Effects of the sulfated polysaccharide ulvan from Ulva ohnoi on the modulation of the immune response in Senegalese sole (Solea senegalensis)

Sulfated polysaccharides derived from green seaweeds exhibit many beneficial biological activities and have great potential to be used as nutraceutical in aquaculture. In this work, we evaluated the effects of the sulfated polysaccharide ulvan from Ulva ohnoi on Senegalese sole (Solea senegalensis) juveniles at the transcriptomic level. Cytotoxicity assay performed in liver primary cell cultures from sole determined that the different ulvan concentrations assayed did not impair cell viability. Juveniles were intraperitoneally (IP) injected with ulvan (0.5 mg/fish) followed by a challenge with Photobacterium damselae subsp. piscicida (Phdp) at 7 days. RNASeq analyses at 2 days post injection (dpi) revealed that 402 transcripts were differentially expressed in liver between ulvan IP injected and control groups before the challenge. Genes related to bacterial and antiviral defence, complement system, chemokines, proteasomes and antigen presentation were upregulated in ulvan treated groups. A detailed expression analysis of sixteen genes related to innate and adaptive immune system was performed in two systemic tissues: liver and spleen. Ulvan injection provoked the upregulation of tlr22 and a transient inflammatory response was initiated in both liver and spleen at 2 dpi. As consequence, expression of acute phase proteins, antimicrobial peptides and complement genes was induced. Moreover, expression of mhcI, mhcII, psmb10 and bcl6 was also induced 2 dpi. M 2 dpi with Phdp, inflammatory cytokines and genes related to bacterial and antiviral defense, iron metabolism, complement system and antigen presentation were differentially modulated in survival juveniles previously IP injected with ulvan. Moreover, mortality was retarded in ulvan treated juveniles. These results provide new evidence about the role of ulvan as a bioactive compound with immunomodulatory activity in Senegalese sole as well as its possible use as vaccine adjuvant against Phdp. This is the first published study that evaluates the transcriptomic response of Senegalese sole IP injected with ulvan.