Recognition by CD8 on cytotoxic T lymphocytes is ablated by several substitutions in the class I alpha 3 domain: CD8 and the T-cell receptor recognize the same class I molecule.

The CD8 molecule on class I-reactive cytotoxic T lymphocytes (CTLs) is believed to function as a coreceptor along with the alpha beta T-cell receptor. Whereas the alpha beta T-cell receptor recognizes polymorphic residues in the alpha 1/alpha 2 domains of the class I molecule, the CD8 molecule is believed to recognize monomorphic class I residues. Our previous experiments suggested that residue 227 in the alpha 3 domain of major histocompatibility complex class I molecules contributes to the determinant recognized by CD8. By using a panel of site-directed mutants of H-2Dd, this observation has been extended herein. Our findings indicate that for recognition by CD8-dependent CTLs, residue 227 must be either glutamic acid or aspartic acid and cannot be either basic or uncharged. However, the recognition by CD8-independent CTLs is unaffected by any of the substitutions at position 227 of H-2Dd. Similarly, alterations of other charged residues at positions 222, 223, and 229 have an analogous effect to substitution at residue 227, whereas substitutions at residues 192 and 232 do not affect the reactivity of CD8-dependent or CD8-independent CTLs. In addition, mutant H-2Dd molecules that are not recognized by CD8-dependent CTLs are unable to stimulate a primary CTL response, yet they can stimulate a secondary CD8-independent H-2Dd-specific CTL response. These findings suggest that CD8 recognition is obligatory for the priming of class I-dependent CTL responses. Since endogenous class I molecules were expressed by all of the transfected cell lines, these findings provide direct genetic evidence that CD8 and the alpha beta T-cell receptor must interact with the same class I molecule.