The CD6 interactome orchestrates ligand-independent T cell inhibitory signaling

BACKGROUND

T-cell membrane scaffold proteins are pivotal in T cell function, acting as versatile signaling hubs. While CD6 forms a large intracellular signalosome, it is distinguished from typical scaffolds like LAT or PAG by possessing a substantial ectodomain that binds CD166, a well-characterized ligand expressed on most antigen-presenting cells (APC), through the third domain (d3) of the extracellular region. Although the intact form of CD6 is the most abundant in T cells, an isoform lacking d3 (CD6∆d3) is transiently expressed on activated T cells. Still, the precise character of the signaling transduced by CD6, whether costimulatory or inhibitory, and the influence of its ectodomain on these activities are unclear.

METHODS

We expressed CD6 variants with extracellular deletions or cytosolic mutations in Jurkat cells containing eGFP reporters for NF-κB and NF-AT transcription factor activation. Cell activation was assessed by eGFP flow cytometry following Jurkat cell engagement with superantigen-presenting Raji cells. Using imaging flow cytometry, we evaluated the impact of the CD6-CD166 pair on cell adhesiveness during the antigen-dependent and -independent priming of T cells. We also examined the role of extracellular or cytosolic sequences on CD6 translocation to the immunological synapse, using immunofluorescence-based imaging.

RESULTS

Our investigation dissecting the functions of the extracellular and cytosolic regions of CD6 revealed that CD6 was trafficked to the immunological synapse and exerted tonic inhibition wholly dependent on its cytosolic tail. Surprisingly, however, translocation to the synapse occurred independently of the extracellular d3 and of engagement to CD166. On the other hand, CD6 binding to CD166 significantly increased T cell:APC adhesion. However, this activity was most evident in the absence of APC priming with superantigen, and thus, in the absence of TCR engagement.

CONCLUSIONS

Our study identifies CD6 as a novel 'on/off' scaffold-receptor capable of modulating responsiveness in two ways. Firstly, and independently of ligand binding, it establishes signaling thresholds through tonic inhibition, functioning as a membrane-bound scaffold. Secondly, CD6 has the capacity for alternative splicing-dependent variable ligand engagement, modulating its checkpoint-like activity.

CD6 and Its Interacting Partners: Newcomers to the Block of Cancer Immunotherapies

Cancer management still requires more potent and safer treatments, of which immunomodulatory receptors on the lymphocyte surface have started to show promise in new cancer immunotherapies (e.g., CTLA-4 and PD-1). CD6 is a signal-transducing transmembrane receptor, mainly expressed by all T cells and some B and NK cell subsets, whose endogenous ligands (CD166/ALCAM, CD318/CDCP-1, Galectins 1 and 3) are overexpressed by malignant cells of different lineages. This places CD6 as a potential target for novel therapies against haematological and non-haematological malignancies. Recent experimental evidence for the role of CD6 in cancer immunotherapies is summarised in this review, dealing with diverse and innovative strategies from the classical use of monoclonal antibodies to soluble recombinant decoys or the adoptive transfer of immune cells engineered with chimeric antigen receptors.

Alcam-a and Pdgfr-α are essential for the development of sclerotome-derived stromal cells that support hematopoiesis

Mesenchymal stromal cells are essential components of hematopoietic stem and progenitor cell (HSPC) niches, regulating HSPC proliferation and fates. Their developmental origins are largely unknown. In zebrafish, we previously found that the stromal cells of the caudal hematopoietic tissue (CHT), a niche functionally homologous to the mammalian fetal liver, arise from the ventral part of caudal somites. We have now found that this ventral domain is the sclerotome, and that two markers of mammalian mesenchymal stem/stromal cells, Alcam and Pdgfr-α, are distinctively expressed there and instrumental for the emergence and migration of stromal cell progenitors, which in turn conditions the proper assembly of the vascular component of the CHT niche. Furthermore, we find that trunk somites are similarly dependent on Alcam and Pdgfr-α to produce mesenchymal cells that foster HSPC emergence from the aorta. Thus the sclerotome contributes essential stromal cells for each of the key steps of developmental hematopoiesis.

Mesenchymal stromal cell-associated migrasomes: a new source of chemoattractant for cells of hematopoietic origin

BACKGROUND

Multipotent mesenchymal stromal cells (MSCs) are precursors of various cell types. Through soluble factors, direct cell-cell interactions and other intercellular communication mechanisms such as extracellular vesicles and tunneling nanotubes, MSCs support tissue homeostasis. In the bone marrow microenvironment, they promote hematopoiesis. The interaction between MSCs and cancer cells enhances the cancer and metastatic potential. Here, we have demonstrated that plastic-adherent MSCs isolated from human bone marrow generate migrasomes, a newly discovered organelle playing a role in intercellular communication.

