DC-SIGN and influenza hemagglutinin dynamics in plasma membrane microdomains are markedly different

DC-SIGN, a Ca²⁺-dependent transmembrane lectin, is found assembled in microdomains on the plasma membranes of dendritic cells. These microdomains bind a large variety of pathogens and facilitate their uptake for subsequent antigen presentation. In this study, DC-SIGN dynamics in microdomains were explored with several fluorescence microscopy methods and compared with dynamics for influenza hemagglutinin (HA), which is also found in plasma membrane microdomains. Fluorescence imaging indicated that DC-SIGN microdomains may contain other C-type lectins and that the DC-SIGN cytoplasmic region is not required for microdomain formation. Fluorescence recovery after photobleaching measurements showed that neither full-length nor cytoplasmically truncated DC-SIGN in microdomains appreciably exchanged with like molecules in other microdomains and the membrane surround, whereas HA in microdomains exchanged almost completely. Line-scan fluorescence correlation spectroscopy indicated an essentially undetectable lateral mobility for DC-SIGN but an appreciable mobility for HA within their respective domains. Single-particle tracking with defined-valency quantum dots confirmed that HA has significant mobility within microdomains, whereas DC-SIGN does not. By contrast, fluorescence recovery after photobleaching indicated that inner leaflet lipids are able to move through DC-SIGN microdomains. The surprising stability of DC-SIGN microdomains may reflect structural features that enhance pathogen uptake either by providing high-avidity platforms and/or by protecting against rapid microdomain endocytosis.     

Microdomains of the C-type lectin DC-SIGN are portals for virus entry into dendritic cells

The C-type lectin dendritic cell (DC)-specific intercellular adhesion molecule grabbing non-integrin (DC-SIGN; CD209) facilitates binding and internalization of several viruses, including HIV-1, on DCs, but the underlying mechanism for being such an efficient phagocytic pathogen-recognition receptor is poorly understood. By high resolution electron microscopy, we demonstrate a direct relation between DC-SIGN function as viral receptor and its microlocalization on the plasma membrane. During development of human monocyte-derived DCs, DC-SIGN becomes organized in well-defined microdomains, with an average diameter of 200 nm. Biochemical experiments and confocal microscopy indicate that DC-SIGN microdomains reside within lipid rafts. Finally, we show that the organization of DC-SIGN in microdomains on the plasma membrane is important for binding and internalization of virus particles, suggesting that these multimolecular assemblies of DC-SIGN act as a docking site for pathogens like HIV-1 to invade the host.     

DC-SIGN mediated sphingomyelinase-activation and ceramide generation is essential for enhancement of viral uptake in dendritic cells

As pattern recognition receptor on dendritic cells (DCs), DC-SIGN binds carbohydrate structures on its pathogen ligands and essentially determines host pathogen interactions because it both skews T cell responses and enhances pathogen uptake for cis infection and/or T cell trans-infection. How these processes are initiated at the plasma membrane level is poorly understood. We now show that DC-SIGN ligation on DCs by antibodies, mannan or measles virus (MV) causes rapid activation of neutral and acid sphingomyelinases followed by accumulation of ceramides in the outer membrane leaflet. SMase activation is important in promoting DC-SIGN signaling, but also for enhancement of MV uptake into DCs. DC-SIGN-dependent SMase activation induces efficient, transient recruitment of CD150, which functions both as MV uptake receptor and microbial sensor, from an intracellular Lamp-1+ storage compartment shared with acid sphingomyelinase (ASM) within a few minutes. Subsequently, CD150 is displayed at the cell surface and co-clusters with DC-SIGN. Thus, DC-SIGN ligation initiates SMase-dependent formation of ceramide-enriched membrane microdomains which promote vertical segregation of CD150 from intracellular storage compartments along with ASM. Given the ability to promote receptor and signalosome co-segration into (or exclusion from) ceramide enriched microdomains which provide a favorable environment for membrane fusion, DC-SIGN-dependent SMase activation may be of general importance for modes and efficiency of pathogen uptake into DCs, and their routing to specific compartments, but also for modulating T cell responses.     

