Private-equity firms TPG Global LLC and Warburg Pincus LLC, which co-own Neiman Marcus Group Inc., announced today that they have reached an agreement to sell the luxury retailer to Ares Management LLC and Canada Pension Plan Investment Board for $6 billion. The retailer says it expects the transaction to close in the fourth quarter.

TPG Global and Warburg Pincus acquired Neiman Marcus, which operates luxury department stores under the Neiman Marcus and Bergdorf Goodman brands, for $5.1 billion in 2005. The retailer conducts its operations through its wholly owned subsidiary, The Neiman Marcus Group Inc.

“This is an excellent opportunity to invest in a leading omnichannel luxury retailer that operates two of the most iconic retail brands in the U.S.,” says André Bourbonnais, senior vice president, private investments, Canada Pension Plan Investment Board. “We believe the company’s strong market position, combined with an expected increase in U.S. luxury goods spending, provide attractive opportunities for future growth.”

The deal comes as the number of consumers buying luxury goods online continues to grow. Luxury e-commerce sales are growing at a rate of 25% per year, according to a recent report from consulting firm Bain & Co.

Neiman Marcus Group operates 79 stores and sells online at a variety of sites, including NeimanMarcus.com, BergdorfGoodman.com and Horchow.com.

Neiman Marcus, No. 39 in the Internet Retailer 2013 Top 500 Guide, in June filed for an initial public offering of stock and said it expects to raise up to $100 million. The retailer did not immediately respond to an inquiry about what led it to shift gears to an outright sale.

The deal is the second major sale of a luxury goods retailer this summer. Hudson’s Bay Co., the Toronto-based company that owns the web and retail store brands Hudson’s Bay and Lord & Taylor, announced a deal in July to acquire Saks Fifth Avenue in a $2.9 billion all-cash transaction.