Crystal structure of the plexin A3 intracellular region reveals an autoinhibited conformation through active site sequestration

Plexin cell surface receptors bind to semaphorin ligands and transduce signals for regulating neuronal axon guidance. The intracellular region of plexins is essential for signaling and contains a R-Ras/M-Ras GTPase activating protein (GAP) domain that is divided into two segments by a Rho GTPase-binding domain (RBD). The regulation mechanisms for plexin remain elusive, although it is known that activation requires both binding of semaphorin to the extracellular region and a Rho-family GTPase (Rac1 or Rnd1) to the RBD. Here we report the crystal structure of the plexin A3 intracellular region. The structure shows that the N- and C-terminal portions of the GAP homologous regions together form a GAP domain with an overall fold similar to other Ras GAPs. However, the plexin GAP domain adopts a closed conformation and cannot accommodate R-Ras/M-Ras in its substrate-binding site, providing a structural basis for the autoinhibited state of plexins. A comparison with the plexin B1 RBD/Rnd1 complex structure suggests that Rnd1 binding alone does not induce a conformational change in plexin, explaining the requirement of both semaphorin and a Rho GTPase for activation. The structure also identifies an N-terminal segment that is important for regulation. Both the N-terminal segment and the RBD make extensive interactions with the GAP domain, suggesting the presence of an allosteric network connecting these three domains that integrates semaphorin and Rho GTPase signals to activate the GAP. The importance of these interactions in plexin signaling is shown by both cell-based and in vivo axon guidance assays.     

Ligand-induced internalization selects use of common receptor neuropilin-1 by VEGF165 and semaphorin3A

Neuropilin-1 (Npn-1) is a receptor shared by class 3 semaphorins and heparin-binding forms of vascular endothelial growth factor (VEGF), protein families that regulate endothelial and neuronal-cell function. Ligand interaction with Npn-1 dictates the choice of signal transducer; plexins transduce semaphorin signals, and VEGF receptors transduce VEGF signals. It is not clear how class 3 semaphorins affect endothelial-cell function and how the shared receptor Npn-1 selects its ligand. We report that semaphorin3A (Sema3A) inhibits endothelial-cell lamellipodia formation, adhesion, survival, proliferation, and cord formation. VEGF(165), but not VEGF(121), could block all these effects of Sema3A. VEGF(165) competed with Sema3A for binding to endothelial cells, effectively reduced cell-surface Npn-1, and promoted its internalization. Use of soluble forms of Npn-1 or VEGF receptor-1 to block VEGF(165) binding to Npn-1 or to VEGF receptors provided evidence that surface Npn-1 and VEGF receptors are required for VEGF(165)-induced Npn-1 internalization. Sema3A also reduced cell-surface Npn-1 in endothelial cells and promoted its internalization, but required a higher concentration than VEGF(165). These results demonstrate that preferential receptor binding and internalization by a ligand are mechanisms by which the common receptor Npn-1 can play an essential role in prioritizing conflicting signals.     

Semaphorin3A signaling controls Fas (CD95)-mediated apoptosis by promoting Fas translocation into lipid rafts

Semaphorins and their receptors (plexins) have pleiotropic biologic functions, including regulation of immune responses. However, the role of these molecules inside the immune system and the signal transduction mechanism(s) they use are largely unknown. Here, we show that Semaphorin3A (Sema3A) triggers a proapoptotic program that sensitizes leukemic T cells to Fas (CD95)-mediated apoptosis. We found that Sema3A stimulation provoked Fas translocation into lipid raft microdomains before binding with agonistic antibody or FasL (CD95L). Disruption of lipid rafts reduced sensitivity to Fas-mediated apoptosis in the presence of Sema3A. Furthermore, we show that plexin-A1, together with Sema3A-binding neuropilin-1, was rapidly incorporated into membrane rafts after ligand stimulation, resulting in the transport of actin-linking proteins into Fas-enriched rafts. Cells expressing a dominant-negative mutant of plexin-A1 did not show Fas clustering and apoptosis on Sema3A/Fas costimulation. This work identifies a novel biologic function of semaphorins and presents an unexpected signaling mechanism linking semaphorin to the tumor necrosis factor family receptors.     

