ABSTRACT
Human CMV has evolved multiple strategies to interfere with immune recognition of the host. A variety of mechanisms target Ag presentation by MHC class I molecules resulting in a reduced class I cell-surface expression. This down-regulation of class I molecules is expected to trigger NK cytotoxicity, which would have to be counteracted by the virus to establish long-term infection. Here we describe that the human CMV open reading frame UL40 encodes a canonical ligand for HLA-E, identical with the HLA-Cw03 signal sequence-derived peptide. Expression of UL40 in HLA-E-positive target cells conferred resistance to NK cell lysis via the CD94/NKG2A receptor. Generation of the UL40-derived HLA-E ligand was also observed in TAP-deficient cells. The presence of a functional TAP-independent HLA-E ligand in the UL40 signal sequence implicates this viral gene as an important negative regulator of NK activity.
Subject(s)
Cytomegalovirus/immunology , Cytotoxicity, Immunologic/immunology , HLA Antigens/metabolism , Histocompatibility Antigens Class I/metabolism , Immunosuppressive Agents/pharmacology , Killer Cells, Natural/immunology , Viral Proteins/immunology , Viral Proteins/metabolism , Amino Acid Sequence , Antibodies, Monoclonal/metabolism , Cells, Cultured , Fibroblasts/immunology , Fibroblasts/metabolism , Fibroblasts/virology , Humans , K562 Cells , Ligands , Molecular Sequence Data , Oligopeptides , Open Reading Frames/immunology , Peptide Fragments/immunology , Peptide Fragments/isolation & purification , Peptides/immunology , Protein Processing, Post-Translational/immunology , Transfection , Viral Proteins/genetics , HLA-E AntigensABSTRACT
The interaction of HLA-E with CD94/NKG2A is dependant on the binding of HLA class I signal sequence derived peptides to HLA-E. In the caucasoid population two HLA-E alleles are observed at equal frequencies. Here we study the functional differences between the two HLA-E molecules with regard to cell surface expression, peptide binding, and potential to inhibit lytic activity of a CD94/NKG2A(+) NK cell line. In contrast to the HLA-E(R) allele, the HLA-E(G) allele shows considerable cell surface expression even in the absence of endogenous HLA class I signal sequence derived HLA-E ligands. Eighteen HLA-E allele/HLA-E ligand combinations were analyzed. No correlation between cell surface expression of HLA-E and NK cell inhibition was observed. The peptides present in the signal sequences of HLA-B15, -Cw0402, and -Cw7 bound to both HLA-E alleles but did not lead to an inhibition of NK cell lysis. In our experimental system the peptides A2 and G were not effective with regard to NK cell inhibition when bound to the HLA-E(R) allele. These results may be of functional significance particularly in the placenta where the only HLA-E ligands are derived from HLA-G and -C.
