Abstract IA007: Dark zone lymphomas

Abstract

Dark zone (DZ) lymphomas are a group of aggressive B-cell lymphomas where the malignant cells display gene expression patterns similar to B cells of the dark zone of the germinal center (DZGC). This represents a refinement of the cell-of-origin classification, separating these tumors from the germinal center B-cell-like (GCB) group. This DZ umbrella group encompasses the recognized pathology entities of Burkitt lymphoma and high-grade B-cell lymphoma with MYC and BCL2 rearrangements (HGBCL-DH-BCL2 - "double hit" lymphoma) alongside tumors that currently are classified as diffuse large B-cell lymphoma (DLBCL), not otherwise specified. In this presentation, after showing the mutational landscape of DZ lymphoma, single cell analyses will be presented that explore the shared biology across these lymphomas. First we performed analyses on disaggregated single nucleus multiome data to identify shared patterns of gene expression and candidate transcription factors (TFs) driving these signatures in DZ lymphomas and the similarities with and differences from physiological germinal center B-cell subsets and GCB- and activated B-cell-like (ABC)-DLBCL. We confirmed that DZ malignant cells express transcriptional programs reminiscent of GCDZ B cells, whereas GCB- and ABC-DLBCL converge on shared programs, including NF-kB activation, not operative in DZ lymphomas. In comparison to GCDZ B cells, DZ lymphoma malignant cells were enriched for MYC target signatures and an mTOR signaling signature. Differential chromatic accessibility and TF motif enrichment analysis identified coordinated increase in FOXO1 expression and motif accessibility and decreases in NFKB1 and RELB in comparison with ABC- and GCB-DLBCL. In aggregate the data support a model in which DZ lymphoma exploit FOXO1 to maintain a DZ-like state, whereas other DLBCL engage NF-kB-driven networks. Next, we performed in situ single transcriptomics on a large cohort of DLBCL tumors to substantially expand our single cell gene expression cohort, quantitate the composition of the tumor microenvironment (TME), identify cellular niches and explore interactions between TME elements and the malignant cells. Single cell and pseudobulk gene expression analyses confirm the patterns of gene expression and biology observed in the single nucleus mutltiome data. DZ lymphomas were defined by an immune-cold TME, marked by enrichment of malignant B cells and depletion of major immune and stromal lineages relative to other DLBCL. Ten cellular niches were identified across the DLBCL cohort with "effacement" being the only niche enriched in DZ lymphoma compared with other DLBCL. In comparison, GCB- and ABC-DLBCL show differential enrichment for the other niches. Consistently, DZ lymphomas harbored the fewest ligand-receptor interactions, consistent with relative autonomy from the TME. In summary, DZ lymphomas are biologically distinct from the remainder of DLBCL with regards to their mutational landscape, gene expression profile, transcription factor usage, tumor microenvironment composition and architecture.