Comparison of the Roles of CD8αα and CD8αβ in Interaction with MHC Class I

The CD8 molecule is expressed either as an α/α homodimer or an α/β heterodimer on thymocytes and cytotoxic T cells, and functions as a coreceptor in concert with TCR for binding the MHC class I/peptide complex. Although CD8α/β heterodimers have been shown to be more effective coreceptors, the precise role of the β-chain in TCR-mediated thymic maturation and T cell activation is not understood. To understand the role of CD8β in mediating CD8/MHC class I interaction, we examined whether cell surface CD8α/β heterodimer promotes better cell-cell adhesion with MHC class I than the CD8α/α homodimer. The abilities of different forms of CD8 to adhere to MHC class I were measured with a cell-cell binding assay. Using a wild-type CD8β and -αa, we found that CD8αβ heterodimers did not mediate greater cell-cell adhesion than CD8αα homodimers. Furthermore, we found that chimeric CD8β-α homodimers afforded no detectable binding. These results do not support the idea that CD8αβ binding to MHC class I is greater than that of CD8αα. Rather, they point to an alternative explanation in which CD8β may play an role in promoting CD8/TCR interaction and/or in signaling/regulatory pathways.