Comparison of the roles of CD8αα and CD8αβ in interaction with MHC class I

The CD8 molecule is expressed either as an alpha/alpha homodimer or an alpha/beta 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 CD8 alpha/beta heterodimers have been shown to be more effective coreceptors, the precise role of the beta-chain in TCR-mediated thymic maturation and T cell activation is not understood. To understand the role of CD8 beta in mediating CD8/MHC class I interaction, we examined whether cell surface CD8 alpha/beta heterodimer promotes better cell-cell adhesion with MHC class 1 than the CD8 alpha/alpha 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 CD8 beta and -alpha, we found that CD8 alpha beta heterodimers did not mediate greater cell-cell adhesion than CD8 alpha alpha homodimers. Furthermore, we found that chimeric CD8 beta-alpha homodimers afforded no detectable binding. These results do not support the idea that CD8 alpha beta binding to MHC class I is greater than that of CD8 alpha alpha. Rather, they point to an alternative explanation in which CD8 beta may play an role in promoting CD8/TCR interaction and/or in signaling/regulatory pathways.