Functions of cell surface galectin-glycoprotein lattices

Programmed remodeling of cell surface glycans by the sequential action of specific glycosyltransferases can control biological processes by generating or masking ligands for endogenous lectins. Galectins, a family of animal lectins with affinity for beta-galactosides, can form multivalent complexes with cell surface glycoconjugates and deliver a variety of intracellular signals to modulate cell activation, differentiation, and survival. Recent efforts involving genetic or biochemical manipulation of O-glycosylation and N-glycosylation pathways, as well as blockade of the synthesis of endogenous galectins, have illuminated essential roles for galectin-glycoprotein lattices in the control of biological processes including receptor turnover and endocytosis, host-pathogen interactions, and immune cell activation and homeostasis.

An emerging role for galectins in tuning the immune response: lessons from experimental models of inflammatory disease, autoimmunity and cancer

Inflammation is a critical process for eliminating pathogens, but can lead to serious deleterious effects if left unchecked. Identifying the endogenous factors that control immune tolerance and inflammation is a key goal in the field of immunology. Galectins, a family of endogenous lectins with affinity for beta-galactoside-containing oligosaccharides, are expressed by several cells of the immune system and tissue-resident stromal cells. According to their architecture, this family of glycan-binding proteins is classified in those containing one-carbohydrate-recognition domain (CRD) (proto-type), those containing two-CRD joined by a linker non-lectin domain (tandem-repeat) and those that have one-CRD attached to an N-terminal peptide (chimera-type). Accumulating evidence indicates that galectins play critical regulatory roles in immune cell response and homeostasis. In this review, we summarize recent developments in our understanding of the galectins' roles within different immune cell compartments, and in the broader context of the inflammatory microenvironments. In particular we illustrate the immunoregulatory role of three representative members of each galectin subfamily: galectin-1, -3 and -9. This body of knowledge, documenting the coming of age of galectins as potential immunosuppressive agents or targets for anti-inflammatory drugs, represents a sound basis to further explore their potential as novel therapies for autoimmune diseases, chronic inflammation and cancer.

Galectins: matricellular glycan-binding proteins linking cell adhesion, migration, and survival

Galectins are a taxonomically widespread family of glycan-binding proteins, defined by at least one conserved carbohydrate-recognition domain with a canonical amino acid sequence and affinity for beta-galactosides. Because of their anti-adhesive as well as pro-adhesive extracellular functions, galectins appear to be a novel class of adhesion-modulating proteins collectively known as matricellular proteins (which include thrombospondin, SPARC, tenascin, hevin, and disintegrins). Accordingly, galectins can display de-adhesive effects when presented as soluble proteins to cells in a strong adhesive state. In this context, the de-adhesive properties of galectins should be considered as physiologically relevant as the proadhesive effects of these glycan-binding proteins. This article focuses on the roles of mammalian galectins in cell adhesion, spreading, and migration, and the crossregulation of these functions. Although careful attention should be paid when examining individual galectin functions due to overlapping distributions, these intriguing glycan-binding proteins offer promising possibilities for the treatment and intervention of a wide variety of pathological processes, including cancer, inflammation, and autoimmunity.

The AP1-dependent secretion of galectin-1 by Reed Sternberg cells fosters immune privilege in classical Hodgkin lymphoma

Classical Hodgkin lymphomas (cHLs) contain small numbers of neoplastic Reed-Sternberg (RS) cells within an extensive inflammatory infiltrate that includes abundant T helper (Th)-2 and T regulatory (Treg) cells. The skewed nature of the T cell infiltrate and the lack of an effective host antitumor immune response suggest that RS cells use potent mechanisms to evade immune attack. In a screen for T cell-inhibitory molecules in cHL, we found that RS cells selectively overexpressed the immunoregulatory glycan-binding protein, galectin-1 (Gal1), through an AP1-dependent enhancer. In cocultures of activated T cells and Hodgkin cell lines, RNAi-mediated blockade of RS cell Gal1 increased T cell viability and restored the Th1/Th2 balance. In contrast, Gal1 treatment of activated T cells favored the secretion of Th2 cytokines and the expansion of CD4+CD25high FOXP3+ Treg cells. These data directly implicate RS cell Gal1 in the development and maintenance of an immunosuppressive Th2/Treg-skewed microenvironment in cHL and provide the molecular basis for selective Gal1 expression in RS cells. Thus, Gal1 represents a potential therapeutic target for restoring immune surveillance in cHL.

Galectin-1 expression in human glioma cells: modulation by ionizing radiation and effects on tumor cell proliferation and migration

Galectins are evolutionarily conserved beta-galactoside-binding lectins which recognize specific glycoconjugates on the cell surface and the extracellular matrix. Accumulating evidence indicates that these proteins are involved in a variety of physiological and pathological processes including tumor growth and metastasis. Up-regulated expression of galectin-1 is a hallmark of a variety of malignant tumors. Here, we examined the expression of galectin-1 in glioma cell lines, the influence of ionizing irradiation and the intracellular and extracellular effects of this protein on tumor cell proliferation and migration. Galectin-1 was detected in both A172 and U118 glioma cells by immunoblot analysis. Ionizing irradiation induced a statistically significant up-regulation in glioma cell lines. RNA-interference-mediated silencing resulted in a significant suppression of the proliferation of the A172 cells, while the addition of recombinant galectin-1 had no effect. On the other hand, the migratory capacity of both cell lines was reduced after galectin-1 down-regulation, and up-regulated by the addition of exogenous galectin-1. Our results provide evidence of a role for galectin-1 in the regulation of glioma cell proliferation and migration. While an intracellular mechanism seemed to prevail in galectin-1-mediated regulation of tumor cell proliferation, the control of cell migration was exerted by both intracellular and extracellular mechanisms. In addition, this protein was up-regulated by ionizing radiation, indicating that the blockade of this protein should be performed before radiotherapy to avoid any undesired stimulating effects. Given the multifactorial role of galectin-1 in the regulation of tumor escape and metastasis, we conclude that targeting galectin-1 may have therapeutic benefits in the treatment of malignant glioma.

Differential glycosylation of TH1, TH2 and TH-17 effector cells selectively regulates susceptibility to cell death

Regulated glycosylation controls T cell processes, including activation, differentiation and homing by creating or masking ligands for endogenous lectins. Here we show that stimuli promoting T helper type 1 (TH1), TH2 or interleukin 17-producing T helper (TH-17) differentiation can differentially regulate the glycosylation pattern of T helper cells and modulate their susceptibility to galectin-1, a glycan-binding protein with anti-inflammatory activity. Although TH1- and TH-17-differentiated cells expressed the repertoire of cell surface glycans critical for galectin-1-induced cell death, TH2 cells were protected from galectin-1 through differential sialylation of cell surface glycoproteins. Consistent with those findings, galectin-1-deficient mice developed greater TH1 and TH-17 responses and enhanced susceptibility to autoimmune neuroinflammation. Our findings identify a molecular link among differential glycosylation of T helper cells, susceptibility to cell death and termination of the inflammatory response.

