Effect of lengthening lymphocyte function-associated antigen 3 on adhesion to CD2

In situ and in silico kinetic analyses of programmed cell death-1 (PD-1) receptor, programmed cell death ligands, and B7-1 protein interaction network

Programmed cell death-1 (PD-1) is an inhibitory receptor with an essential role in maintaining peripheral tolerance and is among the most promising immunotherapeutic targets for treating cancer, autoimmunity, and infectious diseases. A complete understanding of the consequences of PD-1 engagement by its ligands, PD-L1 and PD-L2, and of PD-L1 binding to B7-1 requires quantitative analysis of their interactions at the cell surface. We present here the first complete in situ kinetic analysis of the PD-1/PD-ligands/B7-1 system. Consistent with previous solution measurements, we observed higher in situ affinities for human (h) than murine (m) PD-1 interactions, stronger binding of hPD-1 to hPD-L2 than hPD-L1, and comparable binding of mPD-1 to both ligands. However, in contrast to the relatively weak solution affinities, the in situ affinities of PD-1 are as high as those of the T cell receptor for agonist pMHC and of LFA-1 (lymphocyte function-associated antigen 1) for ICAM-1 (intercellular adhesion molecule 1) but significantly lower than that of the B7-1/CTLA-4 interaction, suggesting a distinct basis for PD-1- versus CTLA-4-mediated inhibition. Notably, the in situ interactions of PD-1 are much stronger than that of B7-1 with PD-L1. Overall, the in situ affinity ranking greatly depends on the on-rate instead of the off-rate. In silico simulations predict that PD-1/PD-L1 interactions dominate at interfaces between activated T cells and mature dendritic cells and that these interactions will be highly sensitive to the dynamics of PD-L1 and PD-L2 expression. Our results provide a kinetic framework for better understanding inhibitory PD-1 activity in health and disease.

Binding constants of membrane-anchored receptors and ligands: A general theory corroborated by Monte Carlo simulations

Adhesion processes of biological membranes that enclose cells and cellular organelles are essential for immune responses, tissue formation, and signaling. These processes depend sensitively on the binding constant K2D of the membrane-anchored receptor and ligand proteins that mediate adhesion, which is difficult to measure in the "two-dimensional" (2D) membrane environment of the proteins. An important problem therefore is to relate K2D to the binding constant K3D of soluble variants of the receptors and ligands that lack the membrane anchors and are free to diffuse in three dimensions (3D). In this article, we present a general theory for the binding constants K2D and K3D of rather stiff proteins whose main degrees of freedom are translation and rotation, along membranes and around anchor points "in 2D," or unconstrained "in 3D." The theory generalizes previous results by describing how K2D depends both on the average separation and thermal nanoscale roughness of the apposing membranes, and on the length and anchoring flexibility of the receptors and ligands. Our theoretical results for the ratio K2D/K3D of the binding constants agree with detailed results from Monte Carlo simulations without any data fitting, which indicates that the theory captures the essential features of the "dimensionality reduction" due to membrane anchoring. In our Monte Carlo simulations, we consider a novel coarse-grained model of biomembrane adhesion in which the membranes are represented as discretized elastic surfaces, and the receptors and ligands as anchored molecules that diffuse continuously along the membranes and rotate at their anchor points.

Nanoscale increases in CD2-CD48-mediated intermembrane spacing decrease adhesion and reorganize the immunological synapse

The relationship between intermembrane spacing, adhesion efficiency, and lateral organization of adhesion receptors has not been established for any adhesion system. We have utilized the CD2 ligand CD48 with two (wild type CD48 (CD48-WT)), four (CD48-CD2), or five (CD48-CD22) Ig-like domains. CD48-WT was 10-fold more efficient in mediating adhesion than CD48-CD2 or CD48-CD22. Electron tomography of contact areas with planar bilayers demonstrated average intermembrane spacing of 12.8 nm with CD48-WT, 14.7 nm with CD48-CD2, and 15.6 nm with CD48-CD22. Both CD48-CD2 and CD48-CD22 chimeras segregated completely from CD48-WT in mixed contact areas. In contrast, CD48-CD2 and CD48-CD22 co-localized when mixed contacts were formed. Confocal imaging of immunological synapses formed between primary T lymphocytes and Chinese hamster ovary cells presenting major histocompatibility complex-peptide complexes, and different forms of CD48 demonstrated that CD48-CD2 and CD48-CD22 induce an eccentric CD2/T cell antigen receptor cluster. We propose that this reorganization of the immunological synapse sequesters the T cell antigen receptor in a location where it cannot interact with its ligand and dramatically reduces T cell sensitivity.