Sulfated polysaccharides identified as inducers of neuropilin-1 internalization and functional inhibition of VEGF165 and semaphorin3A

Neuropilin-1 (NRP1) and NRP2 are cell surface receptors shared by class 3 semaphorins and vascular endothelial growth factor (VEGF). Ligand interaction with NRPs selects the specific signal transducer, plexins for semaphorins or VEGF receptors for VEGF, and promotes NRP internalization, which effectively shuts down receptor-mediated signaling by a second ligand. Here, we show that the sulfated polysaccharides dextran sulfate and fucoidan, but not others, reduce endothelial cell-surface levels of NRP1, NRP2, and to a lesser extent VEGFR-1 and VEGFR-2, and block the binding and in vitro function of semaphorin3A and VEGF(165). Administration of fucoidan to mice reduces VEGF(165)-induced angiogenesis and tumor neovascularization in vivo. We find that dextran sulfate and fucoidan can bridge the extracellular domain of NRP1 to that of the scavenger receptor expressed by endothelial cells I (SREC-I), and induce NRP1 and SREC-I coordinate internalization and trafficking to the lysosomes. Overexpression of SREC-I in SREC-I-negative cells specifically reduces cell-surface levels of NRP1, indicating that SREC-I mediates NRP1 internalization. These results demonstrate that engineered receptor internalization is an effective strategy for reducing levels and function of cell-surface receptors, and identify certain sulfated polysaccharides as "internalization inducers."     

Semaphorins and tumor angiogenesis

Semaphorins belong to a large family of proteins well-conserved along evolution from viruses to mammalians. Secreted and membrane-bound semaphorins participate in a wide range of biological phenomena including development and regeneration of nervous system, cardiovascular development, and immune system activities. Different classes of semaphorins are bifunctional and often exert opposite effects (i.e., repellent or attractive) by acting through the plexin receptor family. However, some classes use other membrane receptors and the same plexin-mediated signals may be modulated by co-receptors, in particular neuropilins or some tyrosine kinase receptors. In cancer, semaphorins have both tumor-suppressor and tumor-promoting functions, by acting on both tumor and stromal components. Here, we review the role of semaphorins in tumor angiogenesis and propose that an unbalance between autocrine loops respectively involving angiogenic inducers and class 3 semaphorin is instrumental for structural and functional abnormalities observed in tumor vasculature. An erratum to this article can be found at     

Besides adhesion: new perspectives of integrin functions in angiogenesis

During angiogenic remodelling in embryo and adult life, endothelial cells lining blood vessel walls dynamically modify their integrin-mediated adhesive contacts with the surrounding extracellular matrix. However, besides regulating cell adhesion and migration, integrins dynamically participate in a network with soluble molecules and their receptors. Angiogenesis is characterized by opposing autocrine and paracrine loops of growth factors and semaphorins that regulate the activation of integrins on the endothelial surface through tyrosine kinase receptors (TKR) and the neuropilin/plexin system. Moreover, pro- and anti-angiogenic factors can directly bind integrins and regulate endothelial cell behaviour. This review summarizes the recent progress in understanding the reciprocal interactions between integrins, TKR, and semaphorin receptors.     

Integrins team up with tyrosine kinase receptors and plexins to control angiogenesis

PURPOSE OF REVIEW: Understanding the role of integrins in the formation of vascular bed is important for designing new therapeutic approaches to ameliorate or inhibit pathological vascularization. Besides regulating cell adhesion and migration, integrins dynamically participate in a network with soluble molecules and their receptors. This study summarizes recent progress in the understanding of the reciprocal interactions between integrins, tyrosine kinase, and semaphorin receptors. RECENT FINDINGS: During angiogenic remodeling, endothelial cells that line blood vessel walls dynamically modify their integrin-mediated adhesive contacts with the surrounding extracellular matrix. During angiogenesis, opposing autocrine and paracrine loops of growth factors and semaphorins regulate endothelial integrin activation and function through tyrosine kinase receptors and the neuropilin/plexins system. Moreover, proangiogenic and antiangiogenic factors can directly bind integrins and regulate endothelial cell behavior. Studies describing these intense research areas are discussed. SUMMARY: Alteration in the balance between the angiogenic growth factors and semaphorins results in an impairment of integrin functions and could account for cardiovascular malformation and structural and functional abnormalities of the tumor vasculature.     

