Peptidoglycan recognition proteins: pleiotropic sensors and effectors of antimicrobial defences

Peptidoglycan recognition proteins (PGRPs) are innate immunity molecules that are present in most invertebrate and vertebrate animals. All PGRPs function in antimicrobial defence and are homologous to the prokaryotic peptidoglycan-lytic type 2 amidases. However, only some PGRPs have the catalytic activity that protects the host from excessive inflammation, and most PGRPs have diversified to carry out other host-defence functions. Insect and mammalian PGRPs defend host cells against infection through very different mechanisms. Insect PGRPs activate signal transduction pathways in host cells or trigger proteolytic cascades in the haemolymph, both of which generate antimicrobial effectors. By contrast, mammalian PGRPs are directly bactericidal. Here, we review these contrasting modes of action.