Abstract
Abstract 3898
Pax5 is an essential transcription factor to maintain B cell identity. Pax5 is expressed in stages from pro-B to mature B cells and promotes the B cell differentiation program by transcriptional activation of many B cell receptor (BCR)-related genes such as CD19, CD79a, and BLNK. On the contrary, it inhibits plasma cell differentiation by suppressing the expression of Blimp1 and XBP-1, transcription factors essential for plasma cell differentiation. After BCR stimulation by antigen, upregulation of Blimp1 and XBP-1 and subsequent suppression of Pax5 by Blimp1 were observed and thought to be the trigger of plasma cell differentiation. However, the first trigger that represses PAX5 function is yet to be revealed. Here, we investigated the relevance between Pax5 and MAP kinase signal, a main component of BCR signal. We demonstrated PAX5 phosphorylation by ERK2 in vitro and in vivo (Figure. A). The sites of PAX5 phosphorylation were identified by PCR mutagenesis assay. In the luciferase reporter assays, PAX5 phosphorylation by MAP kinase signal reduced the transcriptional activity of PAX5 when CD19 promoter sequence was used for reporter gene. On the other hand, PAX5 caused transcriptional repression of Blimp1 promoter, which was released by PAX5 phosphorylation by MAP kinase signal. These data supported the reported findings that PAX5 had dual function, and suggested that the PAX5 function was inhibited by its phosphorylation. We demonstrated that B cell receptor stimulation with anti-IgM antibody induced strong MAP kinase activation, phosphorylation of endogenous Pax5 (Figure. B), and upregulation of Blimp1 mRNA (Figure. C). All of these phenomena were inhibited by U0126, MEK inhibitor. These results suggested that transcriptional repression of Blimp1 by PAX5 was released by its phosphorylation by MAP kinase signal. This might be a trigger of plasma cell differentiation. Our findings give a new insight into the regulation of the terminal differentiation of B cells.
Hayakawa:Otsuka Pharmaceutical Co., Ltd.: Research Funding. Naoe:Chugai pharmaceutical, Zenyaku pharmaceutical, Kyowa-Kirin pharmaceutical, Dainippon-Sumitomo pharmaceutical, Novartis pharmaceutical, Janssen pharmaceutical, Otsuka pharmaceutical: Research Funding.