The formation and stability of DC-SIGN microdomains require its extracellular moiety

Dendritic cell-specific intercellular adhesion molecule (ICAM)-3-grabbing non-integrin (DC-SIGN) is a Ca(2+) -dependent transmembrane lectin that binds a large variety of pathogens and facilitates their uptake for subsequent antigen presentation. This receptor is present in cell surface microdomains, but factors involved in microdomain formation and their exceptional stability are not clear. To determine which domain/motif of DC-SIGN facilitates its presence in microdomains, we studied mutations at key locations including truncation of the cytoplasmic tail, and ectodomain mutations that resulted in the removal of the N-linked glycosylation site, the tandem repeats and the carbohydrate recognition domain (CRD), as well as modification of the calcium sites in the CRD required for carbohydrate binding. Confocal imaging and fluorescence recovery after photobleaching measurements showed that the cytoplasmic domain and the N-linked glycosylation site do not affect the ability of DC-SIGN to form stable microdomains. However, truncation of the CRD results in complete loss of visible microdomains and subsequent lateral diffusion of the mutants. Apart from cell adhesions, membrane domains are thought to be localized primarily via the cytoskeleton. By contrast, we propose that interactions between the CRD of DC-SIGN and the extracellular matrix and/or cis interactions with transmembrane scaffolding protein(s) play an essential role in organizing these microdomains.     

Pseudotypes of vesicular stomatitis virus with CD4 formed by clustering of membrane microdomains during budding

Many plasma membrane components are organized into detergent-resistant membrane microdomains referred to as lipid rafts. However, there is much less information about the organization of membrane components into microdomains outside of lipid rafts. Furthermore, there are few approaches to determine whether different membrane components are colocalized in microdomains as small as lipid rafts. We have previously described a new method of determining the extent of organization of proteins into membrane microdomains by analyzing the distribution of pairwise distances between immunogold particles in immunoelectron micrographs. We used this method to analyze the microdomains involved in the incorporation of the T-cell antigen CD4 into the envelope of vesicular stomatitis virus (VSV). In cells infected with a recombinant virus that expresses CD4 from the viral genome, both CD4 and the VSV envelope glycoprotein (G protein) were found in detergent-soluble (nonraft) membrane fractions. However, analysis of the distribution of CD4 and G protein in plasma membranes by immunoelectron microscopy showed that both were organized into membrane microdomains of similar sizes, approximately 100 to 150 nm. In regions of plasma membrane outside of virus budding sites, CD4 and G protein were present in separate membrane microdomains, as shown by double-label immunoelectron microscopy data. However, virus budding occurred from membrane microdomains that contained both G protein and CD4, and extended to approximately 300 nm, indicating that VSV pseudotype formation with CD4 occurs by clustering of G protein- and CD4-containing microdomains.     

Membrane microdomains in immunity: glycosphingolipid-enriched domain-mediated innate immune responses

Over the last 30 years, many studies have indicated that glycosphingolipids (GSLs) expressed on the cell surface may act as binding sites for microorganisms. Based on their physicochemical characteristics, GSLs form membrane microdomains with cholesterol, sphingomyelin, glycosylphosphatidylinositol (GPI)-anchored proteins, and various signaling molecules, and GSL-enriched domains have been shown to be involved in these defense responses. Among the GSLs, lactosylceramide (LacCer, CDw17) can bind to various microorganisms. LacCer is expressed at high levels on the plasma membrane of human neutrophils, and forms membrane microdomains associated with the Src family tyrosine kinase Lyn. LacCer-enriched membrane microdomains mediate superoxide generation, chemotaxis, and non-opsonic phagocytosis. Therefore, LacCer-enriched membrane microdomains are thought to function as pattern recognition receptors (PRRs) to recognize pathogen-associated molecular patterns (PAMPs) expressed on microorganisms. In contrast, several pathogens have developed infection mechanisms using membrane microdomains. In addition, some pathogens have the ability to avoid degradation by escaping from the vacuolar compartment or preventing phagosome maturation, utilizing membrane microdomains, such as LacCer-enriched domains, of host cells. The detailed molecular mechanisms of these membrane microdomain-associated host-pathogen interactions remain to be elucidated.     

