This trial addresses the use of myeloablative TCD transplantation from HLA matched and mismatched unrelated donors with current day technology and practices. The goals were to provide effective therapy with a low rate of relapse, to facilitate T cell recovery, and to decrease regimen related toxicity. Despite the high risk nature of many of the transplants and modifications to the regimens, the study demonstrated promising results: only a 6% cumulative incidence of relapse, excellent engraftment, and low rates of GVHD. Confounding contributors to GVHD were incomplete TCD and use of DLI. Death due to infection occurred in 20% of patients, most in the peritransplant setting or in association with GVHD therapy.
Limited reports of TCD unrelated BM or PBSC grafts following myeloablation have been published7,23-26
Most studies utilized partial TCD together with immune suppressive medications25
post transplant. They reported slower immune recovery, a higher incidence of infectious complications, 18,27,28
and relapse in certain diseases compared with T-cell replete transplantation 29-31
. Use of ATG has also been associated with similar complications. 1,32
The majority of studies, including the largest trial 33
, described patients treated in the 1980’s-2000 using BM allografts. The current study addresses an older patient population, but incorporates improved supportive care, high resolution HLA typing, improved TCD technology, and PBSC graft source.
Studies suggest that higher doses of CD34+ cells facilitate engraftment and immune reconstitution.34-36
In this study, TCD PBSCs grafts had 5.75 ×106
/kg median CD34+ cell doses, similar to T-cell replete PBSC37
, and much higher than 1.2 × 106
/kg for TCD BM grafts (historical, unpublished results) . Log 10 depletion of CD3+ in PBSC was 5.5 compared with 2.5-3 for BM. The level of TCD in this study is approximately 1/2 log greater than that achieved with the currently available CliniMACS device 38
Median age of the patients was >10 years older than our earlier reports and similar to our recent study of TCD matched, related PBSC transplants.1,2,5
Distribution of diseases was representative of older patients and included many treated at advanced stages. Infectious complications reflect current day pathogens but notably, did not include Streptococcal
bacteremias which were observed with this regimen previously5
. Early initiation of antibiotics, for fevers during conditioning, may have avoided this complication.
This trial evaluated changes in the regimen and graft. Despite reducing the intensity of the conditioning, the OS and DFS for the 17 ‘standard’ risk patients was excellent, >70% and comparable to our previous results5
and those reported by others. The Kaplan-Meier survival curve for this group did not change after the 8th
month. The OS and DFS for the ‘poor’ risk transplant group was 47% and 41%, respectively, again similar to those with T-cell replete grafts, but with less GVHD. Median follow up for this trial was >4 years, encompassing the only 2 relapses. The very low incidence of relapse argues against a loss of graft versus tumor effect with TCD. Deaths on this trial generally occurred within the first 6-8 months and were predominantly infection-related.
The low relapse rate in the setting of a TCD allograft invokes the possibility that NK cells are the primary mediator of the graft-versus-tumor effect. Classic NK alloreactivity due to killer Ig-like receptor (KIR)-mediated recognition of “missing self” occurs when MHC class I KIR ligands present in donor are lacking in the transplant recipient. Only 4 of the 18 HLA-mismatched donor-recipient pairs exhibited KIR ligand incompatibility and only 1 of these pairs had KIR ligand incompatibility in a graft-versus-host vector. For this 1 pair, however, donor KIR genotyping revealed lack of the relevant inhibitory KIR responsible for sensing the class I ligand lacking in the recipient; and thus, no NK alloreactivity due to recognition of missing self would occur. Donor NK alloreactivity due to missing self-MHC, therefore, could not account for the low relapse rate among these patients.
NK alloreactivity due to “missing ligand,” which we have previously described39
, can occur when the patient is lacking any class I ligand for donor inhibitory KIR, irrespective of whether the ligand is present in the donor. This receptor-ligand mismatch occurred in 22 of the 35 pairs. Because the event of relapse is low in this small cohort, however, we cannot conclude that NK alloreactivity due to missing ligand is responsible for protection from relapse. A larger cohort would be necessary to confirm this.
A goal of the current study was to reduce the dosing of ATG and potentially improve immune recovery without increasing the incidence of graft rejection or GVHD. Based on previously identified predictors of graft rejection using TCD BM grafts9,40
, more than half the patients were at increased risk. However, only 1 HLA mismatched graft recipient, in the poor risk group, experienced late graft failure. Enhanced immunosuppression provided by fludarabine coupled with the higher CD34+ dose achieved with PBSCs likely contributed to the high rate of durable engraftment. These results support further reduction in ATG dosing in future trials.
The TCD PBSC contained >1 log10
lower concentration of T cells compared with TCD BM grafts (median 1.2 vs 42.5 × 103
and a 4 log10
lower concentration than T-cell replete PBSC grafts with similar CD34+ numbers . 37
However, the incidence and severity of GVHD in this study, is higher than that previously reported for our TCD BM and PBSC with matched related donors2,5
, and for HLA haplotype disparate related donors reported by Aversa et al. 41
Factors contributing to the GVHD might include: 1) additional allogeneic disparities in unrelated and mismatched donors; 2) qualitatively different T cell and NK populations in TCD PBSC vs. TCD BM; and 3) the relatively low dose of ATG employed.
The goal of utilizing lower dose ATG was to improve T cell recovery and potentially reduce life-threatening OIs. Recovery of CD4 counts to ≥200 and normalization of CD8 T cells were similar for the TCD PBSC (6-9 months), compared with unrelated TCD BM (12-18 months) 18
. CD4+CD45RA+ cell recovery was also faster than for unrelated TCD BM. Thus, despite including ATG, time to recovery of CD4+ and CD4+CD45RA+cell counts in this trial was similar to that for recipients of related TCD PBSC using identical cytoreduction but without ATG.5
Similarly, patients’ PHA response normalized faster on this trial than in recipients of TCD unrelated BM, who had a PHA response <25% LLN for the first year vs the median PHA response in this study of 60% LLN at 12 months, and normal by 18 months. Keever-Taylor et al. recently published the results of lymphoid recovery in patients transplanted on the National Heart, Lung and Blood Institute randomized TCD versus T-cell replete unrelated BM trial which included posttransplant immunosuppression.42
Despite the older age of the current study patients, recovery of CD4+ cells was similar and CD4+ CD45RA+ cells more rapid compared to that reported by Keever-Taylor et al. (median age 31.2 years).
Infection as the primary cause of death occurred in 17%. This is lower than the 29 and 30% incidence of fatal infection reported by Wagner et al. in recipients of T-cell replete or TCD unrelated stem cell transplants who received immunosuppressives for GVHD prophylaxis. 43
Furthermore, the higher incidence of deaths from CMV and fungus reported by van Burik et al. in TCD unrelated transplant recipients who also received posttransplant GVHD prophylaxis44
was not observed in this study. No patient died of CMV disease and only 1died of a fungal infection.
This study supports the use of TCD PBSC transplantation from both matched and mismatched unrelated donors in appropriately selected adult patients, especially those at increased risk of GVHD. Acknowledging the limited numbers of patients, the median survivals compare favorably with those reported by the NMDP for each disease group45
. The risk of relapse was remarkably low, with a median followup of >4 years. The character of complications and death were similar to those observed with T-replete transplantation, but with a lower incidence and severity of GVHD. Adequately TCD PBSC from unrelated donors precludes the need for posttransplant GVHD prophylaxis, allowing more rapid recovery of functional immunity that reduces the risk of lethal opportunistic infections. It provides an alternative form of transplantation for patients without sibling donors including those who are older than 40 years.