Human Mesenchymal Stem Cells as Precursors of Functional Fibroblastic Reticular Cells: Implications for the Physiopathology of Secondary Lymphoid Organs.

Several subsets of stromal cells are found among secondary lymphoid organs where they play a key role in the initiation and maintenance of immune response. In particular, fibroblastic reticular cells (FRC) of the paracortex secrete extracellular matrix (ECM) components that constitute a dense network of conduits allowing antigens carried within the subcapsular afferent lymph to reach the lumen of the medullary high endothelial venules. FRC produce also several chemokines that recruit T, B, and dendritic cells from blood and favour their reciprocal interactions. In addition, follicular dendritic cells (FDC) are located exclusively into germinal centers and allow normal B-cell selection through a complex set of survival signals, including BCR-mediated signal, chemokines and adhesion molecules. FRC and FDC networks are phenotypically and probably functionally altered during development of follicular lymphomas and diffuse large B cell lymphomas, the two most frequent Non-Hodgkin Lymphomas. FRC and FDC are supposed to be of mesenchymal origin even if no conclusive work has been conducted to date in human. We have obtained 15 tonsil-derived stromal cell lines, that displayed all the morphologic, phenotypic, and functional characteristics of FRC, including synthesis of inflammatory (CXCL10, CXCL9, CCL5) and lymph-node specific (CCL19, CCL21) chemokines, and secretion of ECM organized in a reticular meshwork after long-term culture in the presence of TNF-α and lymphotoxin-α1β2 (LT). These cells induced tonsil leukocyte migration and adhesion in vitro. Tonsil-derived stromal cells expressed LTβR, TNFR, and CD40 but were negative for FDC specific markers, such as CD21 or CXCL13, even following in vitro stimulation by TNF-α, LT, and trimeric CD40L. Interestingly, such TNF and LT-dependent FRC differentiation could also be induced in adult bone marrow-derived mesenchymal stem cells (MSC). In addition, MSC-like cells able to differentiate along osteogenic, adipogenic, and chondrogenic lineages at the clonal level were found in normal tonsils. These data shed new lights on our current understanding of lymph node stromal cell origin and strongly suggest that MSC are the precursors of FRC in secondary lymphoid organs, and perhaps in bone marrow in case of FL involvement where ectopic lymph node-like stromal cells are detected in close association with tumor cells. In conclusion, MSC and their progeny trigger differential immune effects, depending on cytokine context, localization and cell contact with immune cells. These properties are probably modified during lymphomas where the contact between malignant B cells and stromal cells is crucial for tumor development.