Immune profiling of rainbow trout (Oncorhynchus mykiss) exposed to Lactococcus garvieae: Evidence in asymptomatic versus symptomatic or vaccinated fish

Lactococcosis, caused by the Gram-positive bacterium Lactococcus garvieae, is a major concern in rainbow trout (Oncorhynchus mykiss) farms, which are regularly affected by outbreaks especially during the summer/fall months. In these farms, unvaccinated healthy and symptomatic fish can coexist with vaccinated fish. In the present study, innate (leukogram, serum lysozyme activity, peroxidase activity, antiprotease activity, bactericidal activity, total IgM and total proteins), and specific immune parameters (serum antibodies to L. garvieae) were assessed in unvaccinated adult rainbow trout naturally exposed to the pathogen, with or without evidence of clinical signs, or subjected to vaccination. Blood was drawn from all three groups, and blood smears were prepared. Bacteria were found in the blood smears of 70% of the symptomatic fish but not in any of the asymptomatic fish. Symptomatic fish showed lower blood lymphocytes and higher thrombocytes than asymptomatic fish (p ≤.05). Serum lysozyme and bactericidal activity did not vary substantially among groups; however, serum antiprotease and peroxidase activity were significantly lower in the unvaccinated symptomatic group than in the unvaccinated and vaccinated asymptomatic groups (p ≤.05). Serum total proteins and total immunoglobulin (IgM) levels in vaccinated asymptomatic rainbow trout were significantly higher than in unvaccinated asymptomatic and symptomatic groups (p ≤.05). Similarly, vaccinated asymptomatic fish produced more specific IgM against L. garvieae than unvaccinated asymptomatic and symptomatic fish (p ≤.05). This preliminary study provides basic knowledge on the immunological relationship occurring between the rainbow trout and L. garvieae, potentially predicting health outcomes. The approach we proposed could facilitate infield diagnostics, and several non-specific immunological markers could serve as reliable indicators of the trout's innate ability to fight infection

In field study on immune-genes expression during a lactococcosis outbreak in rainbow trout (Oncorhynchus mykiss)

Italian rainbow trout (Oncorhynchus mykiss) farms are periodically affected by outbreaks of lactococcosis caused by Lactococcus garvieae, and rearing basins reveal simultaneously asymptomatic and symptomatic fish. The present study, as part of an “in field” sanitary survey, was designed to describe the expression of immune related genes in infected versus healthy fish, as well as to explore a Near Infrared Spectroscopy (NIR) as a novel approach to discriminate between them. Measurements/sampling were performed in a farm of Northern Italy and included two groups of fish (150–300 g), sharing the same basin: asymptomatic (n = 10) and symptomatic (n = 9). Fingerling (n = 5, 15 g) were also sampled from the farm internal hatchery and considered as L. garvieae-free reference group. NIR spectra were collected from the abdominal region of the fish through the SCiO molecular device. Spleen and head kidney were sampled for bacterial detection (PCR). Head kidney was also subjected to gene expression analysis (qPCR). Seven out of nine (7/9) symptomatic trout were L.garvieae positive (bacterial DNA) while all the asymptomatic (10/10) and fingerling (2/2) were L. garvieae negative. SCiO scanning provided two different spectral populations, almost coinciding with the two fish groups considered (asymptomatic and symptomatic). Among cytokines, IL-1β showed significant up-regulation in symptomatic group compared to asymptomatic group (P < 0.05) while IL-8 showed very high (P < 0.01) and high significant (P < 0.05) differences in its up-regulation in symptomatic group compared to asymptomatic group and fingerling, respectively. IL-10 showed a tendency of significant over-expressed in symptomatic fish compared to fingerling (P < 0.1). TCR-β showed significant (P < 0.05) down-regulation in symptomatic fish compared to asymptomatic ones and MHC-II resulted in down-regulation in a very significant (P < 0.01) manner in symptomatic trout compared to fingerling and asymptomatic. Concerning immunoglobulin genes expression, the level of mRNA transcript of IgT was significantly higher in symptomatic trout compared to fingerling (P < 0.05) and tended to be higher in asymptomatic trout compared to fingerling (P < 0.1). No significant differences were observed for IL-6, TNF-α, TLR5, MHC-I and IgM. In this study, a NIR based approach was used for the first time in the field of fish pathology/immunology with the aim of differentiating between healthy and diseased fish. Moreover, our results on immune related genes modulation highlighted how the outcome of a L. garvieae infection might be influenced by a specific gene expression pattern, which is crucial in the mode of action of trout immune system against L. garvieae