CD81 controls immunity to Listeria infection through rac-dependent inhibition of proinflammatory mediator release and activation of cytotoxic T cells

Human tetraspanin CD81 facilitates invasion of Salmonella enterica into human epithelial cells

Human CD81 and CD9 are members of the tetraspanin family of proteins characterized by a canonical structure of four transmembrane domains and two extracellular loop domains. Tetraspanins are known as molecular facilitators, which assemble and organize cell surface receptors and partner molecules forming clusters known as tetraspanin-enriched microdomains. They have been implicated to play various biological roles including an involvement in infections with microbial pathogens. Here, we demonstrate an important role of CD81 for the invasion of epithelial cells by Salmonella enterica. We show that overexpression of CD81 in HepG2 cells enhances invasion of various typhoidal and non-typhoidal Salmonella serovars. Deletion of CD81 by CRISPR/Cas9 in intestinal epithelial cells (C2BBe1 and HT29-MTX-E12) reduces S. Typhimurium invasion. In addition, the effect of human CD81 is species-specific as only human but not rat CD81 facilitates Salmonella invasion. Finally, immunofluorescence microscopy and proximity ligation assay revealed that both human tetraspanins CD81 and CD9 are recruited to the entry site of S. Typhimurium during invasion but not during adhesion to the host cell surface. Overall, we demonstrate that the human tetraspanin CD81 facilitates Salmonella invasion into epithelial host cells.

Human in vivo evidence of reduced astrocyte activation and neuroinflammation in patients with treatment-resistant depression following electroconvulsive therapy

AIM

The current study aimed to investigate the neuroinflammatory hypothesis of depression and the potential anti-inflammatory effect of electroconvulsive therapy (ECT) in vivo, utilizing astrocyte-derived extracellular vesicles (ADEVs) isolated from plasma.

METHODS

A total of 40 patients with treatment-resistant depression (TRD) and 35 matched healthy controls were recruited at baseline, and 34 patients with TRD completed the post-ECT visits. Blood samples were collected at baseline and post-ECT. Plasma ADEVs were isolated and confirmed, and the concentrations of two astrocyte markers (glial fibrillary acidic protein [GFAP] and S100β), an extracellular vesicle marker cluster of differentiation 81 (CD81), and nine inflammatory markers in ADEVs were measured as main analyses. In addition, correlation analysis was conducted between clinical features and ADEV protein levels as exploratory analysis.

RESULTS

At baseline, the TRD group exhibited significantly higher levels of two astrocyte markers GFAP and S100β, as well as CD81 compared with the healthy controls. Inflammatory markers interferon γ (IFN-γ), interleukin (IL) 1β, IL-4, IL-6, tumor necrosis factor α, IL-10, and IL-17A were also significantly higher in the TRD group. After ECT, there was a significant reduction in the levels of GFAP, S100β, and CD81, along with a significant decrease in the levels of IFN-γ and IL-4. Furthermore, higher levels of GFAP, S100β, CD81, and inflammatory cytokines were associated with more severe depressive symptoms and poorer cognitive function.

CONCLUSION

This study provides direct insight supporting the astrocyte activation and neuroinflammatory hypothesis of depression using ADEVs. ECT may exert an anti-inflammatory effect through inhibition of such activation of astrocytes.

Targeting of Tetraspanin CD81 with Monoclonal Antibodies and Small Molecules to Combat Cancers and Viral Diseases

Tetraspanin CD81 plays major roles in cell-cell interactions and the regulation of cellular trafficking. This cholesterol-embarking transmembrane protein is a co-receptor for several viruses, including HCV, HIV-1 and Chikungunya virus, which exploits the large extracellular loop EC2 for cell entry. CD81 is also an anticancer target implicated in cancer cell proliferation and mobility, and in tumor metastasis. CD81 signaling contributes to the development of solid tumors (notably colorectal, liver and gastric cancers) and has been implicated in the aggressivity of B-cell lymphomas. A variety of protein partners can interact with CD81, either to regulate attachment and uptake of viruses (HCV E2, claudin-1, IFIM1) or to contribute to tumor growth and dissemination (CD19, CD44, EWI-2). CD81-protein interactions can be modulated with molecules targeting the extracellular domain of CD81, investigated as antiviral and/or anticancer agents. Several monoclonal antibodies anti-CD81 have been developed, notably mAb 5A6 active against invasion and metastasis of triple-negative breast cancer cells. CD81-EC2 can also be targeted with natural products (trachelogenin and harzianoic acids A-B) and synthetic compounds (such as benzothiazole-quinoline derivatives). They are weak CD81 binders but offer templates for the design of new compounds targeting the open EC2 loop. There is no anti-CD81 compound in clinical development at present, but this structurally well-characterized tetraspanin warrants more substantial considerations as a drug target.

