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

Area of Science:

• Immunology
• Cellular Biology
• Biochemistry

Background:

• The αβ T-cell coreceptor CD4 significantly amplifies immune responses.
• The affinity of CD4 for its ligand, peptide-major histocompatibility class II (pMHC II), has been historically unquantifiable due to its extreme weakness in solution.

Purpose of the Study:

• To quantify the binding affinity of CD4 to pMHC II.
• To map the CD4 binding site on pMHC II.
• To understand how CD4 binding influences T-cell receptor (TCR) signaling.

Main Methods:

• Surface plasmon resonance assays to determine solution binding affinity.
• Multivalent presentation of CD4 on magnetic beads for binding assays with pMHC II-expressing cells.
• Supported lipid bilayer system to measure 2D binding affinity.
• Computational analysis to assess the impact on TCR phosphorylation and affinity.

Main Results:

• Soluble CD4 showed undetectable binding to pMHC II in solution, with an upper affinity limit of 2.5 mM.
• CD4 binding to pMHC II was confirmed using multivalently presented CD4 on beads.
• A 2D dissociation constant (Kd) of ~5,000 molecules/µm² was measured for the CD4/pMHC II interaction on lipid bilayers.
• Calculations showed that CD4/pMHC II binding increases TCR complex phosphorylation rates by threefold via Lck recruitment.

Conclusions:

• The CD4/pMHC II interaction affinity is optimized to enhance T-cell sensitivity through increased phosphorylation.
• This affinity does not compromise the T-cell receptor's ability to discriminate between specific and non-specific ligands.
• The study provides a quantitative understanding of CD4-pMHC II interaction crucial for T-cell activation.