Immunosuppressive strategies that are mediated by tumor cells

Despite major advances in understanding the mechanisms leading to tumor immunity, a number of obstacles hinder the successful translation of mechanistic insights into effective tumor immunotherapy. Such obstacles include the ability of tumors to foster a tolerant microenvironment and the activation of a plethora of immunosuppressive mechanisms, which may act in concert to counteract effective immune responses. Here we discuss different strategies employed by tumors to thwart immune responses, including tumor-induced impairment of antigen presentation, the activation of negative costimulatory signals, and the elaboration of immunosuppressive factors. In addition, we underscore the influence of regulatory cell populations that may contribute to this immunosuppressive network; these include regulatory T cells, natural killer T cells, and distinct subsets of immature and mature dendritic cells. The current wealth of preclinical information promises a future scenario in which the synchronized blockade of immunosuppressive mechanisms may be effective in combination with other conventional strategies to overcome immunological tolerance and promote tumor regression.

Neuroendocrine-immune disequilibrium and endometriosis: an interdisciplinary approach

Endometriosis, a chronic disease characterized by endometrial tissue located outside the uterine cavity, affects one fourth of young women and is associated with chronic pelvic pain and infertility. However, an in-depth understanding of the pathophysiology and effective treatment strategies of endometriosis is still largely elusive. Inadequate immune and neuroendocrine responses are significantly involved in the pathophysiology of endometriosis, and key findings are summarized in the present review. We discuss here the role of different immune mechanisms particularly adhesion molecules, proteinglycan interactions, and pro-angiogenic mediators in the development and progression of the disease. Finally, we introduce the concept of endometrial dissemination as result of a neuroendocrine-immune disequilibrium in response to high levels of perceived stress caused by cardinal clinical symptoms of endometriosis.

A comparative study of the anti-inflammatory, anticoagulant, antiangiogenic, and antiadhesive activities of nine different fucoidans from brown seaweeds

The anti-inflammatory, antiangiogenic, anticoagulant, and antiadhesive properties of fucoidans obtained from nine species of brown algae were studied in order to examine the influence of fucoidan origin and composition on their biological activities. All fucoidans inhibited leucocyte recruitment in an inflammation model in rats, and neither the content of fucose and sulfate nor other structural features of their polysaccharide backbones significantly affected the efficacy of fucoidans in this model. In vitro evaluation of P-selectin-mediated neutrophil adhesion to platelets under flow conditions revealed that only polysaccharides from Laminaria saccharina, L. digitata, Fucus evanescens, F. serratus, F. distichus, F. spiralis, and Ascophyllum nodosum could serve as P-selectin inhibitors. All fucoidans, except that from Cladosiphon okamuranus carrying substantial levels of 2-O-alpha-D-glucuronopyranosyl branches in the linear (1-->3)-linked poly-alpha-fucopyranoside chain, exhibited anticoagulant activity as measured by activated partial thromboplastin time whereas only fucoidans from L. saccharina, L. digitata, F. serratus, F. distichus, and F. evanescens displayed strong antithrombin activity in a platelet aggregation test. The last fucoidans potently inhibited human umbilical vein endothelial cell (HUVEC) tubulogenesis in vitro and this property correlated with decreased levels of plasminogen-activator inhibitor-1 in HUVEC supernatants, suggesting a possible mechanism of fucoidan-induced inhibition of tubulogenesis. Finally, fucoidans from L. saccharina, L. digitata, F. serratus, F. distichus, and F. vesiculosus strongly blocked MDA-MB-231 breast carcinoma cell adhesion to platelets, an effect which might have critical implications in tumor metastasis. The data presented herein provide a new rationale for the development of potential drugs for thrombosis, inflammation, and tumor progression.

Regulation of galectin-1 expression by transforming growth factor beta1 in metastatic mammary adenocarcinoma cells: implications for tumor-immune escape

Tumors escape from immune surveillance by producing immunosuppressive cytokines and proapototic factors, including TGF-beta and galectin-1 (Gal-1). Since immunosuppressive mechanisms might act in concert to confer tumor-immune privilege, we investigated the potential cross talk between TGF-beta and Gal-1 in highly metastatic mammary adenocarcinoma (LM3) cells. While Gal-1 treatment was not capable of regulating TGF-beta synthesis, a pronounced and dose-dependent increase in Gal-1 expression was observed when tumor cells were treated with TGF-beta(1. )This effect was also observed in the murine lung adenocarcinoma LP07 and in the human breast adenocarcinoma MCF-7 cell lines. TGF-beta1-mediated upregulation of Gal-1 expression was specifically mediated by TbetaRI and TbetaRII, since it was abrogated when LM3 cells were infected with retroviral vectors expressing the dominant negative forms of these receptors. In addition, gal-1 gene sequence analysis revealed the presence of three putative binding sites for Smad4 and Smad3 transcription factors, consistent with the ability of TGF-beta(1) to trigger a Smad-dependent signaling pathway in these cells. Thus, TGF-beta(1) may trigger a Smad-dependent pathway to control Gal-1 expression, suggesting that distinct mechanisms might cooperate in tilting the balance toward an immunosuppressive environment at the tumor site.

Low-dose cyclophosphamide modulates galectin-1 expression and function in an experimental rat lymphoma model

In recent years, one of the most important insights into tumor immunity was provided by the identification of negative regulatory pathways and immune escape strategies that greatly influence the magnitude of antitumor responses. Galectin-1 (Gal-1), a member of a family of highly conserved beta-galactoside-binding proteins, has been recently shown to contribute to tumor cell evasion of immune responses by modulating survival and differentiation of effector T cells. However, there is still scarce information about the regulation of Gal-1 expression and function in vivo. Here we show that administration of a single low-dose cyclophosphamide (Cy), which is capable of restraining metastasis in the rat lymphoma model L-TACB, can also influence Gal-1 expression in primary tumor, metastasis, and spleen cells and modulate the effects of this protein on T cell survival. A time-course study revealed a positive correlation between Gal-1 expression and tumor volume in primary tumor cells. Conversely, Gal-1 expression was significantly reduced in spleen cells and lymph node metastasis throughout the period studied. Interestingly, cyclophosphamide treatment was capable of restoring the basal levels of Gal-1 expression in primary tumors and spleens. In addition, this antimetastatic agent rendered spleen T cells from tumor-bearing animals resistant to Gal-1-induced cell death. Our results suggest that, in addition to other well-known functions of cyclophosphamide, this immunomodulatory agent may also modulate Gal-1 expression and function during tumor growth and metastasis with critical implications for tumor-immune escape and immunotherapy.

Dissecting the pathophysiologic role of endogenous lectins: Glycan-binding proteins with cytokine-like activity?

Several families of endogenous glycan-binding proteins have been implicated in a wide variety of immunological functions including first-line defence against pathogens, cell trafficking, and immune regulation. These include, among others, the C-type lectins (collectins, selectins, mannose receptor, and others), S-type lectins (galectins), I-type lectins (siglecs and others), P-type lectins (phosphomannosyl receptors), pentraxins, and tachylectins. This review will concentrate on the immunoregulatory roles of galectins (particularly galectin-1) and collectins (mannose-binding lectins and surfactant proteins) to illustrate the ability of endogenous glycan-binding proteins to act as cytokines, chemokines or growth factors, and thereby modulating innate and adaptive immune responses under physiological or pathological conditions. Understanding the pathophysiologic relevance of endogenous lectins in vivo will reveal novel targets for immunointervention during chronic infection, autoimmunity, transplantation and cancer.

