Interaction of streptavidin-based peptide-MHC oligomers (tetramers) with cell-surface TCRs

The binding of oligomeric peptide-MHC (pMHC) complexes to cell surface TCR can be considered to approximate TCR-pMHCinteractions at cell-cell interfaces. In this study, we analyzed the equilibrium binding of streptavidin-based pMHC oligomers(tetramers) and their dissociation kinetics from CD8 ^pos T cells from 2C-TCR transgenic mice and from T cell hybridomas thatexpressed the 2C TCR or a high-affinity mutant (m33) of this TCR. Our results show that the tetramers did not come close tosaturating cell-surface TCR (binding only 10-30% of cell-surface receptors), as is generally assumed in deriving affinity values(K _D), in part because of dissociative losses from tetramer-stained cells. Guided by a kinetic model, the oligomer dissociation rateand equilibrium constants were seen to depend not only on monovalent association and dissociation rates (k _off and k _on), but also ona multivalent association rate (μ) and TCR cell-surface density. Our results suggest that dissociation rates could account for therecently described surprisingly high frequency of tetramer-negative, functionally competent T cells in some T cell responses.