NaïVe T Cell Depletion of PBSC Grafts Results in Very Low Rates of Chronic Gvhd and High Survival

Background

Graft-versus-host disease (GVHD) frequently causes morbidity and mortality after allogeneic hematopoietic cell transplantation (HCT) as a result of organ damage and infections. In HLA-identical HCT, GVHD results from recognition by donor T cells of minor histocompatibility (H) antigens on recipient tissues. Complete T cell depletion (TCD) of donor hematopoietic cell products is more effective than pharmacologic immunosuppression for preventing GVHD, but is complicated by delayed immune reconstitution and consequent life-threatening infections.Approaches to HCT which preferentially deplete the T cells that primarily cause GVHD and preserve pathogen-specific T cells may improve HCT outcomes.

Mature CD3⁺ CD8⁺ and CD3⁺ CD4⁺ T cells can be classified into CD45RA⁺ CD62L⁺ naïve (T_N) and CD45RO⁺ memory (T_M) subsets, the latter of which includes effector memory (T_EM) and central memory (T_CM) cells. Murine studies in which allogeneic TCD bone marrow (BM) is transplanted with purified T cells from individual T cell subsets to irradiated minor H antigen disparate recipients have demonstrated that the most severe GVHD results from transplanting T cells of the T_N subset. Purified T_CM causes mild GVHD and T_EM do not cause detectable GVHD and can transfer immunity to pathogens.In vitro studies have similarly demonstrated that human donor CD8⁺ T cells specific for recipient minor H antigens are found predominantly within the T_N cell subset, suggesting selective T_N cell depletion may alter the GVHD incidence and/or severity in human HCT.

Methods and results

We developed an effective process for engineering human peripheral blood stem cell (PBSC) grafts that depletes CD45RA⁺ T_N cells and retains CD34⁺ stem cells and functional CD45RO⁺ T_M cells specific for a broad range of opportunistic pathogens (Bleakley BBMT 2014). We are conducting clinical trials to evaluate the selective depletion of T_N cells from HLA-matched allogeneic PBSC grafts for the prevention of GVHD in patients with acute leukemia, the first of which has been published (Bleakley JCI 2015, N=35). Seventy patients have now been treated on three consecutive phase II trials. The median age was 34 years (1-56 years), 56% of patients had a diagnosis of ALL, 46% had previously relapsed or had detectable disease (MRD or relapse) at the time of HCT, and 23% had unrelated donor (URD) grafts. Intensive myeloablative, TBI-containing (13.2Gy) conditioning was used for 63 patients, whilst 7 patients received a medium intensity 'midi' preparative regimen, including 4Gy of TBI. The T_N-depletion procedure was successfully performed on URD PBSC products shipped overnight from donor centers throughout the US, as well as on MRD PBSC collected at our centers. Reliable engraftment with high-level donor chimerism was observed in recipients of 'midi' as well as intensive myeloablative conditioning. The 2-year estimates of overall survival, disease-free survival, survival free of relapse and chronic GVHD (CRFS) and survival free of relapse, grade II-IV acute GVHD, and chronic GVHD (GRFS) are 79%, 73%, 69% and 63% respectively. Median follow-up among survivors is 26 months. The frequency and severity of chronic GVHD is remarkably low (5%) compared to historical rates of 40-60% chronic GVHD in HLA-matched PBSC transplantation with conventional calcineurin inhibitor-based immunosuppression. Relapse and non-relapse mortality (NRM) are acceptably low at 19% and 8%, respectively. No NRM occurred in patients <40 years. Updated results will be presented.

Conclusions

The outcomes of recipients of T_N-depleted PBSC grafts compare very favorably to published results of HCT for patients with acute leukemia. For example, the 69% incidence of CRFS at 2 years in T_N-depleted recipients compares with reported 2-year GRFS rates of 37% and 17% in recipients of allogeneic PBSC from HLA-matched related donors with or without ATG (Kroger et al. NEJM 2016). Our results suggest that T_N-depletion of PBSC grafts may reduce the risk of chronic GVHD without negatively impacting other important HCT outcomes.