GAP-JUNCTIONS PROMOTE THE BYSTANDER EFFECT OF HERPES-SIMPLEX VIRUS THYMIDINE KINASE IN-VIVO

Transfer of the herpes simplex thymidine kinase gene (HSVtk) into tumo r cells followed by the administration of ganciclovir (GCV) provides a potential strategy for the treatment of some malignancies, During GCV treatment, not only the cells that express the HSVtk gene are killed but also frequently neighboring tumor cells that are not genetically a ltered, This has been called the ''bystander effect.'' Although the me chanism of the bystander effect in vivo remains elusive, our results s uggest that gap junction formation between neighboring cells is an imp ortant contributing factor. The C6 rat glioma cell line, which exhibit s a low level of intercellular communication by gap junctions and conn exin43 (Cx43)-transfected clones of this cell line forming gap junctio ns from a moderate level (Cx43-12 and Cx43-14) to a high level (Cx43-1 3), were transduced with HSVtk. Transduced and nontransduced cells wer e mixed in various concentrations and then cultured in vitro or inject ed s.c. into C.B-17/SCID-beige mice follow-ed by i.p. injections of GC V. Cx43-transfected clones showed a significant increase of the bystan der effect compared with the less coupled C6 parental cell line. In 11 of 12 mice injected with cells of Cx43-transfected clones, no tumors were seen at the inoculation site when a mixture of 50% HSVtk-negative and HSVtk-positive cells was used, Moreover, in mice injected with ce lls of clone Cx43-13, which exhibits the highest intercellular communi cation, tumors mere frequently undetectable at the inoculation site wh en using mixtures of 75% HSVtk-negative and 25% HSVtk-positive cells, and even mixtures containing 5% HSVtk-positive cells of Cx43-transfect ed clones showed tumor size reduction, All animals in control groups ( n = 26) developed large tumors at every injection site, These results demonstrate that gap junctions are an important component in mediating the bystander effect in vivo.