Development of a Bispecific Antibody-Releasing Stem Cell System for the Eradication of Acute Myeloid Leukemia Blasts Via Redirected Immune Effector Cells

Despite many years of research and great advances in the field, acute myeloid leukemia (AML) still remains one of the most challenging battle fields in the context of hematologic malignancies treatment. Although AML patients initially respond to conventional chemotherapy, a complete remission is rarely achieved and 5-year survival rates remain low especially in elderly patients. Hence, there is a pressing need for novel effective strategies for AML treatment to prevent relapse and treat minimal residual disease (MRD).

The use of recombinant bispecific antibodies (bsAbs) for retargeting effector T lymphocytes towards cancer cells is recently emerging as a promising immunotherapeutic approach for tumor treatment. This class of small molecules is designed to bind simultaneously to a pre-defined tumor-associated antigen (TAA) on tumor cells and the activating CD3 complex on T cells. The cross-linkage of immune effector cell and tumor cell leads to a tumor-specific T cell activation and efficient target cell killing independently of the T cell receptor specificity.

However, due to their low molecular mass, bsAbs have a short life span in vivo and consequently have to be continuously administrated to patients over prolonged time spans of several weeks to achieve clinical responses. As an alternative to continuous exogenous infusions of short-lived Abs we examined the use of engineered bone marrow-derived human mesenchymal stem cells (hMSCs) as cellular vehicles for the constant production and secretion of a fully humanized anti-CD33-anti-CD3 bsAb that targets the surface molecule CD33, which is widely overexpressed on AML blasts. Our studies demonstrate that gene-modified hMSCs are effective in releasing the bsAb at sufficient amounts to activate and redirect both human primary CD4⁺ and CD8⁺T cells from healthy donors against AML cells expressing varying levels of the CD33 antigen, leading to an efficient T cell-mediated tumor cell killing at low effector to target cell ratios and Ab concentrations. Most importantly, we could demonstrate that patient-derived T cells were able to suppress autologous AML blasts upon Ab-mediated cross-linkage over prolonged period of time without being affected by the presence of the modified hMSCs. Additional improvement of this system was achieved by the artificial expression of T cell co-stimulatory 4-1BB ligand (CD137L) on the hMSCs surface. The additional co-stimulatory signal provided by the engineered hMSCs resulted in an enhanced T cell proliferation, a higher pro-inflammatory cytokine release, and consequently in a more pronounced specific tumor cell killing already at earlier time-points.

Taken together, our data could demonstrate that continuous in situ delivery of the anti-CD33-anti-CD3 bsAb by genetically modified hMSCs facilitates efficient activation of T cells for specific and efficient killing of AML blasts over prolonged period of time.

Furthermore, as promising perspective of this approach for future in vivo application we are currently investigating on the development of biocompatible synthetic scaffolds as transplantable biomaterial-based production platforms for genetically engineered hMSCs as locally confined vehicle of immunotherapeutics. The implantation of these small engineered devices would ensure that the delivery of the anti-cancer agents can be controlled and stopped after tumor clearance by removing the scaffold at a desired time point. In this way, administration of ex vivo gene-modified hMSCs embedded in appropriate scaffolds would result in a continuous in situ production of recombinant Abs for effective and persistent levels of these therapeutic agents over time with low risk of side effects.