Serum levels of the iron binding protein p97 are elevated in Alzheimer's disease.

Alzheimer's disease is a progressive and incurable disease whose prevalence increases dramatically with age. A biochemical marker for monitoring the onset and progression of the disease would be a valuable tool for disease management. In addition, such a marker might be used as an end point in clinical intervention protocols. Here we provide evidence that the soluble form of the iron binding protein p97 is found in elevated amounts in the serum of Alzheimer's patients compared with healthy controls. This biochemical marker has the potential for identifying subjects afflicted with the disease and possibly for monitoring the onset and longitudinal progression of the disease.

E3/19K from adenovirus 2 is an immunosubversive protein that binds to a structural motif regulating the intracellular transport of major histocompatibility complex class I proteins.

We have previously expressed in transgenic mice a chimeric H-2Kd/Kk protein called C31, which contains the extracellular alpha 1 domain of Kd, whereas the rest of the molecule is of Kk origin. This molecule functions as a restriction element for alloreactive and influenza A-specific cytotoxic T lymphocytes (CTL) but is only weakly expressed at the cell surface of splenocytes. Here, we show that the low cell surface expression is the result of slow intracellular transport and processing of the C31 protein. A set of hybrid molecules between Kd and Kk were used to localize the regions in major histocompatibility complex (MHC) molecules that are important for their intracellular transport and to further localize the structures responsible for binding to the adenovirus 2 E3/19K protein. This protein appears to be an important mediator of adenovirus persistence. It acts by binding to the immaturely glycosylated forms of MHC class I proteins in the endoplasmic reticulum (ER), preventing their passage to the cell surface and thereby reducing the recognition of infected cells by virus-specific T cells. We find the surprising result that intracellular transport and E3/19K binding are controlled primarily by the first half of the second domain of Kd, thus localizing these phenomena to the five polymorphic residues in this region of the Kd protein. This result implies that the E3/19K protein may act by inhibiting peptide binding or by disrupting the oligomerization of MHC class I molecules required for transport out of the ER. Alternatively, the E3/19K protein may inhibit the function of a positively acting transport molecule necessary for cell surface expression of MHC class I molecules.

Authentic T helper CD4 (W3/25) antigen on rat peritoneal macrophages.

The rat W3/25 antigen that appears to be equivalent to human CD4 (T4) antigen is expressed on thymocytes and T helper cells and plays a role in the response of T helper cells to antigen. The W3/25 and anti-T4 antibodies also label macrophages. In this paper we examine whether the macrophage antigen is the same as that on T cells. New monoclonal antibodies against the rat CD4 antigen, MRC OX-35 through OX-38, are described, all of which label peritoneal macrophages from normal and athymic rats. The molecular weight of W3/25 antigen on macrophages is indistinguishable from that on T cells. We conclude that macrophages express authentic CD4 (W3/25) antigen. Another new monoclonal antibody, MRC OX-34, labels an antigen of 50-54,000 mol wt that is expressed on rat T but not B cells or peritoneal macrophages. It was used to control for the presence of any T cell products in immunoprecipitation from rat macrophage extracts.

Comparison of cell lines deficient in antigen presentation reveals a functional role for TAP-1 alone in antigen processing.

