Computational Design and Crystal Structure of an Enhanced Affinity Mutant Human CD8 aa Coreceptor

ABSTRACT Human CD8 is a T cell coreceptor, which binds to pHLA I and plays a pivotal role in the activation of cytotoxic T lymphocytes. Soluble recombinant CD8 aa has been shown to antagonize T cell activation, both in vitro and in vivo. How-ever, because of a very low affinity for pHLA I, high concentrations of soluble CD8 aa are re-quired for efficient inhibition. Based upon our knowledge of the wild-type CD8/pHLA I structure, we have designed and produced a mutated form of soluble CD8 aa that binds to pHLA I with approxi-mately fourfold higher affinity. We have character-ized the binding of the high affinity CD8 mutant using surface plasmon resonance and determined its structure at 2.1 A ̊ resolution using X-ray crys-tallography. The analysis of this structure sug-gests that the higher affinity is achieved by pro-viding a larger side chain that allows for an opti-mal contact to be made between the HLA a3 loop and the mutated CDR-like loops of CD8. Protein