Biophysical regulation of Chlamydia pneumoniae-infected monocyte recruitment to atherosclerotic foci

Cholesterol depletion decreases adhesion of non-small cell lung cancer cells to E-selectin

Lipid microdomains, ordered membrane phases containing cholesterol and glycosphingolipids, play an essential role in cancer cell adhesion and ultimately metastasis. Notably, elevated levels of cholesterol-rich lipid microdomains are found in cancer cells relative to their normal counterparts. Therefore, alterations of lipid microdomains through cholesterol modulation could be used as a strategy to prevent cancer metastasis. In this study, methyl-beta-cyclodextrin (MβCD), sphingomyelinase (SMase), and simvastatin (Simva) were used to investigate the effects of cholesterol on the adhesive behaviors of four non-small cell lung cancer (NSCLC) cell lines (H1299, H23, H460, and A549) and a small cell lung cancer (SCLC) cell line (SHP-77) on E-selectin, a vascular endothelial molecule that initiates circulating tumor cell recruitment at metastatic sites. Under hemodynamic flow conditions, the number of adherent NSCLC cells on E-selectin significantly decreased by MβCD and Simva treatments, whereas SMase treatment did not show a significant effect. Significant increases in rolling velocities were detected only for H1299 and H23 cells after MβCD treatment. In contrast, cholesterol depletion did not affect SCLC cell attachment and rolling velocities. Moreover, cholesterol depletion by MβCD and Simva induced CD44 shedding and resulted in an enhanced membrane fluidity in the NSCLC cells, whereas it did not affect the membrane fluidity of the SCLC cells which lacked detectable expression of CD44. Our finding suggests that cholesterol regulates the E-selectin-mediated adhesion of NSCLC cells by redistributing the CD44 glycoprotein and thus modulating the membrane fluidity.NEW & NOTEWORTHY This study investigates the effects of cholesterol on the adhesive behaviors of lung cancer cells in recruitment at metastatic sites. Using cholesterol-modulating compounds, we found that reducing cholesterol decreases the adhesion of non-small cell lung cancer (NSCLC) cells while having no significant effect on small cell lung cancer (SCLC) cells. The study suggests that cholesterol regulates NSCLC cell metastasis by redistributing the adhesion proteins on the cells and modulating cells' membrane fluidity.

A Hidden Organism, Chlamydia in the Age of Atherosclerosis

Atherosclerosis is a chronic inflammatory disease. It is still the leading cause of mortality and morbidity in the world. Inflammation in the vessels plays the most important role in the pathogenesis of atherosclerosis. Many studies have been emphasized that Chlamydia pneumoniae triggers inflammation in the vessels and associated with atherosclerosis. It is stated that most of the chlamydial infections are asymptomatic and around 40% of adult individuals are infected. Chlamydia has different subgroups. It was thought to be a virus due to its intracellular pathogenicity, but it was included in the bacteria genus because it contains DNA and RNA chromosomes and has enzymatic activity. Chlamidya can easily be transmitted through the respiratory tract and sexual transmission. Seroepidemiological and pathological studies of atherosclerotic plaques showed the presence of Chlamydia in the plaque. This section will provide relationship between Chlamydia and atherosclerosis on the recent researces and current information will be discussed.

Interplay of receptor-ligand binding and lipid domain formation during cell adhesion

Cell adhesion involved in biological processes such as cell migration, immune responses, and cancer metastasis, is mediated by the specific binding of receptor and ligand proteins. Some of these proteins exhibit affinity for nanoscale lipid clusters in cell membranes. A key question is how these nanoscale lipid clusters influence and react to the receptor-ligand binding during cell adhesion. In this article, we review recent computational studies that shed new light on the interplay of the receptor-ligand binding and the formation of lipid domains in adhering membranes. These studies indicate that the receptor-ligand binding promotes coalescence of lipid clusters into mesoscale domains, which, in turn, enhances both the affinity and cooperativity of the receptor-ligand binding in cell-cell adhesion with mobile ligands. In contrast, in the case of cell-extracellular matrix adhesion with immobile ligands, the receptor-ligand binding and the lipid cluster coalescence can be correlated or anti-correlated, depending strongly on the ligand distribution. These findings deepen our understanding of correlations between cell adhesion and membrane heterogeneities.