Structural requirements for multimerization of the pathogen receptor dendritic cell-specific ICAM3-grabbing non-integrin (CD209) on the cell surface

The myeloid C-type lectin dendritic cell-specific ICAM3-grabbing non-integrin (DC-SIGN, CD209) recognizes oligosaccharide ligands on clinically relevant pathogens (HIV, Mycobacterium, and Aspergillus). Alternative splicing and genomic polymorphism generate DC-SIGN mRNA variants, which have been detected at sites of pathogen entrance and transmission. We present evidence that DC-SIGN neck variants are expressed on dendritic and myeloid cells at the RNA and protein levels. Structural analysis revealed that multimerization of DC-SIGN within a cellular context depends on the lectin domain and the number and arrangement of the repeats within the neck region, whose glycosylation negatively affects oligomer formation. Naturally occurring DC-SIGN neck variants differ in multimerization competence in the cell membrane, exhibit altered sugar binding ability, and retain pathogen-interacting capacity, implying that pathogen-induced cluster formation predominates over the basal multimerization capability. Analysis of DC-SIGN neck polymorphisms indicated that the number of allelic variants is higher than previously thought and that multimerization of the prototypic molecule is modulated in the presence of allelic variants with a different neck structure. Our results demonstrate that the presence of allelic variants or a high level of expression of neck domain splicing isoforms might influence the presence and stability of DC-SIGN multimers on the cell surface, thus providing a molecular explanation for the correlation between DC-SIGN polymorphisms and altered susceptibility to HIV-1 and other pathogens.     

Proteomic analysis of DC-SIGN on dendritic cells detects tetramers required for ligand binding but no association with CD4

DC-SIGN (dendritic cell specific intracellular adhesion molecule 3 grabbing non-integrin) or CD209 is a type II transmembrane protein and one of several C-type lectin receptors expressed by dendritic cell subsets, which bind to high mannose glycoproteins promoting their endocytosis and potential degradation. DC-SIGN also mediates attachment of HIV to dendritic cells and binding to this receptor can subsequently lead to endocytosis or enhancement of CD4/CCR5-dependent infection. The latter was proposed to be facilitated by an interaction between DC-SIGN and CD4. Endocytosis of HIV virions does not necessarily lead to their complete degradation. A proportion of the virions remain infective and can be later presented to T cells mediating their infection in trans. Previously, the extracellular domain of recombinant DC-SIGN has been shown to assemble as tetramers and in the current study we use a short range covalent cross-linker and show that DC-SIGN exists as tetramers on the surface of immature monocyte-derived dendritic cells. There was no evidence of direct binding between DC-SIGN and CD4 either by cross-linking or by fluorescence resonance energy transfer measurements suggesting that there is no constitutive association of the majority of these proteins in the membrane. Importantly we also show that the tetrameric complexes, in contrast to DC-SIGN monomers, bind with high affinity to high mannose glycoproteins such as mannan or HIV gp120 suggesting that such an assembly is required for high affinity binding of glycoproteins to DC-SIGN, providing the first direct evidence that DC-SIGN tetramers are essential for high affinity interactions with pathogens like HIV.     

Analysis of CD44-containing lipid rafts: Recruitment of annexin II and stabilization by the actin cytoskeleton.

CD44, the major cell surface receptor for hyaluronic acid (HA), was shown to localize to detergent-resistant cholesterol-rich microdomains, called lipid rafts, in fibroblasts and blood cells. Here, we have investigated the molecular environment of CD44 within the plane of the basolateral membrane of polarized mammary epithelial cells. We show that CD44 partitions into lipid rafts that contain annexin II at their cytoplasmic face. Both CD44 and annexin II were released from these lipid rafts by sequestration of plasma membrane cholesterol. Partition of annexin II and CD44 to the same type of lipid rafts was demonstrated by cross-linking experiments in living cells. First, when CD44 was clustered at the cell surface by anti-CD44 antibodies, annexin II was recruited into the cytoplasmic leaflet of CD44 clusters. Second, the formation of intracellular, submembranous annexin II-p11 aggregates caused by expression of a trans-dominant mutant of annexin II resulted in coclustering of CD44. Moreover, a frequent redirection of actin bundles to these clusters was observed. These basolateral CD44/annexin II-lipid raft complexes were stabilized by addition of GTPgammaS or phalloidin in a semipermeabilized and cholesterol-depleted cell system. The low lateral mobility of CD44 in the plasma membrane, as assessed with fluorescent recovery after photobleaching (FRAP), was dependent on the presence of plasma membrane cholesterol and an intact actin cytoskeleton. Disruption of the actin cytoskeleton dramatically increased the fraction of CD44 which could be recovered from the light detergent-insoluble membrane fraction. Taken together, our data indicate that in mammary epithelial cells the vast majority of CD44 interacts with annexin II in lipid rafts in a cholesterol-dependent manner. These CD44-containing lipid microdomains interact with the underlying actin cytoskeleton.     

