期刊
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
卷 113, 期 20, 页码 5682-5687出版社
NATL ACAD SCIENCES
DOI: 10.1073/pnas.1513918113
关键词
protein interactions; TCR phosphorylation; adhesion; T-cell activation; binding equilibrium; kinetics
资金
- Wellcome Trust
- UK Medical Research Council
- Swedish Research Council [623-2014-6387, 621-2014-3907]
- Sir Henry Dale Fellowship - Wellcome Trust [098363]
- Sir Henry Dale Fellowship - Royal Society [098363]
- Medical Research Council [MC_UU_12010/4, MR/N023668/1, G0400720, G0801508] Funding Source: researchfish
- MRC [G0801508, G0400720, MC_UU_12010/4, MR/N023668/1] Funding Source: UKRI
The alpha beta 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 un-quantifiable. 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 K-d of similar to 5,000 molecules/mu m(2). This value is two to three orders of magnitude higher than previously measured 2D K-d 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.
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