Characterization of immunosuppressive surface coat proteins from Steinernema glaseri that selectively kill blood cells in susceptible hosts

Surface coat proteins (SCPs) of entomopathogenic nematodes are implicated in the suppression/evasion of host immune responses, which is required for successful host colonization. Steinernema glaseri NC strain SCPs suppressed immune responses in oriental beetle larvae (Exomala orientalis), a susceptible host for S. glaseri, in a dosage-dependent manner, thus protecting Heterorhabditis bacteriophora from being killed in the same host. Melanization of H. bacteriophora decreased from 92 ± 5% in the untreated check to 1 ± 3% when protected by injection of 230 ng of S. glaseri SCPs. As the SCPs dosage increased, freely moving H. bacteriophora increased from 3 ± 4% in the untreated group to 57 ± 15% with an SCPs dose of 940 ng. At 2 h and in the absence of SCPs, 8% and 11% of hemocytes of E. orientalis were stained by propidium iodide and Hoechst, respectively. When exposed to 300 ng/μl SCPs, 70% and 96% were stained, respectively. At 6 h, propidium iodide stained 37% and 92% of the hemocytes without and with SCPs, respectively. In contrast, more than 90% of the cells were stained by Hoechst with or without SCPs. As native proteins, two isolated S. glaseri SCPs had an immunosuppressive effect; they were each composed of 38 kDa (PI = 4.6) and 56 kDa (PI = 3.6) subunits. SCP peptides were sequenced using LC-MS/MS and the mass fingerprints obtained with MALDI-TOF-MS; there were no significant matches found in peptide databases, which suggests that the SCPs studied are novel proteins. Twelve cDNA sequences were derived based on short peptides and 7 of them had no significant match against the Caenorhabditis elegans protein database. One of the cDNA matched an unknown C. elegans protein and the remaining 4 cDNAs matched proteins of C. elegans and Brugia malayi.