Notch signaling suppresses IgH gene expression in chicken B cells: implication in spatially restricted expression of Serrate2/Notch1 in the bursa of Fabricius.

The bursa of Fabricius is a central organ for chicken B cell development and provides an essential microenvironment for expansion of the B cell pool and for generation of a diversified B cell repertoire. We report here that genes encoding the Notch family of transmembrane proteins, key regulators of cell fate determination in development, are differentially expressed in the bursa of Fabricius: Notch1 is expressed in medullary B cells located close to the basement membrane-associated epithelium (BMAE). In contrast, a Notch ligand, Serrate2, is expressed exclusively in the BMAE, which surrounds bursal medulla. A basic helix-loop-helix-type transcription factor, Hairy1, a downstream target of Notch signaling, is expressed in the bursa coordinately with Notch1 and Serrate2 and an immature B cell line, TLT1, which expresses both Notch1 and Serrate2. Furthermore, stable expression of a constitutively active form of chicken Notch1 or Notch2 in a B cell line results in a down-regulation of surface IgM expression, which is accompanied by the reduction of IgH gene transcripts. Transient reporter assay with the human IgH gene intronic enhancer reveals that an active form of Notch1 inhibits the IgH enhancer activity in chicken B cells, suggesting that Notch-mediated signals suppress the IgH gene expression via influencing the IgH intronic enhancer. These findings raise the possibility that the local activation of Notch1 in a subset of B cells by Serrate2 expressed in BMAE may influence the cell fate decision that is involved in B cell differentiation and selection inside the bursa.