Responses and relapse after immunochemotherapy in mouse models of diffuse large B cell lymphoma

Introduction: Diffuse large B cell lymphoma (DLBCL) is a common haematological cancer, but despite extensive research over the past 20 years the standard of care remains chemoimmunotherapy with R-CHOP, and approximately 30% of patients die from lymphoma. A better understanding of lymphoma biology and its response to treatment is required to design more effective combination therapies. Methods: We bred mice combining B cell-specific deletion of Blimp1 with activation of MYC and Ikk2, driving NF-kB, to model activated B cell (ABC) DLBCL, and characterised these lymphomas by histology, flow cytometry and RNAseq. We devised a murine regimen combining CHOP chemotherapy with the murine anti-CD20 antibody 5d2, and combined with a surgical technique for partial splenectomy, we used this to generate samples of residual and relapsed lymphomas. We analysed these by RNAseq and whole exome sequencing, and determined changes in the microenvironment by flow cytometry and histology. We used public data from human DLBCL to corroborate our findings. Results: After 6-8 months MYC-Ikk2-Blimp1 (MIB) mice developed clonal lymphomas, the majority of which had an antigen-experienced phenotype and an ABC-like transcriptional profile. These MIB lymphomas also exhibited heterogeneity in their anatomy, phenotype and microenvironment, consistent with the heterogeneity of human ABC DLBCL. MIB mice initially responded to treatment with 5d2-CHOP but ultimately relapsed, and by comparing paired relapsed lymphomas with pre- treatment specimens, we established that relapsed lymphomas were clonally related but showed evidence of clonal evolution. They upregulated transcription of genes involved in cell cycling, DNA repair, ribosome synthesis and oxidative phosphorylation, suggesting increased aggressiveness. We speculated that residual cells early after 5d2-CHOP treatment might also be more aggressive but, to our surprise, they downregulated cell cycling- and oxidative phosphorylation-associated genes. This is consistent with published studies of various cancer types which found that, after cytotoxic or targeted treatments, persistent cells are quiescent. However, in MIB lymphomas, ribosomal synthesis genes were upregulated both in early residual cells and in relapsed lymphomas, and using publicly available human data, we demonstrated that high ribosome synthesis is associated with inferior survival in ABC DLBCL, independently of the international prognostic index. Conclusions: Genetic models using conditional mutagenesis are a useful tool for studying immunochemotherapy responses in unique, naïve cancers. Therapies targeting ribosome synthesis may be an effective adjunct to existing first-line therapy for ABC DLBCL and merit further research