Here's a huge one for the ever-growing pile of lawsuits filed by and against Airbnb. Apartment Investment & Management Co. (AIMCO) has sued the rental service for "helping tenants breach their leases," according to The Wall Street Journal. Denver-based AIMCO, one of the biggest landlords in the US, owns and manages over 50,000 apartments across the country. Real-estate research firm Green Street Advisors told the WSJ that this is the first time Airbnb has been sued by a major landlord. Other apartment owners might now feel emboldened to follow suit if the service refuses to cooperate with them.