Inhibition of B-Cell Receptor Complex by Anti-membrane-IgM Antibody mAb4: Targeting Signaling Pathways in Leukemia and Lymphoma

The study confirms the role of the B-cell Receptor Complex (BCRC) in leukemia-lymphoma through clinical evidence. A unique antibody, mAb4, has been developed targeting a novel epitope on the BCRC's membrane IgM (mIgM), which has been shown to prevent cell growth and trigger cell death. Unlike traditional antibodies, mAb4 does not bind to the CDRs or the micro-clustering site of mIgM, which are typically involved in signal amplification. The research is focused on understanding how mAb4 impacts tyrosine kinase activity within several BCRC signaling pathways, with a hypothesis that it may inhibit multiple downstream pathways.

Current investigations involve examining the kinase activity at three BTK sites: Ser180, Tyr223, and Tyr551, as well as the phosphorylation status of other clinical targets like Syk, Lyn, BCL2, and PI3K. The study employs specific antibodies to measure the effect of mAb4 on kinase activation or inhibition compared to control antibodies and drug-treated cells. Preliminary ELISA results indicate that mAb4, along with a BTK inhibitor, can prevent BTK phosphorylation at Tyr223 and Tyr551, but not at Ser180.

In vitro tests show that mAb4-treated cells exhibit growth inhibition and apoptosis, while the BTK inhibitor only inhibits growth. The research suggests that mAb4 binds to a region of mIgM that includes the extracellular proximal domain and the constant domain 4 (μC4), leading to BTK dephosphorylation. Unlike BTK inhibitors, mAb4 also induces BCRC internalization and has anti-clonogenic effects, suggesting it could be a new generation of BTK inhibitors.

The findings indicate that mAb4 targets a critical structural site on mIgM, disrupting ligand binding and signal transduction. The study anticipates that further analysis of other kinases will reveal similar effects following BCRC inhibition by mAb4. Given its high specificity for IgM-expressing B-cells, mAb4's approach to modulating B-cell signaling could open new avenues in clinical treatment.

Reference: Rachel S. Welt, Jonathan A. Welt, Virginia Raymond, David Kostyal, Sydney Welt. Anti-membrane-IgM monoclonal antibody, mAb4, modulates downstream signaling pathways by inhibiting the BCRC [abstract]. AACR Virtual Special Conference: Tumor Immunology and Immunotherapy; 2020 Oct 19-20. Philadelphia (PA): AACR; Cancer Immunol Res 2021;9(2 Suppl):Abstract nr PO020.