THE LEUKOCYTE COMMON ANTIGEN (CD45) ON HUMAN PRE-B LEUKEMIA-CELLS - VARIANT GLYCOPROTEIN FORM EXPRESSION DURING THE CELL EXPOSURE TO PHORBOL ESTER IS BLOCKED BY A NONSELECTIVE PROTEIN-KINASE INHIBITOR H7

The human pre-B acute lymphoblastic leukemia cell line REH6 was utiliz ed for characterization of CD45 glycoprotein by monoclonal antibodies (mAb) recognizing four distinct CD45 antigen specificities, i.e. nonre stricted CD45, restricted CD45RA, CD45RB and CD45R0. Immunoprecipitati on revealed two antigen specificities on REH6 cells of m.w. 220 kDa an d 190 kDa, both presenting wide range of isoelectric point pI similar to 6.0-7.5. Nonrestricted CD45 epitopes were not affected by the sialy l acid cleavage with sodium metaperiodate or neuraminidase, but were s ensitive to both, tunicamycin, the N-glycosylation inhibitor and monen sin, an inhibitor of protein transport through the Golgi compartment. O-sialoglycoprotein endopeptidase from Pasteurella haemolytica Al part ially cleaved CD45RA and CD45RB epitopes, while nonrestricted CD45 det erminants were not affected by this enzyme. Limited proteolysis of thi s antigen resulted in the appearance of 160-180 kDa peptide domains wh ich retained CD45 epitopes. Further, the treatment of cells with phorb ol myristate acetate (PMA) induced marked down-regulation of 220 and 1 90 kDa isoforms and the appearance of new 210, 180 and 170 kDa variant glycoprotein forms which were not found on parental cells. This PMA e ffect was not accompanied by the programmed cell death and was markedl y blocked by a nonselective protein kinase (PK) inhibitor isoquinoline sulfonamide H7. Modulation of CD45 by phorbol esters might serve as a n in vitro model for an additional insight into the function of CD45 i n hematopoietic cells.