CAR-directed cytotoxicity of NK cells: an alternative tool to treat and to study multiple myeloma and NK biology

Treatment options for multiple myeloma (MM) include targeted inhibitors, antibodies, and immunomodulatory drugs (IMiDs). However, eradicating the disease remains a major clinical challenge and new therapeutic strategies are needed. Recently, chimeric antigen receptor (CAR) T-cell therapy has shown striking activity and high rates of complete responses (CR) in other lymphoid malignancies. We speculate that targeted natural killer (NK) cells represent an alternative, powerful therapeutic strategy. While we expect that CAR T cells may be more active on a cell-by-cell basis, the CAR NK cells are off-the-shelf reagents with capacity for in vitro expansion and unprecedented versatility, potentially enabling repeated treatments and justifying their use. We created and assessed the in vitro cytotoxicity of B-cell maturation antigen (BCMA) CAR-expressing NK cells using the NK92 cell line. Then, we utilized the CRISPR/Cas9 screen to understand how MM cells can potentially escape the cytotoxicity of CAR NK92 cells and therefore, the mechanisms of NK-killing. The co-culture of CAR NK92 and a MM cell line, OPM2 triggers activation, release of INFγ, and cytotoxicity of NK92. The specificity of CAR-binding is validated via the knockout of BCMA on OPM2. Subsequently, a genome-wide CRISPR-Cas9 screen on OPM2 reveals additional mechanisms required for CAR NK92 cytotoxicity, such as heparan sulfate, etc. Our results indicate that CAR NK cells represent an attractive, alternative cell-based immunotherapy option for MM. Furthermore, our genetic screen hinted on additional regulators for the cytotoxicity of NK cells.