"Interleukin (IL)-10 is a cytokine that, at least in mammals, plays a central role in innate and adaptive immunity. IL-10 limits inflammation by acting on macrophages and promotes anti-inflammatory responses via T-helper cells. Underlining the regulatory role of IL-10 is the presence of IL-10 homologues in viral genomes (vIL-10).IL-10 has been cloned in several fish species but only recently a vIL-10 has been identified in the genome of koi-herpes-virus (KHV), a highly pathogenic virus of carp. Very little is known about the function of fish IL-10 and nothing is known about the activity of khvIL-10. However, the presence of this cytokine in both fish and their viruses, suggests a conservation of function throughout the vertebrate lineage.The first aim is to identify IL-10-producing leukocytes in carp. The second aim is to study the effect(s) of carp and KHV IL-10 on different leukocytes, in vitro. The third aim is to investigate the immunomodulatory effect(s) of carp and KHV IL-10 in vivo during a natural infection of carp. The overall aim is to investigate the biological activity of both host and viral IL-10 in fish to study the evolutionary conservation of function of this important regulatory cytokine."