AIDSWEEKLY Plus; Monday, January 25, 1999
Daniel J. DeNoon, Senior Editor

Like a movie vampire, HIV exposes its fangs only when it is just about to bite into a victim cell. What if you killed the virus at this crucial moment of virus/cell fusion, wondered a University of Montana research team. Could you then obtain a whole-cell vaccine containing occult viral antigens?

This wildly hopeful strategy appears to have worked.

"In a transgenic mouse immunization model, these formaldehyde- fixed whole-cell vaccines elicited antibodies capable of neutralizing infectivity of 23 of 24 primary HIV isolates from diverse geographic locations and genetic clades A to E," wrote Rachel A. LaCasse, Jack H. Nunberg, and colleagues. "Development of these fusion-dependent immunogens may lead to a broadly effective HIV vaccine."

LaCasse et al. reported their findings in the journal Science ("Fusion-Competent Vaccines: Broad Neutralization of Primary Isolates of HIV," Science, 1999;283:357-62).

The finding completely changes current thinking about AIDS vaccines. No previous HIV vaccine - including those currently in Phase III clinical trials - has ever shown such broad neutralizing activity. Thus it has long been accepted as inevitable that any vaccine capable of eliciting neutralizing antibodies must include antigens from all relevant HIV strains.

"We now show that an appropriately presented clade B envelope protein can elicit potent neutralization against most primary-isolate viruses from multiple HIV clades," LaCasse et al. wrote. "We show that not only is primary-isolate virus neutralization achievable, we suggest that broad vaccine protection may not require an unlimited number of HIV serotypes."

Studies of people with chronic HIV infection show that disease progression can occur despite the presence of naturally occurring neutralizing antibodies. Thus vaccines based on the LaCasse et al. findings are unlikely to have therapeutic applications. But even though there is no precedent with HIV, preexisting neutralizing antibodies to other viral diseases are sufficient to prevent initial infection. Thus the new findings are a huge leap forward in the race to find a vaccine that can stem the AIDS pandemic.

LaCasse et al. began their studies with the observation that antibodies from HIV infected individuals are capable of neutralizing the infectivity of primary-isolate HIV strains, although such neutralization is incomplete (low-level neutralization of some 30 to 50 percent of primary isolates). The researchers hypothesized that these neutralizing antibodies recognized antigens on the viral envelope that are exposed only when the envelope changes shape during the process of fusion with a target cell.

To test this hypothesis, the researchers hit on an elegantly simple strategy: they would kill the virus and the cell at the very moment that fusion occurs.

For their fusion-competent envelope they obtained functional, molecularly cloned HIV-1 envelope protein from a primary clade B isolate (ACH168.10) with the syncytium-inducing (SI) phenotype. This envelope protein (168P) uses both the CCR5 and CXCR4 coreceptors. Transfected COS-7 cells were used to express the protein.

For target cells they used human U87 glioma cells expressing the CD4 and CCR5 receptors (U87-CD4-CCR5). The envelope-transfected COS-7 cells were then cocultured with the U87-CD4-CCR5 cells.

Five hours after the envelope-bearing cells and the receptor- bearing cells were put together - as the HIV envelope and receptor ligands were just beginning to make the cells clump together into the giant, multinucleated cells known as syncytia - LaCasse et al. fixed the cultures in 0.2 percent formaldehyde. The resulting whole-cell preparation, dubbed fusion-competent immunogen, then was used as a vaccine.

At this point, however, the researchers had to face a potentially confounding factor. Earlier this decade a whole-cell HIV vaccine was touted as a breakthrough when it elicited broadly neutralizing antibodies in monkeys. This report had to be retracted when it was found that the vaccine component that elicited neutralization was the human cells used in the vaccine preparation (Montefiore et al., Nature, 1991;354:439).

LaCasse et al. therefore tested their fusion-competent immunogen in transgenic mice that express human CD4 and CCR5 receptors, making them immunologically tolerant to the human components of the vaccine. The mice were inoculated either with the immunogen or with cell-based controls.

None of the cell-control-immunized mice developed antibodies capable of HIV neutralization. But astonishingly broad neutralization was seen in mice that received the immunogen.

