Abstract 2966: Exploring the therapeutic potential of a novel Siglec-6 targeted bispecific antibody to selectively kill leukemia cells
Authors: Nunes Jessica , Mao Charlene , Purcell Matthew , Perry Elizabeth , Zhang Liwen , Mo Xiaokui , Cyr Matthew , Long Meixiao , Byrd John , Rader Christoph , Muthusamy Natarajan 
Published: April 2023

Abstract
Objectives of the Study: Bispecific antibodies (BsAbs) are a class of immunotherapy drugs that facilitate targeted killing of cancer cells. Here, we report therapeutic potential of a novel Siglec-6/CD3 targeted bsAb against chronic lymphocytic leukemia (CLL) cells. We show here for the first time the functional and therapeutic relevance of Siglec-6 in CLL cell adhesion and migration. Mechanistically, we have shown Siglec-6 mediated cell adhesion and migration through Siglec-6 ligand (sialyl Tn) mediated DOCK8 dependent activation of Cdc42 associated with actin polymerization. To evaluate the therapeutic potential of Siglec-6, we generated a human (hu) Siglec-6 transgenic mouse model and crossed it with the TCL1 CLL mouse model to obtain Siglec-6 x TCL1 mice which develop Siglec-6+ leukemia. A pilot in-vivo study with anti-Siglec-6/CD3 targeted bsAb showed a survival benefit in humanized CD3 (huCD3) mice engrafted with Siglec-6+ leukemic cells.
Methods used: Flow cytometry was used to analyze cell surface expression of Siglec-6 and sialyl Tn (sTn). Mass spectrometry analysis was performed to identify Siglec-6 interacting proteins. CRISPR/Cas9 technique was used to generate knock-out (KO) cell lines for mechanistic studies. Phalloidin staining followed by confocal imaging was used to examine actin polymerization. For in-vivo BsAb treatment experiments, huCD3 mice were engrafted with 5 million Siglec-6+ leukemic cells isolated from Siglec-6 x TCL1 mouse splenocytes. Weekly treatment (i.v) was started after mice display 5% circulating leukemia.
Results and Conclusion: Compared to Siglec-6+ CLL cells, Siglec-6- CLL cells exhibited significant reduction in adhesion to (~50%) and migration towards (~50%) CLL-BMSCs. Importantly, a Siglec-6 targeted antibody inhibited homing of Siglec-6+ MEC1 cells and primary CLL cells to the spleen and bone marrow in NSG mice (~35%). Mass spectrometry and co-immunoprecipitation analysis in MEC1 cells revealed interaction of Siglec-6 with DOCK8, a guanine nucleotide exchange factor. Stimulation of Siglec-6+ MEC1 cells with sTn resulted in Cdc42 activation and WASP protein recruitment, followed by increased actin polymerization, that were compromised in Siglec-6 or DOCK8 KO MEC1 cells. Siglec-6+ leukemia engrafted huCD3 mice treated with anti-Siglec-6/CD3 BsAb has so far demonstrated a survival benefit with median survival of 18 weeks versus a median survival of 8 weeks in the control group that received a non-targeting BsAb (n=4/group).
Significance: We have for the first time shown Siglec-6 dependent recruitment of DOCK8 leading to migration and adhesion of B-CLL cells. An anti-Siglec-6/CD3 BsAb provides a survival benefit against Siglec-6+ leukemia, paving the way for development of Siglec-6 targeted therapeutics to treat CLL.
Citation Format: Jessica Nunes, Charlene Mao, Matthew Purcell, Elizabeth Perry, Liwen Zhang, Xiaokui Mo, Matthew Cyr, Meixiao Long, John Byrd, Christoph Rader, Natarajan Muthusamy. Exploring the therapeutic potential of a novel Siglec-6 targeted bispecific antibody to selectively kill leukemia cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2966.