Evolutionary conserved pathway of the innate immune response after a viral insult in Paracentrotus lividus sea urchin

Despite the apparent simplicity of the body organization of echinoderms, their im- mune system is competent to perform a complex innate immune response, which is far from being well understood. The echinoderms represent the most advanced in- vertebrates that form a bridge with the primitive chordates. In fact, they possess numerous receptors and effectors that are used to obtain a fast immune response. After an infection, the humoral and cellular immune response determines a network in which the main protagonists are membrane and endosomal receptors. The recog- nition of nonself molecules by specific membrane receptors triggers the immune re- sponse, stimulating consecutive intracellular events. We have previously shown how the polyinosinic-polycytidylic acid (polyI:C) that mimics a viral infection is able to in- duce an immune response in the sea urchin Paracentrotus lividusimmune cells. It acti- vates a specific membrane receptor belonging to the Toll-like receptor (TLR) family. Here, we show the activated expression pattern of some genes involved in the down- stream cascade of TLR signalling pathway, such as Pl-Tbkand Pl-Irf, whose partial se- quence was isolated from P. lividusimmune cells. Their mRNA expression increases consequentially to the polyI:C stimulation and in a temporal way. In addition, we ana- lysed the expression of Pl-NF-kB and we found that its upregulation was time-depend- ent, preceding Pl-Tbkand Pl-Irfincrease. Protein analysis showed that also some cytokine (TNF-?and IL-1?) expression increased after polyI:C insult. Therefore, the purpose of this study was to discover the molecular mechanisms of the innate de- fence strategies, similarly to vertebrates, implemented by the sea urchins in order to cope with viral infection challenge.