Triggering of natural killer cell mediated cytotoxicity by costimulatory molecules

NK Cell Triggering by the Human Costimulatory Molecules CD80 and CD86

NK cell-mediated effector functions are regulated by a delicate balance between positive and negative signals. Receptors transmitting negative signals upon engagement with target cell MHC class I molecules have been characterized in detail in recent years. In contrast, less information is available about receptor-ligand interactions involved in the transmission of positive or “triggering” signals to NK cells. Recently, it has been described that murine NK cells are triggered by the costimulatory molecules CD80, CD86, and CD40. Using NK cell lines derived from PBMC as effectors, we demonstrate that the human CD80 and CD86 gene products can function as triggering molecules for NK cell-mediated cytotoxicity. Expression of human CD80 or CD86 molecules in murine B16.F1 melanoma cells rendered these significantly more susceptible to lysis by human NK cell lines. Blocking of the transfected gene products with specific mAb reduced lysis levels to that of nontransfected control cell lines. Triggering of human NK cells by CD80 and CD86 appeared to be independent of CD28 and CTLA-4, at least as determined by the reagents used in the present study, because the expression of these molecules could not be detected on the NK cell lines by either flow cytometry or in redirected lysis assays. Thus, human NK cells may use receptors other than CD28 and CTLA-4 in their interactions with CD80 and CD86 molecules. Alternatively, interactions may involve variants of CD28 (and possibly CTLA-4) that are not recognized by certain anti-CD28 mAb.

Activating interactions in human NK cell recognition: the role of 2B4-CD48

2B4 is a cell surface glycoprotein of the immunoglobulin superfamily structurally related to CD2-like molecules. It was originally identified in the mouse as a receptor that mediates non-MHC-restricted cytotoxicity by NK cells and CD8+ T cells. Recently, 2B4 was shown to bind CD48 by molecular binding assays and surface plasmon resonance. Here, we have investigated the cell surface expression, biochemical characteristics and function of human 2B4. Our results show that 2B4 is expressed not only on NK cells and CD8+ T cells, but also on monocytes and basophils, indicating a broader role for 2B4 in leukocyte activation. In NK cells, engagement of 2B4 with a specific monoclonal antibody or with CD48 can trigger NK cell-mediated cytotoxicity. The contribution of 2B4-CD48 interaction to target cell lysis by different NK cell clones varies, probably dependent on the relative contribution of other receptor-ligand interactions. In T cells and monocytes, ligation of 2B4 does not lead to T cell or monocyte activation. Thus, it appears that the primary function of 2B4 is to modulate other receptor-ligand interactions to enhance leukocyte activation.

Host B7-1 and B7-2 Costimulatory Molecules Contribute to the Eradication of B7-1-Transfected P815 Tumor Cells Via a CD8+ T Cell-Dependent Mechanism

B7-1 (CD80)-transfected P815 tumor cells were previously shown to elicit tumor-eradicating immunity that leads to the regression of B7-1+ P815 tumors after transient growth in normal syngeneic (DBA/2) mice. Here, we show that not only the B7-1 molecule but also the B7-2 (CD86) molecule contributed to the eradication of B7-1+ P815 tumors. The B7-1 molecule that contributed to the eradication of B7-1+ P815 tumors was expressed not only on the tumor cells but also on host APCs, including MAC-1+ cells. The B7-2 molecule that contributed to the eradication of B7-1+ P815 tumors was expressed only on host APCs, such as B220+ cells, and not on the tumor cells. In spite of the fact that B7-expressing host APCs contributed to the eradication of B7-1+ P815 tumors, only CD8+ T cells without help from CD4+ T cells were important for tumor eradication. Taken together, these findings indicate that in addition to the ability of B7-1-transfected tumor cells to stimulate CD8+ T cell-mediated tumor-eradicating immunity directly, such tumor cells can also stimulate CD8+ T cell-mediated tumor-eradicating immunity indirectly as a result of cross-priming through B7-expressing host APCs.