Biochemical and immunohistochemical characteristics of CD62 and CD63 monoclonal antibodies. Expression of GMP-140 and LIMP-CD63 (CD63 antigen) in human lymphoid tissues.

During platelet secretion granule membrane glycoproteins are translocated to the plasma membrane. We report here the biochemical and immunohistochemical characterization of a panel of platelet-secretion-specific, CD62 and CD63 monoclonal antibodies (MoAb), which we raised to thrombin-activated platelets. The CD62 MoAb identify the alpha-granule membrane protein GMP-140, also designated platelet activation-dependent granule external membrane protein (PADGEM). The number of epitopes on thrombin-activated platelets ranged from 15,000 to 20,000. The CD63 MoAb recognize a 30-60 kDalton integral membrane protein of lysosomes. Due to its distinct localization, we have designated the CD63 antigen lysosome integral membrane protein, CD63 (LIMP-CD63). The number of epitopes on thrombin-activated platelets ranged from 9000 to 11,000. Expression of GMP-140, a member of the Selectin family (also referred as the LEC-CAM family) of adhesion molecules, and LIMP-CD63 was examined on human spleen, thymus and lymph node by immunohistochemistry. Both GMP-140 and LIMP-CD63 showed a wide distribution in lymphoid tissues; vascular endothelial cells and tissue compartments that were readily accessible to blood-borne components were uniformly positive for GMP-140 and LIMP-CD63. Furthermore, LIMP-CD63 was expressed in polymorphonuclear granulocytes and macrophages.     

Signal transduction in lymphocytic and myeloid cells via CD24, a new member of phosphoinositol-anchored membrane molecules

The CD24 Ag is present on human B cells from the earliest stages of B lineage development and is lost on terminal B cell maturation to plasma cells. This Ag is also expressed on mature forms of granulocytes. We studied the effects of CD24 mAb on the function of both lymphocytic and myeloid cell types. We found a clear-cut increase in free cytoplasmic calcium when tonsil B cells or mononuclear cells from B cell chronic lymphatic leukemia patients were preincubated with CD24 mAb and further stimulated with goat F(ab')2 anti-mouse Ig antibodies. The same experimental setting with granulocytes instead of B lymphocytes led to the triggering of hydrogen peroxide production in these cells. CD24 Ag is expressed at higher levels on activated granulocytes and is anchored to the plasma membrane by a phosphoinositol linkage. In conclusion we provide evidence that the CD24 Ag are functional molecules in cells of different lineage, playing a signal transducing role in cell function.     

Three-dimensional microscopy characterization of death receptor 5 expression by over-activated human primary CD4+ T cells and apoptosis

Activation-induced cell death is a natural process that prevents tissue damages from over-activated immune cells. TNF-Related apoptosis ligand (TRAIL), a TNF family member, induces apoptosis of infected and tumor cells by binding to one of its two death receptors, DR4 or DR5. TRAIL was reported to be secreted by phytohemagglutinin (PHA)-stimulated CD4(+) T cells in microvesicles.We investigate here TRAIL and DR5 regulation by activated primary CD4(+) T cells and its consequence on cell death. We observed that PHA induced CD4(+) T cell apoptosis in a dose-dependent manner. Thus, we investigated molecules involved in PHA-mediated cell death and demonstrated that TRAIL and DR5 were over-expressed on the plasma membrane of PHA-stimulated CD4(+) T cells. Surprisingly, DR5 was constitutively expressed in naive CD4(+) T cells at messenger RNA (mRNA) and protein levels. Thus, using 3 dimensional microscopy and intracellular staining assays, we show that DR5 is constitutively expressed in CD4(+) T cells and is pre-stocked in the cytoplasm. When cells are stimulated by PHA, DR5 is relocalized from cytoplasm to plasma membrane. Small interference RNA (siRNA) and blocking antibody assays demonstrate that TRAIL/DR5 interaction is mainly responsible for PHA-mediated CD4(+) T cell apoptosis. Thus, membrane DR5 expression leading to TRAIL-mediated apoptosis may represent one of the pathways responsible for eradication of over-activated CD4(+) T cells during immune responses.     

