Human eosinophil cationic protein manifests alarmin activity through its basicity and ribonuclease activity

Eosinophil cationic protein (ECP), eosinophil derived neurotoxin (EDN), and human pancreatic ribonuclease (HPR) are members of the RNase A superfamily having similar catalytic residues and diverse functions. Alarmins are the endogenous mediators of innate immunity which activate or alarm the adaptive immune system by activating antigen presenting cells (APCs). EDN acts as an alarmin molecule and plays an important role in innate as well as adaptive immunity. EDN displays chemotactic activity for dendritic cells (DCs) and activates them, has antiviral and antiparasitic activities, and is rapidly released from immune cells. HPR only displays chemotactic activity while no such activity has been reported for ECP. In this study we show that ECP displays the chemotactic activity comparable to that of HPR and EDN. ECP also interacts with TLR-2 to activate NF-{kappa}B/AP-1 expression like EDN. The RNase activity of ECP, EDN and HPR, and basicity of ECP were found to be crucial determinants for their chemotactic activity for APCs, however for the DC maturation activity, RNase activity was not found to be essential. Bovine RNase A did not show any chemotactic activity despite having a very high RNase activity indicating that other determinants in addition to the RNase activity are involved in the chemotactic activity of ECP, EDN and HPR. The current study establishes that ECP also can act like an alarmin.