BLIMP-1 is insufficient to induce antibody secretion in the absence of IRF4

Differentiation of B cells into antibody secreting cells (ASCs), plasmablasts and plasma cells, is regulated by a network of transcription factors. Within this network factors including PAX5 and BCL6 prevent ASC differentiation and maintain the B cell phenotype whereas BLIMP-1 and high IRF4 expression promote plasmacytic differentiation. BLIMP-1 is thought to induce immunoglobulin secretion. While IRF4 is needed for the survival of ASCs, its role in the regulation of antibody secretion has been controversial. BCL6-deficient DT40 B cell line has upregulated BLIMP-1 expression and secrete antibodies. In order to study the role of IRF4 in regulation of antibody secretion we have created a double knockout (DKO) DT40 B cell line deficient in both IRF4 and BCL6. This DKO cell line did not upregulate PRDM1 (the gene encoding for BLIMP-1) expression or secrete IgM. Even enforced BLIMP-1 expression in DKO cells or IRF4-deficient cells could not induce IgM secretion while it did in WT cells. However, enforced IRF4 expression in DKO cells induced strong IgM secretion. Our findings support a model where IRF4 expression in addition to BLIMP-1 expression is required to induce antibody secretion. Gene expression profiles of BCL6 knockout (BCL6KO) and IRF4-BCL6 doubleknockout (DKO) chicken DT40 B cells were generated by Agilent Chicken Gene Expression Microarray V2.