Graft versus host disease (GVHD) remains a major cause of morbidity and mortality following allogeneic hematopoietic stem-cell transplantation (HSCT). Despite the use of prophylactic GVHD regimens, a significant proportion of transplant recipients will develop acute or chronic GVHD following HSCT. Corticosteroids are standard first-line therapy, but are only effective in roughly half of all cases with ~50% of patients going on to develop steroidrefractory disease, which increases the risk of nonrelapse mortality. While progress has been made with improvements in survival outcomes over time, corticosteroids are associated with significant toxicities, and many currently available salvage therapies are associated with increased immunosuppression, infectious complications, and potential loss of the graft versus leukemia (GVL) effect. Thus, there is an unmet need for development of newer treatment strategies for both acute and chronic GVHD to improve long-term post-transplant outcomes and quality of life for HSCT recipients. Here, we provide a concise review of major emerging therapies currently being studied in the treatment of acute and chronic GVHD.

Introduction: We describe a Phase I dose escalation study (NCT03081910) of autologous CD5-directed chimeric antigen receptor T cell (CD5 CAR T) therapy for relapsed or refractory (r/r) T-cell leukemia and lymphoma. Establishing a CAR T cell platform to target neoplasms of T-cell origin has been hindered by the shared expression of most targetable antigens on both malignant and normal T lymphocytes, which can promote CAR T cell fratricide. CD5 is one such pan-T cell surface marker present in ~85% of T-cell malignancies. We developed a second-generation CD5-specific CAR with CD28 costimulatory endodomain that produces minimal and transient fratricide when expressed in T cells. We designed this study to evaluate the safety and feasibility of treating patients with r/r T-cell malignancies with these CD5 CAR T cells as a bridge to allogeneic hematopoietic stem cell transplant (HSCT). Secondary objectives of our study included evaluating the antitumor response, in vivo expansion, persistence of CD5 CAR T cells, as well as their impact on normal T-cell numbers and function. Patients and methods: CD5 CAR T cells were generated from autologous PBMCs using gammaretroviral transduction and cryopreserved. We detected no residual malignant cells in the CD5 CAR T cell products by flow cytometry. To date, we have treated a total of 9 patients (8 adults and 1 adolescent; age 16-71 years [median 62 yrs]) with CD5+ r/r T-acute lymphoblastic leukemia (T-ALL; n=4) or T-non-Hodgkin's lymphoma (T-NHL; n=5) on dose levels 1 and 2. All patients were transplant-eligible with an identified allogeneic HSCT donor, yet unable to proceed due to residual disease. All patients had been heavily pretreated, with a median of 5 (range 2 -18) prior lines of therapy. Two patients had previously failed allogeneic HSCT. Patients received cytoreductive chemotherapy with cyclophosphamide and fludarabine followed by a single dose of CD5 CAR T cells. We evaluated adverse events, clinical responses, and in vivo expansion and persistence pre and post-infusion. Results: Three patients received CD5 CAR T cells on dose level 1 (1x107 CAR T cells/m2) and 6 on dose level 2 (5x107 CAR T cells/m2). In all patients treated, CAR T cells reached peak expansion in peripheral blood (PB) 1-4 weeks following infusion, followed by a gradual contraction in most patients (Figure 1). CD5 CAR T cells were present in lymph node and marrow biopsies in patients with T-NHL and T-ALL, respectively, and were also detected in a CSF sample in 1 T-ALL patient. After cytoreduction and CAR T cell infusion, we observed decreased PB CD3+ cell numbers but this ablation was never complete. Cytokine release syndrome (CRS) occurred in 3/9 patients (all at dose level 2). Grade 1 CRS was observed in 2 patients. One patient experienced Grade 2 CRS and Grade 2 neurotoxicity, which resolved after administration of tocilizumab and supportive care, respectively. Two patients had prolonged cytopenias at 6 weeks, 1 of whom had viral reactivation (CMV and BK virus) requiring antiviral therapy. On disease re-evaluation 4-8 weeks post-CD5 CAR T cell infusion, 4 of 9 evaluable patients obtained an objective response (1 of 3 on DL1 and 3 of 6 on DL2). Complete responses (CR) were achieved in 3 patients, one with angioimmunoblastic T cell lymphoma (AITL), one with peripheral T cell lymphoma (PTCL), and one with T-ALL. Two of these patients did not wish or were unable to proceed to planned HSCT and relapsed with their underlying CD5+ malignancy at 6 weeks and 7 months post-infusion. The remaining patient is currently undergoing work-up for HSCT (Figure 2). An additional patient with extensive AITL was classified as a mixed response (Figure 3) due to the appearance of a new PET-avid lesion. This patient received a second infusion of CD5-CAR T cells, proceeded to HSCT, and remains in CR at 125 days post-transplant. Conclusions: These results demonstrate that CD5 CAR T cells are safe and can induce clinical responses in heavily treated patients with r/r CD5+ T-ALL and T-NHL without inducing complete T-cell aplasia. Importantly, elimination of malignant T cells by CD5 CAR T cells may allow previously ineligible patients to proceed to HSCT. Disclosures Rouce: Novartis: Consultancy, Honoraria; Tessa Therapeutics: Research Funding; Kite, a Gilead Company: Consultancy, Honoraria. Grilley:Allovir: Consultancy, Equity Ownership; Marker Therapeutics: Consultancy; Tessa: Consultancy. Heslop:Marker Therapeutics: Equity Ownership, Membership on an entity's Board of Directors or advisory committees; Allovir: Equity Ownership; Gilead Biosciences: Membership on an entity's Board of Directors or advisory committees; Kiadis: Membership on an entity's Board of Directors or advisory committees; Tessa Therapeutics: Membership on an entity's Board of Directors or advisory committees, Research Funding; Cell Medica: Research Funding. Brenner:Allovir: Equity Ownership, Membership on an entity's Board of Directors or advisory committees; Marker Therapeutics: Equity Ownership; T Scan: Membership on an entity's Board of Directors or advisory committees; Tessa Therapeutics: Equity Ownership; Memgen: Membership on an entity's Board of Directors or advisory committees; Allogene: Membership on an entity's Board of Directors or advisory committees.