CD137 induces proliferation of murine hematopoietic progenitor cells and differentiation to macrophages

CD137 is a member of the TNFR family, and reverse signaling through the CD137 ligand, which is expressed as a cell surface transmembrane protein, costimulates or activates APCs. CD137 and CD137 ligand are expressed on small subsets of bone marrow cells. Activation of bone marrow cells through CD137 ligand induces proliferation, colony formation and an increase in cell numbers. Compared with total bone marrow cells, the small subpopulation of progenitor cells that express no lineage markers but express CD117 cells (or Lin(-), CD117(+) cells) responds with the same activities to CD137 ligand signaling, but at a significantly enhanced rate. Concomitantly to proliferation, the cells differentiate to CFU granulocyte-macrophage and CFU macrophage, and then to monocytes and macrophages but not to granulocytes or dendritic cells. Hematopoietic progenitor cells differentiated in the presence of CD137 protein display enhanced phagocytic activity, secrete high levels of IL-10 but little IL-12 in response to LPS, and are incapable of stimulating T cell proliferation. These data demonstrate that reverse CD137 ligand signaling takes place in hematopoietic progenitor cells, in which it induces proliferation, an increase in cell numbers, colony formation, and differentiation toward monocytes and macrophages.     

CD84 functions as a homophilic adhesion molecule and enhances IFN-gamma secretion: adhesion is mediated by Ig-like domain 1

CD84 is a member of the CD2 subset of the Ig superfamily of cell surface molecules. Its cytoplasmic tail binds to Src homology 2 domain-containing protein 1A (signaling lymphocytic activation molecule-associated protein), a protein encoded by the X-linked lymphoproliferative disease gene. It is preferentially expressed on B lymphocytes, monocytes, and platelets. We show that it is also expressed on thymocytes and T cells. CD84 was positive on CD4-CD8- thymocytes, and its expression decreased with cell maturation. It is expressed on mature T cells preferentially on CD45RO+. To identify the CD84 ligand, we generated a soluble Ig fusion protein containing the human CD84 extracellular domains (CD84-Ig). Because receptor-ligand interactions occur between several members of this subfamily, we assayed CD84-Ig binding with all members of the CD2 family. CD84-Ig bound to CD84-transfected cells, whereas no binding was detected with cells expressing other CD2 subfamily receptors, showing that CD84 binds to itself. Anti-CD84 mAbs recognizing epitopes wholly within domain 1 of CD84 blocked the binding of the CD84-Ig fusion protein to CD84-transfected cells and platelets. Data from CD84 domain human/mouse chimeras further revealed that only the first extracellular domain of the molecule is involved in the ligand receptor recognition. The CD84-CD84 interaction was independent of its cytoplasmic tail. Finally, concurrent ligation of human CD84 with mAbs or CD84-Ig and CD3 enhanced IFN-gamma secretion in human lymphocytes. Thus, CD84 is its own ligand and acts as a costimulatory molecule.     

Endocytosis of oxidized low density lipoprotein through scavenger receptor CD36 utilizes a lipid raft pathway that does not require caveolin-1