Glycosphingolipid (GSL) microdomains as attachment platforms for host pathogens and their toxins on intestinal epithelial cells: activation of signal transduction pathways and perturbations of intestinal absorption and secretion

Glycosphingolipid (GSL)-enriched microdomains are used as cellular binding sites for various pathogens including viruses and bacteria. These attachment platforms are specifically associated with transducer molecules, so that the binding of host pathogens (or their toxins) to the cell surface may result in the activation of signal transduction pathways. In the intestinal epithelium, such pathogen-induced dysregulations of signal transduction can elicit a severe impairment of enterocytic functions. In this study, we demonstrate that the interaction of a bacterial toxin (cholera toxin) and a viral envelope glycoprotein (HIV-1 gp120) with the apical plasma membrane of intestinal cells is mediated by GSL-enriched microdomains that are associated with G regulatory proteins. These microbial proteins induce a GSL-dependent increase of intestinal fluid secretion by two mechanisms: activation of chloride secretion and inhibition of Na+ -dependent glucose absorption. Taken together, these data support the view that GSL-enriched microdomains in the apical plasma membrane of enterocytes are involved in the regulation of intestinal functions.     

Glycosphingolipid (GSL) microdomains as attachment platforms for host pathogens and their toxins on intestinal epithelial cells: Activation of signal transduction pathways and perturbations of intestinal absorption and secretion

Glycosphingolipid (GSL)-enriched microdomains are used as cellular binding sites for various pathogens including viruses and bacteria. These attachment platforms are specifically associated with transducer molecules, so that the binding of host pathogens (or their toxins) to the cell surface may result in the activation of signal transduction pathways. In the intestinal epithelium, such pathogen-induced dysregulations of signal transduction can elicit a severe impairment of enterocytic functions. In this study, we demonstrate that the interaction of a bacterial toxin (cholera toxin) and a viral envelope glycoprotein (HIV-1 gp120) with the apical plasma membrane of intestinal cells is mediated by GSL-enriched microdomains that are associated with G regulatory proteins. These microbial proteins induce a GSL-dependent increase of intestinal fluid secretion by two mechanisms: activation of chloride secretion and inhibition of Na⁺-dependent glucose absorption. Taken together, these data support the view that GSL-enriched microdomains in the apical plasma membrane of enterocytes are involved in the regulation of intestinal functions.     

Microdomain-dependent regulation of Lck and Fyn protein-tyrosine kinases in T lymphocyte plasma membranes

Src family protein-tyrosine kinases are implicated in signaling via glycosylphosphatidylinositol (GPI)-anchored receptors. Both kinds of molecules reside in opposite leaflets of the same sphingolipid-enriched microdomains in the lymphocyte plasma membrane without making direct contact. Under detergent-free conditions, we isolated a GPI-enriched plasma membrane fraction, also containing transmembrane proteins, selectively associated with sphingolipid microdomains. Nonionic detergents released the transmembrane proteins, yielding core sphingolipid microdomains, limited amounts of which could also be obtained by detergent-free subcellular fractionation. Protein-tyrosine kinase activity in membranes containing both GPI-anchored and transmembrane proteins was much lower than in core sphingolipid microdomains but was strongly reactivated by nonionic detergents. The inhibitory mechanism acting on Lck and Fyn kinases in these membranes was independent of the protein-tyrosine phosphatase CD45 and was characterized as a mixed, noncompetitive one. We propose that in lymphocyte plasma membranes, Lck and Fyn kinases exhibit optimal activity when juxtaposed to the GPI- and sphingolipid-enriched core microdomains but encounter inhibitory conditions in surrounding membrane areas that are rich in glycerophospholipids and contain additional transmembrane proteins.     

Modulation of Fas-mediated apoptosis by lipid rafts in T lymphocytes

In type I cells, Fas-mediated cell death requires cytoplasmic membrane subdomains called microdomains or lipid rafts. On the contrary, Fas signaling is independent of these structures in type II cells. We report that in human T cells, CD28, CD59, and CD55 are all localized into lipid rafts and that CD28 is concentrated into microdomains enriched in ganglioside GM1, whereas CD59 and CD55 are not. Moreover, CD28 cross-linking leads to the formation of lipid raft clusters which exclude CD59 and CD55, and reciprocally. Coligation of Fas with CD55 or CD59 inhibits the apoptotic signal, whereas CD28 recruitment amplifies the Fas signaling pathway. Therefore, we conclude that 1) different types of microdomains exist on the cell surface, with distinct functional properties and 2) the recruitment of these distinct structures may differentially modulate the Fas pathway. Moreover, our results demonstrate that Fas-induced apoptosis can be controlled at the level of the cytoplasmic membrane.     

