A recombinant endogenous retrovirus amplified in a mouse neuroblastoma is involved in tumor growth in vivo

The theory of immunoediting postulates that tumor cells exhibit a reduced immunogenicity to escape eradication by the host immune system. It has been proposed that endogenous retroviruses-provided that they are active-could play a role in this process, via the immunosuppressive domain carried by their envelope protein. Here, we demonstrate that the Neuro-2a tumor cell line-originating from a spontaneous A/J mouse neuroblastoma-produces an infectious retrovirus that most probably results from a recombination event between 2 mouse endogenous retroviral elements. This Neuro-2a-associated recombinant retrovirus derives from the unique ecotropic provirus located at the Emv-1 locus, but with a gag sequence conferring B-tropism, thus allowing its high-level amplification in Neuro-2a cells. We show that knocking down -by RNA interference- this endogenous retrovirus in Neuro-2a cells has no effect on the transformed phenotype of the cells, but results in delayed tumor growth and prolonged animal survival, following engraftment of the cells into immunocompetent mice. Recombination between endogenous retroviruses, amplification of the resulting element and high-level expression of its immunosuppressive activity are therefore likely steps of an immunoediting process, leading to an invading tumor.