Although intercellular adhesion molecule-1 (ICAM-1) has been implicated as a ligand in some LFA-1-dependent adhesion, its importance to T cell function has not been established. The present studies investigate the importance of ICAM-1 for human cytotoxic T lymphocytes (CTL), both in their formation of antigen-independent conjugates (AIC) and in their lysis of targets. Analysis of monoclonal antibody (mAb) inhibition of AIC formation indicate that ICAM-1 mAb 1 blocks (a) AIC formation with some but not all targets; (b) the LFA-1 pathway but not the CD2/LFA-3 pathway of adhesion; (c) by binding to the target cell, not the T cell. In studies of cell-mediated lysis (CML) ICAM-1 mAb inhibited lysis of some targets, such as U-937, that use ICAM-1 predominantly in AIC formation; CML on some other targets is not inhibited by ICAM-1 mAb. These data indicate that ICAM-1 is a ligand for AIC formation, antigen-specific CTL recognition and cytolysis of particular target cells. The data also indicate that ICAM-1 is not used in LFA-1-dependent CTL interactions with all kinds of target cells, suggesting the existence of alternative ligands for LFA-1.
Mac-1 (CD 11b/CD18) is a leukocyte adhesion heterodimeric glycoprotein which functions both as a receptor for iC3b (CR3) and in several cell-cell and cell-substrate adhesive interactions. We describe full-length cDNA clones for the alpha subunit of Mac-1. Mac-1 alpha subunit message was detected in blood monocytes and phorbol-12-myristate acetate-induced myeloid cell lines, but not in cells of the T or B lineages, correlating with Mac-1 protein surface expression. The alpha subunit of Mac-1 is a transmembrane protein of 1137 residues with a long extracellular domain (1092 residues) and a 19-amino acid cytoplasmic tail. The extracellular domain contains three putative divalent cation-binding sequences and 19 potential N-glycosylation sites. The amino acid sequence of Mac-1 alpha shows that it is a member of the integrin superfamily; Mac-1 alpha shows 63% identity to the alpha subunit of the leukocyte adhesion glycoprotein p150.95 and 25% to the alpha subunits of the extracellular matrix receptors platelet glycoprotein IIb/IIIa, the fibronectin receptor, and the vitronectin receptor. The Mac-1 alpha subunit putative divalent cation-binding sites and the flanking regions exhibit a high degree of identity both to the p150.95 alpha subunit (87% identity at the amino acid level) and to the rest of the integrin alpha subunits (38%). The alpha subunit of Mac-1, like the p150.95 alpha subunit, contains a domain of 187 amino acids in the extracellular region which is absent in other integrins. This leukocyte or "L" domain is homologous to the A domains of von Willebrand factor, which in turn are homologous to regions of the C3-binding proteins factor B and C2. These findings draw attention to this region of Mac-1 as a potential binding site for iC3b.
Studies of cell-surface molecules involved in human T cell interaction reveal that differential expression of each of three adhesion molecules (LFA-3, CD2, and LFA-1) subdivides human peripheral blood T cells into major subpopulations. Systematic analysis of the relationship between expression of these and other markers of T cell subsets demonstrates a single major subset of human peripheral blood T lymphocytes distinguished by enhanced expression of LFA-3, CD2, LFA-1, and three other markers (CDw29 [4B4], UCHL1, and Pgp-1). Large differences in relative expression are observed for UCHL1 (29-fold) and LFA-3 (greater than 8-fold), and smaller differences (2- to 4-fold) are seen for CDw29, CD2, LFA-1, and Pgp-1. Bimodal distribution of LFA-3 is found on both CD4+ cells and on CD8+ cells as well as on B lymphocytes (CD19+). Neonatal T cells (CD3+) are comprised almost exclusively of the subset expressing low LFA-3, CD2, LFA-1, CDw29, and UCHL1. Activation of cord peripheral blood mononuclear leukocytes with PHA leads to uniform enhanced expression of each of these molecules on CD3+ cells. Functional analyses of these T cell subsets were performed after sorting of adult T cells based on differential LFA-3 expression. Only the LFA-3+ subset proliferated in response to the Ag tetanus toxoid, even though the LFA-3- subset proliferated more strongly to PHA. Furthermore, the LFA-3+ subset made greater than fivefold more IFN-gamma than the LFA-3- subset in response to PHA, despite the fact that both subsets made equivalent amounts of IL-2. This phenotypic and functional analysis of resting and activated newborn and adult T cells indicates that human memory T cells express enhanced levels of LFA-3, CD2, LFA-1, UCHL1, CDw29, and Pgp-1; we speculate that the increase in expression of T cell adhesion molecules LFA-3, CD2, and LFA-1 on memory cells is functionally important in their enhanced responsiveness.