RESULTS

Migrasomes are forming a network with retraction fibers behind the migrating MSCs or surrounding them after membrane retraction. The MSC markers, CD44, CD73, CD90, CD105 and CD166 are present on the migrasome network, the latter being specific to migrasomes. Some migrasomes harbor the late endosomal GTPase Rab7 and exosomal marker CD63 indicating the presence of multivesicular bodies. Stromal cell-derived factor 1 (SDF-1) was detected in migrasomes, suggesting that they play a chemoattractant role. Co-cultures with KG-1a leukemic cells or primary CD34⁺ hematopoietic progenitors revealed that MSC-associated migrasomes attracted them, a process intercepted by the addition of AMD3100, a specific CXCR4 receptor inhibitor, or recombinant SDF-1. An antibody directed against CD166 reduced the association of hematopoietic cells and MSC-associated migrasomes. In contrast to primary CD34⁺ progenitors, leukemic cells can take up migrasomes.

CONCLUSION

Overall, we described a novel mechanism used by MSCs to communicate with cells of hematopoietic origin and further studies are needed to decipher all biological aspects of migrasomes in the healthy and transformed bone marrow microenvironment. Video Abstract.

Gene Variation at Immunomodulatory and Cell Adhesion Molecules Loci Impacts Primary Sjögren's Syndrome

Primary Sjögren's syndrome (pSS) is an autoimmune disease triggered by a combination of environmental and host genetic factors, which results in the focal lymphocytic infiltration of exocrine glands causing eye and mouth dryness. Glandular infiltrates include T and B cell subsets positive for CD5 and/or CD6, two surface scavenger receptors involved in the fine-tuning of intracellular signals mediated by the antigen-specific receptor complex of T (TCR) and B (BCR) cells. Moreover, the epithelial cells of inflamed glands overexpress CD166/ALCAM, a CD6 ligand involved in homo and heterotypic cell adhesion interactions. All this, together with the reported association of functionally relevant single nucleotide polymorphisms (SNPs) of CD5, CD6, and CD166/ALCAM with the risk or prognosis of some immune-mediated inflammatory disorders, led us to investigate similar associations in a local cohort of patients with pSS. The logistic regression analyses of individual SNPs showed the association of CD5 rs2241002^T with anti-Ro/La positivity, CD6 rs17824933^C with neutropenia, and CD6 rs11230563^T with increased leukopenia and neutropenia but decreased peripheral nervous system EULAR Sjögren's syndrome disease activity index (ESSDAI). Further analyses showed the association of haplotypes from CD5 (rs2241002^T-rs2229177^C) with anemia and thrombocytopenia, CD6 (rs17824933^G-rs11230563^C-rs12360861^G) with cutaneous ESSDAI, and CD166/ALCAM (rs6437585^C-rs579565^A-rs1044243^C and rs6437585^C-rs579565^G-rs1044243^T) with disease susceptibility and several analytical parameters (anti-nuclear antibodies, neurological ESSDAI, and hematologic cytopenias). These results support the relevance of gene variation at loci coding for cell surface receptors involved in the modulation of T and B lymphocyte activation (CD5, CD6) and epithelial-immune cell adhesion (CD166/ALCAM) in modulating the clinical and analytical outcomes in patients with pSS.

Activated leukocyte cell adhesion molecule regulates B lymphocyte migration across central nervous system barriers

The presence of B lymphocyte-associated oligoclonal immunoglobulins in the cerebrospinal fluid is a classic hallmark of multiple sclerosis (MS). The clinical efficacy of anti-CD20 therapies supports a major role for B lymphocytes in MS development. Although activated oligoclonal populations of pathogenic B lymphocytes are able to traffic between the peripheral circulation and the central nervous system (CNS) in patients with MS, molecular players involved in this migration have not yet been elucidated. In this study, we demonstrated that activated leukocyte cell adhesion molecule (ALCAM/CD166) identifies subsets of proinflammatory B lymphocytes and drives their transmigration across different CNS barriers in mouse and human. We also showcased that blocking ALCAM alleviated disease severity in animals affected by a B cell-dependent form of experimental autoimmune encephalomyelitis. Last, we determined that the proportion of ALCAM⁺ B lymphocytes was increased in the peripheral blood and within brain lesions of patients with MS. Our findings indicate that restricting access to the CNS by targeting ALCAM on pathogenic B lymphocytes might represent a promising strategy for the development of next-generation B lymphocyte-targeting therapies for the treatment of MS.

