Forsythoside a promotes anti-bacterial immunity through endoplasmic reticulum unfolded protein response and autophagy pathway.

The unfolded protein response (UPR) of the endoplasmic reticulum (ER) is an evolutionarily conserved cellular stress pathway with a key function in innate immunity. Autophagy, a conserved self-degradative process, is pivotal for innate immunity by maintaining cellular energy homeostasis. Caenorhabditis elegans (C. elegans) is a well-established genetic research model and it has been widely used to study animal host immune responses. Forsythoside A (FA), as the main active components isolated from Forsythia suspensa exhibits well-documented anti-inflammatory, antioxidant, and antiviral activities. But the role of FA in modulating immune responses and the molecular mechanisms mediating this process remain elusive. Herein, in Caenorhabditis elegans we demonstrated that 10 μM FA augmented the host's resistance against two Gram-negative pathogens (Pseudomonas aeruginosa and Salmonella enterica) as well as the Gram-positive pathogen Listeria monocytogenes. Concurrently, FA conferred protection against infection on animals by reinforcing their tolerance to pathogenic invasion, without a significant reduction in bacterial burden. Transcriptome sequencing combined with GO enrichment analysis indicated a potential trend that 10 μM FA upregulated the expression of immune response-associated genes as well as those involved in the IRE-1-mediated endoplasmic reticulum unfolded protein response (ER UPR). Moreover, FA upregulated the expression of ER UPR genes such as abu-1, abu-7, abu-10, and pqn-74 by activating the IRE-1/XBP-1 pathway. Furthermore, autophagy was identified as a critical regulator in this process by which Forsythoside A modulated innate immunity. FA activates autophagy by upregulating the expression levels of autophagy-related genes, such as C35E7.5, C35E7.6, sepa-1 and ZK1053.4. Taken together, these findings indicate that Forsythoside A holds potential as an anti-bacterial innate immune modulatory compound for enhancing host defense against virulent pathogen infections.