Structural Evidence of T Cell Xeno-reactivity in the Absence of Molecular Mimicry

The T cell receptor (TCR), from a xeno-reactive murine cytotoxic T lymphocyte clone AHIII12.2, recognizes murine H-2Db complexed with peptide p1027 (FAPGVFPYM), as well as human HLA-A2.1 complexed with peptide p1049 (ALWGFFPVL). A commonly proposed model (the molecular mimicry model) used to explain TCR cross-reactivity suggests that the molecular surfaces of the recognized complexes are similar in shape, charge, or both, in spite of the primary sequence differences. To examine the mechanism of xeno-reactivity of AHIII12.2, we have determined the crystal structures of A2/p1049 and Db/p1027 to 2.5 Å and 2.8 Å resolution, respectively. The crystal structures show that the TCR footprint regions of the two class I complexes are significantly different in shape and charge. We propose that rather than simple molecular mimicry, unpredictable arrays of common and differential contacts on the two class I complexes are used for their recognition by the same TCR

Advances in shape measurement in the digital world

The importance of particle shape in terms of its effects on the behaviour of powders and other particulate systems has long been recognised, but particle shape information has been rather difficult to obtain and use until fairly recently, unlike its better-known counterpart, particle size. However, advances in computing power and 3D image acquisition and analysis techniques have resulted in major progress being made in the measurement, description and application of particle shape information in recent years. Because we are now in a digital era, it is fitting that many of these advanced techniques are based on digital technology. This review article aims to trace the development of these new techniques, highlight their contributions to both academic and practical applications, and present a perspective for future developments

Bridging the Gap Between Random Microstructure and 3D Meshing

There are different ways to mathematically represent three-dimensional (3D) heterogeneous material microstructures. It is desirable to pick the representation that best bridges the gap between heterogeneous microstructure and computer-aided engineering finite element analysis. 3D cubic meshes of brick (voxel) elements can be generated for digital structures (e.g., from X-ray CT), but this work describes how simplified tetrahedral meshes, more suitable for complex mechanical problems, including crack generation, can be implemented. The mathematical representations of heterogeneous material structures considered in this paper include (1) 3D digital image microstructure, exemplified by the Virtual Cement and Concrete Testing Laboratory (VCCTL); (2) X-ray computed tomography (CT) images stacked into a 3D digital image; and (3) real-shaped sand and gravel particles, represented by spherical harmonic series, randomly placed into a 3D box to make a virtual concrete microstructure. The approach used involves a topological structure suitable for stereo lithography file (STL) representation and the development of algorithms for topological and geometric data processing to obtain a 3D simplified tetrahedral mesh that incorporates the random material structure. Mesh simplification is obtained through a set of remeshing tools to improve element quality and reduce the number of elements. During the mesh generation procedure, we combine both the aggregates and the cement paste matrix to ensure perfectly coinciding nodes at their interface. Based on the proposed procedures, a simplified surface and volume mesh of heterogeneous systems can be generated with data consistencies (e.g., no overlapping, no gap at the interfaces)

Linear elastic properties of 2D and 3D models of porous materials made from elongated objects

International audiencePorous materials are formed in nature and by man by many different processes. The nature of the pore space, which is usually the space left over as the solid backbone forms, is often controlled by the morphology of the solid backbone. In particular, sometimes the backbone is made from the random deposition of elongated crystals, which makes analytical techniques particularly difficult to apply. This paper discusses simple two- and three-dimensional porous models in which the solid backbone is formed by different random arrangements of elongated solid objects (bars/crystals). We use a general purpose elastic finite element routine designed for use on images of random porous composite materials to study the linear elastic properties of these models. Both Young's modulus and Poisson's ratio depend on the porosity and the morphology of the pore space, as well as on the properties of the individual solid phases. The models are random digital image models, so that the effects of statistical fluctuation, finite size effect and digital resolution error must be carefully quantified. It is shown howto average the numerical results over random crystal orientation properly. The relations between two and three dimensions are also explored, as most microstructural information comes from two-dimensional images, while most real materials and experiments are three dimensional

Local elastic moduli of simple random composites computed at different length scales

Techniques like nanoindentation and atomic force microscopy can estimate the local elastic moduli in a region surrounding the probe used. For composites with phase regions much larger than the size of the probe, these procedures can identify the phases via their different elastic moduli but identifying phase regions that are on the same size scale as the indent is more problematic. This paper looks at three random 3D 8003 voxel composite models, each consisting of a matrix and spherical inclusions. One model has non-overlapping spheres and two models have overlapping spheres, with two and three distinct phases. The linear elastic problem is solved for each microstructure, and histograms are made of the local Young’s moduli over a number of sub-volumes (SVs), averaged over progressively larger SVs. The number and shape of histogram peaks change from N delta functions, where N is the number of elastically distinct phases, at the 1 voxel SV limit, to a single delta function located at the value of the effective global Young’s modulus, when the SV equals the unit cell volume. The phase volume fractions are also tracked for each bin in the Young’s modulus histograms, showing the phase make-up of bin in the histogram. There are clear differences seen between the non-overlapping and three-phase overlapping models and the two-phase overlapping sphere model, because of different size microstructural features, characterized by the average value of size as computed by the W(q) function. In the three-phase model, a peak that is originally all phase 3 persists at its same location, but as the size of the SVs increase, it is made up of a mixture of phases, so that it cannot be identified with a single phase even though it remains a clear peak. These results give some guidance as to what probe size might be useful in distinguishing different phases by local elastic moduli measurements, and how the length scales of the probe and the microstructure interact.ISSN:1359-5997ISSN:0025-5432ISSN:1871-687

Three dimensional shape analysis of concrete aggregate fines produced by VSI crushing

We studied the 3-D shape of concrete aggregate fines with particle sizes between 3 µm and 250 µm produced by high-speed vertical shaft impact (VSI) crushing of rock types from 10 different quarries representing a wide range of local Norwegian geology with respect to mineralogy and mechanical properties. This included igneous (intrusive and extrusive), metamorphic, and sedimentary rocks that were both mono- and multi-mineralic. VSI crushing seems to be able to generate concrete aggregate fines of very similar equidimensional mean shape characteristics for the whole analysed size range, independent of the mineralogical composition of the rocks included in the study. The effect on normalising the average particle shape was somewhat lower for the particle size range smaller than about 15 µm, where there seems to be a greater influence of the crystallographic structure of the individual minerals. Particles of the rock type containing the highest mica content (5.5 %, by mass) had the least equidimensional shape. The most equidimensional shape in a given particle size range was found for both limestone rock types that were analysed. A new shape parameter, the micro-Flakiness Index (µFI), has been proposed to characterize the shape of the fine crushed concrete aggregate particles to enable practical use of shape parameters