibodies were detected in 7 of 11 (64%) evaluable patients but rarely were against donor antigens. Myeloid engraftment was rapid (100%) at a median of 9 days. At 30 days, donor chimerism was 93-99% and natural killer cell levels were restored. By 60 days, CD19 B cells were normal. CD8 and CD4 T-cells levels were normal by 279 and 365 days, respectively. Activated CD4 and CD8 T-cells were elevated at 100-365 days post-transplant while naïve cells remained below baseline. Tregs were elevated at 100-270 days post-transplant, returning to baseline levels at one year. At one year, C3 and C4 levels were above baseline and CH50 levels were near baseline. At one year, cytokine levels were not significantly different from baseline.

Discussion

These results suggest that haploidentical transplantation with CD34-enriched cells and peripheral blood mononuclear cell addback results in rapid engraftment, sustained donor chimerism and broad-based immune reconstitution.

Innovative Strategies to Improve the Clinical Application of NK Cell-Based Immunotherapy

Natural killer cells constitute a part of the innate immune system that mediates an effective immune response towards virus-infected and malignant cells. In recent years, research has focused on exploring and advancing NK cells as an active immunotherapy platform. Despite major advances, there are several key challenges that need to be addressed for the effective translation of NK cell research to clinical applications. This review highlights some of these challenges and the innovative strategies being developed to overcome them, including in vitro expansion, in vivo persistence, infiltration to the tumor site, and prevention of exhaustion.

Influence of KIR and NK Cell Reconstitution in the Outcomes of Hematopoietic Stem Cell Transplantation

Natural killer (NK) cells play a significant role in immune tolerance and immune surveillance. Killer immunoglobin-like receptors (KIRs), which recognize human leukocyte antigen (HLA) class I molecules, are particularly important for NK cell functions. Previous studies have suggested that, in the setting of hematopoietic stem cell transplantation (HSCT), alloreactive NK cells from the donor could efficiently eliminate recipient tumor cells and the residual immune cells. Subsequently, several clinical models were established to determine the optimal donors who would exhibit a graft-vs. -leukemia (GVL) effect without developing graft-vs. -host disease (GVHD). In addition, hypotheses about specific beneficial receptor-ligand pairs and KIR genes have been raised and the favorable effects of alloreactive NK cells are being investigated. Moreover, with a deeper understanding of the process of NK cell reconstitution post-HSCT, new factors involved in this process and the defects of previous models have been observed. In this review, we summarize the most relevant literatures about the impact of NK cell alloreactivity on transplant outcomes and the factors affecting NK cell reconstitution.

Clinical-scale production of cGMP compliant CD3/CD19 cell-depleted NK cells in the evolution of NK cell immunotherapy at a single institution

BACKGROUND

Allogeneic natural killer (NK) cell adoptive immunotherapy is a growing therapeutic option for patients. Clinical-scale production of NK cells using immunomagnetic selection complies with current good manufacturing practices (cGMPs) and allows for closed-system, automated purification. We report our experience with CD3/CD19 cell-depleted (CD3/CD19_dep ) NK cell production and compare to previous methods of CD3 cell depletion and CD3 cell depletion/CD56 cell enrichment.

STUDY DESIGN AND METHODS

Nonmobilized mononuclear cells collected by apheresis were incubated with anti-CD3/anti-CD19 microbeads and depleted in an automated cell selection system (CliniMACS, Miltenyi). The NK cell-enriched products were incubated overnight in interleukin (IL)-2 or IL-15, washed, and resuspended prior to lot release testing and infusion.

RESULTS

Since 2010, 94 freshly infusible CD3/CD19_dep NK cell products were manufactured in support of eight clinical trials. Sixty-six products were incubated in IL-2 and 28 products in IL-15. Processing resulted in a mean NK cell recovery of 74% and viability of 95.8%; NK cells, T cells, B cells, and monocytes accounted for 47%, 0.2%, 0.08%, and 49% of the final products, respectively. Seven products required dose adjustments to meet lot release. The specification for purity changed throughout the evolution of manufacturing. IL-2 or IL-15 activation enhanced in vitro cytotoxicity compared to preactivated cells. There was no difference in final product composition or cytotoxicity between cytokine cohorts.

CONCLUSION

Clinical-scale/cGMP production of NK cells using CD3/CD19 cell-depletion effectively minimized T-cell and B-cell contamination in a single manipulation without compromise to NK-cell recovery. Cytokine activation increased in vitro cytotoxicity compared to column-depleted, preactivated NK cells.