The role of activated leukocyte cell adhesion molecule (ALCAM) in cancer progression, invasion, metastasis and recurrence: A novel cancer stem cell marker and tumor-specific prognostic marker

Activated leukocyte cell adhesion molecule (ALCAM) or CD166 is a 100 to 105 KDa transmembrane immunoglobulin which is involved in activation of T-cells, hematopoiesis, neutrophils trans-endothelial migration, angiogenesis, inflammation and tumor propagation and invasiveness through formation of homophilic and heterophilic interactions. Recently, many studies have proposed that the expression pattern of ALCAM is highly associated with the grade, stage and invasiveness of tumors. Although ALCAM is a valuable prognostic marker in different carcinomas, similar expression patterns in different tumor types may be associated with completely different prognostic states, making it to be a tumor-type-dependent prognostic marker. In addition, ALCAM isoforms provide ways for primary detection of tumor cells with metastatic potential. More importantly, this prognostic marker has shown to be considerably dependent on the cytoplasmic and membranous expression, indirect and direct regulation of post-transcriptional molecules, pro-apoptotic proteins functionalities and several other oncogenic proteins or signalling pathways. This review mainly focuses on the pathways involved in expression of ALCAM and its prognostic value of in different types of cancers and the way in which it is regulated.

CD6, a Rheostat-Type Signalosome That Tunes T Cell Activation

Following T cell receptor triggering, T cell activation is initiated and amplified by the assembly at the TCR/CD3 macrocomplex of a multitude of stimulatory enzymes that activate several signaling cascades. The potency of signaling is, however, modulated by various inhibitory components already at the onset of activation, long before co-inhibitory immune checkpoints are expressed to help terminating the response. CD5 and CD6 are surface glycoproteins of T cells that have determinant roles in thymocyte development, T cell activation and immune responses. They belong to the superfamily of scavenger receptor cysteine-rich (SRCR) glycoproteins but whereas the inhibitory role of CD5 has been established for long, there is still controversy on whether CD6 may have similar or antagonistic functions on T cell signaling. Analysis of the structure and molecular associations of CD5 and CD6 indicates that these molecules assemble at the cytoplasmic tail a considerable number of signaling effectors that can putatively transduce diverse types of intracellular signals. Biochemical studies have concluded that both receptors can antagonize the flow of TCR-mediated signaling; however, the impact that CD5 and CD6 have on T cell development and T cell-mediated immune responses may be different. Here we analyze the signaling function of CD6, the common and also the different properties it exhibits comparing with CD5, and interpret the functional effects displayed by CD6 in recent animal models.

ALCAM shedding at the invasive front of the tumor is a marker of myometrial infiltration and promotes invasion in endometrioid endometrial cancer

Endometrial cancer (EC) is the sixth deadliest cancer in women. The depth of myometrial invasion is one of the most important prognostic factors, being directly associated with tumor recurrence and mortality. In this study, ALCAM, a previously described marker of EC recurrence, was studied by immunohistochemistry at the superficial and the invasive tumor areas from 116 EC patients with different degree of myometrial invasion and related to a set of relevant epithelial and mesenchymal markers. ALCAM expression presented a heterogeneous functionality depending on its localization, it correlated with epithelial markers (E-cadherin/β-catenin) at the superficial area, and with mesenchymal markers at the invasive front (COX-2, SNAIL, ETV5, and MMP-9). At the invasive front, ALCAM-negativity was an independent marker of myometrial invasion. This negativity, together with an increase of soluble ALCAM in uterine aspirates from patients with an invasive EC, and its positive correlation with MMP-9 levels, suggested that ALCAM shedding by MMP-9 occurs at the invasive front. In vivo and in vitro models of invasive EC were generated by ETV5-overexpression. In those, we demonstrated that ALCAM shedding was related to a more invasive pattern and that full-ALCAM recovery reverted most of the ETV5-cells mesenchymal abilities, partially through a p-ERK dependent-manner.

N-glycan mediated adhesion strengthening during pathogen-receptor binding revealed by cell-cell force spectroscopy

Glycan-protein lateral interactions have gained increased attention as important modulators of receptor function, by regulating surface residence time and endocytosis of membrane glycoproteins. The pathogen-recognition receptor DC-SIGN is highly expressed at the membrane of antigen-presenting dendritic cells, where it is organized in nanoclusters and binds to different viruses, bacteria and fungi. We recently demonstrated that DC-SIGN N-glycans spatially restrict receptor diffusion within the plasma membrane, favoring its internalization through clathrin-coated pits. Here, we investigated the involvement of the N-glycans of DC-SIGN expressing cells on pathogen binding strengthening when interacting with Candida fungal cells by using atomic force microscope (AFM)-assisted single cell-pathogen adhesion measurements. The use of DC-SIGN mutants lacking the N-glycans as well as blocking glycan-mediated lateral interactions strongly impaired cell stiffening during pathogen binding. Our findings demonstrate for the first time the direct involvement of the cell membrane glycans in strengthening cell-pathogen interactions. This study, therefore, puts forward a possible role for the glycocalyx as extracellular cytoskeleton contributing, possibly in connection with the intracellular actin cytoskeleton, to optimize strengthening of cell-pathogen interactions in the presence of mechanical forces.

