Hypercholesterolemia Enhances T Cell Receptor Signaling and Increases the Regulatory T Cell Population

Hypercholesterolemia promotes the inflammation against lipoproteins in atherosclerosis. Development of atherosclerosis is affected by the balance between pro-inflammatory effector T cells and anti-inflammatory regulatory T (Treg) cells. However, phenotype and function of T cell subpopulations in hypercholesterolemia remain to be investigated. Here, we found that cholesterol-containing diet increased the expression of the Treg cell lineage-defining transcription factor FoxP3 among thymocytes and splenocytes. Hypercholesterolemia elevated the FoxP3 expression level and population size of peripheral Treg cells, but did not prevent enhanced proliferation of stimulated T cells. Moreover, cholesterol supplementation in diet as well as in cell culture medium promoted T cell antigen receptor (TCR) signaling in CD4+ T cells. Our results demonstrate that hypercholesterolemia enhances TCR stimulation, Treg cell development as well as T cell proliferation. Thus, our findings may help to understand why hypercholesterolemia correlates with altered CD4+ T cell responses.

Lipid raft-based membrane order is important for antigen-specific clonal expansion of CD4(+) T lymphocytes

Background

Lipid rafts are cholesterol and saturated lipid-rich, nanometer sized membrane domains that are hypothesized to play an important role in compartmentalization and spatiotemporal regulation of cellular signaling. Lipid rafts contribute to the plasma membrane order and to its spatial asymmetry, as well. The raft nanodomains on the surface of CD4⁺ T lymphocytes coalesce during their interaction with antigen presenting cells (APCs). Sensing of foreign antigen by the antigen receptor on CD4⁺ T cells occurs during these cell-cell interactions. In response to foreign antigen the CD4⁺ T cells proliferate, allowing the expansion of few antigen-specific primary CD4⁺ T cell clones. Proliferating CD4⁺ T cells specialize in their function by undergoing differentiation into appropriate effectors tailored to mount an effective adaptive immune response against the invading pathogen.

Results

To investigate the role of lipid raft-based membrane order in the clonal expansion phase of primary CD4⁺ T cells, we have disrupted membrane order by incorporating an oxysterol, 7-ketocholesterol (7-KC), into the plasma membrane of primary CD4⁺ T cells expressing a T cell receptor specific to chicken ovalbumin_323–339 peptide sequence and tested their antigen-specific response. We report that 7-KC, at concentrations that disrupt lipid rafts, significantly diminish the c-Ovalbumin_323–339 peptide-specific clonal expansion of primary CD4⁺ T cells.

Conclusions

Our findings suggest that lipid raft-based membrane order is important for clonal expansion of CD4⁺ T cells in response to a model peptide.

Electronic supplementary material

The online version of this article (doi:10.1186/s12865-014-0058-8) contains supplementary material, which is available to authorized users.

CD28 sensitizes TCR Ca²⁺ signaling during Ag-independent polarization of plasma membrane rafts

T cells become polarized during initial interactions with an APC to form an Ag-independent synapse (AIS) composed of membrane rafts, TCR, and TCR-proximal signaling molecules. AISs occur temporally before TCR triggering, but their role in downstream TCR signaling is not understood. Using both human and murine model systems, we studied the signals that activate AIS formation and the effect of these signals on TCR-dependent responses. We show that CD28 produces AISs detectable by spinning disc confocal microscopy seconds following initial interactions between the T cell and APC. AIS formation by CD28 coincided with costimulatory signaling, evidenced by a cholesterol-sensitive activation of the MAPK ERK that potentiated Ca²⁺ signaling in response to CD3 cross-linking. CD45 also enriched in AISs but to modulate Src kinase activity, because localization of CD45 at the cell interface reduced the activation of proximal Lck. In summary, we show that signaling by CD28 during first encounters between the T cell and APC both sensitizes TCR Ca²⁺ signaling by an Erk-dependent mechanism and drives formation of an AIS that modulates the early signaling until TCR triggering occurs. Thus, early Ag-independent encounters are an important window for optimizing T cell responses to Ag by CD28.

