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.

Plasmacytoid dendritic cells (pDC) are central players in the innate and adaptive immune response against viral infections. The molecular mechanism that underlies pDC development from progenitor cells is only beginning to be elucidated. Previously, we reported that the Ets factor Spi-B and the inhibitors of DNA binding protein 2 (Id2) or Id3, which antagonize E-protein activity, are crucially involved in promoting or impairing pDC development, respectively. Here we show that the basic helix-loop-helix protein E2-2 is predominantly expressed in pDC, but not in their progenitor cells or conventional DC. Forced expression of E2-2 in progenitor cells stimulated pDC development. Conversely, inhibition of E2-2 expression by RNA interference impaired the generation of pDC suggesting a key role of E2-2 in development of these cells. Notably, Spi-B was unable to overcome the Id2 enforced block in pDC development and moreover Spi-B transduced pDC expressed reduced Id2 levels. This might indicate that Spi-B contributes to pDC development by promoting E2-2 activity. Consistent with notion, simultaneous overexpression of E2-2 and Spi-B in progenitor cells further stimulated pDC development. Together our results provide additional insight into the transcriptional network controlling pDC development as evidenced by the joint venture of E2-2 and Spi-B.Key words: E2-2/TCF4 Á E-proteins Á Human development Á Plasmacytoid dendritic cell Á Spi-B Supporting Information available online See accompanying commentary by Esashi and Liu IntroductionThe ability of dendritic cells (DC) to capture and present antigenic peptides has established a function in both the adaptive and innate branches of the immune system. Extensive characterization of DC has revealed the existence of many different DC subsets with distinct cell surface phenotype, cytokine expression profile, and anatomical localization [1]. One member of the DC family is the plasmacytoid DC (pDC), which is hallmarked by their capacity to produce high levels of type I interferons and hence are also known as natural type I interferons producing cells [2]. Eur. J. Immunol. 2008. 38: 2389-2400 DOI 10.1002 HIGHLIGHTS 2389Frontline pDC are detected in blood and most tissues, including spleen, lymph nodes, and thymus [2,3]. Previously, we described that at least two developmental pathways exist for pDC, an extrathymic and an intrathymic pathway [4]. The requirements for the development of DC subsets are not fully understood. In mice it has been shown that conventional DC (cDC) and pDC can develop from minor Flt3 1 subpopulations within the common myeloid and the common lymphoid progenitor pools [5][6][7]. Recently, this pool was narrowed down to a DC committed precursor (pro-DC) that can only develop into cells of the DC lineages [8,9]. It is not clear yet at what point in DC development the commitment to a subpopulation is accomplished. Also, human pDC can be derived from both myeloid and lymphoid progenitor cells [10,11]. A better understanding of the molecular mechanisms that control...

Interleukin (IL)-21-producing CD4+ T cells are central to humoral immunity. Deciphering the signals that induce IL-21 production in CD4+ T cells and those triggered by IL-21 in B cells are, therefore, of importance for understanding the generation of antibody responses. Here, we show that IL-6 increased IL-21 production by human CD4+ T cells, particularly in those that express the transcriptional regulator B cell lymphoma (BCL)6, which is required in mice for the development of CXCR5+ IL-21-producing T follicular helper (TFH) cells. However, retroviral overexpression of BCL6 in total human CD4+ T cells, only transiently increased CXCR5, the canonical TFH–defining surface marker. We show here that IL-21 was required for the induction of antibody production by IL-6. In IL-21–treated B cells, signal transducer and activator of transcription (STAT)3 was required for optimal Ig production and upregulation of PRDM1, the master plasma cell factor. These results, therefore, demonstrate the critical importance of STAT3 activation in B cells during IL-21-driven humoral immunity and suggest that BCL6 expression, while not sufficient, may serve as a platform for the acquisition of a TFH–like phenotype by human CD4+ T cells.