Investigation of proteins identified in the secretory and excretory products (SEPs) of the infectious copepodid stage of the salmon louse Lepeophtheirus salmonis

The salmon louse (Lepeophtheirus salmonis (Krøyer, 1837)) is a caligid ectoparasite of salmonids that feeds on host blood, mucus, and skin. While secreted virulence factors from later life stages have been studied, the protein composition of secretory and excretory products (SEPs) from copepodids, the initial infectious stage of L. salmonis, remains uncharacterized.

Copepodids were hatched and incubated at 10°C until 7 days post-hatch. Batches (n = 4) were then exposed to either 0.45 μm filtered seawater or 0.1 mg mL⁻¹ isophorone to stimulate SEP production. Adult males and females (n = 2 replicates) were similarly treated for comparison. SEPs were filtered, precipitated, trypsin-digested, and analyzed via LC-MS/MS. Proteins were identified using an L. salmonis database and further analyzed with SignalP and InterPro.

In total, 433 distinct proteins were detected in copepodid samples (mean 95.5 ± 146.74), and 117 in adult samples (mean 56 ± 12.70). Signal peptide analysis revealed 164 copepodid and 69 adult proteins as secretory. Among adults, 31 secretory proteins were female-specific and 10 male-specific. Twenty-one secretory proteins were shared across life stages, including 8 proteases, 2 protease inhibitors, and 2 uncharacterized proteins. Of proteins with GO annotations, 75 % were involved in proteolysis and 50 % localized extracellularly. However, secretory profiles differed markedly between life stages. Notably, 67 % of adult-specific secretory proteins were extracellular versus 30.7 % in copepodids. Copepodid and adult SEPs also contained 23 and 4 unique uncharacterized proteins, respectively.

These findings highlight a complex repertoire of copepodid SEPs potentially involved in host invasion and immunomodulation, providing new targets for therapeutic development.