Supramolecular attack particles are autonomous killing entities released from cytotoxic T cells

Cytotoxic T lymphocytes (CTLs) kill infected and cancerous cells. We detected transfer of cytotoxic multiprotein complexes, called supramolecular attack particles (SMAPs), from CTLs to target cells. SMAPs were rapidly released from CTLs and were autonomously cytotoxic. Mass spectrometry, immunochemical analysis, and CRISPR editing identified a carboxyl-terminal fragment of thrombospondin-1 as an unexpected SMAP component that contributed to target killing. Direct stochastic optical reconstruction microscopy resolved a cytotoxic core surrounded by a thrombospondin-1 shell of ~120 nanometer diameter. Cryo-soft x-ray tomography analysis revealed that SMAPs had a carbon-dense shell and were stored in multicore granules. We propose that SMAPs are autonomous extracellular killing entities that deliver cytotoxic cargo targeted by the specificity of shell components.

CX3CR1+ interstitial dendritic cells form a contiguous network throughout the entire kidney

Dendritic cells (DCs) interface innate and adaptive immunity in nonlymphoid organs; however, the exact distribution and types of DC within the kidney are not known. We utilized CX3CR1GFP/+ mice to characterize the anatomy and phenotype of tissue-resident CX3CR1+ DCs within normal kidney. Laser-scanning confocal microscopy revealed an extensive, contiguous network of stellate-shaped CX3CR1+ DCs throughout the interstitial and mesangial spaces of the entire kidney. Intravital microscopy of the superficial cortex showed stationary interstitial CX3CR1+ DCs that continually probe the surrounding tissue environment through dendrite extensions. Flow cytometry of renal CX3CR1+ DCs showed significant coexpression of CD11c and F4/80, high major histocompatibility complex class II and FcR expression, and immature costimulatory but competent phagocytic ability indicative of tissue-resident, immature DCs ready to respond to environment cues. Thus, within the renal parenchyma, there exists little immunological privilege from the surveillance provided by renal CX3CR1+ DCs, a major constituent of the heterogeneous mononuclear phagocyte system populating normal kidney.

Identification of self through two-dimensional chemistry and synapses

Cells in the immune and nervous systems communicate through informational synapses. The two-dimensional chemistry underlying the process of synapse formation is beginning to be explored using fluorescence imaging and mechanical techniques. Early analysis of two-dimensional kinetic rates (k(on) and k(off)) and equilibrium constants (K(d)) provides a number of biological insights. First, there are two regimes for adhesion-one disordered with slow k(on) and the other self-ordered with 10(4)-fold faster k(on). Despite huge variation in two-dimensional k(on), the two-dimensional k(off) is like k(off) in solution, and two-dimensional k(off) is more closely related to intrinsic properties of the interaction than the two-dimensional k(on). Thus difference in k(off) can be used to set signaling thresholds. Early signaling complexes are compartmentalized to generate synergistic signaling domains. Immune antigen receptor components have a role in neural synapse editing. This suggests significant parallels in informational synapse formation based on common two-dimensional chemistry and signaling strategies.

The immunological synapse and CD28-CD80 interactions

According to the two-signal model of T cell activation, costimulatory molecules augment T cell receptor (TCR) signaling, whereas adhesion molecules enhance TCR-MHC-peptide recognition. The structure and binding properties of CD28 imply that it may perform both functions, blurring the distinction between adhesion and costimulatory molecules. Our results show that CD28 on naïve T cells does not support adhesion and has little or no capacity for directly enhancing TCR-MHC-peptide interactions. Instead of being dependent on costimulatory signaling, we propose that a key function of the immunological synapse is to generate a cellular microenvironment that favors the interactions of potent secondary signaling molecules, such as CD28.

The immunological synapse

The adaptive immune response is initiated by the interaction of T cell antigen receptors with major histocompatibility complex molecule-peptide complexes in the nanometer scale gap between a T cell and an antigen-presenting cell, referred to as an immunological synapse. In this review we focus on the concept of immunological synapse formation as it relates to membrane structure, T cell polarity, signaling pathways, and the antigen-presenting cell. Membrane domains provide an organizational principle for compartmentalization within the immunological synapse. T cell polarization by chemokines increases T cell sensitivity to antigen. The current model is that signaling and formation of the immunological synapse are tightly interwoven in mature T cells. We also extend this model to natural killer cell activation, where the inhibitory NK synapse provides a striking example in which inhibition of signaling leaves the synapse in its nascent, inverted state. The APC may also play an active role in immunological synapse formation, particularly for activation of naïve T cells.

Role of adhesion molecules in activation signaling in T lymphocytes

The T cell and antigen-presenting cell communicate to initiate an immune response through formation of an immunological synapse. This specialized cell-cell junction is compartmentalized into adhesion molecule and T cell receptor enriched regions or SMACs. Distinct signals seem to be generated in the T cell receptor and adhesion molecule-dominated regions. This review focuses on how these distinct signaling pathways may be integrated within the T cell to set thresholds for T cell activation, proliferation, and survival.

Reprogramming T cells: the role of extracellular matrix in coordination of T cell activation and migration

The stable immunological synapse between a T cell and antigen-presenting cell coordinates migration and activation. Three-dimensional collagen gels transform this interaction into a series of transient hit-and-run encounters. Here we integrate these alternative modes of interaction in a model for primary T cell activation and effector function in vivo.

Environmental control of immunological synapse formation and duration

The coordination of T-cell migration and antigen recognition is crucial for an effective immune response. We have proposed that this coordination is achieved by formation of an immunological synapse between the T cell and the antigen-presenting cell (APC). Our view contrasts with the serial encounter model also proposed in this issue of Trends in Immunology, which is based on transient T cell-APC interactions when surrounded by collagen. Here, we propose a model that reconciles immunological synapse formation and serial encounters based on environmental control of immunological synapse formation.

A supramolecular basis for CD45 tyrosine phosphatase regulation in sustained T cell activation

Transmembrane protein tyrosine phosphatases, such as CD45, can act as both positive and negative regulators of cellular signaling. CD45 positively modulates T cell receptor (TCR) signaling by constitutively priming p56lck through the dephosphorylation of the C-terminal negative regulatory phosphotyrosine site. However, CD45 can also exert negative effects on cellular processes, including events triggered by integrin-mediated adhesion. To better understand these opposing actions of tyrosine phosphatases, the subcellular compartmentalization of CD45 was imaged by using laser scanning confocal microscopy during functional TCR signaling of live T lymphocytes. On antigen engagement, CD45 was first excluded from the central region of the interface between the T cell and the antigen-presenting surface where CD45 would inhibit integrin activation. Subsequently, CD45 was recruited back to the center of the contact to an area adjacent to the site of sustained TCR engagement. Thus, CD45 is well positioned within a supramolecular assembly in the vicinity of the engaged TCR, where CD45 would be able to maintain src-kinase activity for the duration of TCR engagement.

