Intracellular cleavage of newly synthesized low affinity Fc epsilon receptor (Fc epsilon R2) provides a second pathway for the generation of the 28-kDa soluble Fc epsilon R2 fragment

It has been reported that the 45-kDa low affinity Fc epsilon R (Fc epsilon R2) on B cells is cleaved spontaneously from the cell surface to release a 28-kDa soluble fragment (sFc epsilon R2). This study demonstrates an additional mechanism by which B cells generate this fragment. Data from 35S methionine pulse-chase experiments with the Fc epsilon R2 bearing human B lymphoblastoid cell line, RPMI 8866, and immunoprecipitations of cell lysates and culture supernatants with an Fc epsilon R2 specific mAb, mAb 25, demonstrates the existence of a cell-associated 28-kDa Fc epsilon R2 fragment. This fragment was shown by partial amino(NH2)-terminal sequence analysis to be identical to the previously described 28-kDa sFc epsilon R2. The resistance to cell treatment with trypsin indicated that it was located intracellularly. Its appearance early in the biosynthesis of the Fc epsilon R2 (within a 10-min pulse), before the Fc epsilon R2 reached the cell surface, suggested that some of this fragment was generated intracellularly. Neutralization of acidic organelles with NH4Cl inhibited the formation of this intracellular fragment, strongly suggesting that it was a produce of intracellular cleavage of the Fc epsilon R2. Finally, this 28-kDa intracellular fragment was shown to be released into the culture supernatant, suggesting an intracellular mechanism by which the cells generate sFc epsilon R2.