Amino acid polymorphisms in the fibronectin-binding repeats of fibronectin-binding protein A affect bond strength and fibronectin conformation

The Staphylococcus aureus cell surface contains cell wall-anchored proteins such as fibronectin-binding protein A (FnBPA) that bind to host ligands (e.g. fibronectin; Fn) present in the extracellular matrix of tissue or coatings on cardiac implants. Recent clinical studies have found a correlation between cardiovascular infections caused by S. aureus and nonsynonymous SNPs in FnBPA. Atomic force microscopy (AFM), surface plasmon resonance (SPR), and molecular simulations were used to investigate interactions between Fn and each of eight 20-mer peptide variants containing amino acids Ala, Asn, Gln, His, Ile, and Lys at positions equivalent to 782 and/or 786 in Fn-binding repeat-9 of FnBPA. Experimentally measured bond lifetimes (1/k_off) and dissociation constants (K_d = k_off/k_on), determined by mechanically dissociating the Fn·peptide complex at loading rates relevant to the cardiovascular system, varied from the lowest-affinity H782A/K786A peptide (0.011 s, 747 μm) to the highest-affinity H782Q/K786N peptide (0.192 s, 15.7 μm). These atomic force microscopy results tracked remarkably well to metadynamics simulations in which peptide detachment was defined solely by the free-energy landscape. Simulations and SPR experiments suggested that an Fn conformational change may enhance the stability of the binding complex for peptides with K786I or H782Q/K786I (K_d^app = 0.2-0.5 μm, as determined by SPR) compared with the lowest-affinity double-alanine peptide (K_d^app = 3.8 μm). Together, these findings demonstrate that amino acid substitutions in Fn-binding repeat-9 can significantly affect bond strength and influence the conformation of Fn upon binding. They provide a mechanistic explanation for the observation of nonsynonymous SNPs in fnbA among clinical isolates of S. aureus that cause endovascular infections.

Dual role of ALCAM in neuroinflammation and blood-brain barrier homeostasis

Activated leukocyte cell adhesion molecule (ALCAM) is a cell adhesion molecule found on blood-brain barrier endothelial cells (BBB-ECs) that was previously shown to be involved in leukocyte transmigration across the endothelium. In the present study, we found that ALCAM knockout (KO) mice developed a more severe myelin oligodendrocyte glycoprotein (MOG)_35-55-induced experimental autoimmune encephalomyelitis (EAE). The exacerbated disease was associated with a significant increase in the number of CNS-infiltrating proinflammatory leukocytes compared with WT controls. Passive EAE transfer experiments suggested that the pathophysiology observed in active EAE was linked to the absence of ALCAM on BBB-ECs. In addition, phenotypic characterization of unimmunized ALCAM KO mice revealed a reduced expression of BBB junctional proteins. Further in vivo, in vitro, and molecular analysis confirmed that ALCAM is associated with tight junction molecule assembly at the BBB, explaining the increased permeability of CNS blood vessels in ALCAM KO animals. Collectively, our data point to a biologically important function of ALCAM in maintaining BBB integrity.

Detection of CD28/CD86 co-stimulatory molecules and surface properties of T and dendritic cells: An AFM study