Cytotoxic T lymphocytes (CTL) recognize antigenic peptides bound to major histocompatibility complex class I antigens on the cell surface of virus-infected cells. It is believed that the majority of peptides originate from cytoplasmic degradation of proteins assumed to be mediated by the "20S" proteasome. Cytosolic peptides are then translocated, presumably by transporters associated with antigen processing (TAP-1 and -2), into the lumen of the endoplasmic reticulum (ER) where binding and formation of the ternary complex between heavy chain, beta2-microglobulin (beta 2m) and peptide occurs. In this study, we have analyzed and compared the phenotype of two mutant cell lines, the thymoma cell line RMA-S and a small lung carcinoma cell line CMT.64, in order to address the mechanism that underlies the antigen processing deficiency of CMT.64 cells. Unlike RMA-S cells, vesicular stomatitis virus (VSV)-infected CMT.64 cells are not recognized by specific CTL. Interferon gamma (IFN-gamma) treatment of CMT.64 cells restores the ability of these cells to process and present VSV in the context of Kb. We show that although CMT.64 cells express a low level of beta 2m, the recognition of VSV-specific CTL is not restored by increasing the amount of beta 2m synthesized in CMT.64 cells. In addition, we find that CMT.64 cells express moderate levels of Kb heavy chain molecules, but most of it is unstable and rapidly degraded in the absence of IFN-gamma treatment. We infer that the antigen processing deficiency does not lie at the level of beta 2m or Kb production. We find also that the mRNAs for both TAP-1 and -2 are present in RMA and RMA-S cells but are absent in uninduced CMT.64 cells. Upon IFN-gamma induction, both mRNAs are highly expressed in CMT-64 cells. In addition, we find that the low molecular mass polypeptides 2 and 7, and additional components of the proteasome are induced by IFN-gamma in CMT-64 cells. Finally, introduction of the rat TAP-1 gene in CMT.64 cells restores CTL recognition of VSV-infected cells. These results indicate that a TAP-1 homodimer may translocate peptides in the ER and explain partially the CMT.64 defect and the RMA-S phenotype. These findings link a dysfunction in the transport and/or generation of antigenic peptides to the capacity of tumor cells to evade immunosurveillance and provide a unique model system to dissect this phenomenon.

Adenovirus infection inhibits the phosphorylation of major histocompatibility complex class I proteins.

Major histocompatibility complex (MHC) class I molecules act as peptide receptors to direct the recognition of foreign antigens by cytolytic T cells. The cell surface expression and trafficking of these peptide receptors is thought to be controlled by the conformation of the MHC molecule and possibly by the phosphorylation of the cytoplasmic portion of the heavy chain protein. It is of some interest that adenoviruses (Ads) have evolved proteins that interfere with the expression of MHC molecules. One of these proteins, called E3/19k, binds to newly synthesized MHC molecules in the rough endoplasmic reticulum (RER) and inhibits their trafficking to the cell surface. Here we show that during the infection of a human cell line with Ad2, the phosphorylation of the endogenous MHC molecules is inhibited. We also observe that the phosphorylation of the endogenous HLA molecules is grossly impaired in a human cell line transfected with the Ad2 EcoRI D fragment containing the E3/19k gene. We conclude that the E3/19k protein inhibits the phosphorylation of the MHC heavy chains and that this may be one of the important functions of this protein in infected cells. In addition, we show that a mutant of the E3/19k protein, which lacks an RER retention signal but which retains its ability to bind to HLA molecules, does not inhibit the phosphorylation of HLA molecules and that phosphorylated molecules are not Endo H sensitive. This suggests that HLA molecules are phosphorylated after leaving the medial-Golgi compartment, thus providing the most compelling evidence yet that HLA molecules are phosphorylated at or near the cell surface. Finally, to our knowledge, this is the first study under which the phosphorylation of MHC molecules is shown to be altered and may have some relevance for other pathogenic conditions.

Recognition of class I major histocompatibility complex molecules by Ly-49: specificities and domain interactions.

Ly-49 is a family type II transmembrane proteins encoded by a gene cluster on murine chromosome 6. One member of this family, Ly-49A, is expressed by a natural killer (NK) cell subset, binds to class I major histocompatibility complex (MHC) molecules, and blocks the killing of target cells bearing the appropriate H-2 antigens. Here we show that another member of this family which is expressed by an NK cell subset, Ly-49C, recognizes H-2b and H-2d structures which are distinct from and overlapping with those recognized by Ly-49A. Interactions between Ly-49A and C and their class I ligands are entirely blocked by the antibodies 5E6, YE1/48, YE1/32, and A1, all of which were found to recognize epitopes contained within the carbohydrate recognition domain (CRD). However, cell-cell binding assays revealed that class I binding specificity is conferred by a combination of sequences within both the CRD and a 19-amino acid adjacent region. We also investigated the question of whether Ly-49A and C form dimers on cells which express both receptors. When coexpressed on COS cells, sequential immunoprecipitation demonstrated that these receptors pair exclusively as homodimers, with no evidence for heterodimeric structures. These observations provide insight into both the biochemical nature of the Ly-49 family as well as the receptor functions of Ly-49C on NK cells.

