Galectin-1 induces reversible phosphatidylserine exposure at the plasma membrane

Galectin-1 promotes human neutrophil migration

An important step of innate immune response is the recruitment of polymorphonuclear leukocytes (PMN) to injured tissues through chemotactic molecules. Galectins, a family of endogenous lectins, participate in numerous functions such as lymphoid cell migration, homing, cell-cell and cell-matrix interactions. Particularly, galectin-3 (Gal-3) and -9 have been implicated in the modulation of acute and chronic inflammation by inducing the directional migration of monocytes/macrophages and eosinophils, whereas Gal-1 is considered to function as an anti-inflammatory molecule, capable of inhibiting the influx of PMN to the site of injury. In this study, we assessed the effect of Gal-1 on neutrophil recruitment, in the absence of additional inflammatory insults. Contrasting with its capacity to inhibit cell trafficking and modulate the release of mediators described in models of acute inflammation and autoimmunity, we evidenced that Gal-1 has the capacity to induce neutrophil migration both in vitro and in vivo. This effect is not mediated through a G-protein-coupled receptor but potentially through the sialoglycoprotein CD43, via carbohydrate binding and through the p38 mitogen-activated protein kinase pathway. These results suggest a novel biological function for CD43 on neutrophils and highlight that depending on the environment, Gal-1 can act either as chemoattractant or, as a molecule that negatively regulates migration under acute inflammatory conditions, underscoring the potential of Gal-1 as a target for innovative drug development.

Eaten alive! Cell death by primary phagocytosis: 'phagoptosis'

Phagoptosis, also called primary phagocytosis, is a recently recognised form of cell death caused by phagocytosis of viable cells, resulting in their destruction. It is provoked by exposure of 'eat-me' signals and/or loss of 'don't-eat-me' signals by viable cells, causing their phagocytosis by phagocytes. Phagoptosis mediates turnover of erythrocytes, neutrophils and other cells, and thus is quantitatively one of the main forms of cell death in the body. It defends against pathogens and regulates inflammation and immunity. However, recent results indicate that inflamed microglia eat viable brain neurons in models of neurodegeneration, and cancer cells can evade phagocytosis by expressing a 'don't-eat-me' signal, suggesting that too much or too little phagoptosis can contribute to pathology. This review provides an overview of the molecular signals that regulate phagoptosis and the physiological and pathological circumstances in which it has been observed.

Regulatory circuits mediated by lectin-glycan interactions in autoimmunity and cancer

Numerous regulatory programs have been identified that contribute to the restoration of homeostasis at the conclusion of immune responses and to safeguarding against the detrimental effects of chronic inflammation and autoimmune pathology. Malignant cells may usurp these pathways to create immunosuppressive networks that thwart antitumor responses. Herein we review the role of endogenous lectins (C-type lectins, siglecs, and galectins) and specific N- and O-glycans generated by the coordinated action of glycosyltransferases and glycosidases that together promote regulatory signals that control immune cell homeostasis. We also discuss the mechanisms by which glycan-dependent regulatory programs integrate into canonical circuits that amplify or silence immune responses related to autoimmunity and neoplastic disease.

Primary phagocytosis of neurons by inflamed microglia: potential roles in neurodegeneration

Microglial phagocytosis of dead or dying neurons can be beneficial by preventing the release of damaging and/or pro-inflammatory intracellular components. However, there is now evidence that under certain conditions, such as inflammation, microglia can also phagocytose viable neurons, thus executing their death. Such phagocytic cell death may result from exposure of phosphatidylserine (PS) or other eat-me signals on otherwise viable neurons as a result of physiological activation or sub-toxic insult, and neuronal phagocytosis by activated microglia. In this review, we discuss the mechanisms of phagocytic cell death and its potential roles in Alzheimer’s Disease, Parkinson’s Disease, and Frontotemporal Dementia.