Dynamic modification of sphingomyelin in lipid microdomains controls development of obesity, fatty liver, and type 2 diabetes

Lipid microdomains or caveolae, small invaginations of plasma membrane, have emerged as important elements for lipid uptake and glucose homeostasis. Sphingomyelin (SM) is one of the major phospholipids of the lipid microdomains. In this study, we investigated the physiological function of sphingomyelin synthase 2 (SMS2) using SMS2 knock-out mice, and we found that SMS2 deficiency prevents high fat diet-induced obesity and insulin resistance. Interestingly, in the liver of SMS2 knock-out mice, large and mature lipid droplets were scarcely observed. Treatment with siRNA for SMS2 also decreased the large lipid droplets in HepG2 cells. Additionally, the siRNA of SMS2 decreased the accumulation of triglyceride in liver of leptin-deficient (ob/ob) mice, strongly suggesting that SMS2 is involved in lipid droplet formation. Furthermore, we found that SMS2 exists in lipid microdomains and partially associates with the fatty acid transporter CD36/FAT and with caveolin 1, a scaffolding protein of caveolae. Because CD36/FAT and caveolin 1 exist in lipid microdomains and are coordinately involved in lipid droplet formation, SMS2 is implicated in the modulation of the SM in lipid microdomains, resulting in the regulation of CD36/FAT and caveolae. Here, we established new cell lines, in which we can completely distinguish SMS2 activity from SMS1 activity, and we demonstrated that SMS2 could convert ceramide produced in the outer leaflet of the plasma membrane into SM. Our findings demonstrate the novel and dynamic regulation of lipid microdomains via conformational changes in lipids on the plasma membrane by SMS2, which is responsible for obesity and type 2 diabetes.     

Interaction of influenza virus haemagglutinin with sphingolipid-cholesterol membrane domains via its transmembrane domain.

Sphingolipid-cholesterol rafts are microdomains in biological membranes with liquid-ordered phase properties which are implicated in membrane traffic and signalling events. We have used influenza virus haemagglutinin (HA) as a model protein to analyse the interaction of transmembrane proteins with these microdomains. Here we demonstrate that raft association is an intrinsic property encoded in the protein. Mutant HA molecules with foreign transmembrane domain (TMD) sequences lose their ability to associate with the lipid microdomains, and mutations in the HA TMD reveal a requirement for hydrophobic residues in contact with the exoplasmic leaflet of the membrane. We also provide experimental evidence that cholesterol is critically required for association of proteins with lipid rafts. Our data suggest that the binding to specific membrane domains can be encoded in transmembrane proteins and that this information will be used for polarized sorting and signal transduction processes.     

Membrane microdomains in immunoreceptor signaling

Membrane microdomains denoted commonly as lipid rafts (or membrane rafts) have been implicated in T-cell receptor (TCR), and more generally immunoreceptor, signaling for over 25 years. However, this area of research has been complicated by doubts about the real nature (and even existence) of these membrane entities, especially because of methodological problems connected with possible detergent artefacts. Recent progress in biophysical approaches and functional studies of raft resident proteins apparently clarified many controversial aspects in this area. At present, the prevailing view is that these membrane microdomains are indeed involved in many aspects of cell biology, including immunoreceptor signaling. Moreover, several other types of raft-like microdomains (perhaps better termed nanodomains) have been described, which apparently also play important biological roles.     

