Effects of the aromatase inhibitor Letrozole on serum immunoglobulin and lysozyme levels in immunized rainbow trout (Oncorhynchus mykiss Walbaum) females

Letrozole is a synthetic aromatase inhibitor and interfere in the committed step in the synthesis of endogenous estrogens from androgens. Also estrogens regulate the immune system in teleost. Changes of 17- β- esrtradiol (E₂), serum immunoglobulin and lysozyme levels were measured using a method based on the ability of lysozyme to lyse the bacterium Micrococcus lysodeikticus, enzyme-linked immunosorbent assay (ELISA) and ELISA respectively. Twelve broodstocks were injected weekly with 2.5 mg kg^-1 letrozole (an endocrine disrupter component) two months before spawning season and vaccinated intraperitoneally (i.p) with a bacterin (inactivated L. garviae) one month before spawning. Twelve broodstocks for vaccination and twelve female rainbow trout as control group were also immiunised (i.p) with the bacterin and injected (i.p) with PBS, respectively. In the group received 2.5 mg AI kg^-1 per week, serum E₂ levels were significantly lower than that of other groups. Total immunoglobulin level and lysozyme activity were significantly higher in the parents received 2.5 mg kg^-1 per week and were immunized with 10^-9 cells ml^-1 Lactococcus garvieae  compared to the group which immunized with L. garvieae and the control (non- immunized). The present study, suggests that aromatase inhibitors such as letrozole may be a potential tool to regulate the synthesis of E₂, is involved in the hormone- immune system interaction in rainbow trout.