Secondary PDZ domain-binding site on class B plexins enhances the affinity for PDZ–RhoGEF

PDZ domains are abundant protein interaction modules and typically recognize a short motif at the C terminus of their ligands, with a few residues in the motif endowing the binding specificity. The sequence-based rules, however, cannot fully account for the specificity between the vast number of PDZ domains and ligands in the cell. Plexins are transmembrane receptors that regulate processes such as axon guidance and angiogenesis. Two related guanine nucleotide exchange factors (GEFs), PDZ–RhoGEF and leukemia-associated RhoGEF (LARG), use their PDZ domains to bind class B plexins and play critical roles in signaling. Here, we present the crystal structure of the full-length cytoplasmic region of PlexinB2 in complex with the PDZ domain of PDZ–RhoGEF. The structure reveals that, in addition to the canonical C-terminal motif/PDZ interaction, the 3D domain of PlexinB2 forms a secondary interface with the PDZ domain. Our biophysical and cell-based assays show that the secondary interface contributes to the specific interaction between plexin and PDZ–RhoGEF and to signaling by plexin in the cell. Formation of secondary interfaces may be a general mechanism for increasing affinity and specificity of modular domain-mediated interactions.Plexins are cell surface receptors for semaphorins, extracellular cues that control essential processes such as neuronal axon guidance and vasculature development (1). Binding of semaphorin to the extracellular region of plexin induces formation of the active dimer of the cytoplasmic region, which transduces signal to downstream pathways (2–7). The plexin cytoplasmic region contains a juxtamembrane segment (JM-segment), a RhoGTPase binding domain (RBD), and a GTPase activating protein (GAP) domain (8–10). The GAP domain, activated by the dimerization, transduces signal through converting its substrate GTPase Rap from the GTP-bound active to the GDP-bound inactive state (2, 3). The RBD regulates plexin activity in response to binding of Rho family GTPases, such as Rac1 (reviewed in ref. 11).In addition to the common signaling pathways through the domains shared by all plexins, class B plexins (B1, B2, and B3) mediate a pathway through their unique C terminus. The conserved “VTDL” motif at the C terminus of these plexins binds to the N-terminal PDZ (PSD-95/Discs-large/ZO-1) domains of two related guanine nucleotide exchange factors (GEFs), PDZ–RhoGEF, and leukemia-associated RhoGEF (LARG) (12–17). This interaction recruits PDZ–RhoGEF and LARG to the plasma membrane, where they promote the exchange of GDP for GTP on RhoA. GTP-bound RhoA binds its downstream effectors and contributes to plexin signaling (13–15, 18). A recent study has shown that deletion of the C terminus of PlexinB2 causes defects in the development of the liver vasculature in mice, highlighting the critical role of the PDZ–RhoGEF/LARG–RhoA pathway in plexin function in vivo (19).More than 250 PDZ domains exist in the human proteome, constituting one of the most abundant protein interaction modules (20, 21). Correspondingly, there are ∼600 ligands for the PDZ domains (22). A high degree of mutual specificity is expected between PDZ domains and their respective ligands to ensure fidelity of signaling in the cell. The fold of PDZ domains is composed of a six-stranded β-barrel and two α-helices. The canonical interaction mode between the PDZ domains and their ligands involves the binding of the C terminus of the ligand in an extended β-strand conformation to the groove between βB and αB in the PDZ domain. The C-terminal carboxyl group of the ligand forms two hydrogen bonds with the backbone of the conserved “GΦGF” motif (Φ: hydrophobic residue) in the βA–βB loop of the PDZ domain, which underlies the strong preference of PDZ domains for C termini. Previous studies have established the general rules of ligand specificity for PDZ domains (23). Class I PDZ domains recognize C-terminal motifs with the “T/S-X-Φ” (X: any residue) sequence, whereas class II PDZ domains prefer the “Φ-X-Φ” sequence. The structural basis for this selectivity is relatively well understood (20). However, many PDZ domains are promiscuous toward short peptidic ligands and defy these simple rules of specificity. Particularly, the PDZ domains in PDZ–RhoGEF and LARG, categorized as class I PDZ domains, bind with similar affinities to many peptides of both classes I and II (24).Binding of PDZ–RhoGEF and LARG to full-length class B plexins has been detected by various in vitro and cell-based experiments, whereas many other PDZ domains showed no binding under similar conditions (12–16). In contrast, isolated C-terminal peptides from class B plexins and the PDZ domains of PDZ–RhoGEF/LARG only exhibit modest affinity, with the dissociation constant (K_d) in the range of 10–40 μM (24–26). This discrepancy suggests that the specificity between class B plexins and PDZ–RhoGEF/LARG is not fully recapitulated by the peptide/PDZ interaction. More broadly, the sequence-based rules described above are likely an oversimplification of the mechanisms underlying specificity between PDZ domains and their ligands. Most previous structural and binding analyses focused on isolated peptidic binding motifs derived from PDZ ligands, leaving open the question of whether other regions in the ligands are involved in the interaction.To address these questions, we determined the crystal structure of the complex between the full-length cytoplasmic region of PlexinB2 and the PDZ domain of PDZ–RhoGEF. The structure reveals a secondary interface between PlexinB2 and the PDZ domain, in addition to the canonical interaction mediated by the C-terminal “VTDL” motif of PlexinB2. Our structure-based mutational analyses show that the secondary interface plays an important role in the specific interaction between class B plexins and PDZ–RhoGEF.     

