Enhancing the Cytotoxicity of NK Cells Toward Multiple Myeloma Using an Antibody Specific to SLAMF7

Multiple myeloma (MM) is a malignancy of plasma cells characterized by a monoclonal expansion of tumor cells in bone marrow and accumulation of a monoclonal protein in blood and urine, which ultimately leads to end-stage organ damage. Despite advances in therapy, MM remains an incurable disease. Recent investigations have revealed a new therapeutic target for MM, SLAMF7 (CS1), which is expressed at high levels on MM tumor cells. SLAMF7 is also found on NK, CD8⁺ T, CD4⁺ T and activated B cells. Elotuzumab (HuLuc63) is a humanized IgG1 anti-SLAMF7 antibody, which can facilitate killing of primary MM cells by NK cells via antibody-dependent cellular cytotoxicity (ADCC) and has shown promise in treating relapsed MM. A key question in understanding elotuzumab’s mechanism of action is whether it can directly activate NK cells via SLAMF7 binding, in addition to Fc-mediated activation via binding of CD16 (FcγRIIIA).

We have examined the in vitro effect of elotuzumab on NK cells in fresh human peripheral blood from healthy donors and MM patients incubated with or without MM cell lines that have varied levels of SLAMF7 surface expression. Cytotoxic degranulation of NK cells was quantified by staining for CD107a (marker of degranulation) after 2 hours, and direct effects of elotuzumab on NK cells were assessed by measuring upregulation of the CD69 activation marker after 24 hours of antibody exposure.

Degranulation of fresh primary NK cells toward SLAMF7-bearing MM target cell lines was stimulated by addition of elotuzumab. Intensity of the degranulation response was dependent upon the concentration of elotuzumab and the expression level of SLAMF7 on the MM target cells. While elotuzumab alone could stimulate low-level degranulation of NK cells in fresh PBL from healthy donors, the degranulation response was stronger if SLAMF7-expressing MM target cells were added in combination with elotuzumab. Addition of F(ab’)₂ elotuzumab or an Fc mutant form of elotuzumab that cannot bind CD16 did not stimulate degranulation, implying that binding of the antibody to SLAMF7 on NK cells and MM cells, without concomitant engagement of NK cell Fc receptors, is insufficient to trigger degranulation.

Soluble intact elotuzumab stimulated CD69 expression and degranulation by fresh primary NK cells in the absence of MM target cells, which peaked at 1 µg/ml and only modestly increased up to 100 µg/ml elotuzumab. In the same assays, we only observed modest stimulation of CD69 expression using soluble F(ab’)₂ or Fc mutant elotuzumab. In contrast, plate-bound F(ab’)₂ elotuzumab stimulated robust CD69 expression, indicating that the antibody can induce direct stimulation of NK cells through SLAMF7, but strong stimulation requires aggregation of the antibody. In soluble form, a non-fucosylated version of elotuzumab with greater affinity for CD16 stimulated significantly stronger degranulation and CD69 expression than intact elotuzumab, both in the presence or absence of MM target cells, and with both high and low SLAMF7 expression on target cells. Importantly, elotuzumab was found to induce similar degranulation by NK cells from freshly isolated peripheral blood of MM patients as compared to healthy donors.

Taken together, our data support the hypothesis that binding of soluble elotuzumab to SLAMF7 can weakly activate NK cells, but strong activation and degranulation require antibody aggregation or engagement of the Fc domain with CD16. Elotuzumab variants with enhanced CD16 binding, such as a non-fucosylated version, can augment ADCC against myeloma cell lines, and warrant further study as a potential strategy to improve clinical efficacy in MM patients.