T cell receptor and coreceptor CD8 alphaalpha bind peptide-MHC independently and with distinct kinetics

Wyer JR, Willcox BE, Gao GF, Gerth UC, Davis SJ, Bell JI, van der Merwe PA, Jakobsen BK. (1999), Immunity. 10, 219-25

Abstract

The T cell surface glycoprotein CD8 enhances T cell antigen recognition by binding to MHC class I molecules. We show that human CD8 alphaalpha binds to the MHC class I molecule HLA-A2 with an extremely low affinity (K_d approximately 0.2 mM at 37 degrees C) and with kinetics that are between 2 and 3 orders of magnitude faster than reported for T cell receptor/peptide-MHC interactions. Furthermore, CD8 alphaalpha had no detectable effect on a T cell receptor (TCR) binding to the same peptide-MHC class I complex. These binding properties provide an explanation as to why the CD8/MHC class I interaction is unable to initiate cell-cell adhesion and how it can enhance TCR recognition without interfering with its specificity..

Key figure: The Affinity of CD8αα Binding to HLA-A2

(A) CD8αα was injected at the indicated concentrations for 30 s through flow cells with ∼8000 RUs of either HLA-A2-flu (solid trace) or the control protein OX68 (dotted trace) immobilized. HLA-A2-flu was coupled via biotinylated β2-microglobulin. The amount of CD8αα that bound HLA-A2-flu at each concentration was calculated as the difference between the responses at equilibrium in the HLA-A2-flu and control flow cells and is plotted in (B). (C) A similar experiment was performed at 37°C using HLA-A2-HY (∼2400 RU) coupled via biotinylated heavy chain and OX68 (∼4000 RU) as a control protein. In (B) and (C) the solid lines in the main plots represent nonlinear fits of the Langmuir binding isotherm to the data. These yielded K_d values of 156 and 205 μM, respectively. The insets show Scatchard transformations of the same data; the K_d values shown were obtained from the slopes by linear regression (K_d = −1/slope). These experiments were performed at a flow-rate of 10 μl × min⁻¹.