The scavenger receptor CD36 binds a diverse array of ligands, including thrombospondin-1, oxidized low density lipoprotein (OxLDL), fatty acids, anionic phospholipids, and apoptotic cells. CD36 has been reported to be present in lipid rafts/caveolae, but little is known about the membrane trafficking of this protein at baseline or following ligand binding. Here, we determined that expression of CD36 in Chinese hamster ovary (CHO) cells and endogenous expression of CD36 in C32 cells led to a homogeneous distribution of the protein on the plasma membrane, as judged by confocal fluorescence microscopy. This homogeneous pattern was observed both by anti-CD36 antibody staining and by live cell imaging of CHO cells expressing a chimeric CD36-green fluorescent protein construct. In contrast, caveolin-1 displayed its usual punctate surface distribution. Correspondingly, dual labeling of CD36 and caveolin-1 showed essentially no overlap, neither by immunofluorescence light microscopy nor by immunogold electron microscopy. Furthermore, isolation of lipid rafts by sucrose gradient ultracentrifugation of cold Triton X-100 cell lysates yielded both CD36 and caveolin-1, but immunoprecipitates of caveolin-1 did not contain CD36. Binding of Ox-LDL led to internalization of CD36 and OxLDL into endosomal structures that did not contain caveolin-1 or transferrin but that co-internalized the glycosyl-phosphatidylinositol-anchored protein decay accelerating factor, a lipid raft protein. Furthermore, expression of CD36 in the caveolin-1-negative KB cell line is sufficient for OxLDL-induced internalization of CD36, indicating that caveolin-1 is not required for this endocytic process. Taken together, these data demonstrate that at steady state, CD36 is localized in lipid rafts but not in caveolae, and that binding of OxLDL to CD36 leads to endocytosis through a lipid raft pathway that is distinct from the clathrin-mediated or caveolin internalization pathways.     

Shedding of the CD44 adhesion molecule from leukocytes induced by anti-CD44 monoclonal antibody simulating the effect of a natural receptor ligand.

The CD44 adhesion molecule, playing an important role in leukocyte extravasation, was down-regulated by PMA and ionomycin on granulocytes and by an immobilized or soluble anti-CD44 mAb both on granulocytes and lymphocytes. Soluble labeled CD44 molecules of lower apparent molecular mass as compared to their membrane counterparts were isolated from culture supernatants of stimulated surface iodinated cells. Shedding rather than internalization is the mechanism found to be responsible for the loss of CD44 from the cell surface. The size of the soluble CD44 shed from the cells stimulated in vitro corresponds to soluble CD44 isolated from human serum. These data suggest that shedding, induced by anti-CD44 antibody simulating the effect of a natural CD44 ligand, is an important regulatory mechanism controlling surface CD44 expression on leukocytes in vivo.     

p62dok negatively regulates CD2 signaling in Jurkat cells

p62(dok) belongs to a newly identified family of adaptor proteins. In T cells, the two members that are predominantly expressed, p56(dok) and p62(dok), are tyrosine phosphorylated upon CD2 or CD28 stimulation, but not upon CD3 ligation. Little is known about the biological role of Dok proteins in T cells. In this study, to evaluate the importance of p62(dok) in T cell function, we generated Jurkat clones overexpressing p62(dok). Our results demonstrate that overexpression of p62(dok) in Jurkat cells has a dramatic negative effect on CD2-mediated signaling. The p62(dok)-mediated inhibition affects several biochemical events initiated by CD2 ligation, such as the increase of intracellular Ca(2+), phospholipase C gamma 1 activation, and extracellular signal-regulated kinase 1/2 activation. Importantly, these cellular events are not affected in the signaling cascade induced by engagement of the CD3/TCR complex. However, both CD3- and CD2-induced NF-AT activation and IL-2 secretion are impaired in p62(dok)-overexpressing cells. In addition, we show that CD2 but not CD3 stimulation induces p62(dok) and Ras GTPase-activating protein recruitment to the plasma membrane. These results suggest that p62(dok) plays a negative role at multiple steps in the CD2 signaling pathway. We propose that p62(dok) may represent an important negative regulator in the modulation of the response mediated by the TCR.     

