Insect cells as hosts for the expression of recombinant glycoproteins

Baculovirus-mediated expression in insect cells has become well-established for the production of recombinant glycoproteins. Its frequent use arises from the relative ease and speed with which a heterologous protein can be expressed on the laboratory scale and the high chance of obtaining a biologically active protein. In addition to Spodoptera frugiperda Sf9 cells, which are probably the most widely used insect cell line, other mainly lepidopteran cell lines are exploited for protein expression. Recombinant baculovirus is the usual vector for the expression of foreign genes but stable transfection of - especially dipteran - insect cells presents an interesting alternative. Insect cells can be grown on serum free media which is an advantage in terms of costs as well as of biosafety. For large scale culture, conditions have been developed which meet the special requirements of insect cells. With regard to protein folding and post-translational processing, insect cells are second only to mammalian cell lines. Evidence is presented that many processing events known in mammalian systems do also occur in insects. In this review, emphasis is laid, however, on protein glycosylation, particularly N-glycosylation, which in insects differs in many respects from that in mammals. For instance, truncated oligosaccharides containing just three or even only two mannose residues and sometimes fucose have been found on expressed proteins. These small structures can be explained by post-synthetic trimming reactions. Indeed, cell lines having a low level of N-acetyl-beta-glucosaminidase, e.g. Estigmene acrea cells, produce N- glycans with non-reducing terminal N-acetylglucosamine residues. The Trichoplusia ni cell line TN-5B1-4 was even found to produce small amounts of galactose terminated N-glycans. However, there appears to be no significant sialylation of N-glycans in insect cells. Insect cells expressed glycoproteins may, though, be alpha1,3-fucosylated on the reducing-terminal GlcNAc residue. This type of fucosylation renders the N-glycans on one hand resistant to hydrolysis with PNGase F and on the other immunogenic. Even in the absence of alpha1,3-fucosylation, the truncated N-glycans of glycoproteins produced in insect cells constitute a barrier to their use as therapeutics. Attempts and strategies to "mammalianise" the N-glycosylation capacity of insect cells are discussed.