  We consider a collision of two dust thin shells with a high center-of-mass
(CM) energy including their self-gravity in a Ba\~{n}ados-Teitelboim-Zanelli
(BTZ) spacetime. The shells divide the BTZ spacetime into three domains and the
domains are matched by the Darmois-Israel junction conditions. We treat only
the collision of two shells which corotate with a background BTZ spacetime
because of the junction conditions. The counterpart of the corotating shell
collision is a collision of two particles with vanishing angular momenta. We
compare the dust thin shell collision and the particle collision in order to
investigate the effects of the self-gravity of colliding objects on the high CM
energy collision. We show that the self-gravity of the shells affects the
position of an event horizon and it covers the high-energy collisional event.
Therefore, we conclude that the self-gravity of colliding objects suppresses
its CM energy and that any observer who stands outside of the event horizon
cannot observe the collision with an arbitrary high CM energy.
