Non-Myeloablative Conditioning of Total Lymphoid Irradiation (TLI) and Anti-Thymocyte Globulin (ATG) Protects Against Acute GVHD Following Allogeneic Hematopoietic Cell Transplantation (HCT) but Retains Anti-Tumor Activity.
Authors: Lowsky Robert , Takahashi Tsuyoshi , Ping Yin , Shizuru Judith , Negrin Robert S. , Strober Samuel 
Published: April 2020

Abstract
Separation of GVHD from graft versus tumor (GVT) reactions is critical in improving outcomes for HCT. Murine models of transplantation showed that after conditioning with repeated low doses of irradiation targeted to lymphoid tissues (TLI) are combined with ATG, regulatory natural killer (NK) T cells become the predominant T cell subset. Secretion of high levels of IL-4 by the host NK T cells protects against aGVHD following HCT. Yet tumor killing activity mediated by donor CD8+ T cells via a direct cytolytic pathway involving perforin remains intact. Thus, regulatory T cells can separate GVHD from graft anti-tumor activity. We adapted the murine protocol to a clinical regimen of TLI (10 doses of 80 cGy/dose) and rabbit ATG (5 doses of 1.5 mg/kg/dose) with post-grafting immunosuppression of mycophenylate mofetil (MMF) and cyclosporin (CSP) to determine if the regimen separates aGVHD from GVT reactions in humans. In a completed phase I and an ongoing phase II study 37 patients with extensively pretreated hemato-lymphoid malignancies (22 with lymphoma, 4 with lymphocytic leukemia and 11 with AML) received related (23) or unrelated (14) HLA matched G-CSF mobilized HCT. Twenty nine patients (78%) had advanced stage disease, 12 had received prior autologous transplants, 18 were in a partial remission (PR) at the time of allogeneic transplant, 2 had progressive disease (PD) and 17 were in complete remission (CR). All patients had initial multilineage donor hematopoietic cell engraftment within 56 days post transplantation. The median follow-up (F/U) for all patients is 262 days with 27 of 37 patients alive. Thirty six of 37 patients had grade 0 aGVHD and 1 patient had grade III aGVHD that responded to steroid therapy. Thirty-five patients were alive at day 100 and considered at risk for cGVHD. Six of the 35 developed denovo extensive cGVHD, and one developed extensive cGVHD following aGVHD. Twenty-eight patients had either no or limited cGVHD. Of the 18 patients transplanted in PR, 11 achieved a CR and have not relapsed, 2 did not clear their tumor, 2 are too early to evaluate and 3 died from non-relapse causes. Eleven of 16 patients transplanted in CR continue in CR and of the 5 that relapsed all had advanced stage disease. Evaluation of sorted CD4+ T cells obtained 1–6 months after HCT from fully chimeric recipients conditioned with TLI/ATG showed a statistically significant increase in IL-4 secretion following in vitro stimulation, and a statistically significant decrease in the proliferation response to allogeneic stimulator cells in the mixed leukocyte reaction (MLR) as compared to normal controls or to patients given non-myeloablative TBI conditioning. Sorted CD8+ T cells obtained from TLI/ATG conditioned patients retained vigorous cytolytic activity in the cell mediated lympholysis (CML) assay. In conclusion, TLI/ATG conditioning resulted in a markedly reduced incidence of aGVHD but with retained GVT reactions as the majority of patients with PR converted to CR and did not relapse. We show evidence that as in the pre-clinical model the low incidence of GVHD is associated with increased IL-4 secretion by chimeric donor T cells and a reduced proliferative response to alloantigens but retained anti-tumor activity.