Based on successful animal studies, a novel vaccine that uses immune cells as factories to produce Her2/neu protein may offer a way to treat some human breast cancers, say researchers at The University of Texas M. D. Anderson Cancer Center.
Their study, published in the online journal, Breast Cancer Research, on Nov. 29, 2004, showed that the vaccine protected 86 percent of experimental mice against HER2/neu-associated breast cancer, even though the tumors were implanted directly into mice.
"This is an exciting strategy that seems to elicit a complete immunologic response against HER2/neu," says the study's lead author, Lawrence Lachman, Ph.D., a professor in the Department of Experimental Therapeutics. "Now that we have gone as far as we can in animal studies, I hope this agent might be tested clinically both to treat HER2/neu breast tumors, and to prevent them from spreading."
Up to one-third of human breast cancers are associated with over-expression of the HER2/neu cell surface receptor protein, which continuously "tells" the cancer cell to grow, this producing an aggressive disease that is difficult to stop.
The drug Herceptin treats this kind of breast cancer by "plugging" these receptors with a monoclonal antibody, but this treatment "produces only a passive and transitory immune reaction," Lachman said. "This vaccine, however, like many that treat microbial infections, appears to create a memory in the immune system that produces a lasting protective response." The vaccine used in this study is a "viral-vectored" approach, in which a naturally occurring virus is re-engineered to be propagation defective but still able to function as a delivery system for proteins from the cancer cells that are to be targeted by the animal's immune system.
The basis for this new vaccine is an RNA virus called VEE, for Venezuelan equine encephalitis, which in its natural form can cause disease in horses.
Researchers at AlphaVax, Inc., a privately held biotechnology company in Research Triangle Park, N.C., have deleted certain genes from the VEE virus, thereby making it impossible for the vector version of virus to reproduce itself, but still allowing the vector to express the target protein. For this study, the researchers inserted the gene for the HER2/neu protein in place of the deleted viral genes and then packaged the virus vector into virus-like replicon particles, referred to as "VRP-neu." When these non-propagating particles are injected into an animal, the vector directs the animal's cells to make thousands of copies of the HER2/neu growth protein, says Lachman.
In addition, the VRP-neu preferentially homes to specialized immune system cells known as dendritic cells, and infects them. Inside, VRP-neu produces high levels of the HER2/neu protein, which the dendritic cells then display on the outside of their cell surface. This is, in fact, a neat trick, says Lachman, because the function of dendritic cells is to flag the immune system by presenting "antigen," or pieces of a foreign invader, to activate an immune response. "So now, the same immune system cells that rev up the immune system are displaying HER2/neu proteins as antigens," Lachman says.
What results is a strong response against HER2/neu that incorporates all three arms of the immune system - B cell, T cell and natural killer cells - and which creates a long-lasting immune memory primed to attack breast cancer cells that are studded with HER2/neu proteins, he says.
In the experiments, three groups of seven mice were treated with three injections of a vaccine. The "control" arm received a VRP vaccine that contained an influenza protein, and the other two groups received one of two different doses of the VRP-neu vaccine. Researchers then implanted a HER2/neu-positive tumor into mammary tissue in the mice. All of the control mice developed breast cancer but only one mouse in each of the VRP-neu groups did. In short, 86 percent of mice that received the vaccine were protected against the cancer. The experiment was repeated numerous times with the same result, says Lachman.
Researchers also worked with mice that were genetically engineered to over-express HER2/neu. These mice normally develop breast cancer in all ten of their mammary glands by the time there are 120 days old. In this experiment, they vaccinated the mice three times with VRP-neu, and "not a single mouse developed a tumor," says Lachman.
Lachman suggests the vaccine might prove helpful in keeping human HER2/neu tumors from metastasizing, or spreading, but adds that this notion must be tested in human clinical trials.
Lachman believes that the vaccine will address two additional safety considerations "because it is built of RNA that cannot insert itself into the genome, and the possibility of autoimmunity to it appears to be low, because normal cells express very small amounts of the HER2/neu growth protein."
Contributors to the study are first author Xiaoyan Wang, M.D., Ph.D. and Jian-Ping Wang, M.D., Ph.D., both from M. D. Anderson; and Maureen Maughan, Ph.D., from AlphaVax, Inc.