Poly(I:C) induces anti-inflammatory response against secondary LPS challenge in zebrafish larvae.

Poly(I:C) is known as an agonist of the TLR3 receptor which could prime inflammation and elicit the host immune response, which is widely applied as adjuvant or antivirus treatment. However, the negative effects of poly(I:C) on regulating immune response to protect the host from inflammatory diseases remain largely unknown. Here, we establish an in vivo model to pre-treat zebrafish larvae with poly(I:C) at 2 dpf, then challenge them with LPS at 6 dpf, and find that poly(I:C) training could significantly alleviate the LPS challenge-induced septic shock and inflammatory phenotypes. Moreover, the poly(I:C)-trained larvae exhibit decreased number of macrophages, but not neutrophils, after secondary LPS challenge. Furthermore, training the larvae with poly(I:C) could elevate the transcripts of mTOR signaling and heighten the H3K4me3-mediated epigenetic modifications. And interestingly, we find that inhibiting the H3K4me3 modification, rather than mTOR signaling, could recover the number of macrophages in poly(I:C)-trained larvae, which is consistent with the observations of inflammatory phenotypes. Taken together, these results suggest that poly(I:C) training could induce epigenetic rewiring to mediate the anti-inflammatory response against secondary LPS challenge-induced septic shock through decreasing macrophages' number in vivo, which might expand our understanding of poly(I:C) in regulating fish immune response.