The adapted virus that immunized hundreds of millions of people against smallpox has now been enlisted in the war on cancer. Vaccinia poxvirus joins a herpesvirus and a host of other pathogens on a growing list of engineered viruses entering late-stage human testing against cancer.
After a decade of development of so-called oncolytic viruses, the newest strains hold the most promise yet, researchers say. This new generation of viruses has been genetically "targeted and armed," says Winald Gerritsen of the VU University Medical Center in Amsterdam, who is involved in an early human trial of an engineered adeno-associated virus that attacks glioblastoma, an aggressive form of brain cancer.
In a two-pronged attack, these viruses specifically target tumor cells while delivering a cargo of immune-boosting genes. In contrast, viruses that cause cancer, such as the human papillomavirus that is responsible for most cases of cervical cancer, disrupt a cell's genome, thereby triggering out-of-control growth.
When the engineered viruses recognize and infect cancer cells, they replicate and sometimes destroy their hosts. Several of the viruses also release the gene for granulocyte-macrophage colony-stimulating factor (GM-CSF) an immune system protein. The GM-CSF attracts a swarm of white blood cells and other immune system operatives that mount a further attack on the tumor.