CD8β Increases CD8 Coreceptor Function and Participation in TCR–Ligand Binding

To study the role of CD8β in T cell function, we derived a CD8α/β(−) (CD8(−/−)) T cell hybridoma of the H-2K(d)–restricted N9 cytotoxic T lymphocyte clone specific for a photoreactive derivative of the Plasmodium berghei circumsporozoite peptide PbCS 252-260. This hybridoma was transfected either with CD8α alone or together with CD8β. All three hybridomas released interleukin 2 upon incubation with L cells expressing K(d)–peptide derivative complexes, though CD8α/β cells did so more efficiently than CD8α/α and especially CD8(−/−) cells. More strikingly, only CD8α/β cells were able to recognize a weak agonist peptide derivative variant. This recognition was abolished by Fab′ fragments of the anti-K(d) α3 monoclonal antibody SF11.1.1 or substitution of K(d) D-227 with K, both conditions known to impair CD8 coreceptor function. T cell receptor (TCR) photoaffinity labeling indicated that TCR–ligand binding on CD8α/β cells was ∼5- and 20-fold more avid than on CD8α/a and CD8(−/−) cells, respectively. SF1-1.1.1 Fab′ or K(d) mutation D227K reduced the TCR photoaffinity labeling on CD8α/β cells to approximately the same low levels observed on CD8(−/−) cells. These results indicate that CD8α/β is a more efficient coreceptor than CD8α/α, because it more avidly strengthens TCR–ligand binding.