Experimental approaches to lymphocyte migration in dermatology in vitro and in vivo

Lymphocyte trafficking through the dermal compartment is part of the physiological surveillance process of the adaptive immune system. On the other hand, persistent or recurrent lymphocyte infiltrates are hallmarks of both types of chronic inflammatory skin diseases, Th1-type such as psoriasis or Th2/allergic-type like atopic dermatitis. A better understanding of the mechanisms underlying lymphocyte movements is one of the key prerequisites for developing more effective therapies. In this review, we introduce a range of simple-to-sophisticated experimental in vitro and in vivo approaches to analyze lymphocyte migration. These methods start from static in vitro adhesion and chemotaxis assays, include dynamic endothelial flow chamber, intravital dual photon, and transcutaneous live-video microscopy, and finally encompass specific genetically deficient or engineered animal models. Discussing pros and cons of these assay systems hopefully generates both state-of-the-art knowledge about the factors involved in most common chronic skin diseases as well as an improved understanding of the limitations and chances of new biologic pharmaceuticals that are currently introduced into clinical practice.

T-cell receptor triggering is critically dependent on the dimensions of its peptide-MHC ligand

The binding of a T-cell antigen receptor (TCR) to peptide antigen presented by major histocompatibility antigens (pMHC) on antigen-presenting cells (APCs) is a central event in adaptive immune responses. The mechanism by which TCR-pMHC ligation initiates signalling, a process termed TCR triggering, remains controversial. It has been proposed that TCR triggering is promoted by segregation at the T cell-APC interface of cell-surface molecules with small ectodomains (such as TCR-pMHC and accessory receptors) from molecules with large ectodomains (such as the receptor protein tyrosine phosphatases CD45 and CD148). Here we show that increasing the dimensions of the TCR-pMHC interaction by elongating the pMHC ectodomain greatly reduces TCR triggering without affecting TCR-pMHC ligation. A similar dependence on receptor-ligand complex dimensions was observed with artificial TCR-ligand systems that span the same dimensions as the TCR-pMHC complex. Interfaces between T cells and APCs expressing elongated pMHC showed an increased intermembrane separation distance and less depletion of CD45. These results show the importance of the small size of the TCR-pMHC complex and support a role for size-based segregation of cell-surface molecules in TCR triggering.

Quantifying the effects of molecular orientation and length on two-dimensional receptor-ligand binding kinetics

Surface presentation of adhesion receptors influences cell adhesion, although the mechanisms underlying these effects are not well understood. We used a micropipette adhesion frequency assay to quantify how the molecular orientation and length of adhesion receptors on the cell membrane affected two-dimensional kinetic rates of interactions with surface ligands. Interactions of P-selectin, E-selectin, and CD16A with their respective ligands or antibody were used to demonstrate such effects. Randomizing the orientation of the adhesion receptor or lowering its ligand- and antibody-binding domain above the cell membrane lowered two-dimensional affinities of the molecular interactions by reducing the forward rates but not the reverse rates. In contrast, the soluble antibody bound with similar three-dimensional affinities to cell-bound P-selectin constructs regardless of their orientation and length. These results demonstrate that the orientation and length of an adhesion receptor influences its rate of encountering and binding a surface ligand but does not subsequently affect the stability of binding.

Cell adhesion and polarity during immune interactions

Intercellular interactions are critical for a coordinated function of different cell types involved in the immune response. Here we review the cellular and molecular events occurring during cell-cell immune contacts. Cognate naïve CD4+ T lymphocyte-dendritic cell (DC) and primed T cell-antigen-presenting B lymphocyte interactions are discussed. The engagement of cytotoxic T lymphocytes (CTL) or natural killer cells (NK) with their targets is analyzed and compared to the process of T cell-antigen-presenting cell (APC) conjugate formation. The immunological synapse, a complex cluster of molecules organized at the contact area of cell conjugates, exhibits common features but shows some differences depending on cell types involved. Cellular interactions occur in sequential stages that involve dramatic changes in cell polarity and dynamic redistribution of cell membrane receptors. The role of membrane microdomains, adaptor molecules and the cytoskeleton in the regulation of the molecular reorganization at cell-cell contacts is also discussed.

