Modulation of the non-specific defence of fish by structurally conserved microbial polymers

The non-specific defence (NSD) of vertebrates includes a variety of components such as lysozyme, complement, pentraxins, transferrin, protease-inhibitors, antibacterial peptides, mononuclear and polymorphonuclear phagocytes, natural killer cells and the interferon system. Many of these components have been identified in fish. Although the NSD can be down-regulated by different kinds of stress, it can also be up-regulated by various compounds of microbial or synthetic nature. Several lines of evidence suggest that the NSD has evolved towards recognition of structurally conserved microbial polymers like fungal cell wall β-glucans, bacterial LPS and peptidoglycan, bacterial DNA and viral double-stranded RNA (dsRNA). These components also have the ability to augment the resistance of vertebrates against microbial infections. The macrophage is the key regulator of most of these processes due to its production of various cytokines upon recognition of conserved microbial structures. Fungal β-glucans and peptidoglycan have been shown to enhance the resistance of fish against bacterial infections. Injection of microbial wall polymers into fish appear to result in production of acute phase proteins and activation of macrophages. Ds RNA has been shown to enhance the resistance of fish against infections with viral pathogens which can be explained by induction of type I interferons which in turn induce the production of antiviral proteins. In salmonids dsRNA has been shown to induce the production of Mx proteins which in man and mouse are known to have potent antiviral activity against certain negative-stranded RNA viruses.