Remarkably low affinity of CD4/peptide-major histocompatibility complex class II protein interactions

Peter Jonsson, Jennifer H. Southcombe, Ana Mafalda Santos, Jiandong Huo, Ricardo A. Fernandes, James McColl, Melissa Lever, Edward J. Evans, Alexander Hudson, Veronica T. Chang, Tomas Hanke, Andrew Godkin, Paul D. Dunne, Mathew H. Horrocks, Matthieu Palayret, Gavin R. Screaton, Jan Petersen, Jamie Rossjohn, Lars Fugger, Omer DushekXiao-Ning Xu, Simon J. Davis, David Klenerman

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28 Citations (Scopus)

Abstract

The αβ T-cell coreceptor CD4 enhances immune responses more than 1 million-fold in some assays, and yet the affinity of CD4 for its ligand, peptide-major histocompatibility class II (pMHC II) on antigen-presenting cells, is so weak that it was previously unquantifiable. Here, we report that a soluble form of CD4 failed to bind detectably to pMHC II in surface plasmon resonance-based assays, establishing a new upper limit for the solution affinity at 2.5 mM. However, when presented multivalently on magnetic beads, soluble CD4 bound pMHC II-expressing B cells, confirming that it is active and allowing mapping of the native coreceptor binding site on pMHC II. Whereas binding was undetectable in solution, the affinity of the CD4/pMHC II interaction could be measured in 2D using CD4- and adhesion molecule-functionalized, supported lipid bilayers, yielding a 2D Kd of ∼5,000 molecules/μm2. This value is two to three orders of magnitude higher than previously measured 2D Kd values for interacting leukocyte surface proteins. Calculations indicated, however, that CD4/pMHC II binding would increase rates of T-cell receptor (TCR) complex phosphorylation by threefold via the recruitment of Lck, with only a small, 2–20% increase in the effective affinity of the TCR for pMHC II. The affinity of CD4/pMHC II therefore seems to be set at a value that increases T-cell sensitivity by enhancing phosphorylation, without compromising ligand discrimination.
Original languageEnglish
Pages (from-to)5682-5687
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Volume113
Issue number20
DOIs
Publication statusPublished - 17 May 2016

Keywords

  • protein interactions
  • TCR phosphorylation
  • adhesion
  • T-cell activation
  • binding equilibrium and kinetics

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