The novel human IgE epsilon heavy chain, epsilon tailpiece, is present in plasma as part of a covalent complex

Several splice variants of the secreted human epsilon heavy chain have prev iously been identified by reverse transcription-PCR. The heavy chain of one isoform, IgE tailpiece, differs from the originally identified IgE, IgE cl assic, by the replacement of the 2 carboxy-terminal amino acids by 8 novel amino acids including a carboxy-terminal cysteine residue. Recombinant huma n epsilon tailpiece and epsilon classic heavy chains were expressed and sec reted as H2L2 monomers in Sp2/0 murine myeloma cells. We have investigated the in vitro function and in vivo occurrence of epsilon tailpiece heavy cha ins using receptor binding assays, granule release assays, flow cytometry, half-life studies, immunoprecipitation, SDS-PAGE, two-dimensional SDS-PAGE, and Western blotting. IgE tailpiece and IgE classic exhibited similar in v ivo half-lives in BALB/c mice, bound the human high- and low-affinity IgE r eceptors with similar affinities and triggered equivalent levels of high af finity IgE receptor induced degranulation. In humans, IgE classic is presen t as a 190 kD circulating protein in vivo. In contrast, we found that in pl asma epsilon tailpiece was primarily present as part of covalent complexes of approximately 300 and 338 kD. Dissociation of the complexes revealed tha t two species of epsilon tailpiece heavy chains were present therein and su rprisingly, these in vivo derived epsilon tailpiece heavy chains were appro ximately 5 and 10 kD smaller than the recombinant expressed epsilon tailpie ce or epsilon classic heavy chains. These results show that epsilon tailpie ce is present in novel covalent complexes in humans. (C) 2000 Elsevier Scie nce Ltd. All rights reserved.