**4.3 Strategies to improve baculovirus transduction**

#### **4.3.1. Surface display via gp64 fusion or expression of heterologous protein**

Heterologous peptides can be inserted between the signal peptide and the mature domain of the envelope fusion protein GP64, and this feature has been exploited for surface display of peptides to improve the virus transduction [103, 104], for ligand‐directed targeting if an appropriate ligand is chosen [105, 106]. When a short peptide motif from gp350/220 of Epstein– Barr virus (EBV, which naturally infects B cells) was displayed as GP64 fusion peptide on the baculovirus envelope [107], the efficiency of transduction to B lymphocytes was increased. Another paradigm is the display of the immunoglobulin Fc region on the baculovirus surface [108]. Fc receptors (FcRs) are membrane proteins that bind to the Fc region of antibody and mediate the phagocytosis and antigen presentation. The Fc display allows for specific bacu‐ lovirus targeting to cell lines and antigen presenting cells (APCs) expressing FcRs, hence augmenting the vaccine effect. The display system also allows for the surface presentation of functional membrane proteins to simplify subsequent isolation.

Aside from the gp64‐aided display, expression of vesicular stomatitis virus G protein (VSVG) [109], influenza virus neuraminidase [110], *Spodoptera exigua* multiple nucleopolyhedrovirus F protein, single chain antibody fragments and human endogenous retrovirus envelope protein [111] in insect cells also leads to incorporation of the protein into baculovirus envelope. Among these strategies, display of VSVG or heterologous peptide/protein via the VSVG anchor is the most widely adopted and can tremendously enhance baculovirus transduction in vitro and in vivo.

Serum complement proteins (e.g. C5b‐9) inactivate baculovirus, hence constituting a major hurdle in the in vivo use of baculovirus. The inactivation problem has been circumvented by the use of complement inhibitors [112] or by displaying human DAF (decay accelerating factor) via gp64 [113]. The DAF‐displaying baculovirus caused lower levels of inflammatory cytokines IL‐1β, IL‐6, and IL‐12p40 in macrophages and mitigated liver inflammation in mice when compared with the control virus. These results demonstrate that DAF display offers protection to the baculoviral vector against complement inactivation and attenuates complement‐ mediated inflammation injury.
