Inhibitory receptors of the immune system: functions and therapeutic implications

The immune system has a remarkable ability to respond to seemingly endless antigens. In essence, a productive immune response takes place along a well defined but treacherous line, that is to effectively eradicate pathogens, and at the same time avoid causing damage to self organs. This type of response is fine-tuned, at least in part, by a complex array of pathways that either promote or inhibit the activation of innate and adaptive immune cells. Much effort has been focused on pathways that can support immune activation. In this article, we review specifically pathways that can inhibit immune responses and maintain immune homeostasis, highlighting our recent understanding on the role of inhibitory receptors that selectively engage the self MHC class I molecules and the B7 superfamily members, we also discuss the inhibitory Fc receptors and inhibitory cytokines and how such pathways, either individually or collectively, regulate innate and adaptive immune responses. Finally, we summarize new emerging approaches on how such negative pathways can be therapeutically modulated in various disease settings.