Galectins in acute and chronic inflammation

Galectins are animal lectins that bind to β-galactosides, such as lactose and N-acetyllactosamine, in free form or contained in glycoproteins or glycolipids. They are located intracellularly or extracellularly. In the latter they exhibit bivalent or multivalent interactions with glycans on cell surfaces and induce various cellular responses, including production of cytokines and other inflammatory mediators, cell adhesion, migration, and apoptosis. Furthermore, they can form lattices with membrane glycoprotein receptors and modulate receptor properties. Intracellular galectins can participate in signaling pathways and alter biological responses, including apoptosis, cell differentiation, and cell motility. Current evidence indicates that galectins play important roles in acute and chronic inflammatory responses, as well as other diverse pathological processes. Galectin involvement in some processes in vivo has been discovered, or confirmed, through studies of genetically engineered mouse strains, each deficient in a given galectin. Current evidence also suggests that galectins may be therapeutic targets or employed as therapeutic agents for these inflammatory responses.

Galectin-1 and immunotherapy for brain cancer

The prognosis of patients diagnosed with high-grade glioma continues to be dismal in spite of multimodal treatment. Active specific immunotherapy by means of dendritic cell vaccination is considered to be a new promising concept that aims at generating an anti-tumoral immune response. However, it is now widely accepted that the success of immunotherapeutic strategies to promote tumor regression will rely not only on enhancing the effector arm of the immune response but also on downregulation of the counteracting tolerogenic signals. In this article, we summarize evidence that galectin-1, an evolutionarily conserved glycan-binding protein that is abundantly expressed in high-grade glioma, is an important player in glioma-mediated immune escape.

Galectin-1 research in T cell immunity: past, present and future

Galectin-1 (Gal-1) is one of 15 evolutionarily conserved ß-galactoside-binding proteins that display biologically-diverse activities in pathogenesis of inflammation and cancer. Gal-1 is variably expressed on immune cells and endothelial cells, though is commonly found and secreted at high levels in cancer cells. It induces apoptosis in effector T cells through homodimeric binding of N-acetyllactosamines on membrane glycoproteins (Gal-1 ligands). There is also compelling evidence in models of cancer and autoimmunity that recombinant Gal-1 (rGal-1) can potentiate immunoregulatory function of T cells. Here, we review Gal-1's structural and functional features, while analyzing potential drawbacks and technical difficulties inherent to rGal-1's nature. We also describe new Gal-1 preparations that exhibit dimeric stability and functional activity on T cells, providing renewed excitement for studying Gal-1 efficacy and/or use as anti-inflammatory therapeutics. We lastly summarize strategies targeting the Gal-1-Gal-1 ligand axis to circumvent Gal-1-driven immune escape in cancer and boost anti-tumor immunity.

Galectin-9 regulates T helper cell function independently of Tim-3

β-Galactoside-binding lectin 9 (galectin-9) is a tandem repeat-type member of the galectin family. It was initially characterized as an eosinophil chemoattractant and an inducer of apoptosis in thymocytes. Subsequently, galectin-9 was identified as a ligand for transmembrane immunoglobulin mucin domain 3 (Tim-3), a type I glycoprotein induced on T cells during chronic inflammation. Work in autoimmune diseases and chronic viral infections have led to the current hypothesis that the function of Tim-3 is to limit immune responses. However, it is still not known to what degree these effects are due to the galectin-9/Tim-3 interaction. In this study, we show that galectin-9 is not limited to the role of a pro-apoptotic agent, but that it can also induce the production of pro-inflammatory cytokines from T helper cells. This effect is dose-dependent and does not require Tim-3. These findings suggest that the effects of galectin-9 on T cells are more complex than previously thought and are mediated by additional receptors apart from Tim-3.

Galectins in hematological malignancies

Carbohydrates are traditionally considered to be an important source of energy for living organisms. In the field of biology, they are defined as organic compounds composed of carbon, hydrogen, and oxygen that are organized into ring structures. The analysis of these structures and their functions has led to a new field of biology called "glycobiology." In the biomedical sciences, glycobiology is rapidly emerging to be an integral part of complex biological processes. Changes in glycan structures and the interactions of these structures with endogenous carbohydrate-binding proteins, known as lectins, are now considered to be potential biomarkers on cancer cells for monitoring tumor progression. Evidence suggesting that the interactions between lectins and their ligands have a major role in the different steps of cancer progression has accumulated at a rapid pace and has gained the attention of several oncologists. This is particularly true for galectin family members because changes in their expression levels correlate with alterations in cancer cell growth, apoptosis, and cell-cell and cell-matrix interactions. Here we provide an integrated view of the role of galectins in hematological malignancies.

