Important note: Information in this article was accurate in 1997. The state of the art may have changed since the publication date.

Reuters NewMedia, Inc. - Thursday July 17 1:39 AM EDT

Naming them Bonzo and BOB (Brother of Bonzo), the researchers said they were used by both the human immunodeficiency virus (HIV) and the version seen in monkeys, SIV.

Writing in the science journal Nature, Dan Littman and colleagues at the Howard Hughes Medical Institute at New York University said their findings cast more light on how HIV adapts and evolves.

They were looking for receptors that SIV uses. Finding Bonzo and BOB, they determined that they could also be used by the two strains that cause humans AIDS, HIV-1 and the less virulent HIV-2.

For a virus to infect a cell, it first has to hook on and gain entry. It does this by using receptors that are designed for use by other molecules in the body.

HIV attacks immune system cells and Littman's paper brings to six the number of receptors the virus can use. All the HIV receptors are meant for use by chemokines -- signalling chemicals that are part of the body's immune response.

Littman's group found BOB in colon cells, and both BOB and Bonzo in lymph tissue, and said this could mean they are important in viral transmission.

Ghalib Alkhatib and colleagues at the National Institutes of Health also reported discovering the Bonzo receptor, which they called STRL33.

The discovery of the new receptors could explain how some people who have a genetic mutation that makes them resistant to HIV nonetheless sometimes get infected.

The genetic mutation means they do not have a CCR5 receptor -- the main one used by HIV. But it could be that, in these people, some strains of HIV can use the alternative receptors.

Robin Weiss and Paul Clapham of Britain's Institute of Cancer Research said it was likely more receptors used by HIV would be found.

"All HIV-1 strains so far tested use the CCR5 or CXCR4 co-receptors, or both, and therefore these two are likely to be crucial to HIV infection," Weiss and Clapham wrote.

"Many HIV -1 strains, however, are able to use other co-receptors -- including CCR3, CCR2b and three of the four newly identified ones, as well as CCR5 or CXCR4."

The findings also had implications for drug development, they said.

"The discovery of co-receptors has brought hope that new drugs can be developed that block HIV-1 entry into cells," they said.

It had been found that chemokines could be used to block the receptors, although this did not yet prevent infection.

And if drugs target just one or two of the receptors, HIV strains could evolve that use the other ones, they added.

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