Although the importance of B7/CD28 co-stimulation has been widely studied, little is known about their nano-spatial localization and their corresponding cells' biophysical and biomechanical properties. Here, we investigated the morphological, biophysical, and biomechanical properties of T cells and dendritic cells (DCs) by atomic force microscopy (AFM) and force curves. The nano-spatial distribution of CD28 and CD86 antigen on T cells and DCs was detected by CD86 or CD28 antibody-functionalized AFM tip. Single-molecule force spectroscopy (SMFS)-based force volumes and quantum dots (QDs)-based fluorescence imaging demonstrated that the co-stimulatory molecules were not randomly distributed over the cells' surface, but more than 80% of CD28 and CD86 molecules appeared to be expressed as 100-200 nm nanoclusters and polarize dominantly in the peak of the cell membrane fluctuations. AFM imaging and quantitative analysis showed that the roughness of mature DCs (mDCs) was higher than that of immature DCs (iDCs). The adhesion force distribution of iDCs and mDCs was heterogeneous while the elasticity distribution was homogeneous locally. In addition, mDCs had a fourfold increase of Young's modulus of iDCs, indicating the contribution of the actin cytoskeleton to the elastic properties of the cells. Taken together, the nano-cluster distribution of CD28 and CD86, the rough mDCs surface, the higher adhesion force and elasticity of mDCs may facilitate to the occurrence of B7/CD28 co-stimulation signals and the formation of immune synapse. These nanoscale findings provide new insights into the antigen-presenting function of DCs, the T cell activation and ultimate immune response. SCANNING 38:365-375, 2016. © 2015 Wiley Periodicals, Inc.

Glycosylation-dependent binding of galectin-8 to activated leukocyte cell adhesion molecule (ALCAM/CD166) promotes its surface segregation on breast cancer cells

BACKGROUND

We previously demonstrated that the activated leukocyte cell adhesion molecule (ALCAM/CD166) can interact with galectin-8 (Gal-8) in endothelial cells. ALCAM is a member of the immunoglobulin superfamily that promotes homophilic and heterophilic cell-cell interactions. Gal-8 is a "tandem-repeat"-type galectin, known as a matricellular protein involved in cell adhesion. Here, we analyzed the physical interaction between both molecules in breast cancer cells and the functional relevance of this phenomenon.

METHODS

We performed binding assays by surface plasmon resonance to study the interaction between Gal-8 and the recombinant glycosylated ALCAM ectodomain or endogenous ALCAM from MDA-MB-231 breast cancer cells. We also analyzed the binding of ALCAM-silenced or control breast cancer cells to immobilized Gal-8 by SPR. In internalization assays, we evaluated the influence of Gal-8 on ALCAM surface localization.

RESULTS

We showed that recombinant glycosylated ALCAM and endogenous ALCAM from breast carcinoma cells physically interacted with Gal-8 in a glycosylation-dependent fashion displaying a differential behavior compared to non-glycosylated ALCAM. Moreover, ALCAM-silenced breast cancer cells exhibited reduced binding to Gal-8 relative to control cells. Importantly, exogenously added Gal-8 provoked ALCAM segregation, probably trapping this adhesion molecule at the surface of breast cancer cells.

CONCLUSIONS

Our data indicate that Gal-8 interacts with ALCAM at the surface of breast cancer cells through glycosylation-dependent mechanisms.

GENERAL SIGNIFICANCE

A novel heterophilic interaction between ALCAM and Gal-8 is demonstrated here, suggesting its physiologic relevance in the biology of breast cancer cells.

Cell Adhesion Molecules and Ubiquitination-Functions and Significance

Cell adhesion molecules of the immunoglobulin (Ig) superfamily represent the biggest group of cell adhesion molecules. They have been analyzed since approximately 40 years ago and most of them have been shown to play a role in tumor progression and in the nervous system. All members of the Ig superfamily are intensively posttranslationally modified. However, many aspects of their cellular functions are not yet known. Since a few years ago it is known that some of the Ig superfamily members are modified by ubiquitin. Ubiquitination has classically been described as a proteasomal degradation signal but during the last years it became obvious that it can regulate many other processes including internalization of cell surface molecules and lysosomal sorting. The purpose of this review is to summarize the current knowledge about the ubiquitination of cell adhesion molecules of the Ig superfamily and to discuss its potential physiological roles in tumorigenesis and in the nervous system.

Structures of CD6 and Its Ligand CD166 Give Insight into Their Interaction

CD6 is a transmembrane protein with an extracellular region containing three scavenger receptor cysteine rich (SRCR) domains. The membrane proximal domain of CD6 binds the N-terminal immunoglobulin superfamily (IgSF) domain of another cell surface receptor, CD166, which also engages in homophilic interactions. CD6 expression is mainly restricted to T cells, and the interaction between CD6 and CD166 regulates T-cell activation. We have solved the X-ray crystal structures of the three SRCR domains of CD6 and two N-terminal domains of CD166. This first structure of consecutive SRCR domains reveals a nonlinear organization. We characterized the binding sites on CD6 and CD166 and showed that a SNP in CD6 causes glycosylation that hinders the CD6/CD166 interaction. Native mass spectrometry analysis showed that there is competition between the heterophilic and homophilic interactions. These data give insight into how interactions of consecutive SRCR domains are perturbed by SNPs and potential therapeutic reagents.