Identification of a specific glycoprotein ligand for P-selectin (CD62) on myeloid cells

P-selectin (CD62, GMP-140, PADGEM), a Ca(2+)-dependent lectin on activated platelets and endothelium, functions as a receptor for myeloid cells by interacting with sialylated, fucosylated lactosaminoglycans. P-selectin binds to a limited number of protease-sensitive sites on myeloid cells, but the protein(s) that carry the glycans recognized by P-selectin are unknown. Blotting of neutrophil or HL-60 cell membrane extracts with [125I]P-selectin and affinity chromatography of [3H]glucosamine-labeled HL-60 cell extracts were used to identify P-selectin ligands. A major ligand was identified with an approximately 250,000 M(r) under nonreducing conditions and approximately 120,000 under reducing conditions. Binding of P-selectin to the ligand was Ca2+ dependent and was blocked by mAbs to P-selectin. Brief sialidase digestion of the ligand increased its apparent molecular weight; however, prolonged digestion abolished binding of P-selectin. Peptide:N-glycosidase F treatment reduced the apparent molecular weight of the ligand by approximately 3,000 but did not affect P-selectin binding. Western blot and immunodepletion experiments indicated that the ligand was not lamp-1, lamp-2, or L-selectin, which carry sialyl Le(x), nor was it leukosialin, a heavily sialylated glycoprotein of similar molecular weight. The preferential interaction of the ligand with P-selectin suggests that it may play a role in adhesion of myeloid cells to activated platelets and endothelial cells.     

Involvement of secretory and endosomal compartments in presentation of an exogenous self-glycolipid to type II NKT cells

Natural Killer T (NKT) cells recognize both self and foreign lipid Ags presented by CD1 molecules. Although presentation of the marine sponge-derived lipid alphaGalCer to type I NKT cells has been well studied, little is known about self-glycolipid presentation to either type I or type II NKT cells. Here we have investigated presentation of the self-glycolipid sulfatide to a type II NKT cell that specifically recognizes a single species of sulfatide, namely lyso-sulfatide but not other sulfatides containing additional acyl chains. In comparison to other sulfatides or alphaGalCer, lyso-sulfatide binds with lower affinity to CD1d. Although plate-bound CD1d is inefficient in presenting lyso-sulfatide at neutral pH, it is efficiently presented at acidic pH and in the presence of saposin C. The lysosomal trafficking of mCD1d is required for alphaGalCer presentation to type I NKT cells, it is not important for presentation of lyso-sulfatide to type II NKT cells. Consistently, APCs deficient in a lysosomal lipid-transfer protein effectively present lyso-sulfatide. Presentation of lyso-sulfatide is inhibited in the presence of primaquine, concanamycin A, monensin, cycloheximide, and an inhibitor of microsomal triglyceride transfer protein but remains unchanged following treatment with brefeldin A. Wortmannin-mediated inhibition of lipid presentation indicates an important role for the PI-3kinase in mCD1d trafficking. Our data collectively suggest that weak CD1d-binding self-glycolipid ligands such as lyso-sulfatide can be presented via the secretory and endosomal compartments. Thus this study provides important insights into the exogenous self-glycolipid presentation to CD1d-restricted T cells.     

Wnt signals across the plasma membrane to activate the beta-catenin pathway by forming oligomers containing its receptors, Frizzled and LRP

Wnt-induced signaling via beta-catenin plays crucial roles in animal development and tumorigenesis. Both a seven-transmembrane protein in the Frizzled family and a single transmembrane protein in the LRP family (LDL-receptor-related protein 5/6 or Arrow) are essential for efficiently transducing a signal from Wnt, an extracellular ligand, to an intracellular pathway that stabilizes beta-catenin by interfering with its rate of destruction. However, the molecular mechanism by which these two types of membrane receptors synergize to transmit the Wnt signal is not known. We have used mutant and chimeric forms of Frizzled, LRP and Wnt proteins, small inhibitory RNAs, and assays for beta-catenin-mediated signaling and protein localization in Drosophila S2 cells and mammalian 293 cells to study transmission of a Wnt signal across the plasma membrane. Our findings are consistent with a mechanism by which Wnt protein binds to the extracellular domains of both LRP and Frizzled receptors, forming membrane-associated hetero-oligomers that interact with both Disheveled (via the intracellular portions of Frizzled) and Axin (via the intracellular domain of LRP). This model takes into account several observations reported here: the identification of intracellular residues of Frizzled required for beta-catenin signaling and for recruitment of Dvl to the plasma membrane; evidence that Wnt3A binds to the ectodomains of LRP and Frizzled; and demonstrations that a requirement for Wnt ligand can be abrogated by chimeric receptors that allow formation of Frizzled-LRP hetero-oligomers. In addition, the beta-catenin signaling mediated by ectopic expression of LRP is not dependent on Disheveled or Wnt, but can also be augmented by oligomerization of LRP receptors.     

