Phox (PX) domains are phosphoinositide (PI)-binding domains with broad PI specificity. Two cytosolic components of NADPH oxidase, p40(phox) and p47(phox), contain PX domains. The PX domain of p40(phox) specifically binds phosphatidylinositol 3-phosphate, whereas the PX domain of p47(phox) has two lipid binding sites, one specific for phosphatidylinositol 3,4-bisphosphate and the other with affinity for phosphatidic acid or phosphatidylserine. To delineate the mechanisms by which these PX domains interact with PI-containing membranes, we measured the membrane binding of these domains and respective mutants by surface plasmon resonance and monolayer techniques and also calculated the electrostatic potentials of the domains as a function of PI binding. Results indicate that membrane binding of both PX domains is initiated by nonspecific electrostatic interactions, which is followed by the membrane penetration of hydrophobic residues. The membrane penetration of the p40(phox) PX domain is induced by phosphatidylinositol 3-phosphate, whereas that of the p47(phox) PX domain is triggered by both phosphatidylinositol 3,4-bisphosphate and phosphatidic acid (or phosphatidylserine). Studies of enhanced green fluorescent protein-fused PX domains in HEK293 cells indicate that this specific membrane penetration is also important for subcellular localization of the two PX domains. Further studies on the full-length p40(phox) and p47(phox) proteins showed that an intramolecular interaction between the C-terminal Src homology 3 domain and the PX domain prevents the nonspecific monolayer penetration of p47(phox), whereas such an interaction is absent in p40(phox).

Based on previous findings supporting HLA-F as a ligand for KIR3DL2 and KIR2DS4, we investigated the potential for MHC-I open conformers (OCs) as ligands for KIR3DS1 and KIR3DL1 through interactions measured by surface plasmon resonance. These measurements showed physical binding of KIR3DS1 but not KIR3DL1 with HLA-F and other MHC-I OC while also confirming the allotype specific binding of KIR3DL1 with MHC-I peptide complex. Concordant results were obtained with biochemical pull-down from cell lines and biochemical heterodimerization experiments with recombinant proteins. In addition, surface binding of HLA-F and KIR3DS1 to native and activated NK and T cells was coincident with specific expression of the putative ligand or receptor. A functional response of KIR3DS1 was indicated by increased granule exocytosis in activated cells incubated with HLA-F bound to surfaces. The data extend a model for interaction between MHC-I open conformers and activating KIR receptors expressed during an inflammatory response, potentially contributing to communication between the innate and adaptive immune response.

Killer Immunoglobulin-like receptors (KIR) are innate immune receptors expressed by NK and T cells classically associated with the detection of missing-self through loss of their respective MHC ligand. Some KIR specificities for allelic classical class I MHC (MHC-I) have been described, while other KIR receptor-ligand relationships, including those associated with non-classical MHC-I, have yet to be clearly defined. We report here that KIR3DL2 and KIR2DS4 and the non-classical antigen HLA-F, expressed as a free form devoid of peptide, physically and functionally interact. These interactions extend to include classical MHC class I open conformers as ligands, defining new relationships between KIR receptors and class I MHC. The data collectively suggest a broader, previously unrecognized interaction between MHC-I open conformers – including prototypical HLA-F – and KIR receptors, acting in an immunoregulatory capacity centered on the inflammatory response.

HLA-F has very low levels of polymorphism in humans and is highly conserved among primates suggesting a conserved function in the immune response. In this study we probed the structure of HLA-F on the surface of B-LCLs and activated lymphocytes by direct measurement of peptide binding of native HLA-F. Our findings suggested that HLA-F is expressed independently of bound peptide, at least with respect to peptide complexity profiles similar to those of either HLA-E or classical MHC-I. As a further probe of native HLA-F structure, we used a number of complementary approaches to explore the interactions of HLA-F with other molecules, at the cell surface, intracellularly, and in direct physical biochemical measurements. This analysis demonstrated that HLA-F surface expression was coincident with MHC-I heavy chain (HC) expression and was down regulated upon perturbation of MHC-I HC structure. It was further possible to directly demonstrate that MHC-I would only interact with HLA-F when in the form of open conformer free of peptide and not as trimeric complex. This interaction was directly observed by co-immunoprecipitation and by surface plasmon resonance and indirectly on the surface of cells through coincident tetramer and MHC-I HC co-localization. Together these data suggest that HLA-F is expressed independent of peptide and that a physical interaction specific to MHC-I HC plays a role in the function of MHC-I HC expression in activated lymphocytes.

Peptides that are presented by MHC class I (MHC-I) are processed from two potential sources, as follows: newly synthesized endogenous proteins for direct presentation on the surface of most nucleated cells and exogenous proteins for cross-presentation typically by professional APCs. In this study, we present data that implicate the nonclassical HLA-F and open conformers of MHC-I expressed on activated cells in a pathway for the presentation of exogenous proteins by MHC-I. This pathway is distinguished from the conventional endogenous pathway by its independence from TAP and tapasin and its sensitivity to inhibitors of lysosomal enzymes, and further distinguished by its dependence on MHC-I allotype-specific epitope recognition for Ag uptake. Thus, our data from in vitro experiments collectively support a previously unrecognized model of Ag cross-presentation mediated by HLA-F and MHC-I open conformers on activated lymphocytes and monocytes, which may significantly contribute to the regulation of immune system functions and the immune defense.