Species heterogeneity in macrophage expression of the CD4 antigen.

The CD4 antigen is expressed on T cells of all mammalian species examined and appears to play an important role in the response of T cells to antigen. In humans, the molecule acts as a receptor for the AIDS virus. Previous studies have demonstrated that M phi in the rat and human also express the CD4 antigen, which is indistinguishable from that on T cells. In this paper we demonstrate by FACS analysis, Northern blot hybridization, and immunoperoxidase labeling that, in striking contrast to the rat and human, mouse M phi do not express the CD4 (L3T4) antigen. This species heterogeneity indicates that T cells and M phi regulate CD4 antigen expression independently and that CD4 may not be essential for M phi function.

The MRC OX-44 antigen marks a functionally relevant subset among rat thymocytes.

A monoclonal antibody called MRC OX-44 is described that labels all myeloid cells and peripheral lymphoid cells but only 12% of thymocytes. The OX-44+ thymic cells include most if not all cells found in the medulla but only a small fraction of the cortical cells. Together with CD4 and CD8 antigens, seven subsets of thymic cell were defined and it was notable that most CD4- CD8- cells were OX-44+ whereas almost all CD4+ CD8+ cells were OX-44-. In functional tests, the OX-44+ cells accounted for all proliferation by thymocytes when stimulated by allogeneic spleen cells or concanavalin A plus growth factors and OX-44- cells were completely negative in these assays. Also, in tests for thymopoiesis after intra-thymic injection of cells, all activity was OX-44+. It seems possible that the OX-44+ set may include all functionally relevant cells in the rat thymus.

Pumping iron in the '90s.

The role o f iron in cell division, cell death and human disease has recently gained increased attention. The best studied process for iron uptake into mammalian cells involves traps ferrin and its receptor. This review discusses evidence supporting the existence of other routes by which iron can enter mammalian cells. Specifically, iron uptake by the cell-surface GPI-linked traps ferrin homologue, melanotransferrin or p97, is described and possible functions of this traps ferrin-independent pathway are proposed.

Surrogate antigen processing mediated by TAP-dependent antigenic peptide secretion.

MHC class I proteins assemble with peptides in the ER. The peptides are predominantly generated from cytoplasmic proteins, probably by the action of the proteasome, a multicatalytic proteinase complex. Peptides are translocated into the ER by the transporters associated with antigen processing (TAP), and bind to the MHC class I molecules before transport to the cell surface. Here, we use a new functional assay to demonstrate that peptides derived from vesicular stomatitis virus nucleoprotein (VSV-N) antigen are actively secreted from cells. This secretion pathway is dependent on the expression of TAP transporters, but is independent of the MHC genotype of the donor cells. Furthermore, the expression and transport of MHC class I molecules is not required. This novel pathway is sensitive to the protein secretion inhibitors brefeldin A (BFA) and a temperature block at 21 degrees C, and is also inhibited by the metabolic poison, azide, and the protein synthesis inhibitor, emetine. These data support the existence of a novel form of peptide secretion that uses the TAP transporters, as opposed to the ER translocon, to gain access to the secretion pathway. Finally, we suggest that this release of peptides in the vicinity of uninfected cells, which we term surrogate antigen processing, could contribute to various immune and secretory phenomena.

TAP expression provides a general method for improving the recognition of malignant cells in vivo.