Cutting edge: hierarchy of chemokine receptor and TCR signals regulating T cell migration and proliferation

Chemokines play an important role in establishing the distribution of lymphocyte subpopulations in primary and secondary lymphoid tissues and in the recruitment of leukocytes to sites of inflammation. However, the potential of chemokines to down-regulate immune responses has not been demonstrated. We now show that certain chemokine gradients have the potential to suppress T cell activation by preventing formation of the immunological synapse, the specialized cell-cell junction that forms before a T cell can be fully activated. Our data reveals an immunosuppressive potential of chemokines engaging the CXCR3 and CCR7 receptors, but not the CXCR4, CCR2, CCR4, or CCR5 receptors. These results suggest a novel mechanism for T cell ignorance of agonist MHC-peptide complexes based on dominant chemokine gradients.

The immunological synapse and the actin cytoskeleton: molecular hardware for T cell signaling

The actin cytoskeleton seems to play two critical roles in the activation of T cells. One of these roles is T cell shape development and movement, including formation of the immunological synapse. The other is the formation of a scaffold for signaling components. This review focuses on the recent convergence of cell biology and immunology studies to explain the role of the actin cytoskeleton in creating the molecular basis for immunological synapse formation and T cell signaling.

Cytoskeletal polarization and redistribution of cell-surface molecules during T cell antigen recognition

T cell antigen recognition is accompanied by cytoskeletal polarization towards the APC and large-scale redistribution of cell surface molecules into 'supramolecular activation clusters' (SMACs), forming an organized contact interface termed the 'immunological synapse' (IS). Molecules are arranged in the IS in a micrometer scale bull's eye pattern with a central accumulation of TCR/peptide-MHC (the cSMAC) surrounded by a peripheral ring of adhesion molecules (the pSMAC). We propose that segregation of cell surface molecules on a much smaller scale initiates TCR triggering, which drives the formation of the IS by active transport processes. IS formation may function as a checkpoint for full T cell activation, integrating information on the presence and quality of TCR ligands and the nature and activation state of the APC.

Relative contribution of LFA-1 and Mac-1 to neutrophil adhesion and migration

To differentiate the unique and overlapping functions of LFA-1 and Mac-1, LFA-1-deficient mice were developed by targeted homologous recombination in embryonic stem cells, and neutrophil function was compared in vitro and in vivo with Mac-1-deficient, CD18-deficient, and wild-type mice. LFA-1-deficient mice exhibit leukocytosis but do not develop spontaneous infections, in contrast to CD18-deficient mice. After zymosan-activated serum stimulation, LFA-1-deficient neutrophils demonstrated activation, evidenced by up-regulation of surface Mac-1, but did not show increased adhesion to purified ICAM-1 or endothelial cells, similar to CD18-deficient neutrophils. Adhesion of Mac-1-deficient neutrophils significantly increased with stimulation, although adhesion was lower than for wild-type neutrophils. Evaluation of the strength of adhesion through LFA-1, Mac-1, and CD18 indicated a marked reduction in firm attachment, with increasing shear stress in LFA-1-deficient neutrophils, similar to CD18-deficient neutrophils, and only a modest reduction in Mac-1-deficient neutrophils. Leukocyte influx in a subcutaneous air pouch in response to TNF-alpha was reduced by 67% and 59% in LFA-1- and CD18-deficient mice but increased by 198% in Mac-1-deficient mice. Genetic deficiencies demonstrate that both LFA-1 and Mac-1 contribute to adhesion of neutrophils to endothelial cells and ICAM-1, but adhesion through LFA-1 overshadows the contribution from Mac-1. Neutrophil extravasation in response to TNF-alpha in LFA-1-deficient mice dramatically decreased, whereas neutrophil extravasation in Mac-1-deficient mice markedly increased.

Congenital nephrotic syndrome in mice lacking CD2-associated protein

CD2-associated protein (CD2AP) is an 80-kilodalton protein that is critical for stabilizing contacts between T cells and antigen-presenting cells. In CD2AP-deficient mice, immune function was compromised, but the mice died at 6 to 7 weeks of age from renal failure. In the kidney, CD2AP was expressed primarily in glomerular epithelial cells. Knockout mice exhibited defects in epithelial cell foot processes, accompanied by mesangial cell hyperplasia and extracellular matrix deposition. Supporting a role for CD2AP in the specialized cell junction known as the slit diaphragm, CD2AP associated with nephrin, the primary component of the slit diaphragm.

The immunological synapse: a molecular machine controlling T cell activation

The specialized junction between a T lymphocyte and an antigen-presenting cell, the immunological synapse, consists of a central cluster of T cell receptors surrounded by a ring of adhesion molecules. Immunological synapse formation is now shown to be an active and dynamic mechanism that allows T cells to distinguish potential antigenic ligands. Initially, T cell receptor ligands were engaged in an outermost ring of the nascent synapse. Transport of these complexes into the central cluster was dependent on T cell receptor-ligand interaction kinetics. Finally, formation of a stable central cluster at the heart of the synapse was a determinative event for T cell proliferation.

Making a little affinity go a long way: a topological view of LFA-1 regulation

Lymphocytes utilize adhesion to navigate in the body and to transiently interact with a variety of potential antigen presenting cells. Interactions of adhesion molecules are governed by the law of mass action and the less understood rules of apposed biological membranes. Biochemical parameters such as adhesion molecule affinity only tell part of the story. Factors such as lateral mobility, membrane alignment and cytoskeletal interactions are equally important in determining the final outcome. Therefore it is important to determine mechanisms by which the properties of cell membranes and the cytoskeleton reinforce or hinder adhesion molecule interactions. Work from my lab has shown that one mechanism by which lymphocyte adhesion molecules cooperate is to align adhering membranes with nanometer precision. Here, I discuss a model for LFA-1 regulation that is dependent on three independent processes: LFA-1 lateral mobility, ligand induced generation of a small amount of high affinity LFA-1 and local membrane alignment. I propose that coordination of these processes allows rapid interconversion between stable adhesion and detachment.

