Abstract PR002: Bcl6 -driven Cd70 -deficient Diffuse large B-cell lymphomas originate from innate-like cells with blunted CD4+ cytotoxic T-cell immune surve

Abstract

Diffuse large B cell lymphoma (DLBCL) is a heterogeneous disease comprised of at least 5 molecular subtypes (Clusters 1-5, C1-5). C1 DLBCLs share genetic features with transformed marginal zone lymphomas such as BCL6 translocations and genetic bases of immune evasion including inactivating mutations of CD70, the CD27 costimulatory ligand. We generated a murine model of Bcl6-driven lymphomagenesis in the setting of Cd70 deficiency and perturbed CD70/CD27 costimulation. We observed earlier onset and increased penetrance of the disease in Cd70 -/-;Bcl6 tg/+ compared to Bcl6 tg/+ animals. Almost all Cd70 -/-;Bcl6 tg/+ and Bcl6 tg/+ mice that were euthanized for symptoms (sick) exhibited splenomegaly. Histomorphologic analyses of these spleens revealed architectural disruption, diffuse infiltration of small-to-large highly proliferative monoclonal B cells intermixed with T cells, consistent with a diagnosis of DLBCL. To investigate the process of Bcl6-driven lymphomagenesis in the presence or absence of CD70, we harvested spleens from asymptomatic wild-type (WT), Cd70 -/-, Bcl6 tg/+ and Cd70 -/-;Bcl6 tg/+ animals at 6, 14 and 18 months and performed scRNA-seq of splenic cell suspensions. By serially characterizing asymptomatic and sick Bcl6 tg/+ and Cd70 -/-;Bcl6 tg/+ mice, we identified cell subsets with low-to-absent expression of the marginal zone (Cd21) and follicular (Cd23) B-cell markers. These Cd21 lo/- /Cd23 lo/- cells exhibited transcriptional features of extrafollicular innate-like B cells, including Bhlhe41 and Fcrl5 expression and regulatory features, including Ebi3 and Il10 expression. The earliest detected Cd21 lo/- /Cd23 lo/- aberrant B cells were progressively replaced by clonally related aged/autoimmune and MHC-IIlo cycling B cells. Clonal relationships between lymphoma precursors and malignant DLBCLs were determined by tracking shared BCR clonotypes and analyzing the acquisition of additional genomic alterations (with InferCNV). Transcriptional analysis of 409 human DLBCLs with C1-5 designations confirmed that human C1 tumors selectively expressed the innate-like B-cell markers, BHLHE41 and EBI3. We further characterized the T-cell anti-tumor immune responses in Bcl6 tg/+ and Cd70 -/-;Bcl6 tg/+ mice, which were predominantly mediated by CD4+, rather than CD8+, cytotoxic T cells (CTLs). CD4+ CTL clonal expansion was seen earlier (14 months) in Bcl6 tg/+ mice and was significant at 18 months in both Bcl6 tg/+ and Cd70 -/-; Bcl6 tg/+ cohorts, compared to WT controls. In vitro analyses confirmed that CD4+ CTLs from 14-month-old Bcl6 tg/+ animals killed Bcl6-driven DLBCLs in a MHC-II-dependent manner. However, in both tumor-bearing cohorts, sick animals had decreased CD4+ CTL and increased Treg clonal expansion. Together, these results highlight CD70/CD27 axis disruption as a central mechanism of accelerating lymphoma development, implicate innate-like B cells as the cells-of-origin in C1 DLBCLs and point to MHC-II downregulation and Treg clonal expansion as mechanisms of immune evasion from CD4+ CTL-mediated immunity.