Green macroalgae ( Ulva sp.) and red macroalgae ( Solieria sp.) were processed to produce eight different crude extracts enriched for marine sulphated polysaccharides (MSP). While all extracts showed direct anti-bacterial effects to some degree, two red algal extracts, in particular, showed high activity against several pathogenic fish bacteria in vitro. . Stimulation of head kidney leukocytes (HKLs) in vitro with MSP-rich extracts showed fish species specific differences. In Nile tilapia, HKLs showed a dose dependent production of reactive oxygen species (ROS) following stimulation with Ulva- derived extracts, while stimulation with Solieria-derived extracts did not induce ROS production. In rainbow trout, HKLs showed relatively high reactive ROS potential and Solieria-- derived extracts could induce significant ROS production, albeit without a clear dose dependent response. Gene expression of in vitro stimulated HKLs showed a clear induction of most cytokines measured ( il1b, il10, tnfa, ifny, il12p40), ), with differences between species. Although cytokine gene expressions were more prominently induced in HKLs of Nile tilapia than in HKLs of rainbow trout, immunomodulatory effects of both, Ulva- and Solieriaderived extracts, always appeared evident. It is likely that HKLs have the ability to sense presence of MSP, via so far uncharacterized pattern recognition receptors which have known downstream signalling pathways. Expression of some genes associated with Toll-like Receptor (TLR) signalling, such as irak1 and especially the expression of genes associated with C-type Lectin Receptor (CLR) signalling, such as card9 and bcl10 were evident following stimulation with MSP-rich extracts. Possibly, immunomodulatory effects of MSP could be regulated by at least a CLR-mediated signalling, at least in Nile tilapia, for which most if not all red algae and green algae extracts induced changes in gene expression of card9 and bcl10. . Overall, induction of ROS production and gene expression read-outs suggest immunomodulatory effects of MSP-rich extracts derived from green algae ( Ulva sp.) and red algae ( Solieria sp.), at least in vitro. . The observed effects suggest clear fish species specific differences between the effects of MSP-rich extracts.