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 molecules involved in the human cytolytic T lymphocyte (CTL)-synovial cell interaction may play an important role in T cell interactions with connective tissue mesenchymal cells. To examine the molecular basis for the CTL-synovial cell interaction, we immortalized synovial cell explants to establish the cell line SYN.SPP. The SYN.SPP cell line was compared to the established B lymphoblastoid cell line JY. Cell surface immunofluorescence demonstrated significantly different levels of the immunologically relevant cell surface molecules ICAM-1 and LFA-3. Both cell lines were used to stimulate CTL precursors. After several months in culture, CTL lines stimulated by the SYN.SPP and JY cell lines demonstrated HLA class I-directed cytolytic activity. The cell surface molecules utilized by the anti-SYN.SPP and anti-JY CTL lines were identified by monoclonal antibody (MAb) inhibition. MAb recognizing the CTL cell surface molecules CD3, CD8 and LFA-1 (CD11a) significantly inhibited CTL-mediated lysis of both target cells. An interesting observation was that the anti-SYN.SPP CTL line appeared to utilize the ICAM-1 and not the LFA-3 target cell molecule. In contrast, the anti-JY CTL line utilized the LFA-3 and not the ICAM-1 membrane molecule. These results indicate that CTL interactions with connective tissue mesenchymal cells may be regulated by a unique pattern of antigen nonspecific cell-cell interaction molecules.
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.
Fc receptors on phagocytic cells in the blood mediate binding and clearance of immune complexes, phagocytosis of antibody-opsonized microorganisms, and potently trigger effector functions, including superoxide anion production and antibody-dependent cellular cytotoxicity. The Fc receptor type III (Fc gamma R III, CD 16), present in 135,000 sites per cell 1 on neutrophils and accounting for most of FcR in blood, unexpectedly has a phosphatidylinositol glycan (PIG) membrane anchor. Deficiency of Fc gamma R III is observed in paroxysmal nocturnal haemoglobinuria (PNH), an acquired abnormality of haematopoietic cells affecting PIG tail biosynthesis or attachment, and is probably responsible for circulating immune complexes and susceptibility to bacterial infections associated with this disease. Although a growing number of eukaryotic cell-surface proteins with PIG-tails are being described, none has thus far been implicated in receptor-mediated endocytosis or in triggering of cell-mediated killing. Our findings on the Fc gamma R III raise the question of how a PIG-tailed protein important in immune complex clearance in vivo and in antibody-dependent killing mediates ligand internalization and cytotoxicity. Together with our results, previous functional studies on Fc gamma R III and Fc gamma R II suggest that these two receptors may cooperate and that the type of membrane anchor is an important mechanism whereby the functional capacity of surface receptors can be regulated.
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.
Defining the cellular and molecular mechanisms of interaction of developing thymocytes with nonlymphoid cells of the thymic microenvironment is critical for understanding normal thymus function. We have previously shown that the CD2/LFA-3 adhesion pathway is important in the interaction of thymocytes with a variety of LFA-3+ nonlymphoid thymic microenvironment cell types. Moreover, T cell activation via the CD2 (alternative, Ag independent) pathway is considered an important mechanism for intrathymic T cell proliferation. To study the relevance of CD2/LFA-3 interactions to human thymocyte activation, we have used purified LFA-3 Ag in several in vitro assays of thymocyte proliferation. Whereas LFA-3 Ag alone did not induce thymocyte proliferation, LFA-3 Ag in combination with the anti-CD2 antibody, CD2.1, and rIL-2 induced marked thymocyte proliferation. Additionally, the anti-CD28 antibody, Kolt2, could substitute for rIL-2, resulting in thymocyte activation induced by LFA-3 Ag in combination with antibodies CD2.1 and Kolt2. In both triggering systems, LFA-3 induced thymocyte activation was dependent upon the concentration of LFA-3 Ag. LFA-3 Ag-dependent thymocyte activation was directed primarily toward CD1-, mature thymocytes. Finally, intact SRBC that express the sheep homolog of LFA-3, T11TS, in combination with antibody CD2.1 and rIL-2 could also induce thymocyte activation. These data suggest that interaction of LFA-3 molecules with thymocyte CD2 molecules may provide a component of the stimulus for normal intrathymic thymocyte activation leading to thymocyte proliferation.
