Galectins in Host Defense Against Microbial Infections

Galectins are differentially expressed in a variety of cell types, including immune cells, and characterized by the affinity for β-galactoside-containing glycans. There are fifteen galectin members in mammals. Galectins are primarily located intracellularly, but can be secreted outside the cells. They exhibit pivotal roles during microbial infection, such as pathogen recognition and innate and adaptive immunity, and this review aims to discuss the functions of endogenous galectins during infection by four main types of microbes (bacteria, fungi, viruses, and parasites). Extracellular galectins are known to exert a bacteriostatic effect on some bacteria via association with bacterial glycans, whereas cytosolic galectins are recognized to control antibacterial autophagy by binding to luminal host glycans of ruptured endo-lysosomes. With regard to fungal infections, most studies deal with galectin-3. Galectin-3 modulates fungal burdens, the adaptive immune responses, and mortality in fungi-infected mice, which has been shown to be associated with its ability to manipulate fungicidal functions in neutrophils and cytokine expression in dendritic cells. Some viral infections, such as human immunodeficiency virus (HIV) and influenza virus infections, can be regulated by galectin-1 and -3, and they affect various aspects of viral infections, including viral binding, replication, budding, transmission, and infection-associated inflammation. Functions of galectins during a number of different parasitic infections have been identified in studies using galectin-knockout mice. Different parasitic infections have consistently demonstrated a role of galectins in tuning T helper immune responses in infected hosts.