Recruitment of CD40 and tumor necrosis factor receptor-associated factors 2 and 3 to membrane microdomains during CD40 signaling

Signals delivered to antigen-presenting cells through CD40 are critical for the activation of immune responses. Intracellular tumor necrosis factor (TNF) receptor-associated factors (TRAFs) are key elements of the signal transduction pathways of many TNF receptor family members, including CD40. We show for the first time that engagement of CD40 in intact B cells induces the rapid translocation of TRAF2 from the cytoplasm to the plasma membrane. We found that CD40 engagement also results in its recruitment, together with TRAF2 and TRAF3, to membrane microdomains, regions of the plasma membrane enriched in signaling molecules such as the Src family kinases. Using a membrane-permeable chelator of zinc or a mutant TRAF2 molecule, we show that the putative zinc-binding domains of TRAFs contribute to their recruitment to microdomains and to the downstream activation of c-Jun N-terminal kinase. We suggest that the zinc RING and zinc finger domains of TRAFs are required for communication between CD40 and microdomain-associated signaling molecules and may serve a similar role in the signal transduction pathways of other TNF receptor family members.     

Selective inhibition of T cell activation via CD147 through novel modulation of lipid rafts

The plasma membrane is compartmentalized into microdomains and the association/dissociation of receptors and signaling molecules with/from these membrane domains is a major principle for regulation of signal transduction. By following the reorganization of microdomains on living cells and performing biochemical studies, we show that Ab targeting of the T cell activation-associated Ag CD147 prevents TCR stimulation-dependent reorganization and clustering of microdomains. Triggering CD147 induces a displacement of the GPI-anchored coreceptors CD48 and CD59 from microdomains in human T lymphocytes. This perturbation of microdomains is accompanied by a selective inhibition of TCR-mediated T cell proliferation. The CD147-inhibited cells secret normal levels of IL-2 but acquire reduced amounts of the IL-2 receptor alpha-chain CD25. These results indicate that negative regulating signals can modulate microdomains and suggest a general mechanism for inhibition of receptor signaling.     

Cutting edge: carbohydrate profiling identifies new pathogens that interact with dendritic cell-specific ICAM-3-grabbing nonintegrin on dendritic cells

Dendritic cells (DC) are instrumental in handling pathogens for processing and presentation to T cells, thus eliciting an appropriate immune response. C-type lectins expressed by DC function as pathogen-recognition receptors; yet their specificity for carbohydrate structures on pathogens is not fully understood. In this study, we analyzed the carbohydrate specificity of DC-specific ICAM-3-grabbing nonintegrin (SIGN)/CD209, the recently documented HIV-1 receptor on DC. Our studies show that DC-SIGN binds with high affinity to both synthetic mannose- and fucose-containing glycoconjugates. These carbohydrate structures are abundantly expressed by pathogens as demonstrated by the affinity of DC-SIGN for natural surface glycans of the human pathogens Mycobacterium tuberculosis, Helicobacter pylori, Leishmania mexicana, and Schistosoma mansoni. This analysis expands our knowledge on the carbohydrate and pathogen-specificity of DC-SIGN and identifies this lectin to be central in pathogen-DC interactions.     

Hypercalcemia induces a proinflammatory phenotype in rat leukocytes and endothelial cells

Endothelial plasma membrane lipid microdomains, so called lipid rafts/caveolae, are rich in neutral glycosphingolipid, globotriaosylceramide, Gb3Cer, or CD77. Several plasma membrane Ca²⁺ channels and pumps are located in lipid rafts/caveolae. Increased Ca²⁺ influx could cause the development of an endothelial proinflammatory phenotype. Therefore, the aim of this study was to estimate the effects of hypercalcemia in rats by determination of CD77 expression on CD34+ endothelial cells in the heart, kidney, and vena cava. In addition, potential proinflammatory calcium effect was estimated by CD11b and CD15s expression on leukocytes. To achieve hypercalcemia, Sprague–Dawley male rats were given CaCl₂ solution with a concentration of 1.5 % elemental calcium during 14 days. CD77 expression on CD34+ endothelial cells in cell suspensions of the heart, kidney, and vena cava, as well as leukocyte expression of CD11b and CD15s in hypercalcemic and control rats were determined by flow cytometry. Ionized calcium concentration in plasma was 1.37?±?0.01 mM in hypercalcemic vs. 1.19?±?0.03 mM in control rats. Hypercalcemic group showed statistically significantly decreased proportion of endothelial CD34+ CD77? cells in the kidney and vena cava in parallel with increase of CD11b and CD15s leukocyte proinflammatory markers. In conclusion, it is tempting to speculate that plasma membranes of glycosphingolipid CD77? endothelial cells are poorer in caveolae lipid microdomains and therefore weaker in controlling of Ca²⁺ influx. The percentage of CD11b+ CD15s+ leukocytes could be a measure of proinflammatory effects of mild hypercalcemia.