CD8 kinetically promotes ligand binding to the T-cell antigen receptor

Abstract

The mechanism of CDS cooperation with the TCR in antigen recognition was studied on live T cells. Fluorescence correlation measurements yielded evidence of the presence of two TCR and CDS subpopulations with different lateral diffusion rate constants. Independently, evidence for two subpopulations was derived from the experimentally observed two distinct association phases of cognate peptide bound to class I MHC (pMHC) tetramers and the T cells. The fast phase rate constant ((1.7 ± 0.2) × 105 M-1 s-1) was independent of examined cell type or MHC-bound peptides' structure. Its value was much faster than that of the association of soluble pMHC and TCR ((7.0 ± 0.3) × 103 M-1 s -1), and close to that of the association of soluble pMHC with CDS ((1-2) × 105 M-1 s-1). The fast binding phase disappeared when CDS-pMHC interaction was blocked by a CD8-specific mAb. The latter rate constant was slowed down ∼10-fold after cells treatment with methyl-β-cyclodextrin. These results suggest that the most efficient pMHC-cell association route corresponds to a fast tetramer binding to a colocalized CD8-TCR subpopulation, which apparently resides within membrane rafts: the reaction starts by pMHC association with the CDS. This markedly faster step significantly increases the probability of pMHC-TCR encounters and thereby promotes pMHC association with CD8-proximal TCR. The slow binding phase is assigned to pMHC association with a noncolocalized CD8-TCR subpopulation. Taken together with results of cytotoxicity assays, our data suggest that the colocalized, raft-associated CD8-TCR subpopulation is the one capable of inducing T-cell activation. © 2005 by the Biophysical Society.