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D-Glucose and D-galactose influx and efflux rates in human erythrocytes were studied using infinite-cis and zero-trans assay methods. It was found that insulin decreased the infinite-cis Km for both D-glucose and D-galactose influx by 44 and 56%, respectively, while the Vmax was unchanged. The Km for D-glucose efflux in the presence of insulin decreased by 47% when compared to controls, and the change in Vmax was statistically insignificant. If insulin receptors were first down regulated, and then influx and efflux assays were performed, decreases in the infinite-cis and zero-trans Km values were also observed in the absence of exogenous insulin. These affinity changes were not due to persistent surface insulin receptor occupation by the insulin which was used to induce down-regulation. These affinity changes were comparable to those observed in non-down-regulated cells in the presence of insulin.

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.

The cell surface expression and function of the LFA-1 ligand, intercellular adhesion molecule 1 (ICAM-1), on epidermal keratinocytes (EK) was studied. ICAM-1 expression on the surface of cultured EK was either absent or weak, but was induced by treating EK with rIFN-gamma or TNF for 4-48 h. IFN-gamma and TNF were synergistic. IFN-gamma treatment increased T lymphoblast adhesion from less than 2% to 20-40%, with a concentration dependence similar to that seen for ICAM-1 induction. All of the adhesion to EK was inhibited by LFA-1 and ICAM-1 mAbs, but not by HLA-DR, CD2, or LFA-3 mAbs. There was no difference in the level of T lymphoblast adhesion to IFN-gamma-treated allogeneic or autologous EK. ICAM-1 purified from the HeLa epithelioid cell line and reconstituted into planar membranes also supported efficient adhesion of T lymphoblasts that was blocked by LFA-1 mAb bound to the T lymphoblasts or ICAM-1 mAb bound to the planar membranes. T lymphoblasts adherent to EK or ICAM-1 planar membranes were isolated by panning, and surface markers were analyzed by immunofluorescence flow cytometry. The adherent T cells were a phenotypically skewed subpopulation. They were enriched for CD8+ cells and expressed 1.5-2.5-fold higher LFA-1 and CD2 compared with the unseparated population.

CD2, also known as LFA-2, T11, and the E rosette receptor, is a T lymphocyte surface protein functionally important in adhesion to target cells and T cell triggering. LFA-3 is a widely distributed cell surface protein that functions in adhesion on target cells. We find that LFA-3 is expressed on human E, and that CD2 is a receptor for LFA-3 that mediates T cell adhesion to human E. Pretreatment of T lymphocytes with CD2 mAb or of E with LFA-3 mAb inhibits rosetting. Purified CD2 molecules bind to human E and inhibit rosetting. 125I-CD2 binding to E is inhibited by LFA-3 mAb; reciprocally, binding of LFA-3 mAb to human E is inhibited by pretreatment with purified CD2. Higher concentrations of CD2 aggregate human E; aggregation is inhibited by mAb to LFA-3.

Previous studies have shown that the purified T lymphocyte glycoprotein, cluster differentiation 2 (CD2) (also known as T11, lymphocyte function-associated antigen (LFA)-2, and the erythrocyte (E) rosette receptor) interacts with the LFA-3 molecule on human E. We have examined the interaction of the purified CD2 molecule with the T11 target structure (T11TS) molecule on sheep E, and compared the two interactions. Purified, 125I-labeled CD2 bound to sheep E and the binding was inhibited by anti-T11TS monoclonal antibody (mAb). Reciprocally, the binding of T11TS mAb to sheep E was inhibited by pretreatment of sheep E with purified CD2. High concentrations of purified CD2 aggregated sheep E, possibly by inserting into the membrane, and the aggregation was inhibited by T11TS mAb. The affinity and number of binding sites for purified CD2 on sheep and human E was found to be similar, with Ka of 9 X 10(7)/M and 6 X 10(7)/M and 9800 and 8300 CD2 binding sites/E, respectively. Thus, the human T lymphocyte CD2 molecule is a receptor that cross-reacts between LFA-3 on human E and T11TS on sheep E, suggesting that LFA-3 and T11TS are functionally homologous ligands. As measured by saturation mAb binding, there are 8100 and 3900 ligand molecules/sheep and human E, respectively. Human and sheep E have surface areas of 145 and 54 micron 2, respectively. The 3.2- to 5.6-fold higher ligand density on sheep E appears to account for the ability of sheep but not human E to rosette with certain types of human T lymphocytes.

