The activation of gilthead seabream professional phagocytes by different PAMPs underlines the behavioural diversity of the main innate immune cells of bony fish.

Phagocytic cells form the cellular arm of the innate immune system. A primary role of these cells is an ability to discriminate large number of potential pathogens from self, using a restricted number of receptors. In the gilthead seabream, acidophilic granulocytes and macrophages have been described as the professional phagocytes of this species. However, no direct functional comparisons between these two phagocytic lineages exist for the seabream or for other teleost species. Therefore, purified fractions of acidophilic granulocytes and macrophages were used to characterize the ability of these cells to recognize and respond to different pathogen-associated molecular patterns (PAMPs) and the data obtained were then correlated with the expression of several pattern-recognition receptors (PRRs). The time course of the respiratory burst of acidophilic granulocytes stimulated with different PAMPs showed that muramyldipeptide (MDP), the ligand for NOD2 in mammals, induced maximal activation earlier than several ligands for toll-like receptors (TLRs), including bacterial DNA, flagellin and lipopolysaccharide (LPS). In addition, all these PAMPs strongly increased the phagocytic and bactericidal activities of acidophilic granulocytes, while other PAMPs, including poly I:C, Pam3CSK(4) and zymosan, failed to do so. The stimulation of acidophilic granulocytes and macrophages by PAMPs also resulted in the up-regulation of interleukin-1beta (IL-1beta), tumor necrosis factor alpha (TNFalpha), cyclooxygenase-2 (COX-2) and TLRs, although the kinetics and expression profiles observed for each cell type differed. These results suggest different roles for professional phagocytes of fish in the recognition and elimination of pathogens and in the regulation of adaptive immune responses.