Alternative pathway activation of T cells by binding of CD2 to its cell-surface ligand

Activation of resting T lymphocytes is initiated by the interaction of cell-surface receptors with their corresponding ligands. In addition to activation through the CD3 (T3)-Ti antigen-receptor complex, recent experiments have demonstrated induction of T-cell proliferation through the CD2 (T11) molecule, traditionally known as the erythrocyte(E)-receptor, through which T cells can bind red blood cells (RBC). This 'alternative pathway' of T-cell activation was observed in vitro in response to combinations of anti-CD2 monoclonal antibodies (mAbs) that bind to distinct epitopes of CD2, such as mAbs against T11(2) plus T11(3). The physiological importance of this activation pathway can be assessed only by studying the effects of a naturally occurring ligand of CD2 on T-cell activation. We have recently described such a ligand, a glycoprotein of apparent relative molecular mass 42,000 (Mr 42K) that is expressed on all blood cells and some other tissues. Here we demonstrate that binding of this cell surface molecule, termed T11 target structure or T11TS, to CD2 (T11) induces reactivity in resting T cells to a mitogenic stimulus given by a mAb to the T11(3) determinant or by submitogenic concentrations of anti-T11(2+3) mAbs. Thus, one of the signals required for T-cell activation through the alternative pathway is provided by the interaction of CD2 with a naturally occurring complementary cell-surface molecule.     

The precursor of Alzheimer's disease amyloid A4 protein resembles a cell-surface receptor

Alzheimer's disease is characterized by a widespread functional disturbance of the human brain. Fibrillar amyloid proteins are deposited inside neurons as neurofibrillary tangles and extracellularly as amyloid plaque cores and in blood vessels. The major protein subunit (A4) of the amyloid fibril of tangles, plaques and blood vessel deposits is an insoluble, highly aggregating small polypeptide of relative molecular mass 4,500. The same polypeptide is also deposited in the brains of aged individuals with trisomy 21 (Down's syndrome). We have argued previously that the A4 protein is of neuronal origin and is the cleavage product of a larger precursor protein. To identify this precursor, we have now isolated and sequenced an apparently full-length complementary DNA clone coding for the A4 polypeptide. The predicted precursor consists of 695 residues and contains features characteristic of glycosylated cell-surface receptors. This sequence, together with the localization of its gene on chromosome 21, suggests that the cerebral amyloid deposited in Alzheimer's disease and aged Down's syndrome is caused by aberrant catabolism of a cell-surface receptor.     

Reciprocal regulation of CD4/CD8 expression by SWI/SNF-like BAF complexes

Thymic development produces two sub-lineages of T cells expressing either CD4 or CD8 co-receptors that assist antibody production and mediate cell killing, respectively. The mechanisms for mutually exclusive co-receptor expression remain poorly defined. We find that mutations in the high mobility group (HMG) domain of BAF57--a DNA-binding subunit of the mammalian SWI/SNF-like chromatin-remodelling BAF complexes--or in the BAF complex ATPase subunit Brg, impair both CD4 silencing and CD8 activation. Brg is haploinsufficient for CD8 activation, but not for CD4 silencing, whereas BAF57 mutations preferentially impair CD4 silencing, pointing to target- and subunit-specific mechanisms of chromatin remodelling. BAF complexes directly bind the CD4 silencer, but the BAF57 HMG domain is dispensable for tethering BAF complexes to the CD4 silencer or other chromatin loci in vivo, or for remodelling reconstituted templates in vitro, suggesting that chromatin remodelling in vivo requires HMG-dependent DNA bending. These results indicate that BAF complexes contribute to lineage bifurcation by reciprocally regulating lineage-specific genes, reminiscent of the role of the yeast SWI/SNF complex in mediating mating-type switching.     

Suppressor effects and cyclic AMP accumulation by the CD29 molecule of CD4+ lymphocytes

Integrins are a superfamily of related molecules whose function, where known, is to mediate adhesion. The so-called very-late-activation antigen (VLA) family includes at least five distinct heterodimers, each composed of a unique alpha-subunit non-covalently associated with a common beta-subunit. Several members of the family have been shown to bind extracellular matrix proteins, but the function of VLA-4 is so far unknown. VLA-4 is the only member of the family detected on thymocytes and resting T cells. We show here that an antibody which recognizes the beta-subunit of VLA-4 (CD29) on T cells can inhibit CD4+ cell proliferation triggered by CD2 or CD3, and that binding of this antibody to activated T cells leads to an increase in cyclic AMP levels which is comparable to that elicited by forskolin. These negative signalling effects are unique to this antibody: other CD29 antibodies do not affect the growth of activated CD4 cells but enhance the proliferation of whole T cell populations and abrogate the suppressive effects of mitomycin-treated CD8 cells on CD4-cell growth. Taken together, our results indicate that VLA-4 functions in cell-cell interactions and that it is the target for the suppressive effects of CD8 cells on CD4 cells.     

