Functional characterization of HLA-F and binding of HLA-F tetramers to ILT2 and ILT4 receptors

HLA-F is a human non-classical MHC molecule. Recombinant HLA-F heavy chain was refolded with beta2-microglobulin to form a stable complex. This comple x was used as an immunogen to produce a highly specific, high-affinity mono clonal antibody (FG1) that was used to study directly the cellular biology and tissue distribution of HLA-F. HLA-F has a restricted pattern of tissue expression in tonsil, spleen, and thymus. HLA-F could be immunoprecipitated from B cell lines and from HUT-78, a T cell line. HLA-F binds TAP, but unl ike the classical human class I molecules, was undetected at the cell surfa ce. HLA-F tetramers stain peripheral blood monocytes and B cells. HLA-F tet ramer binding could be conferred on non-binding cells by transfection with the inhibitory receptors ILT2 and ILT4. Surface plasmon resonance studies d emonstrated a direct molecular interaction of HLA-F with ILT2 and ILT4. The se results, together with structural predictions based on the sequence of H LA-F, suggest that HLA-F may be a peptide binding molecule and may reach th e cell surface under favorable conditions, which may include the presence o f specific peptide or peptides. At the cell surface it would be capable of interacting with LIR1 (ILT2)and LIR2 (ILT4) receptors and so altering the a ctivation threshold of immune effector cells.