Defence mechanisms against viral infection in poultry: A review

Defence against viral infections in poultry consists of innate and adaptive mechanisms. The innate defence is mainly formed by natural killer cells, g ranulocytes, and macrophages and their secreted products, such as nitric ox ide and various cytokines, The innate defence is of crucial importance earl y in viral infections. Natural killer cell activity can be routinely determ ined in chickens of 4 weeks and older using the RP9 tumour cell line. In vi tro assays to determine the phagocytosis and killing activity of granulocyt es and macrophages towards bacteria have been developed for chickens, but t hey have not been used with respect to virally infected animals. Cytokines, such Bs interleukin (IL)-1, IL-6 and tumour necrosis factor (TNF)-alpha, a re indicators of macrophage activity during viral infections, and assays to measure IL-1 and IL-6 have been applied to chicken-derived materials. The adaptive defence can be divided into humoral and cellular immunity and both take time to develop and thus are more important later on during viral infections. Various enzyme-linked immunosorbent assays (ELISAs) to measure humoral immunity specific for the viruses that most commonly infect poultr y in the field are now commercially available. These ELISAs are based on a coating of a certain virus on the plate. After incubation with chicken sera , the bound virus-specific antibodies are recognized by conjugates specific for chicken IgM and IgG, Cytotoxic T lymphocyte activity can be measured u sing a recently developed in vitro assay based on reticuloendotheliosis vir us-transformed ta;get cells that are loaded,vith viral antigens, e.g. Newca stle disease virus. This assay is still in an experimental stage, but will offer great opportunities in the near future for research into the cellular defence mechanisms during viral infections.