Mesenchymal Stromal Cells Orchestrate Follicular Lymphoma Cell Niche Through the CCL2-Dependent Recruitment and Polarization of Monocytes

Abstract 1566

Accumulating evidence indicates that infiltrating stromal cells contribute directly and indirectly to tumor growth in a wide range of solid cancers and hematological malignancies. In follicular lymphoma (FL), malignant B cells are found admixed with heterogeneous lymphoid-like stromal cells within invaded lymph nodes and bone marrow (BM). In addition, in vitro functional studies have underlined that mesenchymal cells recruit malignant FL B cells and protect them from spontaneous and drug-induced apoptosis. In particular, we have previously demonstrated that mesenchymal stromal cells (MSC) efficiently support in vitro FL B-cell survival, especially after their engagement towards lymphoid differentiation through treatment with TNF-α and Lymphotoxin-α1β2 (TNF/LT) or after coculture with malignant B cells. However, the mechanisms of this supportive activity remain largely unknown.

In this study, we used Affymetrix U133 Plus 2.0 microarrays, to compare the gene expression profile (GEP) of bone marrow-derived MSC (BM-MSC) obtained from 10 FL patients at diagnosis versus 6 age-matched healthy donors (HD). In these conditions, neither the CFU-F concentration in the BM nor the cumulative population doubling of BM-MSC significantly differed between HD and FL patients. Unsupervised analysis was able to perfectly segregate FL-MSC from HD-MSC and we identified, using supervised analyzes, a list of 408 probesets defining FL-MSC signature, including 320 nonredundant genes upregulated in FL-MSC compared to HD-MSC. We then defined the GEP of human lymphoid-like stroma using HD-MSC treated in vitro by TNF/LT and demonstrated, by a Gene Set Enrichment Analysis (GSEA) approach, that the FL-MSC signature is significantly enriched for genes associated with a lymphoid-like commitment. Interestingly, CCL2 was strongly overexpressed by FL-MSC, was upregulated in HD-MSC by coculture with malignant B cells, and was detected at a higher level in FL BM plasma compared to normal BM plasma (504.4 pg/mL [23.8-4413] versus 33.9 pg/mL [5-126.1]; P <.01). In agreement, FL-MSC triggered a more potent CCL2-dependent monocyte migration than HD-MSC. Moreover, FL-MSC and macrophages cooperated to sustain malignant B-cell growth through both protection from apoptosis and enhancement of cell proliferation. Finally, FL-MSC promoted monocyte differentiation towards a proangiogenic LPS-unresponsive phenotype close to that of tumor-associated macrophages. We unraveled a key role for the Notch pathway in this process and identified an overexpression of JAGGED1 in FL-MSC compared to HD-MSC.

Altogether, these results highlight the complex role of FL stromal cells that promote direct tumor B-cell growth and orchestrate FL cell niche. The identification and characterization of this intricate network of cell interactions may provide novel therapeutic targets in this disease.