Phenotypic and functional changes of circulating monocytes and polymorphonuclear leucocytes from elderly persons

The function and phenotype of monocytes and granulocytes in the elderly is consistently remodelled. Because leucocyte adhesion molecules play important roles in mediating a wide variety of leucocyte functions, age-related changes in their expression on granulocyte and monocyte surfaces could be partially responsible for immune dysfunctions during senescence. Considering the central role of innate immunity in the process of immunosenescence and the involvement of cell adhesion molecules (CAM) in the great majority of leucocyte functions, we studied the expression of CD50 and CD62L adhesion molecules in peripheral blood granulocytes and monocytes from healthy elderly and young subjects. We show here that the percentage of granulocytes and monocytes expressing CD62L is decreased in the elderly, whereas its density expression is unchanged on both cell types. A downregulation of the density expression of CD50 at a per cell level characterizes granulocytes in the elderly, whereas CD50 expression on monocytes from old subjects shows a peculiar attitude: its density expression decreases whereas the number of positive cells is expanded. The downregulation of this receptor on granulocytes from aged people could determine a state of hyperactivation contributing to the proinflammatory status of the elderly, while the lower expression on monocytes could therefore contribute to the impaired antigen presentation in the elderly. On the other hand, the increased number of CD50 positive monocytes in the elderly, despite its decreased density expression at a per cell level, could be interpreted as an attempt to counteract the inability to mount strong immune responses. Both CD50 and CD62L changes in ageing polymorphonuclear (PMN) cells allow recognition as non-self or senescent self to permit macrophages in the liver and spleen to remove them from the circulation. The increased proportion of granulocytes and monocytes lacking CD62L and the downregulation of CD50 intensity expression on both cell types may suggest a state of in vivo activation. Therefore, CD50 and CD62L shedding from the cell surface of activated granulocytes and monocytes could be interpreted as a tentative to counteract the dangerous effects of an excessive chronic inflammation in the elderly. However, the increased proportion of CD62L negative granulocytes in the elderly leads to an impairment in cell adhesion which is the first line of response to acute inflammatory stimuli. This phenomenon likely contributes to the increased susceptibility to acute infections of elderly people.     

Blockade of TGF-beta signaling in T cells prevents the development of experimental glomerulonephritis

Anti-glomerular basement membrane (GBM) Ab-induced glomerulonephritis (GN) at late stage is thought to be mediated by T cells. However, signaling pathways of T cells that are involved in the development of anti-GBM Ab-induced GN are unclear. We have recently established transgenic mice expressing Smad7, an inhibitor of TGF-beta signaling, in mature T cells, where signaling by TGF-beta was blocked specifically in T cells. In this study, we showed that anti-GBM Ab-induced GN was suppressed in several measures in the transgenic mice including the severity of glomerular changes, proteinuria, renal function, and CD4 T cell infiltration into the glomeruli without down-regulation of CD62 ligand (CD62L) (L-selectin) expression on CD4 T cells. Furthermore, treatment with the soluble fusion protein of CD62L and IgG enhanced anti-GBM Ab-induced GN. These findings indicated that blockade of TGF-beta signaling in T cells prevented the development of anti-GBM Ab-induced GN. Because CD62L on T cells appears to be inhibitory for the development of anti-GBM Ab-induced GN, persistent expression of CD62L on CD4 T cells may explain, at least in part, the suppression of anti-GBM Ab-induced GN in the transgenic mice. Our findings suggest that the development of anti-GBM Ab-induced GN requires TGF-beta/Smad signaling in T cells.     

