Human Cytomegalovirus Antigen Presentation by HLA-G in Infected Cells

HLA-E and -G class Ib molecules were considered unrelated to viral antigen presentation. HLA-E binds nonamers from the leader sequences of other HLA-I molecules and the human cytomegalovirus (HCMV) UL40 protein, interacting with CD94/NKG2 NK cell receptors. Yet, evidence that HLA-E may present some pathogen-derived peptides to CD8+ T lymphocytes has been reported. By contrast, HLA-G binds a broad spectrum of endogenous sequences but its role in antigen presentation is unknown. An experimental approach was set up to search for HCMV antigens displayed by HLA-G in infected cells. Among the analysed peptidome, 22 sequences corresponding to 16 HCMV molecules were identified; 17 peptides were confirmed to interact in vitro with HLA-G of which 10 displayed characteristic anchor residues. As compared to the response in short-term (6 h) assays to immunodominant IE-1 and pp65 antigens, none of the HLA-G-binding peptides stimulated cytokine production by CD8+ T cells from HCMV-seropositive blood donors (n = 15). Following a 14-day peptide stimulation of PBMC and expansion with IL-2, CD8+ T cells specifically responding to a subset of these viral antigens were detected in some individuals, yet were not restricted by HLA-G in functional assays. A subset of viral peptides did bind to both HLA-G and -E but were not recognised by CD94/NKG2 NK cell receptors. Our results provide the first evidence that HLA-G may display potentially immunogenic viral peptides in HCMV-infected cells, yet do not support their ability to promote HLA-G-restricted CD8+ T cell responses nor to modulate NK cell functions.