The SIRP family of receptors and immune regulation

The signal-regulatory proteins (SIRPs) can be classed as paired receptors, the most well known of which include the killer-cell immunoglobulin-like receptors, which are expressed by natural killer cells.The SIRP family contains activating, inhibitory and non-signalling members, which have closely related extracellular regions but distinct cytoplasmic tails. They are expressed mainly by myeloid cells and are therefore thought to have a role in immune regulation.Two types of ligand for the inhibitory member, SIRPα, have been identified: the widely expressed cell-surface protein CD47 and surfactant protein A.A third SIRP-family member, SIRPγ, transmits neither activating nor inhibitory signals even though it binds CD47.Like other paired receptors, the SIRPs show evidence of rapid evolution with considerable species differences and polymorphisms.Factors such as ligand availability, binding affinity, protein mobility and expression levels are likely to affect their function in vivo.