Acute myeloid leukemia (AML) is the most frequent acute leukemia in adults. The median age at diagnosis is 72 years and its incidence increases by age peaking at 80 years . While the recent employment of new treatments and hematopoietic cell transplantation protocols improved survival of AML patients, outcomes are still poor in the elderly. In fact, the 2-year overall survival is <20% in patients over the age of 60 . The presence of frequent clinically relevant comorbidities that limit standard therapies is not the only reason of such results. The AML in the elderly is characterized by intrinsic biological factors that favor disease resistance, such as the appearance of clones with adverse cytogenetics and a previous diagnosis of myelodysplastic syndrome (MDS). Despite the poor overall survival, a standard intensive chemotherapy can still achieve a complete remission (CR) in about half of the treated patients and even hypomethylating agents can obtain a CR in about 20% of them. These results suggest that the lack of safe and powerful enough consolidative treatments could be responsible for such outcomes of AML in the elderly.
Hematopoietic stem cell transplantation (HSCT) is a curative treatment in patients with AML at high risk of relapse. Because of their potent antileukemic activity, myeloablative conditioning (MAC) regimens are often preferred in AML patients, but their unacceptably high toxicity and subsequent non-relapse mortality limit their use in the elderly. The development of safe and effective reduced-intensity conditioning regimens (RIC) extended the age limit for HSCT to old patients. Rashidi and coworkers  recently reported a meta-analysis of 13 eligible studies, with a total sample size of 749 AML elderly patients who received a RIC-matched HSCT. The cumulative incidence of relapse (CIR) at 2 and 3 years was 39% and 39%, respectively. Non-relapse mortality (NRM) was 29 and 40%, respectively. Relapse-free survival (RFS) at 2 and 3 years was 44 and 35%, respectively.
Similar CIR, NRM, and RFS were also reported by McClune and colleagues  in a retrospective analysis on a large number of AML patients in first CR undergoing RIC HSCT from a related or unrelated donor.
RIC may reduce morbidity and mortality. However, it may also be less effective than myeloablative regimens with regard to leukemia eradication. Indeed, in a recent phase III study, Scott and colleagues  compared MAC and RIC HLA-matched transplants for AML and MDS. The study was stopped early because of the high relapse incidence with RIC (48.3%). A statistically significant advantage in RFS with MAC was also observed. The lower antileukemia efficacy was likely a result of lower cytotoxicity. The authors concluded that for patients who cannot tolerate MAC, novel regimens are needed.
Focused radiation techniques, such as the helical tomoradiotherapeutic approach, are one such option to potentially increase myeloablative strength while decreasing transplant-related toxicity. Indeed, helical tomoradiotherapy (HT) provides the ability to “sculpt” radiation doses to skeletal bone, major lymph node chains, and spleen, while simultaneously reducing doses to major visceral organs. Researches at City of Hope Medical Center pionereed this technology in T-cell replete matched transplantation for acute leukemia patients [6, 7].
A recent prospective study  included 61 patients treated with total marrow irradiation (TMI) and total lymphoid irradiation (TLI) to a dose of 12 Gy (1.5 Gy BID×4 days), fludarabine (25 mg/m2/d×5 days), and melphalan (140 mg/m2/d×1 day). The majority of patients had acute leukemia (72%) and 67% of them had high-risk disease. They were transplanted with a median age of 55 (range: 9–70). Acute Graft-vs-Host Disease (GvHD) of any grade was 69% and chronic GvHD was 74%. Relapse-free survival was 41%, with a CRI of 26% and a NRM of 33%. Regarding toxicity, mucositis and gastrointestinal toxicity were above grade 2 in 56% and 16% of the patients, respectively.
We employed the same radiotherapy technology for T-cell depleted HLA-haploidentical HSCT in the elderly with high-risk AML. TMI was delivered in nine fractions of 1.5 Gy each from day −15 to day −11, two fractions each day. TLI was delivered in nine doses of 1.2 Gy each from day −15 to day −11, two fractions each day. The skeletal bone, the major lymph node chains, and the spleen were considered target organs. Avoidance-contoured structures included lungs (6.5 Gy), heart (6.2 Gy), kidneys (5.9 Gy), liver (7.7 Gy), small bowel (6.3 Gy), rectum, esophagus, stomach, oral cavity, parotid glands, thyroid gland, eyes, lens, brain, bladder, and testes. The conditioning regimen also included thiotepa (7.5 mg/kg), fludarabine (150 mg/mq), and cyclophospamide (30 mg/kg). Following the conditioning, the patients received a “designed” haploidentical graft , which includes a megadose of purified CD34+ cells, and 1 million per kilogram of conventional T lymphocytes (Tcons) under the protective umbrella of 2 million per kilogram of freshly isolated regulatory T cells (Tregs) (Fig. 1). No post-transplant immunosuppression was given whatsoever.