Recombinant human lymphotoxin (LT) was compared with recombinant human tumor necrosis factor (TNF) for direct actions on cultured human endothelial cells (HEC). At equivalent half-maximal concentrations (based on L929 cytotoxicity units) LT and TNF each caused rapid and transient induction (peak 4 to 6 hr) of an antigen associated with leukocyte adhesion (detected by monoclonal antibody H4/18), a rapid but sustained increased expression (plateau 24 hr) of a lymphocyte adhesion structure (ICAM-1), a gradual (plateau 4 to 6 days) increase in expression of HLA-A,B antigens, and gradual (4 to 6 days) conversion of HEC culture morphology from epithelioid to fibroblastoid, an effect enhanced by immune interferon (IFN-gamma). Induction of H4/18 binding by maximal concentrations of LT or TNF could not be augmented by addition of the other cytokine, and 24 hr pretreatment with LT or TNF produced hyporesponsiveness to both mediators for reinduction. H4/18 binding can be transiently induced by tumor-promoting phorbol esters. Pretreatment with either LT or TNF also fully inhibited induction of H4/18 binding by phorbol ester, whereas phorbol ester pretreatment only variably and partially inhibited reinduction by LT or TNF. These actions of LT on endothelium shared with TNF may serve in vivo to promote lymphocyte and inflammatory leukocyte adhesion and transendothelial migration. Recombinant human interleukin 1 species (IL 1 alpha and IL 1 beta) shared many of the actions of LT and TNF and were indistinguishable from each other. However, IL 1 species could be distinguished from LT/TNF by their relative inability to enhance HLA-A,B expression, by their ability to augment H4/18 binding caused by maximally effective concentrations of LT or TNF, and by their inability to inhibit reinduction of H4/18 binding by LT or TNF. In contrast to the actions of LT or TNF, pretreatment with IL 1 alpha or IL 1 beta only partially inhibited induction of H4/18 binding by phorbol ester, and phorbol ester pretreatment consistently, albeit partially, inhibited induction by IL 1 species. These studies suggest that activated T cells through the secretion of LT can in turn activate the local endothelial lining so as to promote homing and extravasation of inflammatory cells. Furthermore, these LT actions can be augmented or complemented by other locally produced mediators such as IFN-gamma or IL 1.
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.
The p150,95 cell surface protein is a member of a family of heterodimeric leukocyte adhesion proteins that have homologous alpha subunits, each noncovalently associated with a common beta subunit. In this report we have metabolically labeled the U937 cell line at various timepoints during its phorbol myristic acetate-induced maturation to examine the kinetics of synthesis of these proteins during monocytic differentiation, and their maturation and glycosylation. The p150,95 alpha subunit was immunoprecipitated with p150,95-specific monoclonal antibody (MAb), or an antiserum to the denatured, purified alpha X subunit. The glycosylation and polypeptide chain length of the p150,95, Mac-1, and lymphocyte function associated antigen (LFA-1) alpha and beta subunits were compared by immunoprecipitation with subunit specific MAb and antisera, and by digestion with Endo H and N-glycanase. The p150,95 alpha subunit is synthesized as a precursor of 146,000 Mr, has five to six N-linked oligosaccharides, and has a polypeptide chain backbone of 132,000 Mr. Over 50% of the carbohydrate on the mature alpha subunit of 150,000 Mr was sensitive to Endo H digestion. The p150,95 alpha and beta precursors can associate before maturation into the mature form. Conversion to the mature form was accompanied by loss of reactivity with the antiserum to the denatured alpha X subunit, suggesting a change in conformation. Mac-1 and LFA-1 alpha subunits have precursors of 160,000 Mr and 165,000 Mr, respectively, and contain N-linked carbohydrates. The polypeptide chain length for the Mac-1 alpha subunit is 137,000 Mr, and for LFA-1 is 149,000 Mr. Only 14% of the oligosaccharide on the mature LFA-1 alpha subunit was sensitive to Endo H, suggesting that unlike p150,95, most is converted to the complex type. The differences noted in the Mr of the three homologous alpha subunits are therefore due to differences in both polypeptide chain length and carbohydrate processing during biosynthesis.
