HLA-Haploidentical Stem Cell Transplantation with Treg and Tcon Adoptive Immunotherapy promotes a Strong Graft-Versus-Leukemia Effect

In HLA-haploidentical stem cell transplantation we showed adoptive immunotherapy with naturally occurring T regulatory cells (nTregs) followed by conventional T cells (Tcons) prevented acute and chronic graft-versus-host-disease (GvHD), favoured lymphoid reconstitution and immunity against pathogens (Di Ianni et al., Blood 2011). One major concern is whether a graft-versus-leukemia (GvL) effect is maintained since FoxP3⁺Tregs can also suppress immune response against tumour. However, we showed in immunodeficient NSG mice engrafted with human myeloid and lymphoblastic acute leukemia cells (7x10⁶ iv), Treg (3x10⁶)+Tcon(3x10⁶) infusion rescued mice from leukemia death in the absence of GvHD. In contrast, controls died of leukemia (when not treated or infused with Tregs only) or of GvHD (when infused with Tcons only).Since September 2008, 45 consecutive patients (21 male; 24 female; median age 40, range 20-59) with high-risk acute leukemia ( 33 AML, 12 ALL) were transplanted. The conditioning regimen included TBI, thiotepa, fludarabine and cyclophosphamide (25 patients in the 1^st clinical trial) or alemtuzumab/ATG (20 patients in the 2^nd clinical trial). On day -4, patients received an infusion of freshly isolated donor Tregs (mean 2.5/kg ±1; Foxp3 81.01%±16.47; Helios/FoxP3 54%±8.4; CD127 11.72%±7.653) followed on day 0 by a megadose of purified CD34+ cells (9.7x10⁶/kg ± 3.1) and Tcons (mean 1.1 /kg ±0.6; CD3 90.72%± 9.6; CD4 57.77% ± 8.85; CD8 31.21% ± 8.59). No post-transplant GvHD prophylaxis was given. 43 of the 45 patients achieved primary, sustained full-donor type engraftment. Only 6/43 evaluable patients developed ≥ grade II acute GvHD (3 alive, treated and now off-therapy). At a median follow-up of 28 months (range 4-58) no patient has developed chronic GvHD. CD4 and CD8 counts reached 100/µL medianly on days 40 (range 25-150) and 45 (range 18-100) and 200/µL on days 55 (range 45-160) and 60 (range 50-140). In KIR-ligand mismatched transplants (Ruggeri et al., Science 2002; Ruggeri et al., Blood 2007), donor versus recipient alloreactive NK cells were also preserved. At a median follow-up of 46 months (range 7-58) 23/43 patients are alive and disease free with a probability of DFS of 0.56. The cumulative incidence of TRM is 0.37. The cumulative incidence of relapse was 0.06 as only 2 AML patients transplanted from non-NK alloreactive donors have relapsed to date. This incidence is extremely low considering the high-risk patient population (9 with FLT-3/ITD; 8 with complex karyotypes; 4 with t(9:22); 2 PIF; 1 secondary AML; 1CR after second-line induction; 1 with CNS and skin localization at diagnosis; 19 ≥ 2CR). In conclusion, adoptive immunotherapy with Tregs and Tcons exerts a powerful GVT activity in both AML and ALL patients in the absence of GvHD.