Trained immunity in vertebrates involves epigenetic reprogra

Trained immunity in vertebrates involves epigenetic reprogramming through histone post-translational modifications, particularly by histone methyltransferases, that enhance the expression of antimicrobial genes during re-infection (Netea et al., 2016; Pereira et al., 2016). Interestingly, most studies in this field have been performed in vitro with differentiated immune hiv protease such as monocytes, macrophages or natural killer cells (Netea et al., 2016; Pereira et al., 2016). However, these cell types are already trained for immune function. Investigating the details of innate immune memory in undifferentiated cell types, such as stem cell-like neoblasts, is more challenging. Planarians are a classic model system for the study of adult wound healing and tissue regeneration (Reddien, 2013; Elliott and Sanchez Alvarado, 2013). These free-living members of the phylum Platyhelminthes contain a persistent pool of adult pluripotent stem cells, termed neoblasts, that are capable of producing all cell types and regenerating all types of tissues (Wagner et al., 2011). The ablation of neoblasts via irradiation or specific RNA-based depletion of essential gene products compromises the regenerative capacity of these animals (Reddien et al., 2005b). Planarians represent a remarkable system with an unmatched capacity to fight infectious agents, including S. aureus, indicating the presence of remarkably efficient but uncharacterized innate immunity (Abnave et al., 2014). Notably, several new components of the innate immune system that are conserved in humans and absent from Ecdysozoa (e.g., flies and nematodes) were discovered by studying this model organism (Abnave et al., 2014). The varied abilities of planarians underscore the value of studying microbial defences in this model organism to identify components that are common between innate immunity and regeneration processes.
Material and Methods
Results
Discussion Here, we report that planarians initiate a genetic program of instructed immunity during S. aureus infection that allows a sensitized expression of anti-microbial responses upon re-infection to clear the pathogen more efficiently. Mechanistically, we defined the critical role of neoblasts and the expression of the Smed-PGRP-2 peptidoglycan receptor and the Smed-setd8-1 histone methyltransferase. These factors promote the expression of the executor genes Smed-p38 MAPK and Smed-morn2, which display a facilitated expression that is associated with enhanced bacterial clearance. Moreover, we established that Smed-PGRP-2 controls the induction of Smed-setd8-1 and the downstream increase in lysine methylation content in neoblasts. We propose that instructed neoblasts orchestrate the heightened anti-bacterial gene response to S. aureus, which comprises Smed-p38 MAP kinase and Smed-morn2. This raises the question of whether neoblasts also control anti-bacterial responses during primo-infection. In support of this hypothesis, we measured an absence of expression of Smed-p38 MAPK and Smed-morn2 in primo-infected worms that were irradiated. However, irradiated animals retained a capacity to eliminate S. aureus similar to that in control worms (not shown). Together, our findings point for neoblasts as central elements in establishing instructed immunity rather than in contributing directly to pathogen clearance. Combined with our functional analysis, these data indicate an unappreciated role of neoblasts in instructing enhanced resistance to S. aureus in planarians. In the current study, we characterized a form of instructed immunity that involves the expression of innate immune genes. We have characterized the critical function of Smed-PGRP-2 in instructed immunity. This peptidoglycan receptor shows close protein sequence homology with human PGRP-2 (PGRPLY-2), which has a more subtle function in immunity than other PGRP family members (Fournier and Philpott, 2005). In line with our findings, PGRP-2 deficiency in human macrophages has no direct impact on inflammation triggered by S. aureus, leaving possible a hidden role in recurrent staphylococcal infections (Fournier and Philpott, 2005; Xu et al., 2004). Interestingly, instructed immunity in planarians showed some specificity against S. aureus and was linked to Smed-PGRP-2 expression during primo-infection. Indeed, S. aureus did not trigger the expression of Smed-PGRP-1, -3, or -4. Moreover, we recorded an absence of Smed-PGRP-2 expression in animals infected with L. pneumophila and M. avium, two bacterial species that do not induce instructed immunity. Interestingly, we defined a hierarchy of innate immune gene expression driven by Smed-PGRP-2. Depletion of Smed-setd8-1 did not affect Smed-PGRP-2 expression, whereas we measured a 72% decrease of Smed-setd8-1 expression triggered by Smed-PGRP-2 depletion. We observed that depletion of Smed-setd8-1 had a dramatic impact on bacterial clearance, which was less efficient than during primo-infection. By contrast, neither marker had an effect on primo-infection, indicating their specific importance in instructed immunity. Smed-setd8-1 depletion did not affect neoblast populations, as indicated by the stable expression of neoblast markers (Onal et al., 2012), but abrogated the expression of Smed-p38 MAPK and Smed-morn2 executor genes, similar to the depletion of Smed-PGRP-2. Together, these data indicate that Smed-setd8-1 has a broader importance in immunity than Smed-PGRP-2. A hypothesis is that the heightened anti-bacterial responses controlled by Smed-setd8-1 signify the need to overcome a desensitization phase following primo-infection in addition to sensitizing the induction of host defences.