PU.1 is a lineage-specific regulator of tyrosine phosphatase CD45

The hematopoietic cell-specific ets family transcription factor PU.1 regula tes many lymphoid and myeloid genes. We have determined that PU.1 is critic al for lineage-specific expression of the tyrosine phosphatase CD45, CD45 i s expressed exclusively in hematopoietic cells at all stages of development , except for mature red cells and platelets, Although CD45 is normally expr essed in all leukocyte lineages, it is critically regulated by PU.1 only in myeloid cells. Whereas myeloid cells from PU.1 null mice failed to express CD45, lymphoid cells were CD45(+) by flow cytometry, Additionally, mRNA fo r CD45 was absent from PU.1-deficient myeloid cells. To understand the mole cular basis for these observations, we characterized a transcriptional regu latory region of the murine CD45 gene containing exons 1a, 1b, and 2, Disti nct transcriptional initiation sites for CD45 were demonstrated in T and B cells versus myeloid cells. A transcriptional initiation site in exon 1b (P 1b) was principally utilized by myeloid cells. A PU.1 binding site was iden tified upstream of exon Ib by sequence analysis and DNA binding assays. Usi ng this region of the CD45 locus we demonstrated that PU.1 directly transac tivated reporter gene expression, Finally, retrovirus-mediated restoration of PU.1 expression to PU.1-deficient myeloid cells resulted in expression o f cell surface CD45 and restored phosphatase activity, confirming the role of PU.1 in the positive regulation of this well known signaling molecule, W e conclude that CD45 is regulated differentially in myeloid and lymphoid ce lls and that sequences critical to direct myeloid expression include a PU.1 binding site upstream of the P1b transcriptional initiation site.