Optimizing Siglec-8-Directed Immunotherapy for Eosinophilic and Mast Cell Disorders

Current treatments for diseases in which eosinophils and/or mast cells play an important pathogenic role are often insufficiently effective. One promising target for these disorders is sialic acid immunoglobulin (Ig)-like lectin 8 (Siglec-8), but there are only few monoclonal antibodies (mAbs) against Siglec-8 available, and, so far, Siglec-8-directed therapies have largely focused on unconjugated mAbs, which may lack efficacy. To raise new human mAbs as basis for novel therapeutics, we immunized transgenic (“humanized”) mice with 3T3 cells expressing human full-length Siglec-8 (Siglec-8^FL) or a chimeric mouse/human protein consisting of the V-set domain of mouse CD33 fused with the membrane-proximal C2-set domain of human Siglec-8 or the V-set domain of mouse Siglec-F fused to the membrane-proximal C2-set domain of human Siglec-8. Among 9 unique mAbs we identified and recombinantly produced, 6 (1B6, 1H4, 1B4, 1E1, 1E4, 1E2) recognized Siglec-8 via binding of the V-set domain, 1 (2A3) via binding of the membrane-distal C2-set domain, and 2 (2A7, 2F10) via binding to the membrane-proximal C2-set domain. All mAbs tested were efficiently internalized into Siglec-8+ cells (up to 75% within 4 hours), suggesting their potential to deliver cytotoxic payloads. To assess the value of Siglec-8-directed immunotherapies engaging T or NK cells, we generated tool T cell-engaging Siglec-8/CD3 bispecific antibodies (BiAbs) in the scFv-scFv format as well as Siglec-8 CAR NK cells, using binding sequences from Siglec-8^V-set mAbs. Very low concentrations (10-25 pM) of a Siglec-8/CD3 BiAb built on 1B6 elicited dose-dependent cytolytic activity against EOL-1, HL-60 as well as LUVA cells transduced with Siglec-8^FL. This BiAb also exerted Siglec-8-specific cytotoxicity against HMC 1.2 cells that were enriched for Siglec-8 expression as well as parental LUVA cells, which both display very few Siglec-8 molecules. Similarly, KHYG-1 cells transduced with a Siglec-8^V-set-directed 41BB co-stimulatory domain-containing CAR generated from scFv sequences from 1B6 or 1H4 induced significant cytotoxicity against EOL-1 and RS4;11 cells transduced with Siglec-8^FL. To examine the relationship between the efficacy of T cell and NK cell-engaging therapies and the distance from the target epitope on Siglec-8 to the cell membrane, we engineered human AML and ALL cell line pairs expressing comparable levels of either Siglec-8^FL or an artificial variant of Siglec-8 containing only the V-set domain on the extracellular portion of the protein (Siglec-8^V-set) to bring the V-set domain closer to the cell membrane, and used these cell line pairs as targets for Siglec-8^V-set-directed BiAbs and CAR-modified immune effector cells. Siglec-8^V-set/CD3 BiAbs exerted greater cytotoxicity against AML and ALL cells expressing Siglec-8^V-set than cells expressing Siglec-8^FL. Likewise, Siglec-8^V-set-directed CAR NK cells exerted greater cytotoxicity against AML and ALL cells expressing Siglec-8^V-set than cells expressing Siglec-8^FL, demonstrating that altering the position of the Siglec-8 mAb binding epitope changes the effector functions of Siglec-8 mAb-derived therapies and suggesting membrane-proximal targeting via one of the C2-set domains could enhance Siglec-8-targeted therapies that engage T or NK cells as immune effector cells. To test whether mAbs recognizing one of the C2-set domains could indeed serve as basis for potent therapeutics, we generated a Siglec-8^C2-set/CD3 BiAb in the scFv-scFv format using binding sequences from 2A3 and found it effectively killed EOL-1, ML-1, RS4;11, and LUVA cells transduced with human Siglec-8^FL at very low concentrations. This BiAb also exerted Siglec-8-specific cytotoxicity against Siglec-8-enriched HMC 1.2 cells and parental LUVA cells, which both display Siglec-8 at very low copy number. Likewise, KHYG-1 cells transduced with a Siglec-8^C2-set CAR generated from 2A3 or 2F10 induced significant cytotoxicity against EOL-1 and RS4;11 cells transduced with Siglec-8^FL as well as against Siglec-8-enriched HMC 1.2 cells, albeit to a very modest extent. Together, our data demonstrate Siglec-8-directed immunotherapies can be highly potent, even against target cells expressing very limited copy numbers of Siglec-8 molecules, supporting their further development for use in patients with difficult-to-treat eosinophilic and mast cell disorders.