CD81 controls sustained T cell activation signaling and defines the maturation stages of cognate immunological synapses

In this study, we investigated the dynamics of the molecular interactions of tetraspanin CD81 in T lymphocytes, and we show that CD81 controls the organization of the immune synapse (IS) and T cell activation. Using quantitative microscopy, including fluorescence recovery after photobleaching (FRAP), phasor fluorescence lifetime imaging microscopy-Föster resonance energy transfer (phasorFLIM-FRET), and total internal reflection fluorescence microscopy (TIRFM), we demonstrate that CD81 interacts with ICAM-1 and CD3 during conjugation between T cells and antigen-presenting cells (APCs). CD81 and ICAM-1 exhibit distinct mobilities in central and peripheral areas of early and late T cell-APC contacts. Moreover, CD81-ICAM-1 and CD81-CD3 dynamic interactions increase over the time course of IS formation, as these molecules redistribute throughout the contact area. Therefore, CD81 associations unexpectedly define novel sequential steps of IS maturation. Our results indicate that CD81 controls the temporal progression of the IS and the permanence of CD3 in the membrane contact area, contributing to sustained T cell receptor (TCR)-CD3-mediated signaling. Accordingly, we find that CD81 is required for proper T cell activation, regulating CD3ζ, ZAP-70, LAT, and extracellular signal-regulated kinase (ERK) phosphorylation; CD69 surface expression; and interleukin-2 (IL-2) secretion. Our data demonstrate the important role of CD81 in the molecular organization and dynamics of the IS architecture that sets the signaling threshold in T cell activation.

New insight into the oncogenic mechanism of the retroviral oncoprotein Tax

Human T cell leukemia virus type 1 (HTLV-1), an etiological factor that causes adult T cell leukemia and lymphoma (ATL), infects over 20 million people worldwide. About 1 million of HTLV-1-infected patients develop ATL, a highly aggressive non-Hodgkin's lymphoma without an effective therapy. The pX region of the HTLV-1 viral genome encodes an oncogenic protein, Tax, which plays a central role in transforming CD4+ T lymphocytes by deregulating oncogenic signaling pathways and promoting cell cycle progression. Expression of Tax following viral entry is critical for promoting survival and proliferation of human T cells and is required for initiation of oncogenesis. Tax exhibits diverse functions in host cells, and this oncoprotein primarily targets IκB kinase complex in the cytoplasm, resulting in persistent activation of NF-κB and upregulation of its responsive gene expressions that are crucial for T cell survival and cell cycle progression. We here review recent advances for the pathological roles of Tax in modulating IκB kinase activity. We also discuss our recent observation that Tax connects the IκB kinase complex to autophagy pathways. Understanding Tax-mediated pathogenesis will provide insights into development of new therapeutics in controlling HTLV-1-associated diseases.

Analysis of detergent-free lipid rafts isolated from CD4+ T cell line: interaction with antigen presenting cells promotes coalescing of lipid rafts

Background

Lipid rafts present on the plasma membrane play an important role in spatiotemporal regulation of cell signaling. Physical and chemical characterization of lipid raft size and assessment of their composition before, and after cell stimulation will aid in developing a clear understanding of their regulatory role in cell signaling. We have used visual and biochemical methods and approaches for examining individual and lipid raft sub-populations isolated from a mouse CD4⁺ T cell line in the absence of detergents.

Results

Detergent-free rafts were analyzed before and after their interaction with antigen presenting cells. We provide evidence that the average diameter of lipid rafts isolated from un-stimulated T cells, in the absence of detergents, is less than 100 nm. Lipid rafts on CD4⁺ T cell membranes coalesce to form larger structures, after interacting with antigen presenting cells even in the absence of a foreign antigen.

Conclusions

Findings presented here indicate that lipid raft coalescence occurs during cellular interactions prior to sensing a foreign antigen.

MAL protein controls protein sorting at the supramolecular activation cluster of human T lymphocytes

T cell membrane receptors and signaling molecules assemble at the immunological synapse (IS) in a supramolecular activation cluster (SMAC), organized into two differentiated subdomains: the central SMAC (cSMAC), with the TCR, Lck, and linker for activation of T cells (LAT), and the peripheral SMAC (pSMAC), with adhesion molecules. The mechanism of protein sorting to the SMAC subdomains is still unknown. MAL forms part of the machinery for protein targeting to the plasma membrane by specialized mechanisms involving condensed membranes or rafts. In this article, we report our investigation of the dynamics of MAL during the formation of the IS and its role in SMAC assembly in the Jurkat T cell line and human primary T cells. We observed that under normal conditions, a pool of MAL rapidly accumulates at the cSMAC, where it colocalized with condensed membranes, as visualized with the membrane fluorescent probe Laurdan. Mislocalization of MAL to the pSMAC greatly reduced membrane condensation at the cSMAC and redistributed machinery involved in docking microtubules or transport vesicles from the cSMAC to the pSMAC. As a consequence of these alterations, the raft-associated molecules Lck and LAT, but not the TCR, were missorted to the pSMAC. MAL, therefore, regulates membrane order and the distribution of microtubule and transport vesicle docking machinery at the IS and, by doing so, ensures correct protein sorting of Lck and LAT to the cSMAC.

