Abstract 2943: The structural basis for inhibition of MICA shedding and anti-tumor activity of the monoclonal anti-MICA|B antibody, CLN-619
Authors: Whalen Kerry A. , Mehta Naveen K. , Meetze Kristan , Michaelson Jennifer S. , Baeuerle Patrick S. 
Published: April 2023

Abstract
CLN-619 is a humanized IgG1 monoclonal antibody, currently in phase 1 clinical development in cancer patients, that targets the NKG2D ligands MICA and MICB. Both ligands serve as activating signals for NKG2D on NK cells and a subset of T cell populations and can induce target cell lysis. Tumor cells evade formation of the NKG2D-MICA/B axis by shedding of MICA/B ligands. This is mediated by various proteases found in the tumor microenvironment that target cleavage sites proximal to the plasma membrane. Shedding of MICA/B is a two-step process. First, the protein disulfide isomerase ERp5 reduces a disulfide bond in the alpha 3 domain. This then allows proteases to gain access to cleavage sites. CLN-619 has been shown to bind to the alpha 3 domain, prevent shedding of MICA/B, increase the cell surface expression of MICA/B, and enable ADCC and ADCP. In xenograft models, doses as low as 0.03 mg/kg inhibited tumor outgrowth, with efficacy dependent on a functional Fcγ1 domain. Of note, CLN-619 does not interfere with binding of the alpha 1 and 2 domains of MICA/B to NKG2D but rather enhances formation of this axis. We have undertaken a structural analysis at an atomic resolution to elucidate how a Fab fragment of CLN-619 interacts with the recombinant alpha 3 domain of human MICA. A co-crystal was obtained that allowed X-ray crystallography at a resolution of 2.12 Angstroms. The antibody was found to laterally bind to the alpha 3 domain without approaching the distal alpha 1 and 2 domains involved in binding to NKG2D. CLN-619 recognizes a discontinuous epitope, interacts with a total of 19 amino acid residues on MICA, and covers approximately 14% of the alpha 3 domain surface when bound. Several modes of action of CLN-619 may explain inhibition of MICA shedding. In the simplest case, the antibody binds to the cleavage sites on MICA and sterically prevents protease access. However, the cleavage sites appear to be located opposite from where CLN-619 binds the alpha 3 domain of MICA. It is also conceivable that CLN-619 prevents access of ERp5. Location of the ERp5 recognition site in the middle of the discontinuous epitope supports this hypothesis. The rather large footprint of CLN-619 on MICA supports yet another hypothesis. It has been suggested that opening of the alpha 3 domain by reduction of the disulfide bond creates a protease-sensitive conformation. The extended interaction of CLN-619 with MICA may freeze the alpha 3 domain in a protease-insensitive conformation. We will report results from ongoing biophysical and biochemical studies further elucidating the mechanism by which CLN-619 prevents shedding of MICA.
Citation Format: Kerry A. Whalen, Naveen K. Mehta, Kristan Meetze, Jennifer S. Michaelson, Patrick S. Baeuerle. The structural basis for inhibition of MICA shedding and anti-tumor activity of the monoclonal anti-MICA|B antibody, CLN-619 [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 2943.