Review: Soluble innate immune pattern-recognition proteins for clearing dying cells and cellular components: implications on exacerbating or resolving inflammation

Soluble innate immune pattern-recognition proteins (sPRPs) identify non-self or altered-self molecular patterns. Dying cells often display altered-self arrays of molecules on their surfaces. Hence, sPRPs are ideal for recognizing these cells and their components. Dying cell surfaces often contain, or allow the access to different lipids, intracellular glycoproteins and nucleic acids such as DNA at different stages of cell death. These are considered as ‘eat me’ signals that replace the native ‘don’t eat me’ signals such as CD31, CD47 present on the live cells. A programmed cell death process such as apoptosis also generates cell surface blebs that contain intracellular components. These blebs are easily released for effective clearance or signalling. During late stages of cell death, soluble components are also released that act as ‘find me’ signal (e.g. LysoPC, nucleotides). The sPRPs such as collectins, ficolins, pentraxins, sCD14, MFG-E8, natural IgM and C1q can effectively identify some of these specific molecular patterns. The biological end-point is different depending on sPRP, tissue, stage of apoptosis and the type of cell death. The sPRPs that reside in the immune-privileged surfaces such as lungs often act as opsonins and enhance a silent clearance of dying cells and cellular material by macrophages and other phagocytic cells. Although the recognition of these materials by complement-activating proteins could amplify the opsonic signal, this pathway may aggravate inflammation. Clear understanding of the involvement of specific sPRPs in cell death and subsequent clearance of dying cell and their components is essential for devising appropriate treatment strategies for diseases involving infection, inflammation and auto-antibody generation.