Expression and functional study of the mAbG9 against the spore wall protein of Nosema bombycis in BmE cells

Microsporidia are fungal-like unicellular eukaryotes which can develop as obligate intracellular parasites and infect a variety of hosts from protozoa to mammals. Recent reports suggest that the spore wall proteins of microsporidia Nosema bombycis may be involved in adherence to host cells and play an important role in spores infection and proliferation in the host cells. In this study, we obtained a monoclonal antibody (mAbG9) with a high affinity and specificity to SWP30, a spore wall protein of N. bombycis and isolated the full-length cDNAs for the heavy and light chains of mAbG9. Expression plasmids pBac [A3-EGFP + IE1-H-chain + IE1-L-chain + A3-Neo] and pBac [A3-EGFP + IE1-scFV + A3-Neo], which encode full-length heavy/light chains and a single-chain antibody comprising variable regions of mAbG9 heavy and light chains (scFvG9), respectively, were constructed to express monoclonal antibodies in BmE-SWU1 cells. Stable cell lines BmE-G9 and BmE-scFvG9 were obtained after G418 selection. RT-PCR and Western blot analyses showed that stable BmE-G9 and BmE-scFvG9 cell lines could express and secrete antibodies, and the expressed antibodies could specifically recognize a protein band in N. bombycis total proteins, which was also recognized by G9 monoclonal antibody. When N. bombycis spores were added to the cell culture media to infect BmE-SWU1, BmE-G9 and BmE-scFvG9 cells, the number of control cells (BmE-SWU1) decreased since many cells were broken. At 14 day post-infection (DPI), the numbers of BmE-G9 and BmE-scFv cells were 2.83 and 2.66 times more than that of the control cells, while the numbers of N. bombycis spores in the BmE-G9 and BmE-scFv cells lines gradually decreased. At 14 DPI,  N. bombycis infection rates in BmE-SWU1, BmE-G9 and BmE-scFv cells were 45%, 6% and 4%, respectively. Our results suggest that monoclonal antibodies expressed in cells may interact with N. bombycis spore wall proteins and blockbinding of N. bombycis spores to cells, resulting in inhibition of spore proliferation and infection in host cells. This study provides a novel way to further study the role of spore wall proteins of N. bombycis.