Self-nonself discrimination by T cells

The alpha beta T cell receptor (TCR) recognizes antigens that are presented by major histocompatibility complex (MHC)-encoded cell surface molecules by binding to both the antigen and the MHC molecules. Discrimination of self from nonself antigens and MHC molecules is achieved by negative and positive selection of T cells in the thymus: potentially harmful T cells with receptors that bind to self antigens plus self MHC molecules are deleted before they can mount immune responses. In contrast, the maturation of useful T cells with receptors that bind foreign antigens plus self MHC molecules requires the binding of their receptor to MHC molecules on thymic epithelium in the absence of foreign antigen. The binding of the TCR to either class I or class II MHC molecules directs differentiation of the selected cells into either CD4-8+ (killer) or CD4+8- (helper) T cells, respectively.     

Conversion of a PI-anchored protein to an integral membrane protein by a single amino acid mutation

Qa-2, a cell-surface glycoprotein anchored by phosphatidylinositol (PI), is structurally related to the class I transplantation antigens H-2 K, D, and L, which are integral membrane glycoproteins. The predicted transmembrane segment of Qa-2 differs from those of H-2 K, D, and L by the presence of an aspartate in place of a valine at position 295. A single base change that replaced this aspartate with valine resulted in cell-surface Qa-2 molecules that were insensitive to hydrolysis by a PI-specific phospholipase C and more resistant to papain cleavage, properties shared by H-2D. Cells expressing Asp----Val mutant Qa-2 proteins were still able to attach a PI anchor to endogenous proteins such as Thy-1 and J11D. It therefore appears that this single amino acid change converts Qa-2 from a PI-linked form into an integral membrane protein.     

The silica-alumina system: stable and metastable equilibria at 1.0 atmosphere

Diffusion couples, consisting of sapphire and fused silica, which were annealed in the temperature range from 1678 degrees to 2003 degrees C and analyzed by electron beam microprobe, have provided data on the stable phase equilibrium of the silica-alumina system. Under stable equilibrium conditions, the intermediate compound of this system, mullite (3Al(2)O(3) . 2SiO(2)), melts incongruently at 1828 degrees +/- 10 degrees C and its solid solution field extends from 70.5 to 74.0 percent (by weight) alumina. The stable phase diagram is a composite of the two binary eutectic diagrams: silica-mullite in the absence of alumina and silica-alumina in the absence of mullite. Under metastable conditions, mullite melts congruently at approximately 1890 degrees +/- 10 degrees C and its solid solution field extends to approximately 83.2 percent (by weight) alumina.     

Prevention of allogeneic bone marrow graft rejection by H-2 transgene in donor mice

Rejection of bone marrow grafts in irradiated mice is mediated by natural killer (NK) cells and is controlled by genes linked to the major histocompatibility complex (MHC). It has, however, not been possible to identify the genes or their products. An MHC class I (Dd) transgene introduced in C57BL donors prevented the rejection of their bone marrow by NK cells in irradiated allogeneic and F1 hybrid mice expressing the Dd gene. Conversely, H-2Dd transgenic C57BL recipients acquired the ability to reject bone marrow from C57BL donors but not from H-2Dd transgenic C57BL donors. These results provide formal evidence that NK cells are part of a system capable of rejecting cells because they lack normal genes of the host type, in contrast to T cells, which recognize cells that contain abnormal or novel sequences of non-host type.     

Structure and specificity of a class II MHC alloreactive gamma delta T cell receptor heterodimer

Two distinct CD3-associated T cell receptors (TCR alpha beta and TCR gamma delta) are expressed in a mutually exclusive fashion on separate subsets of T lymphocytes. While the specificity of the TCR alpha beta repertoire for major histocompatibility complex (MHC) antigens is well established, the diversity of expressed gamma delta receptors and the ligands they recognize are less well understood. An alloreactive CD3+CD4-CD8- T cell line specific for murine class II MHC (Ia) antigens encoded in the I-E subregion of the H-2 gene complex was identified, and the primary structure of its gamma delta receptor heterodimer was characterized. In contrast to a TCR alpha beta-expressing alloreactive T cell line selected for similar specificity, the TCR gamma delta line displayed broad cross-reactivity for multiple distinct I-E-encoded allogeneic Ia molecules.     

