WASHINGTON, Dec 16 (Reuters) - The popular multiple sclerosis drug Tysabri appears to promote a rare brain infection by suppressing immune system cells in the brain, researchers said on Tuesday.

The researchers, who conducted an autopsy on an MS patient who died while taking the drug, said it may be possible to make the drug safer by giving patients treatment "holidays" from time to time to allow the brain's immune protection to recover.

Tysabri, known generically as natalizumab and made by Biogen Idec Inc <BIIB.O> and Elan Corp <ELN.I><ELN.N> of Ireland, can cause a serious brain infection called progressive multifocal leukoencephalopathy or PML.

It was pulled off the market soon after being introduced in 2004, but sold again beginning in 2006 because there were few good options for patients with MS.

On Monday Biogen reported the fourth case of PML this year -- in a European patient who had been taking Tysabri exclusively for 26 months.

"Whether or not treatments other than prolonged, uninterrupted dosing may benefit patients with MS should be tested in controlled clinical trials," Dr. Olaf Stuve, a neurologist at the University of Texas Southwestern Medical School in Dallas, said in a statement.

Tysabri's package insert label cautions that patients taking the drug have a 1-in-1,000 risk of developing PML.

Writing in the Archives of Neurology, Stuve and colleagues said they found significantly fewer immune cells called CD4 T cells in and around the patient's blood vessels in the brain.

"Natalizumab is very effective in keeping pro-inflammatory cells out of the brain to reduce damage from MS," Stuve said in a statement. But by doing so it may promote infection in some people.

Stuve said doctors should look for biomarkers -- easily tested indicators -- in people who may be developing the syndrome so they may be taken off the drug.

"It's a very effective drug, and it's clear that the vast majority of patients are greatly benefiting from its use," Stuve said. He said about 43,000 people had taken natalizumab since 2006.

MS is an auto-immune disease in which the body mistakenly attacks the fatty myelin coating surrounding nerve cells.

The company says the drug produced a 68 percent reduction in relapses in clinical trials. (Reporting by Maggie Fox, Editing by Howard Goller)

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