"Fusion-competent sera elicited by a functioning clade B envelope protein were able to neutralize 23 of 24 primary-isolate viruses tested - monocytotropic (non-SI; NSI) and T lymphocytotropic (SI viruses) from North America and Europe (clade B), Africa (clades A and D), Thailand (clades B and E), and India (clade C)," LaCasse et al. wrote. "Despite the sequence diversity among these isolates, most were similarly sensitive to neutralization by fusion-competent vaccine sera."

But was this neutralization specific to HIV envelope? Sera from immunogen-vaccinated mice were tested against pseudotyped HIV virions bearing murine leukemia virus (MuLV) envelope and against pathogenic simian immunodeficiency virus (SIV). There was no neutralization.

"[This] crucial control experiment ... implies that the effective antibodies were binding to structures conserved among HIV-1 envelope glycoproteins but not shared by SIV and MuLV," wrote David C. Montefiori of Duke University and John P. Moore of Aaron Diamond AIDS Research Center in an accompanying commentary ("Magic of the Occult?" Science, 1999;283:337-8).

LaCasse et al. warned that their experiments did not entirely rule out the possibility that human cellular determinants - which would not be effective in a human vaccine - affected the broad-based neutralization they described.

"Absolute exclusion of fusion-induced cellular targets, and the ultimate definition of specific envelope protein determinants, must await the analysis of primary-isolate virus-neutralizing MAbs [monoclonal antibodies] to fusion-competent immunogens," they wrote.

Development of such MAbs will be crucial because the immunogen as currently made is not practical for human vaccine production. Even if the cost and complexity of creating the immunogen on a large scale were overcome, the use of killed tumor cells in the immunogen raises complex safety issues. Identification of the crucial fusion-antigen complex could lead to its ultimate use in vaccines.

"Recombinant viral vectors that respectively express envelope and CD4 with coreceptor could be coadministered to drive critical fusion events in vivo," LaCasse et al. suggested. "Alternatively, purified fusion-active complexes could be developed as an inactivated subunit vaccine."

Moreover, the technique might be applicable to other diseases in which viral envelopes conceal relevant epitopes.

Crucial experiments remain to be performed, as Montefiori and Moore observed.

"Any residual concerns will no doubt be alleviated if and when these experiments are repeated in larger animals - such as macaques," they suggested. "Whether the antibody response to these immunogens is strong enough to confer protection from infection can also be addressed in macaques by virus challenge experiments."

This work was supported by the American Foundation for AIDS Research (AmFAR), Concerned Parents for AIDS Research (CPFA), the University of Montana, the NIH, and the M.J. Murdock Charitable Trust.

The corresponding author for the LaCasse et al. study is Jack H. Nunberg, The Montana Biotechnology Center and Division of Biological Sciences, University of Montana, Missoula, Montana 59812. Email: <nunberg@selway.umt.edu>. The authors of the commentary article are: David C. Montefiori, Center for AIDS Research, Department of Surgery, Duke University Medical Center, Durham, North Carolina 27710. Email: <monte005@mc.duke.edu>. John P. Moore, Aaron Diamond AIDS Research Center, Rockefeller University, New York, NY 10021. Email: <jmoore@adarc.org>.

990125
AW990110

Copyright © 1999 - Charles Henderson, Publisher. All rights Reserved. Permission to reproduce granted to AEGIS by Charles W. Henderson. Authorization to reproduce for personal use granted granted by C. W. Henderson, Publisher, provided that the fee of US$4.50 per copy, per page is paid directly to the Copyright Clearance Center, 27 Congress Street, Salem, Massachusetts 01970, USA. Published by Charles Henderson, Publisher. Editorial & Publishing Office: P.O. Box 5528, Atlanta, GA 30307-0528 / Telephone: (800) 633-4931; Subscription Office: P.O. Box 830409, Birmingham, AL 35283-0409 / FAX: (205) 995-1588 http://www.newsrx.net

Copyright ©1990, 2000. AEGiS & the Sisters of Saint Elizabeth of Hungary. All materials appearing on AEGiS are protected by copyright as a collective work or compilation under U.S. copyright and other laws and are the property of ÆGIS, or the party credited as the provider of the content.