The role of galectins in immunity and infection

Abstract

The galectin family consists of carbohydrate (glycan) binding proteins that are expressed by a wide variety of cells and bind to galactose-containing glycans. Galectins can be located in the nucleus or the cytoplasm, or can be secreted into the extracellular space. They can modulate innate and adaptive immune cells by binding to glycans on the surface of immune cells or intracellularly via carbohydrate-dependent or carbohydrate-independent interactions. Galectins expressed by immune cells can also participate in host responses to infection by directly binding to microorganisms or by modulating antimicrobial functions such as autophagy. Here we explore the diverse ways in which galectins have been shown to impact immunity and discuss the opportunities and challenges in the field.

Fig. 1. Galectins are unique regulators of host immunity.

Galectins can be classified into prototypical galectins, chimaera-type galectins and tandem-repeat galectins according to their overall structural features. Prototypical galectins exist in a monomer–dimer equilibrium with the extent of dimerization at steady state differing for each individual galectin. a, Extracellular galectins can engage common glycan motifs that can be present on many different glycoproteins and glycolipids, which are expressed on the surface of most cells. As such, galectins can bind a wide variety of glycoprotein and glycolipid receptors, which can result in distinct signalling outcomes depending on the different types of receptors engaged. Importantly, the glycan modifications of cell surface proteins can change following cellular activation and differentiation. This can result in galectin engagement of completely different glycoprotein or glycolipid targets and, therefore, different signalling outcomes. In immune cells (both innate immune cells such as macrophages and dendritic cells, and adaptive immune cells such as T cells and B cells), such interactions can affect cell activation, cytokine secretion, chemotaxis and cell fate. b, Galectins are synthesized and are either secreted or reside in the cytosol, due to their lack of a classical signal sequence, but can also shuttle to the nucleus. Cytosolic galectins can impact cellular signalling by directly binding intracellular targets such as BCL-2 and ALG2-interacting protein X (ALIX) (Supplementary Table 3) through carbohydrate-independent interactions. This can affect cell signalling, survival, phagocytosis and autophagy. c, Intracellular galectins can also bind exposed glycans following organelle damage, inducing autophagy (shown in detail in Fig. 4).

Fig. 2. Regulation of adaptive immunity by galectins.

Galectins can modulate adaptive immune cells through both intracellular and extracellular mechanisms and via their effects on antigen-presenting cells (APCs) and other innate immune cells. a, Extracellular galectin-3 can modulate T cell activation through engagement of extracellular glycans on co-receptors such as CD8 and other cell surface glycoconjugates, which restricts CD8 interactions with the T cell receptor (TCR) and, ultimately, MHC class I at the immmunological synapse. This, in turn, weakens TCR signalling. Galectin-3-mediated restriction of CD8 in this manner is mediated by β1,6-N-acetylglucosaminyltransferase V (MGAT5), an enzyme that is induced by IL-10 and catalyses the formation of branched N-glycans bearing polylactosamine structures that are common galectin ligands. Intracellular galectin-3 can upregulate the threshold for TCR activation by binding the adaptor protein ALG2-interacting protein X (ALIX), whereas galectin-9 can have the opposite effect on the TCR through engagement of intracellular partners that remain to be described. b, Galectin-1, -8 or -9 knockout mice appear to have a defect in T cell contraction after challenge, and extracellular administration of galectin-1, -2, -3, -8 and -9 has been shown to induce apoptosis in T cells in vitro^–. However, whether and how these galectins regulate T cell fate in vivo remains to be elucidated, especially in view of the issues described in Box 2. In addition, intracellular galectin-3 can inhibit T cell apoptosis through interactions with the anti-apoptotic protein BCL-2 (refs. ^,). c, Galectins have also been shown to regulate B cell differentiation through extracellular and possibly intracellular processes. For example, galectin-3 knockout B cells display an enhanced proclivity to differentiate into germinal centre B cells and plasma cells that produce IgG2c or IgG3, all of which may enhance the probability of antibody-mediated autoimmunity. Galectin-3 expressed by B cells can suppress B cell differentiation, probably via intracellular mechanisms. d, On naive B cells, galectin-9 can recruit glycosylated forms of the transmembrane proteins CD22 and CD45 to the B cell receptor (BCR). CD45 is bound by the phosphatase SHP1, which downregulates BCR signalling^,. In contrast, germinal centre B cells express the carbohydrate I antigen which reduces the ability of galectin-9 to facilitate CD22 and CD45-mediated inhibition of B cell signalling and enhances B cell responses following antigen engagement.

