Methyl-Beta-Cyclodextrin-Induced Macropinocytosis Results in Increased Infection of Sf21 Cells by Bombyx Mori Nucleopolyhedrovirus

Bombyx mori nucleopolyhedrovirus (BmNPV) is closely related to Autographa californica multiple nucleopolyhedrovirus (AcMNPV) with over ~93% amino acid sequence identity. However, their host ranges are essentially nonoverlapping. The mechanism of BmNPV entry into host cells is completely different from that of AcMNPV, and whether the entry mechanism difference relates to the host range remains unclear. BmNPV produces an abortive infection in nonhost cells due to virion nuclear transportation failure. Here, we performed a detailed study by increasing BmNPV infection in Sf21 cells with the aid of methyl-beta-cyclodextrin (MβCD). We found that low-concentration MβCD incubation efficiently activates membrane ruffling in Sf21 cells, which mediates the increase in BmNPV infection. Interestingly, MβCD incubation after virion internalization also increases the infection, which suggests that macropinocytosis is involved in BmNPV infection in Sf21 cells after virion internalization. Further study revealed that clathrin-mediated endocytosis (CME) is employed by BmNPV to facilitate entry into Sf21 cells, and chlorpromazine application abolishes BmNPV infection in cells incubated both with and without MβCD. Based on these studies, we show that BmNPV enters Sf21 cells via CME and that parallel induction of macropinocytosis facilitates BmNPV infection in Sf21 cells. This study reveals the mechanism of BmNPV entry into Sf21 cells and provides clues for improving BmNPV infections in nonpermissive cells.