Abstract 1886: Identifying T-cell engagers with optimal potency and cytokine-release profiles with a diverse panel of CD3-binding antibodies

In this study, we describe the characterization and validation of a diverse panel of fully human CD3-binding antibodies, including hundreds of human and cyno cross-reactive binders. We used two proof-of-concept TCE targets to demonstrate that this panel streamlines CD3 T-cell engager (TCE) development, enabling identification of optimal tumor cell-killing and cytokine-release profiles. CD3 TCEs have potential to be powerful cancer treatments, but the small number of available CD3-binding antibodies and limited multispecific engineering technologies have been barriers to development. Identifying TCEs that balance anti-tumor potency with potential toxicities, such as cytokine release syndrome, requires simultaneous tuning of both the CD3- and tumor-binding arms. Pairs of antibodies that achieve this balance are rare, creating a need for diverse panels of developable antibodies that can be combined and tested to identify optimal clinical candidates. To streamline TCE development, we discovered a diverse panel of CD3-binding antibodies. We screened over 5 million single cells from humanized mice and identified 585 unique CD3-specific antibody sequences. Of these, over 170 were identified as cross-reactive to human and cyno CD3 in primary screening. We then used high-throughput characterization to curate a panel of diverse and developable antibodies. We found a wide range of CD3εδ and CD3εγ binding specificities, affinities, and kinetics. Epitope binning analysis revealed multiple bins containing human and cyno cross-reactive binders, some of which are distinct from previously described cross-reactive antibodies, such as SP34-2. We assessed their biophysical properties and identified antibodies with good developability properties, including high thermal stability and low hydrophobicity, self-association, polyspecificity, and aggregation. To validate these antibodies, we used OrthoMab™ to generate proof-of-concept TCE panels with fixed tumor-binding arms. We identified CD3 x EGFR TCEs with high potency, low cytokine release, functional cross-reactivity in a cyno T cell-mediated tumor killing assay, and good pharmacokinetic properties in Tg32 mice. A second proof-of-concept CD3 x PSMA panel further validated our antibodies in bispecific formats. Together, these studies demonstrate that starting with diverse CD3-binding antibodies streamlines identification of developable TCEs with optimal potency and cytokine release. We leveraged data from our extensive characterization of CD3-binding antibodies in mono- and bispecific formats to develop a strategy for down-selection and pairing of CD3- and tumor-binding antibodies, and a high-throughput method for analysis of resulting TCEs. By categorizing antibodies based on functional properties, we are able to rapidly pinpoint optimal potential clinical candidates for specific tumor targets.

Citation Format: Juntao (Matt) Mai, Kate Caldwell, Lindsay DeVorkin, Grace P. Leung, Karine Herve, Yuri Hwang, Cristina Faralla, Wei Wei, Emma Lathouwers, Valentine de Puyraimond, Lauren Clifford, Rhys S. Chappell, Stefan Hannie, Katherine J. Lam, Harveer Dhupar, Tran N. Tran, Melissa Cid, Lena M. Bolten, Tova Pinsky, Ping Xiang, Courteney Lai, Ahn Lee, Vivian Z. Li, Patrick Chan, Jasmine Chin, Steve Booth, Amy C. Lee, Stephanie Masterman, Sherie Duncan, Aaron Yamniuk, Kush Dalal, Tim M. Jacobs, Raffi Tonikian, Bryan C. Barnhart. Identifying T-cell engagers with optimal potency and cytokine-release profiles with a diverse panel of CD3-binding antibodies [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 1886.

Arp2/3 complex-driven spatial patterning of the BCR enhances immune synapse formation, BCR signaling and B cell activation

When B cells encounter antigens on the surface of an antigen-presenting cell (APC), B cell receptors (BCRs) are gathered into microclusters that recruit signaling enzymes. These microclusters then move centripetally and coalesce into the central supramolecular activation cluster of an immune synapse. The mechanisms controlling BCR organization during immune synapse formation, and how this impacts BCR signaling, are not fully understood. We show that this coalescence of BCR microclusters depends on the actin-related protein 2/3 (Arp2/3) complex, which nucleates branched actin networks. Moreover, in murine B cells, this dynamic spatial reorganization of BCR microclusters amplifies proximal BCR signaling reactions and enhances the ability of membrane-associated antigens to induce transcriptional responses and proliferation. Our finding that Arp2/3 complex activity is important for B cell responses to spatially restricted membrane-bound antigens, but not for soluble antigens, highlights a critical role for Arp2/3 complex-dependent actin remodeling in B cell responses to APC-bound antigens.