Apoptosis inhibitor of macrophage (AIM， encoded by cd5l) is a multi-functional circulating protein that has a beneficial role in the regulation of a broad range of diseases， some of which are ameliorated by AIM administration in mice. In blood， AIM is stabilized by association with IgM pentamers and maintains its high circulating levels. The mechanism regulating the excessive accumulation of blood AIM remains unknown， although it is important， since a constitutive increase in AIM levels promotes chronic inflammation. Here we found a physiological AIM-cleavage process that induces destabilization of AIM and its excretion in urine. In blood， IgM-free AIM appeared to be cleaved and reduced in size approximately 10?kDa. Cleaved AIM was unable to bind to IgM and was selectively filtered by the glomerulus， thereby excreted in urine. Amino acid substitution at the cleavage site resulted in no renal excretion of AIM. Interestingly， cleaved AIM retained a comparable potency with full-length AIM in facilitating the clearance of dead cell debris in injured kidney， which is a key response in the recovery of acute kidney injury. Identification of AIM-cleavage and resulting functional modification could be the basis for designing safe and efficient AIM therapy for various diseases.