Expression in mammalian cells using BacMam viruses

The baculovirus expression vector system has been commonly employed as a rapid and inexpensive approach for recombinant protein production in insect cells (1, 2). Despite the wide applications, baculovirus infection results in insect cell death and lysis in a few days post-infection, which can lead to suboptimal protein processing late in the infection phase. In addition, some proteins that are synthesized initially as large inactive precursor proteins, such as peptide hormones, are not efficiently processed in insect cells (3). Furthermore, glycosylation in insect cells differs in many aspects from that in mammalian cells. In particular, insect cells generally fail to sialylate recombinant N-glycoproteins due to the absence of sialyltransferase activities and CMP-sialic acids (4, 5). The resultant glycoproteins, lacking sialic acids, may thus have altered immunogenicity and extremely short half-lives in vivo. In the mid-1990s, Hofmann et al. (6) and Boyce & Bucher (7) first reported that recombinant baculoviruses harboring a mammalian expression cassette could efficiently transduce mammalian cells. Their data indicated a strong preference of baculovirus to enter hepatocytes of different origins. Later, Shoji et al. (8) showed that cells that are not transduced by a baculovirus expressing β-galactosidase under the control of a cytomegalovirus (CMV) promoter can be transduced efficiently by a baculovirus expressing the same reporter protein under the control of the stronger CAG promoter, comprising the β-actin promoter and CMV immediate-early enhancer. These pioneering studies lead to the discovery of a growing list of permissive cells originating from human, rodent, porcine, bovine, fish (for reviews, see 3 and 9) and even chicken (10). Due to the high efficiency of baculovirus-mediated gene delivery to various nondividing and primary cells, increasing efforts have been directed towards employing baculoviruses for in vitro and in vivo gene delivery. In recent years, baculoviruses have emerged as a new expression vector in mammalian cells, know as the BacMam system, thanks to the high gene delivery efficiency, its nonreplicative nature and low cytotoxicity inside mammalian cells, its large genome (~130 kb) capable of harboring multiple genes or large inserts,