One of the main problems in the worldwide production of marine fish is the high mortality rate during larviculture (Rojo-Cebreros et al., 2018, Austin, 2006, Bergh, 2000). Disease susceptibility may be due to an immature immune defense system at early stages. It has been shown the first feeding represents a milestone of organ development in gadoids, flatfishes and sparids, while a continuous differentiation during the larval stage and metamorphosis occurs (Falk-Petersen, 2005). For many fish species, the immune system is fully operational at the stage of first feeding (Martin and Król, 2017, Yúfera and Darias, 2007).

It is acknowledged that main lymphoid organs of the teleost are the head kidney, thymus, spleen, gill- and mucous-associated lymphoid tissues (GIALTs and MALTs) (Patel et al., 2009, Dalum et al., 2021) – both in larvae and developmental stages later on (Campoverde et al., 2019, Liang et al., 2022). During the early life stages, larvae have to rely solely on innate immune mechanisms and possibly maternal antibodies until they develop a functional adaptive immune system, which may take a few weeks to months (Saurabh and Sahoo, 2008). The cellular innate immunity is partly executed by phagocytic cells such as neutrophils and macrophages (Gasteiger et al., 2016, Hirayama et al., 2017, Lee et al., 2022). They are involved in antibacterial defenses, both by phagocytosis and by subsequent intracellular killing, as well as the release of extracellular traps (ETs) (Ellis, 1999, Malech et al., 2020, Sollberger et al., 2018). Myeloperoxidase (MPO), a highly cationic, heme-containing, and glycosylated enzyme is a major protein in azurophilic granules of neutrophils and is one of the key molecules in immune response (Odobasic et al., 2016). In the phagosomes, MPO is involved in sustaining an alkaline milieu, which is optimal for the activity of serine proteases and other granule components to inactivate and kill microbes (Arnhold, 2020). In the presence of hydrogen peroxide and halides, MPO catalyzes the formation of reactive oxygen intermediates, which facilitates microbial killing by neutrophils (Aratani, 2018). MPO is also a key component in ETs capable of inhibiting the proliferation of bacteria, such as Staphylococcus aureus (Morales-Primo et al., 2022). In teleost, MPO possess antimicrobial activity similar to what found in mammals, where it could inhibit bacterial proliferation as constituent in ETs (Chi and Sun, 2016, Zhao et al., 2017, Gan et al., 2023).

The MPO gene has previously been cloned and the tissue distribution in some teleost has been described (Gan et al., 2023). However, MPO expression and content during flounder Paralichthys olivaceus (PoMPO) ontogeny have not yet been studied. Here, we studied the expression of PoMPO mRNA and protein at various stages during fish development and compared the fraction of MPO positive cells relative to other leukocytes from larvae to juvenile and adult stages. This study provided knowledge on early innate defense mechanisms in flounder, assessed by the occurrence of PoMPO - which is mainly expressed by neutrophils.