Measuring the forces that control protein interactions

Although the force fields and interaction energies that control protein behavior can be inferred indirectly from equilibrium and kinetic measurements, recent developments have made it possible to quantify directly (a) the ranges, magnitudes, and time dependence of the interaction energies and forces between biological materials; (b) the mechanical properties of isolated proteins; and (c) the strength of single receptor-ligand bonds. This review describes recent results obtained by using the atomic force microscope, optical tweezers, the surface force apparatus, and micropipette aspiration to quantify short-range protein-ligand interactions and the long-range, nonspecific forces that together control protein behavior. The examples presented illustrate the power of force measurements to quantify directly the force fields and energies that control protein behavior.

T-lymphocyte coactivator molecules

T-cell recognition and activation occurs within a specialized area of contact known as the immunologic synapse, localized to areas of glycolipid-enriched membrane microdomains. Within this area, T-cell activation is dependent not only upon specific recognition of peptide antigen embedded within molecules of the major histocompatibility complex, but also on a variety of costimulatory receptors and interactions. Engagement of T-cell receptor (TCR) with antigen alone will induce T-cell unresponsiveness; ligation of the coreceptor CD28 will prevent the induction of unresponsiveness. Novel costimulatory molecules belonging to both the CD28 and TNF/TNFR superfamilies have recently been identified. These receptors appear to act at different stages of T-cell differentiation and activation, have been shown to play a role in promoting different T-cell effector functions, and are important for B-cell differentiation and function.

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.

The membrane anchor influences ligand binding two-dimensional kinetic rates and three-dimensional affinity of FcgammaRIII (CD16)

Kinetic rates and affinity are essential determinants for biological processes that involve receptor-ligand binding. By using a micropipette method, we measured the kinetics of human Fcgamma receptor III (CD16) interacting with IgG when the two molecules were bound to apposing cellular membranes. CD16 is one of only four eukaryotic receptors known to exist natively in both the transmembrane (TM, CD16a) and glycosylphosphatidylinositol (GPI, CD16b) isoforms. The biological significance of this anchor isoform coexistence is not clear. Here we showed that the anchor influenced kinetic rates; compared with CD16a-TM, CD16a-GPI bound faster and with higher affinities to human and rabbit IgGs but slower and with lower affinity to murine IgG2a. The same differential affinity patterns were observed using soluble IgG ligands. A monoclonal antibody bound CD16a-GPI with higher affinity than CD16a-TM, whereas another monoclonal antibody reacted strongly with CD16a-TM but weakly with CD16a-GPI. No major differential glycosylation between the two CD16a isoforms was detected by SDS-polyacrylamide gel electrophoresis analysis. We suggest a conformational difference as the mechanism underlying the observed anchor effect, as it cannot be explained by the differing diffusivity, flexibility, orientation, height, distribution, or clustering of the two molecules on the cell membrane. These data demonstrate that a covalent modification of an Ig superfamily receptor at the carboxyl terminus of the ectodomain can have an impact on ligand binding kinetics.

Branched O-linked oligosaccharides ectopically expressed in transgenic mice reduce primary T-cell immune responses

Core 2 beta-1,6-N-acetylglucosaminyltransferase, C2GnT, is a key enzyme in O-linked oligosaccharide (O-glycan) biosynthesis and the resultant core 2 branch serves as a backbone for additional glycosylation to form oligosaccharide ligands such as sialyl Le(x). Since the expression of C2GnT is highly regulated during T-cell development and increases in pathological conditions such as the Wiskott-Aldrich syndrome, we have generated transgenic mice overexpressing C2GnT in the T-cell lineage. Surprisingly, T lymphocytes in the transgenic mice develop normally, but they exhibit a reduced immune response when assayed by delayed-type hypersensitivity, proliferation upon stimulation and cytokine production. Moreover, T lymphocytes from the transgenic mice adhere much less efficiently to ICAM-1 and fibronectin than do T lymphocytes from non-transgenic mice. These results indicate that overexpression of the core 2 branched O-glycans in T lymphocytes results in reduced immune responses due to impaired cell-cell interaction. Such an impaired immune response may be one of the causes for immunodeficiency in the Wiskott-Aldrich syndrome.

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.