Fig. 3. Regulation of macrophages by extracellular and intracellular galectins.

Several different galectins have been shown to affect macrophage polarization, phagocytosis and inflammasome activation. For example, galectin-3 was shown to be induced in cardiac macrophages in models of myocardial infarction where it promoted their polarization into M2-like macrophages, which contribute to tissue repair by promoting fibrosis and clearance of apoptotic cells after myocardial infarction. Here, galectin-3 expression was induced via IL-10-mediated activation of the transcription factor STAT3. In vitro experiments have also shown that IL-4 induces galectin-3 expression and galectin-3 facilitates M2-like macrophage polarization by binding CD98, which induces PI3K (ref. ). By contrast, galectin-12 (which is primarily located intracellularly and expressed by adipocytes, where it has a key role in adipogenesis) is also expressed by myeloid cells, where it was shown to be a positive regulator of M1 macrophage polarization in response to treatment with lipopolysaccharide (LPS). This was related to decreased activation of IKKα/β, AKT and ERK downstream of Toll-like receptor 4 (TLR4) which results in decreased activation of NF-κB, resulting in M1 polarization. Intracellular galectin-3 is also a positive regulator of the NLRP3 inflammasome^, and was shown to physically associate with NLRP3 in isolated hepatic macrophages, promoting IL-1β production. Moreover, intracellular galectin-3 is a positive regulator of macrophage phagocytosis. In vitro experiments showed that it is localized at the cytosolic side of phagosomes where it is involved in actin rearrangement. Similar observations were made in cultured primary microglia. Additional studies suggest that galectin-3 controls the activation state of cofilin, which affects actin filament organization and stability. Intracellular galectin-9 has also been shown to enhance phagocytosis through the regulation of actin filament formation.

Fig. 4. Galectins as sensors for endolysosomal damage.

Under normal conditions, glycoconjugates reside in the lumen of organelles and thus are not accessible to intracellular galectins, which are located in the cytosol or the nucleus. However, these organelles can become disrupted by intracellular bacteria that cause damage to phagosomes as they escape into the cytosol, toxins secreted into cells by extracellular bacteria, certain viruses and intracellular protein aggregates. Lysosomal damage can also be induced with agents such as Leu-Leu-O-Me (LLOMe) and glycyl-l-phenylalanine 2-naphthylamide (GPN). This can lead to the exposure of glycoconjugates to the cytosolic milieu, where these can be bound by various intracellular galectins. Given that galectins can also bind to various signalling molecules via non-carbohydrate interaction, this can lead to the formation of ‘signalling platforms’ that can induce autophagic activation and bacterial destruction. For example, galectin-8 was shown to bind to the autophagy adaptor NDP52 and initiate the formation of antibacterial autophagosomes around Salmonella-containing vesicles. Galectin-3 was shown to direct the recruitment of the interferon-inducible guanylate binding proteins to intracellular vacuoles damaged by intracellular bacteria. In a model of lysosomal damage, galectin-3 and galectin-8 were found to bind to exposed β-galactosides on damaged membranes, where both galectins can become associated with the autophagy regulator TRIM16 and triggered autophagy. In another model, galectin-8 was shown to associate with the mTOR apparatus, which is normally localized on the surface of lysosomes and serves as an inhibitor of autophagy. This results in inactivation of mTOR and thus autophagic activation. Galectin-9 binding to glycoproteins on damaged lysosomes results in displacement of its cytosolic binding partner, the deubiquitinase USP9X (ref. ). This inhibits the enzyme and enhances cellular ubiquitination responses, leading to the activation of a protein kinase TAK1 and, in turn, its downstream AMP-activated protein kinase (AMPK), an inducer of autophagy.