Unrelated Donor Transplants

One of the most widely used strategies for children who need an allograft and do not have an available HLA-identical sibling is to identify an unrelated HLA-matched donor in the registries. Worldwide international registries include more than 13 million HLA-typed volunteer donors. HLA-A, -B, -C, class I loci, and the DRB1 class II locus are the HLA loci that most influence outcome after HSCT from an unrelated volunteer. The roles played by other class II loci (DQB1, DP1 loci) on a patient’s outcome remain controversial.

Data on serologic typing of HLA class I A and B loci are available for all donors, and information on DRB1 typing is available for approximately one third of donors. While in the past serologic (low-resolution) typing was used for HLA-A and -B loci, currently the unrelated donors are selected using high-resolution (allelic) molecular typing of loci HLA-A, -B, -C, and DRB1. The chance of finding an HLA-matched donor depends on the frequency of the HLA phenotype, which is closely linked to the ethnic origin of the registry donors, with a range of 60-70% for white patients to <10% for persons of other ethnic groups (Hispanic, black, etc.).

Identifying a suitable unrelated donor is a complicated and lengthy process, with the median time from the start of search to transplantation being 4-5 mo. During this period, a patient with acute leukemia may relapse and require further therapy, accumulating organ toxicity that unfavorably affects outcome. Moreover, for a variety of reasons, a relevant proportion of donors (sometimes reaching 10-20%) are either no longer available or refuse donation. Despite these limitations, many thousands of matched unrelated donor transplants have been performed.

Initially, HLA polymorphism and the limitations of conventional (i.e., serological) HLA-typing techniques unfavorably affected the accuracy of matching, thus increasing rejection rates and the incidence of acute and chronic graft versus host disease (GVHD). Consequently, because the event-free survival of recipients of an unrelated donor allograft was worse than that observed when the donor was a compatible sibling transplant, there was no consensus on the use of unrelated donor transplants for nonmalignant diseases, such as thalassemia. DNA-based (i.e., high-resolution molecular) techniques for HLA typing have revealed an impressive number of new alleles within antigens that were previously defined by serology. Matching by high-resolution DNA typing reduces the risk of immune complications, namely graft rejection and GVHD, but also decreases the chance of finding a suitable donor. Nevertheless, the advent of both high-resolution molecular HLA class I and II loci-typing coupled with progress in the prophylaxis and treatment of GVHD has resulted in a reduction of transplantation-related mortality and improved outcomes. Indeed, outcomes from a fully matched unrelated volunteer donor are now similar to those of HSCT from an HLA-identical sibling, as indicated by results of unrelated donor transplantation in children with acute lymphoblastic leukemia (ALL) in the 2nd complete remission, juvenile myelomonocytic leukemia, or thalassemia (Fig. 130-1).

Figure 130-1 Kaplan-Meier estimates of event-free survival (EFS) (upper lines) and transplantation-related mortality (TRM) (lower lines) according to the type of donor used, matched family donor (MFD) or unrelated donor (UD). Matched family donor was superior (55% event-free survival and 10% transplantation-related mortality) compared to unrelated donor (49% event-free survival and 16% transplantation-related mortality).

(From Locatelli F, Nöllke P, Zecca M, et al: Hematopoietic stem cell transplantation [HSCT] in children with juvenile myelomonocytic leukemia [JMML]: results of the EWOG-MDS/EBMT trial, Blood 105:410–419, 2005.)

In patients with leukemia, a single locus disparity does not substantially affect the probability of event-free survival, as the increased risk of toxic death may be compensated by a reduction in the relapse rate. In contrast, in patients with non-malignant disorders, optimal results are obtained only when a donor matched at the allelic level with the recipient is selected. In general, a single HLA disparity in the donor/recipient pair, irrespective of whether antigenic or allelic, predicts a greater risk of nonleukemia mortality; multiple allelic disparities at different HLA loci have an additive detrimental effect and are associated with an even worse outcome. Ex vivo T-cell depletion of the graft has reduced the risk of acute GVHD but has not significantly affected patient outcomes.

The survival rates of unrelated donor HSCT include only patients who are transplanted and do not take into account patients for whom a donor is not found. For patients who urgently need a transplant, the time required to identify a suitable donor from a potential panel, establish eligibility, and harvest the cells may lead to relapse and failure to transplant. For patients who do not have a matched donor or who urgently need a transplant, attention has focused on unrelated cord blood and the HLA-haploidentical, mismatched family member.