A novel adaptor protein orchestrates receptor patterning and cytoskeletal polarity in T-cell contacts

Recognition of antigen by T cells requires the formation of a specialized junction between the T cell and the antigen-presenting cell. This junction is generated by the recruitment and the exclusion of specific proteins from the contact area. The mechanisms that regulate these events are unknown. Here we demonstrate that ligand engagement of the adhesion molecule, CD2, initiates a process of protein segregation, CD2 clustering, and cytoskeletal polarization. Although protein segregation was not dependent on the cytoplasmic domain of CD2, CD2 clustering and cytoskeletal polarization required an interaction of the CD2 cytoplasmic domain with a novel SH3-containing protein. This novel protein, called CD2AP, is likely to facilitate receptor patterning in the contact area by linking specific adhesion receptors to the cytoskeleton.

Low affinity interaction of human or rat T cell adhesion molecule CD2 with its ligand aligns adhering membranes to achieve high physiological affinity

The mechanism by which low affinity adhesion molecules function to produce stable cell-cell adhesion is unknown. In solution, the interaction of human CD2 with its ligand CD58 is of low affinity (500 mM-1) and the interaction of rat CD2 with its ligand CD48 is of still lower affinity (40 mM-1). At the molecular level, however, the two systems are likely to be topologically identical. Fluorescently labeled glycosylphosphatidylinositol-anchored CD48 and CD58 were prepared and incorporated into supported phospholipid bilayers, in which the ligands were capable of free lateral diffusion. Quantitative fluorescence imaging was used to study the binding of cell surface human and rat CD2 molecules to the fluorescent ligands in contact areas between Jurkat cells and the bilayers. These studies provide two major conclusions. First, CD2/ligand interactions cooperate to align membranes with nanometer precision leading to a physiologically effective two-dimensional affinity. This process does not require the intact cytoplasmic tail of CD2. Second, the degree of membrane alignment that can be achieved by topologically similar receptors deteriorates with decreasing affinity. This suggests an affinity limit for the ability of this mode of cooperativity to achieve stable cell-cell adhesion at approximately 10 mM-1.

Survival of FimH-expressing enterobacteria in macrophages relies on glycolipid traffic

Strains of Escherichia coli persist within the human gut as normal commensals, but are frequent pathogens and can cause recurrent infection. Here we show that, in contrast to E. coli subjected to opsonic interactions stimulated by the host's immune response, E. coli that bind to the macrophage surface exclusively through the bacterial lectin FimH can survive inside the cell following phagocytosis. This viability is largely due to the attenuation of intracellular free-radical release and of phagosome acidification during FimH-mediated internalization, both of which are triggered by antibody-mediated internalization. This different processing of non-opsonized bacteria is supported by morphological evidence of tight-fitting phagosomes compared with looser, antibody-mediated phagosomes. We propose that non-opsonized FimH-expressing E. coli co-opt internalization of lipid-rich microdomains following binding to the FimH receptor, the glycosylphosphatidylinositol-linked protein CD48, because (1) the sterol-binding agents filipin, nystatin and methyl beta-cyclodextrin specifically block FimH-mediated internalization; (2) CD48 and the protein caveolin both accumulate on macrophage membranes surrounding bacteria; and (3) antibodies against CD48 inhibit FimH-mediated internalization. Our findings bring the traditionally extracellular E. coli into the realm of opportunistic intracellular parasitism and suggest how opportunistic infections with FimH-expressing enterobacteria could occur in a setting deprived of opsonizing antibodies.

Low affinity of cell surface lymphocyte function-associated antigen-1 (LFA-1) generates selectivity for cell-cell interactions

We examined binding of soluble intercellular adhesion molecule-1 (ICAM-1) dimers and a range of ICAM-1-coated particles to activated T cells. Lymphocyte function-associated antigen-1 (LFA-1) on the surface of activated T cells did not bind soluble ICAM-1 dimers with high affinity. In contrast, activated T cells adhered avidly to ICAM-1-coated planar surfaces. Between these two extremes, a range of ICAM-1-bearing particles was tested for binding. Activated T cells bound particles of 1-microm diameter or larger, but did not bind particles of 0.5-microm diameter or smaller. This threshold was eliminated, and all forms of ICAM-1 bound to LFA-1 when LFA-1 was converted to a high affinity form with an activating antibody. We show that high affinity LFA-1 is generated only as a consequence of an initial low affinity interaction of LFA-1 with ICAM-1 under physiological conditions. Therefore, the selectivity of cell surface LFA-1 for cell-sized particles bearing ICAM-1 appears to be due to the maintenance of low affinity LFA-1 on the surface of activated T cells. These findings alter the definition of inside-out signaling for LFA-1.

Low affinity of cell surface lymphocyte function-associated antigen-1 (LFA-1) generates selectivity for cell-cell interactions

We examined binding of soluble intercellular adhesion molecule-1 (ICAM-1) dimers and a range of ICAM-1-coated particles to activated T cells. Lymphocyte function-associated antigen-1 (LFA-1) on the surface of activated T cells did not bind soluble ICAM-1 dimers with high affinity. In contrast, activated T cells adhered avidly to ICAM-1-coated planar surfaces. Between these two extremes, a range of ICAM-1-bearing particles was tested for binding. Activated T cells bound particles of 1-microm diameter or larger, but did not bind particles of 0.5-microm diameter or smaller. This threshold was eliminated, and all forms of ICAM-1 bound to LFA-1 when LFA-1 was converted to a high affinity form with an activating antibody. We show that high affinity LFA-1 is generated only as a consequence of an initial low affinity interaction of LFA-1 with ICAM-1 under physiological conditions. Therefore, the selectivity of cell surface LFA-1 for cell-sized particles bearing ICAM-1 appears to be due to the maintenance of low affinity LFA-1 on the surface of activated T cells. These findings alter the definition of inside-out signaling for LFA-1.

The lymphocyte function-associated antigen 1 I domain is a transient binding module for intercellular adhesion molecule (ICAM)-1 and ICAM-3 in hydrodynamic flow

The I domain of lymphocyte function-associated antigen (LFA)-1 contains an intercellular adhesion molecule (ICAM)-1 and ICAM-3 binding site, but the relationship of this site to regulated adhesion is unknown. To study the adhesive properties of the LFA-1 I domain, we stably expressed a GPI-anchored form of this I domain (I-GPI) on the surface of baby hamster kidney cells. I-GPI cells bound soluble ICAM-1 (sICAM-1) with a low avidity and affinity. Flow cell experiments demonstrated a specific rolling interaction of I-GPI cells on bilayers containing purified full length ICAM-1 or ICAM-3. The LFA-1 activating antibody MEM-83, or its Fab fragment, decreased the rolling velocity of I-GPI cells on ICAM-1-containing membranes. In contrast, the interaction of I-GPI cells with ICAM-3 was blocked by MEM-83. Rolling of I-GPI cells was dependent on the presence of Mg2+. Mn2+ only partially substituted for Mg2+, giving rise to a small fraction of rolling cells and increased rolling velocity. This suggests that the I domain acts as a transient, Mg2+-dependent binding module that cooperates with another Mn2+-stimulated site in LFA-1 to give rise to the stable interaction of intact LFA-1 with ICAM-1.

