Optimal therapeutic activity of monoclonal antibodies against chikungunya virus requires Fc-FcR interaction on monocytes

Chikungunya virus (CHIKV) is an emerging mosquito-borne virus that has caused explosive outbreaks worldwide. Although neutralizing monoclonal antibodies (mAbs) against CHIKV inhibit infection in animals, the contribution of Fc effector functions to protection remains unknown. Here, we evaluated the activity of therapeutic mAbs that had or lacked the ability to engage complement and Fc receptors (FcR). When administered as post-exposure therapy in mice, the Fc effector functions of mAbs promoted virus clearance from infected cells and reduced joint swelling—results that were corroborated in antibody-treated transgenic animals lacking activating FcR. The control of CHIKV infection by antibody-FcR engagement was associated with an accelerated influx of monocytes. A series of immune cell depletions revealed that therapeutic mAbs required monocytes for efficient clearance of CHIKV infection. Overall, our study suggests that in mice, FcR expression on monocytes is required for optimal therapeutic activity of antibodies against CHIKV and likely other related viruses.