Umbilical Cord Blood Transplants

UCB transplantation (UCBT) is a viable option for children who need allogeneic HSCT (Fig. 130-2). Several hundred children have been cured through transplantation with either related or unrelated UCB units. UCBT offers the advantages of absence of risks to donors, reduced risk of transmitting infections, and, for transplants from unrelated donors, immediate availability of cryopreserved cells, with the median time from start of search to transplantation being only 3-4 wk. In comparison to bone marrow transplantation (BMT), advantages of UCBT include lower incidence and severity of GVHD, easier procurement and prompter availability of cord blood cells, and potential to use donors that have HLA disparities with the recipient. Despite these advantages, UCBT patients may be exposed to an increased risk of early fatal complications, mainly due to a lower engraftment rate of donor hematopoiesis, delayed kinetics of neutrophil recovery, and lack of adoptive transfer of pathogen-specific memory T-cells. Transfer of donor-derived, memory T cells significantly contributes to early immunologic reconstitution of children after unmanipulated allogeneic bone marrow or peripheral blood stem cell transplantation.

Figure 130-2 Probability of leukemia-free survival after bone marrow (BM) and cord blood (CB) transplantation adjusted for disease status at transplantation. AG, antigen; MM, mismatched.

(From Eapen M, Rubinstein P, Zhang NJ, et al: Outcomes of transplantation of unrelated donor umbilical cord blood and bone marrow in children with acute leukaemia: a comparison study, Lancet 369:1947–1954, 2007.)

There is a strong inverse correlation between the number of nucleated cord blood cells infused per kg recipient body weight and the risk of dying from transplantation-related causes. In particular, engraftment is a major concern when the nucleated cells are <2.5 × 10⁷/kg of recipient body weight. As a cord blood unit usually contains between 8 × 10⁸ and 1.5 × 10⁹ cells, it is not surprising that UCB transplantation has been less frequently employed for adolescents or adults with body weight >40 kg. Indeed, it can be estimated that only 20% of the UCB units available in the bank inventory could suffice for a 75 kg patient according to the recommended threshold cell dose (namely more than 2.5 × 10⁷ total nucleated cells/kg recipient body weight before thawing the unit). In view of these findings, it is not surprising that efforts have been focused on approaches capable of increasing the number of UCB cells to be transplanted. Selection of the richest cord blood units, infusion of 2 units in the same recipient (i.e., double UCBT), and transplantation of ex vivo expanded progenitors have contributed to improving the results of UCBT, opening new scenarios for a wider application of the procedure. In particular, double UCBT is largely employed, as it significantly increases the engraftment rate when compared to single-unit UCBT. In the majority of double UCBT, the 2 UCB units are partially HLA-matched with the recipient and with each other. Sustained hematopoiesis after double UCBT is usually derived from a single donor.

Direct intrabone transplantation of UCB cells is a feasible and safe approach, capable of overcoming the problem of graft failure and increasing prompt platelet recovery, even when low numbers of HLA-mismatched cord-blood cells are transplanted. This technique is of interest to pediatricians because it extends the applicability of UCBT to patients with a body weight exceeding 40-50 kg, including adolescents. The decreased incidence of both acute and chronic GVHD observed after intrabone UCBT is particularly intriguing and can be interpreted as a consequence of the immediate contact of UCB lymphocytes with mesenchymal stem cells and osteoblasts present in the marrow niches.

Despite the low incidence of acute and chronic GVHD observed after UCB transplantation, the risk of recurrence of leukemia is not increased. The long-term results of UCB transplants are similar to those after transplantation with other sources of hematopoietic stem cells. In particular, several reports have compared the outcome of UCBT and BMT from unrelated donors in children with hematological malignancies. Recipients of UCBT were transplanted from donors with greater HLA-disparities, received 1-log fewer nucleated cells, had delayed neutrophil and platelet recovery, and showed reduced incidence of GVHD as compared to children given BMT. Nevertheless, both the relapse rate and the overall survival probability did not differ in unrelated UCBT or BMT pediatric recipients. There is no doubt that, in the absence of an HLA-identical family donor, unrelated UCBT can be considered a suitable option for children with malignant and nonmalignant disorders. Results of UCBT have been of particular interest in children with Hurler syndrome, or Krabbe disease transplanted with cord blood cells from an unrelated donor, as well as in children with hemoglobinopathies given a related UCBT.

Haploidentical Transplants

HSCT from an HLA-haploidentical (haplo-HSCT) individual offers an immediate source of hematopoietic stem cells to almost all leukemia patients who fail to find a matched donor, whether related or unrelated, or a suitable cord blood unit. Indeed, almost all of children have at least 1 haploidentical, 3 loci mismatched family member who is promptly available as donor. Moreover, the few patients who reject the haploidentical transplant have the advantage of another immediately available donor within the family.

