"Trojan Horse" Virus Controls HIV Infection
National Institute of Allergy and Infectious Diseases (NIAID) grantees at Yale University have converted a common livestock virus into a Trojan horse that selectively targets HIV-infected cells and then destroys them. As reported Sept. 5 in the journal Cell, this strategy effectively controlled HIV infection in laboratory-grown T cells and dramatically reduced infectious HIV to levels that were barely or no longer detectable.
"This is a completely new approach, targeting a virus to an infected cell," explains the study’s senior scientist, John K. Rose, Ph.D., from Yale’s Departments of Pathology and Cell Biology. "The concept could be used to develop a whole new class of agents that are useful for controlling disease."
"Although additional in vitro and animal studies need to be performed before this novel virus can be tested in humans," comments NIAID Director Anthony S. Fauci, M.D., "this concept of cell-targeted delivery has enormous potential applications for HIV, cancer or other diseases."
In their report, Dr. Rose, Matthias J. Schnell, Ph.D., and their colleagues describe how they modified the vesicular stomatitis virus (VSV) genome, deleting its envelope gene and replacing it with the genes for a pair of cell surface receptor -- CD4 and the coreceptor CXCR4 -- normally found on human T cells. These receptors enable HIV to attach to, enter and infect T cells.
These receptors also permit cell-to-cell HIV infection to occur. HIV-infected cells flag themselves for destruction by the body’s immune system by displaying HIV’s outer coat protein. But this protein, HIV gp120, is the same one that attaches to the T-cell receptors and leads to infection. Cell-to-cell infection occurs when the HIV gp120 on an infected cell first hitches up to the receptors on an uninfected T cell, resulting in the fusion of the cell and viral membranes, and transfer of virus from the infected to the uninfected cell.
Turning around what occurs naturally, the remodeled shell of VSV -- which now looks like an uninfected T cell -- tricks HIV-infected cells into fusing with it instead. This enables VSV, which easily kills cells, to gain entry into the HIV-infected cell and destroy it. The modified VSV cannot infect normal cells because it lacks its normal surface protein. Thus, it targets, enters, multiplies in and kills only T cells that, through the display of HIV gp120, signal that they are infected.
In their experiments, the Yale team infected human T cell lines with a laboratory strain of HIV. To these cells lines -- in which about half of the cells were now HIV-infected -- they added the novel VSV at either three or five days postinfection. This ultimately slashed infectious HIV to extremely low or undetectable levels, at least 300-fold to 10 thousand-fold lower than the levels of HIV produced in control cells.
"Until there are data from animal models," Dr. Rose cautions, "we cannot gauge how well the potential treatment might work in people." But he regards it as "likely to be safe," and would like to see the concept tested in human clinical trials as soon as possible. Such discussions are already under way, but Dr. Rose estimates the possibility is at least a year away and that trials in animal models are a necessary first step.
The report says the novel VSV described would be most appropriate for limiting HIV production in people with late-stage disease, but the Yale team has moved on to develop VSV constructs that incorporate other HIV coreceptors such as CCR5 and CCR3 in an attempt to affect HIV strains that target macrophages and typically predominate in early HIV infection.
The virus involved, VSV, causes vesicular stomatitis, a disease mainly of cattle, horses and pigs that causes blister-like bumps on the hoofs and tongue. Nearly all animals recover completely from the illness.
Occasionally, people become infected with VSV through close contact with infected livestock or via laboratory exposure. Many people with VSV have no symptoms, and those who become ill usually have a mild, limited flu-like disease. No human deaths linked to VSV infection have been reported.
The modified VSV is defective because it no longer has its normal coat protein. Therefore, it can not enter normal cells and cause infection in livestock or humans.
In their paper, the authors note several positive features of their system. For example, levels of the novel VSV would be expected to decline as HIV declines, since the VSV only targets and multiples in HIV-infected cells. Moreover, resistance to the novel VSV would not be expected to develop, because that "would require loss of HIV’s ability to bind CD4 or [the] coreceptor and would therefore not be selected," the authors write.
Nava Sarver, Ph.D., chief of the targeted interventions branch in NIAID’s Division of AIDS says, "This is a very exciting advance. We are getting closer to solving one of the major problems in targeted delivery of genes to specific cells for treatment and possibly disease prevention: specifically, how to deliver what you want to the cell you want it to go to."
Currently, most gene delivery is done by removing certain cells from the body, modifying and then growing more of them, and, finally, reintroducing them back into the body. "This is a very labor-intensive, time-consuming task," says Dr. Sarver. In addition, there’s a tremendous dilution effect because the amount of cells reintroduced is very small.
She adds, "The Yale group has crossed a major hurdle that may allow direct, in vivo delivery of a vector that can find its destined target in the body. There should be no need for ex-vivo manipulation of cells." She envisions many potential applications of this research. Surface-modified live vectors like VSV could be used to shuttle into the body viruses or toxins to destroy infected or cancerous cells, or therapeutic genes to protect uninfected cells against an invading virus. Moreover, such vectors could be used as novel vaccines to deliver antigenic genes to antigen-presenting cells, such as dendritic cells, for mounting immune protection against an invading pathogen such as HIV.
Reference: Schnell MJ, Johnson JE, Buonocore L and Rose JK. Construction of a novel virus that targets HIV-1-infected cells and controls HIV-1 infection. Cell 1997;90(5):849-857.
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