To improve the safety of experimental AIDS vaccines based on live but weakened HIV, scientists at the National Institute of Allergy and Infectious Diseases (NIAID) have made a prototype of such a vaccine drug-susceptible so it can be eliminated from the body soon after use.
We know how to attenuate HIV, but now we've also given it an off-switch that may make live-attenuated HIV vaccines even safer," comments Stephen M. Smith, M.D., an infectious diseases specialist from NIAID's Laboratory of Molecular Microbiology (LMM).
An article in the July 23, 1996 issue of the Proceedings of the National Academy of Sciences describes the research to date on developing this novel strategy. Lead author Dr. Smith and LMM retrovirologist Kuan-The Jeang, M.D., Ph.D., conceived of the idea and performed all the experiments except the tests in mice, which were carried out by Richard B. Markham, M.D., at Johns Hopkins University School of Hygiene and Public Health.
Most currently licensed vaccines against viral diseases such as polio, measles and mumps consist of live viruses that have been disabled just enough so they cannot cause disease but can still replicate and thereby stimulate protective immunity.
But many scientists think that such a vaccine based on HIV, which causes a fatal disease, may be too hazardous. Unlike other viruses, HIV, a retrovirus, multiplies by permanently inserting its genetic material into that of its hosts' cells. This raises concerns that even if the weakened HIV vaccine cannot itself cause disease, it could lead to long-term, unanticipated side effects.
To overcome this problem, Drs. Smith and Jeang added to an attenuated HIV a foreign gene that makes thymidine kinase (TK). TK is a protein found in herpes simplex virus 1 (HSV-1), the cause of cold sores. Cells that produce TK can be selectively killed by the antiviral drug ganciclovir.
The attenuated HIV to which they added TK had been stripped of its nef gene because animal studies by other scientists indicate that this deletion might result in an effective live-virus vaccine. Monkeys vaccinated with a nef-deleted form of simian immunodeficiency virus (SIV), the monkey equivalent of HIV, have been completely protected against subsequent challenge with pathogenic SIV.
In theory, TK's susceptibility to ganciclovir allows scientists to effectively eliminate the HIV-TK vaccine from the body after it has stimulated protective immunity but before it might mutate and cause problems. Ganciclovir can not only eliminate the production of free HIV-TK, but also HIV-TK provirus integrated in the genome.
To define the time period during which ganciclovir might inhibit virus replication, the NIAID team added the drug to HIV-TK-infected cells at different time points. The HIV-TK grew well without drug but did not grow if the cells were pretreated with ganciclovir two hours before infection or treated up to six hours after infection. "These findings suggest there is a window of time during which ganciclovir treatment could be withheld without compromising efficacy," Dr. Smith states. When given 48 hours after infection, ganciclovir markedly reduced but did not eradicate the virus below detectable levels.
Concerned about the emergence of drug-resistant mutants, the NIAID scientists also tested whether drug combinations might help maintain drug susceptibility. Although ganciclovir is the only drug that specifically targets TK, their experiments suggest that a multidrug regimen that includes ganciclovir may be a better choice if their concept advances to human trials. Ganciclovir plus zidovudine (AZT) completely inhibited HIV-TK growth in cell cultures for up to 60 days when AZT therapy was started two hours before infection followed by ganciclovir therapy at 48 hours after infection.
Finally, they extended their experiments to the hu-PBL-SCID mouse model, a strain of mice that suffer severe immunodeficiency and into which have been transplanted human peripheral blood lymphocytes, which can be infected with HIV. Dr. Markham infected eight mice with HIV-TK and seven mice with a wild strain of HIV. Approximately half of each group received ganciclovir treatment and the other half did not. The four HIV-TK-infected animals given ganciclovir twice a day for 10 days sh owed no sign of virus up to 21 days after infection.
Critics contend that the HIV-TK concept is intriguing but may be impractical because HIV-TK will kick out non-essential genes like TK as it multiples. Well aware of this problem, Dr. Smith is currently focused on devising a way to make HIV require the addition of TK so it doesn't reject the gene.
The experiments described in the new paper are based on a concept proposed in a letter to Science in November 1995 by Harry W. Kestler, Ph.D., of the Cleveland Clinic Foundation, and Dr. Jeang. They suggested adding to an already attenuated HIV a gene(s) that confers beneficial functions. In theory, depending on what new gene is added, this approach can be used to create safer or more immunogenic vaccines.
NIAID and the National Institutes of Health (NIH) Office of AIDS Research supported this research. NIAID, a component of NIH, supports research on immunology, allergy and AIDS, tuberculosis and other infectious diseases. NIH is an agency of the U.S. Public Health Service, U.S. Department of Health and Human Services.