CD8αβ Has Two Distinct Binding Modes of Interaction with Peptide-Major Histocompatibility Complex Class I

Abstract Interaction of CD8 (CD8αα or CD8αβ) 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 CD8αα·pMHCI complex revealed that two symmetric CD8α subunits interact with pMHCI asymmetrically, with one subunit engaged in more extensive interaction than the other, the details of the interaction between the CD8αβ heterodimer and pMHCI remained unknown. The Ig-like domains of mouse CD8αβ and CD8αα 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 CD8α variants CD8αR8A and CD8αE27A, which were functionally inactive as homodimers, could form an active co-receptor with wild-type (WT) CD8β as a CD8αR8Aβ or CD8αE27Aβ heterodimer. We also identified CD8β variants that could form active receptors with WT CD8α but not with CD8αR8A. This observation is consistent with the notion that the CD8β subunit may replace either CD8α subunit in CD8αα·pMHCI complex. In addition, we showed that both anti-CD8α and anti-CD8β antibodies were unable to completely block the co-receptor activity of WT CD8αβ. We propose that CD8αβ binds to pMHCI in at least two distinguishable orientations.