Coreceptor affinity for MHC defines peptide specificity requirements for TCR interaction with coagonist peptide-MHC

Area of Science:

• Immunology
• Molecular Biology
• Computational Biology

Background:

• Nonstimulatory endogenous peptides can enhance T cell recognition of antigen.
• MHCI and MHCII-restricted systems show differing results regarding peptide specificity for coagonists.
• Previous studies suggested CD8 binding to noncognate MHCI is crucial for coagonism in MHCI systems.

Purpose of the Study:

• To resolve the dichotomy in coagonism results between MHCI and MHCII systems.
• To elucidate the mechanisms by which CD8 influences peptide specificity in coagonism.
• To investigate the roles of CD8 and TCR binding affinities in T cell activation.

Main Methods:

• Experimental variation of CD8 and TCR binding to agonist and coagonist peptides.
• Utilized computer simulations to model T cell recognition and coagonism.
• Analyzed the relationship between CD8 affinity, TCR binding, and coagonist peptide requirements.

Main Results:

• Identified two distinct mechanisms of CD8 influence on coagonism peptide specificity.
• Mechanism 1: CD8 binding to noncognate ligand directly correlates coagonism magnitude with CD8 affinity for coagonist pMHCI.
• Mechanism 2: CD8 affinity for agonist pMHCI alters the requirement for specific coagonist peptides, with stronger CD8 binding allowing broader peptide tolerance.

Conclusions:

• CD8 binding to MHCI plays a critical role in determining coagonist peptide specificity.
• Findings explain peptide-specific coagonism in MHCII-restricted cells due to generally weaker CD4-MHCII interactions compared to CD8-MHCI.
• This study resolves the apparent differences between MHCI and MHCII systems in T cell coagonism.