Adoptive Immunotherapy with Tregs and Tcons Ensures Low TRM and a Low Incidence of Post Transplant Leukaemia Relapse After HLA Haploidentical Transplants for Acute Leukemia

Abstract 154

In full haplotype mismatched (HLA-haploidentical) stem cell transplantation we showed adoptive transfer of freshly isolated donor CD4+CD25+ FoxP3+ T regulatory cells (Tregs) followed by donor T cells (Tcons) prevented acute and chronic GvHD without any post-transplant immunosuppression, promoted lymphoid reconstitution and improved immunity against opportunistic pathogens (Di Ianni et al., Blood 2011). The major drawback was the extra-haematological toxicity of the conditioning regimen which included TBI, thiotepa, fludarabine and cyclophosphamide. To reduce regimen related toxicity we replaced cyclophosphamide with alemtuzumab, given 22 days before the Treg infusion to prevent it from interfering with adoptive T cell immunotherapy (Fig 1). The graft consisted of immunoselected Tregs (median 2×10⁶/kg; range 1.6–4.8; FoxP3+ cells 92% ± 8 SD;), CD34+ cells (median 9.1×10⁶/kg; range 8.1–10.9) and Tcons (median 1×10⁶/kg; range 0.5–3). No post-transplant prophylaxis against GvHD was given.

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Since May 2010 18 patients (median age 43 years, range 23–61) with high risk acute leukaemia (16 AML, 2 ALL) have been transplanted. All sustained full donor-type engraftment. Neutrophils reached 0.5×10⁹/L at a median of 12 days (range 9–28 days). Platelets reached 20×10⁹/L and 50×10⁹/L at median of 12 and 15 days, respectively (range 10–36 days and 11–55 days). CD4⁺ and CD8⁺ peripheral blood counts reached, respectively, 50/μL medianly on days 36 (range 27 – 120 days) and 34 (range 15– 85); 100/μL medianly on days 55 (range 27 – 147 days) and 48 (range 27 – 114); 200/μL on days 62 (range 37 – 177 days) and 49 (range 28 – 147). We observed a rapid development of a wide T-cell repertoire with specific CD4+ and CD8+ T cells for opportunistic pathogen antigen such as Aspergillus, Candida, CMV, ADV, HSV, VZV, Toxoplasma. Treg immunotherapy did not compromise post-transplant generation of donor-vs-recipient alloreactive natural killer (NK) cell repertoires in patients who received transplants from NK alloreactive donors (Ruggeri et al., Science 2002).

Three of 16 valuable patients developed acute GvHD. Two responded to a short course of immunosuppressive therapy and at present (288 and 360 days after transplant) are alive and well with very good immunological reconstitution. The 3^rd patient died of infectious complications. Two other patients died of non-leukemic causes (1 fulminant hepatitis 17 days post-transplant, 1 pneumonia 14 days post-transplant). The incidence of TRM is 17% (3/18). As hoped, extra-haematological toxicity was mild. One AML patient, who received a transplant from a non-NK alloreactive donor, relapsed 77 days post-transplant. Fourteen of the 18 patients are alive and well at a minimum follow-up of 3 months.

This study shows adoptive immunotherapy with freshly isolated, naturally occurring Tregs is a feasible option in HLA-haploidentical stem cell transplantation since alloantigen-specific Tregs were efficiently activated in vivo and controlled alloreactivity of at least 1×10⁶/kg Tcons without clinically significant inhibition of general immunity.

Moreover Treg infusion did not weaken the GvL effect. The incidence of post-transplant leukaemia relapse was surprisingly low as only 1 patient has relapsed to date and even in our previous series no patient who was transplanted in CR has relapsed at a median follow-up of 25 months. Infusion of high numbers of Tcons in the absence of post-transplant immunosuppression can be hypothesized to exert a GvL effect. In addition, in patients who were transplanted from NK alloreactive donors, preservation of alloreactive NK cell repertoires played a key role in reducing the incidence of relapse.