Short-lived plasmablasts express intermediate level of Blimp-1, whereas long-lived plasma cells express high amounts of Blimp-1 [19, 20]. Blimp-1 is universally required for the formation of competent plasma cells. Blimp-1-deficient mice fail to generate antibody-secreting cells [18, 20, 21], and ectopic

expression of Blimp-1 is sufficient to induce antibody-secreting cell differentiation [22]. Blimp-1 can efficiently shut down the B cell gene expression programme and promote the exit from the cell cycle by repressing mature B cell–associated transcription factor genes such as Pax5, CIITA, SpiB, c-Myc and genes important Sorafenib manufacturer for GC formation including Bcl6 and activation-induced cytidine deaminase (AID) [15, 23–25]. However, Blimp-1 is not only needed to drive the plasmacytic properties but is also required for the maintenance of long-lived plasma cells [26]. These findings led to the conclusion that Blimp-1 is a master regulator of the initiation of plasma cell differentiation. This concept,

however, is challenged by a parallel mouse model, where Blimp-1 gene is engineered to harbour a green fluorescent protein reporter gene [20]. This model was used to discover a subset of cells called preplasmablast that have downregulated the expression of a central B cell transcription factor Pax5 but not yet induced the expression of Blimp-1 [27]. This finding fits with other models, BAY 80-6946 mw where deletion of Pax5 Edoxaban gene in DT40 B cell line induced spontaneous plasma cell differentiation [8, 9] and inactivation of Pax5 in mature mouse B cells induces Blimp-1 expression [28]. Collectively

these findings suggest that Blimp-1 drives the differentiation of plasma cells, but the initiation of plasma cell differentiation precedes the induction of Blimp-1 and is caused by downregulation of B cell properties. IRF4 has a two-phase expression pattern during the B cell development. While it is expressed in immature B cells in the bone marrow, it is lost in proliferating GC centroblasts [29, 30]. However, its expression starts to gradually increase again in some centrocytes and plasmablasts and reaches its highest level in plasma cells [30, 31]. In addition to Blimp-1, IRF4 is generally required for plasma cell differentiation. IRF4-deficient mice lack plasma cells, their serum Ig levels are low and their B cells cannot form plasma cells in vitro [16, 32, 33]. IRF4 seems to act upstream of Blimp-1, as IRF4 can bind to Blimp-1 gene and B cells cannot express Blimp-1 in the absence of IRF4 [33]. Xbp1 is also necessary for effective plasma cell formation [17], but it cannot initiate the process in the absence of Blimp-1 [18]. Xbp1 is required for secretion of antibody in plasma cells [34]. Within the B cells, the expression of Xbp1 is suppressed by Pax5 [35] and its overexpression in B cells expands the protein secretory apparatus [34]. Xbp1 acts downstream of IRF4 and Blimp-1 [18, 32].