Multiparametric biophysical profiling of red blood cells in malaria infection

Biophysical separation promises label-free, less-invasive methods to manipulate the diverse properties of live cells, such as density, magnetic susceptibility, and morphological characteristics. However, some cellular changes are so minute that they are undetectable by current methods. We developed a multiparametric cell-separation approach to profile cells with simultaneously changing density and magnetic susceptibility. We demonstrated this approach with the natural biophysical phenomenon of Plasmodium falciparum infection, which modifies its host erythrocyte by simultaneously decreasing density and increasing magnetic susceptibility. Current approaches have used these properties separately to isolate later-stage infected cells, but not in combination. We present biophysical separation of infected erythrocytes by balancing gravitational and magnetic forces to differentiate infected cell stages, including early stages for the first time, using magnetic levitation. We quantified height distributions of erythrocyte populations-27 ring-stage synchronized samples and 35 uninfected controls-and quantified their unique biophysical signatures. This platform can thus enable multidimensional biophysical measurements on unique cell types.

Interplay Between Receptor-Ligand Binding and Lipid Domain Formation Depends on the Mobility of Ligands in Cell-Substrate Adhesion

Cell-cell adhesion and the adhesion of cells to extracellular matrix are mediated by the specific binding of receptors on the cell membrane to their cognate ligands on the opposing surface. The adhesion receptors can exhibit affinity for nanoscale lipid clusters that form in the cell membrane. Experimental studies of such adhesion systems often involve a cell adhering either to a solid surface with immobile ligands or a supported lipid bilayer with mobile ligands. A central question in these cell-substrate adhesions is how the mobility of the ligands physically affects their binding to the adhesion receptors and thereby the behavior of the nanoscale lipid clusters associated with the receptors. Using a statistical mechanical model and Monte Carlo simulations for the adhesion of cells to substrates with ligands, we find that, for mobile ligands, binding to adhesion receptors can promote the formation of mesoscale lipid domains, which in turn enhances the receptor-ligand binding. However, in the case of immobile ligands, the receptor-ligand binding and the tendency for the nanoscale lipid clusters to further coalesce depend on the distribution of the ligands on the substrate. Our findings help to explain why different adhesion experiments for identifying the interplay between receptor-ligand binding and heterogeneities in cell membranes led to contradictory results.

The association of Chlamydia pneumoniae infection with atherosclerosis: Review and update of in vitro and animal studies

Previous studies have tended to relate Chlamydia pneumoniae (Cpn) infection to atherosclerosis. However, while serological studies have mostly reinforced this hypothesis, inconsistent and even contradictory findings have been reported in various researches. Recent papers have pointed to the significance of Cpn in atherosclerotic lesions, which are regarded as the initiator and cause of chronic inflammation. This bacterium develops atherosclerosis by phenotypic changes in vascular smooth muscle cells, dysregulation of endothelin-1 in the vascular wall, and releasing pro-inflammatory cytokines from Toll-like receptor-2 (TLR2). Furthermore, Cpn infection, particularly under hyperlipidemic conditions, enhances monocyte adhesion to endothelium; changes the physiology of the host, e.g., cholesterol homeostasis; and activates the Low-density lipoprotein (LDL) receptor, which is the initial step in atherogenesis. On the other hand, it has been reported that Cpn, even without the immune system of the host, has the ability to stimulate arterial thickening. Moreover, there is evidence that Cpn can increase the impact of the classical risk factors such as hyperlipidemia, pro-inflammatory cytokines, and smoking for atherosclerosis. Furthermore, animal studies have shown that Cpn infection can induce atherosclerotic, which alongside hyperlipidemia is a co-risk factor for cardiovascular disease. Although the exact link between Cpn and atherosclerosis has not been determined yet, previous studies have reported possible mechanisms of pathogenesis for this bacterium. Accordingly, investigating the exact role of this infection in causing atherosclerosis may be helpful in controlling the disease.