Efficient T-cell depletion of the graft has been demonstrated to prevent acute and chronic GVHD even when using haploidentical parental bone marrow differing at the 3 major HLA loci. The benefits of T-cell depletion were first demonstrated in transplantation of children with severe combined immunodeficiency (SCID). More than 300 transplants in SCID patients using haploidentical donors have been performed worldwide, with a high rate of long-term partial or complete immune reconstitution.

As patients with acute leukemia have a high chance of rejecting a haploidentical bone marrow graft, a “megadose” of granulocyte colony-stimulating factor (G-CSF)–mobilized peripheral blood stem cells has been demonstrated to be essential for overcoming the barrier of HLA incompatibility in the donor/recipient pair and for eluding residual anti-donor cytotoxic T-lymphocyte precursor (CTL-p) activity in the recipient. Indeed, in leukemia patients, the combination of high-intensity immune-suppressive/myeloablative conditioning regimens with the infusion of great numbers of highly purified, peripheral blood CD34+ cells has been demonstrated to (1) enhance the successful and sustained engraftment of donor hematopoiesis across the HLA barrier, and (2) minimize the incidence of grade II-IV acute GVHD without the need for any post-transplant immune suppression as prophylaxis. The physical elimination of mature T cells from the graft reduces the likelihood of GVHD in the context of great immune genetic disparity but comes at a cost, namely loss of the benefit of the adoptive transfer of donor memory T lymphocytes that provide protection from infections in the 1st months after transplantation. A state of profound immune deficiency lasts for at least 4-6 mo after transplantation in haplo-HSCT recipients. Sophisticated strategies of adoptive infusions of T-cell lines or clones specific for the most common and life-threatening pathogens (EBV, HCMV, Aspergillus, adenovirus) have been envisaged and successfully tested in a few pilot trials to protect the recipients in the early post-transplant period. Co-infusion of donor-derived mesenchymal stem cells, aimed at completely eliminating the risk of graft failure, seems also promising to optimize post-transplant outcome.

The outcomes of haplo-HSCT have been more extensively reported in adults than in children. The reported probability of survival at 3-4 yr after a haplo-HSCT in children with acute leukemia ranges from 48% to 18% and is influenced by many factors, especially the state of remission at time of transplantation and the general prognosis of the specific leukemia. In haplotype mismatched parent-to-child HSCT for children with acute leukemia, patients grafted from the mother have reduced relapse rates compared to patients grafted from the father, translating into improved event-free survival.

For many years recipients of a T-cell depleted allograft have been considered more susceptible to leukemia relapse, reflecting the absence of the T-cell mediated graft versus leukemia (GVL) effect. A GVL effect displayed by donor natural killer (NK) cells can compensate for the lack of T-cell specific alloreactivity when an HLA-disparate NK alloreactive relative is employed as a donor.

Donor Versus Recipient NK-Cell Alloreactivity

Donor versus recipient NK-cell alloreactivity is a biologic phenomenon that is unique to the mismatched transplant. It derives from a mismatch between donor NK clones, carrying specific inhibitory receptors for self-major histocompatibility complex (MHC) class I molecules, and MHC class I ligands on recipient cells. NK cells are primed to kill by several activating receptors. Human NK cells discriminate allelic forms of MHC molecules via killer cell Ig-like receptors (KIR), with each cell in the repertoire bearing at least 1 receptor that is specific for self-MHC class I molecules. Because NK cells co-express inhibitory receptors for self-MHC class I molecules, autologous cells are not killed. When faced with mismatched allogeneic targets, NK cells sense the missing expression of self-class I alleles and mediate alloreactions. In mismatched transplants, there are many donor-recipient pairs in which the donor NK KIRs do not recognize the recipient’s class I alleles as self. Consequently, the donor NK cells are not blocked and are activated to lyse the recipient’s lymphohematopoietic cells.

Haplo-HSCT trials demonstrate that MHC class I mismatches, which generate an alloreactive NK-cell response in the graft versus host direction, eradicate leukemia cells, improve engraftment, and protect from T cell–mediated GVHD (Fig. 130-3). Lack of an NK-alloreactive donor is the strongest independent risk factor for leukemia relapse after adjustment for disease status at transplant. The potential for donor versus recipient NK-cell alloreactivity, which can be predicted by standard HLA typing, is recommended when selecting the donor of choice from among the mismatched family members.