Different patterns of peripheral migration by memory CD4+ and CD8+ T cells

Infections localized to peripheral tissues such as the skin result in the priming of T-cell responses that act to control pathogens. Activated T cells undergo migrational imprinting within the draining lymph nodes, resulting in memory T cells that provide local and systemic protection. Combinations of migrating and resident memory T cells have been implicated in long-term peripheral immunity, especially at the surfaces that form pathogen entry points into the body. However, T-cell immunity consists of separate CD4(+) helper T cells and CD8(+) killer T cells, with distinct effector and memory programming requirements. Whether these subsets also differ in their ability to form a migrating pool involved in peripheral immunosurveillance or a separate resident population responsible for local infection control has not been explored. Here, using mice, we show key differences in the migration and tissue localization of memory CD4(+) and CD8(+) T cells following infection of the skin by herpes simplex virus. On resolution of infection, the skin contained two distinct virus-specific memory subsets; a slow-moving population of sequestered CD8(+) T cells that were resident in the epidermis and confined largely to the original site of infection, and a dynamic population of CD4(+) T cells that trafficked rapidly through the dermis as part of a wider recirculation pattern. Unique homing-molecule expression by recirculating CD4(+) T effector-memory cells mirrored their preferential skin-migratory capacity. Overall, these results identify a complexity in memory T-cell migration, illuminating previously unappreciated differences between the CD4(+) and CD8(+) subsets.     

Interleukin-4 mediates CD8 induction on human CD4+ T-cell clones

CD4 and CD8 antigens are simultaneously expressed on most of the cortical thymocytes, that weakly express the T-cell antigen receptor(TCR)/CD3 complex. Mature peripheral T cells, however, strongly express the TCR complex and are positive for either CD4 or CD8. Nevertheless, a small percentage of peripheral CD3+ T cells express CD4 and CD8 simultaneously. These mature, double positive cells could be intermediates between CD4+CD8+ thymocytes and mature, single positive T cells, or they may originate from single positive T cells that acquire either CD4 or CD8. Here we report that activation and culturing of cloned CD4+ T cells in interleukin-4 (IL-4), results in the acquisition of CD8 due to its de novo synthesis. The IL-4-induced co-expression of CD8 on CD4+ T cells is reversible, in that CD8 disappeared from double positive T-cell clones isolated in IL-4, when they were cultured in IL-2. CD8 induced by IL-4 can be functional as a monoclonal antibody to CD8 inhibited anti-CD3-mediated cytotoxicity by a double positive T-cell clone.     

HIV infection is blocked in vitro by recombinant soluble CD4

The T-cell surface glycoprotein, CD4 (T4), acts as the cellular receptor for human immunodeficiency virus, type 1 (HIV-1), the first member of the family of viruses that cause acquired immunodeficiency syndrome. HIV recognition of CD4 is probably mediated through the virus envelope glycoprotein (gp120) as shown by co-immunoprecipitation of CD4 and gp120 (ref.5) and by experiments using recombinant gp120 as a binding probe. Here we demonstrate that recombinant soluble CD4(rsT4) purified from the conditioned medium of a stably transfected Chinese hamster ovary cell line is a potent inhibitor of both virus replication and virus-induced cell fusion (syncytium formation). These results suggest that rsT4 is sufficient to bind HIV, and that it represents a potential anti-viral therapy for HIV infection.     

Prevention of HIV-1 IIIB infection in chimpanzees by CD4 immunoadhesin

The first step in infection by the human immunodeficiency virus (HIV) is the specific binding of gp120, the envelope glycoprotein of HIV, to its cellular receptor, CD4. To inhibit this interaction, soluble CD4 analogues that compete for gp120 binding and block HIV infection in vitro have been developed. To determine whether these analogues can protect an uninfected individual from challenge with HIV, we used the chimpanzee model system of cell-free HIV infection. Chimpanzees are readily infected with the IIIB strain of HIV-1, becoming viraemic within about 4-6 weeks of challenge, although they do not develop the profound CD4+ T-cell depletion and immunodeficiency characteristic of HIV infection in humans. CD4 immunoadhesin (CD4-IgG), a chimaeric molecule consisting of the N-terminal two immunoglobulin-like regions of CD4 joined to the Fc region of human IgG1, was selected as the CD4 analogue for testing because it has a longer half-life than CD4, contributed by the IgG Fc portion of the molecule. In humans, this difference results in a 25-fold increased concentration of CD4-IgG in the blood compared with recombinant CD4. Here we report that pretreatment with CD4-IgG can prevent the infection of chimpanzees with HIV-1. The need for a preventative agent is particularly acute in perinatal HIV transmission. As recombinant CD4-IgG, like the parent IgG molecule, efficiently crosses the primate placenta, it may be possible to set up an immune state in a fetus before HIV transfer occurs, thus preventing infection.     

