MYD88L265P augments proximal B-cell receptor signaling in large B-cell lymphomas via an interaction with DOCK8

•In DLBCLs with MYD88L265P and CD79BY196F alterations, MYD88L265P selectively increased proximal BCR signaling and survival via DOCK8.•MYD88L265P/DOCK8–enhanced proximal BCR signaling is a basis for the increased sensitivity of MYD88L265P/CD79BY196F DLBCLs to BTK blockade. [Display omitted] Diffuse large B-cell lymphoma (DLBCL) is a clinically and genetically heterogeneous disease with at least 5 recognized molecular subtypes. Cluster 5 (C5)/MCD tumors frequently exhibit concurrent alterations in the toll-like receptor (TLR) and B-cell receptor (BCR) pathway members, MYD88L265P and CD79B, and have a less favorable prognosis. In healthy B cells, the synergy between TLR and BCR signaling pathways integrates innate and adaptive immune responses and augments downstream NF-κB activation. In addition, physiologic TLR9 pathway engagement via MYD88, protein tyrosine kinase 2 (PYK2), and dedicator of cytokinesis 8 (DOCK8) increases proximal BCR signaling in healthy murine B cells. Although C5/MCD DLBCLs are selectively sensitive to Bruton tyrosine kinase (BTK) inhibition in in vitro studies and certain clinical trials, the role of mutated MYD88 in proximal BCR signaling remains undefined. Using engineered DLBCL cell line models, we found that concurrent MYD88L265P and CD79B alterations significantly increased the magnitude and duration of proximal BCR signaling, at the level of spleen tyrosine kinase and BTK, and augmented PYK2-dependent DOCK8 phosphorylation. MYD88L265P DLBCLs have significantly increased colocalization of DOCK8 with both MYD88 and the proximal BCR-associated Src kinase, LYN, in comparison with MYD88WT DLBCLs, implicating DOCK8 in MYD88L265P/proximal BCR cross talk. Additionally, DOCK8 depletion selectively decreased proximal BCR signaling, cellular proliferation, and viability of DLBCLs with endogenous MYD88L265P/CD79BY196F alterations and increased the efficacy of BTK blockade in these lymphomas. Therefore, MYD88L265P/DOCK8-enhanced proximal BCR signaling is a likely mechanism for the increased sensitivity of C5/MCD DLBCLs to BTK blockade. One molecular subset of diffuse large B-cell lymphoma (DLBCL) with an inferior prognosis carries concurrent alterations of the toll-like receptor and B-cell receptor (BCR) pathway members, MYD88L265P and CD79BY196F. Mandato and colleagues used genetically engineered cell lines to reveal that MYD88L265P selectively increases proximal BCR signaling and survival via the adaptor protein dedicator of cytokinesis 8