Substitution in the alpha 3 domain of class I molecules can ablate the recognition of target cells by CD8 dependent cytotoxic T lymphocytes. This effect has been attributed to a destruction of the CD8 alpha binding site on the class I molecule, a hypothesis which is consistent with results obtained in conjugate binding assays. To assess the relative contribution to CTL activation of CD8 functioning as either a coreceptor or an accessory molecule, we have compared the ability of H-2Kb
ovalbumin reactive CTL to lyse M12.C3 or T2 cells transfected with an H-2Kb gene encoding a wild type or mutant (CD8 nonbinding) alpha 3 domain. To establish that the substitution in the alpha 3 domain does not alter the ability of the H-2Kb molecule to bind the antigenic
peptide, we have compared the binding of the
ovalbumin derived H-2Kb restricted
peptide (SIINFEKL) to T2 cells expressing either the CD8 binding or the CD8 nonbinding form of H-2Kb. This
peptide conjugated with
FITC bound equally well to T2 cells expressing either form of H-2Kb. Upon binding of this
peptide, both forms of the H-2Kb molecule underwent the same conformational change as revealed by increases in the expression of particular serological
epitopes. Furthermore, inhibition of the binding of the
SIINFEKL peptide to both the wild type and mutant H-2Kb was observed following pretreatment of the cells with similar amounts of other H-2Kb restricted
peptides derived from Sendai and
Vesicular Stomatitis viruses. When the transfected M12 cells were tested for their ability to serve as targets for an anti-H-2Kb
ovalbumin CTL clone, cells expressing the mutant H-2Kb molecule required the addition of 100-fold more exogenous
peptide than did cells expressing the wild type molecule in order to obtain significant lysis. These data strengthen the previous hypothesis that CD8 functions much more efficiently as a coreceptor than as an accessory molecule for T cell effector function.