A major class of tumors lack expression of the transporters associated with antigen processing (TAP). These proteins are essential for delivery of antigenic peptides into the lumen of the endoplasmic reticulum (ER) and subsequent assembly with nascent major histocompatibility complex (MHC) class I, which results in cell surface presentation of the trimeric complex to cytolytic T lymphocytes. Cytolytic T lymphocytes are major effector cells in immunosurveillance against tumors. Here we have tested the hypothesis that TAP downregulation in tumors allows immunosubversion of this effector mechanism, by establishing a model system to examine the role of TAP in vivo in restoring antigen presentation, immune recognition, and effects on malignancy of the TAP-deficient small-cell lung carcinoma, CMT.64. To test the potential of providing exogenous TAP in cancer therapies, we constructed a vaccinia virus (VV) containing the TAP1 gene and examined whether VV-TAP1 could reduce tumors in mice. The results demonstrate that TAP should be considered for inclusion in cancer therapies, as it is likely to provide a general method for increasing immune responses against tumors regardless of the antigenic complement of the tumor or the MHC haplotypes of the host.

Iron metabolism in mammalian cells.

Most living things require iron to exist. Iron has many functions within cells but is rarely found unbound because of its propensity to catalyze the formation of toxic free radicals. Thus the regulation of iron requirements by cells and the acquisition and uptake of iron into tissues in multicellular organisms is tightly regulated. In humans, understanding iron transport and utility has recently been advanced by a "great conjunction" of molecular genetics in simple organisms, identifying genes involved in genetic diseases of metal metabolism and by the application of traditional cell physiology approaches. We are now able to approach a rudimentary understanding of the "iron cycle" within mammals. In the future, this information will be applied toward modulating the outcome of therapies designed to overcome diseases involving metals.

Antigens of activated rat T lymphocytes including a molecule of 50,000 Mr detected only on CD4 positive T blasts.

Mouse monoclonal antibodies (MAbs) have been prepared against rat T cell blasts. One MAb called MRC OX-40 recognized an antigen that differed from any previously described in that its expression was detected only on T blasts that also expressed the CD4 antigen. The OX-40 MAb did not detect an activation determinant of CD2 or CD4 molecules but recognized a distinct chain of mol. wt 50,000. The OX-40 MAb augmented T cell proliferation at late stages on in vitro responses. Other MAbs without obvious counterparts in other species were MRC OX-48 and MRC OX-49,50 which recognized cell surface molecules of mol. wts of about 95,000 and 90,000, respectively. The OX-48 antigen was not expressed on resting lymphocytes but was found on a subset of T and B blasts and also on other leucocytes. The OX-49,50 antigen was found on most haemopoietic cells but was expressed at greatly increased levels after lymphocyte activation and this was also the case for MRC OX-47 antigen which is of unknown Mr. The MRC OX-39 MAb was found to bind the rat IL-2 receptor; expression of this antigen was detected on thymic dendritic cells as well as on T blasts. The phenotype of rat T blasts compared to resting cells was also examined and changes in expression of L-CA, Thy-1, OX-2 and CD8 antigens were seen in addition to the changes found with the above MAbs.

Coincident expression and distribution of melanotransferrin and transferrin receptor in human brain capillary endothelium.

One method of iron transport across the blood brain barrier (BBB) involves the transferrin receptor (TR), which is localized to the specialized brain capillary endothelium. The melanotransferrin (MTf) molecule, also called p97, has been widely described as a melanoma specific molecule, however, its expression in brain tissues has not been addressed. MTf has a high level of sequence homology to transferrin (Tf) and lactoferrin, but is unusual because it predominantly occurs as a membrane bound, glycosylphosphatidylinositol (GPI) anchored molecule, but can also occur as a soluble form. We have recently demonstrated that GPI-anchored MTf provides a novel route for cellular iron uptake which is independent of Tf and its receptor. Here we consider whether MTf may have a role in the transport of iron across the BBB. The distributions of MTf, Tf and the TR were studied immunohistochemically in human brain tissues. The distributions of MTf and TR were remarkably similar, and quite different from that of Tf. In all brain tissues examined, MTf and the TR were highly localized to capillary endothelium, while Tf itself was mainly localized to glial cells. These data suggest that MTf may play a role in iron transport within the human brain.

