Identification and Characterization of Two Salmon Louse ( Lepeophtheirus salmonis, Krøyer, 1838) Heme Peroxidases and Their Potential As Vaccine Antigens

Abstract

Salmon louse, Lepeophtheirus salmonis, represents a major challenge for salmon farming as current treatments impose welfare issues and are costly, and no prophylactic measures are available. Two salmon louse heme peroxidases (LsPxtl-1 and LsPxtl-2) were tested for their importance in parasite development in vitro and as potential vaccine candidates. LsPxtl-1 possesses two heme peroxidase domains and is expressed in ovaries and gut. LsPxtl-1 knockdown in nauplius II stage resulted in poor swimming performance and parasite death when reaching the copepodid stage, indicating LsPxtl-1 being indispensable for parasite development. LsPxtl-2 encodes one peroxidase domain and was predicted to contain an N-terminal signal peptide and an Eph receptor ligand binding domain. LsPxtl-2 knockdown did not impact survival or phenotype. Immunizations included a single DNA plasmid i.m. injection encoding the peroxidases or a heterologous prime (DNA) and boost (recombinant LsPxtl-2 protein), gave non-significant reduction in lice numbers post challenge. Single injection gave low specific antibody levels while prime-boost vaccination induced higher antibody response and reduced lice numbers post challenge but not significantly different from controls. The findings suggest LsPxtl-1 to play a role for parasite development but formulations used, and vaccination modalities employed, did not give significant reduction in lice infestation.