Correlation of Cd2 Binding and Functional Properties of Multimeric and Monomeric Lymphocyte Function-associated Antigen 3 By

Lymphocyte function-associated antigen 3 (LFA-3)' is a widely distributed cell surface glycoprotein that is a ligand for the T lymphocyte CD2 glycoprotein (1-4). This receptor-ligand pair is used in many intercellular adhesion interactions, including binding ofcytolytic Tlymphocytes to target cells (3) and thymocytes to thymic epithelial cells (4). Both CD2 and one form ofLFA-3 have membrane-spanning polypeptide segments that anchor them to the membrane (5-7), while a distinct form of LFA-3 is anchored to the membrane by aglycosylphosphatidylinositol (GPI) moiety at its COOH terminus (7-9). Despite these hydrophobic components, purified CD2 and LFA-3 can be manipulated in the absence of detergents and can be shown to bind to cells expressing LFA-3 and CD2, respectively, in radioreceptor assays (10-12). CD2 also mediates rosetting of human T lymphocytes with sheep and human erythrocytes; the sheep ligand for CD2, T11-target structure (TUTS), is a homologue of LFA-3 and can also be used in soluble form to bind to CD2 on cells (13, 14). In addition to mediating adhesion, early studies suggested that CD2 could deliver activating signals (15, 16). Subsequently it was found that several distinct pairwise combinations of CD2 mAbs that bind to different classes of epitopes could trigger T lymphocyte proliferation and function (17-21). Hunig et al . (22) showed that sheep erythrocytes bearing T11TS were strongly mitogenic in the presence of a single CD2 mAb. Tiefenthaler et al . (23) demonstrated that purified LFA-3 or TUTS synergize with a submitogenic dose of a mitogenic CD2 mAb combination. In that study the form of LFA-3 or T11TS used was not defined and neither LFA-3 nor T11TS could stimulate T cells in combination with a single nonmitogenic CD2 mAb. We have been interested in defining the physiochemical properties of LFA-3 important in stimulation ofT cell function . There are 40,000 CD2 molecules per resting T cell (24) and 200,000 LFA-3 sites per B lymphoblast (10) . Hence, signaling attendant upon interaction via these adhesion molecules might involve engagement of ahigher number ofsurface receptors than required for signaling through many lymphokine/hormone receptors, on the order of 102 to 103 (25, 26). Furthermore, a mul-