A Synchrotron X-ray Study of a Solid-Solid Phase Transition in a Two-Dimensional Crystal

A measurement and interpretation on a molecular level of a phase transition in an ordered Langmuir monolayer is reported. The diagram of surface pressure (pi) versus molecular area of a monolayer of chiral (S)-[CF(3)-(CF(2))(9)-(CH(2))(2)-OCO-CH(2)-CH (NH(3)(+))CO(2)(-)] over water shows a change in slope at about pi(s)= 25 millinewtons per meter. Grazing-incidence x-ray diffraction and specular reflectivity measurements indicate a solid-solid phase transition at pi(s). The diffraction pattren at low pressures reveals two diffraction peaks of equal intensities, with lattice spacings d of 5.11 and 5.00 angstroms; these coalesce for pi >/=pi(s). Structural models that fit the diffraction data show that at pi> pi(s) the molecules pack in a two-dimensional crystal with the molecules aligned vertically. At pi < pi(s) there is a molecular tilt of 16 degrees +/- 7 degrees . Independent x-ray reflectivity data yield a tilt of 26 degrees +/- 7 degrees . Concomitant with the tilt, the diffraction data indicate a transition from a hexagonal to a distorted-hexagonal lattice. The hexagonal arrangement is favored because the -(CF(2))(9)CF(3) moiety adopts a helical conformation. Compression to 70 millinewtons per meter yields a unit cell with increased crystallinity and a coherence length exceeding 1000 angstroms.     

Lateral diffusion of visual pigment in photorecptor disk membranes

Visual pigment molecules are found to move transversely, but not longitudinally, in both rod and cone outer segments of mud puppy and frog. This is consistent with the idea that they are immersed in a two-dimensional fluid disk membrane. The diffusion coefficient for the motion is about 5 x 10(-9) square centimeters per second at 20 degrees C, corresponding to a root-mean-square molecular displacement of 0.3 micrometer in 1 second.     

Sodium and potassium activities in normal and "sodium-rich" frog skeletal muscle

In frog sartorius muscles immersed for 2 hours at 26 degrees C in normal Ringer solution, the intrafiber potassium concentration, C(K)(in millimoles per liter), was 123 +/- 2 (mean value plus or minus standard error), and the potassium activity, a(K) (in millimoles per liter), was 90 +/- 1.0. The corresponding sodium concentration and activity were 20 +/- 1 and 6.5 +/- 0.4, respectively. After overnight immersion in K+-free Ringer solution the values were: CK, 97 +/- 2; aK, 81.5 +/- 1.6; CNa, 47 +/- 2; and aNa, 11.2 +/- 0.6. The changes in aK and aNa during storage were not consistent with an exchange between predominantly "free" fiber K+ and external Na+. These results suggest that the Na+ taken up during overnight immersion largely replaced adsorbed or sequestered K+ in the fibers.     

Crystal Structure of the High-Pressure Phase of Solid CO2

X-ray diffraction study of solid CO(2) at room temperature has shown that the powder pattern of the high-pressure phase, which supersedes the low-pressure cubic Pa3 phase at about 10 gigapascals, is consistently interpreted in terms of an orthorhombic Cmca structure. The orthorhombic cell at 11.8 gigapascals has dimensions of 4.330 +/- 0.015, 4.657 +/- 0.005, 5.963 +/- 0.009 angstroms for its a, b, and c faces, respectively, and a volume of 120.3 +/- 0.5 cubic angstroms. Four molecules contained in the unit cell are located at the base-centered positions with their molecular axes inclined at about 52 degrees with respect to the crystallographic c axis. The volume change associated with the Pa3-Cmca transition is close to zero. The structural dimensions obtained for the high-pressure crystalline phase of CO(2) are of great importance for a theoretical understanding of the role of intermolecular interactions, including quadrupole-quadrupole interactions, in molecular condensation.     

