"Out of Africa" is one theory that seeks to explain where, when and how the AIDS virus suddenly erupted two decades ago.

Ostensibly, subhuman primates — chimps, monkeys and their ilk — had long harbored simian immunodeficiency virus, SIV, which somehow mutated to the human form. Supporting this notion is the fact that monkeys, notably the familiar rhesus macaque, don't readily contract either the human or the simian form of symptomatic AIDS.

Presumably, over eons of evolution, they developed immunity.

Now, a Harvard Medical School-affiliated team of immunologists, virologists and biochemists has discovered that a twist in a single amino acid in the rhesus genome can make it susceptible to infection. By the contrary token, mutating the homologous gene in the human genome prevents HIV from entering its target cells, and spreading the viral infection.

Last week's Science, dated Nov. 21, 1997, reports the phenomenon in an article noncommittally titled: "CD4-independent binding of SIV gp120 to rhesus CCR5." Its co-senior authors are Norma and Craig Gerard, at Children's Hospital, a teaching facility of Harvard Medical School, in Boston.

CCR5 is the hot-button chemokine receptor that surfaced two years ago as the indispensable co-conspirator, along with the CD4 cell-surface receptor on immune-system T cells, whereby HIV can penetrate its target T cells and macrophages. (See BioWorld Today, Sept. 27 and Nov. 14, 1996, p. 1.)

"We were able to show in this Science paper," Craig Gerard told BioWorld Today, "using rhesus macaque's SIV — a cousin of HIV, and probably the original entry of the virus into humans — that in fact the CCR5 chemokine receptor is the critical event in viral penetration of the cells."

He made the added salient point that "you can now dispense with the need for CD4 in the viral interactions involved, by mutations that differ between rhesus and humans."

Those interactions resemble a gang of burglars deploying their tools to pick the locks and force the door into a T cell or macrophage.

First, though, the victim's proteases break down HIV's envelope glycoprotein 160 (gp 160) to subunits gp120 and gp 41.

Second, gp120 binds to its CD4 receptor on the cell surface.

Third, thus engaged, the gp120 bends itself out of shape, and its epitope complexes with the CCR5 co-factor.

Fourth, gp120 then pulls its former remnant, gp41, into position to fuse with CCR5 and carry out the actual break-in, hustling the rest of the infective virus inside.

"What this does," Gerard pointed out, "is reduce to a high-throughput screen the ability to search for anti-HIV drugs. You take that chemokine receptor, CCR5, and gp120 and CD4s, all of which are pure, non-infectious simple reagents, and throw them together in a 96-well format.

"Then," he continued, "you start looking for antibodies or small-molecule drugs that disrupt the binding interaction."

What the Gerards and their co-authors found on the rhesus monkeys' CCR5 genes was that a single point-mutation shift from one amino acid to another — specifically, aspartic acid to asparagine — occurs "in the exact region that triggers gp120 to make the critical fusion event.

"So we said," Craig recalled: "'Oh my God, the important part for gp120 binding is different between monkey and man!' So we made the point mutation in rhesus," he went on, "which reverted infectivity, to man, and made the same mutation in the human CCR5 gene, which reverted non-infectivity to rhesus."

The take-home lesson from this experiment, he stated, "was that we could confer or take away the necessity for CD4 in the binding interaction."

From LeukoSite To Warner-Lambert

Gerard is a scientific adviser to LeukoSite Inc., of Cambridge, Mass., which, he said, "is obviously interested in this area and has participated in some of our experiments." A year ago this month, LeukoSite concluded a collaborative research and licensing agreement with Warner-Lambert Co., of Morris Plains, N.J., for CCR5 development.

"People haven't yet picked up the message," Gerard observed, "that screening for chemokine antagonists is not the same as screening for HIV antagonists. In fact," he added, "in a paper we published last January in the Journal of Biological Chemistry, we demonstrated that the chemokine receptor binding site and the HIV gp120 binding site are on different parts of CCR5.

"Therefore," Gerard pointed out, "if you screened and found a small molecule that inhibited chemokine binding, it might not inhibit HIV binding, for which you'd need to screen directly."

He made the final point that "if you're not a virologist, in fact, a retrovirologist, with a bazillion dollars' worth of special equipment and containment facilities, you can't work in the field. It's very exclusionary. But the real impact of our finding is that any receptor pharmacologist, or anybody who's interested in drug discovery, now has the simpler system."

LeukoSite's chief financial officer and vice president of corporate development, Gus Waller, told BioWorld Today that his company's collaboration with the Parke-Davis division of Warner-Lambert "is a small-molecule program, in the stage of identifying compounds, and doing chemistry on those compounds."

He added that "Leukocyte brings principally the biology, but it's also involved in the chemistry. Of course, the folks at Parke-Davis are doing a lot of the chemistry and also taking the steps necessary to validate some of the biological results that we see." *

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