Recent advances on the complement system of teleost fish

The complement system plays an essential role in alerting the host of the presence of potential pathogens, as well as in their clearing. In addition, activation of the complement system contributes significantly in the orchestration and development of an acquired immune response. Although the complement system has been studied extensively in mammals, considerably less is known about complement in lower vertebrates, in particular teleost fish. Here we review our current understanding of the role of fish complement in phagocytosis, respiratory burst, chemotaxis and cell lysis. We also thoroughly review the specific complement components characterized thus far in various teleost fish species. In addition, we provide a comprehensive compilation on complement host-pathogen interactions, in which we analyze the role of fish complement in host defense against bacteria, viruses, fungi and parasites. From a more physiological perspective, we evaluate the knowledge accumulated on the influence of stress, nutrition and environmental factors on levels of complement activity and components, and how the use of this knowledge can benefit the aquaculture industry. Finally, we propose future directions that are likely to advance our understanding of the molecular evolution, structure and function of complement proteins in teleosts. Such studies will be pivotal in providing new insights into complement-related mechanisms of recognition and defense that are essential to maintaining fish homeostasis.