Abstract: Infective juveniles (IJs) of Steinernema ceratophorum strain D43 are all ensheathed in a transparent sheath. In order to investigate the effect of sheaths on infectivity of S. ceratophorum D43 and determine the roles of sheath proteins (SPs) in immune suppression of Galleria mellonella larvae, we first obtained the desheathed IJs with chemical exsheathment method. And then, the infection rate of desheathed and ensheathed IJs to G. mellonella larvae was compared according to the host mortality and number of penetration sites on the host. Moreover, the SPs from IJs were extracted with ethanol and characterized by two dimensional electrophoresis (2-DE) and MALDI-TOF-MS. Finally, the influence of SPs on hemocyte number and phenoloxidase (PO) activity of G. mellonella larvae was evaluated. The results indicated that over 95% ensheathed nematodes lost their sheaths after being exposed to 0.5% sodium hypochlorite at room temperature for 20 min and kept alive. Desheathed nematodes caused lower mortality of G. mellonella larvae, longer lethal time and less penetration sites compared to ensheathed nematodes. The SPs extracted in cold 35% ethanol showed six protein spots on 2-DE, and only one was successfully identified as a serine protease by peptide mass fingerprinting. The hemocyte number and PO activity in G. mellonella larvae injected with the extracted SPs were significantly reduced and suppressed, respectively. These results suggest that sheaths have an important role in pathogenicity of S. ceratophorum D43, and the SPs are implicated in the suppression of host immune responses.

Key words: Entomopathogenic nematode, Steinernema ceratophorum, Galleria mellonella, sheath proteins (SPs), immune responses