Delicate Role of PD-L1/PD-1 Axis in Blood Vessel Inflammatory Diseases: Current Insight and Future Significance

Immune checkpoint molecules are the antigen-independent generator of secondary signals that aid in maintaining the homeostasis of the immune system. The programmed death ligand-1 (PD-L1)/PD-1 axis is one among the most extensively studied immune-inhibitory checkpoint molecules, which delivers a negative signal for T cell activation by binding to the PD-1 receptor. The general attributes of PD-L1's immune-suppressive qualities and novel mechanisms on the barrier functions of vascular endothelium to regulate blood vessel-related inflammatory diseases are concisely reviewed. Though targeting the PD-1/PD-L1 axis has received immense recognition-the Nobel Prize in clinical oncology was awarded in the year 2018 for this discovery-the use of therapeutic modulating strategies for the PD-L1/PD-1 pathway in chronic inflammatory blood vessel diseases is still limited to experimental models. However, studies using clinical specimens that support the role of PD-1 and PD-L1 in patients with underlying atherosclerosis are also detailed. Of note, delicate balances in the expression levels of PD-L1 that are needed to preserve T cell immunity and to curtail acute as well as chronic infections in underlying blood vessel diseases are discussed. A significant link exists between altered lipid and glucose metabolism in different cells and the expression of PD-1/PD-L1 molecules, and its possible implications on vascular inflammation are justified. This review summarizes the most recent insights concerning the role of the PD-L1/PD-1 axis in vascular inflammation and, in addition, provides an overview exploring the novel therapeutic approaches and challenges of manipulating these immune checkpoint proteins, PD-1 and PD-L1, for suppressing blood vessel inflammation.

Chronic Inflammatory Diseases at Secondary Sites Ensuing Urogenital or Pulmonary Chlamydia Infections

Chlamydia trachomatis and C. pneumoniae are members of the Chlamydiaceae family of obligate intracellular bacteria. The former causes diseases predominantly at the mucosal epithelial layer of the urogenital or eye, leading to pelvic inflammatory diseases or blindness; while the latter is a major causative agent for pulmonary infection. On top of these well-described diseases at the respective primary infection sites, Chlamydia are notoriously known to migrate and cause pathologies at remote sites of a host. One such example is the sexually acquired reactive arthritis that often occurs at few weeks after genital C. trachomatis infection. C. pneumoniae, on the other hand, has been implicated in an extensive list of chronic inflammatory diseases which include atherosclerosis, multiple sclerosis, Alzheimer’s disease, asthma, and primary biliary cirrhosis. This review summarizes the Chlamydia infection associated diseases at the secondary sites of infection, and describes the potential mechanisms involved in the disease migration and pathogenesis.

B cell mechanosensing: A mechanistic overview

B cells are essential to the adaptive immune system for providing the humoral immunity against cohorts of pathogens. The presentation of antigen to the B cell receptor (BCR) leads to the initiation of B cell activation, which is a process sensitive to the stiffness features of the substrates presenting the antigens. Mechanosensing of the B cells, potentiated through BCR signaling and the adhesion molecules, efficiently regulates B cell activation, proliferation and subsequent antibody responses. Defects in sensing of the antigen-presenting substrates can lead to the activation of autoreactive B cells in autoimmune diseases. The use of high-resolution, high-speed live-cell imaging along with the sophisticated biophysical materials, has uncovered the mechanisms underlying the initiation of B cell activation within seconds of its engagement with the antigen presenting substrates. In this chapter, we reviewed studies that have contributed to uncover the molecular mechanisms of B cell mechanosensing during the initiation of B cell activation.

