Abstract 5473: Isoform-selective HDAC inhibition up-regulates CD26 expression on multiple myeloma cells and augments cytotoxic efficacy by humanized monoclonal antibody

Abstract Objective: CD26, a 110-kDa transmembrane glycoprotein, which is expressed on several tumor cells including malignant lymphoma, has been implicated in tumorigenesis, whereas, its roles in plasma cell malignancies remain elusive. Recently, we have identified that CD26 is uniformly and intensely expressed in osteoclasts, while its expression in plasma cells of patients with multiple myeloma (MM) reveals heterogenous. Decreased expression levels of CD26 in MM cells are one of the mechanisms underlying resistance to humanized anti-CD26 monoclonal antibody (mAb) therapy. In the present study, we clarify the impact of epigenetic modification by HDAC inhibition (HDACi) with isoform-selective or broad inhibitors on the regulation of CD26 in MM cells and its mechanisms, thereby affecting the performance of humanized CD26mAb. Methods and Results: Immunostaining of bone marrow tissues of MM showed that CD26pos/CD138pos plasma cells were detected in several patients but not in others. So, first, we investigated the impact of HDACi on CD26 expression of MM cells. Although the cell surface expression of CD26 was relatively low or not detected on 5 MM cell lines (KMS26, KMS27, KMS28, KMS11, RPMI8226), the increased expression in CD26 levels was detectable within 24 h of the treatment with HDAC1i; FK228, HDAC3i; BG45, MS-275, RG2833 or HDAC6i; nexturastat A, tubastatin A, ACY-1215 as well as broad HDACi; LBH-589, SAHA. It increased further and maximum increase was observed at 72 h of the treatment by each HDACi. Then, subsequent removal of HDACi resulted in a decline of CD26 expression on MM cells to pre-treated levels. In addition, the levels of CD26mRNA were concomitantly enhanced, which was paralleled with an increase in the induction of CD26 protein in MM cells. Next, we examined the effect of HDACi on the viability of CD26neg MM cells in the absence or presence of CD26mAb. The mAb alone did not induce lysis of CD26neg MM cell lines at any doses, whereas, combining HDACi plus CD26mAb (10 μg/ml) synergistically facilitated lysis of CD26neg MM cells via direct effects as well as NK cell-mediated ADCC by mAb. Of note, HDACi plus CD26mAb in combination readily augmented lysis of CD26neg cell populations, refractory to HDACi or mAb alone. Furthermore, to elucidate the mechanisms of CD26 up-regulation in MM cells by HDACi, we performed chromatin immunoprecipitation assay on CD26 gene promoter. Each HDACi increased acetylation of histone 3 lysine 27 (H3K27Ac), concomitant with enhanced binding of Sp1 at the 5’ flanking region of the CD26 gene containing Sp1 binding sites in CD26neg MM cells, which is suggestive of transactivation of CD26 gene by HDACi targeting HDAC1, 3 or 6. Conclusion: Combination with isoform-selective HDACi not only shows anti-MM activity but supports as immunopotentiators by sensitizing CD26neg MM cells to CD26mAb and augment its cytotoxicity against MM cells. Citation Format: Hiroko Nishida, Mari Fujiwara, Mutsumi Hayashi, Michiie Sakamoto. Isoform-selective HDAC inhibition up-regulates CD26 expression on multiple myeloma cells and augments cytotoxic efficacy by humanized monoclonal antibody [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 5473.