Adhesive bond dynamics in contacts between T lymphocytes and glass-supported planar bilayers reconstituted with the immunoglobulin-related adhesion molecule CD58

The interaction of the T cell glycoprotein CD2 and its ligand CD58 is important for T cell interaction with antigen-presenting and target cells. The binding interaction is of low affinity and has a fast off-rate (>5 s-1) in solution. However, solution measurements may not accurately predict the behavior of molecules in an adhesive contact area. Interaction between T cells that express CD2 and glass-supported planar bilayers containing purified and fluorescently labeled CD58 leads to accumulation of CD58 (fluorescence) in the cell/bilayer contact area. CD58 molecules accumulated within the contact area in excess of the CD58 density in the bilayer outside the contact area can be considered as bound by cell surface CD2. Here, this phenomena and fluorescence photobleaching recovery were utilized to determine whether CD2-CD58 bonds are transient in contact areas. Fluorescent CD58 molecules accumulated in the T cell-bilayer interface were completely bleached. The bleached CD58 molecules accumulated in the contact area were rapidly replaced by fluorescent CD58 that diffused into the contact area from adjacent bilayer regions outside the contact area. Rapid recovery of the accumulated fluorescence directly demonstrates that the CD2-CD58 bonds are dissociating and that the dissociation leads to partner exchange, rather than rebinding of the same CD2-CD58 pairs. This suggests that the solution off-rate provides an accurate description of CD2-CD58 interaction in contact areas. Accumulated fluorescent IgG in contacts between K562 cells expressing low affinity Fc receptors and planar bilayers with fluorescent IgG bound to hapten-derivitized phospholipids displayed slower recovery than CD58 by a factor of 10. This suggests that the Fc receptor-IgG interaction has a longer lifetime than the CD2-CD58 interaction. These findings have implications for the mechanism of signaling by CD2 and the mechanism of cell detachment from large numbers of transient interactions.

Antigen receptor engagement delivers a stop signal to migrating T lymphocytes

We investigated the role of the T cell antigen receptor (TcR) in control of T cell migration in an in vitro system. We used T cells from transgenic mice bearing a TcR for the lysozyme peptide 48-62 bound to I-A(k) (3A9). T cells from the 3A9 TcR transgenic mice crawled on purified intercellular adhesion molecule-1 substrates, but strikingly, stopped upon interaction with the physiological ligand, i.e., the mouse I-A(k) with covalently attached hen egg white lysozyme peptide residues 48-62 complex. TcR-triggered stopping was reversible by treatment with adhesion-strengthening phorbol esters. The microtubule organizing center of stopped cells was positioned adjacent to the site of stable cell anchorage. Direct conversion of lymphocyte function associated-1 to the high-affinity conformation with antibodies also stopped T cells in a similar manner to antigen. Thus, physiological TcR engagement triggers a stop signal through lymphocyte function associated-1. We propose that the stop signal is an early and essential event in T cell activation that also will play an important role in control of T cell migration.

TCR-mediated adhesion of T cell hybridomas to planar bilayers containing purified MHC class II/peptide complexes and receptor shedding during detachment

T cell recognition of foreign Ag/MHC class II complexes is sensitive down to approximately 100 complexes per cell or approximately 0.2 complexes/micron2. To better understand the physical basis of the recognition stage of Ag presentation, we examined adhesion of the lysozyme- specific T cell hybridoma, 3A9, to artificial bilayers containing covalent MHC class II/peptide complexes or adhesion molecules. Adhesion of 3A9 cells required a superphysiologic density of the MHC class II/peptide complex and was partly dependent on CD4; cells adhered but did not crawl. No adhesion was observed to bilayers containing MHC class II molecules without the lysozyme peptide. Activated 3A9 cells adhered and crawled on bilayers containing ICAM-1. The physical strength of contacts was tested with fluid shear. 3A9 cells adherent to bilayers containing MHC class II/peptide complexes shed their contact, which remained on the substrate and contained TCR. In contrast, 3A9 cells peeled from the ICAM-1 bilayer, and held firmly on LFA-1 bilayers; in a manner dependent on filamentous actin. When ICAM-1 and the MHC/peptide complexes were combined, the 3A9 cells adhered tightly and spread, but did not crawl, on the bilayers and TCR clustered at the center of the contact area. Physiologically, the TCR is unlikely to directly initiate adhesion. TCR clusters formed with the assistance of adhesion mechanisms may have to be shed to allow de-adhesion, and this may contribute to TCR down-regulation.

TCR-mediated adhesion of T cell hybridomas to planar bilayers containing purified MHC class II/peptide complexes and receptor shedding during detachment

T cell recognition of foreign Ag/MHC class II complexes is sensitive down to approximately 100 complexes per cell or approximately 0.2 complexes/micron2. To better understand the physical basis of the recognition stage of Ag presentation, we examined adhesion of the lysozyme- specific T cell hybridoma, 3A9, to artificial bilayers containing covalent MHC class II/peptide complexes or adhesion molecules. Adhesion of 3A9 cells required a superphysiologic density of the MHC class II/peptide complex and was partly dependent on CD4; cells adhered but did not crawl. No adhesion was observed to bilayers containing MHC class II molecules without the lysozyme peptide. Activated 3A9 cells adhered and crawled on bilayers containing ICAM-1. The physical strength of contacts was tested with fluid shear. 3A9 cells adherent to bilayers containing MHC class II/peptide complexes shed their contact, which remained on the substrate and contained TCR. In contrast, 3A9 cells peeled from the ICAM-1 bilayer, and held firmly on LFA-1 bilayers; in a manner dependent on filamentous actin. When ICAM-1 and the MHC/peptide complexes were combined, the 3A9 cells adhered tightly and spread, but did not crawl, on the bilayers and TCR clustered at the center of the contact area. Physiologically, the TCR is unlikely to directly initiate adhesion. TCR clusters formed with the assistance of adhesion mechanisms may have to be shed to allow de-adhesion, and this may contribute to TCR down-regulation.

