A treatment for Aids which is more effective and less expensive than those currently available could have moved closer, according to scientists.

They have successfully inserted a beneficial gene into blood immune cells taken from patients infected with HIV.

The gene blocked the Aids virus from replicating in the cells, pointing to a possible treatment which would lengthen sufferers' survival rates.

Though the action does not eliminate the Aids virus, it stops it activating.

Dr Wenzhe Ho, at the Children's Hospital of Philadelphia, said: "These results suggest that, with further work, this technique may keep HIV-infected patients free of disease symptoms."

Knowledge

Using the knowledge that an HIV gene called tat is essential for the Aids virus to replicate in infected cells, Dr Ho and colleagues at the Research Institute for Genetic and Human Therapy in Washington DC designed an anti-tat gene to block it.

The anti-tat gene was inserted into a mouse retrovirus which can enter cells that are potential sites for HIV replication, and prevented activation.

More importantly, say the researchers, virus activation was also prevented in blood immune cells taken from HIV positive patients.

Additionally, the anti-tat gene prolonged the survival of immune system cells known as CD4+T lymphocytes which are targeted by the Aids virus.

If further studies prove the gene therapy could be used in HIV patients, it could provide an alternative to the standard Aids treatment - active antiretroviral therapy.

The current treatments are expensive and require a difficult regime of four to six pills, two to three times daily, and do not eradicate HIV. They also requires lifelong use.

Dr Stuart Starr, head of immunologic and infectious diseases at the Children's Hospital of Philadelphia, said: "The anti-tat gene offers the possibility of prolonging the latency period indefinitely without the need for long-term antiretroviral treatment.

"Early indications are that the anti-tat gene does not affect uninfected cells or cause toxic side effects."