Circulating extracellular vesicles of patients with steroid-sensitive nephrotic syndrome have higher RAC1 and induce recapitulation of nephrotic syndrome phenotype in podocytes

Since previous research suggests a role of a circulating factor in the pathogenesis of steroid-sensitive nephrotic syndrome (NS), we speculated that circulating plasma extracellular vesicles (EVs) are a candidate source of such a soluble mediator. Here, we aimed to characterize and try to delineate the effects of these EVs in vitro. Plasma EVs from 20 children with steroid-sensitive NS in relapse and remission, 10 healthy controls, and 6 disease controls were obtained by serial ultracentrifugation. Characterization of these EVs was performed by electron microscopy, flow cytometry, and Western blot analysis. Major proteins from plasma EVs were identified via mass spectrometry. Gene Ontology classification analysis and Ingenuity Pathway Analysis were performed on selectively expressed EV proteins during relapse. Immortalized human podocyte culture was used to detect the effects of EVs on podocytes. The protein content and particle number of plasma EVs were significantly increased during NS relapse. Relapse NS EVs selectively expressed proteins that involved actin cytoskeleton rearrangement. Among these, the level of RAC-GTP was significantly increased in relapse EVs compared with remission and disease control EVs. Relapse EVs were efficiently internalized by podocytes and induced significantly enhanced motility and albumin permeability. Moreover, relapse EVs induced significantly higher levels of RAC-GTP and phospho-p38 and decreased the levels of synaptopodin in podocytes. Circulating relapse EVs are biologically active molecules that carry active RAC1 as cargo and induce recapitulation of the NS phenotype in podocytes in vitro.NEW & NOTEWORTHY Up to now, the role of extracellular vesicles (EVs) in the pathogenesis of steroid-sensitive nephrotic syndrome (NS) has not been studied. Here, we found that relapse NS EVs contain significantly increased active RAC1, induce enhanced podocyte motility, and increase expression of RAC-GTP and phospho-p38 expression in vitro. These results suggest that plasma EVs are biologically active molecules in the pathogenesis of NS.

CD9 inhibition reveals a functional connection of extracellular vesicle secretion with mitophagy in melanoma cells

Tetraspanins are often used as Extracellular Vesicle (EV) detection markers because of their abundance on these secreted vesicles. However, data on their function on EV biogenesis are controversial and compensatory mechanisms often occur upon gene deletion. To overcome this handicap, we have compared the effects of tetraspanin CD9 gene deletion with those elicited by cytopermeable peptides with blocking properties against tetraspanin CD9. Both CD9 peptide or gene deletion reduced the number of early endosomes. CD9 peptide induced an increase in lysosome numbers, while CD9 deletion augmented the number of MVB and EV secretion, probably because of compensatory CD63 expression upregulation. In vivo, CD9 peptide delayed primary tumour cell growth and reduced metastasis size. These effects on cell proliferation were shown to be concomitant with an impairment in mitochondrial quality control. CD9 KO cells were able to compensate the mitochondrial malfunction by increasing total mitochondrial mass reducing mitophagy. Our data thus provide the first evidence for a functional connection of tetraspanin CD9 with mitophagy in melanoma cells.