Cytoplasmic domain of P-selectin (CD62) contains the signal for sorting into the regulated secretory pathway.

P-selectin (CD62), formerly called GMP-140 or PADGEM, is a membrane protein located in secretory storage granules of platelets and endothelial cells. To study the mechanisms responsible for the targeting of P-selectin to storage granules, we transfected its cDNA into COS-7 and CHO-K1 cells, which lack a regulated exocytic pathway, or into AtT20 cells, which are capable of regulated secretion. P-selectin was expressed on the plasma membrane of COS-7 and CHO-K1 cells but was concentrated in storage granules of AtT20 cells. Immunogold electron microscopy indicated that the electron-dense granules containing P-selectin in AtT20 cells also stored the endogenous soluble hormone ACTH. Activation of AtT20 cells with 8-Br-cAMP increased the surface expression of P-selectin, consistent with agonist-induced fusion of granule membranes with the plasma membrane. Deletion of the last 23 amino acids of the 35-residue cytoplasmic domain resulted in delivery of P-selectin to the plasma membrane of AtT20 cells. Replacement of the cytoplasmic tail of tissue factor, a plasma membrane protein, with the cytoplasmic domain of P-selectin redirected the chimeric molecule to granules. We conclude that the cytoplasmic domain of P-selectin is both necessary and sufficient for sorting of membrane proteins into the regulated pathway of secretion.     

Evidence for transmembrane orientation of acylated simian virus 40 large T antigen.

In mKSA cells (a simian virus 40-transformed BALB/c mouse tumor cell line), plasma membrane-associated large T antigen (large T) is found in two subfractions of the plasma membrane; a minor amount of large T is recovered from the Nonidet P-40 (NP-40)-soluble plasma membrane fraction, whereas the majority is tightly bound to a substructure of the plasma membrane, the plasma membrane lamina (PML). Only PML-associated large T is fatty acid acylated (U. Klockmann and W. Deppert, EMBO J. 2:1151-1157, 1983). We have analyzed whether these two forms of plasma membrane-associated large T might differ in features like cell surface expression or metabolic stability. In addition, we have asked whether one of the two large Ts might represent the hypothetic, large T-related protein T* (D. F. Mark and P. Berg, Cold Spring Harbor Symp. Quant. Biol. 44:55-62, 1979). We show that in mKSA cells grown in suspension culture, large T associated with the PML is also exposed on the cell surface. This form of large T, therefore, exhibits properties of a transmembrane protein. Large T in the NP-40-soluble plasma membrane fraction could not be labeled with radioiodine on the cell surface and, for this reason, does not seem to be oriented towards the cell surface. In contrast, when mKSA cells were grown on substratum (culture dish), we found that in these cells both NP-40-soluble large T as well as large T anchored in the PML could be cell surface iodinated. We also have analyzed the plasma membrane association of surface T antigen in mKSA cells grown in a mouse as ascites tumor. In tumor cells, only PML-bound large T is cell surface associated. We conclude that differences in extractibility of cell surface-associated large T most likely depend on cell shape and are not an artifact of cell culture. Both NP-40-soluble and PML-bound large Ts are associated with the plasma membrane in a metabolically stable fashion. Neither of the two large Ts represents T*.     

A monoclonal antibody to VLA-4 alpha-chain (CDw49d) induces homotypic lymphocyte aggregation

The complex processes of cellular adhesion involve a variety of receptor to ligand interactions that are extremely important during the development of immune function. Lymphocyte activation by Ag or mitogen, CTL- and NK-mediated cytolysis, homing to lymphoid-associated tissue, and the attachment of lymphocytes to extracellular matrix proteins are all governed, at least in part, by cell surface adhesion receptors. During the analysis of mAb for the ability to block human cytotoxic T lymphocyte-mediated killing an inhibitory mAb was noted that caused rapid and vigorous aggregation among the CTL. This antibody, mAb L25, also induced aggregation among human T and B tumor cell lines. mAb L25 binds to an epitope on the alpha 4 subunit of the integrin protein VLA-4 and induced an adhesion event requiring divalent cations, energy, a fluid plasma membrane, and an intact cytoskeleton. The Ag-independent homotypic adhesion induced by mAb L25 was not inhibited by mAb to the lymphocyte function associated Ag-1 (CD11a/CD18), CD2, CD4, and CD8, or to their ligands ICAM-1, LFA-3, MHC class I, or MHC class II. We believe that these experiments suggest a role for VLA-4 in a novel system of leukocyte adhesion.     

