Identification and gene cloning of a new phosphatidylinositol-linked antigen expressed on mature lymphocytes. Down-regulation by lymphocyte activation.

Lymphocytes are shown to express a limited number of a unique category of membrane Ag, such as Thy-1, Ly-6, Ly-31, and Qa-2, that are covalently linked to the membrane phosphatidylinositol (PI). We have identified a new glycosyl-phosphatidylinositol (GPI)-anchored lymphocyte Ag, B7, by using a mAb and have determined the primary structure by cDNA cloning. B7 Ag was expressed on the majority, if not all, of the mature lymphocytes of both T and B lineages, including strongly CD3+ thymocytes, most splenic T cells, and approximately 60% of splenic IgM+ B cells, whereas the expression of B7 Ag on bone marrow cells was negligible. The expression of B7 Ag was nearly completely abolished with as little as 2 mU of PI-specific phospholipase C per ml, which did not completely eliminate Ly-6C and Thy-1 expression. Unlike the expression of other GPI-linked lymphocyte Ag, the expression of B7 was rapidly down-regulated upon the activation of T cells by mitogens or IL-2 both in vitro and in vivo. Immunoprecipitation analysis revealed that B7 Ag was an approximately 12-kDa protein. With a CDM8 expression vector, a cDNA encoding B7 Ag was cloned, and it was confirmed that the B7 Ag on cDNA-transfected cells was indeed PI-specific phospholipase C sensitive. The B7 cDNA contained an open reading frame of 222 bp including a typical N-terminal leader sequence and a characteristic sequence at the C terminus encoding hydrophobic amino acids. A computer search revealed no significant homology to any known molecule at both DNA and amino acid sequence levels. Northern blot analysis indicated that the B7 transcript was expressed on lymphohematopoietic tissues, including thymus, spleen, and bone marrow, but not on other organs, such as liver, kidney, and brain. The results indicated that B7 Ag is a new member of the GPI-anchored proteins which is selectively expressed on mature resting but not activated lymphocytes.