Christian St-Pierre

Galectins in innate immunity: dual functions of host soluble β‐galactoside‐binding lectins as damage‐associated molecular patterns (DAMPs) and as receptors for pathogen‐associated molecular patterns (PAMPs)

Summary:  The glycocalyx is a glycan layer found on the surfaces of host cells as well as microorganisms and enveloped virus. Its thickness may easily exceed 50 nm. The glycocalyx does not only serve as a physical protective barrier but also contains various structurally different glycans, which provide cell‐ or microorganism‐specific ‘glycoinformation’. This information is decoded by host glycan‐binding proteins, lectins. The roles of lectins in innate immunity are well established, as exemplified by collectins, dectin‐1, and dendritic cell (DC)‐specific intracellular adhesion molecule‐3‐grabbing non‐integrin (DC‐SIGN). These mammalian lectins are synthesized in the secretory pathway and presented on the cell surface to bind to specific glycan ‘epitopes’. As they recognize non‐self glycans presented by microorganisms, they can be considered as receptors for pathogen‐associated molecular patterns (PAMPs), i.e. pattern recognition receptors (PRRs). One notable exception is the galectin family. Galectins are synthesized and stored in the cytoplasm, but upon infection‐initiated tissue damage and/or following prolonged infection, cytosolic galectins are either passively released by dying cells or actively secreted by inflammatory activated cells through a non‐classical pathway, the ‘leaderless’ secretory pathway. Once exported, galectins act as PRR, as well as immunomodulators (or cytokine‐like modulators) in the innate response to some infectious diseases. As galectins are dominantly found in the lesions where pathogen‐initiated tissue damage signals appear, this lectin family is also considered as potential damage‐associated molecular pattern (DAMP) candidates that orchestrate innate immune responses alongside the PAMP system.

Galectin-3 interacts with naïve and primed neutrophils, inducing innate immune responses

The neutrophil is the first line of defense against infection. As a part of the innate immune response, neutrophils start to emigrate from blood to an affected site and their state is altered from passively circulating naïve to primed, and then to fully activated. The extent of neutrophil activation and their subsequent response varies depending on the stimuli and environment that neutrophils encounter. Because neutrophils can also induce deleterious effects on host tissues, tight regulation of recruitment and functions of neutrophils is required for efficient recovery. Galectin‐3, a soluble β‐galactoside binding protein, of which expression is up‐regulated during inflammation/infection, is suggested to be involved in various inflammatory responses. However, the precise roles of this lectin in innate immunity remain unknown, while it has been demonstrated that galectin‐3 binds to naïve and primed neutrophils. Here we report that galectin‐3 can induce L‐selectin shedding and interleukin‐8 production in naïve and primed neutrophils. These activities were shown to be dependent on the presence of the C‐terminal lectin domain and the N‐terminal nonlectin domain of galectin‐3, which is involved in oligomerization of this lectin. We also found that, after galectin‐3 binds to neutrophils, primed but not naïve neutrophils can cleave galectin‐3, mainly through elastase, which results in the formation of truncated galectin‐3 lacking the N‐terminal domain. Together, these results suggest that galectin‐3 activates naïve and primed neutrophils, and galectin‐3‐activated primed neutrophils have an ability to inactivate galectin‐3.

Role of Galectin-3 in Leukocyte Recruitment in a Murine Model of Lung Infection by Streptococcus pneumoniae

Pneumonia can be caused by a variety of pathogens, among which Streptococcus pneumoniae causes one of the most common forms of community-acquired pneumonia. Depending on the invading pathogen, the elements of the immune response triggered will vary. For most pathogens, such as Escherichia coli, neutrophil recruitment involves a well-described family of adhesion molecules, β2-integrins. In the case of streptococcal pneumonia, however, neutrophil recruitment occurs mainly through a β2-integrin-independent pathway. Despite decades of research on this issue, the adhesion molecules involved in neutrophil recruitment during lung infection by S. pneumoniae have not been identified. We have previously shown that galectin-3, a soluble mammalian lectin, can be found in lungs infected by S. pneumoniae, but not by E. coli, and can mediate the adhesion of neutrophils on the endothelial cell layer, implying its role in the recruitment of neutrophils to lungs infected with S. pneumoniae. In this study, using galectin-3 null mice, we report further evidence of the involvement of this soluble lectin in the recruitment of neutrophils to S. pneumonia-infected lungs. Indeed, in the absence of galectin-3, lower numbers of leukocytes, mainly neutrophils, were recruited to the infected lungs during infection by S. pneumoniae. In the case of β2-integrin-dependent recruitment induced by lung infection with E. coli, the number of recruited neutrophils was not reduced. Thus, taken together, our data suggest that galectin-3 plays a role as a soluble adhesion molecule in the recruitment of neutrophils to lungs infected by S. pneumoniae, which induces β2-integrin-independent migration.