Adhesion-activating phorbol ester increases the mobility of leukocyte integrin LFA-1 in cultured lymphocytes

Lymphocytes activate adhesion to intracellular adhesion mlecule 1 (ICAM-1) via leukocyte function associated antigen 1 (LFA-1), their major beta 2 integrin, in response to PMA (phorbol 12-myristate 13-acetate) without an increase in the number of receptors expressed. The molecular details of the mechanism are unknown. To determine the effect of PMA activation on LFA-1 movement within the plasma membrane, we used the single particle tracking technique to measure the diffusion rate of LFA-1 molecules on EBV-transformed B cells before and after PMA activation. Diffusion of LFA-1 on unactivated cells was restricted compared to CR1 (CD35), another transmembrane protein of equivalent size. PMA caused a 10-fold increase in the diffusion rate of LFA-1 without any effect on CD35. The increased LFA-1 motion induced by PMA was random, not directed, indicating that it was due to a release of constraints rather than the application of forces. The diffusion rates of LFA-1 are consistent with cytoskeletal attachment before and free diffusion after PMA. Cytochalasin D led to an equivalent increase in mobility and, at low doses, stimulated adhesion, implying that the nonadhesive state of LFA-1 is actively maintained by the lymphocyte cytoskeleton.

A mannose 6-phosphate-containing N-linked glycopeptide derived from lysosomal acid lipase is bound to MHC class II in B lymphoblastoid cell lines

Binding of peptides to MHC class II Ag generates ligands for TCR of Th lymphocytes. We have identified a novel class of peptides bound to MHC class II: mannose 6- phosphate (Man-6-P) containing glycopeptides from lysosomal enzymes. These species were identified in the process of characterizing mannose 6-phosphate/insulin-like growth factor II (M-6-P/IGF-II) receptor binding to the surface of B lymphoblasts. Surface iodination and Man-6-P/IGF-II receptor affinity chromatography implicated MHC class II as a carrier of Man-6-P-modified oligosaccharides. These oligosaccharides were found to be primarily associated with the bound peptide. Peptides eluted from the Man-6-P/IGF-II receptor-binding fraction of immunoaffinity-purified MHC class II from the Swei cell line contained a sequence derived from the propiece of lysosomal acid lipase. Partial sequences were also obtained for peptides from other HLA-DR alleles but none of these were attributable to known proteins. This study defines a novel approach for isolating rare glycan-modified peptides from MHC class II and demonstrates that very large secondary modifications are tolerated in peptides bound to MHC class II.

A mannose 6-phosphate-containing N-linked glycopeptide derived from lysosomal acid lipase is bound to MHC class II in B lymphoblastoid cell lines

Binding of peptides to MHC class II Ag generates ligands for TCR of Th lymphocytes. We have identified a novel class of peptides bound to MHC class II: mannose 6- phosphate (Man-6-P) containing glycopeptides from lysosomal enzymes. These species were identified in the process of characterizing mannose 6-phosphate/insulin-like growth factor II (M-6-P/IGF-II) receptor binding to the surface of B lymphoblasts. Surface iodination and Man-6-P/IGF-II receptor affinity chromatography implicated MHC class II as a carrier of Man-6-P-modified oligosaccharides. These oligosaccharides were found to be primarily associated with the bound peptide. Peptides eluted from the Man-6-P/IGF-II receptor-binding fraction of immunoaffinity-purified MHC class II from the Swei cell line contained a sequence derived from the propiece of lysosomal acid lipase. Partial sequences were also obtained for peptides from other HLA-DR alleles but none of these were attributable to known proteins. This study defines a novel approach for isolating rare glycan-modified peptides from MHC class II and demonstrates that very large secondary modifications are tolerated in peptides bound to MHC class II.

Visualization of CD2 interaction with LFA-3 and determination of the two-dimensional dissociation constant for adhesion receptors in a contact area

Many adhesion receptors have high three-dimensional dissociation constants (Kd) for counter-receptors compared to the KdS of receptors for soluble extracellular ligands such as cytokines and hormones. Interaction of the T lymphocyte adhesion receptor CD2 with its counter-receptor, LFA-3, has a high solution-phase Kd (16 microM at 37 degrees C), yet the CD2/LFA-3 interaction serves as an effective adhesion mechanism. We have studied the interaction of CD2 with LFA-3 in the contact area between Jurkat T lymphoblasts and planar phospholipid bilayers containing purified, fluorescently labeled LFA-3. Redistribution and lateral mobility of LFA-3 were measured in contact areas as functions of the initial LFA-3 surface density and of time after contact of the cells with the bilayers. LFA-3 accumulated at sites of contact with a half-time of approximately 15 min, consistent with the previously determined kinetics of adhesion strengthening. The two-dimensional Kd for the CD2/LFA-3 interaction was 21 molecules/microns 2, which is lower than the surface densities of CD2 on T cells and LFA-3 on most target or stimulator cells. Thus, formation of CD2/LFA-3 complexes should be highly favored in physiological interactions. Comparison of the two-dimensional (membrane-bound) and three-dimensional (solution-phase) KdS suggest that cell-cell contact favors CD2/LFA-3 interaction to a greater extent than that predicted by the three-dimensional Kd and the intermembrane distance at the site of contact. LFA-3 molecules in the contact site were capable of lateral diffusion in the plane of the phospholipid bilayer and did not appear to be irreversibly trapped in the contact area, consistent with a rapid off-rate. These data provide insights into the function of low affinity interactions in adhesion.

Intercellular adhesion molecule-1 dimerization and its consequences for adhesion mediated by lymphocyte function associated-1

Intercellular adhesion molecule-1 (ICAM-1, CD54) is a ligand for the integrins lymphocyte function associated-1 (LFA-1, CD11a/CD18) and complement receptor-3 (Mac-1, CD11b/CD18) making it an important participant in many immune and inflammatory processes. Modified recombinant soluble ICAM-1 formed dimers. This result indicated that the ectodomain of ICAM-1 contains homophilic interaction sites. Soluble ICAM-1 dimers bind to solid-phase purified LFA-1 with high avidity (dissociation constant [Kd] = 8 nM) in contrast to soluble ICAM-1 monomers whose binding was not measurable. Cell surface ICAM-1 was found to be dimeric based on two distinct criteria. First, a monoclonal antibody specific for monomeric soluble ICAM-1, CA7, binds normal ICAM-1 poorly at the cell surface; this antibody, however, binds strongly to two mutant forms of ICAM-1 when expressed at the cell surface, thus identifying elements required for dimer formation. Second, chemical cross-linking of cell surface ICAM-1 on transfected cells and tumor necrosis factor-activated endothelial cells results in conversion of a portion of ICAM-1 to a covalent dimer. Cell surface ICAM-1 dimers are more potent ligands for LFA-1-dependent adhesion than ICAM-1 monomers. While many extracellular matrix-associated ligands of integrins are multimeric, this is the first evidence of specific, functionally important homodimerization of a cell surface integrin ligand.

