Activating and inhibitory Fc receptors in immunotherapy: being the actor or being the target

Membrane Fc receptors (Fc Rs) can act either as potent activators of effector cell functions or as inhibitors of receptor-mediated cell activation following engagement by IgG antibodies bound to their target molecules. The remarkable ability of activating Fc Rs to trigger antibody-dependent cellular cytotoxicity, cytokine release and phagocytosis/endocytosis followed by antigen presentation has stimulated the development of a number of therapeutic monoclonal antibodies whose Fc regions have been engineered to optimize their effector functions, mostly their killing activities. Conversely, the demonstration that inhibitory Fc Rs can block or downmodulate effector functions has led to the concept that targeting these receptors is of interest in a number of pathologies. The use of bispecific antibodies leading to the crosslinking of Fc RIIB with activating receptors could induce immunomodulation in autoimmune or allergic diseases. Alternatively, the use of cytotoxic/antagonist anti-Fc RIIB antibodies could kill Fc RIIB-positive tumor cells or prevent the downmodulation of activating receptors. Thus, antibodies engineered to preferentially target activating or inhibitory Fc Rs are currently being designed for therapeutic use.