Lectocyte secrete novel leukolectins in ovo for first-line innate immunity defence.

Atlantic salmon hatching fluid (HF) contains numerous polypeptides. A component unidentified by proteomics, was cloned from tryptic peptides and characterized as lectin-like (LL-) proteins in the tectonin-family. Purified salmon LL-proteins elicited high-titre, LL-specific polyclonal antibodies. This study aims to delineate the cellular and genetic basis of fish embryonic LL-expression. LL-proteins were detected in salmon, cod, rainbow trout and zebrafish HFs. LL-immunoreactive cells were numerous in salmon and rainbow trout embryos, but fewer in zebrafish, cod and halibut. Peridermal salmon LL-positive cells (lectocytes) corresponded to non-eosinophilic cells stained by PAS-reagent. Northern blots revealed two transcripts in salmon and zebrafish embryos, and LL-transcripts were detected specifically in lectocytes. Dual in situ hybridization distinguished lectocytes from hatching glands. BAC-library screening yielded salmon Leukolectins gene-structure with 4 introns, 5 exons, TATA-box, multiple upstream putative transcription-factor binding-sites, and polyadenylation site. Sequence-analysis indicated zebrafish LLs conserved nt-sequences and gene-structure, which exhibited mature and truncated LL-transcripts. Zebrafish LL-expression was detected at 6 hpf (yolk syncytium) and 19 hpf (lectocytes and PVF). In dermal mucus, Leukolectins with TECPR-domains may function as pathogen-recognition receptors in first-line innate immunity defence. SUMMARY STATEMENTAt hatching, embryos lose maternal chorions, their first-line innate immuno-protection. Novel leukolectin-genes specifically expressed in non-eosinophilic peridermal cells (lectocytes) help explain how embryos develop innate immuno-competency to survive as larvae.