Natural killer (NK) cells are believed to play an important role in a variety of disease pathologies, including transplant rejection and autoimmunity. None of the therapeutic modalities currently available are known to potently interfere with NK cell activity. Here we demonstrate for the first time that low molecular weight inhibitors of the integrin lymphocyte function-associated antigen-1 (LFA-1) readily block NK cell adhesion, activation, and NK cell-mediated cytolysis in vitro, in contrast to other immunosuppressive agents. These effects were independent of the type of allosteric mechanism by which LFA-1 inhibition was achieved. In addition, we describe a simple, nonradioactive whole-blood assay that should be suitable to monitor NK cell activation in clinical practice. Taken together, our study underlines the importance of LFA-1 in NK cell effector functions and indicates that allosteric LFA-1 inhibitors may become important tools to further elucidate the therapeutic potential of NK cell modulation in immunological diseases.

Standard adhesion assays measure cell binding either to immobilized ligands or to cell monolayers in flat-well microtiter plates under static conditions. Typically, these test systems require several washing steps to separate adherent from nonadherent cells. Here, we describe an adhesion assay which avoids these washing steps by employing V-bottom 96-well plates. In this assay, fluorescently labeled leukocytes are allowed to adhere to V-well plates coated with soluble ligand for a fixed time. Thereafter, centrifugal force is applied to separate adherent cells from nonadherent cells. Nonadherent cells accumulate in the nadir of the V-shaped wells and are quantified using a fluorometer with a narrow aperture. This simple and reproducible method has been validated with different classes of adhesion molecule families (selectins and integrins) and is adaptable to several other adhesive interactions. The assay format is suitable for screening applications and may also be used for diagnostic testing. The receptor/ligand interaction chosen as an example to describe the assay methodology is the interaction between the integrin lymphocyte function-associated molecule-1 (LFA-1, α(L)β(2)) and intercellular adhesion molecule-1 (ICAM-1).