Introducing genes encoding major antigens found on tumor cells into attenuated viruses results in the attenuated virus acting as a vector, replicating within cells whilst expressing cancer antigens on the cells. Infection by a recombinant virus allows co-representation of the tumor antigen with host histocompatibilty molecules (MHC molecules) to generate a cell-mediated immune response. Common vectors used in recombinant gene therapy have been vaccinia virus and canarypox virus.
Another approach involves targeting tumor antigens encoded by viral oncogenes. Human Papiloma Virus (HPV) is a virus associated with certain forms of cervical and anogenital cancers. Vaccinia virus expressing E6 and E7 proteins of HPV have induced tumor specific immune responses in both animal and human models. They are presently in clinical trial phase.
A third approach involves recombinant vaccinia virus encoding B7 genes. When tested with murine cervical cells, the virus prevented cancer cell outgrowth upon transplantation to immunocompetant hosts.