Expression and polarization of intercellular adhesion molecule-1 on human intestinal epithelia: consequences for CD11b/CD18-mediated interactions with neutrophils

BACKGROUND

Epithelial dysfunction and patient symptoms in inflammatory intestinal diseases such as ulcerative colitis and Crohn's disease correlate with migration of neutrophils (PMN) across the intestinal epithelium. In vitro modeling of PMN transepithelial migration has revealed distinct differences from transendothelial migration. By using polarized monolayers of human intestinal epithelia (T84), PMN transepithelial migration has been shown to be dependent on the leukocyte integrin CD11b/CD18 (Mac-1), but not on CD11a/CD18 (LFA-1). Since intercellular adhesion molecule-I (ICAM-1) is an important endothelial counterreceptor for these integrins, its expression in intestinal epithelia and role in PMN-intestinal epithelial interactions was investigated.

MATERIALS AND METHODS

A panel of antibodies against different domains of ICAM-1, polarized monolayers of human intestinal epithelia (T84), and natural human colonic epithelia were used to examine the polarity of epithelial ICAM-1 surface expression and the functional role of ICAM-1 in neutrophil-intestinal epithelial adhesive interactions.

RESULTS

While no surface expression of ICAM-1 was detected on unstimulated T84 cells, interferon-gamma (IFN gamma) elicited a marked expression of ICAM-1 that selectively polarized to the apical epithelial membrane. Similarly, apically restricted surface expression of ICAM-1 was detected in natural human colonic epithelium only in association with active inflammation. With or without IFN gamma pre-exposure, physiologically directed (basolateral-to-apical) transepithelial migration of PMN was unaffected by blocking monoclonal antibodies (mAbs) to ICAM-1. In contrast, PMN migration across IFN gamma-stimulated monolayers in the reverse (apical-to-basolateral) direction was inhibited by anti-ICAM-1 antibodies. Adhesion studies revealed that T84 cells adhered selectively to purified CD11b/CD18 and such adherence, with or without IFN gamma pre-exposure, was unaffected by ICAM-1 mAb. Similarly, freshly isolated epithelial cells from inflamed human intestine bound to CD11b/CD18 in an ICAM-1-independent fashion.

CONCLUSIONS

These data indicate that ICAM-1 is strictly polarized in intestinal epithelia and does not represent a counterreceptor for neutrophil CD11b/CD18 during physiologically directed transmigration, but may facilitate apical membrane-PMN interactions after the arrival of PMN in the intestinal lumen.

P-selectin must extend a sufficient length from the plasma membrane to mediate rolling of neutrophils

Under physiological shear stress, neutrophils roll on P-selectin on activated endothelial cells or platelets through interactions with P-selectin glycoprotein ligand-1 (PSGL-1). Both P-selectin and PSGL-1 are extended molecules. Human P-selectin contains an NH2-terminal lectin domain, an EGF domain, nine consensus repeats (CRs), a transmembrane domain, and a cytoplasmic tail. To determine whether the length of P-selectin affected its interactions with PSGL-1, we examined the adhesion of neutrophils to CHO cells expressing membrane-anchored P-selectin constructs in which various numbers of CRs were deleted. Under static conditions, neutrophils attached equivalently to wild-type P-selectin and to constructs containing from 2-6 CRs. Under shear stress, neutrophils attached equivalently to wild-type and 6 CR P-selectin and nearly as well to 5 CR P-selectin. However, fewer neutrophils attached to the 4 CR construct, and those that did attach rolled faster and were more readily detached by increasing shear stress. Flowing neutrophils failed to attach to the 3 CR and 2 CR constructs. Neutrophils attached and rolled more efficiently on 4 CR P-selectin expressed on glycosylation-defective Lec8 CHO cells, which have less glycocalyx. We conclude that P-selectin must project its lectin domain well above the membrane to mediate optimal attachment of neutrophils under shear forces. The length of P-selectin may: (a) facilitate interactions with PSGL-1 on flowing neutrophils, and (b) increase the intermembrane distance where specific bonds form, minimizing contacts between the glycocalyces that result in cell-cell repulsion.

Influence of surface charges on cell adhesion: difference between static and dynamic conditions

We tested the hypothesis that nonspecific repulsion, as a result of electrostatic forces and (or) steric stabilization effects, impaired adhesion more efficiently under dynamic than under static conditions. Cells from the human monocytic line THP1 were plated on a glass surface. Spherical particles bearing monoclonal antibodies specific for antigens expressed by THP1 cells (CD11b, CD18, CD35, CD64) were then added and adhesion was quantified. The effect of neuraminidase treatment of THP1 cells was also studied. Adhesion was then measured in a flow chamber under low shear flow (wall shear rate was 11 or 22 s-1), allowing a quantitative determination of cell adhesion frequency. The following conclusions were obtained: (i) under static conditions, neuraminidase treatment had little effect on adhesion (only CD18-mediated interaction was significantly increased at 4 degrees C after enzyme treatment); (ii) under dynamic conditions, neuraminidase treatment significantly increased binding; (iii) surprisingly, there was no clear relationship between the length of adhesion molecules involved in the interaction and binding efficiency; and (iv) such parameters as cell shape and topographical distribution of adhesion molecules may strongly influence adhesion under flow. It is concluded that a dynamic reorganization of the pericellular matrix following intercellular contact may play an important role in regulating adhesion.

