Differential Ibrutinib Sensitivity in Cd79b‐mutant and Wildtype Subtypes of a Novel Myd88‐driven DLBCL Mouse Model

Introduction: Recent efforts have established distinct subtypes of DLBCL by clustering cases based on their mutational profiles. Mutations associated with the MCD/C5 cluster include MYD88 p.L265P, CD79B p.Y196X and BCL2 amplifications. Several recurring MCD-associated mutations affect transcription factors involved in regulating B cell differentiation (BCL6, SPIB, PRDM1, TBL1XR1). We previously established an autochthonous mouse model mimicking B cell-specific expression of MYD88L265P, BCL2 amplification and loss of PRDM1. These animals develop a disease that resemble many features of MCD DLBCL and thereby provide a valuable tool for DLBCL research. Here, we aimed to advance this existing model by introducing a Cd79b ITAM mutation. Methods: We generated an allele that allows the conditional expression of Cd79b p.Y195H (the murine orthologue of CD79B p.Y196H) from the endogenous locus and bred it to our established DLBCL mouse models. The developing lymphomas were characterized by exome, transcriptome and B cell receptor sequencing. We performed flow cytometric analyses, phosphoproteomics and proximity ligation assays to assess the activation status of the BCR pathway in our models. Lastly, we conducted MRI-guided treatment experiments with the BTK inhibitor ibrutinib. Results: By including a B-cell-specific Cd79b p.Y195H mutation, we refined our mouse model to gain novel insights into MCD/C5 DLBCL characteristics. Mice developed highly proliferative, (oligo-)clonal lymphomas. While mouse models with an engineered loss of Prdm1 formed lymphomas that were B220+CD138- and enriched for pre-memory and light zone gene signatures, the B220-CD138+ tumors developing in Prdm1-deficient lines showed plasmablastic features on a transcriptional level. The Cd79b status had no effect on the putative precursor population of the malignant B cells, however the highest frequency of spontaneous mutations in genes associated with MCD DLBCL was observed Prdm1-deficient lymphomas carrying both Myd88 and Cd79b activating mutations. Futhermore, the presence of the Cd79b p.Y195H allele increased BCR pathway activation levels in both the Prdm1-proficient and -deficient mouse lines. This activated state of the BCR pathway in Cd79b-mutated murine lymphoma translated into an increased sensitivity of those tumors to BTK inhibition by ibrutinib. Conclusions: Taken together, we refined existing Myd88 p.L252p and BCL2-driven MCD/C5 DLBCL mouse models by co-expressing the Cd79b p.Y195H mutation. Cd79b-mutant murine lymphomas exhibited increased BCR activation levels, resulting in an increased sensitivity towards BTK inhibition, when compared to Cd79b wt control tumors. These findings indicate that patients with CD79B ITAM mutations might be particularly sensitive to BTK inhibitor treatment. Keywords: Aggressive B-cell non-Hodgkin lymphoma, Basic and Translational Science Conflicts of interests pertinent to the abstract. H. C. Reinhardt Employment or leadership position: CDL Therapeutics Consultant or advisory role: Roche, Novartis, BMS, AbbVie, Vertex, Merck Research funding: AstraZeneca, Gilead