The roles of tetraspanins in bacterial infections

Tetraspanins, a wide family composed of 33 transmembrane proteins, are associated with different types of proteins through which they arbitrate important cellular processes such as fusion, adhesion, invasion, tissue differentiation and immunological responses. Tetraspanins share a comparable structural design, which consists of four hydrophobic transmembrane domains with cytoplasmic and extracellular loops. They cooperate with different proteins, including other tetraspanins, receptors or signalling proteins to compose functional complexes at the cell surface, designated tetraspanin-enriched microdomains (TEM). Increasing evidences establish that tetraspanins are exploited by numerous intracellular pathogens as a doorway for entering and replicating within human cells. Although previous surveys focused mainly on viruses and parasites, it is now becoming clear that bacteria interact with tetraspanins, using TEM as a "gateway" to infection. In this review, we examine the biological functions of tetraspanins that are relevant to bacterial infective procedures and consider the available data that reveal how different bacteria benefit from host cell tetraspanins in infection and in the pathogenesis of diseases. We will also emphasise the stimulating potentials of targeting tetraspanins for preventing bacterial infectious diseases, using specific neutralising antibodies or anti-adhesion peptide-based therapies. Such innovative therapeutic opportunities may deliver alternatives for fighting difficult-to-manage and drug-resistant bacterial pathogens.

Tetraspanin 33 (TSPAN33) regulates endocytosis and migration of human B lymphocytes by affecting the tension of the plasma membrane

B lymphocytes are a leukocyte subset capable of developing several functions apart from differentiating into antibody-secreting cells. These processes are triggered by external activation signals that induce changes in the plasma membrane properties, regulated by the formation of different lipid-bilayer subdomains that are associated with the underlying cytoskeleton through different linker molecules, thus allowing the functional specialization of regions within the membrane. Among these, there are tetraspanin-enriched domains. Tetraspanins constitute a superfamily of transmembrane proteins that establish lateral associations with other molecules, determining its activity and localization. In this study, we identified TSPAN33 as an active player during B-lymphocyte cytoskeleton and plasma membrane-related phenomena, including protrusion formation, adhesion, phagocytosis, and cell motility. By using an overexpression model of TSPAN33 in human Raji cells, we detected a specific distribution of this protein that includes membrane microvilli, the Golgi apparatus, and extracellular vesicles. Additionally, we identified diminished phagocytic ability and altered cell adhesion properties due to the aberrant expression of integrins. Accordingly, these cells presented an enhanced migratory phenotype, as shown by its augmented chemotaxis and invasion rates. When we evaluated the mechanic response of cells during fibronectin-induced spreading, we found that TSPAN33 expression inhibited changes in roughness and membrane tension. Contrariwise, TSPAN33 knockdown cells displayed opposite phenotypes to those observed in the overexpression model. Altogether, our data indicate that TSPAN33 represents a regulatory element of the adhesion and migration of B lymphocytes, suggesting a novel implication of this tetraspanin in the control of the mechanical properties of their plasma membrane.

EspH Suppresses Erk by Spatial Segregation from CD81 Tetraspanin Microdomains

Enteropathogenic Escherichia coli (EPEC) belongs to a group of enteric human pathogens known as attaching-and-effacing (A/E) pathogens, which utilize a type III secretion system (T3SS) to translocate a battery of effector proteins from their own cytoplasm into host intestinal epithelial cells. Here we identified EspH to be an effector that prompts the recruitment of the tetraspanin CD81 to infection sites. EspH was also shown to be an effector that suppresses the mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (Erk) signaling pathway at longer infection times. The inhibitory effect was abrogated upon deletion of the last 38 amino acids located at the C terminus of the protein. The efficacy of EspH-dependent Erk suppression was higher in CD81-deficient cells, suggesting that CD81 may act as a positive regulator of Erk, counteracting Erk suppression by EspH. EspH was found within CD81 microdomains soon after infection but was largely excluded from these domains at a later time. Based on our results, we propose a mechanism whereby CD81 is initially recruited to infection sites in response to EspH translocation. At a later stage, EspH moves out of the CD81 clusters to facilitate effective Erk inhibition. Moreover, EspH selectively inhibits the tumor necrosis factor alpha (TNF-α)-induced Erk signaling pathway. Since Erk and TNF-α have been implicated in innate immunity and cell survival, our studies suggest a novel mechanism by which EPEC suppresses these processes to promote its own colonization and survival in the infected gut.

