Novel Autochthonous Mouse Models of Serve As Preclinical Tools to Investigate Pathogenesis and Treatment of MCD/C5 DLBCL

Diffuse large B cell lymphoma (DLBCL) is the most common lymphoid malignancy in adults. Both, biologically and clinically, DLBCL represents a highly heterogeneous disease. Based on gene expression profiling DLBCL is subdivided into germinal center B cell (GCB)-like and non-GCB- or activated B cell (ABC)-like DLBCL, a classifier which distinguishes DLBCL sub-entities displaying distinct biological features and clinical response to frontline anthracycline-based chemo-immune therapy. Plasmablasts have long been regarded as the cell of origin (COO) of ABC DLBCL, however, recent evidence points towards aberrant memory B cells (MB) as the true COO of these lymphomas.

Several recent studies attempted to give structure to the genetic heterogeneity observed in DLBCL by clustering cases based on their mutational profiles. Clusters MCD by Schmitz et al. and C5 by Chapuy et al. are amongst the clusters enriched for ABC-DLBCL cases with particularly inferior prognosis. Both clusters are defined by mutations in MYD88 and CD79B as well as high levels of BCL2 expression and recurring mutations in transcriptional regulators of the plasmacytic differentiation process (PRDM1, SPIB).

We previously reported an autochthonous mouse model of DLBCL mimicking hallmark mutations of MCD/C5 DLBCL: Combination of Myd88 p.L252P (the murine orthologue of human MYD88 L265P) and overexpression of BCL2 leads to the generation of ABC-DLBCL-like lymphomas in the Myd88^p.L252P/wt;R26^LSL.BCL2/wt;Cd19^Cre/wt (MBC) mouse line. MBC Lymphoma cells show a plasmablastic immunophenotype with expression of CD138 and lack of expression of B220/CD19. In an attempt to refine our genetically-engineered mouse model, we incorporated other key alterations of MCD/C5 DLBCL in the MBC background: B cell specific loss of PRDM1, overexpression of SPIB and hallmark mutations in CD79B. Young Prdm1^fl/fl;Myd88^p.L252P/wt;R26^LSL.BCL2/wt;Cd19^Cre/wt (PPMBC) and Myd88^p.L252P/wt;R26^{LSL.BCL2/LSL.Spib};Cd19^Cre/wt (SMBC) compound animals display a strong reduction of the CD138⁺ population compared to control, accompanied by a decrease in serum immunoglobulins, in agreement with a block in plasma cell differentiation mediated by these additional genetic alterations. In contrast, cells with a marker profile of age-associated B cells (ABC, B220⁺/CD19⁺/CD21^-/CD23^-) as well as Memory B cells (MB, B220⁺/IgD^-/Fas^-/CD38⁺) accumulate markedly in all lymphoma lines, but particularly in PPMBC animals, compared to control. This is in line with recent studies hinting at ABC/ MB as the cells of origin of ABC-DLBCL.

While MBC lymphomas are largely CD138⁺ and represent a plasmablastic/-cytic differentiation stage, PPMBC tumors express B-cell markers B220 and CD19 and lack expression of CD138. Additionally, PPMBC lymphomas frequently show a marker profile reminiscent of memory B cells, while CD21/CD23 – negativity suggests a relation to age-associated B cells, a population enlarged also in the pre-malignant setting. PPMBC tumors furthermore show an enrichment for B-cell receptor (BCR), NFkB and PI3K signaling gene signatures, when compared to MBC lesions. These signs of enhanced B cell receptor signaling translate into an increased sensitivity towards inhibition of Bruton’s Tyrosine Kinase (BTK) by ibrutinib. The sensitivity of ibrutinib is further enhanced in lymphomas additionally expressing Cd79b p.Y195H, both in the Prdm1-deficient and the Prdm1-proficient model systems.

We have established and characterized novel refined autochthonous mouse models of MCD/C5 DLBCL which serve as a preclinical tool to test novel therapeutic strategies in this sub-entity and pave the way to a deeper understanding of molecular mechanisms of pathogenesis, treatment response and resistance in highly aggressive DLBCL.