AIDSWEEKLY Plus, 9 Sep 1996
Daniel J. DeNoon, Senior Editor

Identification of co-receptors necessary for HIV infection of CD4(+) T cells opens up new targets for vaccines.

AIDS researcher Paolo Lusso and colleagues recently reported that several cellular factors known as (beta)- chemokines - RANTES, MIP-1(alpha), and MIP-1(beta) - have potent anti-HIV activity.

"This was an extremely important paper that will influence research for years to come," said John P. Moore of The Aaron Diamond AIDS Research Center, New York City.

Moore reported the discovery that the (beta)-chemokines block the C-C CKR-5 receptor at the XI International Conference on AIDS, held July 7-12, 1996, in Vancouver, British Columbia, Canada.

"Five groups more or less simultaneously described CKR-5 as the primary virus second receptor," Moore said. "All of the papers were published within an eight-day span."

Other groups that described CKR-5 as the HIV co-receptor included B.J. Doranz of the University of Pennsylvania and colleagues (Cell, 1996;85(7):1149-58); H. Choe of the Dana Farber Cancer Institute and colleagues (Cell, 1996;85(7):1135- 48); G. Alkhatib of NIAID and colleagues (Science, 1996;272(5270):1955-8); and H. Deng of New York University and colleagues (Nature, 19960;381(6584):661-6).

The discovery was based on the Lusso et al. findings and on another major piece of research by a team led by NIAID researcher Edward Berger. Berger and colleagues reported that HIV requires not only the CD4 receptor, but also another co- receptor in order to infect T cells (Feng et al., Science, 1996;272(5263):872-7).

"Building on these, the first work we did was to look at whether the (beta)-chemokines were acting at the level of virus entry," Moore said.

The researchers showed that fusion of HIV-1 with permissive CD4(+) T cells was completely inhibited by the (beta)-chemokines. But when they added excess HIV-1 gp120 envelope glycoprotein, the (beta)-chemokines lost their ability to block virus/cell fusion.

"This suggested pretty strongly to us that the (beta)- chemokines were blocking virus/cell fusion," Moore said. "We still to this day do not understand the cell-dependent variables that impact on macrophages compared to T cells. But there's no doubt that the fusion reaction is blocked by the (beta)-chemokines in T cells which is the most relevant cell for HIV pathogenesis."

Once they had formally proven that the (beta)-chemokines exert their anti-HIV activity at the level of fusion, Moore and colleagues began searching for the B-chemokine receptor.

They had two major hints. The so-called "fusin" receptor described by Berger et al. is "a member of the IL-8 C-X-C receptor family, but it's sufficiently similar to the (beta)- chemokine receptors to attract our attention," Moore said.

The second major hint was the fact that of the (beta)- chemokine receptors described in the literature, the only one that bound MIP-1(beta) was CKR-5.

"This was a shining light to us, and when the sequence became available we and a number of other groups chewed at the carcass that Ed Berger had brought down for us," Moore said.

The researchers therefore cloned the CKR-5 receptor gene, transfected it into CD4(+) cells (murine, simian, and human cells were used), and assayed the cells for permissiveness to HIV infection.

In the presence of the (beta)-chemokines, there was no viral entry mediated by CKR-5.

"The degree of inhibition by the (beta)-chemokines was dependent on the level of [cellular] expression of CKR-5," Moore said.

"If you overexpress CKR-5 ... the longer you have the cell churning out CKR-5 the less inhibition you get by the (beta)- chemokines. When you have a lot of CKR-5 expression the (beta)-chemokines don't block. When you have relatively little, you get virus fusion but it's still blockable by the (beta)-chemokines. So this was suggestive of a competitive mechanism."

With different HIV strains, different (beta)-chemokine receptors functioned as fusion co-receptors to different extents. But Moore said that all the groups agree that CKR-5 is the most physiologically relevant; CDR-3 for example is only expressed in eosinophils which are not the major target for HIV.

Moore noted that his group has gone on to show that gp120 blocks chemokine binding to CKR-5.

"That together with a number of data sets that have not fully matured yet indicates to me that this is a gp120 binding protein," he said. "CKR-5 binds to gp120 after CD4 binding."

Moore emphasized that the complexity of the HIV binding to T cells means that the so-called CD4-binding site on the V3 loop of the HIV-1 gp120 envelope protein may not be unique.

"If it turns out that the V3 loop is the only binding site, I should be extremely surprised," Moore said.

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