Vitamin E transport, membrane incorporation and cell metabolism: Is alpha-tocopherol in lipid rafts an oar in the lifeboat?

Vitamin E is composed of closely related compounds, including tocopherols and tocotrienols. Studies of the last decade provide strong support for a specific role of alpha-tocopherol in cell signalling and the regulation of gene expression. It produces significant effects on inflammation, cell proliferation and apoptosis that are not shared by other vitamin E isomers with similar antioxidant properties. The different behaviours of vitamin E isomers might relate, at least in part, to the specific effects they exert at the plasma membrane. alpha-Tocopherol is not randomly distributed throughout the phospholipid bilayer of biological membranes, and as compared with other isomers, it shows a propensity to associate with lipid rafts. Distinct aspects of vitamin E transport and metabolism is discussed with emphasis on the interaction between alpha-tocopherol and lipid rafts and the consequences of these interactions on cell metabolism.

Cytoskeleton-membrane interactions in membrane raft structure

Cell membranes are structurally heterogeneous, composed of discrete domains with unique physical and biological properties. Membrane domains can form through a number of mechanisms involving lipid-lipid and protein-lipid interactions. One type of membrane domain is the cholesterol-dependent membrane raft. How rafts form remains a current topic in membrane biology. We review here evidence of structuring of rafts by the cortical actin cytoskeleton. This includes evidence that the actin cytoskeleton associates with rafts, and that many of the structural and functional properties of rafts require an intact actin cytoskeleton. We discuss the mechanisms of the actin-dependent raft organization, and the properties of the actin cytoskeleton in regulating raft-associated signaling events. We end with a discussion of membrane rafts and the actin cytoskeleton in T cell activation, which function synergistically to initiate the adaptive immune response.

An essential role for the MAL protein in targeting Lck to the plasma membrane of human T lymphocytes

The MAL protein is an essential component of the specialized machinery for apical targeting in epithelial cells. The src family kinase Lck plays a pivotal role in T cell signaling. We show that MAL is required in T cells for efficient expression of Lck at the plasma membrane and activation of IL-2 transcription. To investigate the mechanism by which MAL regulates Lck targeting, we analyzed the dynamics of Lck and found that it travels to the plasma membrane in specific transport carriers containing MAL. Coimmunoprecipitation experiments indicated an association of MAL with Lck. Both carrier formation and partitioning of Lck into detergent-insoluble membranes were ablated in the absence of MAL. Polarization of T cell receptor for antigen (TCR) and microtubule-organizing center to immunological synapse (IS) were also defective. Although partial correction of the latter defects was possible by forced expression of Lck at the plasma membrane, their complete correction, formation of transport vesicles, partitioning of Lck, and restoration of signaling pathways, which are required for IL-2 transcription up-regulation, were achieved by exogenous expression of MAL. We concluded that MAL is required for recruitment of Lck to specialized membranes and formation of specific transport carriers for Lck targeting. This novel transport pathway is crucial for TCR-mediated signaling and IS assembly.

Signal initiation in T-cell receptor microclusters

Although dynamic imaging technologies have provided important insights into the underlying processes responsible for T-cell activation, the processes that link antigen recognition to downstream signaling remain poorly defined. Converging lines of inquiry indicate that T-cell receptor (TCR) microclusters are the minimal structures capable of directing effective TCR signaling. Furthermore, imaging studies have determined that these structures trigger the assembly of oligomeric signaling scaffolds that contain the adapters and effectors required for T-cell activation. Existing models of T-cell activation accurately explain the sensitivity and selectivity of antigen recognition. However, these models do not account for important properties of microclusters, including their peripheral formation, size, and movement on the actin cytoskeleton. Here we examine how lipid rafts, galectin lattices, and protein scaffolds contribute to the assembly, function, and fate of TCR microclusters within immune synapses. Finally, we propose a 'mechanical segregation' model of signal initiation in which cytoskeletal forces contribute to the lateral segregation of molecules and cytoskeletal scaffolds provide a template for microclusters assembly.