Functional cloning of ICAM-2, a cell adhesion ligand for LFA-1 homologous to ICAM-1

The leukocyte adhesion molecule LFA-1 mediates a wide range of lymphocyte, monocyte, natural killer cell, and granulocyte interactions with other cells in immunity and inflammation. LFA-1 (CD11a/CD18) is a receptor for intercellular adhesion molecule 1 (ICAM-1, CD54), a surface molecule which is constitutively expressed on some tissues and induced on other in inflammation. Induction of ICAM-1 on epithelial cells, endothelial cells and fibroblasts mediates LFA-1-dependent adhesion of lymphocytes. Several lines of evidence have suggested the existence of a second LFA-1 ligand: homotypic adhesion of one cell line was inhibited by a monoclonal antibody to LFA-1, but not by one to ICAM-1; there exists an LFA-1-dependent, ICAM-1-independent pathway of adhesion to endothelial cells; and also, there are some types of target cells in which LFA-1-dependent T-lymphocyte adhesion and lysis are independent of ICAM-1. We have cloned this second ligand, designated ICAM-2, using a novel method for identifying ligands of adhesion molecules. ICAM-2 is an integral membrane protein with two immunoglobulin-like domains, whereas ICAM-1 has five. Remarkably, ICAM-2 is much more closely related to the two most N-terminal domains of ICAM-1 (34% identity) than either ICAM-1 or ICAM-2 is to other members of the immunoglobulin superfamily, demonstrating the existence of a subfamily of immunoglobulin-like ligands that bind the same integrin receptor.     

Purification and structural characterisation of human HLA-linked B-cell antigens.

The human B cell-specific alloantigen which is closely linked genetically to HLA contains two non-covalently associated, sialogycoprotein subunits of molecular weight (MW) 29,000 (p29), and 34,000 (p34). Although p29 and p34 have different amino-terminal sequences, their tyrosine peptide maps indicate considerable similarity in other portions of their polypeptide chains. Thus the genes for their proteins may have evolved by duplication of a common ancestral gene. Another lymphocyte cell surface protein of MW 16,000 (p16) has also been characterised. Both p16 and p44 (the heavy chain of HLA-A,B antigens) have been compared with p29 and p34.     

Structural basis for allostery in integrins and binding to fibrinogen-mimetic therapeutics

Integrins are important adhesion receptors in all Metazoa that transmit conformational change bidirectionally across the membrane. Integrin alpha and beta subunits form a head and two long legs in the ectodomain and span the membrane. Here, we define with crystal structures the atomic basis for allosteric regulation of the conformation and affinity for ligand of the integrin ectodomain, and how fibrinogen-mimetic therapeutics bind to platelet integrin alpha(IIb)beta3. Allostery in the beta3 I domain alters three metal binding sites, associated loops and alpha1- and alpha7-helices. Piston-like displacement of the alpha7-helix causes a 62 degrees reorientation between the beta3 I and hybrid domains. Transmission through the rigidly connected plexin/semaphorin/integrin (PSI) domain in the upper beta3 leg causes a 70 A separation between the knees of the alpha and beta legs. Allostery in the head thus disrupts interaction between the legs in a previously described low-affinity bent integrin conformation, and leg extension positions the high-affinity head far above the cell surface.     

Summer food habits of coyotes in central Wyoming

Summer food habits of coyotes ( Canis latrans ) were investigated on a 3100-km 2 area in central Wyoming, divided into one deer-use area and five non-deer areas. Analysis of 404 scats (fecal samples) revealed an overall average of 63 percent occurrence of native ungulates, 63 percent leporids, 46 percent rodents, 14 percent livestock, and 11 percent birds. Pronghorn ( Antilocapra Americana ) was the ungulate most frequently consumed, occurring in about 87 percent of the scats. Mule deer ( Odocoileus hemionus ) occurred in only 8 percent, and in 5 percent the native ungulate remains were not identifiable beyond order. This large percentage of big game in the diet is apparently unusual, because big game has been of minor importance in most coyote food-habit studies. The high incidence of leporids is consistent with other studies performed in arid intermountain areas. Although cricetines, especially deer mice ( Peromyscus maniculatus ), were trapped consistently in all habitats, months, and trapping areas, they were found in scats at a lower frequency than microtines and sciurids. This suggests a coyote hunting strategy that selected for the latter two groups.     

