Platelet Glycoproteins LIb and Lila Associated With Blood Monocytes Are Derived From Platelets*

Platelet glycoprotein lIb/Illa (GP lIb/Illa). the receptor complex for fibrinogen, has been regarded as a megakaryo-cyte/platelet lineage-restricted antigen. Recently. how-ever. it has been reported that GP lIb/Illa is expressed in blood monocytes. Studies were performed to establish the origin and immunological characteristics of monocyte-associated glycoproteins lIb and lIla (GPs lIb and lIla). Preparations of blood monocytes containing varying plate-let-monocyte ratios were metabolically labeled with r5Sjmethionine with the expectation that any newly syn-thesized GPs lIb and lIla would be monocyte-derived. since platelets have only rudimentary protein synthetic appara-tuses. Analyses of sodium dodecyl sulfate (SDS) gels of homogenates of cell preparations containing from 200 to 5:1 platelet-monocyte ratios revealed that unlabeled GP