A Novel Function for Galectin-1 at the Crossroad of Innate and Adaptive Immunity: Galectin-1 Regulates Monocyte/Macrophage Physiology through a Nonapoptotic ERK-Dependent Pathway

Several environmental factors can differentially regulate monocyte and macrophage response patterns, resulting in the display of distinct functional phenotypes. Galectin-1, an endogenous lectin found at peripheral lymphoid organs and inflammatory sites, has shown immunoregulatory activity in vivo in experimental models of autoimmunity and cancer. Whereas compelling evidence has been accumulated regarding the effects of galectin-1 on T cell fate, limited information is available on how galectin-1 may impact other immune cell types. In the present study, we report a novel role for galectin-1 in the regulation of monocyte and macrophage physiology. Treatment with galectin-1 in vitro differentially regulates constitutive and inducible FcgammaRI expression on human monocytes and FcgammaRI-dependent phagocytosis. In addition, galectin-1 inhibits IFN-gamma-induced MHC class II (MHC-II) expression and MHC-II-dependent Ag presentation in a dose-dependent manner. These regulatory effects were also evident in mouse macrophages recruited in response to inflammatory stimuli following treatment with recombinant galectin-1 and further confirmed in galectin-1-deficient mice. Investigation of the mechanisms involved in these functions showed that galectin-1 does not affect survival of human monocytes, but rather influences FcgammaRI- and MHC-II-dependent functions through active mechanisms involving modulation of an ERK1/2-dependent pathway. Our results provide evidence of a novel unrecognized role for galectin-1 in the control of monocyte/macrophage physiology with potential implications at the crossroad of innate and adaptive immunity.

Galectin-1 Suppresses Autoimmune Retinal Disease by Promoting Concomitant Th2- and T Regulatory-Mediated Anti-Inflammatory Responses

Intraocular inflammatory diseases are a common cause of severe visual impairment and blindness. In this study, we investigated the immunoregulatory role of galectin-1 (Gal-1), an endogenous lectin found at sites of T cell activation and immune privilege, in experimental autoimmune uveitis (EAU), a Th1-mediated model of retinal disease. Treatment with rGal-1 either early or late during the course of interphotoreceptor retinoid-binding protein-induced EAU was sufficient to suppress ocular pathology, inhibit leukocyte infiltration, and counteract pathogenic Th1 cells. Administration of rGal-1 at the early or late phases of EAU ameliorated disease by skewing the uveitogenic response toward nonpathogenic Th2 or T regulatory-mediated anti-inflammatory responses. Consistently, adoptive transfer of CD4(+) regulatory T cells obtained from rGal-1-treated mice prevented the development of active EAU in syngeneic recipients. In addition, increased levels of apoptosis were detected in lymph nodes from mice treated with rGal-1 during the efferent phase of the disease. Our results underscore the ability of Gal-1 to counteract Th1-mediated responses through different, but potentially overlapping anti-inflammatory mechanisms and suggest a possible therapeutic use of this protein for the treatment of human uveitic diseases of autoimmune etiology.

Impact of protein–glycan interactions in the regulation of autoimmunity and chronic inflammation

Protein-glycan interactions control essential immunological processes, including T-cell activation, differentiation and survival. Galectins, carbohydrate-binding proteins, defined by shared consensus amino acid sequences and affinity for beta-galactose-containing oligosaccharides, participate in a wide spectrum of immunological processes. These carbohydrate-binding proteins regulate the development of pathogenic T-cell responses by influencing T-cell survival, activation and cytokine secretion. Administration of recombinant galectins or their genetic delivery modulate the development and severity of chronic inflammatory responses in experimental models of autoimmunity by triggering different and potentially overlapping immunoregulatory mechanisms. Given the potential use of galectins as novel anti-inflammatory agents or targets for immunosuppressive drugs, we will summarize here recent findings on the influence of these carbohydrate-binding proteins in autoimmune and chronic inflammatory disorders.

Circulating anti-galectin-1 antibodies are associated with the severity of ocular disease in autoimmune and infectious uveitis

PURPOSE

Galectin (Gal)-1, an endogenous lectin found at sites of immune privilege, plays a critical role in the regulation of the immune response. Therapeutic administration of Gal-1 or its genetic delivery suppresses chronic inflammation in experimental models of autoimmunity. The purpose of this work was to investigate the occurrence of circulating anti-Gal-1 antibodies in patients with autoimmune and infectious uveitis as potential determinant factors of disease progression.

METHODS

IgG, IgE, and IgA anti-Gal-1 antibodies were assessed by ELISA and Western blot in sera from patients with autoimmune (n = 47) and infectious (n = 15) uveitis compared with healthy control subjects (n = 30). The frequency of anti-Gal-1 antibodies was examined in patients experiencing poor clinical outcome (n = 21) or good evolution (n = 9). Anti-Gal-1 antibodies were eluted by incubating patient sera with nitrocellulose filters adsorbed with rGal-1. The ability of these antibodies to recognize retinal tissue was assessed by ELISA, Western blot, and immunohistochemistry.

RESULTS

IgE, IgG, and IgA anti-Gal-1 antibodies were increased in sera from patients with autoimmune uveitis (P < 0.001 vs. controls) and toxoplasmic retinochoroiditis (P < 0.001). The level of anti-Gal-1 IgE and IgG antibodies was associated with progressive disease and poor outcome in autoimmune and infectious uveitis. Furthermore, these antibodies strongly immunoreacted with retinal lysates and recognized retinal structures mainly photoreceptors in retinal sections.

CONCLUSIONS

Anti-retinal Gal-1 antibodies are present in sera from patients with uveitis and can be associated with the progression of ocular disease, suggesting their potential use in follow-up observations of these patients.

Regulated expression of galectin-1 after in vitro productive infection with herpes simplex virus type 1: implications for T cell apoptosis

Apoptosis of cytotoxic T lymphocytes by herpes simplex virus type-1 (HSV-1) has been reported to be a relevant mechanism of viral immune evasion. Galectin-1 (Gal-1), an endogenous lectin involved in T-cell apoptosis, has recently gained considerable attention as a novel mechanism of tumor-immune evasion. Here we investigated whether infection of cells with HSV-1 can modulate the expression of Gal-1. Results show that pro-apoptotic Gal-1, but not Gal-3, is remarkably up-regulated in cell cultures infected with HSV-1. In addition, this protein is secreted to the extracellular milieu, where it contributes to apoptosis of activated T cells in a carbohydrate-dependent manner. Since many viruses have evolved mechanisms to counteract the antiviral response raised by the infected host, our results suggest that HSV-1 may use galectin-1 as a weapon to kill activated T cells and evade specific immune responses.

[How do protein-glycan interactions regulate T-cell physiology?]

Recent evidence indicates that protein-glycan interactions play a critical role in different events associated with the physiology of T-cell responses including thymocyte maturation, T-cell activation, lymphocyte migration and T-cell apoptosis. Glycans decorating T-cell surface glycoproteins can modulate T-cell physiology by specifically interacting with endogenous lectins including selectins and galectins. These endogenous lectins are capable of recognizing sugar structures localized on T-cell surface glycoproteins and trigger different signal transduction pathways leading to differentiation, proliferation, cell cycle regulation or apoptosis. Protein-carbohydrate interactions may be controlled at different levels, including regulated expression of lectins during T-cell maturation and differentiation and the spatio-temporal regulation of glycosyltransferases and glycosidases, which create and modify sugar structures present in T-cell surface glycoproteins. This article briefly reviews the mechanisms by which protein-carbohydrate interactions modulate immunological processes such as T-cell activation, migration and apoptosis.

