Inflammatory environment created by fibroblast aggregates induces growth arrest and phenotypic shift of human myeloma...

Multiple myeloma (MM) is characterized by accumulation of clonal plasma cells (PCs) predominantly in the bone marrow but tumor cells appear in the circulation in significant numbers as the disease progress. The occurrence of circulating multiple myeloma cells raises question concerning interactions between these cells and stroma of peripheral organs specifically under certain pathophysiological conditions, e.g., inflammation. Therefore, in the present study we exposed three human multiple myeloma cell lines to sterile inflammation produced in a culture dish by clusters of cell-cell contact-activated dermal fibroblasts. We now observed that myeloma cells responded differently to this particular type of stromal cell activation, nemosis. Two cell lines U-266 and LP-1 were minimally affected by the proinflammatory signalling, while the third cell line RPMI 8226 responded with growth arrest and altered expression of three phenotypic markers CD38, CD45, and CD138, indicating dedifferentiation shift of these cells to less mature PC-like phenotype. In a preliminary study we identified a subclone of cells having similar phenotype in 14 out of 23 analysed specimens of MM patients. This set of data indicates that the observed phenomenon might be clinically relevant. Our results emphasize the potential role of activated stromal fibroblasts and subsequent inflammation in altering phenotype of PCs and directing myeloma progression towards dormancy. Given the significant implication of dormant myeloma cells that might serve as a major cellular basis for the relapse, understanding their unique biology and precise elucidation of the underlying molecular mechanisms for the maintenance of quiescence is important. Therefore, we consider this study as a particular contribution to development of experimental model for in vitro studies of cancer dormancy.