Osteoporosis causing bone thinning

The absence of the female sex hormone oestrogen triggers an immune reaction which leads to the loss of bone, and they now believe they know how this happens.

The team, from research centres in Atlanta and St Louis in the US, hope that revealing how this happens could eventually help produce drugs to counter the process.

This could lead to the development of new drugs Dr Roberto Pacifici

Thousands of women in the UK suffer from thinning bones in old age.

It makes it more likely that they will break bones in falls, severely hindering their future mobility and costing the NHS millions in hip replacements and other operations.

Doctors know that when a woman's body gradually stops producing the sex hormone oestrogen, this bone loss accelerates.

One of the main benefits of hormone replacement therapy is to temporarily halt this process, but there is demand for a more carefully targeted drug which can halt or reverse bone loss without any side effects.

Mouse tests

The US research focuses on what actually happens when the supply of oestrogen is cut off.

When this happens, they found, two proteins are produced by the body which trigger the damaging immune over-response.

The immune cells activated by these proteins make other chemicals which contribute to the breakdown of bone.

One of the proteins, called IFN-g, has already been found to play a role in other "autoimmune" diseases, in which overactivity by the body's own immune system is causing the problem.

So far the research has only focused on osteoporosis in laboratory mice - but researchers are hopeful that the same effects will be seen in humans.

They say that, if that proves the case, researchers can start to work on drugs which interrupt the "chain reaction", and perhaps slow down bone loss.

'Major advance'

Dr Roberto Pacifici, who led the study, said: "This study represents a major advance in our understanding of the mechanism of action of oestrogen in bone, including the essential link between the immune system and bone stability.

"If we observe the same results in humans, this could lead to the development of new drugs that work in bone like oestrogen, but do not have negative effects on reproductive organs and the cardiovascular system.

"The study also helps explain why certain autoimmune diseases, such as rheumatoid arthritis, are improved by oestrogen and exacerbated by menopause."

However, British expert Professor Jim Gallagher, from the Human Bone Cell Research Group at Liverpool University, told BBC News Online that while the study did represent a step forward, it might not translate into new treatments.

He said: "This is an interesting paper, but any treatment for osteoporosis as a result is a long way off."

The study was published in the journal Proceedings of the National Academy of Sciences.