Binding constant of cell adhesion receptors and substrate-immobilized ligands depends on the distribution of ligands

Cell-cell adhesion and the adhesion of cells to tissues and extracellular matrix, which are pivotal for immune response, tissue development, and cell locomotion, depend sensitively on the binding constant of receptor and ligand molecules anchored on the apposing surfaces. An important question remains of whether the immobilization of ligands affects the affinity of binding with cell adhesion receptors. We have investigated the adhesion of multicomponent membranes to a flat substrate coated with immobile ligands using Monte Carlo simulations of a statistical mesoscopic model with biologically relevant parameters. We find that the binding of the adhesion receptors to ligands immobilized on the substrate is strongly affected by the ligand distribution. In the case of ligand clusters, the receptor-ligand binding constant can be significantly enhanced due to the less translational entropy loss of lipid-raft domains in the model cell membranes upon the formation of additional complexes. For ligands randomly or uniformly immobilized on the substrate, the binding constant is rather decreased since the receptors localized in lipid-raft domains have to pay an energetic penalty in order to bind ligands. Our findings help to understand why cell-substrate adhesion experiments for measuring the impact of lipid rafts on the receptor-ligand interactions led to contradictory results.

Influence of membrane cholesterol on monocyte chemotaxis

Cholesterol content influences several important physiological functions due to its effect on membrane receptors. In this work, we tested the hypothesis that cellular cholesterol alters chemotactic response of monocytes to Monocyte Chemoattractant Protein-1 (MCP-1) due to their effect on the receptor, CCR2. We used Methyl-β-cyclodextrin (MβCD) to alter the baseline cholesterol in human monocytic cell line THP-1, and evaluated their chemotactic response to MCP-1. Compared to untreated cells, cholesterol enrichment increased the number of monocytes transmigrated in response to MCP-1 while depletion had opposite effect. Using imaging flow cytometry, we established that these differences were due to alterations in expression levels, but not the surface distribution, of CCR2.

Manipulation of Host Cholesterol by Obligate Intracellular Bacteria

Cholesterol is a multifunctional lipid that plays important metabolic and structural roles in the eukaryotic cell. Despite having diverse lifestyles, the obligate intracellular bacterial pathogens Chlamydia, Coxiella, Anaplasma, Ehrlichia, and Rickettsia all target cholesterol during host cell colonization as a potential source of membrane, as well as a means to manipulate host cell signaling and trafficking. To promote host cell entry, these pathogens utilize cholesterol-rich microdomains known as lipid rafts, which serve as organizational and functional platforms for host signaling pathways involved in phagocytosis. Once a pathogen gains entrance to the intracellular space, it can manipulate host cholesterol trafficking pathways to access nutrient-rich vesicles or acquire membrane components for the bacteria or bacteria-containing vacuole. To acquire cholesterol, these pathogens specifically target host cholesterol metabolism, uptake, efflux, and storage. In this review, we examine the strategies obligate intracellular bacterial pathogens employ to manipulate cholesterol during host cell colonization. Understanding how obligate intracellular pathogens target and use host cholesterol provides critical insight into the host-pathogen relationship.

Cholesterol Regulates Monocyte Rolling through CD44 Distribution

Cholesterol is an important risk factor of atherosclerosis, due to its active uptake by monocytes/macrophages. Monocyte recruitment from flowing blood to atherosclerotic foci is the key first step in the development of atherosclerosis. Cholesterol content alters cell membrane stiffness, and lateral lipid and protein diffusion. We hypothesized that cholesterol content will modulate the recruitment of monocytes to inflamed endothelial surface by altering the dynamics of adhesion receptors. We depleted or enriched the cellular cholesterol levels using methyl-β-cyclodextran in freshly isolated human monocytes. We investigated the effect of these changes on the mechanics of monocyte rolling on E-selectin surfaces at 1 dyn/cm² in microchannels. Using imaging flow cytometry and atomic force microscopy, we characterized the distribution of lipid rafts and the E-selectin counterreceptor CD44 on the monocyte surface. We observed that lower levels of cholesterol resulted in the uniform, CD44-mediated rolling of monocytes on the E-selectin-coated surfaces. We also observed that cells depleted of cholesterol had higher membrane fluidity, and more uniform distribution of CD44 counterreceptor, which resulted in smooth motion of the cells compared to cells enriched with cholesterol. This work demonstrates that cholesterol can modulate monocyte adhesion by regulating the receptor mobility, and our results provide insights into the biophysical regulation of inflammation for the better understanding of diseases like atherosclerosis and hypercholesterolemia.