Signal for T-cell differentiation to a CD4 cell lineage is delivered by CD4 transmembrane region and/or cytoplasmic tail.

Mature T cells express either CD4 or CD8 on their surface. Most helper T cells express CD4, which binds to class II major histocompatibility complex (MHC) proteins, and most cytotoxic T cells express CD8, which binds to class I MHC proteins. In the thymus, mature CD4+CD8- and CD4-CD8+ T cells expressing alpha beta T-cell antigen receptors (TCR) develop from immature thymocytes through CD4+CD8+ alpha beta TCR+ intermediates. Experiments using mice transgenic for alpha beta TCR suggest that the specificity of the TCR determines the CD4/CD8 phenotype of mature T cells. These results, however, do not indicate how a T cell differentiates into the CD4 or CD8 lineage. Here we show that the CD4 transmembrane region and/or cytoplasmic tail mediates the delivery of a specific signal that directs differentiation of T cells to a CD4 lineage. We generated transgenic mice expressing a hybrid molecule composed of the CD8 alpha extracellular domains linked to the CD4 transmembrane region and cytoplasmic tail. We predicted that this hybrid molecule would bind to class I MHC proteins through the extracellular domains but deliver the intracellular signals characteristic of CD4. By crossing our transgenic mice with mice expressing a transgenic alpha beta TCR specific for a particular antigen plus class I MHC protein, we were able to express the hybrid molecule in developing thymocytes expressing the class I MHC-restricted TCR. Our results show that the signal transduced by the hybrid molecule results in the differentiation of immature thymocytes expressing a class I-restricted TCR into mature T cells expressing CD4.     

O-GlcNAc丝氨酸单体的合成

O-GlcNAc是最常见的翻译后修饰之一,几乎在从原始细菌到哺乳动物和植物的所有的生物系统中都能发现.参考Schmidt的合成方法,利用乙硫基3,4,6-三-O-乙酰基-2-脱氧-2-(2,2,2-三氯乙氧)甲酰胺基-β-D-葡萄吡喃糖苷作为糖苷供体,合成了O-GlcNAc修饰丝氨酸单体Fmoc-Ser(Ac3-β-O-GlcNAc)-OH.得到的反应中间体和最终产物,通过电喷雾质谱(ESI-MS)鉴定.     

Biological properties of a CD4 immunoadhesin

Molecular fusions of CD4, the receptor for human immunodeficiency virus (HIV), with immunoglobulin (termed CD4 immunoadhesins) possess both the gp120-binding and HIV-blocking properties of recombinant soluble CD4, and certain properties of IgG, notably long plasma half-life and Fc receptor binding. Here we show that a CD4 immunoadhesin can mediate antibody-dependent cell-mediated cytotoxicity (ADCC) towards HIV-infected cells, although, unlike natural anti-gp120 antibodies, it does not allow ADCC towards uninfected CD4-expressing cells that have bound soluble gp120 to the CD4 on their surface. In addition, CD4 immunoadhesin, like natural IgG molecules, is efficiently transferred across the placenta of a primate. These observations have implications for the therapeutic application of CD4 immunoadhesins, particularly in the area of perinatal transmission of HIV infection.     

Interconversion of CD45R subsets of CD4 T cells in vivo

T lymphocytes express multiple forms of the leukocyte common antigen CD45, transcribed by alternative usage of leukocyte-common antigen exons 4-6. Species-specific monoclonal antibodies against restricted epitopes (CD45R) of the antigen subdivide CD4 T cells into reciprocal subsets expressing either the high molecular weight isoforms CD45RA or RB or a molecule in which exons 4-6 have been spliced out (CD45R0). CD45R+ or RB+ CD4 T cells are potent in graft-versus-host reactions, and interleukin-2 related activities, whereas the CD45R0+ subset responds in vitro to recall antigens and provides help for antibody synthesis. It is unclear whether CD45R subsets derive from separate lineages, or are products of unidirectional or reversible differentiation. We show by transferring CD45R+ or CD45R- allotype-marked CD4 T cells into athymic nude rats that both subsets routinely generate cells of the opposite phenotype with a function that follows phenotype, not parentage. The recent equation of CD45R subsets as maturation stages representing 'naive' and 'memory' T cells is difficult to reconcile with this finding.     

