Chipotle Mexican Grill Inc, which has been under pressure from activist shareholder Bill Ackman, said on Friday four of its 12 directors would not stand for re-election to the burrito chain's board at its upcoming shareholder meeting.

John Charlesworth, Patrick Flynn, Darlene Friedman and Stephen Gillett will opt out of re-election this year, Chipotle said in a regulatory filing.

Friedman is the Denver-based company's longest-serving director with a 22-year tenure. Charlesworth and Flynn have served on the board for nearly two decades, while Gillett has been a director for about two years.

Chipotle said in December it had appointed four new members to its board, including one from Ackman's hedge fund Pershing Square Capital Management.

With a 10 percent stake as of March 3, Pershing is Chipotle's largest shareholder, according to the company.

Chipotle, once majority-owned by McDonald's Corp , has faced shareholder calls for an overhaul after a string of food safety stumbles that bruised its sales and reputation.

The restaurant chain was linked in November 2015 to a multistate E.coli outbreak and the next month to a norovirus outbreak that sickened at least 80 Boston College students.

Following the food-safety lapses, which battered the company's shares and racked up high legal bills, Chipotle abandoned its dual-CEO structure and named Steve Ells the sole chief executive.

Chipotle has apologized, offered free and discounted food to regain lost customers and hired two high-profile food-safety experts to help address quality issues.

The company last week won the dismissal of a lawsuit claiming it defrauded shareholders about its ability to protect customers from the outbreaks.

In its latest quarter, the once high-flying Mexican chain managed to grow sales for the first time since the scandals, before which its shares and sales had hit record levels.

Three months prior to the outbreaks, Chipotle shares had climbed to a record high of about $758 from a $37 low during the 2008 financial crisis.