Chemotherapeutic drugs increase CS1 (CD319) expression on multiple myeloma cells enhancing NK cell mediated killing

Background: Multiple myeloma (MM) is characterized by malignant plasma cells. The clinical diagnosis includes end-organ damage, such as hypercalcemia and renal insufficiency. CS1, a member of the Signaling Lymphocyte Activation Molecule (SLAM) family of receptors, is expressed on natural killer (NK) cells as an activation receptor. CS1-CS1 binding could activate natural killer cell cytotoxicity. In multiple myeloma cells, CS1 is highly expressed and therefore offers a remarkable target for antibody dependent cell-mediate cytotoxicity (ADCC) by NK cells. Elotuzumab (Empliciti), is a humanized monoclonal antibody against CS1. Clinical trials suggest Empliciti combined with chemotherapeutic drugs has increased efficacy than using Empliciti only. In this study we investigated the mechanism of enhanced killing of MM by NK cells. Hypothesis: Chemotherapeutic drugs up-regulate the CS1 expression on multiple myeloma cell surface and enhance NK cell mediated ADCC. Material and Methods: Multiple myeloma cells, NCI-H929 were treated with vehicle control, lenalidomide and anti-CS1 mAb, doxorubicin and dexamethasone, or dexamethasone and anti-CS1 mAb for 24h and 48h. NCI-H929 cells were labeled with antibody to detect CS1 expression on cell surface by flow cytometry. CS1 mRNA expression in multiple myeloma cells were investigated by RT-qPCR. NK cell mediated killing was determined by chromium release assay. Results: Compare with vertical group, CS1 expression levels are increased in all experiment groups, but only doxorubicin and dexamethasone treatment 24h group show significantly increase, same results in anti-CS1 mAb and lenalidomide treatment 48h group. After treating NCI-H929 cell with dexamethasone and anti-CS1 mAb for 24h and 48h, mRNA expression levels are significantly decreased. No statistically variation is observed among the other groups. Natural killer cell killing abilities are significantly improved in chemotherapeutic drugs combination with anti-CS1 mAb treatment groups, no significant increase in doxorubicin and dexamethasone group. Conclusion: In conclusion, our results suggest chemotherapeutic drugs could increase the CS1 expression on NCI-H929 cell. Combining clinical trial observations and our data, it appears chemotherapeutic drugs may increase the anti-mAb treatment efficacy by up-regulating the CS1 expression level in multiple myeloma cells resulting in enhanced NK cell mediated cytotoxicity.