THE ETS-PROTEIN-SPI-B IS EXPRESSED EXCLUSIVELY IN B-CELLS AND T-CELLSDURING DEVELOPMENT

Spi-B and PU.1 are hematopoietic-specific transcription factors that c onstitute a subfamily of the Ets family of DNA-binding proteins. Here we show that contrary to previous reports, PU.1 and Spi-B have very di fferent expression patterns. PU.1 is expressed at high levels in B cel ls, mast cells, megakaryocytes, macrophages, neutrophils, and immature erythroid cells and at lower levels in mature erythrocytes. PU.1 is c ompletely absent from peripheral T cells and most T cell lines based o n sensitive RT-PCR assays. In contrast, Spi-B is expressed exclusively in lymphoid cells and can be detected in early fetal thymus and splee n. In situ hybridizations of adult murine tissues demonstrate Spi-B mR NA in the medulla of the thymus, the white pulp of the spleen, and the germinal centers of lymph nodes. Spi-B expression is very abundant in B cells and both Spi-B mRNA and protein are detected in some T cells. In situ hybridization and Northern blot analysis suggest that Spi-B g ene expression increases during B cell maturation and decreases during T cell maturation. Gel-retardation experiments show that Spi-B can bi nd to all putative PU.1 binding sites, but do not reveal any preferred Spi-B binding site. Finally, both PU.1 and Spi-B function as transcri ptional activators of the immunoglobulin light-chain enhancer E(lambda 2.4) when coexpressed with Pip (PU.1-interaction partner) in NIH-3T3 cells. Taken together, these data suggest that differences in patterns of expression between Spi-B and PU.1 distinguish the function of each protein during development of the immune system.