Transforming growth factor-β response to mycobacterial infection in striped bass Morone saxatilis and hybrid tilapia Oreochromis spp.

Striped bass (Morone saxatilis) and hybrid tilapia (Oreochromis spp.) were experimentally infected with Mycobacterium marinum. Splenic mononuclear cell transforming growth factor-β (TGF-β) mRNA was measured by reverse transcription quantitative-competitive PCR (RT-qcPCR). In histologic sections of liver and anterior kidney, the area of each section that was occupied by granulomas and the total area of each section were measured by computer-assisted image analysis and compared as a proportion (the granuloma proportion). Infected striped bass splenic mononuclear cell TGF-β mRNA expression was significantly lower than uninfected controls, while for tilapia there was no significant difference between infected and control fish. Mycobacterial granuloma proportion of liver and anterior kidney sections was significantly greater for infected striped bass than tilapia. Three (of 10) infected tilapia with the most pronounced inflammatory response displayed a decrease in TGF-β mRNA expression, similar to the overall striped bass response to mycobacterium challenge. Downregulation of TGF-β and failure to modulate the immune response may be related to excessive inflammatory damage to organs observed in mycobacteria-sensitive fish species.