B lymphocyte-induced maturation protein-1 (Blimp-1) is a transcriptional repressor that plays an important role during plasmacytic differentiation and is expressed in normal and transformed plasma cells. We here investigated the importance of continuous Blimp-1 expression. We found that knockdown of Blimp-1 expression by lentiviral vector-delivered short hairpin RNA causes apoptosis in multiple myeloma cell lines and plasmacytoma cells, indicating that continued expression of Blimp-1 is required for cell survival. We examined the mechanism underlying Blimp-1 knockdown-mediated apoptosis and found that the Blimp-1 knockdown neither reversed the phenotypic markers of plasma cells nor caused cell cycle arrest. Instead, our results show that knockdown of Blimp-1 induced the proapoptotic protein Bim, reduced the antiapoptotic protein Mcl-1, and activated caspase-9 and caspase-3. We further link apoptosis in transformed plasma cells mediated by proteasome inhibitors, the effective therapeutic agent for multiple myeloma patients, with reduced expression of Blimp-1. Lastly, we show that Blimp-1-dependent cell survival may act downstream of IFN regulatory factor 4 (IRF4) because IRF4 knockdown leads to down-regulation of Blimp-1 and apoptosis in multiple myeloma cells and plasmacytoma cells. Together, our data suggest that Blimp-1 ensures the survival of transformed plasma cells.

Terminally differentiated, antibody-secreting plasma cells are the end-stage effectors of humoral immune responses. Understanding the regulatory mechanisms that maintain the longevity of plasma cells provides a rational basis for designing strategies to enhance the levels of vaccine-induced antibodies. Blimp-1 orchestrates plasma cell differentiation by silencing the gene expression program of mature B cells. We showed that long-lived plasma cells in the bone marrow require the continuous presence of Blimp-1. Inhibition of Blimp-1 in plasma cells caused apoptosis. We performed microarray study examining the differential gene expression following depletion of Blimp-1 in plasma cells and uncovered that Blimp-1 suppresses apoptosis signal-regulating kinase 1 (ASK1). Blimp-1 directly suppresses ASK1 transcription. Knocking down ASK1 and Blimp-1 together in plasma cells blocked Blimp-1 knockdown-mediated apoptosis. Furthermore, ASK1 plays a negative role in survival of long-lived and short-lived plasma cells. ASK1 activity was induced during differentiation of short-lived plasma cells, and when produced by ASK1-deficient mice these cells survived better than those of control mice. Similarly, antigen-specific long-lived plasma cells generated by immunization accumulated in ASK1-deficient mice. Given that ASK1 induces apoptosis in response to stress, we rationalized that long-lived plasma cells may require sustained suppression of ASK1 by Blimp-1 to maintain survival.