Signals delivered via the Qa-2 molecule can synergize with limiting anti-CD3-induced signals to cause T lymphocyte activation

New Insights Into the Role of Qa-2 and HLA-G Non-classical MHC-I Complexes in Malignancy

It is well established that the immune system can identify and destroy neoplastic transformed cells in a process known as immunosurveillance. Most studies have focused on the classical major histocompatibility complex (MHC) class Ia molecules, which are known to play an important role on the presentation of tumor antigens to the immune system in order to activate a response against tumor cells. However, a larger family of non-classical MHC class Ib-related molecules has received less attention. In this mini-review, we discuss the role of class Ib murine Qa-2 and its proposed human HLA-G homolog on immunosurveillance during embryogenesis and cancer. Whereas, both HLA-G and Qa-2 are involved in immune tolerance in pregnancy, the current evidence suggests that they play opposite roles in cancer. HLA-G appears to promote tumor progression while Qa-2 acts as a tumor suppressor awaking the immune system to reject tumor cells, as suggested by studies on different cancer cell models, such as melanoma, lymphoma, lung carcinoma, and our own results in mammary carcinoma.

Activation of T cells by cross-linking Qa-2, the ped gene product, requires Fyn

PROBLEM

Qa-2, the product of the Ped (preimplantation development) gene, regulates the rate of cell division of preimplantation mouse embryos by an unknown mechanism. Due to the limited availability of preimplantation embryos, T cells were used as a model system to assess the possible roles of Fyn and Lck, and two downstream effectors, PI-3 kinase and Akt, in Qa-2 induced cell proliferation.

METHOD OF STUDY

Resting T cells were stimulated to proliferate by treating with mouse anti-Qa-2 antibody, cross-linking with anti-mouse immunoglobulin, and adding PMA. The effects of kinase inhibitors on this proliferation were studied. Co-immunoprecipitates of T-cell lysates were analyzed for possible associations between Qa-2 and Fyn or Lck. Fyn knockout mice (Fyn-/-) were used to determine whether Fyn is required for T-cell activation induced by cross-linking Qa-2.

RESULTS

An inhibitor of Src family kinases and inhibitors of PI-3 kinase and Akt suppressed proliferation of resting T cells induced by cross-linking Qa-2. Fyn, but not Lck, co-immunoprecipitated with Qa-2. Fyn-/- T cells failed to proliferate in response to Qa-2 cross-linking.

CONCLUSION

Fyn, PI-3 kinase, and Akt are required for the activation of T cells by cross-linking Qa-2.

HLA-G is found in lipid rafts and can act as a signaling molecule

HLA-G protein is the functional homolog of Qa-2, the product of the mouse preimplantation embryo development (Ped) gene. Embryos expressing Qa-2 on the cell surface exhibit a faster rate of preimplantation cleavage and preferential survival in utero compared with Qa-2-negative embryos. Qa-2 is glycosylphosphatidylinositol (GPI) linked in the cell membrane. As a result, Qa-2 proteins cluster in cholesterol- and sphingolipid-rich lipid raft microdomains in the cell membrane and can signal via raft-associated intracellular signaling molecules. Using T cells as a model system, cross-linking of Qa-2 on the cell membrane has been shown to induce proliferation of resting cells. HLA-G, like Qa-2, lacks a cytoplasmic domain capable of transducing signals from the cell surface to the nucleus, but unlike Qa-2, HLA-G has a short six-amino acid cytoplasmic tail rather than a GPI anchor. To test whether HLA-G, like Qa-2, is located in lipid rafts and can act as a signaling molecule, we used an HLA-G transgenic mouse system. T cells were isolated and tested for HLA-G expression by immunofluorescence and for localization of HLA-G in lipid rafts by immunofluorescence and Western blotting. Next, the T cells were cross-linked with anti-HLA-G antibody to test for induction of proliferation. Our novel results show that HLA-G, like GPI-linked Qa-2, is present in lipid rafts in the cell membrane and can act as a signaling molecule to induce proliferation of resting T cells.

Outer membrane monolayer domains from two-dimensional surface scanning resistance measurements

Cellular plasma membranes have domains that are defined, in most cases, by cytoskeletal elements. The outer half of the bilayer may also contain domains that organize glycosylphosphatidylinositol (GPI)-linked proteins. To define outer membrane barriers, we measured the resistive force on membrane bound beads as they were scanned across the plasma membrane of HEPA-OVA cells with optical laser tweezers. Beads were bound by antibodies to fluorescein-phosphatidylethanolamine (Fl-PE) or to the class I major histocompatibility complex (MHC class I) Qa-2 (a GPI-anchored protein). Two-dimensional scans of resistive force showed both occasional, resistive barriers and a velocity-dependent, continuous resistance. At the lowest antibody concentration, which gave specific binding, the continuous friction coefficient of Qa-2 was consistent with that observed by single-particle tracking (SPT) of small gold particles. At high antibody concentrations, the friction coefficient was significantly higher but decreased with increasing temperature, addition of deoxycholic acid, or treatment with heparinase I. Barriers to lateral movement (>3 times the continuous resistance) were consistently observed. Elastic barriers (with elastic constants from 1 to 20 pN/microm and sensitive to cytochalasin D) and small nonelastic barriers (<100 nm) were specifically observed with beads bound to the GPI-linked Qa-2. We suggest that GPI-linked proteins interact with transmembrane proteins when aggregated by antibody-coated beads and the transmembrane proteins encounter cytoplasmic barriers to lateral movement. The barriers to lateral movement are dynamic, discontinuous, and low in density.

Signal transduction in leucocytes via GPI-anchored proteins: an experimental artefact or an aspect of immunoreceptor function?

Membrane proteins anchored in the membrane via a glycolipid glycosylphosphatidylinositol (GPI) as well as some glycolipids are able to transduce signals and induce diverse functional responses in cells upon their cross-linking via antibodies or natural ligands. In some cases this signaling capacity seems to be due to associations of these molecules with specific transmembrane proteins. GPI-anchored proteins are components of membrane microdomains enriched in glycosphingolipids and cholesterol and devoid of most transmembrane proteins. These membrane specializations are relatively resistant to solubilization in solutions of some mild detergents at low temperatures. These 'GPI-microdomains' contain also cytoplasmic signaling molecules such as Src-family protein tyrosine kinases and trimeric G-proteins. Thus, at least some signaling elicited upon cross-linking of GPI-anchored proteins and glycolipids may be due to perturbation of the signaling molecules associated with these microdomains. It is suggested that these specialized areas of the membrane rich in signaling molecules interact with immunoreceptors (TCR, BCR, Fc receptors) cross-linked upon their interactions with ligands and importantly contribute to initiation of proximal phases of their signaling pathways.