Figure 130-3 Function of donor-versus-recipient alloreactive natural killer (NK) cells in hematopoietic transplantation, as derived from mouse models and human studies. Alloreactive NK cells are generated in vivo from engrafted stem cells. They eradicate leukemia and prevent rejection (through killing of recipient T lymphocytes) and T cell-mediated graft versus host disease (GVHD) (through killing of recipient-type dendritic cells [DC], which trigger GVHD). NK-cell alloreactivity is restricted to lympho-hematopoietic cells and does not attack other tissues as it does not cause GVHD.

(From Ruggeri L, Capanni M, Urbani E, et al: Effectiveness of donor natural killer cell alloreactivity in mismatched hematopoietic transplants, Science 295:2097–2100, 2002.)

The chance of finding a “perfect mismatch” NK-alloreactive donor in the family is on the order of 50%. From a practical point of view, first the transplantation recipient is HLA typed. Recipients expressing class I alleles belonging to the three class I groups recognized by KIRs (HLA-C group 1, HLA-C group 2, and HLA-Bw4 alleles) will block all NK cells from every donor and belong to the one third of the population that is resistant to alloreactive NK killing. Patients who express only 1 or 2 of these allele groups may find NK-alloreactive donors.

Donor HLA typing identifies family members who do not express the class I group(s) expressed by the patient and, therefore, have the potential for NK alloreactivity. Not all inhibitory KIRs are present in 100% of the population. KIR2DL2/3, the receptor for HLA-C group 1, is present in all persons; KIR2DL1, the receptor for HLA-C group 2, is present in 97% of persons; and KIR3DL1, the receptor for HLA-Bw4 alleles, is present in ≈80%. Donor KIR genotyping ensures that the donor expresses the relevant NK cells.

In HLA-Bw4 mismatches, even when the KIR3DL1 gene is present, NK repertoire studies show alloreactive NK cells in approximately two thirds of individuals. This may be because they occur in highly variable frequencies or because allelic KIR3DL1 variants may not allow receptor expression at the cell membrane. Therefore, for HLA-Bw4 mismatches, direct assessment of the donor NK repertoire is necessary.

Autologous Hematopoietic Stem Cell Transplantation

Autologous transplantation, using the patient’s own stored marrow, is associated with a very low risk of life-threatening transplant-related complications, although the main cause of failure is disease recurrence due to the lack of the immune-mediated GVL effect. Bone marrow was once the only source of stem cells employed in patients given an autograft; currently the vast majority of patients treated with autologous HSCT receive hematopoietic progenitors mobilized in peripheral blood by either cytokines alone (mainly G-CSF) or by cytokines plus cytotoxic agents. When compared to bone marrow, the use of peripheral blood progenitors is associated with a faster hematopoietic recovery and a comparable outcome. A major concern in patients with malignancies given autologous HSCT is the risk of reinfusing malignant cells with the graft; tumor progenitors contained in the graft can contribute to recurrence of the original malignant disease. This observation has provided the rationale for tumor purging using elaborate strategies aimed at reducing or eliminating tumor contamination of the graft.

Autologous HSCT is employed primarily to prevent relapse in patients with AML who achieve complete remission after induction therapy and also for selected children with relapsed lymphomas and selected solid tumors (Table 130-1).

Whereas some randomized studies suggest an advantage in terms of event-free survival for patients with AML in the 1st complete remission given an autologous HSCT as compared to those treated with chemotherapy alone, other reports have not confirmed this observation. The probability of event-free survival for children with AML in the 1st complete remission given autologous HSCT has been reported to range from 40 to 60%. Ex vivo purging of bone marrow cells with mafosfamide has been shown to reduce the risk of disease recurrence in children with AML in the 1st complete remission given an autologous transplantation.

Patients with sensitive lymphomas and little tumor burden have favorable outcomes after autologous HSCT, with disease-free survival rates of 50-60%, whereas high-risk patients with bulky tumor or poorly responsive disease have a dismal outcome, with survival rates of 10-20%.

Some studies suggest that autologous HSCT may offer an advantage over conventional chemotherapy and radiotherapy in terms of event-free survival in children with ALL in the second complete remission after an isolated extramedullary relapse (CNS, testicular relapse).

Autologous HSCT in patients with high-risk neuroblastoma is associated with a better outcome compared to conventional chemotherapy, especially in patients treated with 13-cis-retinoic acid after the transplant procedure.

For children with brain tumors at high risk of relapse or resistant to conventional chemotherapy and irradiation, the dose-limiting toxicity for intensifying therapy is myelosuppression, thus providing a role for stem cell rescue. Several studies have provided encouraging results for patients with different histologic types of brain tumors treated with autologous HSCT.