Identification and immunolocalization of two isoforms of ATP-diphosphohydrolase (ATPDase) in the pig immune system

The occurrence of a variety of purine receptors in the immune system indicates that extracellular purines play important functional roles. Extracellular purine concentrations are, in great part, determined by ectonucleotidases, namely, the ATP diphosphohydrolase, also identified as CD39, a lymphocyte cell surface marker. The latter enzyme converts triphospho- and diphosphonucleosides to nucleoside monophosphates. In this study, high levels of ATPase and ADPase activities have been found in homogenates of the different pig lymphoid organs. Specific activities decreased in the following order: spleen > bone marrow > thymus > lymph glands. The parallel decrease in ATPase and ADPase activities, in the presence of sodium azide, indicated that an ATP diphosphohydrolase (ATPDase) was responsible for these activities. Particulate fractions, prepared from the different lymphoid organs by ultracentrifugation on a sucrose cushion, showed about a 10-fold enrichment of ATPDase activity. Identity of ATPDase was confirmed by electrophoretograms of the particulate fractions and Western immunoblots, with an antibody that recognizes ATPDases from different sources. Two isoforms of ATPDase were found (I and II), corresponding to molecular masses of 78,000 and 54,000, respectively, as estimated by SDS-PAGE. Immunohistochemical localization was carried out on these different organs: In spleen, reaction was found in both white and red pulps. A particularly intense reaction was put in evidence in nervous fibers of this organ. Immunolocalization also showed positive reactions with tonsilar lymphoid structures, diffuse lymphoid tissues, and nodules associated with stomach, duodenum, jejunum, and ileum. In addition, our observations establish the presence of ATPDase in lymphocytes and macrophages of the pig immune system.