Modified cell ELISA to determine the solubilization of cell surface proteins: Applications in GPI-anchored protein purification

Neuronal Wiring Receptors Dprs and DIPs Are GPI Anchored and This Modification Contributes to Their Cell Surface Organization

The Drosophila Dpr and DIP proteins belong to the immunoglobulin superfamily of cell surface proteins (CSPs). Their hetero- and homophilic interactions have been implicated in a variety of neuronal functions, including synaptic connectivity, cell survival, and axon fasciculation. However, the signaling pathways underlying these diverse functions are unknown. To gain insight into Dpr-DIP signaling, we sought to examine how these CSPs are associated with the membrane. Specifically, we asked whether Dprs and DIPs are integral membrane proteins or membrane anchored through the addition of glycosylphosphatidylinositol (GPI) linkage. We demonstrate that most Dprs and DIPs are GPI anchored to the membrane of insect cells and validate these findings for some family members in vivo using Drosophila larvae, where GPI anchor cleavage results in loss of surface labeling. Additionally, we show that GPI cleavage abrogates aggregation of insect cells expressing cognate Dpr-DIP partners. To test if the GPI anchor affects Dpr and DIP localization, we replaced it with a transmembrane domain and observed perturbation of subcellular localization on motor neurons and muscles. These data suggest that membrane anchoring of Dprs and DIPs through GPI linkage is required for localization and that Dpr-DIP intracellular signaling likely requires transmembrane coreceptors.

Cell painting with an engineered EPCR to augment the protein C system

The protein C (PC) system conveys beneficial anticoagulant and cytoprotective effects in numerous in vivo disease models. The endothelial protein C receptor (EPCR) plays a central role in these pathways as cofactor for PC activation and by enhancing activated protein C (APC)-mediated protease-activated receptor (PAR) activation. During inflammatory disease, expression of EPCR on cell membranes is often diminished thereby limiting PC activation and APC's effects on cells. Here a caveolae-targeting glycosylphosphatidylinositol (GPI)-anchored EPCR (EPCR-GPI) was engineered to restore EPCR's bioavailability via "cell painting." The painting efficiency of EPCR-GPI on EPCR-depleted endothelial cells was time- and dose-dependent. The EPCR-GPI bioavailability after painting was long lasting since EPCR surface levels reached 400 % of wild-type cells after 2 hours and remained > 200 % for 24 hours. EPCR-GPI painting conveyed APC binding to EPCR-depleted endothelial cells where EPCR was lost due to shedding or shRNA. EPCR painting normalised PC activation on EPCR-depleted cells indicating that EPCR-GPI is functional active on painted cells. Caveolin-1 lipid rafts were enriched in EPCR after painting due to the GPI-anchor targeting caveolae. Accordingly, EPCR painting supported PAR1 and PAR3 cleavage by APC and augmented PAR1-dependent Akt phosphorylation by APC. Thus, EPCR-GPI painting achieved physiological relevant surface levels on endothelial cells, restored APC binding to EPCR-depleted cells, supported PC activation, and enhanced APC-mediated PAR cleavage and cytoprotective signalling. Therefore, EPCR-GPI provides a novel tool to restore the bioavailability and functionality of EPCR on EPCR- depleted and -deficient cells.

Influenza virus-like particles engineered by protein transfer with tumor-associated antigens induces protective antitumor immunity

Delivery of antigen in particulate form using either synthetic or natural particles induces stronger immunity than soluble forms of the antigen. Among naturally occurring particles, virus-like particles (VLPs) have been genetically engineered to express tumor-associated antigens (TAAs) and have shown to induce strong TAA-specific immune responses due to their nano-particulate size and ability to bind and activate antigen-presenting cells. In this report, we demonstrate that influenza VLPs can be modified by a protein transfer technology to express TAAs for induction of effective antitumor immune responses. We converted the breast cancer HER-2 antigen to a glycosylphosphatidylinositol (GPI)-anchored form and incorporated GPI-HER-2 onto VLPs by a rapid protein transfer process. Expression levels on VLPs depended on the GPI-HER-2 concentration added during protein transfer. Vaccination of mice with protein transferred GPI-HER-2-VLPs induced a strong Th1 and Th2-type anti-HER-2 antibody response and protected mice against a HER-2-expressing tumor challenge. The Soluble form of GPI-HER-2 induced only a weak Th2 response under similar conditions. These results suggest that influenza VLPs can be enriched with TAAs by protein transfer to develop effective VLP-based subunit vaccines against cancer without chemical or genetic modifications and thus preserve the immune stimulating properties of VLPs for easier production of antigen-specific therapeutic cancer vaccines.

