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

¹ Department of Immunology, the Weizmann Institute of Science, P.O.B. 26, 76100 Rehovot, Israel
² Ludwig Institute for Cancer Research, Lausanne Branch, University of Lausanne, Epalinges,Switzerland
³ Department of Medical Biophysics, MBB, Karolinska Institute, S - 17177 Stockholm, Sweden
⁴ Department of Materials and Interfaces, Weizmann Institute of Science, P.O.B. 26, 76100 Rehovot, Isr
⁵ Department SIDA-Retrovirus, Institut Pasteur, 25 rue du Doctor Roux, 75724 Paris, Cedex 15, France
⁶ Laboratory of Physical Chemistry of Polymers and Membranes, Department of Chemistry, Swiss Federal I

^* To whom correspondence should be addressed. E-mail: lidima{at}wisemail.weizmann.ac.il.

Submitted on February 23, 2005
Revised on March 27, 2005
Accepted on 19 May 2005

	Abstract

The mechanism of CD8 cooperation with the TCR in antigen recognition was studied on live T-cells. Fluorescence correlation measurements yielded evidence for the presence of two TCR and CD8 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)x105 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)x103 M-1·s-1), and close to that of the association of soluble pMHC with CD8 ((1 - 2)x105 M-1·s-1). The fast binding phase disappeared when CD8-pMHC interaction was blocked by a CD8-specific mAb. The latter rate constant was slowed down ~10-fold following cells treatment with methyl-b-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 rafts: The reaction starts by pMHC association with the CD8. 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 non-colocalized 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.

Key Words: Antigen recognition, CD8, Fluorescence correlation spectroscopy (FCS), Lateral diffusion, Major histocompatibility complex (MHC), T-cell receptor (TCR)

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