Background: ALT-803 is a novel IL-15 superagonist complex consisting of an IL-15 mutant (IL-15N72D) bound to an IL-15 receptor α/IgG1 Fc fusion protein. It has been reported that ALT-803 exerted a significant anti-tumor activity in murine myeloma, breast, colon carcinomas, in rat bladder cancer, and in human bladder, ovarian carcinomas in vivo. ALT-803 was also found to enhance ADCC against wide range of human carcinoma cells in vitro. NEO-201 is a novel humanized IgG1 monoclonal antibody (mAb) which was found to react against a variety of cultured human carcinoma cell lines and tumor tissues. NEO-201 can mediate ADCC against human carcinoma cells.

Methodology: We investigate the ability of ALT-803 to modulate the ADCC mediated by NE0-201, employing NK cells isolated from normal donors as effectors. Human NK cells were treated with ALT-803 at different concentrations for 48h prior to be used as effector cells, and human carcinoma cell lines expressing the NE0-201 antigen were utilized as targets in an in vitro non-radioactive ADCC assay. The ability of ALT-803 to affect the phenotype of NK cells and to modulate NK cells gene expression was evaluated by flow cytometry and by using the Nanostring analysis respectively.

Results: The treatment with ALT-803 significantly enhanced the ADCC activity mediated by NEO-201 against NEO-201 positive carcinoma cells. The effect ALT-803 was dose-dependent and achieved statistical significance at all doses tested compared to vehicle control treatment. Treatment of NK cells with ALT-803 enhanced ADCC activity also from donors with minimal ADCC activity and lowered the effective dose of NEO-201 required to initiate the ADCC response compared to untreated NK cells. ADCC activity can be blocked by using anti-CD16 and anti-TIM3 blocking antibody. Phenotypic analysis of NK cells treated with 25 ng/ml of ALT-803 for 48h demonstrated that ALT-803 can enhance the expression of TIM3 and NKG2D and the mean fluorescence intensity (MFI) of granzyme B and CD107a in CD16/CD56 positive NK cells. Nanostring analysis of human NK cells treated with ALT-803 at different concentrations for 48h, showed that ALT-803 was able to modulate mRNA expression of 62 genes (1.6 log2 fold change compared to vehicle control was considered significant). ALT-803 up-regulated the mRNA expression of 43 genes, including NK activating receptors, factors involved in the NK cytotoxicity, cytokines and their receptors, and down-regulated the mRNA expression of 19 genes, including NK inhibiting receptors and factors involved in the activation of apoptosis.

Conclusions: ALT-803 can enhance ADCC activity mediated by NEO-201 against human carcinoma cells. The enhancement of the ADCC activity may be in part due to the increase in the expression of TIM3, NKG2D, granzyme B, and CD107a positive NK cells, as well as to the modulation of transcripts that are involved in the NK activation and cytotoxicity.

Citation Format: Massimo Fantini, Justin M. David, Hing C. Wong, Christina M. Annunziata, Philip M. Arlen, Kwong Yok Tsang. ALT-803 enhances antibody-dependent cell-mediated cytotoxicity (ADCC) mediated by NEO-201 against human carcinoma cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 619.