Network geometry of evidence from randomised controlled trials addressing donor selection and source of haematopoietic progenitor cells used in allogeneic transplantation: a systematic scoping review

BACKGROUND AND METHODS

A scoping review of randomised controlled trials (RCTs) addressing source of cells and choice of donor for allogeneic haematopoietic cell transplantation (HCT) was performed to create a network of best evidence that allows us to identify new potential indirect comparisons for the strategic development of future studies that connect to the existing evidence network.

RESULTS

A total of 19 eligible RCTs (2589 total patients) were identified. Nine studies (1566 patients) compared clinical outcomes following the use of peripheral blood progenitor cells (PBPCs) with bone marrow (BM) from matched related donors (eight studies) or matched unrelated donors (one study). The remaining studies compared BM or PBPCs with various methods of BM stimulation or manipulation (six studies), compared different methods of surface molecule-based selection and/or depletion of grafts (two studies) or compared the optimal number of units for paediatric cord blood transplantation (two studies). No published RCTs compared different types of donors. The geometry of the evidence network was analysed to identify opportunities for potential novel indirect comparisons and to identify opportunities to expand the network. Few indirect comparisons are currently feasible due to small sample size and heterogeneity in patient diagnoses and demographics between treatment nodes in the network.

CONCLUSION

More RCTs that enrol greater numbers of similar patients are needed to leverage the current evidence network concerning donor choice and source of cells used in allogeneic HCT.

GVHD prevents NK-cell-dependent leukemia and virus-specific innate immunity

Allogeneic bone marrow transplantation (allo-BMT) is a curative therapy for hematological malignancies, but is associated with significant complications, principally graft-versus-host disease (GVHD) and opportunistic infections. Natural killer (NK) cells mediate important innate immunity that provides a temporal bridge until the reconstruction of adaptive immunity. Here, we show that the development of GVHD after allo-BMT prevented NK-cell reconstitution, particularly within the maturing M1 and M2 NK-cell subsets in association with exaggerated activation, apoptosis, and autophagy. Donor T cells were critical in this process by limiting the availability of interleukin 15 (IL-15), and administration of IL-15/IL-15Rα or immune suppression with rapamycin could restore NK-cell reconstitution. Importantly, the NK-cell defect induced by GVHD resulted in the failure of NK-cell-dependent in vivo cytotoxicity and graft-versus-leukemia effects. Control of cytomegalovirus infection after allo-BMT was also impaired during GVHD. Thus, during GVHD, donor T cells compete with NK cells for IL-15 thereby inducing profound defects in NK-cell reconstitution that compromise both leukemia and pathogen-specific immunity.

Added effects of dexamethasone and mesenchymal stem cells on early Natural Killer cell activation

Graft rejection and graft-versus-host disease are leading causes of transplant related mortality despite advancements in immunosuppressive therapy. Mesenchymal stem cells (MSCs) offer a promising addition to immunosuppressive drugs (ISD), while NK-cells are increasingly used as effector cells in graft-versus-leukemia. Combined therapy of ISD, NK-cells and/or MSCs is used in clinical practice. Here, we examined the effects of MSCs and selected ISD (tacrolimus, cyclosporin A, mycophenolic acid, dexamethasone) treatment on early NK-cell activation. We assessed STAT4 and STAT5 phosphorylation triggered by IL-12 and IL-2, respectively. Furthermore, we determined IFNγ, perforin production and the expression pattern of selected NK-cell receptors. Of all drugs tested, only dexamethasone inhibited NK-cell STAT4 and STAT5 phosphorylation. All ISD, with the exception of MPA, significantly inhibited IFNγ, and only dexamethasone inhibited upregulation of early activation markers CD69 and CD25 (IL-2 condition only). MSCs inhibited IL-2 induced NK cell STAT5 phosphorylation, IFNγ production and CD69 upregulation, and IL-12 induced IFNγ and perforin production. While MSCs mediated inhibition of CD69 expression was cell contact dependent, inhibition of IFNγ and perforin production, as well as STAT5 phosphorylation was cell-contact independent. Importantly, dexamethasone augmented MSCs mediated inhibition of both IL-12 and IL-2 induced CD69 expression and IFNγ production, as well as IL-2 induced STAT5 phosphorylation. Taken together, these novel insights may help the design of future NK-cell and MSCs based immunotherapy.