When galectins recognize glycans: from biochemistry to physiology and back again

In the past decade, increasing efforts have been devoted to the study of galectins, a family of evolutionarily conserved glycan-binding proteins with multifunctional properties. Galectins function, either intracellularly or extracellularly, as key biological mediators capable of monitoring changes occurring on the cell surface during fundamental biological processes such as cellular communication, inflammation, development, and differentiation. Their highly conserved structures, exquisite carbohydrate specificity, and ability to modulate a broad spectrum of biological processes have captivated a wide range of scientists from a wide spectrum of disciplines, including biochemistry, biophysics, cell biology, and physiology. However, in spite of enormous efforts to dissect the functions and properties of these glycan-binding proteins, limited information about how structural and biochemical aspects of these proteins can influence biological functions is available. In this review, we aim to integrate structural, biochemical, and functional aspects of this bewildering and ancient family of glycan-binding proteins and discuss their implications in physiologic and pathologic settings.

Coupling pathogen recognition to innate immunity through glycan-dependent mechanisms

Innate immune cells have evolved to sense microbial pathogens through pattern recognition receptors (PRRs), which interact with conserved pathogen-associated molecular patterns (PAMPs) to convey microbial information into immune cell signaling and activation events. PRRs also recognize endogenous damage-associated molecular patterns (DAMPs), including alarmins released during microbial invasion, initiation of autoimmune inflammation or tumor growth. In spite of the well-established role of Toll-like receptors (TLRs) in mediating these recognition events, compelling evidence supports a central function for lectin-glycan interactions in promoting microbial sensing and evoking immune responses. Here we discuss the role of glycans and lectins (particularly galectins) in mediating microbial recognition and initiation of innate immune responses. Both microbes and host cells are sources of glycan-containing information which is, at least in part, decoded by endogenous glycan-binding proteins or lectins, including C-type lectins, siglecs and galectins. Although C-type lectins and siglecs can recognize microbial glycans when expressed on the cell surface of innate immune cells, galectins mainly function as soluble mediators that bridge microbial or host glycans to amplify or attenuate immune responses. Galectins are widely expressed in host cells and play important roles during different steps of infection such as pathogen recognition, invasion and resolution. In addition, recent studies report the presence of conserved 'galectin-like' domains in certain pathogens including helminths and protistan parasites, suggesting that they could also serve as potential virulence factors that influence the outcome and course of infection. Understanding the role of lectin-glycan interactions and the relevance of PRR or PAMP glycosylation in microbial recognition might contribute to the design of novel prophylactic and therapeutic strategies.

Fine-tuning antitumor responses through the control of galectin-glycan interactions: an overview

In recent years, we have witnessed critical advances in genomics and proteomics which contributed to delineate the "tumor progression signature". This includes the altered expression of genes and proteins not only in tumor cells, but also in tumor-associated stromal, endothelial, and immune cells. Adding more complexity to this bewildering information, efforts are being made to define the "glycosylation signature" of the tumor microenvironment, which results from the abnormal expression and activity of glycosyltransferases, glycosidases, and enzyme chaperons. The multiple combinatorial possibilities of glycan structures expressed by neoplastic versus normal tissue provide enormous potential for information display and expand potential therapeutic opportunities. The responsibility of deciphering the biological information encoded by the tumor-associated glycome is partially assigned, to distinct families of endogenous glycan-binding proteins or lectins, whose expression and function are regulated in cancerous tissues. Galectins, a family of evolutionarily conserved glycan-binding proteins, can control tumor progression by directly influencing tumor growth or by modulating cell migration, angiogenesis, and tumor-immune escape. In this review, we will highlight recent findings on how galectin-glycan lattices control the dialogue between tumor and immune cells and how these interactions could be exploited for therapeutic purposes.

