HEAVY-CHAIN DIMERS AS WELL AS COMPLETE ANTIBODIES ARE EFFICIENTLY FORMED AND SECRETED FROM DROSOPHILA VIA A BIP-MEDIATED PATHWAY

We have constructed a stable Drosophila cell line coexpressing heavy c hain (HC) and Light chain (LC) immunoglobulins of a humanized monoclon al antibody (mAb) that recognizes the F antigen of respiratory syncyti al virus Tempest, P. R, Bremmer, P., Lambert, M., Taylor, G., Furze, J . M, Carr, F. J., and Harris, W. J. (1991) Bio/Technology 9, 266-271. These cells efficiently secrete antibody with substrate binding activi ty indistinguishable from that produced from vertebrate cell lines. Si gnificantly, the Drosophila homologue of the immunoglobulin binding ch aperone protein (BiP), hsc72, was found to interact specifically with the immunoglobulin HC in an ATP-dependent fashion, similar to the BiP- HC interaction known to occur in vertebrate cells. This is, in fact, t he first substrate ever shown to interact specifically with Drosophila hsc72. Most surprisingly, expression of heavy chains in the absence o f LC led to the efficient secretion of heavy chain dimers. Moreover, t his secretion occurred in association with hsc72. This dramatically co ntrasts with what is seen in vertebrate cells where in the absence of LC, HC remains sequestered inside the cell in stable association with BiP. Our results clearly suggest that Drosophila Hip can substitute fo r its mammalian counterpart and chaperone the secretion of active IgG. However, the finding that Drosophila Hip can also uniquely chaperone heavy chain dimers indicates mechanistic differences that may relate t o the evolved need for retaining immature IgGs in vertebrates.