Semaphorin 4D cooperates with VEGF to promote angiogenesis and tumor progression

The semaphorins and plexins comprise a family of cysteine-rich proteins implicated in control of nerve growth and development and regulation of the immune response. Our group and others have found that Semaphorin 4D (SEMA4D) and its receptor, Plexin-B1, play an important role in tumor-induced angiogenesis, with some neoplasms producing SEMA4D in a manner analogous to vascular endothelial growth factor (VEGF) in order to attract Plexin-B1-expressing endothelial cells into the tumor for the purpose of promoting growth and vascularity. While anti-VEGF strategies have been the focus of most angiogenesis inhibition research, such treatment can lead to upregulation of pro-angiogenic factors that can compensate for the loss of VEGF, eventually leading to failure of therapy. Here, we demonstrate that SEMA4D cooperates with VEGF to promote angiogenesis in malignancies and can perform the same function in a setting of VEGF blockade. We also show the potential value of inhibiting SEMA4D/Plexin-B1 signaling as a complementary mechanism to anti-VEGF treatment, particularly in VEGF inhibitor-resistant tumors, suggesting that this may represent a novel treatment for some cancers.     

Genetic dissection of plexin signaling in vivo

Mammalian plexins constitute a family of transmembrane receptors for semaphorins and represent critical regulators of various processes during development of the nervous, cardiovascular, skeletal, and renal system. In vitro studies have shown that plexins exert their effects via an intracellular R-Ras/M-Ras GTPase-activating protein (GAP) domain or by activation of RhoA through interaction with Rho guanine nucleotide exchange factor proteins. However, which of these signaling pathways are relevant for plexin functions in vivo is largely unknown. Using an allelic series of transgenic mice, we show that the GAP domain of plexins constitutes their key signaling module during development. Mice in which endogenous Plexin-B2 or Plexin-D1 is replaced by transgenic versions harboring mutations in the GAP domain recapitulate the phenotypes of the respective null mutants in the developing nervous, vascular, and skeletal system. We further provide genetic evidence that, unexpectedly, the GAP domain-mediated developmental functions of plexins are not brought about via R-Ras and M-Ras inactivation. In contrast to the GAP domain mutants, Plexin-B2 transgenic mice defective in Rho guanine nucleotide exchange factor binding are viable and fertile but exhibit abnormal development of the liver vasculature. Our genetic analyses uncover the in vivo context-dependence and functional specificity of individual plexin-mediated signaling pathways during development.Plexins constitute a family of transmembrane proteins that serve as receptors for semaphorins (1). They function as key regulators of a multitude of developmental processes, including axon guidance, pattern and synapse formation in the nervous system (2), vasculogenesis and angiogenesis (3, 4), and morphogenesis of the heart, kidney, and skeletal system (5). In the adult organism, plexins play crucial roles in the physiology and pathophysiology of the immune and cardiovascular system, as well as in bone homeostasis and in cancer (6–9). Nine plexins have been identified in the mammalian system, which are grouped into four subfamilies, A–D, according to sequence homologies.The activation of plexins by their semaphorin ligands triggers several intracellular signaling cascades, most of which modulate the activity of small GTPases (10). The intracellular domain of all plexins shares homology with GTPase-activating proteins (GAPs) and confers the deactivation of R-Ras, M-Ras, and Rap1 (11–17). The GAP activity toward R-Ras and M-Ras, but not toward Rap1, requires binding of Rnd GTPases to the plexin receptor (11, 12, 15, 16). Plexins of the B-subfamily differ from all other plexins in that they carry a C-terminal PDZ domain interaction motif that mediates a stable interaction with the Rho guanine nucleotide exchange factor (RhoGEF) proteins PDZ-RhoGEF and LARG (18–20). Activation of B-plexins by semaphorin ligands results in activation of the RhoGEF proteins and subsequent activation of RhoA and RhoC (18–21). This process and the GAP function of B-plexins are independent of each other, because mutations in the GAP domain do not interfere with the activation of RhoA, and deletion of the PDZ domain interaction motif does not influence GAP activity (11).The plexin-mediated signal transduction pathways have been identified and characterized mainly in in vitro systems using recombinant proteins, purified membranes, and cell culture models. These in vitro approaches have often yielded conflicting results with respect to the functional relevance of individual plexin-mediated signaling pathways. For example, the axonal growth cone collapse of primary neurons has been suggested to be caused by plexin-mediated deactivation of R-Ras (11, 12), activation of RhoA (20, 22), and deactivation of Rap1 (15). Which plexin signaling pathways are functionally relevant in vivo is largely unknown.Here we report on the generation of an allelic series of BAC transgenic mice carrying subtle mutations in the Plexin-B2 and Plexin-D1 gene, which specifically affect particular downstream signaling functions of these plexins. Our genetic analyses reveal a key function for the GAP domain of plexins during mouse development, which is independent of R-Ras and M-Ras inactivation. Furthermore, we identify a requirement of Plexin-B2–mediated RhoA activation for the development of the liver vasculature.     