Transient intercellular adhesion: the importance of weak protein-protein interactions

Intercellular adhesion is a complex phenomenon central to the development, structure and functioning of all multicellular organisms. Adhesion is mediated by distinct families of cell-adhesion molecules (CAMs), and recent studies have identified key characteristics of CAMs that influence their function. Affinity and kinetic analyses using a novel technique based on surface plasmon resonance have shown that CAM interactions that mediate transient cell adhesion may have surprisingly low affinities and extremely fast dissociation rate constants.

The I domain is a major recognition site on the leukocyte integrin Mac-1 (CD11b/CD18) for four distinct adhesion ligands

Despite the identification and characterization of several distinct ligands for the leukocyte integrin (CD11/CD18) family of adhesion receptors, little is known about the structural regions on these molecules that mediate ligand recognition. In this report, we use alpha subunit chimeras of Mac-1 (CD11b/CD18) and p150,95 (CD11c/CD18), and an extended panel of newly generated and previously characterized mAbs specific to the alpha chain of Mac-1 to map the binding sites for four distinct ligands for Mac-1: iC3b, fibrinogen, ICAM-1, and the as-yet uncharacterized counter-receptor responsible for neutrophil homotypic adhesion. Epitopes of mAbs that blocked ligand binding were mapped with the chimeras and used to localize the ligand recognition sites because the data obtained from functional assays with the Mac-1/p150,95 chimeras were not easily interpreted. Results show that the I domain on the alpha chain of Mac-1 is an important recognition site for all four ligands, and that the NH2-terminal and perhaps divalent cation binding regions but not the COOH-terminal segment may contribute. The recognition sites in the I domain appear overlapping but not identical as individual Mac-1-ligand interactions are distinguished by the discrete patterns of inhibitory mAbs. Additionally, we find that the alpha subunit NH2-terminal region and divalent cation binding region, despite being separated by over 200 amino acids of the I domain, appear structurally apposed because three mAbs require the presence of both of these regions for antigenic reactivity, and chimeras that contain the NH2 terminus of p150,95 require the divalent cation binding region of p150,95 to associate firmly with the beta subunit.

Molecular isoforms of murine CD44 and evidence that the membrane proximal domain is not critical for hyaluronate recognition

We previously found that the CD44 glycoprotein on some lymphocytes can mediate adhesion to hyaluronate (HA) bearing cells. However, many questions remain about the molecular heterogeneity of CD44 and mechanisms which control its recognition of this ligand. In vitro mutagenesis and DNA sequencing have now been used to investigate the importance of the membrane proximal region of murine CD44 for recognition of soluble or cell surface HA. CD44 with an 83 amino acid deletion in this region mediated binding to soluble ligand and the apparent avidity increased markedly in the presence of a particular antibody to CD44, IRAWB14. The shortened CD44 was however inefficient in mediating adhesion of transfected cells to HA immobilized on cell surfaces. Four new murine isoforms of CD44 were isolated from a carcinoma line by use of the polymerase chain reaction. Only two of them correspond to ones recently discovered in rat and human cells. The longest variant nearly doubled the length of the extracellular portion of the molecule and introduced an additional 20 potential sites for glycosylation. When expressed on T lymphoma cells, all four of the new murine CD44 isoforms were capable of mediating adhesion to HA bearing cells. This result contrasts with a report that a related human CD44 isoform lacks this ability when expressed on B lineage lymphoma cells. The new murine isoforms also conferred the ability to recognize soluble HA and were very responsive to the IRAWB14 antibody. A brief survey of normal murine cell lines and tissues revealed that the hemopoietic isoform was the most abundant species. These findings indicate that the NH2-terminal portion of CD44 is sufficient for HA recognition and that this function is not necessarily abrogated by variations which occur in the membrane proximal domain. They add to the known molecular diversity of CD44 and provide another experimental model in which isoform specific functions can be investigated.