Intercellular adhesion molecule-1 (ICAM-1) on the surface of cultured umbilical vein and saphenous vein endothelial cells was upregulated between 2.5- and 40-fold by rIL-1, rTNF, LPS and rIFN gamma corresponding to up to 5 X 10(6) sites/cell. Endothelial cell ICAM-1 was a single band of 90 kD in SDS-PAGE. Purified endothelial cell ICAM-1 reconstituted into liposomes and bound to plastic was an excellent substrate for both JY B lymphoblastoid cell and T lymphoblast adhesion. Adhesion to endothelial cell ICAM-1 in planar membranes was blocked completely by monoclonal antibodies to lymphocyte function associated antigen-1 (LFA-1) or ICAM-1. Adhesion to artificial membranes was most sensitive to ICAM-1 density within the physiological range found on resting and stimulated endothelial cells. Adhesion of JY B lymphoblastoid cells, normal and genetically LFA-1 deficient T lymphoblasts and resting peripheral blood lymphocytes to endothelial cell monolayers was also assayed. In summary, LFA-1 dependent (60-90% of total adhesion) and LFA-1-independent basal adhesion was observed and the use of both adhesion pathways by different interacting cell pairs was increased by monokine or lipopolysaccharide stimulation of endothelial cells. The LFA-1-dependent adhesion could be further subdivided into an LFA-1/ICAM-1-dependent component which was increased by cytokines and a basal LFA-1-dependent, ICAM-1-independent component which did not appear to be affected by cytokines. We conclude that ICAM-1 is a regulated ligand for lymphocyte-endothelial cell adhesion, but at least two other major adhesion pathways exist.

The manner in which a membrane protein is anchored to the lipid bilayer may have a profound influence on its function. Most cell surface membrane proteins are anchored by a membrane-spanning segment(s) of the polypeptide chain, but another type of anchor has been described for several proteins: a phosphatidyl inositol glycan moiety, attached to the protein C terminus. This type of linkage has been identified on membrane proteins involved in adhesion and transmembrane signalling and could be important in the execution of these functions. We report here that an immunologically important adhesion glycoprotein, lymphocyte function-associated antigen 3 (LFA-3), can be anchored to the membrane by both types of mechanism. These two distinct cell-surface forms of LFA-3 are derived from different biosynthetic precursors. The existence of a phosphatidyl-inositol-linked and a transmembrane anchored form of LFA-3 has important implications for adhesion and transmembrane signalling by LFA-3.

T11 target structure (T11TS) and lymphocyte function-associated antigen (LFA) 3 are the cell-surface glycoproteins on sheep and human erythrocytes (E) binding to cluster differentiation 2 (the E-receptor) on T cells in E rosette formation. Here we show that this functional cross-reactivity is most likely due to a structural homology of these molecules. A rabbit antiserum to sheep T11TS is shown to cross-react with LFA-3 in several independent assays: (a) rabbit anti-T11TS antiserum blocks the formation of E rosettes by human T cells with both autologous and xenogeneic (sheep) E by binding to the respective E; (b) the antiserum blocks the binding of anti-LFA-3 monoclonal antibody to human E; and (c) it reacts with purified LFA-3 in Western blotting. Together, these findings demonstrate that T11TS on sheep E and LFA-3 on human E are serologically related, providing further support for the notion that T11TS and LFA-3 are the sheep and human forms of the same cell interaction molecule.

Effective interaction between T cells and their targets requires that recognition of specific antigen be coordinated with increased cell-cell adhesion. We show that antigen-receptor cross-linking increases the strength of the adhesion mechanism between lymphocyte function-associated molecule-1 (LFA-1) and intercellular adhesion molecules (ICAMs), with intracellular signals transmitted from the T-cell antigen receptor to the LFA-1 adhesion molecule. The increase in avidity is rapid and transient, providing a dynamic mechanism for antigen-specific regulation of lymphocyte adhesion and de-adhesion.

Intercellular adhesion molecule 1 (ICAM-1) is a 90 kd inducible surface glycoprotein that promotes adhesion in immunological and inflammatory reactions. ICAM-1 is a ligand of lymphocyte function-associated antigen-1 (LFA-1), an alpha beta complex that is a member of the integrin family of cell-cell and cell-matrix receptors. ICAM-1 is encoded by an inducible 3.3 kb mRNA. The amino acid sequence specifies an integral membrane protein with an extracellular domain of 453 residues containing five immunoglobulin-like domains. Highest homology is found with neural cell adhesion molecule (NCAM) and myelin-associated glycoprotein (MAG), which also contain five Ig-like domains. NCAM and MAG are nervous system adhesion molecules, but unlike ICAM-1, NCAM is homophilic. The ICAM-1 and LFA-1 interaction is heterophilic and unusual in that it is between members of the immunoglobulin and intergrin families. Unlike other integrin ligands, ICAM-1 does not contain an RGD sequence.

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.

In this study we have used cells expressing LFA-3 or T11TS, the human and sheep forms of the ligand of CD2, as well as the purified LFA-3 and T11TS molecules themselves to study their effects on T cell activation via the CD2-mediated "alternative pathway". Sheep red blood cells, which bind to CD2 via T11TS in E-rosette formation, and human autologous monocytes, which express the LFA-3 molecule, both induce proliferation of resting T cells in the presence of per se submitogenic concentrations of anti-T11(2) plus anti-T11(3) monoclonal antibodies (mAb). This effect is blocked by mAb to LFA-3, T11TS and CD2 known to inhibit CD2-ligand interaction. In addition, purified LFA-3 and T11TS, when added at ng amounts to cultures containing submitogenic concentrations of anti-T11(2 + 3) mAb, are also strongly mitogenic for resting human T cells. Thus, both LFA-3 and T11TS are potent co-stimulators of the alternative pathway of T cell activation but by themselves do not provide a mitogenic signal. This finding is discussed with regard to a physiological role of CD2-LFA-3 interaction in T cell activation.