We previously showed that adoptive immunotherapy with naturally occurring polyclonal Tregs is a feasible option in HLA-haploidentical HSCT, since this strategy controls alloreactivity of a significant number of coinfused Tcons with low incidence of GvHD, favors immune recovery, and is associated with a powerful GvL effect [10, 11]. From June 2015 to August 2017, 14 AML patients with a median age of 62 years (range 55–68) underwent HSCT from a full haplotype-mismatched family member. Six patients were in CR1, seven in CR2, and one in PR. HSCT was indicated for all patients in CR1 because of adverse cytogenetic or molecular features. Seven patients had composite comorbidity/age scores  of 1 or 2, while eight patients had scores of 3 or 4. Donor vs. recipient NK cell alloreactivity  occurred in eight donor–recipient pairs. All patients achieved primary and sustained full donor-type engraftment. Neutrophil and platelet recoveries were very fast. As expected, regimen-related toxicity was mild enough in the majority of patients. One patient died of treatment-related veno-occlusive disease. Six patients (43%) developed grade II–IV acute GvHD. Five of these patients are alive and off therapy. No patient developed chronic GvHD. A lower incidence of acute GvHD (15%) was observed in a previous study, which included 60 acute leukemia patients with a median age of 40 years . These patients were infused with a similar designed graft (megadose of CD34+ cells and Tregs/Tcons), following a total body irradiation (TBI)-based conditioning regimen. Perhaps, the difference in acute GvHD incidence between the two studies is related to the different median age of the recipients (62 years vs. 40 years).
The pattern of post-transplant immune reconstitution was very different to what we usually see in a standard T-cell depleted haploidentical transplantation. We observed a rapid recovery of CD4+ and CD8+ T-cell subpopulations, as well as a rapid development of a wide T-cell repertoire, which approached normal scores in a few months. Soon after transplant, high frequencies of pathogen-specific CD4+ and CD8+ lymphocytes against Aspergillus, Candida, CMV, and other opportunistic pathogens occurred. Such levels were usually achieved many months after transplant in our historical series of haploidentical transplants. As expected, during the first 6 months post transplant, the immunological reconstitution relies on peripheral expansion of memory T lymphocytes. A rapid and sustained reconstitution of peripheral blood B-cell number, B-cell repertoire, and levels of immunoglobulins in the sera of the patients also occurred.
Overall the cumulative incidence of NRM was 29%. The causes of non-relapse death were veno-occlusive disease (1), GvHD (2), and infection (3). At median follow-up of 24 months (range 12–36), no patient relapsed so far. In total, 10/14 patients are alive and well with a RFS probability of 71%.
In conclusion, this pilot study shows the addition of TMI/TLI to relatively low doses of chemotherapy (fludarabine/thiotepa/cyclophosphamide) induces a powerful myelo- and immuno-ablation with mild enough extra medullary toxicity in the elderly with AML. This regimen ensures a high rate of engraftment of the T-cell depleted haploidentical transplant. Furthermore, its myeloablative intensity contributes to eradicate leukemia, in combination with cGvHD-free allogeneic effects of Treg/Tcon-adoptive immunotherapy and NK-cell alloreactivity. A very low incidence of leukemia relapse was already reported in a large cohort of AML patients, who received a TBI-based conditioning, followed by a CD34+ cell megadose and Treg/Tcon-adoptive immunotherapy . Thus, both studies showed that Tregs (2 × 106/kg) interfere with the pathophysiology of GvHD, with no need of any post-transplant pharmacological immunosuppression and permit co-transplantation of enough Tcons (1 × 106/kg) to eradicate minimal residual disease.
The mechanisms underlying Treg suppression of GvHD with no loss of Graft-vs-Leukemia (GvL) activity are still obscure. However, recent studies suggest that donor CD45RO + Treg/Tcon-adoptive immunotherapy mediates the GvL effect in the absence of GvHD, because Tcons that home to the bone marrow exert unopposed alloantigen recognition and kill leukemia, while Tcons that home to the periphery are blocked by CD45RO + Tregs (Ruggeri and colleagues, unpublished observations). In other words, Treg/Tcon- adoptive immunotherapy confines graft vs. host reaction only to the hematopoietic system, consequently separating the GvHD from the GvL effect.
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Publication of this supplement was sponsored by Gilead Sciences Europe Ltd, Cell Source, Inc., The Chorafas Institute for Scientific Exchange of the Weizmann Institute of Science, Kiadis Pharma, Miltenyi Biotec, Celgene, Centro Servizi Congressuali, and Almog Diagnostic. AP and AV were funded by the Associazione italiana per la Ricerca sul Cancro (AIRC).
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Pierini, A., Ruggeri, L., Carotti, A. et al. The “ultimate” haploidentical transplantation for the elderly with high-risk acute myeloid leukemia. Bone Marrow Transplant 54, 803–805 (2019). https://doi.org/10.1038/s41409-019-0618-x