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First structure of consecutive scavenger receptor cysteine rich domains in CD6

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Structure of the two N-terminal domains of CD166 which is the ligand for CD6

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Mapping binding sites on CD6 and CD166

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Insight into how CD6 and its interactions are perturbed by polymorphisms and mAbs

Assay for characterizing the recovery of vertebrate cells for adhesion measurements by single-cell force spectroscopy

Single-cell force spectroscopy (SCFS) is becoming a widely used method to quantify the adhesion of a living cell to a substrate, another cell or tissue. The high sensitivity of SCFS permits determining the contributions of individual cell adhesion molecules (CAMs) to the adhesion force of an entire cell. However, to prepare adherent cells for SCFS, they must first be detached from tissue-culture flasks or plates. EDTA and trypsin are often applied for this purpose. Because cellular properties can be affected by this treatment, cells need to recover before being further characterized by SCFS. Here we introduce atomic force microscopy (AFM)-based SCFS to measure the mechanical and adhesive properties of HeLa cells and mouse embryonic kidney fibroblasts while they are recovering after detachment from tissue-culture. We find that mechanical and adhesive properties of both cell lines recover quickly (<10 min) after detachment using EDTA, while trypsin-detached fibroblasts require >60 min to fully recover. Our assay introduced to characterize the recovery of mammalian cells after detachment can in future be used to estimate the recovery behavior of other adherent cell types.

Syntenin-1 and ezrin proteins link activated leukocyte cell adhesion molecule to the actin cytoskeleton

Activated leukocyte cell adhesion molecule (ALCAM) is a type I transmembrane protein member of the immunoglobulin superfamily of cell adhesion molecules. Involved in important pathophysiological processes such as the immune response, cancer metastasis, and neuronal development, ALCAM undergoes both homotypic interactions with other ALCAM molecules and heterotypic interactions with the surface receptor CD6 expressed at the T cell surface. Despite biochemical and biophysical evidence of a dynamic association between ALCAM and the actin cytoskeleton, no detailed information is available about how this association occurs at the molecular level. Here, we exploit a combination of complementary microscopy techniques, including FRET detected by fluorescence lifetime imaging microscopy and single-cell force spectroscopy, and we demonstrate the existence of a preformed ligand-independent supramolecular complex where ALCAM stably interacts with actin by binding to syntenin-1 and ezrin. Interaction with the ligand CD6 further enhances these multiple interactions. Altogether, our results propose a novel biophysical framework to understand the stabilizing role of the ALCAM supramolecular complex engaged to CD6 during dendritic cell-T cell interactions and provide novel information on the molecular players involved in the formation and signaling of the immunological synapse at the dendritic cell side.

Dynamic coupling of ALCAM to the actin cortex strengthens cell adhesion to CD6

At the immunological synapse, the activated leukocyte cell adhesion molecule (ALCAM) on a dendritic cell (DC) and CD6 molecules on a T cell contribute to sustained DC-T-cell contacts. However, little is known about how ALCAM-CD6 bonds resist and adapt to mechanical stress. Here, we combine single-cell force spectroscopy (SCFS) with total-internal reflection fluorescence microscopy to examine ALCAM-CD6-mediated cell adhesion. The combination of cells expressing ALCAM constructs with certain cytoplasmic tail mutations and improved SCFS analysis processes reveal that the affinity of ALCAM-CD6 bonds is not influenced by the linking of the intracellular domains of ALCAM to the actin cortex. By contrast, the recruitment of ALCAM to adhesion sites and the propensity of ALCAM to anchor plasma membrane tethers depend on actin cytoskeletal interactions. Furthermore, linking ALCAM to the actin cortex through adaptor proteins stiffens the cortex and strengthens cell adhesion. We propose a framework for how ALCAMs contribute to DC-T-cell adhesion, stabilize DC-T-cell contacts and form a mechanical link between CD6 and the actin cortex to strengthen cell adhesion at the immunological synapse.

Magnetic tweezers-based force clamp reveals mechanically distinct apCAM domain interactions

Cell adhesion molecules of the immunoglobulin superfamily (IgCAMs) play a crucial role in cell-cell interactions during nervous system development and function. The Aplysia CAM (apCAM), an invertebrate IgCAM, shares structural and functional similarities with vertebrate NCAM and therefore has been considered as the Aplysia homolog of NCAM. Despite these similarities, the binding properties of apCAM have not been investigated thus far. Using magnetic tweezers, we applied physiologically relevant, constant forces to apCAM-coated magnetic particles interacting with apCAM-coated model surfaces and characterized the kinetics of bond rupture. The average bond lifetime decreased with increasing external force, as predicted by theoretical considerations. Mathematical simulations suggest that the apCAM homophilic interaction is mediated by two distinct bonds, one involving all five immunoglobulin (Ig)-like domains in an antiparallel alignment and the other involving only two Ig domains. In summary, this study provides biophysical evidence that apCAM undergoes homophilic interactions, and that magnetic tweezers-based, force-clamp measurements provide a rapid and reliable method for characterizing relatively weak CAM interactions.

