Affinity/Avidity and co-receptors: CD8, rheostats for T cells?

J Frelinger, S Kerry, L Hensley, E Collins & R Maile.

Dept of Micro & Immuno, University of North Carolina, Chapel Hill NC 27599-7290

During the last 20 years many studies have invoked differences in T cell receptor- peptide-MHC (pMHC) affinity to explain altered biological outcomes, ranging form thymic selection to activation to anergy. In spite of this few quantitative studies have been undertaken. Our group has undertaken an analysis of the effect of measured biochemical TCR/pMHC affinity (both in vitro and on live cells) and subsequent biological responses. We have used the ability of MHC class I tetramers to directly stimulate CD8 T cells to control both the peptide presented and the interaction with CD8 on the responding cells.

SPR experiments showed that the affinity of TCR monomer for a high affinity pMHC (C9M) is about 17 mM. In contrast binding to live cells by tetramer is about 1000 fold better. For P14 T cells, blocking CD8 engagement during high affinity tetramer treatment results in a 20 fold decrease in binding. This is accompanied by the absence of early signaling events. In contrast, later events are unaffected. For the lower affinity natural ligand, the initial avidity is lower and abrogation of CD8 binding lowers it to about the same level as the blocked high avidity pMHC. Strikingly, abrogation of CD8 binding results in loss of both early and late T cell activation measures for the lower affinity ligand. Thus engagement of CD8 results in powerful biological effects that are affinity dependent.

This suggested that the amount of CD8 on the cell surface might act as a rheostat to regulate T cell activation. HY-TCR tg males produce a substantial number of transgene expressing T cells in the periphery, but they cells had significantly lower levels of CD8. Similar CD8 cells could be elicited in females by injection of HY/D^b tetramers. CD8^lo cells not only do not respond to stimulation themselves, but inhibit the proliferation of naïve CD8^hi cells in vitro. Normal female mice treated with tetramer to produce these CD8^lo cells and show prolonged acceptance of male skin grafts, consistent with a regulatory function of these cells.

Thus we have described a new type of regulatory T cell marked both phenotypicaly by low CD8 expression.