Death by neglect as a deletional mechanism of peripheral tolerance

In contrast to most organs, the anatomy of the liver may allow naive CD8(+) T cells to make direct contact with liver parenchymal cells. We have previ ously shown, using a combination of TCR transgenic T cells specific for H-2 K-b and hepatocytes expressing a transgenic H-2 K-b molecule, that hepatoc ytes can induce antigen-specific activation and proliferation of naive CD8( +) T cells independently of CD28 co-stimulation, However, T cell activation by hepatocytes leads to premature T cell death and tolerance, both in vivo and in vitro. In this study, we investigated the mechanisms of T cell deat h induced by hepatocytes in vitro using primary hepatocytes to activate pur ified CD8(+) T cells, Neither Fas nor tumor necrosis factor receptor were i nvolved, indicating that hepatocyte-induced death was distinct from activat ion-induced cell death. Before they started to divide, T cells activated by hepatocytes expressed lower levels of the bcl-x(L) survival gene and 30 ti mes less IL-2 mRNA than CD8(+) cells activated by splenic antigen-presentin g cells, Since CD28 co-stimulation increases both IL-2 and bcl-x(L) express ion, this suggests that hepatocyte-activated T cells die by neglect because they fail to receive effective co-stimulatory signals. In agreement with t his model, premature death promoted by hepatocytes could be prevented by cr oss-linking CD28, Survival after CD28 crosslinking correlated with increase d IL-2 and bcl-x(L) expression, and sustained T cell proliferation, while c ytotoxic T lymphocyte activity was prolonged as compared with cells stimula ted without CD28 co-stimulation. This study confirms that high-affinity TCR transgenic antigen-specific CD8(+) T cells can be activated to proliferate and differentiate into cytotoxic effector cells. However, prolonged T cell survival and cytotoxicity required CD28 co-stimulation as well. To our kno wledge, this is the first report suggesting that tolerance in the context o f lack of CD28 co-stimulation can result from Fas-independent peripheral de letion rather than from anergy.