Antitumor Immunity Is Controlled by Tetraspanin Proteins

Antitumor immunity is shaped by the different types of immune cells that are present in the tumor microenvironment (TME). In particular, environmental signals (for instance, soluble factors or cell–cell contact) transmitted through the plasma membrane determine whether immune cells are activated or inhibited. Tetraspanin proteins are emerging as central building blocks of the plasma membrane by their capacity to cluster immune receptors, enzymes, and signaling molecules into the tetraspanin web. Whereas some tetraspanins (CD81, CD151, CD9) are widely and broadly expressed, others (CD53, CD37, Tssc6) have an expression pattern restricted to hematopoietic cells. Studies using genetic mouse models have identified important immunological functions of these tetraspanins on different leukocyte subsets, and as such, may be involved in the immune response against tumors. While multiple studies have been performed with regards to deciphering the function of tetraspanins on cancer cells, the effect of tetraspanins on immune cells in the antitumor response remains understudied. In this review, we will focus on tetraspanins expressed by immune cells and discuss their potential role in antitumor immunity. New insights in tetraspanin function in the TME and possible prognostic and therapeutic roles of tetraspanins will be discussed.

Tetraspanins as Organizers of Antigen-Presenting Cell Function

Professional antigen-presenting cells (APCs) include dendritic cells, monocytes, and B cells. APCs internalize and process antigens, producing immunogenic peptides that enable antigen presentation to T lymphocytes, which provide the signals that trigger T-cell activation, proliferation, and differentiation, and lead to adaptive immune responses. After detection of microbial antigens through pattern recognition receptors (PRRs), APCs migrate to secondary lymphoid organs where antigen presentation to T lymphocytes takes place. Tetraspanins are membrane proteins that organize specialized membrane platforms, called tetraspanin-enriched microdomains, which integrate membrane receptors, like PRR and major histocompatibility complex class II (MHC-II), adhesion proteins, and signaling molecules. Importantly, through the modulation of the function of their associated membrane partners, tetraspanins regulate different steps of the immune response. Several tetraspanins can positively or negatively regulate the activation threshold of immune receptors. They also play a role during migration of APCs by controlling the surface levels and spatial arrangement of adhesion molecules and their subsequent intracellular signaling. Finally, tetraspanins participate in antigen processing and are important for priming of naïve T cells through the control of T-cell co-stimulation and MHC-II-dependent antigen presentation. In this review, we discuss the role of tetraspanins in APC biology and their involvement in effective immune responses.

HDAC6 controls innate immune and autophagy responses to TLR-mediated signalling by the intracellular bacteria Listeria monocytogenes

Recent evidence on HDAC6 function underlines its role as a key protein in the innate immune response to viral infection. However, whether HDAC6 regulates innate immunity during bacterial infection remains unexplored. To assess the role of HDAC6 in the regulation of defence mechanisms against intracellular bacteria, we used the Listeria monocytogenes (Lm) infection model. Our data show that Hdac6-/- bone marrow-derived dendritic cells (BMDCs) have a higher bacterial load than Hdac6+/+ cells, correlating with weaker induction of IFN-related genes, pro-inflammatory cytokines and nitrite production after bacterial infection. Hdac6-/- BMDCs have a weakened phosphorylation of MAPK signalling in response to Lm infection, suggesting altered Toll-like receptor signalling (TLR). Compared with Hdac6+/+ counterparts, Hdac6-/- GM-CSF-derived and FLT3L-derived dendritic cells show weaker pro-inflammatory cytokine secretion in response to various TLR agonists. Moreover, HDAC6 associates with the TLR-adaptor molecule Myeloid differentiation primary response gene 88 (MyD88), and the absence of HDAC6 seems to diminish the NF-κB induction after TLR stimuli. Hdac6-/- mice display low serum levels of inflammatory cytokine IL-6 and correspondingly an increased survival to a systemic infection with Lm. The impaired bacterial clearance in the absence of HDAC6 appears to be caused by a defect in autophagy. Hence, Hdac6-/- BMDCs accumulate higher levels of the autophagy marker p62 and show defective phagosome-lysosome fusion. These data underline the important function of HDAC6 in dendritic cells not only in bacterial autophagy, but also in the proper activation of TLR signalling. These results thus demonstrate an important regulatory role for HDAC6 in the innate immune response to intracellular bacterial infection.

