(Reuters) - Johnson & Johnson on Thursday said it licensed rights from a Chinese drugmaker to drugs that spur the immune system to help fight diseases, which it hopes will become a key part of a cure for chronic hepatitis B.

The deal, with a unit of Sino Biopharmaceutical Ltd that focuses on treatments for liver diseases, gives J&J exclusive rights to develop, manufacture and sell the medicines outside China.

Unlike new hepatitis C drugs that boast extremely high cure rates and short treatment durations, current medicines for the hepatitis B virus (HBV) can hold it in check but must be taken for life. If untreated, the virus, which is spread by contact with infected bodily fluids, can be fatal and is a leading cause of liver cancer.

Hepatitis B has been difficult to cure because antiviral medicines that block replication of the virus have proved insufficient at clearing it from the blood.

J&J and other researchers believe an assault by components of the immune system will be needed as part of a cure.

The deal with Sino's Chia Tai Tiaqing Pharmaceutical unit includes an immune-stimulating agent that could potentially be combined with antiviral drugs J&J acquired when it purchased Novira Therapeutics in November.

"With the Novira acquisition we have the lead direct antiviral in the world for HBV," Lawrence Blatt, head of infectious diseases and vaccines for J&J's Janssen pharmaceutical unit, said in an interview.

"If we can in the same time we're giving direct antivirals and blocking (virus) replication, activate the immune system and wake it up, maybe we can cause patients to clear HBV," Blatt said.

Hepatitis B is an enormous market and far more prevalent than hepatitis C, particularly in Asia, with more than 350 million sufferers worldwide compared with about 130 million with hepatitis C.

"Our aim is to develop a cure for hepatitis B by building the best internal and external pipeline of promising, complementary solutions, and advancing them into the clinic as rapidly as possible," Janssen research chief William Hait said in a statement.

Several companies are working on hepatitis B treatments, including Arrowhead Research Corp, Tekmira Pharmaceuticals, and privately held OnCore Biopharma, which is led by the scientists who discovered Gilead Sciences' market-leading hepatitis C treatment.

Blatt said hepatitis B will be much tougher to crack than hepatitis C. "It's going to get cracked," he said. "There's enough critical mass right now."

(Reporting by Bill Berkrot; Editing by Cynthia Osterman)