The role of cell adhesion molecules for navigating axons: density matters

Cell adhesion molecules of the immunoglobulin-super-family (IgSF-CAMs) do not only have a physical effect, mediating merely attachment between cell surfaces. For navigating axons, IgSF-CAMs also exert an instructive impact: Upon activation, they elicit intracellular signalling cascades in the tip of the axon, the growth cone, which regulate in a spatio-temporally concerted action both speed and direction of the axon. Density and distribution of IgSF-CAMs in the growth cone plasma membrane play important roles for the activation of IgSF-CAMs, their clustering, and the adhesive forces they acquire, as well as for the local restriction and effective propagation of their intracellular signals.

Nano-characterization of Jagged-1-educated dendritic cells

Jagged-1-educated dendritic cells (DCs) are of distinct phenotypes different from Lipopolysaccharide (LPS)-educated DCs. However, to date, little is known as to nano-features of Jagged-1-educated DCs. In this study, nanostructure of Jagged-1-educated DCs was observed through atomic force microscopy. Our results showed that the volume, surface area and width of Jagged-1-educated DCs, and the number of protrusion, pseudopodia and lamellapodia on the surface of Jagged-1-educated DCs were significantly more than those of GM-CSF-induced DCs, but less than those of LPS-educated DCs. Compared with GM-CSF-induced DCs, the roughness on the surface of Jagged-1-educated DCs was greatly increased, similar to LPS-educated DCs, but the particle size and number of them on the membrane were markedly less than the latter. The change of DC nanostructure caused by Jagged-1 was abrogated with N-[N-(3,5-difluorophenacetyl)-l-alanyl]-S-phenylglycinet-butyl ester, a γ-secretase inhibitor of Notch signaling pathway. Compared to LPS-educated DCs, Jagged-1-educated DCs highly expressed NICD, Deltex-1 and Hes-1 proteins, moderately produced IL-12, but not IFN-γ, and the level of CD40 molecules on the surface of them was much lower, suggesting that Jagged-1-educated DCs are in a semi-mature status and may have unique functions distinguished from LPS-educated DCs.

Binding strength and dynamics of invariant natural killer cell T cell receptor/CD1d-glycosphingolipid interaction on living cells by single molecule force spectroscopy

Invariant natural killer T (iNKT) cells are a population of T lymphocytes that play an important role in regulating immunity to infection and tumors by recognizing endogenous and exogenous CD1d-bound lipid molecules. Using soluble iNKT T cell receptor (TCR) molecules, we applied single molecule force spectroscopy for the investigation of the iNKT TCR affinity for human CD1d molecules loaded with glycolipids differing in the length of the phytosphingosine chain using either recombinant CD1d molecules or lipid-pulsed THP1 cells. In both settings, the dissociation of the iNKT TCR from human CD1d molecules loaded with the lipid containing the longer phytosphingosine chain required higher unbinding forces compared with the shorter phytosphingosine lipid. Our findings are discussed in the context of previous results obtained by surface plasmon resonance measurements. We present new insights into the energy landscape and the kinetic rate constants of the iNKT TCR/human CD1d-glycosphingolipid interaction and emphasize the unique potential of single molecule force spectroscopy on living cells.

Rationale for Targeting CD6 as a Treatment for Autoimmune Diseases

CD6 is a 105–130 kDa surface glycoprotein expressed on the majority of T cells and a subset of B cells. The human cd6 gene maps to chromosome 11, and the expression of its protein product is tightly regulated. CD6 mediates cellular adhesion migration across the endothelial and epithelial cells. In addition, it participates in the antigen presentation by B cells and the subsequent proliferation of T cells. CD6 may bind in trans to surface glycoproteins (such as ALCAM and 3A11), or to microbial lipopolysaccharides, and may bind in cis to endogenous ligands (such as CD3 and CD5), and thereby deliver a costimulatory signal. Transinteractions are reinforced during autoimmune diseases (e.g., rheumatoid arthritis (RA), Sjögren's syndrome, and multiple sclerosis) and some cancers. Based on experimental data and on clinical results in RA and psoriasis, we believe that the recent humanized anti-CD6-specific mAb T1h may act as a regulator of the immunological response in addition to its function as an anti-T- and -B cell agent.