Carbohydrate triazoles and isoxazoles as inhibitors of galectins-1 and -3

Galactosides and lactosides bearing triazoles or isoxazoles, regiospecifically prepared by [1,3]-dipolar cycloadditions between alkynes, azides or nitrile oxides, provided specific galectin-1 and -3 inhibitors with potencies as low as 20 microM.

Synthesis of stable and selective inhibitors of human galectins-1 and -3.

The syntheses of glycolytically stable galactosides and lactosides have been made toward the selective inhibition of human galectins-1 and -3. Transition metal-catalyzed cross-coupling reactions were used to create carbon-carbon bond formation (Sonogashira, Suzuki, Heck, Glaser). Additionally, Hantzsch condensation was used to create novel 2-aminothiazoles which reacted with a panel of acylating and sulfonylating reagents. Moreover, dimeric galactosides and lactosides bearing triazoles, regiospecifically prepared using copper-catalyzed Huisgen azide-alkyne [1,3]-dipolar cycloaddition, provided efficient galectins-1 and -3 inhibitors. Best monovalent inhibitor among the tested series was (E)-methyl 2-phenyl-4-(beta-D-galactopyranosyl)-but-2-enoate 15 with inhibitory potency of 313 microM against galectin-1 and best dimers were bis-lactoside 68 and 75 having both inhibitory properties of 160 microM against Galectin-3.

Host-Soluble Galectin-1 Promotes HIV-1 Replication through a Direct Interaction with Glycans of Viral gp120 and Host CD4

ABSTRACT Sexual transmission of HIV-1 requires virus adsorption to a target cell, typically a CD4+ T lymphocyte residing in the lamina propria, beneath the epithelium. To escape the mucosal clearance system and reach its target cells, HIV-1 has evolved strategies to circumvent deleterious host factors. Galectin-1, a soluble lectin found in the underlayers of the epithelium, increases HIV-1 infectivity by accelerating its binding to susceptible cells. By comparison, galectin-3, a family member expressed by epithelial cells and part of the mucosal clearance system, does not perform similarly. We show here that galectin-1 directly binds to HIV-1 in a β-galactoside-dependent fashion through recognition of clusters of N-linked glycans on the viral envelope gp120. Unexpectedly, this preferential binding of galectin-1 does not rely on the primary sequence of any particular glycans. Instead, glycan clustering arising from the tertiary structure of gp120 hinders its binding by galectin-3. Increased polyvalency of a specific ligand epitope is a common strategy for glycans to increase their avidity for lectins. In this peculiar occurrence, glycan clustering is instead exploited to prevent binding of gp120 by galectin-3, which would lead to a biological dead-end for the virus. Our data also suggest that galectin-1 binds preferentially to CD4, the host receptor for gp120. Together, these results suggest that HIV-1 exploits galectin-1 to enhance gp120-CD4 interactions, thereby promoting virus attachment and infection events. Since viral adhesion is a rate-limiting step for HIV-1 entry, modulation of the gp120 interaction with galectin-1 could thus represent a novel approach for the prevention of HIV-1 transmission.

