A review of CpGs and their relevance to aquaculture.

CpG oligodeoxynucleotides (ODN) have been described as functioning as natural adjuvants because they promote professional antigen presenting cell (APC) function and co-stimulate lymphocytes. The majority of studies into the immune effects of CpG ODN to date have been carried out on mammals where they are proving very successful at stimulating innate and adaptive immune responses in a variety of species as well as protecting them from bacterial, viral and protozoan pathogens. Fish also possess the ability to raise both innate and adaptive immune responses to invading pathogens and interest in the effect of CpG ODN on the piscine immune system is growing. Various studies have now been carried out to elicit the effects of CpG ODN on diverse fish species showing that 31 different B-class CpG ODN exert various immune responses both in vivo and in vitro in salmonids, cyprinids and pleuronectiformes. These responses include activation of macrophages, proliferation of leucocytes and stimulation of cytokine expression. CpG ODN have also been shown to be protective against bacterial and viral challenge as well as against pathogenic amoebae. As would be expected these effects are all dependent on not only the ODN sequence and length but on the concentration and the species in which it is being used. This review provides the first comprehensive overview of all CpG ODN tested in fish to date and brings together all the work carried out in this field.