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 1, results in killing of infected cells. The CD8 alpha beta heterodimer acts as a coreceptor with the TCR, to enhance sensitivity of the T cells to peptide/MHC class 1, and is two orders of magnitude more efficient as a coreceptor than the CD8 alpha alpha. To understand the important interaction between CD8 alpha beta and MHC class 1, we created a panel of CD8 beta mutants and identified mutations in the CDRI, 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. CD8 beta-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 CD8 alpha beta with MHC class I using CD8 alpha mutants expressed as a heterodimer with wild-type CD8 alpha or CD8 beta. The partial rescuing of binding with wild-type CD8 beta compared with wild-type CD8 alpha is consistent with models in which either the topology of CD8aa and CD8 alpha beta binding to MHC class I is different or CD8 alpha beta is capable of binding in both the T cell membrane proximal and distal positions.