How Adoptive Immunotherapy with Conventional T and Regulatory T Cells Exerts a Gvl Effect without GvHD, after Haploidentical Hematopoietic Transplantation

▪ Post-transplant relapse is still a major cause of treatment failure in high-risk acute leukemia (AL) patients. Attempts to manipulate donor T cell alloreactivity to spare normal tissues while killing leukemic cells have been largely unsuccessful. In the search for strategies to separate the GvL effect from GVHD, we investigated the role of a thymic-derived CD4+CD25+ FoxP3+ regulatory T-cell subpopulation (Tregs) that physiologically helps maintain immunological self-tolerance and immune homeostasis. Evidence from murine bone marrow (BM) transplantation across major histocompatibility barriers showed co-infusion of conventional T lymphocytes (Tcons) with Tregs suppressed lethal GVHD without impairing Tcon activity against malignant diseases. In 69 high-risk AL patients who had received an HLA haploidentical T cell-depleted hematopoietic transplant and no post-transplant immunosuppressive GvHD prophylaxis, adoptive immunotherapy with donor Tregs (2 × 106/kg) and Tcons (1 × 106/kg) protected patients from GvHD (Di Ianni et al., Blood 2011) and largely prevented post-transplant leukemia relapse. In fact, only 5% of patients relapsed (Martelli et al., Blood 2014). However, also because such patients had advanced stage disease, TRM was still in the range of that of T cell-depleted haplo transplants, (i.e. 40%, Aversa et al., JCO 2005). A current cohort of AL patients was conditioned with total body irradiation (TBI) (8Gy as single dose TBI or 13.5 Gy as fractionated TBI), cyclophosphamide (30mg/kg), thiotepa (8mg/kg) and fludarabine (200mg/m2). To date, 24 high-risk AL patients (7 ALL, 17 AML) have been enrolled. Twenty-two of the 24 patients engrafted, one AML patient relapsed, 4/22 developed aGvHD (4/4 are alive) and one developed cGvHD, TRM was exceptionally low (Fig. 1). At a median follow up of 2 years, 19 patients are alive. Consequently, probabilities of leukemia-free and leukemia/cGVHD-free survivals are good (Fig. 1). Hypotheses have been put forward to explain the ability of Tregs to prevent of GvHD while preserving the GvL effect mediated by Tcons. However, the mechanism is still unknown. In humans, naïve CD45RA+ Tregs express CXCR4 BM homing receptor and preferentially localize to the BM while memory CD45RO+ Tregs display lower CXCR4 expression and home to the periphery (Booth et al., J Immunol. 2010). Since Tregs that are recovered from peripheral blood and are used for adoptive immunotherapy are CD45RO+ and largely CxCR4 negative, we hypothesized