A SOLUBLE FORM OF THE HUMAN FC RECEPTOR FC-GAMMA-RIIA - CLONING, TRANSCRIPT ANALYSIS AND DETECTION

The Fc gamma receptors (Fc gamma R) are glycoproteins that bind the Fc region of immunoglobulin G. Human hematopoietic cells express three b iochemically distinct classes of Fc gamma receptors: Fc gamma RI(CD64) , Fc gamma RII (CD32) and Fc gamma RIII (CD16). Complementary DNA (cDN A) clones for each of the human Fc gamma receptors have been isolated from myeloid and lymphoid cells. We describe the isolation and charact erization of four Fc gamma RII clones from a cDNA library obtained fro m a megakaryocyte-like cell line, human erythroleukemia (HEL). Three c lones encode the Fc gamma RIIA transmembrane (TM) form, while one nove l clone lacks the TM region but retains the cytoplasmic domain. By con ducting reverse transcription coupled to polymerase chain reaction (PC R), we found transcripts coding for this unique form of receptor in RN A from platelets, HEL cells and a second megakaryocyte-like cell line, CHRF-288-11. These results were confirmed by RNase protection analysi s of RNA from HEL cells. The structure of the novel cDNA suggested tha t it codes for a soluble form of Fc gamma RIIA. A soluble Fc gamma RII protein was detected in the conditioned medium from HEL cells but not from the Fc gamma RII-negative T cell line, Jurkat, by immunoprecipit ation with the anti-Fc gamma RII monoclonal antibody (mAb), IV.3. The immunoprecipitated protein was of the expected size for a soluble Fc g amma RII lacking the TM region but retaining the cytoplasmic domain. S oluble Fc gamma RIIA may be important in modulating the interaction be tween immune complexes and membrane-associated Fc gamma RII.