* Deal potentially worth more than $500 million

* Covers experimental vaccines against abnormal tau protein

* AC Immune secures upfront sum and milestone payments

* CEO says listing is among options for company's future (Adds comments from AC Immune CEO)

By Ben Hirschler and Caroline Copley

LONDON/ZURICH, Jan 12 (Reuters) - Johnson & Johnson boosted its research efforts into battling Alzheimer's on Monday, striking a deal potentially worth more than $500 million to develop anti-tau vaccines with Swiss biotech firm AC Immune.

Tau is a protein known for forming twisted fibres inside brain cells and is linked to cell death. It is one of two abnormal proteins tied to the memory-robbing disease. The other is beta amyloid.

The hope is that therapeutic vaccines targeting tau will offer a way to treat Alzheimer's patients early in the disease.

It is a decade since the last drug was approved for Alzheimer's, and there is still no treatment that can slow its progression, with current drugs only easing some of its symptoms.

Unlisted AC Immune said on Monday it would receive an upfront sum and further payments based on scientific and commercial progress under the worldwide exclusive licence agreement and research collaboration with J&J's Janssen Pharmaceuticals unit.

J&J will further develop AC Immune's lead therapeutic vaccine, ACI-35, which is currently in an early-stage Phase Ib clinical trial. ACI-35 is designed to stimulate the patient's immune system to produce a response against tau protein.

The deal is the fourth licensing agreement for AC Immune, which has another major tie-up with Roche for a beta amyloid-fighting drug called crenezumab.

Roche is due to make a decision about whether to advance crenezumab into late-stage testing, which will be crucial factor in determining how AC Immune's future pans out.

"We will leave our options open and see what the best steps for the company are. An IPO is one of the options," AC Immune's Chief Executive Andrea Pfeifer told Reuters in an interview.

AC Immune has a further vaccine, ACI-24, for preventing and clearing amyloid plaques in Phase I/IIa clinical trials.

Pfeifer said the company plans to start testing that vaccine in people with Down syndrome in the course of this year. Up to 75 percent of adults with the condition develop dementia.

(Reporting by Ben Hirschler and Caroline Copley; Editing by Jason Neely and John Stonestreet)