Expanding the universe of cytokines and pattern recognition receptors: galectins and glycans in innate immunity

Effective immunity relies on the recognition of pathogens and tumors by innate immune cells through diverse pattern recognition receptors (PRRs) that lead to initiation of signaling processes and secretion of pro- and anti-inflammatory cytokines. Galectins, a family of endogenous lectins widely expressed in infected and neoplastic tissues have emerged as part of the portfolio of soluble mediators and pattern recognition receptors responsible for eliciting and controlling innate immunity. These highly conserved glycan-binding proteins can control immune cell processes through binding to specific glycan structures on pathogens and tumors or by acting intracellularly via modulation of selective signaling pathways. Recent findings demonstrate that various galectin family members influence the fate and physiology of different innate immune cells including polymorphonuclear neutrophils, mast cells, macrophages, and dendritic cells. Moreover, several pathogens may actually utilize galectins as a mechanism of host invasion. In this review, we aim to highlight and integrate recent discoveries that have led to our current understanding of the role of galectins in host-pathogen interactions and innate immunity. Challenges for the future will embrace the rational manipulation of galectin-glycan interactions to instruct and shape innate immunity during microbial infections, inflammation, and cancer.

Secondary necrosis: the natural outcome of the complete apoptotic program

The predominant definition of apoptosis considers that the elimination of the apoptosing cell is by heterolytic degradation following phagocytosis by an assisting scavenger (efferocytosis). However, an alternative and largely underestimated outcome of apoptosis is secondary necrosis, an autolytic process of cell disintegration with release of cell components that occurs when there is no intervention of scavengers and the full apoptotic program is completed. Secondary necrosis is the typical outcome of apoptosis in unicellular eukaryotes but, importantly, it may also occur in multicellular animals and has been implicated in the genesis of important human pathologies. Secondary necrosis is a mode of cell elimination with specific molecular and morphological features and should be considered the natural outcome of the complete apoptotic program.

Transfusion-related acute lung injury: from bedside to bench and back

Over the past 60 years, the transfusion medicine community has attained significant knowledge regarding transfusion-related acute lung injury (TRALI) through the bedside to bench and back to the bedside model. First, at the bedside, TRALI causes hypoxia and noncardiogenic pulmonary edema, typically within 6 hours of transfusion. Second, bedside studies showed a higher incidence in plasma and platelet products than in red blood cell products (the fatal TRALI incidence for plasma is 1:2-300 000 products; platelet, 1:3-400 000; red blood cells, 1:25 002 000), as well as an association with donor leukocyte antibodies (∼ 80% of cases). Third, at the bench, antibody-dependent and antibody-independent mechanisms have been described, requiring neutrophil and pulmonary endothelial cell activation. Antibodies, as well as alternate substances in blood products, result in neutrophil activation, which, in a susceptible patient, result in TRALI (2-hit hypothesis). Fourth, back to the bedside, policy changes based on results of these studies, such as minimizing use of plasma and platelet products from donors with leukocyte antibodies, have decreased the incidence of TRALI. Thus, steps to mitigate TRALI are in place, but a complete mechanistic understanding of the pathogenesis of TRALI and of which patients are at highest risk remains to be elucidated.

Development of a nascent galectin-1 chimeric molecule for studying the role of leukocyte galectin-1 ligands and immune disease modulation

