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Robust transcriptional responses are critical for defense against infection. However, unrestrained immune responses can cause negative impacts such as damaging inflammation and slowed development. Here, we find that a class of transcriptional regulators previously associated with regulation of development inCaenorhabditis elegans, is also involved in repressing immune responses. Specifically, through forward genetics, we find that loss oflin-15Bleads to constitutive expression of Intracellular Pathogen Response (IPR) genes.lin-15Bencodes a transcriptional repressor with a conserved THAP domain that is associated with the DRM chromatin remodeling complex that regulatesC. elegansdevelopment. We show thatlin-15Bmutants have increased resistance to natural intracellular pathogens, and the induction of IPR genes inlin-15Bmutants relies on the MES-4 histone methyltransferase. We extend our analyses to other DRM and NuRD chromatin remodeling factors, as well as SUMOylation histone modifiers, showing that a broad range of chromatin-related factors can repress IPR gene expression. Altogether these findings suggest that conserved chromatin regulators may facilitate development in part by repressing damaging immune responses against intracellular pathogens. 

The immune system continually battles against pathogen-induced pressures, which often leads to the evolutionary expansion of immune gene families in a species-specific manner. For example, thepalsgene family expanded to 39 members in theCaenorhabditis elegansgenome, in comparison to a single mammalianpalsortholog. Our previous studies have revealed that two members of this family,pals-22andpals-25, act as antagonistic paralogs to control the Intracellular Pathogen Response (IPR). The IPR is a protective transcriptional response, which is activated upon infection by two molecularly distinct natural intracellular pathogens ofC.elegans–the Orsay virus and the fungusNematocida parisiifrom the microsporidia phylum. In this study, we identify a previously uncharacterized member of thepalsfamily,pals-17, as a newly described negative regulator of the IPR.pals-17mutants show constitutive upregulation of IPR gene expression, increased immunity against intracellular pathogens, as well as impaired development and reproduction. We also find that two other previously uncharacterizedpalsgenes,pals-20andpals-16, are positive regulators of the IPR, acting downstream ofpals-17. These positive regulators reverse the effects caused by the loss ofpals-17on IPR gene expression, immunity, and development. We show that the negative IPR regulator protein PALS-17 and the positive IPR regulator protein PALS-20 colocalize inside and at the apical side of intestinal epithelial cells, which are the sites of infection for IPR-inducing pathogens. In summary, our study demonstrates that severalpalsgenes from the expandedpalsgene family act as ON/OFF switch modules to regulate a balance between organismal development and immunity against natural intracellular pathogens inC.elegans.