Immunocapture-Selected Reaction Monitoring Screening Facilitates the Development of ELISA for the Measurement of Native TEX101 in Biological Fluids

Monoclonal antibodies that bind the native conformation of proteins are indispensable reagents for the development of immunoassays, production of therapeutic antibodies and delineating protein interaction networks by affinity purification-mass spectrometry. Antibodies generated against short peptides, protein fragments, or even full length recombinant proteins may not bind the native protein form in biological fluids, thus limiting their utility. Here, we report the application of immunocapture coupled with selected reaction monitoring measurements (immunocapture-SRM), in the rapid screening of hybridoma culture supernatants for monoclonal antibodies that bind the native protein conformation. We produced mouse monoclonal antibodies, which detect in human serum or seminal plasma the native form of the human testis-expressed sequence 101 (TEX101) protein-a recently proposed biomarker of male infertility. Pairing of two monoclonal antibodies against unique TEX101 epitopes led to the development of an ELISA for the measurement of TEX101 in seminal plasma (limit of detection: 20 pg/ml) and serum (limit of detection: 40 pg/ml). Measurements of matched seminal plasma samples, obtained from men pre- and post-vasectomy, confirmed the absolute diagnostic specificity and sensitivity of TEX101 for noninvasive identification of physical obstructions in the male reproductive tract. Measurement of male and female serum samples revealed undetectable levels of TEX101 in the systemic circulation of healthy individuals. Immunocapture-SRM screening may facilitate development of monoclonal antibodies and immunoassays against native forms of challenging protein targets.

Immunoenzymatic quantitative analysis of antigens expressed on the cell surface (cell-ELISA)

Cell-enzyme-linked immunosorbent assay (cell-ELISA) is an useful technique for the quantitative analysis of cell surface antigen expression that was developed on the basis of enzyme immunohistochemistry (EIH) and ELISA. Since its development, which was made possible by the establishment of monoclonal antibody technology, a wide range of cell types and surface molecules were analyzed by cell-ELISA. Here we show four variants of this method and provide a brief comparison of cell-ELISA with flow cytometry (FACS) and radioimmunobinding assay (RIA), which are other methods for the quantitative detection of cell-surface molecules. We describe step-by-step procedures for both direct and indirect cell-ELISA using either adherent or nonadherent live cells.

Adsorbed anthranilic acid molecules cause charge reversal of nonionic micelles

The effect of anthranilic acid, an aromatic amino acid, on the structural characteristics of nonionic micelles of Triton X-100 at different pH values was investigated by light scattering and small-angle neutron scattering (SANS) measurements. The scattered light intensity decreases as pH is increased or decreased on either side of the isoelectric point (IEP = 3.4) of the amino acid. Analysis of the SANS data using a sticky hard-sphere model shows that the micelles are oblate ellipsoids with an axial ratio of approximately 2.3. No significant change could be observed in the size of the micelles with a change in the pH, while the stickiness parameter (tau), which is related to the interaction potential (u(0)) increases on either side of the IEP. As tau increases, u(o) becomes less negative, indicating a decrease in the attractive interaction between nonionic micelles. This can be explained in terms of the changes in the surface charge of the micelles resulting from a shift in the acid-base equilibrium of the adsorbed amino acid. The variation of the intermicellar interaction as calculated from the stickiness parameter is consistent with the picture of reversal of charge of amino acids with pH. This is further supported by the observed variation of the cloud point of the solutions at different pH values. The change in the interparticle interaction is also reflected in the diffusion coefficient of the micelles measured by dynamic light scattering.

