Selective reactivity of CD8-independent T lymphocytes to a cytotoxic T lymphocyte-selected H-2Kb mutant altered at position 222 in the alpha 3 domain

Role of alpha3 domain of class I MHC molecules in the activation of high- and low-avidity CD8+ CTLs

CD8 can serve as a co-receptor or accessory molecule on the surface of CTL. As a co-receptor, CD8 can bind to the alpha3 domain of the same MHC class I molecules as the TCR to facilitate TCR signaling. To evaluate the role of the MHC class I molecule alpha3 domain in the activation of CD8(+) CTL, we have produced a soluble 227 mutant of H-2D(d), with a point mutation in the alpha3 domain (Glu227 --> Lys). 227 mutant class I-peptide complexes were not able to effectively activate H-2D(d)-restricted CD8 T cells in vitro, as measured by IFN-gamma production by an epitope-specific CD8(+) CTL line. However, the 227 mutant class I-peptide complexes in the presence of another MHC class I molecule (H-2K(b)) (that cannot present the peptide) with a normal alpha3 domain can induce the activation of CD8(+) CTL. Therefore, in order to activate CD8(+) CTL, the alpha3 domain of MHC class I does not have to be located on the same molecule with the alpha1 and alpha2 domains of MHC class I. A low-avidity CD8(+) CTL line was significantly less sensitive to stimulation by the 227 mutant class I-peptide complexes in the presence of the H-2K(b) molecule. Thus, low-avidity CTL may not be able to take advantage of the interaction between CD8 and the alpha3 domain of non-presenting class I MHC molecules, perhaps because of a shorter dwell time for the TCR-MHC interaction.

Evidence that the antigen receptors of cytotoxic T lymphocytes interact with a common recognition pattern on the H-2Kb molecule

Recognition of class I MHC antigens involves interaction between TCRs of cytotoxic T lymphocytes (CTL) and the two alpha helices of MHC molecules. Using a combined panel of H-2Kb mutants selected by either a CTL clone or MAbs, we have shown evidence that the TCRs of 59 Kb-specific CTL clones shared a common binding pattern on the H-2Kb molecule. Mutations of amino acid residues at the C-terminal regions, but not the N-terminal regions, of the alpha helices abrogated the recognition by the majority of the clones. The data suggests that TCRs predominantly recognize the class I MHC molecule with an orientation that is parallel to the beta-pleated strands and diagonal to the alpha helices.

Analysis of coreceptor versus accessory molecule function of CD8 as a correlate of exogenous peptide concentration

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.

The cysteine residues in the cytoplasmic tail of CD8 alpha are required for its coreceptor function

The cytoplasmic segment of the CD8 alpha polypeptide includes both a cysteine-containing motif that is required for its association with the tyrosine kinase p56lck, and two serine residues which are likely to be phosphorylated and involved in inside-out signaling phenomena. To determine the relative importance of these residues for CD8 function, a mouse T cell hybridoma expressing a T cell receptor specific for the class I major histocompatibility product H-2Kb was transfected with a set of CD8 alpha chain genes encoding polypeptides in which the cytoplasmic cysteine or serine residues were substituted with alanine. When challenged with Kb-transfected L cells, T cell transfectants expressing CD8 alpha beta or CD8 alpha alpha dimers with substituted cytoplasmic serine residues responded nearly as well as wild-type CD8 transfectants. In marked contrast, the CD8 alpha polypeptides bearing substitutions of both cytoplasmic cysteine residues were totally impaired in their ability to complement the co-expressed T cell receptor.