Human Mesenchymal Stem Cells Isolated from Bone Marrow and Lymphoid Organs Support Tumor B-Cell Growth: Implications in Follicular Lymphoma Pathogenesis.

There is accumulating evidence that cellular microenvironment plays a key role in follicular lymphoma (FL) pathogenesis, both within tumor lymph nodes (LN) and in infiltrated bone marrow (BM) where ectopic LN-like reticular cells are integrated within malignant B-cell nodular aggregates. In normal secondary lymphoid organs, specific stromal cell subsets provide a highly specialized microenvironment that supports immune response. In particular, fibroblastic reticular cells (FRC) mediate immune cell migration, adhesion, and reciprocal interactions. The role of FRC and their postulated progenitors, i.e. bone marrow mesenchymal stem cells (MSC), in FL remains unexplored. In this study, we have investigated the relationships between FRC and MSC and their capacity to sustain malignant B-cell growth. Our findings strongly suggest that secondary lymphoid organs contain bona-fide MSC able to give rise at single-cell level to adipocytes, chondrocytes, and osteoblasts. These LN-derived MSC could also differentiate, in response to a combination of tumor necrosis factor-α (TNF) and lymphotoxin-α1β2 (LT), into fully functional FRC, able to construct a dense extracellular reticular meshwork positive for transglutaminase and fibronectin staining, to produce inflammatory (CXCL9, CXCL10, CCL5, CCL2) and LN-specific (CCL19) chemokines, and to favour lymphoma B-cell growth. Bone marrow-derived MSC (BM-MSC) acquire in vitro a complete FRC phenotype in the same culture conditions. As an exemple, BM-MSC had a strong, although not complete, protective effect on serum deprivation-induced apoptosis of BL2 cell line (mean percentage of CD20^posCaspase-3^pos cells: 24.8 +/− 17.5% in coculture with BM-MSC versus 80.7 +/- 10.4% in medium alone; P < .05; n =5) and pretreatment with TNF/LT fully restored BL2 viability (mean percentage of CD20^posCaspase-3^pos cells: 7.4 +/− 4.7%; P < .05; n = 5). Moreover, stimulation of stromal cells by TNF/LT before coculture enhanced the number of viable CD19^pos primary FL B cells by 2.4-fold for BM-MSC and 2.3 fold for LN-MSC compared with the culture without stromal cells (P < .05; n = 6). Interestingly, cell contact with lymphoma B-cell lines or purified FL B cells trigger the differentiation of BM-MSC into FRC that, in turn, support malignant B-cell migration, adhesion and survival. Altogether, these new insights into the interactions between lymphoma cells and their microenvironment could offer original therapeutic strategies.