Characterization of intercellular adhesion molecule-1 ectodomain (sICAM-1) as an inhibitor of lymphocyte function-associated molecule-1 interaction with ICAM-1

Intercellular adhesion molecule-1 (ICAM-1) is a member of the Ig superfamily, contains five Ig-like domains comprising the extracellular portion of the molecule, and interacts with lymphocyte function-associated molecule-1 (LFA-1), a member of the beta 2-integrin family. LFA-1/ICAM-1 interaction is important in a variety of cellular events including Ag-specific T cell activation and leukocyte transendothelial migration. Recently, a soluble circulating form of ICAM-1 has been detected in human serum that appears to result from the proteolytic cleavage of membrane ICAM-1. Native and recombinant soluble forms of ICAM-1 have been reported to inhibit LFA-1/ICAM-mediated adhesion in vitro, and it is conceivable that circulating forms of soluble ICAM-1 are regulators of LFA-1/ICAM-1-mediated cell-cell interaction in vivo. We have investigated the properties of the ICAM-1 ectodomain (sICAM453) as an inhibitor of LFA-1 interaction with ICAM-1 in cell- and molecule-based systems. The results show clearly that recombinant sICAM453 can inhibit LFA-1/ICAM-1 interaction. Soluble ICAM-1 inhibited LFA-1-mediated cell adhesion to immobilized sICAM453 and homotypic T-cell aggregation with IC50 in the 20 to 40 microM range. Definitive evidence that sICAM-1 can inhibit LFA-1 interaction with ICAM-1 was obtained by showing that the sICAM-1 inhibited the interaction between LFA-1 protein micelles and ICAM-1 immobilized on plastic. These results clearly show that sICAM453 can bind to LFA-1 and competitively inhibit ICAM-1/LFA-1-mediated cell-cell interaction, albeit at concentrations much greater than found in plasma. As a consequence, it is unlikely that sICAM-1 would antagonize ICAM-1/LFA-1-mediated cellular events in vivo.

Characterization of intercellular adhesion molecule-1 ectodomain (sICAM-1) as an inhibitor of lymphocyte function-associated molecule-1 interaction with ICAM-1

Intercellular adhesion molecule-1 (ICAM-1) is a member of the Ig superfamily, contains five Ig-like domains comprising the extracellular portion of the molecule, and interacts with lymphocyte function-associated molecule-1 (LFA-1), a member of the beta 2-integrin family. LFA-1/ICAM-1 interaction is important in a variety of cellular events including Ag-specific T cell activation and leukocyte transendothelial migration. Recently, a soluble circulating form of ICAM-1 has been detected in human serum that appears to result from the proteolytic cleavage of membrane ICAM-1. Native and recombinant soluble forms of ICAM-1 have been reported to inhibit LFA-1/ICAM-mediated adhesion in vitro, and it is conceivable that circulating forms of soluble ICAM-1 are regulators of LFA-1/ICAM-1-mediated cell-cell interaction in vivo. We have investigated the properties of the ICAM-1 ectodomain (sICAM453) as an inhibitor of LFA-1 interaction with ICAM-1 in cell- and molecule-based systems. The results show clearly that recombinant sICAM453 can inhibit LFA-1/ICAM-1 interaction. Soluble ICAM-1 inhibited LFA-1-mediated cell adhesion to immobilized sICAM453 and homotypic T-cell aggregation with IC50 in the 20 to 40 microM range. Definitive evidence that sICAM-1 can inhibit LFA-1 interaction with ICAM-1 was obtained by showing that the sICAM-1 inhibited the interaction between LFA-1 protein micelles and ICAM-1 immobilized on plastic. These results clearly show that sICAM453 can bind to LFA-1 and competitively inhibit ICAM-1/LFA-1-mediated cell-cell interaction, albeit at concentrations much greater than found in plasma. As a consequence, it is unlikely that sICAM-1 would antagonize ICAM-1/LFA-1-mediated cellular events in vivo.

A novel mutagenesis strategy identifies distantly spaced amino acid sequences that are required for the phosphorylation of both the oligosaccharides of procathepsin D by N-acetylglucosamine 1-phosphotransferase

A novel combinatorial mutagenesis strategy (shuffle mutagenesis) was developed to identify sequences in the propiece and amino lobe of cathepsin D which direct oligosaccharide phosphorylation by UDP-GlcNAc:lysosomal enzyme N-acetylglucosamine 1-phosphotransferase. Propiece restriction fragments and oligonucleotide cassettes corresponding to 13 regions of the cathepsin D and glycopepsinogen amino lobes were randomly shuffled together to generate a large library of chimeric molecules. The library was inserted into an expression vector encoding the carboxyl lobe of cathepsin D with a carboxyl-terminal myc epitope and a CD8 transmembrane extension. Transfected COS1 cells expressing the membrane-anchored forms of the cathepsin D/glycopepsinogen chimeras at the cell surface were selected with solid phase mannose 6-phosphate receptor or an antibody to the myc epitope. Plasmids were rescued in Escherichia coli and sequenced by hybridization to the original oligonucleotide cassettes. Two regions of the cathepsin D amino lobe (segments 7 and 12) were found to contribute to proper folding, surface expression, and selective phosphorylation of the carboxyl lobe oligosaccharide. Two different cathepsin D regions (the propiece and segment 5) cooperated with a previously identified recognition element in the carboxyl lobe to allow efficient phosphorylation of both the amino and carboxyl lobe oligosaccharides. Three general models for extending the catalytic reach of N-acetylglucosamine 1-phosphotransferase to widely spaced oligosaccharides are presented.

Micromanipulation of adhesion of phorbol 12-myristate-13-acetate-stimulated T lymphocytes to planar membranes containing intercellular adhesion molecule-1

This paper presents an analytical and experimental methodology to determine the physical strength of cell adhesion to a planar membrane containing one set of adhesion molecules. In particular, the T lymphocyte adhesion due to the interaction of the lymphocyte function associated molecule 1 on the surface of the cell, with its counter-receptor, intercellular adhesion molecule-1 (ICAM-1), on the planar membrane, was investigated. A micromanipulation method and mathematical analysis of cell deformation were used to determine (a) the area of conjugation between the cell and the substrate and (b) the energy that must be supplied to detach a unit area of the cell membrane from its substrate. T lymphocytes stimulated with phorbol 12-myristate-13-acetate (PMA) conjugated strongly with the planar membrane containing purified ICAM-1. The T lymphocytes attached to the planar membrane deviated occasionally from their round configuration by extending pseudopods but without changing the size of the contact area. These adherent cells were dramatically deformed and then detached when pulled away from the planar membrane by a micropipette. Detachment occurred by a gradual decrease in the radius of the contact area. The physical strength of adhesion between a PMA-stimulated T lymphocyte and a planar membrane containing 1,000 ICAM-1 molecules/micron 2 was comparable to the strength of adhesion between a cytotoxic T cell and its target cell. The comparison of the adhesive energy density, measured at constant cell shape, with the model predictions suggests that the physical strength of cell adhesion may increase significantly when the adhesion bonds in the contact area are immobilized by the actin cytoskeleton.

