Development of a sandwich immunoassay for the detection of soluble bovine interleukin-2 receptor-alpha (sIL-2R-alpha) and its use to measure cell-mediated immunity in cattle.

Stimulation of T-lymphocytes with mitogens or antigens results in the production of the cytokine interleukin-2, which exerts its physiological effect by interacting with a specific IL-2 receptor on the cell surface. The alpha-chain of this receptor is induced and expressed on the cell surface after lymphocyte activation. Following continuous antigen stimulation, a smaller soluble form of this alpha-subunit (sIL-2R-alpha) is shed from the membrane of activated cells. This study describes a sandwich ELISA for bovine sIL-2R-alpha that was developed using monoclonal antibodies specific for bovine IL-2R-alpha (CD 25). The feasibility of using sIL-2R-alpha released by activated T-lymphocytes as an in vitro marker of cell-mediated immunity (CMI) in cattle is demonstrated. Calves were immunized with the foreign protein keyhole limpet haemocyanin (KLH) and the development of CMI was followed using sIL-2R-alpha release, IFN-gamma production and lymphocyte proliferation assay. The results showed that the release of sIL-2R-alpha by previously sensitized cells following stimulation with antigen is likely to be a useful marker of CMI in infectious diseases, and in the study of T cell antigens and/or novel vaccines. Using appropriate detection systems, the measurement of sIL-2R-alpha may also prove to be a useful marker of CMI in other species.