Background: Obexelimab is a bifunctional, non-cytolytic, humanized monoclonal antibody that binds CD19 and ligates FcγRIIb with high affinity. Data from a phase 2 clinical trial of patients with IgG4-related disease (IgG4-RD) demonstrated rapid, and sustained clinical improvement, including complete remission in most patients following treatment with obexelimab. Although shown to not drive apoptotic death of B cells, obexelimab treatment does result in an approximate 50% reduction in circulating B cell numbers, as well as near complete reduction in circulating plasmablasts. We have shown that obexelimab inhibits the phosphorylation of BTK, thereby interfering with BCR signalling.

Objectives: The aim of this study is to examine the molecular effects of obexelimab on B cells from patients with IgG4-RD.

Methods: Peripheral blood was collected from patients with IgG4-RD before and after treatment with obexelimab. PBMCs were isolated using standard density gradient centrifugation. Naïve (CD19 ⁺ IgD ⁺ CD27 ^- ) and memory (CD19 ⁺ IgD ^- CD27 ⁺ ) B cell populations were FACS purified and used for subsequent ATAC-sequencing (N = 3 with 3 technical replicates for each sample) and RNA-sequencing (N = 16).

Results: We observed subtle changes in gene expression and chromatin accessibility attributed to treatment with obexelimab. RNA sequencing identified 775 genes upregulated and 808 genes downregulated in post- compared to pre-obexelimab treated B cells. ATAC sequencing analysis identified 1040 increased and 708 decreased differentially accessible genes in post- versus pre-obexelimab treated B cells. Gene ontology biological process analysis of ATAC sequencing data demonstrated key elements associated with phosphoinositide regulation, essential for cytoskeleton organization, chemotaxis and migration. In addition, nucleotide and purine metabolic processes were impacted after treatment. Furthermore, KEGG pathway analysis demonstrated key components of actin cytoskeleton regulation associated with altered phosphoinositide regulation. Gene ontology biological process analysis of RNA-seq data aligned with ATAC-seq analysis, confirming alteration of cell-cell adhesion pathways.

Conclusion: Collectively, these gene expression and chromatin accessibility data suggest that obexelimab has a broad inhibitory effect on B cells consistent with the role of phosphoinositides in regulating the cellular and activation mechanisms in B cells. Paired with the previous published data demonstrating interference with BCR signalling, obexelimab likely dampens crosslinked BCR-induced BTK phosphorylation in antigen-experienced B cell subsets from IgG4-RD patients and impacts phosphoinositide mediated B cell migration. Together, these two mechanisms may eliminate the potential of pathogenic, self-reactive B cells to respond to self-antigen, migrate out of secondary lymphoid organs, and infiltrate inflamed tissues.

REFERENCES: NIL.

Acknowledgements: NIL.

Disclosure of Interests: Thomas Guy: None declared, Hang Liu An employee of UCB, Vinay Mahajan: None declared, Cory Perugino Has received consulting fees from Horizon Therapeutics, Zachary S. Wallace Has received consultancy fees from Viela Bio, Zenas BioPharma, Horizon Therapeutics, Sanofi, MedPace, BioCryst, Amgen and PPD. Served as member of a Scientific Advisory Board for Sanofi, Horizon Therapeutics, Novartis, Shionogi, Otsuka/Visterra, and Amgen, Has received research support from BMS, Sanofi, and Amgen, Shauna Quinn Holds stock/options in Zenas BioPharma, Employee of Zenas BioPharma, Mark Matijevic Holds stock/options in Zenas BioPharma, Employee of Zenas BioPharma, Allen Poma Holds stock/options in Zenas BioPharma, Employee of Zenas BioPharma, Debra Zach: None declared, John H. Stone Has received consultancy fees from Zenas BioPharma, Has received financial grants from Roche, Sanofi and BMS, Shiv Pillai Served as member of a Scientific Advisory Board for Abpro Inc, BeBio, Paratus Sciences and Octagon Therapeutics