The macrophage soluble receptor AIM/Api6/CD5L displays a broad pathogen recognition spectrum and is involved in early response to microbial aggression

Apoptosis inhibitor of macrophages （AIMs）, a homologue of human Spa, is a mouse soluble member of the scavenger receptor cysteine-rich superfamily （SRCR-SF）. This family integrates a group of proteins expressed by innate and adaptive immune cells for which no unifying function has yet been described. Pleiotropic functions have been ascribed to AIM, from viability support in lymphocytes during thymic selection to lipid metabolism and anti-inflammatory effects in autoimmune pathologies. In the present report, the pathogen binding properties of AIM have been explored. By using a recombinant form of AIM （rAIM） expressed in mammalian cells, it is shown that this protein is able to bind and aggregate Gram-positive and Gram-negative bacteria, as well as pathogenic and saprophytic fungal species. Importantly, endogenous AIM from mouse serum also binds to microorganisms and secretion of AIM was rapidly induced in mouse spleen macrophages following exposure to conserved microbial cell wall components. Cytokine release induced by well-known bacterial and fungal Toll-like receptor （TLR） ligands on mouse splenocytes was also inhibited in the presence of rAIM. Furthermore, mouse models of pathogen-associated molecular patterns （PAMPs）-induced septic shock of bacterial and fungal origin showed that serum AIM levels changed in a time-dependent manner. Altogether, these data suggest that AIM plays a general homeostatic role by supporting innate humoral defense during pathogen aggression.

AIM associated with the IgM pentamer: attackers on stand-by at aircraft carrier

Circulating immunoglobulin M(IgM)exists in a pentameric form,possessing a polyreactive nature that responds not only to foreign antigens but also to autoantigens;thus,it is involved in both beneficial and detrimental immune responses,including protection from infection and the progression of autoimmunity.On the other hand,IgM also behaves as a carrier of the apoptosis inhibitor of macrophage(AIM)protein,storing a large amount of the inactivated form of AIM in the blood through this association.Under different disease conditions,AIM can dissociate from IgM locally or systemically to exert its function,inducing the removal of various biological debris such as excess fat,bacteria,cancer cells or dead cell debris.Most typically,upon induction of acute kidney injury(AKI),IgM-free AIM is filtered by the glomerulus in the kidney,which stimulates the clearance of intraluminal dead cells debris at the obstructed proximal tubules,thereby facilitating the repair of kidney injury.Interestingly,cats exhibit a deficiency in AIM release from IgM,which may increase their susceptibility to renal failure.Conversely,association with AIM inhibits IgM binding to the Fcα/μreceptor on follicular dendritic cells at the splenic germinal center,thereby protecting the IgM immune complex from Fcα/μreceptor-mediated internalization,which supports IgM-dependent antigen presentation to B cells and stimulates high-affinity IgG antibody production.The regulation of AIM–IgM binding,resulting from the discovery of reciprocal actions between AIM and IgM,could lead to the development of novel therapies against different diseases.