Progressive natural killer cell dysfunction associated with alterations in subset proportions and receptor expression in soft-tissue sarcoma patients

Immunotherapy is currently investigated as treatment option in many types of cancer. So far, results from clinical trials have demonstrated that significant benefit from immunomodulatory therapies is restricted to patients with select histologies. To broaden the potential use of these therapies, a deeper understanding for mechanisms of immunosuppression in patients with cancer is needed. Soft-tissue sarcoma (STS) presents a medical challenge with significant mortality even after multimodal treatment. We investigated function and immunophenotype of peripheral natural killer (NK) cells from chemotherapy-naive STS patients (1st line) and STS patients with progression or relapse after previous chemotherapeutic treatment (2nd line). We found NK cells from peripheral blood of both STS patient cohorts to be dysfunctional, being unable to lyse K562 target cells while NK cells from renal cell cancer (RCC) patients did not display attenuated lytic activity. Ex vivo stimulation of NK cells from STS patients with interleukin-2 plus TKD restored cytotoxic function. Furthermore, altered NK cell subset composition with reduced proportions of CD56(dim) cells could be demonstrated, increasing from 1st- to 2nd-line patients. 2nd-line patients additionally displayed significantly reduced expression of receptors (NKG2D), mediators (CD3ζ), and effectors (perforin) of NK cell activation. In these patients, we also detected fewer NK cells with CD57 expression, a marker for terminally differentiated cytotoxic NK cells. Our results elucidate mechanisms of NK cell dysfunction in STS patients with advanced disease. Markers like NKG2D, CD3ζ, and perforin are candidates to characterize NK cells with effective antitumor function for immunotherapeutic interventions.

Functional Reconstitution of Natural Killer Cells in Allogeneic Hematopoietic Stem Cell Transplantation

Natural killer (NK) cells are the first lymphocyte population to reconstitute following allogeneic hematopoietic stem cell transplantation (HSCT) and are important in mediating immunity against both leukemia and pathogens. Although NK cell numbers generally reconstitute within a month, the acquisition of mature NK cell phenotype and full functional competency can take 6 months or more, and is influenced by graft composition, concurrent pharmacologic immunosuppression, graft-versus-host disease, and other clinical factors. In addition, cytomegalovirus infection and reactivation have a dominant effect on NK cell memory imprinting following allogeneic HSCT just as it does in healthy individuals. Our understanding of NK cell education and licensing has evolved in the years since the "missing self" hypothesis for NK-mediated graft-versus-leukemia effect was first put forward. For example, we now know that NK cell "re-education" can occur, and that unlicensed NK cells can be more protective than licensed NK cells in certain settings, thus raising new questions about how best to harness graft-versus-leukemia effect. Here, we review current understanding of the functional reconstitution of NK cells and NK cell education following allogeneic HSCT, highlighting a conceptual framework for future research.

Negative depletion of CD3(+) and TcRαβ(+) T cells

PURPOSE OF REVIEW

In contrast to CD34(+) positive selection, negative depletion strategies retain large numbers of effector cells in allogeneic peripheral stem cell grafts, such as natural killer (NK) and other cells. This review summarizes the clinical experience obtained using negative depletion approaches of CD3(+) and T-cell receptor (TcR)αβ(+) T lymphocytes.

RECENT FINDINGS

Attempts to improve immune reconstitution and to better exploit the graft-versus-malignancy effect after transplantation of T-cell-depleted grafts through the preservation of immune effector cells led to the development of CD3-, CD3/CD19- and more recently TcRαβ/CD19-negative depletion strategies of mobilized peripheral stem cell grafts. A faster immune reconstitution has been observed in patients with negatively depleted grafts after haploidentical transplantation, although no prospective randomized trials have been reported to date. In a randomized study of matched sibling and matched unrelated transplantation, CD3/CD19-depleted peripheral stem cell grafts led to a faster recovery of NK cells compared with the CD34(+)-positive selection group.

SUMMARY

New technologies allow the large-scale graft engineering of peripheral stem cells for clinical use in matched and mismatched stem cell transplantation. Further clinical trials are necessary to decide which of these methods is associated with a faster immune reconstitution and a better outcome after transplantation.