Mariner 9 celestial mechanics experiment: gravity field and pole direction of Mars

Analysis of the Mariner 9 radio-tracking data shows that the Martian gravity field is rougher than that of Earth or the moon, and that the accepted direction of Mars's rotation axis is in error by about 0.5 degrees . The new value for the pole direction for the epoch 1971.9, referred to the mean equatorial system of 1950.0, is right ascension alpha= 317.3 degrees +/- 0.3 degrees , declination delta = 52.6 degrees +/- 0.2 degrees . The values found for the coefficients of the low-order harmonics of Mars's gravity field are as follows: J(2)=(1.96+/-0.01)x10(-3), referred to an equatorial radius of 3394 kilometers; C(22) = -(5 +/- 1) x 10(-5); and S(22) = (3 +/- 1) x 10(-5). The value for J(2) is in excellent agreement with the result from, Wilkins' analysis of the observations of Phobos. The other two coefficients imply a value of (2.5 +/- 0.5) x 10(-4) for the fractional difference in the principal equatorial moments of inertia; the axis of the minimum moment passes near 105 degrees W.     

The MHC-binding and gp120-binding functions of CD4 are separable

CD4 is a cell surface glycoprotein that is thought to interact with nonpolymorphic determinants of class II major histocompatibility (MHC) molecules. CD4 is also the receptor for the human immunodeficiency virus (HIV), binding with high affinity to the HIV-1 envelope glycoprotein, gp120. Homolog-scanning mutagenesis was used to identify CD4 regions that are important in class II MHC binding and to determine whether the gp120 and class II MHC binding sites of CD4 are related. Class II MHC binding was abolished by mutations in each of the first three immunoglobulin-like domains of CD4. The gp120 binding could be abolished without affecting class II MHC binding and vice versa, although at least one mutation examined reduced both functions significantly. These findings indicate that, while there may be overlap between the gp120 and class II MHC binding sites of CD4, these sites are distinct and can be separated. Thus it should be possible to design CD4 analogs that can block HIV infectivity but intrinsically lack the ability to affect the normal immune response by binding to class II MHC molecules.     

Transport of peptide-MHC class II complexes in developing dendritic cells

Major histocompatibility complex class II (MHC II) molecules capture peptides within the endocytic pathway to generate T cell receptor (TCR) ligands. Immature dendritic cells (DCs) sequester intact antigens in lysosomes, processing and converting antigens into peptide-MHC II complexes upon induction of DC maturation. The complexes then accumulate in distinctive, nonlysosomal MHC II+ vesicles that appear to migrate to the cell surface. Although the vesicles exclude soluble lysosomal contents and antigen-processing machinery, many contain MHC I and B7 costimulatory molecules. After arrival at the cell surface, the MHC and costimulatory molecules remain clustered. Thus, transport of peptide-MHC II complexes by DCs not only accomplishes transfer from late endocytic compartments to the plasma membrane, but does so in a manner that selectively concentrates TCR ligands and costimulatory molecules for T cell contact.     

Crystal Structure of Benzene II at 25 Kilobars

Crystals of a high-pressure form of benzene (benzene 11) were grown in the diamond-anvil pressure cell at elevated temperature and pressure from the transition of solid I to solid II. X-ray precession data were obtained from a single-crystal in the high-pressure cell. At 21 degrees C and about 25 kilobars, benzene II crystallizes in the monoclinic system with a = 5.417 +/- 0.005 angstroms (S.D.), b = 5.376 +/- 0.019 angstroms, c = 7.532 +/- 0.007 angstroms, beta = 110.00 degrees +/- 0.08 degrees , space group P2(1)/ c, Pc= 1.26 grams per cubic centimeter. The crystal structure was solved by generating all possible molecular packing configurations and calculating structure factors, reliability factors, and packing energies for each configuration. This procedure produced a unique solution for the molecular packing of benzene II.     

Field measurement of slow metamorphic reaction rates at temperatures of 500 degrees to 600 degrees C

High-temperature metamorphic reaction rates were measured using strontium isotopic ratios of garnet and whole rock from a field site near Simplon Pass, Switzerland. For metamorphic conditions of cooling from 612 degrees +/- 17 degrees C to 505 degrees +/- 15 degrees C at pressures up to 9.1 kilobars, the inferred bulk fluid-rock exchange rate is 1.3(-0.4)(+1.1) x 10(-7) grams of solid reacted per gram of solid per year, several orders of magnitude lower than laboratory-based estimates. The inferred reaction rate suggests that mineral chemistry may lag the evolving conditions in Earth's crust during mountain building.     