The coming of age of galectins as immunomodulatory agents: impact of these carbohydrate binding proteins in T cell physiology and chronic inflammatory disorders

Immune cell homoeostasis is attributed to multiple distinct safety valves that are interconnected and intervene at defined checkpoints of the life cycle of immunocytes to guarantee clonal expansion and functional inactivation of self-reactive potentially autoaggressive lymphocytes. Galectins, animal lectins defined by shared consensus amino acid sequence and affinity for beta-galactose containing oligosaccharides, are found on various cells of the immune system, and their expression is associated with the differentiation and activation status of these cells. Over the past few years, galectins have been implicated in the regulation of many aspects of T cell physiology such as cell activation, differentiation, and apoptosis. In addition, a growing body of experimental evidence indicates that galectins may play critical roles in the modulation of chronic inflammatory disorders, autoimmunity, and cancer. Given the increased interest of immunologists in this field, the growing body of information raised during the past few years and the potential use of galectins as novel anti-inflammatory agents or targets for immunosuppressive drugs, we will summarise recent advances on the role of galectins in different aspects of T cell physiology and their impact in the development and/or resolution of chronic inflammatory disorders, autoimmunity, and cancer.

Synthetic lactulose amines: novel class of anticancer agents that induce tumor-cell apoptosis and inhibit galectin-mediated homotypic cell aggregation and endothelial cell morphogenesis

Galectins, a family of structurally related carbohydrate-binding proteins, contribute to different events associated with cancer biology, including apoptosis, homotypic cell aggregation, angiogenesis and tumor-immune escape. To interfere with galectin-carbohydrate interactions during tumor progression, a current challenge is the design of specific galectin inhibitors for therapeutic purposes. Here, we report the synthesis of three novel low molecular weight synthetic lactulose amines (SLA): (1) N-lactulose-octamethylenediamine (LDO), (2) N,N'-dilactulose-octamethylenediamine (D-LDO), and (3) N,N'-dilactulose-dodecamethylenediamine (D-LDD). These compounds showed a differential ability to inhibit binding of galectin-1 and/or galectin-3 to the highly glycosylated protein 90K in solid-phase assays. In addition, each compound demonstrated selective regulatory effects in different events linked to tumor progression including tumor-cell apoptosis, homotypic cell aggregation, and endothelial cell morphogenesis. Our results suggest that galectin inhibitors with subtle differences in their carbohydrate structures may be potentially used to specifically block different steps of tumor growth and metastasis.

Overexpression of inducible nitric oxide synthase and cyclooxygenase-2 in rat zinc-deficient lung: Involvement of a NF-kappaB dependent pathway

Reactive oxygen and nitrogen species have been implicated in the pathogenesis of pulmonary diseases. The goal of this study was to measure the response of inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 enzymes (COX-2) in lung with moderate zinc deficiency. Adult male Wistar rats were divided into two groups receiving (1) a zinc-deficient diet (ZD) or (2) a zinc-adequate control diet. After 2 months of treatment, the zinc-deficient group showed a significant pulmonary edema. This was associated to a reduction of protein thiols and to a significant increase of metallothionein and glutathione disulfide levels. In addition, a higher serum and lung NO production in ZD group was positively related to the higher activity and expression of iNOS and COX-2 found in lungs. Western blot analysis revealed increased IkappaBalpha degradation, an indicator of NF-kappaB activation in ZD lungs. Anatomopathologic analysis of ZD lungs showed an increase of connective tissue fibers with an influx of polymorphonuclear cells. These cells and type II cells from the alveoli showed specific immunohistochemical signals for iNOS. The conclusion is that, during the development of zinc-deficiency, iNOS activity increases in lung and contributes to lung injury. Zinc deficiency implications must be taken into account to design therapies and public health interventions involving targeted zinc supplementation for high-risk subjects or certain diseases, such as asthma.

Galectins as immunoregulators during infectious processes: from microbial invasion to the resolution of the disease

Recent evidence has implicated galectins, a family of evolutionarily conserved carbohydrate-binding proteins, as regulators of immune cell homeostasis and host-pathogen interactions. Galectins operate at different levels of innate and adaptive immune responses, by modulating cell survival and cell activation or by influencing the Th1/Th2 cytokine balance. Furthermore, galectins may contribute to host-pathogen recognition and may serve as receptors for specific interactions of pathogens with their insect vectors. Here we will explore the influence of galectins in immunological processes relevant to microbial infection and will summarize exciting recent work related to the specific interactions between galectins and their glycoconjugate ligands as critical determinants of pathogen recognition. Understanding the role of galectin-sugar interactions during the course of microbial infections might contribute to defining novel targets for disease prevention and immune intervention.

Administration of a peptide inhibitor of alpha4-integrin inhibits the development of experimental autoimmune uveitis

PURPOSE

Recruitment of lymphocytes into the retina and to the vitreous during the development of experimental autoimmune uveitis (EAU) is governed by factors such as the state of activation of inflammatory cells and the repertoire of adhesion molecules expressed by the local vascular endothelia. alpha4 Integrins and their receptors play an important role during homing of cells to the inflammatory site. In the present study, the effect of alpha4-integrin inhibitor on the development of EAU was investigated.

METHODS

EAU was induced either by immunizing B10.RIII mice with the 161-180 peptide or by adoptive transfer of interphotoreceptor retinoid-binding protein (IRBP)-specific uveitogenic T cells. Animals were treated with an active peptide inhibitor (alpha4-api) or a peptide control at different time points after induction of disease. EAU was evaluated by histology 21 to 49 days after immunization. Antigen-specific cell proliferation was evaluated by thymidine incorporation. Cytokine synthesis in culture supernatants and anti-IRBP-specific serum IgG1 and IgG2a were evaluated by ELISA. Delayed-type hypersensitivity was evaluated by ear challenge 2 days before the termination of the experiment.

RESULTS

Treatment with alpha4-api had a significant ameliorating effect on EAU. The anti-IRBP antibody response and cellular proliferation were not affected by the treatment, whereas delayed-type hypersensitivity was significantly diminished. Cytokine synthesis was not changed by treatment, except for a decrease in IL-10 levels.

CONCLUSIONS

The results show that small-molecule inhibitors of alpha4-integrins can act therapeutically in EAU, possibly by interfering with cell adhesion events involved in the development of the disease.

Galectin-1 as a potential cancer target

Galectins are a family of structurally related carbohydrate-binding proteins, which are defined by their affinity for poly-N-acetyllactosamine-enriched glycoconjugates and sequence similarities in the carbohydrate recognition domain. Galectin-1, a member of this family, contributes to different events associated with cancer biology, including tumour transformation, cell cycle regulation, apoptosis, cell adhesion, migration and inflammation. In addition, recent evidence indicates that galectin-1 contributes to tumour evasion of immune responses. Given the increased interest of tumour biologists and clinical oncologists in this field and the potential use of galectins as novel targets for anticancer drugs, we summarise here recent advances about the role of galectin-1 in different events of tumour growth and metastasis.

