Integration and Iteration: Using Advanced, High-Content Imaging and Single-Cell Gene Expression Analysis to Uncover Unique Aspects of Follicular Lymphoma Biology

BACKGROUND: Follicular lymphoma (FL) is an indolent malignancy of germinal center B-cell origin. FL patients experience remarkable heterogeneity in their disease trajectory, with many patients slowly progressing over several years, and a subset of patients experiencing an aggressive clinical course. Uncovering the cell-intrinsic and -extrinsic factors that govern differential progression and outcome in FL patients is thus essential. Beyond the genetic and epigenetic aberrations that contribute to FL oncogenesis, the tumor microenvironment (TME) plays an integral role in supporting the proliferation and survival of malignant cells. In a process described as "re-education", FL tumor cells may actively subvert the normal functions of non-malignant cells present in the TME, including T cells, follicular dendritic cells (FDCs), macrophages, dendritic cells, and stromal cells to support their survival and growth.

METHODS: To address the importance of the TME in FL, we molecularly profiled excisional lymph node biopies from untreated FL patients using multiple platforms: bulk RNA sequencing (RNAseq), single-cell RNA sequencing (scRS), and a unique high content imaging method, Iterative Bleaching Extends MultipleXity (IBEX), which utilizes chemical bleaching to image 40+ proteins in the same tissue section by antibody staining. In combination with advanced computational tools for the quantitative analysis of cell types and distribution in tissues, we have used this approach to evaluate the TME:FL interaction within 8 FL samples, with 4 normal lymph nodes as controls.

RESULTS: Both FL and TME components reconstructed from bulk RNA-seq were similar to the cellular composition revealed by scRS and IBEX analyses. Moreover, the bulk RNAseq and scRS identified the expression of genes involved in tumorigenesis and oncogenic signaling, often unique to each case. However, RNAseq-based approaches often miss important cellular and acellular components not readily extracted from dense tissues. For example, IBEX imaging can trace the pattern of blood vessels within a section, which cannot be achieved with non-imaging methods. In one case, our multi-parameter imaging studies revealed the close spatial interaction between clonal FL B cells, expressing a B-cell receptor (BCR) possessing a de novo N-linked glycosylation site introduced by somatic hypermutation, and cells expressing dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin (DC-SIGN). This contact may activate pro-survival signaling in the malignant B cells.

CONCLUSIONS: Integration of bulk RNAseq, scRS, clonotype analysis, and IBEX reveals both shared and unique aspects of different FL tumors. These data highlight the importance of integrating direct tissue analysis by high-content imaging with methods examining aspects of isolated cells. This approach may provide a more complete understanding of tumor biology, which in turn will identify patients at risk for developing aggressive disease and rationally improve treatment strategies for FL.

This research was supported in part by the Intramural Research Program of the NIH, NIAID and NCI

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