Semaphorin家族在动脉粥样硬化中的作用

动脉粥样硬化是一类慢性炎症性血管疾病,其复杂的发病机制涉及多种细胞和分子的参与。Semaphorin是一类由分泌、跨膜以及糖基化磷脂酰肌醇锚定的旗语蛋白组成的轴突导向分子家族,通过与其受体Neuropilin和Plexin结合,在许多病理生理过程中发挥作用,包括神经生长、免疫反应、肿瘤转移、血管生成以及代谢性疾病等。近十年来,有关Semaphorin参与动脉粥样硬化机制的研究逐渐增多。本文主要介绍Semaphorin家族通过影响血管细胞、血液细胞、血管新生,参与动脉粥样硬化发生发展的研究进展。     

Identification of semaphorin E as a non-MDR drug resistance gene of human cancers

To improve cancer chemotherapy, a better understanding of the molecular mechanisms of drug resistance is essential. To identify the molecules responsible for drug resistance that is unrelated to MDR1 or MRP gene products, a eukaryotic expression cDNA library of cis-diamminedichloroplatinum(II) (CDDP)-resistant ovarian cancer TYKnuR cells was introduced into Cos-7 cells. After repeated CDDP selection, cDNA homologous to murine semaphorin E was isolated from surviving cells. Human semaphorin E (H-sema E) was overexpressed in CDDP-resistant cell lines and was readily induced not only by diverse chemotherapeutic drugs but also by x-ray and UV irradiation. Transfection of H-sema E conferred a drug-resistant phenotype to CDDP-sensitive cells. In addition, the aberrant expression of H-sema E protein was detected immunohistochemically in 14 of 42 (33.3%) recurrent squamous cell carcinomas removed at autopsy after extensive radiochemotherapy. Recently, another member of the semaphorin family, CD100, was shown to significantly improve the viability of B lymphocytes. These results suggest the involvement of semaphorins in diverse cell survival mechanisms.     

Cloning, expression, and genetic mapping of Sema W, a member of the semaphorin family

The semaphorins comprise a large family of membrane-bound and secreted proteins, some of which have been shown to function in axon guidance. We have cloned a transmembrane semaphorin, Sema W, that belongs to the class IV subgroup of the semaphorin family. The mouse and rat forms of Sema W show 97% amino acid sequence identity with each other, and each shows about 91% identity with the human form. The gene for Sema W is divided into 15 exons, up to 4 of which are absent in the human cDNAs that we sequenced. Unlike many other semaphorins, Sema W is expressed at low levels in the developing embryo but was found to be expressed at high levels in the adult central nervous system and lung. Functional studies with purified membrane fractions from COS7 cells transfected with a Sema W expression plasmid showed that Sema W has growth-cone collapse activity against retinal ganglion-cell axons, indicating that vertebrate transmembrane semaphorins, like secreted semaphorins, can collapse growth cones. Genetic mapping of human SEMAW with human/hamster radiation hybrids localized the gene to chromosome 2p13. Genetic mapping of mouse Semaw with mouse/hamster radiation hybrids localized the gene to chromosome 6, and physical mapping placed the gene on bacteria artificial chromosomes carrying microsatellite markers D6Mit70 and D6Mit189. This localization places Semaw within the locus for motor neuron degeneration 2, making it an attractive candidate gene for this disease.     