Regulated expression of galectin-1 during T-cell activation involves Lck and Fyn kinases and signaling through MEK1/ERK, p38 MAP kinase and p70S6 kinase

Recent evidence has implicated galectins and their carbohydrate ligands as novel regulators of T-cell homeostasis. Galectin-1 (Gal-1), a member of this family, inhibits clonal expansion, induces apoptosis of antigen-primed T lymphocytes and suppresses the development of T-cell-mediated autoimmune diseases in vivo. Because the beta-galactoside-binding protein is expressed in activated but not resting T cells, it has been hypothesized that Gal-1-induced apoptosis may constitute an autocrine suicide mechanism to eliminate activated T cells contributing to the termination of an effector immune response. We undertook this study to investigate the signals and intracellular pathways leading to Gal-1 expression during T-cell activation. When T cells were stimulated either with anti-CD3 or anti-CD28 monoclonal antibody plus PMA in the presence of accessory cells, a sustained up-regulation of Gal-1 was observed, reaching a plateau between days 3 and 5 following CD3 engagement or costimulation through CD28. Investigation of the signal transduction events involved in this process revealed a role for Lck and Fyn kinases, since the Src kinase inhibitor PP1 inhibited the up-regulated expression of Gal-1 following T-cell activation. Downstream signaling routes involve mitogen-activated protein kinase (MAPK) kinase (MEK)1/extracellular signal-regulated kinase (ERK) and p38 MAPK, as Gal-1 expression was prevented by U0126 and SB202190. In addition, expression of Gal-1 involves interleukin (IL)-2-dependent signaling routes triggered by p70S6 kinase, as it could be inhibited by rapamycin. This is the first demonstration of the intracellular pathways that control activation-induced expression of Gal-1, which may reveal potential targets for immune intervention to modulate expression of this beta-galactoside-binding protein in pathological disorders.

Galectin-1 Sensitizes Resting Human T Lymphocytes to Fas (CD95)-mediated Cell Death via Mitochondrial Hyperpolarization, Budding, and Fission

Galectins have emerged as a novel family of immunoregulatory proteins implicated in T cell homeostasis. Recent studies showed that galectin-1 (Gal-1) plays a key role in tumor-immune escape by killing antitumor effector T cells. Here we found that Gal-1 sensitizes human resting T cells to Fas (CD95)/caspase-8-mediated cell death. Furthermore, this protein triggers an apoptotic program involving an increase of mitochondrial membrane potential and participation of the ceramide pathway. In addition, Gal-1 induces mitochondrial coalescence, budding, and fission accompanied by an increase and/or redistribution of fission-associated molecules h-Fis and DRP-1. Importantly, these changes are detected in both resting and activated human T cells, suggesting that Gal-1-induced cell death might become an excellent model to analyze the morphogenetic changes of mitochondria during the execution of cell death. This is the first association among Gal-1, Fas/Fas ligand-mediated cell death, and the mitochondrial pathway, providing a rational basis for the immunoregulatory properties of Gal-1 in experimental models of chronic inflammation and cancer.

Galectins as modulators of tumour progression

Galectins are a family of animal lectins with diverse biological activities. They function both extracellularly, by interacting with cell-surface and extracellular matrix glycoproteins and glycolipids, and intracellularly, by interacting with cytoplasmic and nuclear proteins to modulate signalling pathways. Current research indicates that galectins have important roles in cancer; they contribute to neoplastic transformation, tumour cell survival, angiogenesis and tumour metastasis. They can modulate the immune and inflammatory responses and might have a key role helping tumours to escape immune surveillance. How do the different members of the Galectin family contribute to these diverse aspects of tumour biology?

Galectin-3 and soluble fibrinogen act in concert to modulate neutrophil activation and survival: involvement of alternative MAPK pathways

Galectin-3 (Gal-3), a member of a family of highly conserved carbohydrate-binding proteins, has recently emerged as a novel cellular modulator at inflammatory foci. Here we investigated the effects of Gal-3 on central effector functions of human neutrophils, including phagocytosis, exocytosis of secretory granules, and survival. We examined the effects of Gal-3 alone or in combination with soluble fibrinogen (sFbg), an extracellular mediator that plays a key role during the early phase of the inflammatory response through binding to integrin receptors. In addition we evaluated the intracellular signals triggered by these mediators in human neutrophils. Human neutrophils incubated with recombinant Gal-3 alone increased their phagocytic activity and CD66 surface expression. In contrast to the known antiapoptotic effect of Gal-3 on many cellular types, Gal-3 enhanced PMN apoptotic rate. Preincubation with Gal-3 primed neutrophils to the effects of sFbg, resulting in a synergistic action on degranulation. On the other hand, Gal-3 and sFbg had opposite effects on PMN survival, and the simultaneous action of both agonists partially counteracted the proapoptotic effects of Gal-3. In addition, although sFbg induced its effects through the activation of the ERKs, Gal-3 led to p38 phosphorylation. Disruption of this signaling pathway abrogated Gal-3-mediated modulation of neutrophil degranulation, phagocytosis, and apoptosis. Together, our results support the notion that Gal-3 and sFbg are two physiological mediators present at inflammatory sites that activate different components of the MAPK pathway and could be acting in concert to modulate the functionality and life span of neutrophils.

NF-kappa B regulates expression of the MHC class I-related chain A gene in activated T lymphocytes

MHC class I-related chain A gene (MICA) is a stress-regulated, HLA-related molecule which exhibits a restricted pattern of expression. MICA protein is up-regulated on different tumor cells, and is recognized by the lectin-like NKG2D molecule expressed by cytotoxic gammadelta T lymphocytes, CD8+ alphabeta T lymphocytes, and NK cells. Although MICA is not expressed on resting lymphocytes, we demonstrated that it is induced on activated T cells. Because NF-kappaB is actively involved in T cell activation, and is constitutively activated in many tumors, here we investigated whether NF-kappaB may modulate MICA expression. Treatment with the NF-kappaB inhibitor sulfasalazine (Sz) resulted in a dose-dependent inhibition of MICA expression in anti-CD3- and anti-CD28/PMA-activated T lymphocytes, as assessed by Western blot and RT-PCR analysis. Moreover, Sz also down-regulated MICA expression on epithelial tumor HeLa cells. MICA expression was accompanied by a Sz-sensitive IkappaBalpha degradation. EMSA with nuclear extracts from anti-CD3- and anti-CD28/PMA-stimulated T lymphocytes demonstrated the binding of a potential NF-kappaB family transcription factor to a MICA gene intron 1-derived oligonucleotide that contains a putative kappaB binding site. Supershift assays demonstrated the presence of p65(RelA)/p50 heterodimers and p50/p50 homodimers in the NF-kappaB complexes bound to the kappaB-MICA oligonucleotide. Transient transfection of HeLa cells with p65(RelA) up-regulated MICA expression, as assessed by Western blot and flow cytometry analysis. Hence, we conclude that NF-kappaB regulates MICA expression on activated T lymphocytes and HeLa tumor cells, by binding to a specific sequence in the long intron 1 of the MICA gene. This constitutes the first description of a transcription factor that regulates MICA gene expression.

