Treatment of acute myeloid leukemia with chimeric antigen receptor T cells

URPP Fellow: Renier Myburgh

Adoptive T cell therapy has made major progress in the treatment of hematological malignancies. One particularly effective approach is the use of chimeric antigen receptors (CAR), which are single-chain monoclonal antibodies linked to the signaling machinery of the T-cell receptor and co-stimulatory molecules.  Several groups reported significant clinical responses to CAR therapy in CD19+ B-cell malignancies, including acute lymphoblastic leukemia and lymphoma (Maude et al., 2014).

In contrast to lymphoid malignancies, the situation in acute myeloid leukemia (AML) is more multifarious: Acute myeloid leukemias (AML) and myelodysplastic syndromes (MDS) are highly heterogeneous diseases. Intensive chemotherapy can cure 30-40% of patients, however, is limited to clinically fit patients without significant comorbidities (Kimby et al., 2001; Schiller, 2013). More aggressive chemotherapy in combination with total body irradiation and allogeneic hematopoietic cell transplantation (HCT) can increase cure rates by an additional 15-20% (Appelbaum, 2012; Kanate et al., 2014).

AML is a clonal disorder of the hematopoietic stem and very early progenitor cell (HSC/HPC).  Leukemia-initiating cells (LIC) are contained in the phenotypic HSC/HPC pool, carry many features of HSCs, can self-renew and give rise to a hierarchy of maturing blasts.  While the proliferating mature blast pool is highly sensitive to chemotherapy, the more quiescent LICs are frequently relatively resistant, and are a source of frequent relapses. Overall, about 40% of young (usually <60 yrs) and otherwise fit AML patients can be cured by conventional chemotherapy, often escalated by allogeneic hematopoietic cell transplantation (HCT). However, about half of the young patient population and most of the elderly patients eventually die from progressive disease.

We propose that the best way to further success in AML therapy is to radically eliminate LICs, accept collateral HSC destruction, and subsequently perform allogeneic HCT to sustain long-term hematopoiesis. We hypothesize that this approach has the potential to be successful for AML patients, irrespective of the genetic heterogeneity of the diseases. Currently, no antigen has been identified that would specifically and exclusively mark the LIC, although the feasibility of AML CAR T-cell trials has already been established and some AML membrane antigens are at an early stage of evaluation as CAR T-cell targets.  We aim to create a versatile platform for the generation of CAR T cells directed against a variety of known and novel leukemic and hematopoietic stem cell antigens. We will test the CAR therapy with respect to tumor eradication and safety in mouse leukemia models and in mice bearing grafted human AML.

Specifically, we will pursue the following aims:

  1. Development of five different CAR T cells directed against cKit (CD117,) the IL-3 receptor alpha (CD123), Thy1 (CD90), CD96, and the myeloid marker CD33.
  2. Investigation of the impact of cKit-specific CAR T cells on leukemic cells versus healthy hematopoiesis.
  3. Design CAR constructs, which have limited and controllable time of action to shut down the CAR+ T-cell driven immune response allowing the repopulation of the bone marrow after eradication of the LIC.
  4. We will also address the dynamics of expansion, persistence, viability and function of CAR-T cells over time and the relevance of genetic disparity between the CAR T cell and the host.
  5. Testing CAR T cells in humanized mice bearing primary human AMLs.
  6. Evaluate the use of mRNA to generate transient CAR T cells.

We expect that our project will pave the way to a clinical trial, implementing one or several CAR T cells simultaneously or sequentially in patients with refractory or relapsed AML.