A nergy is a cellular state in which a lymphocyte is ahve but fails to display certain functional responses when optimally stimulated through both its antigen-specific receptor and any other receptors that are normally required for full activation. The term was initially used by Nossal and Pike (1) to describe an unresponsive state induced by the injection of soluble protein antigens in vivo, in which the antigen-specific B cells were still found to be present in the animal, but these cells could not be reactivated by antigen or mitogen to make Ig. The first observation of proliferative unresponsiveness induced in purified T cells using peptide antigens was made on human CD4+ clones (2). The results were initially interpreted as a direct inactivation of the T cells through recognition of free antigen; however, subsequent blocking studies with anti-Ia antibodies revealed the involvement of MHC class II molecules (expressed on the T cells; 3). Downregulation of T cell antigen receptor expression was noted after the stimulation and postulated to be the molecular mechanism for the blocking of reactivation (4). Studies with mouse CD4 § T cell clones uncovered other ways of inducing an unresponsive state, which at first appeared to be similar to the nonprohferating state seen with human T cell clones (5, 6). Presentation of peptide antigens either on chemically fixed APCs (5) or in planar lipid membranes containing only MHC class II molecules (6) was successful, as was stimulation of highly purified T cells with either concanavalin A (7) or anti-CD3 antibodies coated on a plastic surface (8). These results suggested that occupancy of the T cell antigen receptor alone, in the absence of other signals, was responsible for inducing the unresponsive state. Proof for this came from so called" allogeneic add-back" experiments in which live APCs bearing allogeneic MHC class II molecules were used to reconstitute the ability to stimulate a prohferative response and prevent the induction of unresponsiveness, even though the allogeneic cells themselves could not present the antigen to the T cell clone (9). The allogeneic APCs were postulated to be delivering a costimulatory signal (s) needed for both effects.

When similar allogeneic add-back experiments were carried out with purified human T cell clones inactivated by exposure to high concentrations of soluble peptides, addition of either allogeneic or syngeneic APC failed to prevent the induction of the unresponsiveness (10). This puzzling observation was further compounded when other laboratories were able to set up mixed leukocyte responses