The proteoglycan lectin domain binds sulfated cell surface glycolipids and promotes cell adhesion

The lecticans are a group of chondroitin sulfate proteoglycans characterized by the presence of C-type lectin domains. Despite the suggestion that their lectin domains interact with carbohydrate ligands, the identity of such ligands has not been elucidated. We previously showed that brevican, a nervous system-specific lectican, binds the surface of B28 glial cells (Yamada, H., Fredette, B., Shitara, K., Hagihara, K., Miura, R., Ranscht, B., Stallcup, W. B., and Yamaguchi, Y. (1997) J. Neurosci. 17, 7784-7795). In this paper, we demonstrate that two classes of sulfated glycolipids, sulfatides and HNK-1-reactive sulfoglucuronylglycolipids (SGGLs), act as cell surface receptors for brevican. The lectin domain of brevican binds sulfatides and SGGLs in a calcium-dependent manner as expected of a C-type lectin domain. Intact, full-length brevican also binds both sulfatides and SGGLs. The lectin domain immobilized as a substrate supports adhesion of cells expressing SGGLs or sulfatides, which was inhibited by monoclonal antibodies against these glycolipids or by treatment of the substrate with SGGLs or sulfatides. Our findings demonstrate that the interaction between the lectin domains of lecticans and sulfated glycolipids comprises a novel cell substrate recognition system, and suggest that lecticans in extracellular matrices serve as substrate for adhesion and migration of cells expressing these glycolipids in vivo.     

The basolateral domain of the hepatocyte plasma membrane bears receptors for the circumsporozoite protein of Plasmodium falciparum sporozoites.

Minutes after injection into the circulation, malaria sporozoites enter hepatocytes. The speed and specificity of the invasion process suggest that it is receptor mediated. We show here that recombinant Plasmodium falciparum circumsporozoite protein (CS) binds specifically to regions of the plasma membrane of hepatocytes exposed to circulating blood in the Disse space. No binding has been detected in other organs, or even in other regions of the hepatocyte membrane. The interaction of CS with hepatocytes, as well as sporozoite invasion of HepG2 cells, is inhibited by synthetic peptides representing the evolutionarily conserved region II of CS. We conclude that region II is a sporozoite ligand for hepatocyte receptors localized to the basolateral domain of the plasma membrane. Our findings provide a rational explanation for the target cell specificity of malaria sporozoites.     

Costimulation via CD55 on human CD4+ T cells mediated by CD97

Decay-accelerating factor (CD55) is a complement regulatory protein, which is expressed by most cells to protect them from complement-mediated attack. CD55 also binds CD97, an EGF-TM7 receptor constitutively expressed on granulocytes and monocytes and rapidly up-regulated on T and B cells upon activation. Early results suggested that CD55 could further enhance T cell proliferation induced by phorbol ester treatment. The present study demonstrates that coengagement of CD55, using either cross-linking mAbs or its natural ligand CD97, and CD3 results in enhanced proliferation of human peripheral blood CD4(+) T cells, expression of the activation markers CD69 and CD25, and secretion of IL-10 and GM-CSF. Recently, an increase in T cell responsiveness in CD55(-/-) mice was shown to be mediated by a lack of complement regulation. In this study, we show that direct stimulation of CD55 on CD4(+) T cells with CD97 can modulate T cell activation but does not interfere with CD55-mediated complement regulation. Our results support a multifaceted role for CD55 in human T cell activation, constituting a further link between innate and adaptive immunity.