Reactive microglia specifically associated with amyloid plaques in Alzheimer's disease brain tissue express melanotransferrin.

Several investigations have implicated the involvement of metals in neuropathologies. In particular, the disruption of iron metabolism and iron transport molecules have been demonstrated in Alzheimer's disease (AD). We have identified a novel pathway of iron uptake into mammalian cells involving melanotransferrin, or p97, which is independent of the transferrin receptor. Here we investigated whether there is a possible link between this molecule and the pathology of AD. The distributions of melanotransferrin, transferrin and the transferrin receptor were studied immunohistochemically in brain tissues from AD cases. In brain tissues from AD, melanotransferrin and the transferrin receptor were highly localized to capillary endothelium, while transferrin itself was mainly localized to glial cells. In brain tissue derived from AD patients, melanotransferrin was additionally detected in a subset of reactive microglia associated with senile plaques. Our demonstration that melanotransferrin mediates iron uptake through a pathway independent of the transferrin receptor indicates that this mechanism may have a role in AD.

Melanotransferrin is produced by senile plaque-associated reactive microglia in Alzheimer's disease.

Melanotransferrin (MTf), also known as p97, has been localized in capillary endothelial cells of human brain. In Alzheimer's-diseased (AD) brain tissues, reactive microglial cells located in senile plaques exhibit elevated levels of MTf. The localization of the p97 protein may reflect its site of synthesis or could reflect a paracrine site of action. We examined the expression of MTf mRNA by in situ hybridization histochemistry using AD and healthy brain tissues. We also examined normal liver tissues by immunohistochemistry and in situ hybridization. In all the brain tissues examined, capillaries had positive signals for MTf mRNA. In AD tissues, expression of MTf mRNA appeared in reactive microglial cells in the grey matter specifically associated with dense plaques. In liver tissues, immunohistochemistry using anti-p97 antibody demonstrated that sinusoids were positively stained. In addition, in situ hybridization histochemistry revealed that hepatocytes had positive signals. These results suggest that p97 expression in reactive microglial cells are closely related to AD pathology. These results also support the notion that p97, which appears elevated in the cerebral spinal fluid and serum of AD patients, originates in the reactive microglia associated with dense senile plaques. Thus, p97 is a unique cellular hallmark of AD and further suggests that metal transport mechanisms play a role in this disease.

The macrophage cell surface glycoprotein F4/80 is a highly glycosylated proteoglycan.

Molecules whose expression is limited to particular leukocyte populations are of interest since they may perform unique functions for these cells. We therefore examined the biochemical nature of the F4/80 molecule, which is expressed solely on macrophage and dendritic cell subpopulations. Our study clearly indicates that post-translational modifications, which can influence both a protein's structural and functional features, constitute a major component of the 160-kDa cell-surface F4/80 molecule. The F4/80 molecule is synthesized as a single polypeptide chain which acquires numerous intramolecular disulfide bonds and requires an extended time period (T1/2 = 60 min) for transport to an endoglycosidase H-resistant form. The F4/80 molecule contains extensive N-linked glycosylation which contributes approximately 40 kDa to the mature molecule. The N-linked carbohydrates are of the branched, complex type, containing repeating N-acetylglycosamine or N-acetyllactosamine units which mediate the reactivity of the F4/80 molecule with Datura stramonium lectin. O-linked glycosylation is also present and contributes approximately 10 kDa to the F4/80 molecule. Furthermore, the sialic acid modifications of the F4/80 molecule are primarily through alpha 2-6 linkages to galactose. Finally, we demonstrate that the F4/80 molecule is a proteoglycan modified by chondroitin sulfate glycosaminoglycans. In addition to clarifying the nature of the F4/80 molecule biochemically, these post-translational modifications have specific implications for molecular recognition processes. We conclude that the modifications of the F4/80 molecule may mediate cell-cell recognition, cell adhesion, or ligand binding independently of the F4/80 molecule protein core.

