Expression of phosphatidylinositol-dependent phospholipase C sensitive Qa-2 antigen is increased on peripheral blood lymphocytes of aging mice

Qa-2 antigen, a nonclassical MHC antigen, is one of a group of cell surface antigens that may be anchored to the cell surface by a glycophosphatidylinositol (GPI) linkage rather than, as is the case with the classical MHC antigens, K, D, and L, by a transmembrane linkage. Recent studies have shown that T cells may be activated through the cross-linking of GPI anchored Qa-2 antigens by anti-Qa-2 antiserum. The Qa-2 antigens involved in signal transduction in activated T cells are sensitive to lipolytic cleavage by phosphatidylinositol-dependent-phospholipase C (PI-PLC). Since T cell function is known to decline with age, a study was undertaken to determine whether the relative amount of lipase sensitive Qa-2 antigen changes with age. Ficoll- Hypaque purified peripheral blood lymphocytes were obtained from C57BL/6 and A strain mice from 6-30 months of age. The cells were incubated with or without phosphatidylinositol- dependent-phospholipase C followed by incubation with anti-Qa-2 monoclonal antibodies and a fluorescein labeled second antibody. Analysis on a FACScan flow cytometer demonstrated that the A strain mice had a single population of Qa-2 positive lymphocytes which increased in lipase sensitivity with age. The C57BL/6 strain mice of all ages had two populations of Qa-2 positive cells, one staining with high fluorescence intensity (high antigen density) and the other with low fluorescence intensity (low antigen density). The proportion of cells found in the high density peak increased markedly with age, suggesting on overall increase in Qa-2 expression. In young animals, only the high density peak was sensitive to PI-PLC treatment. In older animals, the cells originally appearing in the high density population shifted to the low density population after treatment with PI-PLC, suggesting that there must be both lipase sensitive and lipase insensitive forms of Qa-2 present on lymphocytes in these aging C57BL/6 mice. Overall, these data suggest that the proportion of lymphocytes expressing lipase sensitive Qa-2 is increased on lymphocytes of old mice. Since a role for lipase sensitive GPI-linked Qa-2-antigen has recently been postulated in T cell activation, it is possible that the increased lipase sensitive Qa-2 antigen may play a role in age-related changes in T cell function.