Expression of recombinant soluble Fc epsilon RI: function and tissue distribution studies

Recombinant soluble IgE Fc receptors (rsFc epsilon RI) are potent inhibitors of type I hypersensitivity reactions tested in a local inflammatory setting. However, the fate of these receptors in vivo is dependent on the cellular source of the rsFc epsilon RI. We have produced these by transiently transfecting Cos-7 cells with a cDNA encoding the extracellular domains of human Fc epsilon RI alpha-chain. Following affinity purification, the rsFc epsilon RI was characterized as 58,000 MW, which was reduced to 23,000 MW following endoglycosidase F treatment. The purified rsFc epsilon RI could inhibit mouse IgE binding to Fc epsilon RI+ transfected CHO-K1 cells in vitro, bind sIgE+ B lymphoma cells in vitro, and inhibit the passive cutaneous anaphylaxis model in vivo in Sprague-Dawley rats. Pharmacokinetic studies in vivo involving intravenous injection of radiolabelled rsFc epsilon RI in mice revealed the receptor to have a rapid initial blood clearance (t1/2 early phase of 15 min) and to accumulate in the liver before being detected in urine. The localization of rsFc epsilon RI in the liver could be blocked by administration of mannose glycosylated ovalbumin and mannan, demonstrating that liver uptake involved the mannose receptor that is expressed on liver sinusoid cells and Kupffer cells. The production of rsFc epsilon RI using a stable expression system in CHO-K1 cells produced functional receptor of the same molecular weight as the Cos-7 system by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE). However, biodistribution studies demonstrated differences; the CHO-K1 cell-produced material did not localize to the liver in comparison to the Cos-7-produced rsFc epsilon RI.