Galectin-1 (Gal-1), a β-galactoside-binding lectin, plays a profound role in modulating adaptive immune responses by altering the phenotype and fate of T cells. Experimental data showing recombinant Gal-1 (rGal-1) efficacy on T cell viability and cytokine production, nevertheless, is controversial due to the necessity of using stabilizing chemicals to help retain Gal-1 structure and function. To address this drawback, we developed a mouse Gal-1 human Ig chimera (Gal-1hFc) that did not need chemical stabilization for Gal-1 ligand recognition, apoptosis induction, and cytokine modulation in a variety of leukocyte models. At high concentrations, Gal-1hFc induced apoptosis in Gal-1 ligand(+) Th1 and Th17 cells, leukemic cells, and granulocytes from synovial fluids of patients with rheumatoid arthritis. Importantly, at low, more physiologic concentrations, Gal-1hFc retained its homodimeric form without losing functionality. Not only did Gal-1hFc-binding trigger IL-10 and Th2 cytokine expression in activated T cells, but members of the CD28 family and several other immunomodulatory molecules were upregulated. In a mouse model of contact hypersensitivity, we found that a non-Fc receptor-binding isoform of Gal-1hFc, Gal-1hFc2, alleviated T cell-dependent inflammation by increasing IL-4(+), IL-10(+), TGF-β(+), and CD25(high)/FoxP3(+) T cells, and by decreasing IFN-γ(+) and IL-17(+) T cells. Moreover, in human skin-resident T cell cultures, Gal-1hFc diminished IL-17(+) T cells and increased IL-4(+) and IL-10(+) T cells. Gal-1hFc will not only be a useful new tool for investigating the role of Gal-1 ligands in leukocyte death and cytokine stimulation, but for studying how Gal-1-Gal-1 ligand binding shapes the intensity of immune responses.

Anionic phospholipid interactions of the prion protein N terminus are minimally perturbing and not driven solely by the octapeptide repeat domain

Although the N terminus of the prion protein (PrP(C)) has been shown to directly associate with lipid membranes, the precise determinants, biophysical basis, and functional implications of such binding, particularly in relation to endogenously occurring fragments, are unresolved. To better understand these issues, we studied a range of synthetic peptides: specifically those equating to the N1 (residues 23-110) and N2 (23-89) fragments derived from constitutive processing of PrP(C) and including those representing arbitrarily defined component domains of the N terminus of mouse prion protein. Utilizing more physiologically relevant large unilamellar vesicles, fluorescence studies at synaptosomal pH (7.4) showed absent binding of all peptides to lipids containing the zwitterionic headgroup phosphatidylcholine and mixtures containing the anionic headgroups phosphatidylglycerol or phosphatidylserine. At pH 5, typical of early endosomes, quartz crystal microbalance with dissipation showed the highest affinity binding occurred with N1 and N2, selective for anionic lipid species. Of particular note, the absence of binding by individual peptides representing component domains underscored the importance of the combination of the octapeptide repeat and the N-terminal polybasic regions for effective membrane interaction. In addition, using quartz crystal microbalance with dissipation and solid-state NMR, we characterized for the first time that both N1 and N2 deeply insert into the lipid bilayer with minimal disruption. Potential functional implications related to cellular stress responses are discussed.

Innate immune lectins kill bacteria expressing blood group antigen

The expression of ABO(H) blood group antigens causes deletion of cells that generate self-specific antibodies to these antigens but this deletion limits adaptive immunity toward pathogens bearing cognate blood group antigens. To explore potential defense mechanisms against such pathogens, given these limitations in adaptive immunity, we screened for innate proteins that could recognize human blood group antigens. Here we report that two innate immune lectins, galectin-4 (Gal-4) and Gal-8, which are expressed in the intestinal tract, recognize and kill human blood group antigen-expressing Escherichia coli while failing to alter the viability of other E. coli strains or other Gram-negative or Gram-positive organisms both in vitro and in vivo. The killing activity of both Gal-4 and Gal-8 is mediated by their C-terminal domains, occurs rapidly and independently of complement and is accompanied by disruption of membrane integrity. These results demonstrate that innate defense lectins can provide immunity against pathogens that express blood group-like antigens on their surface.