The role of galectins in the initiation, amplification and resolution of the inflammatory response

Inflammation involves the sequential activation of signalling pathways leading to the production of both pro-inflammatory and anti-inflammatory mediators. Galectins constitute a family of structurally related beta-galactoside-binding proteins, which are defined by their affinity for poly-N-acetyllactosamine-enriched glycoconjugates and sequence similarities in the carbohydrate recognition domain. By crosslinking specific glycoconjugates, different members of the galectin family behave as pro-inflammatory or anti-inflammatory agents, acting at different levels of acute and chronic inflammatory responses. Recent studies highlighted immunomodulatory roles for galectins in vivo in several experimental models of chronic inflammation, suggesting that these carbohydrate-binding proteins may be potential targets for the design of a novel generation of anti-inflammatory agents. In this study, we review recent advances on the role of galectins in the initiation, amplification and resolution of the inflammatory response. In particular, we examine the influence of individual members of this family in regulating cell adhesion, migration, chemotaxis, antigen presentation, immune cell activation and apoptosis. From a better understanding of the molecular basis of galectin-induced immune regulation, we may become able to exploit the potential of these sugar-binding proteins and their glycoligands as suitable therapeutic agents in acute and chronic inflammatory disorders.

Identification of RANTES as a novel immunomodulator of the maternal allogeneic response

We investigated the immunomodulatory role of chemokines in the maternal allogeneic T-cell response. In comparison with fertile women, we found in patients with recurrent spontaneous abortions (RSA), a significant decreased sera level of RANTES that increased after immunization with paternal leukocytes. Since blocking factors with unknown identity are detected in sera from fertile women, we hypothesized that RANTES might function as a novel blocking factor and therefore we explored its cell growth inhibitory properties during the allogenic T-cell response. We demonstrated that RANTES inhibits the mixed lymphocyte reaction (MLR) in a dose-dependent manner. Investigation of the mechanisms involved in cell growth inhibition revealed that this beta-chemokine induces T-cell apoptosis through modulation of Bcl-2 protein levels and by a caspase-independent mechanism and does not involve modulation of Fas (CD95) antigen expression. Our results provides experimental evidence implicating RANTES as a suppressor of alloantigen specific T-cell responses and indicates that this beta-chemokine might function as a novel blocking factor and reliable marker for successful allotreatment of RSA patients.

Targeted inhibition of galectin-1 gene expression in tumor cells results in heightened T cell-mediated rejection; A potential mechanism of tumor-immune privilege

Despite the existence of tumor-specific immune cells, most tumors have devised strategies to avoid immune attack. We demonstrate here that galectin-1 (Gal-1), a negative regulator of T cell activation and survival, plays a pivotal role in promoting escape from T cell-dependent immunity, thus conferring immune privilege to tumor cells. Blockade of immunosuppressive Gal-1 in vivo promotes tumor rejection and stimulates the generation of a tumor-specific T cell-mediated response in syngeneic mice, which are then able to resist subsequent challenge with wild-type Gal-1-sufficient tumors. Our data indicate that Gal-1 signaling in activated T cells constitutes an important mechanism of tumor-immune escape and that blockade of this inhibitory signal can allow for and potentiate effective immune responses against tumor cells, with profound implications for cancer immunotherapy.

Galectin-3 Mediates IL-4-Induced Survival and Differentiation of B Cells: Functional Cross-Talk and Implications during Trypanosoma cruzi Infection

The role of transcription factors in B cell survival and differentiation has been delineated during the last years. However, little is known about the intermediate signals and the intracellular pathways that control these events. In this study, we provide evidence both in vitro and in vivo, showing that galectin-3 (Gal-3), a beta-galactoside-binding protein, is a critical mediator of B cell differentiation and survival. Although Gal-3 is not expressed in resting B cells from normal mice, its expression is markedly induced after activation with stimuli such as IL-4 and CD40 cross-linking. These signals promote survival and block the final differentiation of these cells, thus allowing the rising of a memory B cell phenotype. In addition, Gal-3 is expressed in B cells from Trypanosoma cruzi-infected mice, which received signals for activation and differentiation in vivo. By using an antisense strategy, we determined that Gal-3 is a critical signal mediating the effects of IL-4 on B cell fate. Blockade of intracellular Gal-3 in vitro abrogated IL-4-induced survival of activated B cells, favoring the differentiation toward a plasma cell pathway. Moreover, B cells with restrained endogenous Gal-3 expression failed to down-regulate the Blimp-1 transcription factor after IL-4 stimulation. Finally, inhibition of Gal-3 in vivo skewed the balance toward plasma cell differentiation, which resulted in increased Ig production and parasite clearance during T. cruzi infection. Thus, the present study provides evidence of a novel role for Gal-3 as an intracellular mediator of B cell survival and a checkpoint in IL-4-induced B cell commitment toward a memory phenotype.

Characterization and functional significance of glucocorticoid receptors in patients with major depression: modulation by antidepressant treatment

Hyperactivity of the hypothalamic pituitary adrenal (HPA) axis in patients with major depression is one of the most consistent findings in biological psychiatry. Experimental data support the idea that glucocorticoid-mediated feedback via glucocorticoid receptors (GR) is impaired in major depression. The aim of the present work was to assess the putative changes in GR density of peripheral blood mononuclear cells (PBMCs) in a group of patients with major depression and to determine modulation of these GR sites by antidepressant treatment. In addition, susceptibility of PBMCs to glucocorticoid effects was also studied using a functional end-point analysis in vitro, such as cortisol inhibition of mitogen-induced lymphocyte proliferation. Cortisol levels were also measured before and after dexamethasone suppression test (DST). The results showed a decrease in GR density in depressed patients compared with healthy subjects, mainly in those patients that showed basal cortisol levels in the upper normal range and were refractory to DST. Regarding the functional significance of this variation, two representative groups emerged from our study: a) free-medication patients with GR function comparable to healthy controls, and b) patients showing diminished GR activity. These results suggest a lack of relationship between GR density and cortisol-induced inhibition of lymphocyte proliferation. Patients treated with different antidepressant drugs showed a marked increase in the number of GR sites per cell compared to non-treated. Interestingly, this increase was even higher than in normal subjects. Hence, restoration of GR density after an efficient antidepressant treatment could be an index of an effective modulatory action of drugs on GR expression and highlights the possibility that GR levels might be used as markers of a successful treatment.

Up-regulated expression of MICA on activated T lymphocytes involves Lck and Fyn kinases and signaling through MEK1/ERK, p38 MAP kinase, and calcineurin

Major histocompatibility complex class I-related chain (MICA) is a cell stress-regulated molecule recognized by cytotoxic cells expressing the NKG2D molecule. MICA can be induced on T cells after CD3 or CD28 engagement. Here, we investigated the intracellular pathways leading to activation-induced expression of MICA. The Src kinase inhibitor PP1 inhibited up-regulated expression of MICA on anti-CD3-stimulated T cells. Downstream signaling routes involved mitogen-activated protein kinase (MAPK) kinase (MEK)1/extracellular signal-regulated kinase (ERK), p38 MAPK, and calcineurin, as MICA expression was prevented by U0126, SB202190, cyclosporin A, and FK506. Also, Lck and Fyn as well as MEK1/ERK and p38 MAPK were found to regulate MICA expression in anti-CD28/phorbol 12-myristate 13-acetate-stimulated T cells. Expression of MICA on activated T cells involved interleukin-2-dependent signaling routes triggered by Janus tyrosine kinases/signal transducer and activators of transcription and p70(S)(6) kinase, as it could be inhibited by AG490 and rapamycin. This is the first demonstration of the intracellular pathways involved in activation-induced expression of MICA, which may reveal potential targets for immune intervention to modulate MICA expression in pathological disorders.