Sequence homology of the LFA-1 and Mac-1 leukocyte adhesion glycoproteins and unexpected relation to leukocyte interferon

Cell-surface adherence reactions are fundamental to the biology of lymphocytes, monocytes and granulocytes. The lymphocyte function-associated 1 (LFA-1) and macrophage 1 (Mac-1) glycoproteins mediate differing types of adhesion reactions on these cells. LFA-1 participates in T-lymphocyte and natural killer-cell adhesion to target cells, whereas the Mac-1 antigen is identical to the complement receptor type 3, which mediates adhesion of monocytes and granulocytes to C3bi-sensitized particles. Deficiency of these proteins, in a heritable disease, results in multiple adhesion-related leukocyte defects. LFA-1 and Mac-1 resemble one another in overall structure, having alpha-subunits of relative molecular mass (Mr) 180,000 and 170,000, respectively, which are non-covalently associated with beta-subunits of Mr 95,000 in alpha 1 beta 1 complexes. Peptide mapping and immunological cross-reactivity have shown that the beta-subunits are highly related if not identical, but have revealed no similarities between the alpha-subunits. Nonetheless, the shared beta-subunit suggested that LFA-1 and Mac-1 might be members of a protein family containing diversified but evolutionarily related alpha-subunits. Therefore, we examine here the structure of the alpha-subunits by N-terminal amino-acid sequencing. Sequence homology shows that the alpha-subunits are members of a novel leukocyte adhesion protein family, and suggests that their evolution occurred by gene duplication. A search for similarities to previously sequenced proteins reveals a further unexpected homology between LFA-1 and leukocyte (alpha) interferons.     

Two antigen-independent adhesion pathways used by human cytotoxic T-cell clones

Cell-cell adhesion is essential for many immunological functions, including interaction of cytotoxic T lymphocytes (CTLs) with their targets. We have explored CTL-target interactions using well-characterized cloned human CTLs. Conjugate formation between these CTLs and many antigen-negative targets is almost as efficient as with specific target cells, but does not lead to target-cell lysis. Thus, on specific target cells, adhesion by antigen-independent pathways may occur concurrently with or precede antigen recognition. The molecules LFA-1, CD2 (T11, LFA-2) and LFA-3 have been shown to be involved in human CTL conjugation with and lysis of specific target cells. Here we describe monoclonal antibody inhibition studies using individual monoclonal antibodies and mixes which demonstrate (1) that LFA-1, CD2 and LFA-3 are involved in antigen-independent conjugate formation; and (2) suggest that CD2 and LFA-3 are involved in one pathway and LFA-1 in another. We confirmed the existence of distinct pathways by the demonstration that LFA-1-dependent adhesion requires divalent cations and is temperature-sensitive whereas CD2- and LFA-3-dependent adhesion does not require divalent cations and is temperature-insensitive. Together with previous data, our studies suggest that CD2 on the effector interacts with LFA-3 as its ligand on targets.     

ICAM-1 a ligand for LFA-1-dependent adhesion of B, T and myeloid cells

Cell-cell adhesion is essential for many immunological functions. The LFA-1 molecule, a member of a superfamily of adhesion molecules, participates in adhesion which is critical to the function of each of the three major subsets of leukocytes: lymphocytes, monocytes and granulocytes. Putative LFA-1 ligands have been identified functionally in different laboratories using three different monoclonal antibodies that inhibit LFA-1-mediated leukocyte adhesion in particular model systems; however, there may be more than one LFA-1 ligand. We have directly compared the three relevant monoclonal antibodies, and show that each binds to the same molecule, intercellular-adhesion molecule-1 (ICAM-1). Most important, B, T and myeloid cells adhere specifically to purified ICAM-1-coated surfaces; such adhesion has distinctive requirements for Mg2+ and Ca2+. This constitutes biochemical evidence that ICAM-1 functions as a ligand for LFA-1-dependent adhesion by a variety of leukocytes.     

Architecture and material properties of diatom shells provide effective mechanical protection

Diatoms are the major contributors to phytoplankton blooms in lakes and in the sea and hence are central in aquatic ecosystems and the global carbon cycle. All free-living diatoms differ from other phytoplankton groups in having silicified cell walls in the form of two 'shells' (the frustule) of manifold shape and intricate architecture whose function and role, if any, in contributing to the evolutionary success of diatoms is under debate. We explored the defence potential of the frustules as armour against predators by measuring their strength. Real and virtual loading tests (using calibrated glass microneedles and finite element analysis) were performed on centric and pennate diatom cells. Here we show that the frustules are remarkably strong by virtue of their architecture and the material properties of the diatom silica. We conclude that diatom frustules have evolved as mechanical protection for the cells because exceptional force is required to break them. The evolutionary arms race between diatoms and their specialized predators will have had considerable influence in structuring pelagic food webs and biogeochemical cycles.