Decreased levels of circulating IL17-producing CD161+CCR6+ T cells are associated with graft-versus-host disease after allogeneic stem cell transplantation

The C-type lectin-like receptor CD161 is a well-established marker for human IL17-producing T cells, which have been implicated to contribute to the development of graft-versus-host disease (GVHD) after allogeneic stem cell transplantation (allo-SCT). In this study, we analyzed CD161⁺ T cell recovery, their functional properties and association with GVHD occurrence in allo-SCT recipients. While CD161⁺CD4⁺ T cells steadily recovered, CD161^hiCD8⁺ T cell numbers declined during tapering of Cyclosporine A (CsA), which can be explained by their initial growth advantage over CD161^neg/lowCD8⁺ T cells due to ABCB1-mediated CsA efflux. Interestingly, occurrence of acute and chronic GVHD was significantly correlated with decreased levels of circulating CD161⁺CD4⁺ as well as CD161^hiCD8⁺ T cells. In addition, these subsets from transplanted patients secreted high levels of IFNγ and IL17. Moreover, we found that CCR6 co-expression by CD161⁺ T cells mediated specific migration towards CCL20, which was expressed in GVHD biopsies. Finally, we demonstrated that CCR6⁺ T cells indeed were present in these CCL20⁺ GVHD-affected tissues. In conclusion, we showed that functional CD161⁺CCR6⁺ co-expressing T cells disappear from the circulation and home to GVHD-affected tissue sites. These findings support the hypothesis that CCR6⁺CD161-expressing T cells may be involved in the immune pathology of GVHD following their CCL20-dependent recruitment into affected tissues.

Defining early human NK cell developmental stages in primary and secondary lymphoid tissues

A better understanding of human NK cell development in vivo is crucial to exploit NK cells for immunotherapy. Here, we identified seven distinctive NK cell developmental stages in bone marrow of single donors using 10-color flow cytometry and found that NK cell development is accompanied by early expression of stimulatory co-receptor CD244 in vivo. Further analysis of cord blood (CB), peripheral blood (PB), inguinal lymph node (inLN), liver lymph node (liLN) and spleen (SPL) samples showed diverse distributions of the NK cell developmental stages. In addition, distinctive expression profiles of early development marker CD33 and C-type lectin receptor NKG2A between the tissues, suggest that differential NK cell differentiation may take place at different anatomical locations. Differential expression of NKG2A and stimulatory receptors (e.g. NCR, NKG2D) within the different subsets of committed NK cells demonstrated the heterogeneity of the CD56(bright)CD16⁺/⁻ and CD56(dim)CD16⁺ subsets within the different compartments and suggests that microenvironment may play a role in differential in situ development of the NK cell receptor repertoire of committed NK cells. Overall, differential in situ NK cell development and trafficking towards multiple tissues may give rise to a broad spectrum of mature NK cell subsets found within the human body.

Role of natural killer cells in hematopoietic stem cell transplantation: myth or reality?

Natural killer (NK) cells play a crucial role in the innate immune system and are responsible for the initial responses in the surveillance against malignant cells and virally infected cells. NK cells express their own repertoire of receptors, including activating and inhibitory receptors, which bind to major histocompatibility complex class I or class-I-related molecules. Binding of NK cell inhibitory receptors to their major histocompatibility complex class I ligands protects the target cells from NK cell-mediated cytotoxicity. NK cell alloreactivity has been put to use in allogeneic hematopoietic stem cell transplantation to reduce the rate of relapse and of graft-versus-host disease. A variety of findings have been observed in clinical studies, showing either beneficial or deleterious effects on clinical outcome. This article reviews the results of major clinical trials in relation to the model used to define NK cell alloreactivity.

Rapamycin and MPA, but not CsA, impair human NK cell cytotoxicity due to differential effects on NK cell phenotype

Cyclosporin A (CsA), rapamycin (Rapa) and mycophenolic acid (MPA) are frequently used for GVHD prophylaxis and treatment after allogeneic stem cell transplantation (SCT). As NK cells have received great interest for immunotherapeutic applications in SCT, we analyzed the effects of these drugs on human cytokine-stimulated NK cells in vitro. Growth-kinetics of CsA-treated cultures were marginally affected, whereas MPA and Rapa severely prevented the outgrowth of CD56(bright) NK cells. Single-cell analysis of NK cell receptors using 10-color flow cytometry, revealed that CsA-treated NK cells gained a similar expression profile as cytokine-stimulated control NK cells, mostly representing NKG2A(+) KIR(-) NCR(+) cells. In contrast, MPA and Rapa inhibited the acquisition of NKG2A and NCR expression and NK cells maintained an overall NKG2A(-) KIR(+) NCR(+/-) phenotype. This was reflected in the cytolytic activity, as MPA- and Rapa-treated NK cells, in contrast to CsA-treated NK cells, lost their cytotoxicity against K562 target cells. Upon target encounter, IFN-γ production was not only impaired by MPA and Rapa, but also by CsA. Overall, these results demonstrate that CsA, MPA and Rapa each have distinct effects on NK cell phenotype and function, which may have important implications for NK cell function in vivo after transplantation.