CD9 Regulates Major Histocompatibility Complex Class II Trafficking in Monocyte-Derived Dendritic Cells

Antigen presentation by dendritic cells (DCs) stimulates naive CD4⁺ T cells, triggering T cell activation and the adaptive arm of the immune response. Newly synthesized major histocompatibility complex class II (MHC-II) molecules accumulate at MHC-II-enriched endosomal compartments and are transported to the plasma membrane of DCs after binding to antigenic peptides to enable antigen presentation. In DCs, MHC-II molecules are included in tetraspanin-enriched microdomains (TEMs). However, the role of tetraspanin CD9 in these processes remains largely undefined. Here, we show that CD9 regulates the T cell-stimulatory capacity of granulocyte-macrophage colony-stimulating factor (GM-CSF)-dependent bone marrow-derived DCs (BMDCs), without affecting antigen presentation by fms-like tyrosine kinase 3 ligand (Flt3L)-dependent BMDCs. CD9 knockout (KO) GM-CSF-dependent BMDCs, which resemble monocyte-derived DCs (MoDCs), induce lower levels of T cell activation than wild-type DCs, and this effect is related to a reduction in MHC-II surface expression in CD9-deficient MoDCs. Importantly, MHC-II targeting to the plasma membrane is largely impaired in immature CD9 KO MoDCs, in which MHC-II remains arrested in acidic intracellular compartments enriched in LAMP-1 (lysosome-associated membrane protein 1), and MHC-II internalization is also blocked. Moreover, CD9 participates in MHC-II trafficking in mature MoDCs, regulating its endocytosis and recycling. Our results demonstrate that the tetraspanin CD9 specifically regulates antigenic presentation in MoDCs through the regulation of MHC-II intracellular trafficking.

miRNA profiling during antigen-dependent T cell activation: A role for miR-132-3p

microRNAs (miRNAs) are tightly regulated during T lymphocyte activation to enable the establishment of precise immune responses. Here, we analyzed the changes of the miRNA profiles of T cells in response to activation by cognate interaction with dendritic cells. We also studied mRNA targets common to miRNAs regulated in T cell activation. pik3r1 gene, which encodes the regulatory subunits of PI3K p50, p55 and p85, was identified as target of miRNAs upregulated after T cell activation. Using 3′UTR luciferase reporter-based and biochemical assays, we showed the inhibitory relationship between miR-132-3p upregulation and expression of the pik3r1 gene. Our results indicate that specific miRNAs whose expression is modulated during T cell activation might regulate PI3K signaling in T cells.

Manipulation of host membranes by the bacterial pathogens Listeria, Francisella, Shigella and Yersinia

Bacterial pathogens display an impressive arsenal of molecular mechanisms that allow survival in diverse host niches. Subversion of plasma membrane and cytoskeletal functions are common themes associated to infection by both extracellular and intracellular pathogens. Moreover, intracellular pathogens modify the structure/stability of their membrane-bound compartments and escape degradation from phagocytic or autophagic pathways. Here, we review the manipulation of host membranes by Listeria monocytogenes, Francisella tularensis, Shigella flexneri and Yersinia spp. These four bacterial model pathogens exemplify generalized strategies as well as specific features observed during bacterial infection processes.

Function and Dynamics of Tetraspanins during Antigen Recognition and Immunological Synapse Formation

Tetraspanin-enriched microdomains (TEMs) are specialized membrane platforms driven by protein–protein interactions that integrate membrane receptors and adhesion molecules. Tetraspanins participate in antigen recognition and presentation by antigen-­presenting cells (APCs) through the organization of pattern-recognition receptors (PRRs) and their downstream-induced signaling, as well as the regulation of MHC-II–peptide trafficking. T lymphocyte activation is triggered upon specific recognition of antigens present on the APC surface during immunological synapse (IS) formation. This dynamic process is characterized by a defined spatial organization involving the compartmentalization of receptors and adhesion molecules in specialized membrane domains that are connected to the underlying cytoskeleton and signaling molecules. Tetraspanins contribute to the spatial organization and maturation of the IS by controlling receptor clustering and local accumulation of adhesion receptors and integrins, their downstream signaling, and linkage to the actin cytoskeleton. This review offers a perspective on the important role of TEMs in the regulation of antigen recognition and presentation and in the dynamics of IS architectural organization.