Regulation of locomotion and cell-cell contact area by the LFA-1 and ICAM-1 adhesion receptors

We demonstrate complementary differences in the behavior of B lymphoblastoid cells adhering to LFA-1 or its counter-receptor ICAM-1. The interaction of B lymphoblastoid cells with glass-supported planar bilayers bearing LFA-1 or ICAM-1 was observed by time-lapse video microscopy, and the distribution of adhesion receptors on cells interacting with the planar bilayers was studied by immunofluorescence microscopy. B lymphoblasts formed a large contact area and crawled rapidly (up to 25 microns/min) on planar bilayers bearing ICAM-1. In contrast, these cells attached to planar bilayers bearing LFA-1 through a fixed point about which the cells actively pivoted, using a single stalk-like projection. Phorbol ester-stimulated lymphoblasts, which adhere more strongly to ICAM-1-bearing substrates than unstimulated lymphoblasts, were still capable of locomotion on ICAM-1. Phorbol ester stimulation of B lymphoblasts on planar bilayers bearing LFA-1 promoted a rapid conversion from "stalk" attachment to symmetrical spreading of the cell on the substrate. Cellular LFA-1 remained uniformly distributed on the cell surface during interaction with bilayers bearing purified ICAM-1 as determined by immunofluorescence. In contrast, ICAM-1 was concentrated in the stalk-like structure through which the unstimulated B lymphoblasts adhered to LFA-1 in planar bilayers, but ICAM-1 immunofluorescence became more uniformly distributed over the cell surface within minutes of phorbol ester addition. Neither LFA-1 or ICAM-1 colocalized with the prominent staining of filamentous actin in the ruffling membrane regions. Interaction through cell surface LFA-1 and ICAM-1, 2, or 3 promotes different cellular morphologies and behaviors, the correlation of which with previously observed patterns of lymphocyte interaction with different cell types is discussed.

Regulation of locomotion and cell-cell contact area by the LFA-1 and ICAM-1 adhesion receptors

We demonstrate complementary differences in the behavior of B lymphoblastoid cells adhering to LFA-1 or its counter-receptor ICAM-1. The interaction of B lymphoblastoid cells with glass-supported planar bilayers bearing LFA-1 or ICAM-1 was observed by time-lapse video microscopy, and the distribution of adhesion receptors on cells interacting with the planar bilayers was studied by immunofluorescence microscopy. B lymphoblasts formed a large contact area and crawled rapidly (up to 25 microns/min) on planar bilayers bearing ICAM-1. In contrast, these cells attached to planar bilayers bearing LFA-1 through a fixed point about which the cells actively pivoted, using a single stalk-like projection. Phorbol ester-stimulated lymphoblasts, which adhere more strongly to ICAM-1-bearing substrates than unstimulated lymphoblasts, were still capable of locomotion on ICAM-1. Phorbol ester stimulation of B lymphoblasts on planar bilayers bearing LFA-1 promoted a rapid conversion from "stalk" attachment to symmetrical spreading of the cell on the substrate. Cellular LFA-1 remained uniformly distributed on the cell surface during interaction with bilayers bearing purified ICAM-1 as determined by immunofluorescence. In contrast, ICAM-1 was concentrated in the stalk-like structure through which the unstimulated B lymphoblasts adhered to LFA-1 in planar bilayers, but ICAM-1 immunofluorescence became more uniformly distributed over the cell surface within minutes of phorbol ester addition. Neither LFA-1 or ICAM-1 colocalized with the prominent staining of filamentous actin in the ruffling membrane regions. Interaction through cell surface LFA-1 and ICAM-1, 2, or 3 promotes different cellular morphologies and behaviors, the correlation of which with previously observed patterns of lymphocyte interaction with different cell types is discussed.

Micromanipulation of adhesion of a Jurkat cell to a planar bilayer membrane containing lymphocyte function-associated antigen 3 molecules

Cell adhesion plays a fundamental role in the organization of cells in differentiated organs, cell motility, and immune response. A novel micromanipulation method is employed to quantify the direct contribution of surface adhesion receptors to the physical strength of cell adhesion. In this technique, a cell is brought into contact with a glass-supported planar membrane reconstituted with a known concentration of a given type of adhesion molecules. After a period of incubation (5-10 min), the cell is detached from the planar bilayer by pulling away the pipette holding the cell in the direction perpendicular to the glass-supported planar bilayer. In particular, we investigated the adhesion between a Jurkat cell expressing CD2 and a glass-supported planar bilayer containing either the glycosyl- phosphatidylinositol (GPI) or the transmembrane (TM) isoform of the counter-receptor lymphocyte function-associated antigen 3 (LFA-3) at a concentration of 1,000 molecules/microns 2. In response to the pipette force the Jurkat cells that adhered to the planar bilayer containing the GPI isoform of LFA-3 underwent extensive elongation. When the contact radius was reduced by approximately 50%, the cell then detached quickly from its substrate. The aspiration pressure required to detach a Jurkat cell from its substrate was comparable to that required to detach a cytotoxic T cell from its target cell. Jurkat cells that had been separated from the substrate again adhered strongly to the planar bilayer when brought to proximity by micromanipulation. In experiments using the planar bilayer containing the TM isoform of LFA-3, Jurkat cells detached with little resistance to micromanipulation and without changing their round shape.

Motility and ultrastructure of large granular lymphocytes on lipid bilayers reconstituted with adhesion receptors LFA-1, ICAM-1, and two isoforms of LFA-3

