We investigated the suitability of transformed rice cell lines as a system for 
the production of therapeutic recombinant antibodies. Expression constructs 
encoding a single-chain Fv fragment (scFvT84.66, specific for CEA, the 
carcinoembryonic antigen present on many human tumours) were introduced into 
rice tissue by particle bombardment. We compared antibody production levels when 
antibodies were either secreted to the apoplast or retained in the endoplasmic 
reticulum (ER) using a KDEL retention signal. Production levels were up to 14 
times higher when antibodies were retained in the ER. Additionally, we compared 
construct sencoding different leader peptides (plant codon optimised murine 
immunoglobulin heavy and light chain leader peptides from mAb24) and carrying 
alternative 5' untranslated regions (the petunia chalcone synthase gene 5' UTR 
and the tobacco mosaic virus omega sequence). We observed no significant 
differences in antibody production levels among cell lines transformed with 
these constructs. The highest level of antibody production we measured was 3.8 
micrograms g-1 callus (fresh weight). Immunological analysis of transgenic rice 
callus confirmed the presence of functional scFvT84.66. We discuss the potential 
merits of cell culture for the production of recombinant antibodies and other 
valuable macromolecules.