Galectin-1 suppresses experimental colitis in mice

BACKGROUND & AIMS

Uncontrolled T-cell activation plays a critical role in the pathogenesis of inflammatory bowel diseases. Therefore, pharmacologic strategies directed to restore the normal responsiveness of the immune system by deleting inappropriately activated T cells could be efficacious in the treatment of these pathologic conditions. Galectin-1 is an endogenous lectin expressed in lymphoid organs that plays a role in the maintenance of central and peripheral tolerance. The aim of the present study was to evaluate the therapeutic effects of galectin-1 on T-helper cell type 1-mediated experimental colitis induced by intrarectal administration of 2,4,6-trinitrobenzene sulfonic acid (TNBS) in mice.

METHODS

Cells and tissues from mice with TNBS colitis receiving treatment with several doses of human recombinant galectin-1 (hrGAL-1) were analyzed for morphology, cytokine production, and apoptosis.

RESULTS

Prophylactic and therapeutic administration of rhGAL-1 resulted in a striking improvement in the clinical and histopathologic aspects of the disease. hrGAL-1 reduced the number of hapten-activated spleen T cells, decreased inflammatory cytokine production, and profoundly reduced the ability of lamina propria T cells to produce IFN gamma in vitro. Moreover, hrGAL-1 led to the appearance of apoptotic mononuclear cells in colon tissue when administered in vivo and induced selective apoptosis of TNBS-activated lamina propria T cells in vitro.

CONCLUSION

Collectively, these data show that hrGAL-1 exerts protective and immunomodulatory activity in TNBS-induced colitis and it might be effective in the treatment of inflammatory bowel diseases.

Opposite effects of galectin-1 on alternative metabolic pathways of L-arginine in resident, inflammatory, and activated macrophages

Recent evidence has implicated galectins and their carbohydrate ligands as master regulators of the inflammatory response. Galectin-1, a member of this family, has shown specific anti-inflammatory and immunoregulatory effects. To gain insight into the potential mechanisms involved in these effects, we investigated the effects of galectin-1 in L-arginine metabolism of peritoneal rat macrophages. Pretreatment of macrophages with galectin-1 resulted in a dose- and time-dependent inhibition of lipopolysaccharide-induced nitric oxide (NO) production, accompanied by a decrease in inducible nitric oxide synthase (iNOS) expression (the classic pathway of L-arginine). On the other hand, galectin-1 favored the balance toward activation of L-arginase, the alternative metabolic pathway of L-arginine. Inhibition of NO production was not the result of increased macrophage apoptosis because addition of this beta-galactoside-binding protein to macrophages under the same experimental conditions did not affect the apoptotic threshold of these cells. To understand how endogenous galectin-1 is regulated in macrophages under inflammatory stress, we finally explored the ultrastructural distribution, expression, and secretion of galectin-1 in resident, inflammatory, and activated macrophages. This study provides an alternative cellular mechanism based on the modulation of L-arginine metabolism to understand the molecular basis of the anti-inflammatory properties displayed by this carbohydrate-binding protein.

Regulated expression and ultrastructural localization of galectin-1, a proapoptotic beta-galactoside-binding lectin, during spermatogenesis in rat testis

Galectin-1, a highly conserved beta-galactoside-binding protein, induces apoptosis of activated T cells and suppresses the development of autoimmunity and chronic inflammation. To gain insight regarding the potential role of galectin-1 as a novel mechanism of immune privilege, we investigated expression and ultrastructural localization of galectin-1 in rat testis. Galectin-1 expression was assessed by Western blot analysis and immunocytochemical localization in testes obtained from rats aged from 9 to 60 days. Expression of this carbohydrate-binding protein was developmentally regulated, and its immunolabeling exhibited a stage-specific pattern throughout the spermatogenic process. Immunogold staining using the anti-galectin-1 antibody revealed the typical Sertoli cell profile in the seminiferous epithelium, mainly at stages X-II. During spermiation (stages VI-VIII), a strong labeling was observed at the luminal pole of seminiferous epithelium, localized on apical stalks of Sertoli cells, on heads of mature spermatids, and on bodies of residual cytoplasm. Moreover, spermatozoa released into the lumen showed a strong immunostaining. Following spermiation (stage VIII), galectin-1 expression was restored at the basal portion of Sertoli cells and progressively spread out through the whole cells as differentiation of germinal cells proceeded. Immunoelectron microscopy confirmed distribution of galectin-1 in nuclei and cytoplasmic projections of Sertoli cells and on heads and tails of late spermatids and residual bodies. Surface localization of galectin-1 was evidenced in spermatozoa from caput epididymis. Thus, the regulated expression of galectin-1 during the spermatogenic cycle suggests a novel role for this immunosuppressive lectin in reproductive biology.

Bone-specific antibodies in sera from patients with celiac disease: characterization and implications in osteoporosis

Osteopenia and osteoporosis are well-known complications detected in celiac disease patients with still obscure pathogenesis. In the present study we investigated the presence of circulating anti-bone autoantibodies in patients with celiac disease and explored their role in the associated bone disease. We evaluated serum samples from 33 patients at the time of diagnosis and from 20 of them after treatment. Sera from patients with inflammatory bowel disease (n = 9), nonceliac osteoporotic (n = 18), and healthy individuals (n = 10) were used as controls. The presence of IgA specific anti-bone antibodies was first investigated using indirect immunofluorescence on cryosections of fetal rat tibia (20-day pregnancy). Furthermore, samples were homogenized and total tissue extracts were subjected to Western blot analysis to confirm immunoreactivity. At diagnosis, sera from 51.5% (17/33) of celiac patients had antibodies that recognized antigenic structures in chondrocytes and the extracellular matrix along mature cartilage, bone interface, and perichondrium of fetal rat bone. Among controls, only two osteoporotic patients showed very low titles of anti-bone autoantibodies. The immunostaining was localized in areas where an active mineralization process occurred and was similar to the distribution of the native bone tissue transglutaminase. The frequency of patients with positive baseline titers of anti-bone antibodies diminished significantly after treatment (P = 0.048). Western blot assays confirmed the presence of autoantibodies in sera from patients with a positive immunofluorescence staining. Autoantibodies recognized a major protein band on tissue extracts with a molecular weight of 77-80 kDa, which could be displaced when sera were preadsorbed with human recombinant tissue transglutaminase. We provide original evidence that patients with celiac disease have IgA-type circulating autoantibodies against intra- and extracellular structures of fetal rat tibia. Our findings suggest that these antibodies recognize bone tissue transglutaminase as the autoantigen, and based on the localization of the immunoreactivity we speculate that they might have an active role in the pathophysiology of celiac disease-associated bone complications.

Role of galectins in inflammatory and immunomodulatory processes

Galectins are members of a highly conserved family of beta-galactoside-binding animal lectins. Presently, more than 14 members have been identified and additional homologues are likely to be discovered. Given their conservation throughout animal evolution, it is not surprising that they could play key roles in innate and adaptive immune responses, through sugar-dependent and -independent mechanisms. Recently, it has become increasingly clear that galectins can differentially affect cellular activation and function. These biological effects attracted attention of researchers in cell biology, biochemistry, glycobiology and immunology, not only in the mode of action of galectins, but also in their role as putative modulators of immune surveillance, apoptosis, cell adhesion and chemotaxis. Here we will summarize the state-of-the-art of the effects of galectins in inflammatory and immunomodulatory processes. In addition, we will discuss in-depth the current knowledge about the effects of this enigmatic family of animal lectins and their glycoligands in the progression, diagnosis and treatment of different pathological processes such as autoimmunity, allergy, infection and chronic inflammation.