The leukocyte adhesion receptors, p150,95, Mac-1 and LFA-1 are integral membrane glycoproteins which contain distinct alpha subunits of 180,000-150,000 Mr associated with identical beta subunits of 95,000 Mr in alpha beta complexes. p150,95 alpha subunit tryptic peptides were used to specify oligonucleotide probes and a cDNA clone of 4.7 kb containing the entire coding sequence was isolated from a size-selected myeloid cell cDNA library. The 4.7-kb cDNA clone encodes a signal sequence, an extracellular domain of 1081 amino acids containing 10 potential glycosylation sites, a transmembrane domain of 26 amino acids, and a C-terminal cytoplasmic tail of 29 residues. The extracellular domain contains three tandem homologous repeats of approximately 60 amino acids with putative divalent cation-binding sites, and four weaker repeats which lack such binding sites. The cDNA clone hybridizes with a mRNA of 4.7 kb which is induced during in vitro differentiation of myeloid cell lines. The p150,95 alpha subunit is homologous to the alpha subunits of receptors which recognize the RGD sequence in extracellular matrix components, as has previously been shown for the beta subunits, supporting the concept that receptors involved in both cell-cell and cell-matrix interactions belong to a single gene superfamily termed the integrins. Distinctive features of the p150,95 alpha subunit include an insertion of 126 residues N-terminal to the putative metal binding region and a deletion of the region in which the matrix receptors are proteolytically cleaved during processing.
A novel mobilizable intracellular compartment was identified in human neutrophils by latent alkaline phosphatase activity. This compartment is mobilized to the plasma membrane much more readily than any identified granule subset and has kinetics of up-regulation in the membrane similar to those reported for a variety of receptor proteins. Triton X-100 permeabilization of both intact human neutrophils and subcellular fractions obtained by density-gradient centrifugation revealed that 70 percent of the alkaline phosphatase is located in an intracellular compartment distinct from primary, secondary, and gelatinase granules and from the plasma membrane. This compartment fully translocates to the plasma membrane after stimulation with nanomolar concentrations of the chemotactic peptide N-formylmethionylleucylphenylalanine.
We have isolated cDNA clones encoding the beta subunit of the human LFA-1, Mac-1, and p150,95 family of leukocyte adhesion proteins. The deduced 769-amino-acid sequence defines a cysteine-rich polypeptide with the characteristic features of an integral membrane protein. Peptide sequence data, Northern blot analysis, and Southern blot analysis suggest that a single gene encodes the beta subunit of all three leukocyte adhesion proteins. There is 45% homology between the beta subunit sequence and band III of integrin, a chick fibronectin and laminin receptor. This homology defines a new supergene family of cellular adhesion proteins.
Lymphocyte function-associated antigen 3 (LFA-3) is a widely distributed cell surface glycoprotein that binds to the T lymphocyte CD2 surface glycoprotein. This interaction mediates CTL-target cell conjugate formation and adhesion of thymocytes to thymic epithelial cells. CD2 is also the E rosette receptor of T lymphocytes and mediates rosetting with autologous E by binding to LFA-3. We describe deficient expression of LFA-3 on E from paroxysmal nocturnal hemoglobinuria (PNH) patients. PNH is an acquired defect affecting phosphatidylinositol-anchored membrane proteins, of which decay-accelerating factor (DAF) is most important in the clinical symptoms of PNH. LFA-3-negative, weakly positive, and positive populations were found among PNH E. There was a good correlation with DAF deficiency. PNH E exhibited decreased binding of 125I-CD2 and rosetting with a human T lymphoma cell line. PNH E readily incorporated purified LFA-3, restoring LFA-3 expression and the CD2 binding and rosetting activity to normal levels. The expression of DAF was not restored after the incorporation of purified LFA-3 into PNH E, showing that LFA-3 and DAF are different molecules. Phosphatidylinositol-specific phospholipase C (PIPLC) treatment of a B lymphoma cell line released 35% of the cell surface LFA-3 and 62% of DAF. LFA-3 on E was resistant to PIPLC. However, when LFA-3 purified from human E was reconstituted in sheep E or human E and subjected to PIPLC treatment, 40-50% of LFA-3 was released from the cell membrane. The results show that LFA-3 is attached to the cell membrane by a phosphatidylinositol glycolipid moiety, and confirm previous findings (37-41) that LFA-3 is a cell adhesion molecule that mediates adhesion by interacting with CD2 antigen.
Leukocyte adhesion deficiency (LAD) is a heritable disease involving deficient expression of three related leukocyte adhesion glycoproteins: LFA-1, Mac-1, and p150,95. These proteins are alpha beta heterodimers containing identical 95,000 dalton beta subunits. Here we demonstrate that the primary defect in LAD is in the beta subunit gene. We identified five distinct beta subunit phenotypes in LAD patients: undetectable beta subunit mRNA and protein precursor; low levels of beta subunit mRNA and precursor; an aberrantly large beta subunit precursor, probably due to an extra glycosylation site; an aberrantly small precursor; and a grossly normal precursor. Mutant beta subunit precursors from LAD patients failed to associate with the LFA-1 alpha subunit. In family studies, inheritance of the aberrant precursors correlates with the known inheritance of the LAD defect.