Establishment of cell lines latently infected with non-oncogenic murine gammaherpesvirus 76

Murine gammaherpesviruses 68 (MHV-68) and 78 (MHV-78), both inducing tumors in mice and a latent infection in cells in vitro, serve as models for study of human oncogenic gammaherpesviruses, namely Epstein-Barr virus (EBV) and Kaposi's sarcoma-associated herpesvirus (KSHV). In this work, we succeeded in establishing a latent infection of HeLa and CGL1 cell lines with non-oncogenic murine gammaherpesvirus 76 (MHV-76), which differs from MHV-68 and MHV-78 besides by oncogenicity also by deletion of M1-M4 genes and eight tRNA-like sequences. Viral latency in these cell lines, λHeLa and λCGL1, was demonstrated by the presence of viral DNA, suppression of viral latency-associated ORF73 gene and appearance of low amounts of infectious virus following treatment with phorbol 12-myristate 13-acetate (PMA). Both latently infected cell lines showed irregular presence of viral antigen originating apparently from spontaneous reactivation. The growth of latently infected cells in culture was similar to that of non-infected ones. The latently-infected λHeLa cells did not induce tumors in mice following subcutaneous inoculation. These results (i) confirm that MHV-76 is the only non-oncogenic murine gammaherpesvirus of all the so far tested ones, (ii) suggest that some of the genes deleted in MHV-76 might be responsible for the oncogenicity of murine gammaherpesviruses, (iii) confirm that viral ORF73 is one of major latency-associated genes that is suppressed during virus reactivation, and (iv) present MHV-76 as another murine gammaherpesvirus useful as a model for study of gammaherpesvirus pathogenesis, oncogenicity, latency and reactivation.