Immune function and galectins
Lectins have diverse functions in a wide variety of life phenomena; however, their roles in the immune system are particularly significant. Collectins belonging to C-type lectins have been drawing attention as a host defense factor that is involved in antibody-independent innate immunity. Selectins, membrane proteins belonging to C-type lectins, are involved in adhesion of leukocytes to vascular endothelial cells. Selectins are molecules that are required for leukocytes in the blood stream or the expressway to land on the tissue of interest (extravasation1) (Figure 2).
Galectins are known to have various roles related to the immune function. Galectin-8 and galectin-9, which are subjects of our interest and research, are classified into the tandem-repeat type. They are also known to have various functions, but their roles in the immune response seem to be most important.
Among leukocytes, neutrophils belong to a group called phagocytes and have a function to uptake bacteria (and other foreign substances) into cells and destroy them. Galectin-8 appears to play a role in the firm adhesion of neutrophils to vascular endothelial cells after weak adhesion of neutrophils to vascular endothelial cells via selectins. Then, neutrophils infiltrate tissues through vascular endothelial cells to reach the site of the lesion. Migration of neutrophils in tissues requires degradation of substances called extracellular matrix around cells in the tissues. Galectin-8 may possibly be involved in the degradation of the extracellular matrix by neutrophils.
Strong adhesion of neutrophils to vascular endothelial cells is mediated by binding between the integrin on neutrophils and the intercellular adhesion molecule (ICAM) on endothelial cells (both integrins and ICAM are cell membrane-integrated glycoproteins). Integrins usually exist as an inactive form without binding activity, and the binding of galectin-8 to the carbohydrate chain of integrins activates integrins. Tissue migration of neutrophils requires degradation of the extracellular matrix, and a group of proteases called matrix metalloproteinases (MMPs) are involved in the degradation. MMPs are secreted from cells as an inactive form without degradation activity, and galectin-8 is known to promote activation of proMMP-9.
Leukocytes are guided by substances produced at the site of the lesion (chemotactic factors) to infiltrate from blood vessels into tissues. Galectin-9, another galectin we are interested in, has been found to act as a chemotactic factor for eosinophils, which belong to phagocytes like neutrophils and are known to be involved in allergic reactions.
Although galectin-9 selectively stimulates migration of eosinophils, other galectins have little or no eosinophil chemotactic activity. The activity requires a tandem-repeat-type structure with two carbohydrate binding domains linked together, and the two carbohydrate binding domains are found to bind to carbohydrate chains with the same or very similar structures.
More recently, study results of a new function of galectin-9 was reported by a group headed by Professor Vijay Kuchroo at Harvard University. Tim-3 identified by the group is specifically expressed in differentiated CD4+ Th1 cells and CD8+ Tc1 cells, but not in CD4+ Th2 or CD8+ Tc2 cells. Tim-3 is believed to play an important role in the modulation of autoimmunity and allergies. Their recent data indicate that galectin-9 may be a physical ligand for Tim-3 and suppress the autoimmune response by inducing Tim-3-positive cell death.
Galectins have features of typical intracellular proteins, such as no disulfide bond, unglycosylated, no secretion signal, and N-terminal acetylation in general. It is believed, however, that some may be secreted by a non-classical pathway and function as intercellular signal transducers like cytokines. However, galectins differ fundamentally from other intercellular signal transducers in that a single galectin can interact with many types of molecules with different properties. Cytokines and hormones bind only to a single type or family of few receptors with high affinity to activate their receptors. In contrast, galectins and other lectins in general recognize and bind to carbohydrate chains with a wide range of selectivity for carbohydrate structures,2 and therefore oligosaccharide structures of binding partners are not uniform. For example, analysis of carbohydrate chains at a certain site of a specific glycoprotein usually reveals many types of carbohydrates. Thus, galectins may use many more types of molecules as receptors than do cytokines, and new receptors and relevant roles are likely to be found in the future.3
Note 1: Invasion of bacteria or other pathogens in animal body is followed by leukocyte extravasation and accumulation at the infection site to attack the pathogens for self-defense. In the process, circulating leukocytes weakly bind and roll on vascular endothelial cells (cells lining the inside surface of the blood vessel) near the infection site. Selectins, especially E-selectin and P-selectin, help such weak adhesion by binding to carbohydrate chains of glycoproteins on leukocytes.
Note 2: Similar affinities for multiple carbohydrate chains may be found in a study of carbohydrate binding properties with model carbohydrate chains. However, not only carbohydrate chains but the interaction between protein and lectins, potential effects of conformation of carbohydrate chains on their affinity, and influence of the macromolecular status on the cell membrane should be considered in investigating actual binding of lectins to membrane-bound glycoproteins.
Note 3: In order to use galectins, particularly galectins-8 and -9, as a therapeutic agent, we have been conducting research and development, through which we have found that galectins have diverse effects and action sites (targets). These properties are mediated by carbohydrate-protein interactions, and it may be difficult to understand them with the common knowledge based on conventional protein-protein interactions (common knowledge about the effects of intercellular signal transducers). Galectins exert a wide variety of physiological actions but the actions are not disordered. As physiologically active intrinsic substances, galectins may produce a series of well-coordinated effects required to maintain favorable biological functions and conditions.