CD8αβ has two distinct binding modes of interaction with peptide-major histocompatibility complex class I

Interaction of CD8 (CD8 alpha alpha or CD8 alpha beta) with the peptide-major histocompatibility complex (MHC) class I (pMHCI) is critical for the development and function of cytolytic T cells. Although the crystal structure of CD8 alpha alpha(.)pMHCI complex revealed that two symmetric CD8 alpha subunits interact with pMHCI asymmetrically, with one subunit engaged in more extensive interaction than the other, the details of the interaction between the CD8 alpha beta heterodimer and pMHCI remained unknown. The Ig-like domains of mouse CD8 alpha beta and CD8 alpha alpha are similar in the size, shape, and surface electrostatic potential of their pMHCI-binding regions, suggesting that their interactions with pMHCI could be very similar. Indeed, we found that the CD8 alpha variants CD8 alpha(R8A) and CD8 alpha(E27A), which were functionally inactive as homodimers, could form an active co-receptor with wildtype (WT) CD8 beta as a CD8 alpha(R8A)beta or CD8 alpha(E27A)beta heterodimer. We also identified CD8 beta variants that could form active receptors with WT CD8 beta but not with CD8 alpha(R8A). This observation is consistent with the notion that the CD8 beta subunit may replace either CD8 beta subunit in CD8 alpha alpha(.) pMHCI complex. In addition, we showed that both anti-CD8 alpha and anti-CD8 beta antibodies were unable to completely block the co-receptor activity of WT CD8 alpha beta. We propose that CD8 alpha beta binds to pMHCI in at least two distinguishable orientations.