A comparative study on innate immune parameters in the epidermal mucus of various fish species

Fish epidermal mucus and its components provide the first line of defense against pathogens. Little is known about the role of epidermal mucus enzymes in the innate immune system of fish species such as Arctic char (Salvelinus alpinus), brook trout (S. fontinalis), koi carp(Cyprinus carpio), striped bass (Morone saxatilis), haddock, (Melanogrammus aeglefinus), Atlantic cod (Gadus morhua) and hagfish (Myxine glutinosa). The epidermal mucus samples from these fish were analysed for the specific activities of various hydrolytic enzymes including lysozyme, alkaline phosphatase, cathepsin B and proteases and the enzyme levels were compared among the fish species. Of all the species hagfish mucus showed a high activity for lysozyme and proteases and koi carp mucus had the highest levels of alkaline phosphatase and cathepsin B. A wide variation in enzyme activities was observed among the seven species and also between species of same family such as Arctic char and brook trout (salmonidae), haddock and cod (gadidae). Only lysozyme levels showed a marked variation with salinity where seawater fish showed approximately two times higher lysozyme activity than freshwater-reared fish species. Characterization of proteases with specific inhibitors showed Arctic char, brook trout, haddock and cod having higher levels of serine over metalloproteases whereas koi carp and striped bass had higher levels of metalloproteases over serine proteases. In contrast, hagfish had almost equal proportion of both serine and metalloproteases. This study demonstrates variation in the level of hydrolytic enzymes in the epidermal mucus of fish. These results provide preliminary information for a better understanding of the role of epidermal mucus and its components in the fish innate immune system.