Positive selection of CD4+ thymocytes controlled by MHC class II gene products

The mature T-cell antigen receptor repertoire is characterized by lack of reactivity to self-components as well as by preferential reactivity to foreign antigens in the context of polymorphic self-proteins encoded within the major histocompatibility complex. Whereas the former characteristic (referred to as negative selection or tolerance) is associated with intrathymic deletion of T cells expressing T-cell antigen receptor beta-chain variable (V beta) domains, which confer a preferential reactivity to self antigens, the existence of the latter (referred to as positive selection or MHC restriction) has so far only been inferred indirectly from functional studies. We show here that intrathymic deletion of V+beta 6 T cells (reactive with a self-antigen encoded by the Mlsa locus) is controlled by polymorphic MHC class II determinants. Furthermore, in mice lacking expression of Mlsa, the same class II MHC loci control the frequency of occurrence of V+beta 6 cells among mature CD4+ T lymphocytes. These data are direct evidence for positive selection by MHC determinants in the thymus in unmanipulated animals.     

Selective killing of HIV-infected cells by recombinant human CD4-Pseudomonas exotoxin hybrid protein

It is projected that in the absence of effective therapy, most individuals infected with human immunodeficiency virus (HIV) will develop acquired immune deficiency syndrome (AIDS) and ultimately succumb to a combination of opportunistic microbial infections, malignancies and direct pathogenic effects of the virus. Anti-viral agents, immunomodulators, and inhibitors of specific HIV functions are being tested as potential treatments to alleviate the high morbidity and mortality. An alternative therapeutic concept involves the development of cytotoxic agents that are targeted to kill HIV-infected cells. Here we describe the purification and characterization of a recombinant protein produced in Escherichia coli that contains the HIV-binding portion of the human CD4 molecule linked to active regions of Pseudomonas exotoxin A. This hybrid protein displays selective toxicity toward cells expressing the HIV envelope glycoprotein and thus represents a promising novel therapeutic agent for the treatment of AIDS.     

Chemokines enhance immunity by guiding naive CD8+ T cells to sites of CD4+ T cell-dendritic cell interaction

CD8+ T cells have a crucial role in resistance to pathogens and can kill malignant cells; however, some critical functions of these lymphocytes depend on helper activity provided by a distinct population of CD4+ T cells. Cooperation between these lymphocyte subsets involves recognition of antigens co-presented by the same dendritic cell, but the frequencies of such antigen-bearing cells early in an infection and of the relevant naive T cells are both low. This suggests that an active mechanism facilitates the necessary cell-cell associations. Here we demonstrate that after immunization but before antigen recognition, naive CD8+ T cells in immunogen-draining lymph nodes upregulate the chemokine receptor CCR5, permitting these cells to be attracted to sites of antigen-specific dendritic cell-CD4+ T cell interaction where the cognate chemokines CCL3 and CCL4 (also known as MIP-1alpha and MIP-1beta) are produced. Interference with this actively guided recruitment markedly reduces the ability of CD4+ T cells to promote memory CD8+ T-cell generation, indicating that an orchestrated series of differentiation events drives nonrandom cell-cell interactions within lymph nodes, optimizing CD8+ T-cell immune responses involving the few antigen-specific precursors present in the naive repertoire.     

Inhibition of HIV replication by pokeweed antiviral protein targeted to CD4+ cells by monoclonal antibodies

Functional impairment and selective depletion of CD4+ T cells, the hallmark of AIDS, are at least partly caused by human immunodeficiency virus (HIV-1) type 1 binding to the CD4 molecule and infecting CD4+ cells. It may, therefore, be of therapeutic value to target an antiviral agent to CD4+ cells to prevent infection and to inhibit HIV-1 production in patients' CD4+ cells which contain proviral DNA. We report here that HIV-1 replication in normal primary CD4+ T cells can be inhibited by pokeweed antiviral protein, a plant protein of relative molecular mass 30,000, which inhibits replication of certain plant RNA viruses, and of herpes simplex virus, poliovirus and influenza virus. Targeting pokeweed antiviral protein to CD4+ T cells by conjugating it to monoclonal antibodies reactive with CD5, CD7 or CD4 expressed on CD4+ cells, increased its anti-HIV potency up to 1,000-fold. HIV-1 replication is inhibited at picomolar concentrations of conjugates of pokeweed antiviral protein and monoclonal antibodies, which do not inhibit proliferation of normal CD4+ T cells or CD4-dependent responses. These conjugates inhibit HIV-1 protein synthesis and also strongly inhibit HIV-1 production in activated CD4+ T cells from infected patients.