Large granular lymphocytes, mediators of NK activity, bind to other cells using both the LFA (lymphocyte function-associated)-1-ICAM and the CD2-LFA-3 adhesion pathways. Here we have studied the motility and ultrastructure of large granule lymphocyte (LGL) on lipid bilayers containing purified LFA-1, ICAM-1, and the transmembrane and glycophosphatidylinositol isoforms of LFA-3. LGLs moved at 8 microns/min on ICAM-1 but poorly (less than 1 microns/min) on its receptor pair LFA-1. TM-LFA-3 promoted locomotion at a rate close to ICAM-1, whereas the cells were less motile on GPI-LFA-3. The difference in the rates of locomotion on the two isoforms of LFA-3 is presumably attributable to their difference in anchoring and lateral mobility in the bilayer. In spite of the variation in motility the ultrastructure of the adhering cells was similar on all four ligands. LGLs contacted the membrane variably, i.e., cells adhering only in a few small areas or in larger areas were detected on each ligand. The relative percentage of the plasma membrane facing the lipid bilayer was greatest on ICAM-1 and least on the transmembrane isoform of LFA-3, demonstrating no correlation with motility. The ratio of adjacent plasma membrane to lipid bilayer was virtually constant for all four ligands. Activation of the LGLs with a combination of CD2 mAb T11(2) and T11(3) (T11(2/3) mAb) reduced the movement on ICAM-1 and virtually immobilized the cells on the other bilayers. In the presence of T11(2/3) mAb, the area of cell membrane attaching to bilayers containing ICAM-1 and GPI-LFA-3 was decreased and the percentage of plasma membrane facing other cells was increased. No preferential orientation of the Golgi apparatus or degranulation was detected in the absence or presence of T11(2/3) mAb, but a significantly lower percentage of LGLs on ICAM-1 contained a profile of the Golgi apparatus after exposure to T11(2/3) mAb. The results demonstrate that the motility of LGLs depends on the type of receptor in the opposing bilayer, the receptor mobility in the bilayer, and the activation of the cells. The ultrastructure of LGLs binding to any of the adhesion molecules does not have the characteristics of LGLs in cytolytic contact with target cells, suggesting that the mediation of an attack on a target requires more complex stimulus than any one of the single adhesion proteins tested here.

Influence of receptor lateral mobility on adhesion strengthening between membranes containing LFA-3 and CD2

We have used an in vitro model system of glass-supported planar membranes to study the effects of lateral mobility of membrane-bound receptors on cell adhesion. Egg phosphatidylcholine (PC) bilayers were reconstituted with two anchorage isoforms of the adhesion molecule lymphocyte function-associated antigen 3 (LFA-3). The diffusion coefficient of glycosyl phosphatidylinositol (GPI)-anchored LFA-3 approached that of phospholipids in the bilayers, whereas the transmembrane (TM)-anchored isoform of LFA-3 was immobile. Both static and laminar flow assays were used to quantify the strength of adherence to the lipid bilayers of the T lymphoma cell line Jurkat that expresses the counter-receptor CD2. Cell adhesion was dependent on LFA-3 density and was more efficient on membranes containing the GPI isoform than the TM isoform. Kinetic measurements demonstrated an influence of contact time on the strength of adhesion to the GPI isoform at lower site densities (25-50 sites/microns2), showing that the mobility of LFA-3 is important in adhesion strengthening. At higher site densities (1,500 sites/microns2) and longer contact times (20 min), Jurkat cell binding to the TM and GPI isoforms of LFA-3 showed equivalent adhesion strengths, although adhesion strength of the GPI isoform developed twofold more rapidly than the TM isoform. Reduction of CD2 mobility on Jurkat cells at 5 degrees C greatly decreased the rate of adhesion strengthening with the TM isoform of LFA-3, resulting in a 30-fold difference between the two LFA-3 isoforms. Our results demonstrate that the ability of a membrane receptor and its membrane-bound counter-receptor to diffuse laterally enhances cell adhesion both by allowing accumulation of ligands in the cell contact area and by increasing the rate of receptor-ligand bond formation.

Role of lymphocyte adhesion receptors in transient interactions and cell locomotion

Lymphocytes adhere to other cells and extracellular matrix in the process of immunological recognition and lymphocyte recirculation. This review focuses on regulation of lymphocyte adhesion and the use of adhesion mechanisms by lymphocytes to obtain information about their immediate environment. The CD2 and LFA-1 adhesion receptors appear to have distinct roles in the regulation of adhesion and modulation of T lymphocyte activation. Adhesion mediated by interaction of CD2 with LFA-3 is dramatically altered by surface charge and adhesion receptor density in such a way that this pathway is latent in resting T lymphocytes but becomes active over a period of hours following T-cell activation. CD2 ligation can mediate or enhance T-cell activation, suggesting that signals from CD2/LFA-3 adhesive interactions are integrated with signals from the T-cell antigen receptor during immunological recognition. A model for the role of LFA-3 lateral diffusion in adhesion is presented, based on the lateral diffusion of different LFA-3 forms in glass supported planar membranes. Interaction of LFA-1 with ICAMs is also regulated by cell activation but in a different way than in interaction of CD2 with LFA-3. LFA-1 avidity for ICAMs is transiently increased by T-cell activation over a period of minutes. Cycles of avidity change are also observed for other T lymphocyte integrins which bind to extracellular matrix components. We propose that integrin avidity cycles may have an important role in the interconnected phenomena of locomotion, initial cell-cell adhesion, and cell-cell deadhesion. Recent observations on recirculation of T lymphocyte subpopulations are discussed in the context of general lessons learned from study of the CD2/LFA-3 and LFA-1/ICAM adhesion mechanisms.

Two-way signalling through the LFA-1 lymphocyte adhesion receptor

T lymphocyte recognition of foreign antigens and migration throughout the body require the regulated adhesion of lymphocytes to diverse types of cells and to the extracellular matrix. The lymphocyte adhesion 'receptor' LFA-1, a member of the integrin family, interacts with ICAM-1 and other counter-receptors to mediate adhesion. The LFA-1/ICAM-1 interaction is regulated by signals transmitted from the cytoplasm to the extracellular space. Conversely, LFA-1 transmits signals from the extracellular space to the cytoplasm to regulate T lymphocyte activation. The observed properties of LFA-1 and related adhesion 'receptors' are incorporated into a general model for adhesion during immune surveillance and recognition of foreign antigens.

On the species specificity of the interaction of LFA-1 with intercellular adhesion molecules

Species restrictions in immune cell interactions have been demonstrated both in Ag-specific responses of T lymphocytes and the phenomenon of natural attachment. To determine the possible contribution of adhesion receptors to these restrictions, we have studied binding between the murine and human homologues of LFA-1 (CD11a/CD18) and ICAM employing purified human LFA-1 and ICAM-1 (CD54) bound to solid substrates. Murine cell lines bind to purified human LFA-1 through ICAM-1 and at least one other counter-receptor. This provides evidence for multiple counter-receptors for LFA-1 in the mouse as well as in the human. In contrast to binding of murine ICAM-1 to human LFA-1, murine LFA-1 does not bind to human ICAM-1. The species specificity maps to the LFA-1 alpha subunit, because mouse x human hybrid cells expressing the human alpha subunit associated with a mouse beta subunit bind to human ICAM-1, whereas those with a human beta subunit associated with a murine alpha subunit do not. Increased adhesiveness for ICAM-1 stimulated by phorbol esters could be demonstrated for hybrid LFA-1 molecules with human alpha and murine beta subunits.