AIDSWEEKLY Plus, 18 March 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

High doses of a very effective, live attenuated HIV prototype vaccine can kill newborn monkeys, but normal doses are safe, new studies show.

The vaccine, SIV[mac239] delta 3, is a live simian immunodeficiency virus (SIV) that has been genetically altered so that it is missing three vital genes: vpr, nef, and the gene encoding the Nef-responsive element (NRE).

The vaccine was developed at Harvard University Medical School and in 1993 was licensed to Therion Biologics Corp., which is developing the vaccine in cooperation with Harvard and TSI Mason Laboratories, TSI Corporation, Worcester, Massachusetts.

A single dose of SIV[mac239] delta 3 safely protects adult monkeys from challenge with lethal strains of SIV. But researchers exploring the effects of the vaccine on mother-to- infant SIV transmission found that very high doses of the vaccine caused lethal SIV infection of neonates.

This finding cast serious doubt on whether live HIV vaccines could ever be made safe enough for human use.

But new findings suggest that this fatal infection does not happen when the neonates receive normal doses of the vaccine.

"It appears that this phenomenon occurs only under the extreme condition of high virus inoculum and lack of maternal antibody," reported Michael Wyand, president and chief scientific officer of TSI Mason Laboratories.

Mason announced his findings at the Eighth Annual Meeting of the National Cooperative Vaccine Development Groups for AIDS, held February 11-15, 1996, in Bethesda, Maryland.

Wyand noted that earlier studies with SIV[mac239] delta 3 showed that:

* The vaccine does not cause AIDS or other disease.

* SIV[mac239] delta 3 establishes a persistent infection characterized by rising titers of SIV neutralizing antibodies.

* Virus load after infection with SIV[mac239] delta 3 is lower than that seen following infection with wild-type SIV[mac239].

* SIV[mac239] delta 3 protects animals against intravenous challenge with lethal doses of wild-type virus. Only partial protection is seen 8 and 20 weeks after vaccination; complete protection occurs 79 weeks after vaccination.

A study presented at a plenary session of the 1994 Nineteenth AIDS Clinical Trial Group (ACTG) Meeting raised serious safety concerns about SIV[mac239] delta 3.

Harvard researcher Ruth Ruprecht and colleagues explored whether the SIV[mac239] delta 3 vaccine could elicit mucosal immunity, as most HIV infection occurs across mucosal surfaces. Ruprecht et al. hypothesized that intrapartum mother-to-infant SIV/HIV transmission occurs when the baby's oral mucosa are exposed to the mother's blood and fluids. They reasoned that by administering the vaccine directly into the infant's mouth, they might stimulate protective mucosal immunity.

In their initial experiment, a newborn monkey was given an oral dose of SIV[mac239] delta 3 immediately after delivery by cesarean section. To ensure infectivity, Ruprecht et al. used 300 times the adult IV infectious dose. The vaccine virus established an infection not in the tonsils, as anticipated, but in the ilium and in local regional lymph nodes.

Unexpectedly, the baby monkey failed to clear the vaccine virus and quickly lost antibodies to the SIV Gag protein. It soon developed a lethal hemolytic anemia.

Virus isolated from the infant animal showed no reversion to wild type: the virus was somehow pathogenic despite retaining its attenuated genotype.

To see whether the vaccine virus had somehow acquired a new pathogenic phenotype, Ruprecht et al. orally administered 2.5 ml of the infant's blood (containing 1.7 adult IV infectious doses of the virus, far less than the original inoculum) to another newborn monkey and, as a control, to its mother. The infant developed pathogenic SIV[mac239] delta 3 infection and died of AIDS while its mother developed an intense antibody response to the viral Gag protein and cleared the virus.

Analysis of factors that could explain these phenomena turned up the fact that both mothers in these experiments - unbeknownst to the researchers - had been infected with simian T-cell leukemia virus type 1 (STLV-1). Even though the infants were free of STLV infection (as demonstrated both by serology and PCR assay), the experiments were repeated with other mother/infant pairs free of STLV. Again the babies developed pathogenic SIV[mac239] delta 3 infection while the mothers cleared the vaccine virus without detrimental effect.

In her ACTG presentation, Ruprecht noted that there are other cases in which neonates and adults exhibit differential susceptibility to retroviral infection. She also noted that mice protected from RLV infection by a live, pharmacologically attenuated vaccine develop pathogenic infection when co- infected with hepatitis virus.

"What would happen if a person infected with attenuated virus were later infected with hepatitis or HTLV?" she asked. "Vaccine virus must be attenuated for pathogenicity, not for replication mechanics. Because virus deleted in nef, vpr, and NRE have retained their pathogenic potential, their use as live vaccines is dangerous."

Although Ruprecht admitted that she and her colleagues had no explanation for why the attenuated virus was pathogenic upon oral administration to neonates, she suggested that HIV and SIV cause immune deficiency only when viremia expands beyond a certain level.

"I believe that in every individual there is a threshold of viral replication that must be reached before disease becomes apparent," she said. "If this hypothesis is true, then anything you do to help keep the viremia below this level could act as an attenuated-virus vaccine - even if you do it with drugs."

At the 1996 NCVDG, Wyand reported partial confirmation of the Ruprecht et al. findings. Three neonatal macaques delivered by cesarian section were given oral doses of SIV[mac239] delta 3 that were 500 times higher (1600 mg) than the protective vaccine dose (5 mg).

One of the three animals developed fatal infection.

Wyand et al. then divided nine C-section-delivered macaques into three groups. Within four hours of delivery, each received oral SIV[mac239] delta 3 at doses of either 283 mg, 50 mg, or 5 mg.

All three of the animals that received the high dose became infected. One died at six months, and all had low CD4(+) T-cell counts.

But none of the animals in the medium- or low-dose groups developed the persistent high-level viral loads seen in the high-dose animals. In fact, the dynamics of SIV[mac239] delta 3 infection of the medium- and low-dose animals resembled that seen in adult animals.

Wyand et al. then explored whether maternal infection with SIV[mac239] delta 3 affected neonates.

They gave four pregnant monkeys intravenous injections of 11.3 mg of SIV[mac239] delta 3 at 80 to 100 days' gestation. When the baby monkeys were born, each received an oral dose of 283 mg of the vaccine.

One of the four monkeys died at birth of causes apparently unrelated to vaccine administration. None of the neonates was infected with SIV[mac239] delta 3 at birth.

All three living neonates became infected after oral challenge, but none developed the high viral load characteristic of fatal infection.

Wyand made the following conclusions:

* SIV[mac239] delta 3 virus load is dependent on the size of the inoculum.

* The phenomenon of fatal infection is limited to neonates and is not seen in juvenile or adult monkeys.

* Infection with SIV[mac239] delta 3 during gestation does not transmit the vaccine virus to the fetus in utero.

* Vaccination of mothers does not protect neonates against vaccination/infection with the vaccine strain.

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.