Galectins and their ligands: amplifiers, silencers or tuners of the inflammatory response?

Recent evidence has implicated galectins and their ligands as master regulators of immune cell homeostasis. Whereas some members of this family, such as galectin-3, behave as amplifiers of the inflammatory cascade, others, such as galectin-1, trigger homeostatic signals to shut off T-cell effector functions. These carbohydrate-binding proteins, identified by shared consensus amino acid sequences and affinity for beta-galactoside-containing sugars, participate in the homeostasis of the inflammatory response, either by regulating cell survival and signaling, influencing cell growth and chemotaxis, interfering with cytokine secretion, mediating cell-cell and cell-matrix interactions or influencing tumor progression and metastasis. The current wealth of new information promises a future scenario in which individual members of the galectin family or their ligands will be used as powerful anti-inflammatory mediators and selective modulators of the immune response.

Induction of allogenic T-cell hyporesponsiveness by galectin-1-mediated apoptotic and non-apoptotic mechanisms

Galectin-1, a beta-galactoside-binding protein expressed at sites of T-cell activation and immune privilege, has shown specific immunosuppressive properties. Because of the implications of this protein in T-cell tolerance and its potential use to avoid graft rejection, we investigated the immunosuppressive effects of galectin-1 in the course of the human allogenic T-cell response. Galectin-1 induced a dose- and carbohydrate-dependent inhibition of the allogenic T-cell response. Addition of galectin-1 to alloreactive lymphocytes resulted in significant apoptosis of CD45R0-positive cells. This negative regulatory effect was accompanied by caspase activation, Bcl-2 downregulation and was prevented by addition of exogenous IL-2. In addition, a significant decrease of IFN-gamma production was detected in the non-apoptotic cell population, following exposure of alloreactive lymphocytes to galectin-1. Moreover, the immunosuppressive activity of this protein did not involve TGF-beta-mediated mechanisms. Since galectin-1 is expressed by activated T cells and could be acting by an autocrine negative loop to control human T-cell reactivity, we finally examined the regulated expression of this protein throughout the allogenic T-cell response. Expression of endogenous galectin-1 was detected at 24 h of cell culture, reaching its maximal levels after 72 h of allostimulation. The present study sets the basis for a potential use of galectin-1 as a selective immunosuppressive agent to limit T-cell-mediated reactivity during the effector phase of the alloimmune response.

Unlocking the secrets of galectins: a challenge at the frontier of glyco-immunology

Over the last decade, we have witnessed an explosion of information regarding the function of glycoconjugates, carbohydrate-binding proteins, and the elucidation of the sugar code. This progress has yielded not only important insights into fundamental areas of glycobiology but has also influenced other fields such as immunology and molecular medicine. A family of galactoside-binding proteins, called galectins, has emerged recently as a novel kind of bioactive molecules with powerful, immunoregulatory functions. Different members of this family have been shown to modulate positively or negatively multiple steps of the inflammatory response, such as cell-matrix interactions, cell trafficking, cell survival, cell-growth regulation, chemotaxis, and proinflammatory cytokine secretion. To introduce a comprehensive overview of these new advances, here we will explore the molecular mechanisms and biochemical pathways involved in these functions. We will also examine the role of these proteins in the modulation of different pathological processes, such as chronic inflammation, autoimmunity, infection, allergic reactions, and tumor spreading. Understanding the intimate mechanisms involved in galectin functions will help to delineate selective and novel strategies for disease intervention and diagnosis.

Activation-induced expression of MICA on T lymphocytes involves engagement of CD3 and CD28

MICA is an HLA-related cell stress-regulated antigen recognized by cytotoxic cells expressing the NKG2D molecule. Although resting lymphocytes do not express MICA, it can be induced on PHA-activated T cells. Here, we demonstrate by Western blot that MICA is induced on allogeneic-activated CD4(+) and CD8(+) T lymphocytes. Blocking activation with anti-HLA class I, anti-HLA-DR, or anti-CD86 mAb affected the expression of MICA slightly. When T cells were stimulated with anti-CD3 or anti-CD28 mAb plus PMA, a sustained up-regulation of MICA was observed by Western blot, RT-PCR, and flow cytometry. The expression of MICA reached a plateau at day 4 after CD3 engagement and at day 3 after anti-CD28/PMA stimulation. Conversely, the proliferative response reached a peak at day 4. Hence, CD3 or CD28 engagement induces MICA expression on T lymphocytes. This activation-induced expression might participate in NKG2D-mediated cytotoxicity toward activated T cells to maintain homeostasis during an ongoing immune response.

Shedding light on the immunomodulatory properties of galectins: Novel regulators of innate and adaptive immune responses

Galectins are a large family of structurally related beta-galactoside-binding proteins that play a pivotal role in the control of cell differentiation, proliferation, activation and apoptosis of many different cell types including immune cells. By crosslinking specific glycoconjugates, different members of the galectin family behave as pro-inflammatory or anti-inflammatory "cytokine-like" mediators, acting at different levels of innate and adaptive immune responses. Here we will review recent advances on the role of galectins in key events of the immune and inflammatory response, such as tolerance induction, cell cycle progression, cell adhesion, chemotaxis, antigen presentation and apoptosis. In particular we will examine the influence of individual members of the galectin family in the physiology of different immune cell types involved in innate and adaptive immune responses. Moreover, we will discuss the importance of these sugar-binding proteins as therapeutic targets in Th1- and Th2-mediated immune disorders, an exciting area for future research.

The antimetastatic effect of a single low dose of cyclophosphamide involves modulation of galectin-1 and Bcl-2 expression

We have demonstrated that a single low dose of cyclophosphamide has an antimetastatic effect on lymphoma (L-TACB)-bearing rats by modulating the host immune response. Galectin-1, a member of the galectin family with specificity for beta-galactosides, has potent immunomodulatory properties by regulating cell-matrix interactions and T-cell apoptosis. Since galectin-1 is expressed by highly metastatic tumors, in the present study we investigated the participation of this beta-galactoside-binding protein in cyclophosphamide-induced immunomodulation. Inbred " e" rats were s.c. challenged with L-TACB. After 14 days, half of the animals received an i.p. injection of cyclophosphamide (10 mg/kg), and on day 21 tumors and spleens were excised. Cell extracts were prepared and galectin-1 expression was determined by Western blot analysis and correlated with Bcl-2 expression levels and the DNA fragmentation profile. Expression of galectin-1 was significantly decreased in tumors from cyclophosphamide-treated rats compared to non-treated rats. The same effect was observed regarding expression of Bcl-2 by tumors. In contrast, expression of Bcl-2 was significantly higher in spleens from treated animals than in non-treated rats. This effect correlated with a decreased intensity in the pattern of DNA fragmentation of spleen cells from cyclophosphamide-treated animals. Our results suggest that a single low dose of cyclophosphamide modulates the expression of galectin-1 and Bcl-2 by tumors, which could in turn influence the apoptotic threshold of spleen mononuclear cells. This mechanism could explain, at least in part, the antimetastatic effect evidenced in our tumor experimental model.