Abstract

Differential expression of transforming growth factor (TGF)-ß by fish splenic mononuclear cells exposed to Mycobacterium marinum was investigated. Data published recently indicated reduced susceptibility to M. marinum infection in hybrid tilapia when compared to striped bass. Histomorphologic changes were also different for each of these species, as well as macrophage population types. M. marinum classically causes granulomatous lesions, containing a large number of macrophages. TGF-ß is an important immunoregulatory cytokine, and altered expression of TGF-ß has been associated with different disease states and varied responses to intracellular pathogens in animals. Relative TGF-ß expression in the two species of fish was therefore likely to be dissimilar. In this study, we experimentally infected striped bass and hybrid tilapia with a single intramuscular injection of M. marinum. Additional fish from each species were injected intramuscularly with phosphate buffered saline-tween (PBS-T) as controls. Splenic mononuclear cells were harvested at 9 days post-injection from each group of fish and mRNA analyzed by reverse transcription quantitative-competitive polymerase chain reaction (RT-qc-PCR) in order to determine TGF-ß production. TGF-ß expression was significantly (P = 0.0002) decreased in infected striped bass, compared to the control striped bass. There was no significant difference in TGF-ß expression between infected hybrid tilapia and the PBS-T injected control hybrid tilapia. Based on these results, it is possible that TGF-ß expression by macrophages from striped bass and tilapia exposed to M. marinum may play a role in the immune mechanisms that cause increased susceptibility in striped bass to infection by M. marinum.