Expression of the W6/32 HLA epitope by cells of rat, mouse, human and other species: critical dependence on the interaction of specific MHC heavy chains with human or bovine beta 2-microglobulin.

The HLA class I epitope W6/32 is conformationally dependent on both heavy chain and beta 2-microglobulin (beta 2M). Previously, the W6/32 epitope has been detected in humans and other primates as well as from bovine sources. Two controversial reports suggest the W6/32 epitope is constitutively expressed by either normal or transformed murine cells expressing the Db allele. Here we show that the appearance of the W6/32 epitope in murine cells results from the association of either the Db or Kd gene products with either bovine or human beta 2M. We use congenic mouse strains and hybrid H-2 class I genes between Db and Kb to map the W6/32 epitope to particular amino acid residues in the alpha 2 domain. Subsequently, we show that beta 2M exchange is not confined to murine or human cells in vitro but can be detected after beta 2M injection into a mouse. The data presented suggests that beta 2M exchange takes place at the cell surface under physiological conditions and indicates that MHC class I heavy chains are in an equilibrium between the bound and unbound form of beta 2M.

IFN-gamma-induced recognition of the antigen-processing variant CMT.64 by cytolytic T cells can be replaced by sequential addition of beta 2 microglobulin and antigenic peptides.

The present study describes the functional nature of the MHC class I determinants expressed in CMT.64 cells and was undertaken to define and further analyze the deficiency in the cell line CMT.64 in the hope of elucidating the relative functional importance of constituent parameters in the recognition of these cells by CTL. We show that induction of Kb in CMT.64 cells with IFN-gamma results in molecules capable of presenting VSV epitopes to the appropriate CTL. However, cells untreated with IFN-gamma and infected with VSV are not recognized by VSV-specific CTL. This study reveals that beta 2 m3 is synthesized in limiting amounts in uninduced CMT.64 and becomes highly expressed after IFN-gamma induction. Thus, the limiting amount of beta 2 m expressed in uninduced cells may partially explain the inability of the cells to present viral components of CTL recognition. This concept is reinforced by the experiments identifying two functional effects upon the addition of immunogenic peptides to uninduced CMT.64 cells: at high peptide concentrations in excess of 5 nM, CMT.64 cells are recognized efficiently after 5 min of incubation; at the limiting peptide concentration of 500 pM, uninduced CMT.64 cells are only recognized providing beta 2m is added before or simultaneously with the antigenic peptide. BFA, an inhibitor of protein transport, and emetine, an inhibitor of protein synthesis, were used to show that at high peptide concentrations, 25 microM, recognition takes place after the peptide has stabilized the limited amount of newly arriving MHC/beta 2m complexes, devoid of peptides, at the cell surface of uninduced CMT.64 cells. These experiments thereby exclude the possibility that peptides are taken up into CMT.64 cells for assembly, transport and surface expression of functional MHC/beta 2m/peptide complexes. In summary, our data expands previous research showing the importance of exogenous beta 2m in sensitizing cells for CTL recognition with peptides added exogenously. These functional experiments also imply that the concentration of endogenous beta 2m may regulate the amount of MHC class I expressed at the cell surface and receptive to exogenous peptides. Finally, the phenotype of CMT.64 cells we describe provides evidence of the complexity of the Ag-presenting capacity of this cell line not previously identified in other studies on these cells, thus revising our understanding of the Ag-processing deficiency in these cells.

Analysis of lymphopoietic stem cells with a monoclonal antibody to the rat transferrin receptor.

A mouse monoclonal IgG2a antibody, designated MRC OX-26, is shown to be specific for the rat transferrin receptor, but does not block transferrin binding. The antibody labelled a myeloma, three leukaemia cell lines and normal dividing cells of various types, but also bound to a number of nondividing normal tissues. No labelling of lymphopoietic stem cells could be detected, even though approximately 25% of bone marrow and over 95% of fetal liver cells were clearly labelled.