Sema4D induces angiogenesis through Met recruitment by Plexin B1

Semaphorins, a large family of membrane-bound and secreted proteins, signal through their transmembrane receptors, the plexins. Semaphorins and plexins share structural homologies with scatter factor receptors, a family of tyrosine kinase receptors for which Met is the prototype. Semaphorins have been studied primarily in the developing nervous system, where they act as repelling cues in axon guidance. However, they are widely expressed in several tissues, and their role in epithelial morphogenesis has been recently established. Not much is known about their role in angiogenesis, a key step during embryonic development and adulthood. Here we demonstrate that a semaphorin, Sema4D, is angiogenic in vitro and in vivo and that this effect is mediated by its high-affinity receptor, Plexin B1. Moreover, we prove that biologic effects elicited by Plexin B1 require coupling and activation of the Met tyrosine kinase. In sum, we identify a proangiogenic semaphorin and provide insight about the signaling machinery exploited by Plexin B1 to control angiogenesis.     

The semaphorin receptor plexin-B1 signals through a direct interaction with the Rho-specific nucleotide exchange factor,LARG

Semaphorins are axon guidance molecules that signal through the plexin family of receptors. Semaphorins also play a role in other processes such as immune regulation and tumorigenesis. However, the molecular signaling mechanisms downstream of plexin receptors have not been elucidated. Semaphorin 4D is the ligand for the plexin-B1 receptor and stimulation of the plexin-B1 receptor activates the small GTPase RhoA. Using the intracellular domain of plexin-B1 as an affinity ligand, two Rho-specific guanine nucleotide exchange factors, leukemia-associated Rho GEF (LARG; GEF, guanine nucleotide exchange factors) and PSD-95/Dlg/ZO-1 homology (PDZ)-RhoGEF, were isolated from mouse brain as plexin-B1-specific interacting proteins. LARG and PDZ-RhoGEF contain several functional domains, including a PDZ domain. Biochemical characterizations showed that the PDZ domain of LARG is directly involved in the interaction with the carboxy-terminal sequence of plexin-B1. Mutation of either the PDZ domain in LARG or the PDZ binding site in plexin-B1 eliminates the interaction. The interaction between plexin-B1 and LARG is specific for the PDZ domain of LARG and LARG does not interact with plexin-A1. A LARG-interaction defective mutant of the plexin-B1 receptor was created and was unable to stimulate RhoA activation. The data in this report suggest that LARG plays a critical role in plexin-B1 signaling to stimulate Rho activation and cytoskeletal reorganization.     

Fas ligand deficiency in HIV disease

Apoptosis is postulated to be involved as an anti-viral immune mechanism by killing infected cells before viral replication has occurred. The Fas–Fas ligand interaction is a powerful regulator of T cell apoptosis and could potentially act as a potent anti-viral immune mechanism against T cell tropic virus such as human immunodeficiency virus (HIV). We investigated the status of Fas ligand in peripheral blood mononuclear cells (PBMCs) obtained from persons infected with HIV. We found that monocytes in freshly isolated PBMCs from healthy individuals possess cell surface Fas ligand. In contrast, monocytes in freshly isolated PBMCs from HIV-infected patients had no detectable Fas ligand on the cell surface. Consistent with these findings of surface expression, Fas ligand activity was deficient in the cells from HIV-infected persons. The effect of replacing Fas ligand activity on HIV production by patients’ cells was assessed in an in vitro assay. The addition of a functional anti-Fas antibody to PBMCs from HIV-infected individuals inhibited viral production by greater than 90% without affecting lymphocytic function. These findings suggest the possibility of a new therapeutic modality for the treatment of HIV-infected individuals based on the reconstitution of Fas ligand activity.     

NKG2D-independent suppression of T cell proliferation by H60 and MICA

The activating receptor NKG2D recognizes a wide range of different ligands, some of which are primarily expressed in "stressed" tissues or on tumor cells. Until now, similar stimulatory effects on natural killer and CD8+ T cells have been described for all NKG2D ligands, and the NKG2D receptor/ligand system has therefore been interpreted as a sensor system involved in tumor immune surveillance and activation of immune responses. We show here that the NKG2D ligands H60 and MIC class 1 chain-related protein A (MICA) can also mediate strong suppressive effects on T cell proliferation. Responsiveness to H60- and MICA-mediated suppression requires IL-10 and involves a receptor other than NKG2D. These findings might provide explanations for the observation that strong in vivo NKG2D ligand expression, such as that on tumor cells, sometimes fails to support effective immune responses and links this observation to a distinct subgroup of NKG2D ligands.