Adenosine: an endogenous regulator of innate immunity.

Although inflammatory and immunological reactions protect the host from invasion by microorganisms and eliminate debris at sites of tissue injury, they can also be responsible for significant tissue damage. Thus, regulatory mechanisms that limit damage from an overly exuberant immune response have evolved. It is increasingly apparent that adenosine, a purine nucleoside that is elaborated at injured and inflamed sites, has a central role in the regulation of inflammatory responses and in limiting inflammatory tissue destruction. Adenosine, called a 'retaliatory metabolite' because it is a regulatory autocoid that is generated as a result of cellular injury or stress, interacts with specific G protein-coupled receptors on inflammatory and immune cells to regulate their function. The effects of adenosine, acting at its receptors, on the functions of the cells that mediate innate immune responses, will be reviewed.