ZURICH, June 6 (Reuters) - Law firm Quinn Emanuel Urquhart & Sullivan said on Thursday it had filed a lawsuit against Switzerland in the U.S. District Court for the Southern District of New York on behalf of a group of Credit Suisse Additional Tier 1 (AT1) bondholders.

AT1 bonds, which act as a shock absorber if a bank's capital levels fall below a certain threshold, have been encouraged by regulators since the 2008-09 global financial crisis.

(Reporting by Dave Graham;Editing by Elaine Hardcastle)