Caenorhabditis elegans N-glycan core beta-galactoside confers sensitivity towards nematotoxic fungal galectin CGL2

The physiological role of fungal galectins has remained elusive. Here, we show that feeding of a mushroom galectin, Coprinopsis cinerea CGL2, to Caenorhabditis elegans inhibited development and reproduction and ultimately resulted in killing of this nematode. The lack of toxicity of a carbohydrate-binding defective CGL2 variant and the resistance of a C. elegans mutant defective in GDP-fucose biosynthesis suggested that CGL2-mediated nematotoxicity depends on the interaction between the galectin and a fucose-containing glycoconjugate. A screen for CGL2-resistant worm mutants identified this glycoconjugate as a Galbeta1,4Fucalpha1,6 modification of C. elegans N-glycan cores. Analysis of N-glycan structures in wild type and CGL2-resistant nematodes confirmed this finding and allowed the identification of a novel putative glycosyltransferase required for the biosynthesis of this glycoepitope. The X-ray crystal structure of a complex between CGL2 and the Galbeta1,4Fucalpha1,6GlcNAc trisaccharide at 1.5 A resolution revealed the biophysical basis for this interaction. Our results suggest that fungal galectins play a role in the defense of fungi against predators by binding to specific glycoconjugates of these organisms.

Multiple functional targets of the immunoregulatory activity of galectin-1: Control of immune cell trafficking, dendritic cell physiology, and T-cell fate

In the postgenomic era, the study of the glycome-the whole repertoire of saccharides in cells and tissues-has enabled the association of unique glycan structures with specific physiological and pathological processes. The responsibility for deciphering this biological information belongs to endogenous glycan-binding proteins or lectins. Galectin-1, a prototypic member of a family of structurally related proteins, has demonstrated selective antiinflammatory and immunoregulatory effects either by controlling immune cell trafficking, "fine-tuning" dendritic cell physiology and regulating T-cell fate. These regulatory functions mediated by an endogenous glycan-binding protein may contribute to fulfill the needs for immune cell homeostasis, including preservation of fetomaternal tolerance and prevention of collateral damage as a result of microbial invasion or autoimmune pathology. We will discuss here the conceptual framework which led to the study of galectin-glycan lattices as a novel paradigm of immune cell communication in physiological and pathological processes and will highlight selected methods and experimental strategies which have contributed to the study of the immunoregulatory activities of this multifaceted glycan-binding protein both in in vitro and in vivo biological settings.

Conveying glycan information into T-cell homeostatic programs: a challenging role for galectin-1 in inflammatory and tumor microenvironments

The immune system has evolved sophisticated mechanisms composed of several checkpoints and fail-safe processes that enable it to orchestrate innate and adaptive immunity, while at the same time limiting aberrant or unfaithful T-cell function. These multiple regulatory pathways take place during the entire life-span of T cells including T-cell development, homing, activation, and differentiation. Galectin-1, an endogenous glycan-binding protein widely expressed at sites of inflammation and tumor growth, controls a diversity of immune cell processes, acting either extracellularly through specific binding to cell surface glycan structures or intracellularly through modulation of pathways that remain largely unexplored. In this review, we highlight the discoveries that have led to our current understanding of the role of galectin-1 in distinct immune cell process, particularly those associated with T-cell homeostasis. Also, we emphasize findings emerging from the study of experimental models of autoimmunity, chronic inflammation, fetomaternal tolerance, and tumor growth, which have provided fundamental insights into the critical role of galectin-1 and its specific saccharide ligands in immunoregulation. Challenges for the future will embrace the rational manipulation of galectin-1-glycan interactions both towards attenuating immune responses in autoimmune diseases, graft rejection, and recurrent fetal loss, while at the same overcoming immune tolerance in chronic infections and cancer.

The liaison between apoptotic cells and macrophages--the end programs the beginning

The efficient execution of apoptotic cell death with the clearance of apoptotic debris by phagocytes is a key regulatory mechanism ensuring tissue homeostasis. Failure in this execution program contributes to the pathogenesis of many human diseases. In this review, we describe the current knowledge regarding the interaction of apoptotic cells with their professional 'captors', the macrophages, with special emphasis on the immunological outcome. Removal of apoptotic cells must be considered as a process that actively delivers signals to polarize macrophages, which are fundamental for the resolution of inflammation. However, the sculpting of macrophage responses by apoptotic cells can be misused under certain inflammatory disease conditions, including tumor development.