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Astronomical black holes often launch highly collimated bipolar outflows known as relativistic jets. Although their formation has been studied for decades, the physical mechanism driving their launch remains incompletely understood. The interaction between dilute plasma and electromagnetic fields in black-hole magnetospheres is widely believed to play a key role.
Over the past decade, general relativistic particle-in-cell (GRPIC) simulations have emerged as a powerful tool for investigating plasma dynamics in black-hole magnetospheres. In this talk, I will overview recent progress in GRPIC simulations of black-hole magnetospheres. I will then present the results of our recent studies on variability and high-energy emission of the magnetosphere. Our study has shown that particles accelerated via large-amplitude electric fields in the polar region of the system efficiently produce inverse-Compton photons extending into the GeV-TeV regime, providing a plausible origin for gamma-ray flares in AGNs such as M87. Possible multiwavelength signature from the system may provide variable information on the magnetic field geometry or dynamical concordance of the system.