Rejection of Lepeophtheirus salmonis driven in part by chitin sensing is not impacted by seawater acclimitization in Coho salmon (Oncorhynchus kisutch)

Cell death induced by Lepeophtheirus salmonis labial gland protein 3 in salmonid fish leukocytes: A mechanism for disabling host immune responses

The salmon louse (Lepeophtheirus salmonis) is an ectoparasite feeding on mucus, skin, and blood of salmonids. On parasitised fish erosions and, at later lice stages, ulcerations appear at the louse feeding site. In susceptible species like Atlantic salmon (Salmo salar) with a limited rejection of lice, only a mild inflammatory response with minor influx of immune cells is seen at these lesions, as the salmon louse secrete proteins that can dampen immune responses. In a previous study, Lepeophtheirus salmonis labial gland protein 3 (LsLGP3) was suggested to dampen cellular responses, and the present study aimed at increasing our understanding of its mode of action. LsLGP3 was found to be secreted on to the host skin, and both in vivo and in vitro experiments were performed to elucidate its function. Histological analysis of the louse attachment site revealed an epidermal and dermal influx of mainly macrophages and granulocytes after 5 days post infestation. The immune cell influx was deeper in the dermis throughout the louse infestation, and LsLGP3 may be involved in dampening this response. Enriched populations of Atlantic salmon B-cells, T-cells, granulocytes, and monocytes were exposed to recombinant LsLGP3 (recLGP3) in vitro, resulting in a significant decrease in cell viability compared to non-exposed controls. An apoptotic cell morphology with "beads-on-a-string" like protrusions was seen in all leukocyte cell fractions after recLGP3 exposure, but not in erythrocytes or keratocytes. A decreased viability was also detected in pink salmon leucocytes, which was not in leucocytes from non-salmonid species. These functional insights suggest that LsLGP3 specifically induces apoptosis of salmonid leukocytes and is likely a key protein secreted by the lice that disables the Atlantic salmon ability to mount an adequate immune response towards the salmon louse. In vivo LsLGP3 knock down studies indicated that the effect is localised primarily at the lice feeding site, without affecting immune cells that are not situated adjacent to the lice-inflicted lesion. The findings from this study could significantly aid in the development of new immune based anti-salmon louse prophylactic measures and treatments.

Keratinocytes drive the epithelial hyperplasia key to sea lice resistance in coho salmon

BACKGROUND

Salmonid species have followed markedly divergent evolutionary trajectories in their interactions with sea lice. While sea lice parasitism poses significant economic, environmental, and animal welfare challenges for Atlantic salmon (Salmo salar) aquaculture, coho salmon (Oncorhynchus kisutch) exhibit near-complete resistance to sea lice, achieved through a potent epithelial hyperplasia response leading to rapid louse detachment. The molecular mechanisms underlying these divergent responses to sea lice are unknown.

RESULTS

We characterized the cellular and molecular responses of Atlantic salmon and coho salmon to sea lice using single-nuclei RNA sequencing. Juvenile fish were exposed to copepodid sea lice (Lepeophtheirus salmonis), and lice-attached pelvic fin and skin samples were collected 12 h, 24 h, 36 h, 48 h, and 60 h after exposure, along with control samples. Comparative analysis of control and treatment samples revealed an immune and wound-healing response that was common to both species, but attenuated in Atlantic salmon, potentially reflecting greater sea louse immunomodulation. Our results revealed unique but complementary roles of three layers of keratinocytes in the epithelial hyperplasia response leading to rapid sea lice rejection in coho salmon. Our results suggest that basal keratinocytes direct the expansion and mobility of intermediate and, especially, superficial keratinocytes, which eventually encapsulate the parasite.

CONCLUSIONS

Our results highlight the key role of keratinocytes in coho salmon's sea lice resistance and the diverged biological response of the two salmonid host species when interacting with this parasite. This study has identified key pathways and candidate genes that could be manipulated using various biotechnological solutions to improve Atlantic salmon sea lice resistance.

Salmon louse labial gland enzymes: implications for host settlement and immune modulation

Salmon louse (Lepeophtheirus salmonis) is a skin- and blood-feeding ectoparasite, infesting salmonids. While feeding, labial gland proteins from the salmon louse may be deposited on the Atlantic salmon (Salmo salar) skin. Previously characterized labial gland proteins are involved in anti-coagulation and may contribute to inhibiting Atlantic salmon from mounting a sufficient immune response against the ectoparasite. As labial gland proteins seem to be important in the host-parasite interaction, we have, therefore, identified and characterized ten enzymes localized to the labial gland. They are a large group of astacins named L. salmonis labial gland astacin 1-8 (LsLGA 1-8), one serine protease named L. salmonis labial gland serine protease 1 (LsLGSP1), and one apyrase named L. salmonis labial gland apyrase 1 (LsLGAp1). Protein domain predictions showed that LsLGA proteins all have N-terminal ShK domains, which may bind to potassium channels targeting the astacins to its substrate. LsLGA1 and -4 are, in addition, expressed in another gland type, whose secrete also meets the host-parasite interface. This suggests that LsLGA proteins may have an anti-microbial function and may prevent secondary infections in the wounds. LsLGAp1 is predicted to hydrolyze ATP or AMP and is, thereby, suggested to have an immune dampening function. In a knockdown study targeting LsLGSP1, a significant increase in IL-8 and MMP13 at the skin infestation site was seen under LsLGSP1 knockdown salmon louse compared to the control, suggesting that LsLGSP1 may have an anti-inflammatory effect. Moreover, most of the identified labial gland proteins are expressed in mature copepodids prior to host settlement, are not regulated by starvation, and are expressed at similar or higher levels in lice infesting the salmon louse-resistant pink salmon (Oncorhynchus gorbuscha). This study, thereby, emphasizes the importance of labial gland proteins for host settlement and their immune dampening function. This work can further contribute to anti-salmon louse treatment such as vaccine development, functional feed, or gene-edited salmon louse-resistant Atlantic salmon.