EXPRESSION OF BRIGHT AT 2 DISTINCT STAGES OF B-LYMPHOCYTE DEVELOPMENT

The B cell regulator of Ig heavy chain transcription (Bright) is a DNA -binding protein that was originally discovered in a mature Ag-specifi c B cell line after stimulation with IL-5 and Ag, It binds to the intr onic heavy chain enhancer and 5' of the V1 S107 family V-H promoter, S everal studies suggested that Bright may increase transcription of the heavy chain locus, and expression in cell lines was!limited to those representing mature B cells. We have now analyzed normal hemopoietic t issues for the expression of Bright during B lymphocyte differentiatio n. We expected to find Bright expression in a subset of mature spleen cells, but also observed Bright in a subset of normal B lymphocytic pr ogenitors in both adult bone marrow (BM) and in fetal liver as early a s day 12 of gestation. Bright was also expressed in the small percenta ge of CD4(low) cells in the thymus that are newly arrived from the BM and are not yet committed to the T lymphocyte lineage, but was not obs erved at later stages of T cell differentiation in either the spleen o r thymus, Bright mRNA was not detected in the immature B lymphocytes t hat initially populate the spleen after migration from the BM, In addi tion, new splice variants of Bright were observed in fetal tissues. Th us, Bright expression is highly regulated in normal murine lymphocytes and occurs both early and late during B cell differentiation. These f indings may have important implications for the function of Bright in regulating Ig transcription.