Classical and nonclassical class I major histocompatibility complex molecules exhibit subtle conformational...

Area of Science:

• Immunology
• Molecular Biology
• Structural Biology

Background:

• CD4 and CD8 are critical co-receptors enhancing T-cell antigen recognition by binding Major Histocompatibility Complex (MHC) molecules.
• This interaction is vital for the T-cell receptor (TCR) to effectively recognize peptide-MHC complexes.

Purpose of the Study:

• To investigate the binding affinity of the CD8alphaalpha co-receptor to various class I MHC molecules.
• To understand how variations in MHC alleles and CD8 structure influence binding interactions.

Main Methods:

• Surface Plasmon Resonance (SPR) was employed to quantify the binding kinetics between CD8alphaalpha and different MHC class I molecules.
• Site-directed mutagenesis was used to identify key amino acid residues in CD8 responsible for binding affinity variations.

Main Results:

• CD8alphaalpha exhibited moderate affinities (90-220 microm) for classical MHC class I alleles (e.g., HLA-A*0201, -B*3501), distinct from TCR affinities.
• Significantly lower affinities (>/=1 mm) were observed for certain alleles (HLA-A*6801, -B*4801), correlating with a lack of cell adhesion.
• Normal binding to nonclassical HLA-G (approx. 150 microm) and weak binding to HLA-E (>/=1 mm) were noted.
• Amino acid variations within the CD8 alpha3 domain, particularly in the solvent-exposed loop (residues 223-229), were found to explain differential binding.

Conclusions:

• The affinity of CD8alphaalpha binding to classical MHC class I molecules is generally lower than TCR/peptide-MHC interactions and may be optimized for T-cell function.
• Conformational flexibility within the CD8 alpha3 domain loop is a key determinant of differential binding to diverse MHC class I molecules, including classical and nonclassical types.