SummaryPlant-based transient expression is potentially the most rapid and cost-efficient system for the production of recombinant pharmaceutical proteins, but safety concerns associated with plant-specific N -glycosylation have hampered its adoption as a commercial production system. In this article, we describe an approach based on the simultaneous transient co-expression of an antibody, a suppressor of silencing and a chimaeric human β 1,4-galactosyltransferase targeted for optimal activity to the early secretory pathway in agroinfiltrated Nicotiana benthamiana leaves. This strategy allows fast and high-yield production of antibodies with human-like N -glycans and, more generally, provides solutions to many critical problems posed by the large-scale production of therapeutic and vaccinal proteins, specifically yield, volume and quality.

Summary Compared with other plant expression systems used for pharmaceutical protein production, alfalfa offers the advantage of very homogeneous N‐glycosylation. Therefore, this plant was selected for further attempts at glycoengineering. Two main approaches were developed in order to humanize N‐glycosylation in alfalfa. The first was a knock‐down of two plant‐specific N‐glycan maturation enzymes, β1,2‐xylosyltransferase and α1,3‐fucosyltransferases, using sense, antisense and RNA interference strategies. In a second approach, with the ultimate goal of rebuilding the whole human sialylation pathway, human β1,4‐galactosyltransferase was expressed in alfalfa in a native form or in fusion with a targeting domain from N‐acetylglucosaminyltransferase I, a glycosyltransferase located in the early Golgi apparatus in Nicotiana tabacum. Both knock‐down and knock‐in strategies strongly, but not completely, inhibited the biosynthesis of α1,3‐fucose‐ and β1,2‐xylose‐containing glycoepitopes in transgenic alfalfa. However, recombinant human β1,4‐galactosyltransferase activity in transgenic alfalfa completely prevented the accumulation of the Lewis a glycoepitope on complex N‐glycans.