Tuning the mechanical properties of bioreducible multilayer films for improved cell adhesion and transfection activity

A simple approach to the mechanical modulation of layer-by-layer (LbL) films is through manipulation of the film assembly. Here, we report results based on altering the salt concentration during film assembly and its effect on film rigidity. Based on changes in film rigidity, cell adhesion characteristics and transfection activity were investigated in vitro. LbL films consisting of reducible hyperbranched poly(amide amine) (RHB) have been implemented along with DNA for investigating fibroblast adhesion on [RHB/DNA](n/2) films with varying rigidities. The rigidity was varied by changing the ionic concentration of the deposition solution between 0.01 m NaCl and 1.0 m NaCl. Molecular force probe (MFP) measurements were performed to measure the apparent Young's modulus, E(APP), of the films in situ. Cell adhesion and stress-fiber characteristics were investigated using total internal reflectance microscopy (TIRF-M). The average cell peripheral area, fiber density and average fiber length during 5 days of cell growth on films with either low (below 2.0 MPa) or high (above 2.0 MPa) film elastic modulus were investigated. Transfection studies were performed using gfpDNA and SEAP-DNA to investigate if changes in cell adhesion affect transfection activity. Furthermore, cell proliferation and cytotoxicity studies were used to investigate cellular viability over a week. The results have shown that surface modification of bioreducible LbL films of controlled thickness and roughness promotes cellular adhesion, stress-fiber growth and increased transfection activity without the need for an additional adhesive protein pre-coating of the surface or chemical cross-linking of the film.

ALCAM regulates mediolateral retinotopic mapping in the superior colliculus

ALCAM [activated leukocyte cell adhesion molecule (BEN/SC-1/DM-GRASP)] is a transmembrane recognition molecule of the Ig superfamily (IgSF) containing five Ig domains (two V-type, three C2-type). Although broadly expressed in the nervous and immune systems, few of its developmental functions have been elucidated. Because ALCAM has been suggested to interact with the IgSF adhesion molecule L1, a determinant of retinocollicular mapping, we hypothesized that ALCAM might direct topographic targeting to the superior colliculus (SC) by serving as a substrate within the SC for L1 on incoming retinal ganglion cell (RGC) axons. ALCAM was expressed in the SC during RGC axon targeting and on RGC axons as they formed the optic nerve; however, it was downregulated distally on RGC axons as they entered the SC. Axon tracing with DiI revealed pronounced mistargeting of RGC axons from the temporal retina half of ALCAM null mice to abnormally lateral sites in the contralateral SC, in which these axons formed multiple ectopic termination zones. ALCAM null mutant axons were specifically compromised in medial orientation of interstitial branches, which is known to require the ankyrin binding function of L1. As a substrate, ALCAM-Fc protein promoted L1-dependent attachment of acutely dissociated retinal cells and an L1-expressing, ALCAM-negative cell line, consistent with an ALCAM-L1 heterophilic molecular interaction. Together, these results suggest a model in which ALCAM in the SC interacts with L1 on RGC axons to promote medial extension of RGC axon branches important for mediolateral axon targeting in the formation of retinocollicular maps.

Attenuation of melanoma invasion by a secreted variant of activated leukocyte cell adhesion molecule

Activated leukocyte cell adhesion molecule (ALCAM/CD166/MEMD), a marker of various cancers and mesenchymal stem cells, is involved in melanoma metastasis. We have exploited a secreted NH(2)-terminal fragment, sALCAM, to test the hypothesis that ALCAM coordinates tissue growth and cell migration. Overexpression of sALCAM in metastatic melanoma cells disturbed clustering of endogenous ALCAM and inhibited activation of matrix metalloproteinase-2 (MMP-2). Exposure of HT1080 fibrosarcoma cells to sALCAM similarly inhibited MMP-2, suggesting a broader effect on ALCAM-positive tumor cells. In contrast to the previously reported, promotive effects of an NH(2)-terminally truncated, transmembrane variant (DeltaN-ALCAM), sALCAM impaired the migratory capacity of transfected cells in vitro, reduced basement membrane penetration in reconstituted human skin equivalents, and diminished metastatic capacity in nude mice. Remarkably, L1 neuronal cell adhesion molecule (L1CAM/CD171), another progression marker of several cancers including melanoma, was suppressed upon sALCAM overexpression but was up-regulated by DeltaN-ALCAM. The partially overlapping and opposite effects induced by alternative strategies targeting ALCAM functions collectively attribute an integrative role to ALCAM in orchestrating cell adhesion, growth, invasion, and proteolysis in the tumor tissue microenvironment and disclose a therapeutic potential for sALCAM.