Glycans, galectins, and HIV‐1 infection

During sexual transmission, HIV‐1 must overcome physiological barriers to establish a founder cell population. Viral adhesion represents a bottleneck for HIV‐1 propagation that the virus widens by exploiting some specific host factors. Recognition of oligomannosyl glycans of gp120 by C‐type lectins is one such example. Recent works suggest that complex glycans of gp120 are recognized by another host lectin, galectin‐1. This interaction results in rapid association of HIV‐1 to susceptible cells and facilitates infection. The peculiar presentation of complex glycans on gp120 seems to impart specificity for galectin‐1, as another member of the same family, galectin‐3, is unable to bind gp120 or enhance HIV‐1 infection. Other studies have shown that galectin‐9 could also increase HIV‐1 infectivity but via an indirect mechanism. Thus, current research suggests that galectins play various roles in HIV‐1 pathogenesis. Drug discovery approaches targeting host lectins at early steps could benefit the current arsenal of antiretrovirals.

Galectin-1-Specific Inhibitors as a New Class of Compounds To Treat HIV-1 Infection

ABSTRACT Despite significant improvements, antiretroviral therapies against HIV-1 are plagued by a high frequency of therapeutic failures that have been associated with acquisition of drug resistance. We recently reported that HIV-1 exploits a host glycan binding protein, galectin-1, to increase its attachment to host cells, thereby increasing its overall infectivity in susceptible cells. This finding suggests that host molecules such as galectin-1 could reduce the expected efficiency of HIV-1 drugs targeting early steps of the replicative cycle, such as attachment and entry processes. Thus, new classes of drugs that would interfere with galectin-1/HIV-1 interactions could benefit the current antiretroviral therapy. To further explore this possibility, experiments were conducted to discover leading compounds showing specific inhibition of galectin-1 activity in a cellular model of HIV-1 infection. Three lactoside compounds were found to modestly inhibit the interaction of galectin-1 with primary human CD4+ T cells. Interestingly, these same inhibitors reduced the galectin-1-mediated increase in HIV-1 attachment to target cells in a much more efficient manner. More important, the tested lactoside derivatives also significantly decreased the galectin-1-dependent enhancement of HIV-1 infection. These observations deserve further attention when considering that the development of new drugs to prevent and treat HIV-1 infection remains a priority.

Galectin-1 and HIV-1 Infection

Initial binding of human immunodeficiency virus-1 (HIV-1) to its susceptible CD4(+) cells is the limiting step for the establishment of infection as the avidity of viral envelope gp120 for CD4 is not high and the number of viral envelope spikes on the surface is found to be low compared to highly infectious viruses. Several host factors, such as C-type lectins, are listed as being able to enforce or facilitate the crucial interaction of HIV-1 to the susceptible cell. Recent works suggest that a host soluble beta-galactoside-binding lectin, galectin-1, also facilitates both virion binding and the infection of target cells in a manner dependent on lactose but not mannose, suggesting that this soluble galectin can be considered as a host factor that influences HIV-1 pathogenesis. In this chapter, we describe methods used to investigate the potential role of the galectin family in HIV-1-mediated disease progression.

Galectin-3 Facilitates Neutrophil Recruitment as an Innate Immune Response to a Parasitic Protozoa Cutaneous Infection

When infection occurs, neutrophils rapidly migrate to the affected site. Although the neutrophils neutralize microorganisms, they can also cause tissue damage or render invasion pathways to pathogens. Thus, the migration could be either beneficial or unfavorable in the initial control of infection. Studies on neutrophil recruitment revealed its complexity, especially in terms of the regulation of its initiation. Galectin-3 is a member of the galectin family that has an affinity for β-galactoside containing glycoconjugates. In this study, we investigated the role of galectin-3 in neutrophil migration and the biological significance of the rapid migration of neutrophils in an experimental parasitic cutaneous infection with Leishmania major. When the substrain of L. major, LV39, was infected, lack of galectin-3 impaired neutrophil recruitment in the footpads and the draining lymph nodes 1 d following infection. Reduced number of recruited neutrophils correlated with local high parasite burdens. In contrast, neutrophil migration, induced by the other L. major substrain, Friedlin, was unaffected, and the initial parasite burden remained similar in galectin-3 null mice as compared with wild-type mice. Infection with L. major LV39 but not Friedlin induced higher levels of extracellular release of galectin-3. Further, galectin-3 alone was able to initiate neutrophil migration even though galectin-3 is not a chemoattractant for neutrophils. Thus, our data suggest that once extracellularly released, galectin-3 can act as a damage-associated molecular pattern to facilitate early neutrophil migration, which is beneficial in the initial control of the Leishmania infection.