Abstract P40: Therapeutic anti-B-cell Receptor antibody, WBMP-4, modulates malignant B-cell signaling and induces apoptosis
Authors: Welt Rachel S , Kostyal David , Lobo Jose M , Raymond Virginia , Welt Sydney 
Published: March 2024

The B-cell Receptor (BCR), comprising CD79a/b and membrane Ig, initiates a driver pathway of cell survival and proliferation, and is a rational drug target for B-cell-derived leukemia/lymphoma. Furthermore, within these cancers, the malignant population of B-cells is restricted to one Ig class (e.g. IgE, IgM, IgD), which provides an opportunity for targeted treatment of the disease, while sparing healthy B-cells that express other Igs. However, homology between the BCR’s membrane Ig (mIg) subunit and circulating serum Ig (sIg) has so far prevented the development of drugs to this target. Previously, we reported the generation of a first-in-class antibody, WBMP-4, to a neo-epitope specific to the mIgM-BCR, which, upon binding to this target, inhibits cell growth and induces apoptosis in IgM-expressing B-cell malignancies. Here we characterize the mechanism of action of WBMP-4. Using western blot, ELISA, and flow cytometric analyses, we measure the effect of treatment of Burkitt lymphoma (CA46) cells with WBMP-4, on the phosphorylation and expression of key B-cell signaling proteins (i.e. Syk, BTK, PI3K, etc.). Additionally, we evaluate kinase activity following treatment with WBMP-4 via the Pamgene phosphotyrosine kinase assay, which uses a microarray chip to measure the kinase activity at predefined phosphotyrosine sites found on a wide variety of signaling proteins. With this assay, we measured kinase activity at 189 phosphosites, and together with directed analyses of key B-cell signaling proteins, we identify the intracellular impacts of treatment with WBMP-4. The Pamgene kinome analysis reveals widespread kinase downregulation following WBMP-4 treatment, with 182 of 189 phosphosites showing significant difference in phosphorylation signal intensity. Western blot analyses of downstream BCR kinases similarly show changes in both phosphorylation and expression of target proteins (CD79b, Syk, BTK, mTOR, and Myc), the latter indicating that modulation of these signaling pathways reaches protein transcription factors, and effects may be sustained via protein downregulation. We show that kinase activity of numerous proteins with varied functions is mediated upon WBMP-4 binding to the BCR and that protein expression is impacted. Key pathways (CD79b, Syk, BTK, PI3K, mTOR, Myc, etc.) are simultaneously downregulated through a decrease in protein expression and kinase activity. As the BCR is the dominant driver pathway of cell survival, these results suggest that this widespread BCR signal disruption leads to the observed cytotoxic effect of WBMP-4 when given at adequate dosages. Due to the specific targeting of WBMP-4 to IgM-expressing B-cells, we expect that the toxicities associated with approved therapies upon their interaction with healthy B-cells and non-lymphatic tissue (immune disorders, bleeding, infection, cytopenia, arrhythmia, and secondary malignancies) will be avoided with WBMP-4 treatment. With potent anti-tumor activity and expected reduced toxicities, WBMP-4 represents a compelling candidate for clinical development.

Citation Format: Rachel S Welt, David Kostyal, Jose M Lobo, Virginia Raymond, Sydney Welt. Therapeutic anti-B-cell Receptor antibody, WBMP-4, modulates malignant B-cell signaling and induces apoptosis [abstract]. In: Proceedings of the Blood Cancer Discovery Symposium; 2024 Mar 4-6; Boston, MA. Philadelphia (PA): AACR; Blood Cancer Discov 2024;5(2_Suppl):Abstract nr P40.