Natural antibodies (NAbs) are immunoglobulins found in animals without prior antigenic experience. NAbs are believed to provide an immediate protection against a wide variety of pathogens. In teleost fish, innate and adaptive immune responses are species-specific; characteristics of innate immune system in European sea bass (Dicentrarchus labrax L.) have never been studied. Moreover, hypothesizing the use of innate immunity traits in indirect selective breeding approaches, we investigated genetic variation of NAbs and their relationships with: i) antibody titer against nodavirus antigens (NNV-AT), ii) mortality caused by nodavirus (MORT), and iii) body weight at a fixed age (BW).
The experimental population was produced by artificial fertilization using a commercial breeding stock. 650 randomly chosen fish (age: 548 d post-hatching; average BW (CV): 146.4 g (0.29)) were individually PIT-tagged, blood sampled and then subjected to a 29 days-NNV challenge test. MORT was recorded as 1 (dead) or 0 (survived). The survival rate at the end of the challenge test was 52.2%. Indirect ELISA assay for NAbs and NNV-AT was performed on the blood serum of each fish. All the experimental fish and their parents were genotyped to reconstruct a high-likelihood pedigree for each tested individual. Genetic parameters, heritability, genetic and phenotypic correlations between traits were estimated using Bayesian procedures and bivariate animal models (gibbsf90+/BLUPf90+ software suite).
Mean (CV) of the sample-to-positive ratio of the optical density values 450 nm of NAbs titer was 15.86 (1.50). The frequency distribution was skewed and it was normalized through Box-Cox transformation of the original data (mean (CV) of the transformed data: 2.29 (0.60)). No significant differences between means of NAbs titer of dead and survivor animals (2.28 (0.61) vs. 2.30 (0.60)) were detected ( posterior difference = 0.02, loge (bayes factor) = 2.34). Heritability, genetic and phenotypic correlations as medians of the marginal posterior density distributions are reported in Table 1. The results evidenced the presence of a genetic basis for NAbs. Genetic correlations between NAbs and both NNV-AT and BW were significantly positive. The high negative genetic correlation between NAbs and MORT suggests the possibility to develop indirect selective strategies to enhance VNN disease resistance using NAbs measures.