Crystal Structure of the Solid Electrolyte(C5H5NH) Ag5I6 at -30{degrees}C

The crystal structure of pyridinium hexaiodopentaargentate, (C(5)H(5)NH) Ag(5)l(6), is unique among those of the halide and chalcogenide solid electrolytes in that face-sharing iodide octahedra as well as face-sharing tetrahedra and face-sharing between octahedra and tetrahedra provide the paths for silver ion transport. There are two formula units in a hexagonal cell, space group P6/mcc (D6h(2)). At -30 degrees C, the lattice constants are a = 11.97 +/- 0.02, c = 7.41 +/- 0.01 A. The structure has three sets of sites for the silver ions. At -30 degrees C two of these sets are apparently filled with the ten silver ions per unit cell, while the third set of tetrahedrally coordinated general positions is empty. Therefore, the conductivity at this temperature is limited by the thermal excitation of the silver ions into the empty tetrahedra.     

三倍体西瓜种子萌发障碍及吸水促萌技术研究

 为克服三倍体西瓜种子的萌发障碍，采用过氧化氢结合吸水促萌技术处理了6个无籽西瓜品种。种子用0.1%过氧化氢、水、水加气浸泡2 h，并在饱和相对湿度下保持24 h、34 h和48 h后回干，3种处理都显著提高了种子的萌发速度和萌发能力，发芽率分别提高了38%～56%，过氧化氢浸种结合34 h和48 h培养为最佳处理组合。吸水促萌提高了所有参试品种的萌发能力，提高程度从14%至66%因品种而异。促萌后的种子用适当的速度回干保存后仍能保持促萌效果。用软-X射线对无籽西瓜种子和胚结构的分析表明，萌发障碍的形成主要与种皮有关，吸水促萌提高了种胚的活力，克服了种皮造成的萌发障碍。     

The minimal number of class II MHC-antigen complexes needed for T cell activation

Major histocompatibility complex (MHC) molecules are exposed to large quantities of self and nonself antigens. It is not known what fraction of MHC molecules needs to be occupied by antigen to induce a T cell response. A quantitative study of naturally processed antigen indicated that T cells could be activated when only 0.03 percent of the total I-Ed purified from antigen-presenting cells (APCs) was occupied with antigen. B cells and macrophages processed hen egg lysozyme (HEL) with different efficiencies, but similar degrees of occupancy were required for T cell stimulation. Higher occupancy was needed for I-Ed-transfected L cells, possibly reflecting the requirement for other accessory molecules for efficient APC-T cell interaction.     

Coral record of equatorial sea-surface temperatures during the penultimate deglaciation at huon peninsula

Uplifted coral terraces at Huon Peninsula, Papua New Guinea, preserve a record of sea level, sea-surface temperature, and salinity from the penultimate deglaciation. Remnants have been found of a shallow-water reef that formed during a pause, similar to the Younger Dryas, in the penultimate deglaciation at 130,000 +/- 2000 years ago, when sea level was 60 to 80 meters lower than it is today. Porites coral, which grew during this period, has oxygen isotopic values and strontium/calcium ratios that indicate that sea-surface temperatures were much cooler (22 degrees +/- 2 degreesC) than either Last Interglacial or present-day tropical temperatures (29 degrees +/- 1 degreesC). These observations provide further evidence for a major cooling of the equatorial western Pacific followed by an extremely rapid rise in sea level during the latter stages of Termination II.     

The Density of Hydrous Magmatic Liquids

Density measurements on several hydrous (相似文献   

Bulk isolation in nonaqueous media of nuclei from lyophilized cells

Intact lyophilized nuclei are obtainable from a variety of tissues, either in situ or in culture, by freezing at -156 degrees C, drying at -25 degrees C, and mechanical disassociation in glycerol at 2 degrees C. Centrifugal separation of nuclei is accomplished in an 85 : 15 by volume mixture of glycerol and 3-chloro-1,2 propanediol at 2 degrees C. The method gives homogeneous nuclear preparations in high yield with preservation of labile and water-soluble constituents.