From Antiviral Agents Bulletin, September 1996, p. 233
Drug-Eradicated Live HIV Vaccines
A number of researchers are investigating the use of live attenuated HIV for use as prophylactic vaccines. One approach was reported by Dr. S.M. Smith (National Inst. of Allergy and Infectious Diseases; NIAID; Bethesda, MD) and collaborators in the July 23 issue of the Proceedings of the National Academy of Sciences describing genetically modified HIV capable of being effectively eradicated using drug therapy. Non-pathogenic live attenuated virus vaccines capable of low-level in vivo replication and induction of cellular and humoral immune responses are currently available for polio, measles and mumps. However, there is no precedent for a live lentivirus or retrovirus vaccine. Many researchers assert that this approach is too dangerous, particularly if latent or residual virus somehow mutates and becomes pathogenic. To overcome this problem, the NIAID researchers added a gene encoding herpes simplex type 1 (HSV-1) thymidine kinase (TK) to an already attenuated HIV with the nef gene deleted. TK expression in cells enables metabolism (monophosphorylation) of TK-activated drugs, such as ganciclovir, to active forms resulting in cell death upon exposure to these drugs. Addition of the TK gene also provides a selectable marker for transformed cells, useful for research purposes.
This TK expression approach to eliminating transformed cells has already been successfully tested in various gene therapy trials using retroviral vectors for TK gene delivery. This approach may enable elimination of live attenuated HIV vaccines from the body after induction of immunity, alleviating much of the concern that prophylactic live HIV vaccines may mutate and become pathogenic in vivo. A nef-deleted simian immunodeficiency virus (SIV) vaccine has demonstrated prophylactic efficacy in monkeys (as reported in the February 1993 Bulletin, p. 38), and Therion Biologics Corp. is developing candidate vaccines based on this approach (as reported in the May 1993 Bulletin, p. 138). Along the same lines, various studies suggest that infection with one strain of HIV may confer protection against subsequent infection with another strain. For example, a study in high-risk African prostitutes infected with HIV-2, found these women to have a 70% lower rate of HIV-1 infection compared to a control cohort.
Theoretically, insertion of the TK gene into HIV should effectively enable elimination of the HIV-TK vaccine after it has stimulated protective immunity but before it might mutate to a pathogenic form. Theoretically, ganciclovir can eliminate both free HIV-TK virions and integrated HIV-TK provirus. To test their approach and define the time during which ganciclovir might inhibit HIV-TK replication, the NIAID researchers added ganciclovir at various times to HIV-TK infected cells. HIV-TK replicated well without ganciclovir exposure but did not replicate and was below the detection limit (6 copies/million cells by one method) if cells were pretreated with ganciclovir two hours before infection or treated up to six hours after infection. The investigators note, ÒThese findings suggest there is a window of time during which ganciclovir treatment could be withheld without compromising [vaccine] efficacy.Ó Ganciclovir treatment of cells 48 hours after infection reduced virus levels by over six logs.
Due to concerns about the emergence of ganciclovir-resistant HIV-TK mutants, various drug combinations were also tested. ÒGanciclovir plus 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.Ó In tests run by Dr. R.B. Markham, Johns Hopkins Univ. School of Hygiene and Public Health, in hu-PBL-SCID mice (immune deficient mice engrafted with human peripheral blood lymphocytes), no virus was detectable for up to 21 days after infection among four HIV-TK infected mice given ganciclovir twice daily for ten days. The investigators note, ÒSomewhat reassuringly, even under robust tissue culture replication conditions, HIV-TK when doubly selected with AZT and GCV [ganciclovir] produced no escape mutants nor did we find evidence for TK escapes from infections in treated hu-PBL-SCID mice,Ó and Òone distinct advantage of GCV-HSV-TK [ganciclovir HIV-TK] treatment is the elimination of integrated proviruses, a goal not easily achieved with other antiretrovirals.Ó Work on this approach is continuing and currently focusing on devising ways to make HIV require TK to prevent it from deleting TK as a nonessential gene as it replicates.