Mapping the Binding Site on CD8β for MHC Class I Reveals Mutants with Enhanced Binding

In an effective immune response, CD8+ T cell recognition of virally derived Ag, bound to MHC class I, results in killing of infected cells. The CD8αβ heterodimer acts as a coreceptor with the TCR, to enhance sensitivity of the T cells to peptide/MHC class I, and is two orders of magnitude more efficient as a coreceptor than the CD8αα. To understand the important interaction between CD8αβ and MHC class I, we created a panel of CD8β mutants and identified mutations in the CDR1, CDR2, and CDR3 loops that decreased binding to MHC class I tetramers as well as mutations that enhanced binding. We tested the coreceptor function of a subset of reducing and enhancing mutants using a T cell hybridoma and found similar reducing and enhancing effects. CD8β-enhancing mutants could be useful for immunotherapy by transduction into T cells to enhance T cell responses against weak Ags such as those expressed by tumors. We also addressed the question of the orientation of CD8αβ with MHC class I using CD8α mutants expressed as a heterodimer with wild-type CD8α or CD8β. The partial rescuing of binding with wild-type CD8β compared with wild-type CD8α is consistent with models in which either the topology of CD8αα and CD8αβ binding to MHC class I is different or CD8αβ is capable of binding in both the T cell membrane proximal and distal positions.