Well-bred cells: poor hosts to viruses.

Well-bred cells: Poor hosts to viruses

Using genetic engineering, scientists have for the first time modified the DNA of mammalian cells to inactivate viral replication within the cells.

The experimental technique, reported this week, dramatically interrupts herpesvirus replication within cultured mouse cells, but has much greater practical potential against other viruses, such as the AIDS-causing HIV, say these and other researchers. Similar experiments with HIV may be completed soon in other laboratories, says Steven L. McKnight, who performed the herpesvirus research with Steven J. Triezenberg and Alan D. Friedman at the Carnegie Institution of Washington in Baltimore.

The novel defense strategy, dubbed "cellular immunization" by molecular biologist and Nobel laureate David Baltimore, represents a new avenue of antiviral research in which scientists program host cells to produce mutant proteins that specifically interfere with viral reproductive machinery.

"Will intracellular immunization really work [against AIDS]? I see no theoretical barriers, only practical questions," Baltimore says in a commentary accompanying the research results in the Sept. 29 NATURE. "I believe intracellular immunization has as good a chance as any other procedure of becoming a real AIDS therapy."

Normally, when a cell becomes infected by a herpesvirus, a viral protein called VP16 enters the host cell along with viral DNA. Once inside the cell, VP16 binds to certain host cell proteins, forming an "activating" complex that can bind to and "turn on" viral genes. These genes in turn regulate transcription--the first stage of viral replication.

McKnight and his colleagues reported this summer that it is possible to disable this self-starting mechanism by chopping off a particular portion of the virus' VP16 protein. In the new research, they inserted into cultured mouse cells a gene that codes for the production of such a truncated version of VP16, causing the cells to build up a supply of the functionally crippled viral protein. When these engineered host cells became infected with herpesviruses, their vast supply of bogus VP16 outcompeted the virus' own VP16 for the limited number of binding sites on the viral DNA, thus blocking viral replication.

Despite success in the laboratory, Triezenberg says, "I don't think this is going to have much applicability for the herpes simplex infection. That virus infects skin cells, and there's no way we'll be able to put a truncated version of VP16 into everybody's skin cells." Rather, he says, the technique is suited to viruses that infect a population of cells whose progenitors are easily located and engineered -- such as HIV.

"You could take bone marrow cells that are not yet infected with the HIV ... and introduce into them some altered version of an HIV activator gene, then reinject those back into the patient so they set up shop back in the bone marrow," says Triezenberg, now at Michigan State University in East Lansing. "Those cells, if all works well, might resist HIV infection."

Carl O. Pabo, of the Johns Hopkins School of Medicine in Baltimore, is one of several researchers experimenting with the primary activator gene in HIV -- called tat -- which is in some ways equivalent to the VP16 gene. He says scientists have identified regions of the tat protein that appear critical to its proper functioning, and in theory it is possible to engineer human bone marrow cells to produce appropriately crippled versions of that HIV protein. However, he and David Baltimore note, substantial obstacles -- including possibilities of toxicity--must be overcome before genetic therapies of this nature become feasible.