A number of studies have suggested that resistance of target cells to natural killing (NK) may be correlated with their level of expression of major histocompatibiity complex (MHC) class I antigens. To examine this hypothesis directly, a NK-sensitive class I-deficient human B-cell line was transfected with MHC class I genes. The expression of transfected HLA, but not H-2, class I gene products resulted in loss of susceptibility to human NKmediated conjugation and cytolysis. Furthermore, this protection did not extend to cytotoxicity mediated by interleukin 2-stimulated human NK effector cells.Natural killer (NK) cells are defined operationally by their potent endogenous and major histocompatibility complex (MHC)-unrestricted cytolytic activity against selected target cells (1). As yet, the recognition elements involved in NKmediated cytolysis remain poorly defined. At the target cell level, elucidation of the NK ligand(s) has been complicated by recently discovered cell adhesion molecules such as the LFA-1 family of ligands (I-CAM and others; ref.2) and LFA-3 (2). Monoclonal antibodies directed against these molecules are capable of partially inhibiting NK-mediated conjugation and cytolysis (2). These accessory molecules do not appear to represent NK ligands themselves but rather appear to strengthen effector-target cell conjugates after the initial cognate interaction (2).Further difficulty in determining the NK ligand(s) may reside in the potential masking effects of target cell MHC class I antigens. While target cell MHC class I antigens may not act as restricting elements for NK cells (in contrast to cytotoxic T lymphocytes; CTLs), recent reports suggest an inverse correlation between the level of class I antigen expression by target cells and their sensitivity to NK cytotoxicity in vitro (3-6) and in vivo (6, 7). This is consistent with a putative role for NK cells in vivo; i.e., to monitor and regulate low class I-expressing tumor cells and metastases (1,7,8) that are refractory to CTL lysis (6,7).To examine this phenomenon in a controlled and quantitative fashion, we have transfected human and murine MHC class I genes into an HLA-A,-B null B-cell line that is sensitive to human NK-mediated conjugation and cytolysis. We report that target cell surface expression of the exogenous HLA, but not H-2, class I antigens is sufficient to impart resistance to human NK and that this resistance varies quantitatively with the level of HLA class I antigens expressed by the target cell. MATERIAL AND METHODSCells and Media. All cell lines were cultured in Iscove's modified Dulbecco's medium (IDMEM) supplemented with 10% heat-inactivated fetal bovine serum and 2 mM glutamine (all reagents from GIBCO) at 370C in 5% C02/95% air. The B-lymphoblastoid cell line (B-LCL) CiR was derived from the HMy.2 B-LCL (9) by y-irradiation and selected for class I loss by antibody and complement as described (10). CiR expresses no HLA-A or -B locus gene products. The CEM.NKR T-jCL is an immunoselected NK-resistant variant of the CE...

Structural characteristics of major histocompatibiity complex class I antigens associated with natural killer