Imaging flow cytometry analysis of intracellular pathogens

Imaging flow cytometry has been applied to address questions in infection biology, in particular, infections induced by intracellular pathogens. This methodology, which utilizes specialized analytic software makes it possible to analyze hundreds of quantified features for hundreds of thousands of individual cellular or subcellular events in a single experiment. Imaging flow cytometry analysis of host cell-pathogen interaction can thus quantitatively addresses a variety of biological questions related to intracellular infection, including cell counting, internalization score, and subcellular patterns of co-localization. Here, we provide an overview of recent achievements in the use of fluorescently labeled prokaryotic or eukaryotic pathogens in human cellular infections in analysis of host-pathogen interactions. Specifically, we give examples of Imagestream-based analysis of cell lines infected with Toxoplasma gondii or Mycobacterium tuberculosis. Furthermore, we illustrate the capabilities of imaging flow cytometry using a combination of standard IDEAS™ software and the more recently developed Feature Finder algorithm, which is capable of identifying statistically significant differences between researcher-defined image galleries. We argue that the combination of imaging flow cytometry with these software platforms provides a powerful new approach to understanding host control of intracellular pathogens.

CCL19-CCR7-dependent reverse transendothelial migration of myeloid cells clears Chlamydia muridarum from the arterial intima

Regions of the normal arterial intima predisposed to atherosclerosis are sites of ongoing monocyte trafficking and also contain resident myeloid cells with features of dendritic cells. However, the pathophysiological roles of these cells are poorly understood. Here we found that intimal myeloid cells underwent reverse transendothelial migration (RTM) into the arterial circulation after systemic stimulation of pattern-recognition receptors (PRRs). This process was dependent on expression of the chemokine receptor CCR7 and its ligand CCL19 by intimal myeloid cells. In mice infected with the intracellular pathogen Chlamydia muridarum, blood monocytes disseminated infection to the intima. Subsequent CCL19-CCR7-dependent RTM was critical for the clearance of intimal C. muridarum. This process was inhibited by hypercholesterolemia. Thus, RTM protects the normal arterial intima, and compromised RTM during atherogenesis might contribute to the intracellular retention of pathogens in atherosclerotic lesions.

Biophysical and Biochemical Outcomes of Chlamydia pneumoniae Infection Promotes Pro-atherogenic Matrix Microenvironment

Multiple studies support the hypothesis that infectious agents may be involved in the pathogenesis of atherosclerosis. Chlamydia pneumoniae is strongly implicated in atherosclerosis, but the precise role has been underestimated and poorly understood due to the complexity of the disease process. In this work, we test the hypothesis that C. pneumoniae-infected macrophages lodged in the subendothelial matrix contribute to atherogenesis through pro-inflammatory factors and by cell-matrix interactions. To test this hypothesis, we used a 3D infection model with freshly isolated PBMC infected with live C. pneumoniae and chlamydial antigens encapsulated in a collagen matrix, and analyzed the inflammatory responses over 7 days. We observed that infection significantly upregulates the secretion of cytokines TNF-α, IL-1β, IL-8, MCP-1, MMP, oxidative stress, transendothelial permeability, and LDL uptake. We also observed that infected macrophages form clusters, and substantially modify the microstructure and mechanical properties of the extracellular matrix to an atherogenic phenotype. Together, our data demonstrates that C. pneumoniae-infection drives a low-grade, sustained inflammation that may predispose in the transformation to atherosclerotic foci.