ER-Stress-Induced Suppression of HLA-E on Bortezomib-Evading Malignant Plasma Cells Dramatically Enhances Their Susceptibility to NK Cell Killing: Identification of an Achilles Heel in Myeloma Cells That Can be Utilized to Prevent Disease Relapse Following Bortezomib Treatment

Although the proteasome inhibitor bortezomib has significantly improved the survival of patients with multiple myeloma (MM), most patients treated with this drug eventually develop progressive disease. Recent data indicate that MM cells can evade bortezomib-induced cell death by undergoing autophagy as a consequence of endoplasmatic reticulum (ER)-stress triggered by proteasome inhibition. Here we show that bortezomib-evading MM cells become highly sensitized to killing by natural killer (NK) cells via ER-stress-induced reduction of the NK cell inhibitory molecule HLA-E that is normally expressed at high levels on the surface of MM cells. High-resolution flow cytometry-based assays revealed augmented NK cell recognition and degranulation against bortezomib-exposed MM cells (3 fold higher compared to untreated MM controls) was restricted to NK cells exclusively controlled by the HLA-E-binding inhibitory receptor NKG2A (NKG2A^SP NK cells) (Figure 1). In contrast, due to unchanged high expression of other HLA class I molecules on the surface of bortezomib-exposed MM cells there was no augmentation in degranulation by NK cells controlled by other inhibitory HLA class I-binding receptors, such as killer immunoglobulin-like receptors (KIRs). Compared to their non-expanded counterparts, ex vivo expanded NK cells have previously been shown to have an increased proportion of NKG2A^SP NK cells (50% vs 25%, p<0.01) and bolstered cytotoxic function against tumor cells. Using NK cells expanded in accordance with our ongoing FDA-approved clinical trial evaluating adoptive infusion of autologous ex vivo expanded NK cells in patients with refractory cancers at the NIH, we established these highly cytotoxic NKG2A^SP dominant NK cells induced substantially higher lysis of bortezomib-exposed MM cells compared to non-expanded matched control NK cells (38% vs 18%, p<0.05) (Figure 2). Based on these findings, we hypothesize that adoptive infusions of ex vivo expanded autologous NK cells following treatment with bortezomib could eradicate the fraction of MM cells that would normally evade killing through proteasome inhibition alone. The increased vulnerability of bortezomib-evading cells to NK cell cytotoxicity identifies a critical Achilles heel in myeloma cells and suggests adoptive NK cell infusions following bortezomib therapy could potentially be utilized as a strategy to improve long-term survival in MM patients.

Figure 1. Degranulation of NK cell subsets against myeloma cells exposed to bortezomib. NT; No target

Figure 2. NK cell killing of bortezomib-exposed myeloma cells.