In studies of patients who receive myeloablative conditioning and unrelated marrow grafts, the incidence of graft rejection was higher among patients given grafts mismatched for one or more class I HLA antigens [5
]. We sought to determine whether graft rejection was increased when class I HLA-mismatched grafts were transplanted after nonmyeloablative conditioning and found that sustained donor engraftment was observed in 95% of recipients. Although initially designed as a dose escalation trial of alemtuzumab according to the number of patients experiencing graft rejection, the dose-escalation rule was never activated. Thus the combination of fludarabine and 2 Gy TBI, followed by a post-transplant combination of MMF given three times a day (TID) and CSP, was sufficient to ensure a high rate of sustained full donor T-cell chimerism and engraftment even with HLA-class I mismatched grafts.
We previously had used 15 mg/kg MMF BID administrated from day 0 to day 40 with taper through day 96 in combination with CSP as GVHD prophylaxis after nonmyeloablative HLA-matched unrelated donor HCT. However, a high graft rejection rate of 21% was observed. By increasing the dosing of MMF to 15 mg/kg TID (because of the short half-life (t½) of only 3 hours of the active metabolite of MMF) [7
] and using only G-PBSC as a stem cell source, we successfully reduced the rejection rate to 5 % in nonmyeloablative HCT from HLA-matched unrelated donors [32
]. Therefore, in the current study we used MMF 15 mg/kg TID and extended the duration of MMF given until day 100 for the HLA-mismatched setting. As a result, sustained engraftment was observed in 95% of evaluable patients without the use of alemtuzumab.
We were unable to detect any enhancement of graft-versus-tumor effects with the use of HLA-mismatched unrelated grafts. Observed cumulative probabilities of relapse/progression of 22% at 1 year and 36% at 2 years were very similar to our previous HLA-matched unrelated HCT data (26% at 1 year and 31% at 2 years) [32
]. If limited to patients not in CR at HCT, the CR rate in this study [8 of 30 (27%)] was lower rather than 48% of the previous our study.
An association between HLA-class I mismatches and a high incidence of GVHD has been described in recipients of ablative conditioning regimens followed by unrelated donor HCT [33
]. Previous studies reported that the cumulative incidences of grades II to IV acute GVHD varied from 43% to 74% after myeloablative HCT with HLA-matched unrelated marrow and 63% to 80% after myeloablative HCT with HLA-one antigen mismatched unrelated marrow [36
]. Chronic GVHD was observed in more than 55% of patients after myeloablative HCT with HLA-matched unrelated marrow and in 70% of patients after myeloablative HCT with HLA-one antigen mismatched unrelated marrow [21
]. In a previous study using the same nonmyeloablative conditioning regimen (fludarabine and 2 Gy TBI), 103 patients received either 10/10 HLA-matched (n=93) or single HLA class I allele-level mismatched (single each; n=10) unrelated donor HCT. In that study, we reported cumulative incidences of grades II to IV acute and chronic extensive GVHD were 53% and 56%, respectively [32
]. Interestingly, the observed 41% incidence of chronic extensive GVHD in the current study is comparable to or less than that observed in the HLA-matched unrelated setting. The prolonged CSP and MMF administration in this study may have contributed to the similar incidence of chronic GVHD. On the other hand, the observed 69% rate of grades II to IV acute GVHD is higher than that seen in the HLA-matched unrelated setting [32
]. The cumulative probability of NRM of 47% at 2 years in the current study is also higher than that seen in the HLA-matched unrelated setting. Additionally, in the current study acute GVHD contributed to death in 9 of 26 non-relapse deaths (35%), and 6 of 7 (86%) grade IV gastrointestinal episodes were associated with gut GVHD which directly or indirectly caused mortality.
As might be expected, there was concern that the prolonged CSP and MMF administration in the present protocol caused an increased risk of infection. Indeed, the incidence of infection was increased compared to our previous results in unrelated HCT (the documented rates per 100 patients days of viral, fungal, and bacterial infection were 0.86, 0.26, and 1.05, respectively, in the previous study [32
]). In 12 of 26 (46%) patients, the causes of NRM were associated with infection.
Since excess immunosuppression can lead to high rates of relapse and infection due to delayed immune reconstitution, an optimal prophylactic regimen for GVHD in HLA-mismatched or unrelated HCT has been explored by other investigators. Alemtuzumab (total 50, 100 mg/person or 1.2 mg/kg) has been applied in HLA haploidentical related, and HLA-matched and mismatched unrelated HCT [40
]. Some investigators reported that alemtuzumab-containing regimens were highly effective in preventing chronic as well as acute GVHD without an increased risk of relapse [40
]. However, high relapse rates, particularly in patients with active disease at HCT (5 of 8 patients [63%]) [42
] and high infection rates (serious infective complication 62.2%) [43
] due to a delay in immune reconstitution were also reported. Furthermore, in a recent publication, we reported that HLA-haploidentical nonmyeloablative related HCT with high-dose posttransplantation cyclophosphamide showed an acceptable sustained engraftment rate of more than 95% and better control of acute GVHD (34% for grades II–IV and 6% for grades II–IV 6%) without increased severe infection. However, a relatively high relapse rate of 51% at 1 year was seen [44
]. In our previous retrospective study, extensive chronic GVHD but not grade II to IV acute GVHD was significantly associated with a decreased risk of relapse or progression without increased NRM [45
]. These data indicate that an intensified prophylaxis of acute GVHD but not chronic GVHD by an additional immunosuppressive agent such as low-dose alemtuzumab [46
] or sirolimus [47
] may be a reasonable strategy to improve outcomes.
The observed overall survivals in all the three groups stratified by HCT-CI score were inferior to recipients of HLA-matched related and/or unrelated HCT. These data suggested that HLA-class I disparity could be another independent outcome factor in the nonmyeloablative setting.
Immune reconstitution was similar to that after HLA-matched unrelated donor nonmyeloablative HCT [48
], with the exception of slower recovery of CD8+ T cells and B cells. Both naive and memory/effector CD8+ T cells recovered slowly, suggesting both reduced thymopoiesis as well as reduced peripheral expansion, possibly due to the fact that many patients had GVHD (clinical or subclinical) and were treated with prolonged immunosuppression drugs. The slow recovery of B cells may also be due to the fact that many patients had GVHD, as GVHD and/or its treatment hamper B-lymphopoiesis [49
]. The slow recovery of CD8+ T cells and B cells might have contributed to the relatively high infection rates.
In conclusion, this study demonstrated the feasibility of nonmyeloablative HCT from HLA-class I mismatched donors using fludarabine and low-dose TBI conditioning. While almost 30% of patients experienced long-term survival with this approach, there was a high incidence of acute GVHD and NRM decreased survival compared to fully HLA-matched unrelated patients. Future studies of more intense early prophylaxis of GVHD in this HLA-class I mismatched setting may decrease severe acute GVHD and improve survival.