The alpha 1 and alpha 2 domains of H-2 class I molecules interact to form unique epitopes

Limited regions of the alpha 2-domain alpha-helix control anti-A2 allorecognition: an analysis using a panel of A2 mutants

The regions of the HLA-A2 molecule controlling anti-A2 alloreactivity were explored using naturally occurring allelic variants of HLA-A, and a panel of transfectants expressing the products of A2.1 genes that had been mutated at multiple positions encoding residues in the alpha 2 domain alpha-helix. As a means of detecting distant conformational effects, these altered A2.1 molecules were also examined serologically. Amino acid substitutions at the carboxy-terminal end of the alpha 2 domain alpha-helix led to diminished staining with the monoclonal antibody (mAb) MA2.1. The epitope for this antibody has previously been mapped to the alpha 1 domain alpha-helix (residues 62-65). This suggests that interdomain contacts may cause conformational alteration, and that mutants can have distant, as well as local effects. Of the 24 positions where substitutions were made, only six led to loss of the anti-A2 alloresponse by the three clones and three lines that were tested. In addition, the mutations that altered the MA2.1 epitope, located on the alpha 1 domain alpha-helix, did not inhibit allorecognition. This suggests that a limited number of regions on the A2.1 molecule are responsible for allodeterminant expression. The most influential substitutions were those at positions 152, 154, 162, and 166. It is notable that three of these are predicted to be T-cell receptor (Tcr)-contacting residues, and one (152) to contribute to peptide binding. These results suggest that the specificity of alloreactive T cells is determined by exposed polymorphisms, directly contacted by the Tcr, and by concealed polymorphisms which influence peptide binding.

Mutations in the alpha 1 domain of a class I gene define residues important for specific allorecognition

Our strategy to use saturation mutagenesis to produce an unbiased collection of major histocompatibility class I mutants has resulted in unpredicted mutant phenotypes. First, we have shown data supporting our earlier work of the Dp20(Y27N) mutant. Allorecognition is altered at the clonal level while no variation in lymphocytic choriomeningitis virus (LCMV)-restricted recognition is observed. The defect does not destroy the integrity of this class I protein on the basis of three observations: (i) LCMV self-restricted recognition is not impaired, (ii) beta 2 microglobulin still associates with Dp20(Y27N) at the cell surface, and (iii) this mutant can stimulate a primary MLR. Thus, we believe Dp20(Y27N) specifically affects allorecognition, perhaps by altering self peptide associations. The Dp14(A11V;E32Q) mutant appears to interact with T cell receptors (TCR) from a cloned cytotoxic T lymphocyte, but is altered in inducing a wild type signal into the responding cell. This is presumably due to decreased interaction at the cell surface between Dp14(A11V;E32Q) and wild type-specific TCR such that variations are detected in how a cell perceives extracellular signals. Analysis of additional mutants suggests that mutant Dp163(N66S) alters the binding site for monoclonal antibodies 7-16.10 and 135, while leaving unaltered the binding site for monoclonal antibodies 34-1.2 and 11-20.3. This maps the residue responsible for 7-16.10 and 135 binding to the region of Dp163(N66S).

Random oligonucleotide mutagenesis: application to a large protein coding sequence of a major histocompatibility complex class I gene, H-2DP

We have used random oligonucleotide mutagenesis (or saturation mutagenesis) to create a library of point mutations in the alpha 1 protein domain of a Major Histocompatibility Complex (MHC) molecule. This protein domain is critical for T cell and B cell recognition. We altered the MHC class I H-2DP gene sequence such that synthetic mutant alpha 1 exons (270 bp of coding sequence), which contain mutations identified by sequence analysis, can replace the wild type alpha 1 exon. The synthetic exons were constructed from twelve overlapping oligonucleotides which contained an average of 1.3 random point mutations per intact exon. DNA sequence analysis of mutant alpha 1 exons has shown a point mutant distribution that fits a Poisson distribution, and thus emphasizes the utility of this mutagenesis technique to "scan" a large protein sequence for important mutations. We report our use of saturation mutagenesis to scan an entire exon of the H-2DP gene, a cassette strategy to replace the wild type alpha 1 exon with individual mutant alpha 1 exons, and analysis of mutant molecules expressed on the surface of transfected mouse L cells.

Cytotoxic T lymphocyte recognition of hybrid MHC class I molecules

We have utilized a group of MHC class I genes produced by in vitro recombination between Dp and Dd to study recognition of MHC class I molecules by cytolytic T cells (CTLs). Both polyclonal allo-specific and H-2-restricted CTLs require that alpha 1 and alpha 2 of the target class I molecule be derived from the same haplotype for efficient killing. By using T-cell lines we showed that within the bulk population there must exist a fraction of T cells which can recognize epitopes in alpha 1 or alpha 2. Critical residues for T-cell recognition have been identified using these chimeric genes.

Saturation mutagenesis of a major histocompatibility complex protein domain: identification of a single conserved amino acid important for allorecognition

The interactive association between T lymphocytes and their target cells is an important system of cell-cell interactions. Major histocompatibility complex class I molecules are the cell surface structures recognized by cytolytic T lymphocytes. To define the molecular structures recognized by cytotoxic T lymphocytes, we have saturated the 270-base-pair alpha 1 exon of the H-2Dp gene with point mutations, rapidly producing a "library" of 2.5 x 10(3) independent mutants. The library contains enough recombinant clones (each clone encoding approximately one amino acid replacement mutation) to predict a mutation at each nucleotide position of the alpha 1 exon. The functional analysis of the first five transfected gene products tested has shown that mutation of a conserved tyrosine at position 27 to asparagine destroys recognition of the H-2Dp gene product by polyclonal alloreactive cytotoxic T lymphocytes. Recognition of the same mutant molecule by three monoclonal antibodies and H-2-restricted lymphocytic choriomenengitis virus-specific cytotoxic T lymphocytes is unaffected.