STAT3-Mediated Up-Regulation of BLIMP1 Is Coordinated with BCL6 Down-Regulation to Control Human Plasma Cell Differentiation

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Abstract
STAT family members have been implicated in regulating the balance between B cell lymphoma (BCL)6 and B lymphocyte induced maturation protein (BLIMP)1 to control plasma cell differentiation. We previously showed that STAT5 induces BCL6 to block plasma cell differentiation and extend the life span of human B cells. The heterogeneity in STAT activation by cytokines and their effects on B cell differentiation prompted us to investigate the effect of STAT3 activation in plasma cell differentiation. First stimulation with IL-21, which promotes plasma cell differentiation, induced robust and prolonged STAT3 activation in primary human B cells. We then investigated effects of direct STAT3 activation on regulation of plasma cell genes, cellular phenotype, and Ig production. Activation of a tamoxifen-regulated STAT3-estrogen receptor fusion protein triggered BLIMP1 mRNA and protein up-regulation, plasma cell phenotypic features, and Ig secretion. When STAT3 was activated by IL-21 in B cells ectopically expressing BCL6, BLIMP1 was up-regulated, but only partial plasma cell differentiation was achieved. Lastly, through coexpression of BCL6 and STAT3-ER, we verified that STAT3 activation functionally mimicked IL-21 treatment and that STAT3-mediated BLIMP1 up-regulation occurred despite high BCL6 expression levels indicating that BCL6 is not the dominant repressor of BLIMP1. Thus, up-regulation of BLIMP1 alone is not sufficient for differentiation of primary human B cells into plasma cells; concomitant down-regulation of BCL6 is absolutely required for completion of the plasma cell differentiation program.