ABSTRACT
Natural T cell receptors (TCRs) generally bind to their cognate pMHC molecules with weak affinity and fast kinetics, limiting their use as therapeutic agents. Using phage display, we have engineered a high affinity version of the A6 wild-type TCR (A6wt), specific for the human leukocyte antigen (HLA-A(∗)0201) complexed with human T cell lymphotropic virus type 111-19 peptide (A2-Tax). Mutations in just 4 residues in the CDR3ß loop region of the A6wt TCR were selected that improved binding to A2-Tax by nearly 1000-fold. Biophysical measurements of this mutant TCR (A6c134) demonstrated that the enhanced binding was derived through favorable enthalpy and a slower off-rate. The structure of the free A6c134 TCR and the A6c134/A2-Tax complex revealed a native binding mode, similar to the A6wt/A2-Tax complex. However, concordant with the more favorable binding enthalpy, the A6c134 TCR made increased contacts with the Tax peptide compared with the A6wt/A2-Tax complex, demonstrating a peptide-focused mechanism for the enhanced affinity that directly involved the mutated residues in the A6c134 TCR CDR3ß loop. This peptide-focused enhanced TCR binding may represent an important approach for developing antigen specific high affinity TCR reagents for use in T cell based therapies.
ABSTRACT
Using directed mutagenesis and phage display on a soluble fragment of the human immunoglobulin super-family receptor ILT2 (synonyms: LIR1, MIR7, CD85j), we have selected a range of mutants with binding affinities enhanced by up to 168,000-fold towards the conserved region of major histocompatibility complex (MHC) class I molecules. Produced in a dimeric form, either by chemical cross-linking with bivalent polyethylene glycol (PEG) derivatives or as a genetic fusion with human IgG Fc-fragment, the mutants exhibited a further increase in ligand-binding strength due to the avidity effect, with resident half-times (t(1/2)) on the surface of MHC I-positive cells of many hours. The novel compounds antagonized the interaction of CD8 co-receptor with MHC I in vitro without affecting the peptide-specific binding of T-cell receptors (TCRs). In both cytokine-release assays and cell-killing experiments the engineered receptors inhibited the activation of CD8(+) cytotoxic T lymphocytes (CTLs) in the presence of their target cells, with subnanomolar potency and in a dose-dependent manner. As a selective inhibitor of CD8(+) CTL responses, the engineered high affinity ILT2 receptor presents a new tool for studying the activation mechanism of different subsets of CTLs and could have potential for the development of novel autoimmunity therapies.
