Cytosolic galectins and their release and roles as carbohydrate-binding proteins in host–pathogen interaction

Abstract

The glycocalyx is a layer of glycoconjugates found on the surface of both host cells and microorganisms. Information presented on some of the glycans on glycoconjugates is recognized by mammalian glycan-binding proteins, lectins, and these interactions modulate various physiological processes, including host innate immune responses. Lectins and host glycoconjugates are synthesized in the same secretory pathway. One notable exception is a family of soluble β-galactoside-binding lectins, galectins, which are synthesized and accumulated in the cytosol and thereby segregated from their glycan ligands. In cases where pathogenic infection per-sists and tissue injury occurs, galectins are passively released from injured cells. In addition, galectins are actively secreted through unconventional secretory pathways by inflammation-activated or differentiating cells. Thus, extracellular emergence of galectins is associated with the presence of pathogenic hazards. Evidence from a series of studies suggests that galectins exert multiple im-munological effects. Extracellular galectin-3 acts as a damage-associated molecular pattern (DAMP) and adhesion molecule for neu-trophils in lungs to initiate a proinflammatory response and to mediate rapid neutrophil migration in lungs infected with pathogenic microorganisms. In this review, the roles of galectin-3 in initial innate immune responses and resolution are discussed together with a historical overview of research on galectins in the secretory pathway and innate immunology.