SCIENTISTS have found that the ability of certain rats to reject a tumour can be transferred to other rats using a simple technique that could one day be used to enhance human resistance to cancers.

Ashok Khar of Hyderabad's Centre for Cellular and Molecular Biology (CCMB) discovered a certain type of tumour prevalent only in rats. Called AK-5, the tumour killed some rats while others rejected it totally. When Khar and his colleagues injected some rats with cultured cells of AK-5, they found the rodents either rejected the tumour and became healthy again or they died because the tumour continued to grow unchecked.

The response of the rats to the tumour depended on the strength of the animal's immune response. Says Khar, "By investigating this response to AK-5, we hope to obtain clues for cancer therapy in humans."

Most tumours evoke a response from the immune system, but usually it is too weak to induce the body to reject the tumour.
Induced immunity Khar says unlike other tumours in rats, AK-5 is not induced by viruses or chemicals but arises spontaneously. "This makes it a valuable system for study," says Khar, "because most human cancers are caused by such tumours."

Khar found that certain natural killer cells, found in rat's blood, play an important role in fighting tumours. The cancerous cells produce an antigen -- a protein, which is foreign to the host and stimulates its immune system to produce what is called an anti-tumour antibody. The antibody binds to the tumour cells, which are then recognised by the natural killer cells and destroyed. The tumour necrosis factor (TNF) -- another chemical substance produced by the immune system -- also plays an important role in fighting tumour cells.

In the experiments carried out by Khar, all the rats rejecting tumours had high levels of the anti-tumour antibody and TNF, while those whose tumours continued to grow had low levels of these substances. Says Khar, TNF and the antibody were found even in test-tubes to be toxic to the tumour cells.

The scientists have also found that immunity to AK-5 can be induced in a rat by injecting into its body cells taken from the spleen of animals that had once successfully rejected the tumour.

Khar and his colleagues are now trying to determine the structure of the gene responsible for the AK-5 antigen. By comparing this antigen with those observed in other kinds of tumours, they will know whether their results can be extended to other forms of cancer, especially those found in humans.