Ontogeny of myeloperoxidase (MPO) positive cells in flounder (Paralichthys olivaceus).

Neutrophils represent an important asset of innate immunity. Neutrophils express myeloperoxidase (MPO) which is a heme-containing peroxidase involved in microbial killing. In this study, by using real-time quantitative PCR and Western blot analysis, the flounder MPO (PoMPO) was observed to be highly expressed in the head kidney, followed by spleen, gill, and intestine during ontogeny - during developmental stages from larvae to adults. Furthermore, PoMPO positive cells were present in major immune organs of flounder at all developmental stages, and the number of neutrophils was generally higher as the fish grew to a juvenile stage. In addition, flow cytometry analysis revealed that the proportion of PoMPO positive cells relative to leukocytes, in the peritoneal cavity, head kidney, and peripheral blood of flounder juvenile stage was 18.3 %, 34.8 %, and 6.0 %, respectively, which is similar to the adult stage in flounder as previously reported. The presence and tissue distribution of PoMPO during ontogeny suggests that PoMPO positive cells are indeed a player of the innate immunity at all developmental stages of flounder.