Galectins as molecules of Danger signal, which could evoke an immune response to infection

Galectins are members of a family of proteins defined by their conserved peptide sequence elements, which are crucial for their affinities to beta-galactosides. Unlike other lectins, galectins are cytosolic proteins but can be actively secreted from inflammatory macrophages and can also be passively released when cells expressing galectins are damaged. In recent years, numbers of papers have demonstrated that galectins can act as immunomodulators or proinflammatory factors, implying their roles in immunity against infections. In the initial stage of infection, innate immunity must recognize invasive infections and initiate a defence system efficiently to clear the infection without the aid of acquired immunity. Until recently, discrimination between self and nonself by innate immunity has been considered as the most important element, which can trigger immunity against infections. A novel model, called 'the Danger model', has been proposed recently and has since gained much attention and support as the initial recognition mechanism for infection. In the Danger model, innate immunity is more concerned with damage induced by invading pathogens than with the 'foreignness' of invading pathogen, immunity is then called into action by alarm signals (danger signals) from injured cells. As galectins can be considered as a new type of proinflammatory factors, secretion and release of which is closely associated with the timing in which the immune system is required to send 'danger signals', in this review, the potential roles of galectins as molecules of danger signals will be discussed.