AIDSWEEKLY Plus, 13 May 1996 issue; 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
Daniel J. DeNoon, Senior Editor

A human monoclonal antibody and a CD4/IgG chimera potently neutralize HIV in plasma from late-stage AIDS patients.

The findings show that both molecules may potentially be used to treat and even to prevent HIV disease.

"These studies suggest that IgG1b12 and CD4-IgG2 have broad and potent neutralizing activity in both in vitro and ex vivo neutralization assays and should be considered for use as potential immunoprophylactic or therapeutic agents," wrote Aaron Diamond AIDS Research Center researcher Marie-Claire Gauduin and colleagues.

Gauduin et al. reported their findings in the Journal of Virology ("Effective Ex Vivo Neutralization of Human Immunodeficiency Virus Type 1 in Plasma by Recombinant Immunoglobulin Molecules," J Virol, 1996;70(4):2586-92).

The monoclonal antibody is IgG1b12 (also known as G1b12). It was produced from a combinatorial phage display library and recognizes the HIV-1 gp120 envelope glycoprotein.

The CD4/IgG chimeric molecule, CD4-IgG2, is a tetrameric human antibody created by replacing each of the IgG heavy- and light-chain variable regions with the first two domains of the human CD4 molecule (the T-cell receptor to which HIV binds). The tetramer therefore contains two chains of a CD4/human IgG2 heavy chain fusion protein and two chains of a CD4/human k light chain fusion protein, giving it four gp120 binding sites to enhance its avidity for HIV virions or HIV infected cells.

In the last decade, in vitro studies showed that recombinant soluble CD4 (rsCD4) potently neutralized HIV. But clinical trials failed to show any effect for the molecule. Subsequent studies revealed that standard neutralization studies failed to predict how rsCD4 would act against primary HIV isolates in vivo.

Because a number of monoclonal antibodies (mAbs) and have been developed that potently neutralize HIV in vitro, Gauduin et al. decided to test two representative anti-HIV mAbs (IgG1b12 and 19b, a V3-specific antibody that recognizes a segment of gp120) and CD4-IgG2 against sCD4 in both in vitro and ex vivo studies.

In their first study the researchers performed standard in vitro neutralization studies of IgG1b12, CD4-IgG2, and sCD4 against four different HIV-1 isolates:

* LAI, an HIV-1 laboratory isolate adapted to grow in transformed T-cell lines;

* JR-CSF, a molecularly cloned isolate similar to primary HIV-1 isolates that does not grow in transformed T-cell lines;

* AD6 and WH91-330, two primary HIV-1 isolates passaged twice in peripheral blood mononuclear cells (PBMC), neither of which grows in transformed T-cell lines.

As expected, sCD4 neutralized LAI and JR-CSF. It was moderately effective against AD6 but did not neutralize WH91- 330.

While all three molecules tested were equally effective against LAI and JR-CSF, the primary isolates were far more sensitive to IgG1b12 and CD4-IgG2 (although still not as sensitive as LAI and JR-CSF).

The researchers then tested sCD4, 19b, IgG1b12, and CD4- IgG2 against plasma samples drawn from six patients with late- stage HIV disease. All plasma samples contained HIV-1 titers of at least 250 TCID[50]/ml.

Based on the detection limit of their assay, neutralization was defined as a >5-fold decrease in virus titer (as measured by p24) in the plasma sample.

"Both IgG1b12 and CD4-IgG2 neutralized HIV-1 in five of six plasma samples," Gauduin et al. reported. "The degree of neutralization ranged from a 25- to a 625-fold reduction in the original infectious titer."

sCD4 was unable to neutralize any measurable HIV-1 in any of the plasma samples, and 19B neutralized virus in only two of the six samples.

"IgG1b12 and CD4-IgG2 appear to be more effective in neutralizing plasma HIV-1 isolates in ex vivo neutralization assays than are sCD4 and 19b," Gauduin et al. concluded. "It is also important to note that viruses within one of the six plasma samples were resistant to neutralization by IgG1b12 but sensitive to CD4-IgG2, while viruses within another plasma sample were resistant to CD4-IgG2 but sensitive to IgG1b12."

Further study of IgG1b12 and CD4-IgG2 against seven different plasma samples containing HIV-1 showed that the two molecules are effective at concentrations between 1 and 25 (micro)g/ml.

"No correlation between the neutralization sensitivities of IgG1b12 and CD4-IgG2 in ex-vivo assays and those in in- vitro neutralization assays performed on P1 isolates from those plasma samples was observed," the researchers found.

Gauduin et al. offered several explanations why the in vitro and ex vivo neutralization results differed:

* The process of expanding a primary isolate from plasma may select for minor variants within the original sample.

* Ex vivo assays are performed in the presence of plasma, and hence in the presence of prebound antibodies that may compete for viral epitopes.

* As plasma samples are not heated, they contain active complement. The ex vivo assays may then measure antibody- dependent complement-mediated neutralization in addition to direct neutralization.

* As the primary isolates are expanded in PHA-activated PBMC for the in vitro assay, high-level expression of adhesion and class II molecules probably are induced.

"The heterogeneity of gp120 among HIV-1 isolates and the lack of sensitivity of primary isolates of HIV-1 to neutralization have been major obstacles to vaccine development and the use of antibody-based therapeutic or prophylactic strategies," Gauduin et al. wrote.

"The breadth and potency of CD4-IgG2 and IgG1b12 in neutralizing HIV-1 directly from plasma make them good candidates for future studies of HIV-1 prophylaxis in animal studies and in human trials."

Such advanced studies are already in the works.

At the December 1995 Twentieth ACTG meeting, Diane Wara of the University of California, San Francisco, and George McSherry of the University of Medicine and Dentistry, New Jersey, discussed plans to conduct clinical trials to determine whether mAbs can be part of a strategy to prevent HIV disease - or even infection - in the children of HIV(+) women.

"Clearly we are going to have to combine passive antibodies plus vaccine if we are going to prevent [mother-to- infant] transmission," Wara said. "This is where the future lies: to give vaccines within two weeks of birth combined with appropriate HIVIG [the anti-HIV hyperimmune serum manufactured by North American Biologicals] spiked with monoclonal antibodies."

To test the feasibility of this concept, Wara and co- workers propose to infuse newborns with escalating concentrations of anti-HIV mAbs.

"There are three requirements for the use of monoclonal antibodies: that they exquisitely neutralize HIV, that they are available, and that they have a long enough half-life," Wara said.

The corresponding author for Gauduin et al. study is Richard A. Koup, The Aaron Diamond AIDS Research Center, 455 First Ave., 7th Floor, New York, New York 10016. Phone: (212) 725- 0018. Fax: (212) 725-1126. Email: koup@adarc.nyu.edu.

Copyright (c) 1995 - 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.

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Copyright © 1996 - 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.