A core set of venom proteins is released by entomopathogenic nematodes in the genus Steinernema

Parasitic helminths release molecular effectors into their hosts and these effectors can directly damage host tissue and modulate host immunity. Excreted/secreted proteins (ESPs) are one category of parasite molecular effectors that are critical to their success within the host. However, most studies of nematode ESPs rely on in vitro stimulation or culture conditions to collect the ESPs, operating under the assumption that in vitro conditions mimic actual in vivo infection. This assumption is rarely if ever validated. Entomopathogenic nematodes (EPNs) are lethal parasites of insects that produce and release toxins into their insect hosts and are a powerful model parasite system. We compared transcriptional profiles of individual Steinernema feltiae nematodes at different time points of activation under in vitro and in vivo conditions and found that some but not all time points during in vitro parasite activation have similar transcriptional profiles with nematodes from in vivo infections. These findings highlight the importance of experimental validation of ESP collection conditions. Additionally, we found that a suite of genes in the neuropeptide pathway were downregulated as nematodes activated and infection progressed in vivo, suggesting that these genes are involved in host-seeking behavior and are less important during active infection. We then characterized the ESPs of activated S. feltiae infective juveniles (IJs) using mass spectrometry and identified 266 proteins that are released by these nematodes. In comparing these ESPs with those previously identified in activated S. carpocapsae IJs, we identified a core set of 52 proteins that are conserved and present in the ESPs of activated IJs of both species. These core venom proteins include both tissue-damaging and immune-modulating proteins, suggesting that the ESPs of these parasites include both a core set of effectors as well as a specialized set, more adapted to the particular hosts they infect.
Author summary In this study we found a core set of 52 venom proteins conserved between two insect-parasitic nematodes Steinernema feltiae and Steinernema carpocapsae, that are released when initially exposed to host tissue. Most of these proteins are conserved in mammalian-parasitic nematodes suggesting that this core set of proteins is important for parasitic nematodes in general. We show that the relevance of in vitro model systems to in vivo model systems needs to be optimized and experimentally measured. Using an in vitro model of parasitic nematode activation, we stimulated protein release from S. feltiae and evaluated its activity in vivo. This activation model was previously developed using S. carpocapsae and we conclude that this method is robust and can be generalized to other EPNs (entomopathogenic nematodes). We found notable characteristics of S. feltiae venom including time-dependent decreases in protein amount and toxicity after exposure to host tissue, which differs from what has been previously reported for other EPNs, illustrating diversity in parasitic strategies among EPNs. Additionally, naïve S. feltiae infective juveniles (IJs) not exposed to host tissue release considerable amounts of protein. These proteins however are not toxic and differ in composition from those of activated IJs.