Generation of CAR-NK Cells Using a Notch-Mediated Cell Expansion and Differentiation Platform

An important challenge in the generation of therapeutic chimeric antigen receptor expressing natural killer (CAR-NK) cells is poor gene transfer efficiencies in mature NK cells requiring large quantities of viral vectors, significantly increasing the production cost. We have an established clinical stem cell expansion platform used to generate off-the-shelf hematopoietic stem/progenitor cell products that have been evaluated in 6 clinical trials to date. This proprietary Notch-mediated expansion platform has now been optimized to generate large numbers of functional NK cells by first ex vivo expansion and priming of CD34+ cells isolated from cord blood (CB), followed by directed differentiation to NK cells. The final NK cell product can be cryopreserved with post-thaw maintenance of viability (≥ 80%) and function. With respect to generation of CAR-NK cells, this manufacturing process is scalable and cost-effective and importantly, allows transduction using a viral vector early in the expansion phase of culture without compromising the robust expansion or phenotype of the developing cells. This significantly reduces both the number of cells to be transduced and the amount of viral vector needed to <5% of that required to transduce the final NK product.

As proof of concept, we developed CB-derived CAR NK (CB-CAR-NK) cells with specificity against mesothelin (MSLN). Mesothelin is a cell surface adhesion molecule that is highly upregulated in 30-50% of AML cases in pediatric and adult cohorts and lacks expression in normal hematopoiesis. The AML-restricted expression of mesothelin makes it an ideal tumor-associated antigen for immunotherapeutic targeting in AML. CB derived CD34+ cells are transduced with a mesothelin-directed CAR vector during the expansion phase in serum-free expansion media supplemented with cytokines. Transduction efficiencies were assessed periodically in bulk, CD34+ and CD56+ cells throughout the culture duration by flow cytometry analysis of truncated CD19, which is linked to CAR expression by a cleavable T2A. We measured cytolytic activity of CB-CAR-NK and CB-NK cells against NOMO-1 AML cells, which express endogenous MSLN, or NOMO-1 MSLN knockout cells by flow cytometry analysis of cell viability in the target cells.

Our manufacturing process generates over 2000 CD56+ NK cells per CD34+ cell initiated in culture. Following transduction and differentiation, we achieve a robust cell product that is >75% CD56+ NK cells and ~22% CD56+CAR+ using a multiplicity of infection of 10 virus particles per cell. The ex vivo generated CAR-NK cells demonstrate an immunophenotype similar to the unmodified NK cells, expressing the NK canonical surface markers NKG2A, NKG2D, NKp46, CD16, NKp44 and NKp30. Despite a modest transduction efficiency, the CB-CAR-NK cells demonstrate superior cytotoxicity against NOMO-1 cells over the unmodified CB NK cells (~4-fold increase). This enhanced killing is highly specific as CB-CAR-NK and CB-NK exhibited similar tumor-killing efficiency in NOMO-1 MSLN KO cells.

Having demonstrated the functional potential of our CAR-NK cells in vitro, we are now working toward assessing their function in vivo in a mouse xenograft model. We will also explore other optimal leukemia-specific antigens as targets for our CAR-NK cells and optimize our CAR construct to further enhance transduction efficiency.

The vast majority of anti-cancer cellular immunotherapies are patient specific, require apheresis, high volumes of viral vector and complex manufacturing processes that are expensive with a risk of delayed or no treatment if the production fails, all contributing to poor clinical adoption despite transformative outcomes. We have developed our platform to overcome these hurdles with the goal of delivering safe and cost-effective allogeneic off-the-shelf immunotherapies for improved clinical access.

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