NKG2D ligand expression in AML increases in response to HDAC inhibitor valproic acid and contributes to allorecognition by NK-cell lines with single KIR-HLA class I specificities

This study exploited alloreactivity of natural killer (NK) cells for augmenting the recognition of human acute myeloid leukemia (AML). To circumvent the inhibitory effect of killer immunoglobulin receptor (KIR) signaling, we generated NK-cell lines with single KIR specificities for major human leukocyte antigen (HLA) class I allotypes. We demonstrated efficient cytolysis of KIR-HLA class I-mismatched primary AML blasts even at low effector-to-target ratios. To define the impact of tumor-associated activating NKG2D-ligands (NKG2D-L), 66 AML patients at diagnosis were analyzed. NKG2D-L were selectively expressed on monoblastic cells in AML M4 and M5 yet absent or weakly expressed on myeloblastic cells in all AML subtypes. Paucity of cell-surface NKG2D-L was not the result of shedding because levels of soluble ULBP1 ligand measured in AML plasma were in the normal range. Notably, purified NKG2D-L(+) monoblastic cells were more susceptible to NK-mediated killing than NKG2D-L(-) myeloblastic cells. Accordingly, induction of cell-surface NKG2D-L by treatment with the histone deacetylase inhibitor, valproic acid, rendered cells more sensitive to NK cytolysis. These data suggest that adoptive transfer of selected populations of alloreactive HLA class I-mismatched NK cells in combination with pharmacologic induction of NKG2D-L merits clinical evaluation as novel approaches to immunotherapy of human AML.

Soy isoflavones attenuate human monocyte adhesion to endothelial cell-specific CD54 by inhibiting monocyte CD11a

Soy-based diets have been shown to protect against the development of atherosclerosis; however, the underlying mechanism(s) remain unknown. Interaction between activated monocytes and inflamed endothelial cells is an early event in atherogenesis. Therefore, we examined whether treatment of monocytes with soy phytochemicals could inhibit their adhesion to the endothelial cell-specific protein, CD54, a key factor in monocyte adhesion. Female Sprague-Dawley rats were fed AIN-93G diets containing soy protein isolate or casein. Sera from soy-fed rats inhibited CD54-dependent monocyte adhesion, whereas sera from casein-fed rats did not. To determine whether isoflavones in the sera of soy-fed rats were involved in this inhibition, monocytes were preincubated with soy isoflavones. Isoflavone treatment inhibited monocyte adhesion to CD54 protein, as well as to endothelial cells expressing CD54. Monocyte expression of CD11a, the cognate receptor for CD54, was unaffected by isoflavones. However, binding of the activation epitope-specific antibody mAb24, which binds specifically to the active form of CD11a, was significantly lower in soy isoflavone-treated monocytes than in media-treated cells. These findings suggest that inhibition of CD54-dependent monocyte adhesion by soy isoflavones is mediated in part by affinity regulation of CD11a. Inhibition of monocyte adhesion to endothelial cells by isoflavones resulted in reduced expression of the inflammatory cytokines IL-6 and IL-8. Collectively, these data suggest that the athero-protective effect of soy diets may be mediated by blocking monocyte-endothelial cell interaction.

Human tumor membrane vesicles modified to express glycolipid-anchored IL-12 by protein transfer induce T cell proliferation in vitro: a potential approach for local delivery of cytokines during vaccination

Administration of soluble human interleukin-12 (hIL-12) has been shown to induce a potent anti-tumor response. However, the use of soluble hIL-12 is hindered by its cytotoxicity when systemically administered and the difficulty of transferring multiple genes into primary tumor cells. In this study, we developed a membrane-anchored hIL-12 and expressed it on tumor membrane vesicles to deliver and confine IL-12 to the vaccination site. We constructed a glycolipid-anchored hIL-12 (GPI-hIL-12) by fusing the coding region of p35 and p40 subunits of hIL-12 with the GPI-signal sequence of CD59 at the C-terminal ends. The two subunits were processed correctly and expressed as a GPI-anchored disulfide-linked heterodimeric protein on the cell surface. GPI-hIL-12 cells induced proliferation of activated T cells and augmentation of allogeneic T cell generation in an MLR assay. Purified GPI-hIL-12 was efficiently intercalated onto isolated tumor cell membrane vesicles prepared from various human tumor cell lines. Further, the incorporation of GPI-hIL-12 onto tumor membrane vesicles induced proliferation of T cells and the release of IFN-gamma by activated T cells. Notably, GPI-hIL-12 enhanced the proliferative response initiated by CD80, a principal costimulatory molecule for T cell activation. These studies suggest that tumor membrane vesicles modified with GPI-anchored cytokines can be used to create potent immunogenic tumor vaccines for use in human immunotherapy. Since protein transfer can be used to modify tumor membrane vesicles obtained from surgical specimens, this approach offers a useful alternative to gene therapy as a means of developing cancer vaccines.