Malignant human papillomavirus type 18 (HPV18)-positive cervical carcinoma cells can be reverted to a nonmalignant phenotype by generation of somatic cell hybrids with normal human fibroblasts. Although nontumorigenic hybrids, their tumorigenic segregants, and the parental HeLa cells have similar in vitro properties, inoculation only of nontumorigenic cells into nude mice results in a selective suppression of HPV18 transcription which precedes cessation of cellular growth. Our present study, aimed at understanding the differential regulation in vitro and in vivo, shows that the JE gene, encoding the monocyte chemoattractant protein (MCP-1), is expressed only in nontumorigenic hybrids. Although the gene, including its regulatory region, is intact, no JE (MCP-1) mRNA is detected in the tumorigenic segregants and in other malignant HPV-positive cervical carcinoma cell lines. Tests of several monocyte-derived cytokines showed that only tumor necrosis factor alpha strongly induces the JE (MCP-1) gene in nontumorigenic cells and that this is accompanied by a dose-dependent reduction of HPV transcription. The JE (MCP-1) up-regulation occurs within 2 h and does not require de novo protein synthesis. The response to tumor necrosis factor alpha seems to be mediated by an NF-KB-related mechanism, since the induction can be completely abrogated by pretreating the cells with an antioxidant such as pyrrolidine dithiocarbamate. Interestingly, cocultivation of nonmalignant hybrids with monocyte-enriched fractions from human peripheral blood also results in an induction of the JE (MCP-1) gene and a concomitant suppression of HPV18 transcription. Neither effect is observed in malignant cells. These data suggest that JE (MCP-1) may play a pivotal role in the intercellular communication by triggering an intracellular pathway which negatively interferes with viral transcription in HPV-positive nontumorigenic cells.

To investigate the effect of tumor-associated macrophages on the in vivo growth properties of cervical carcinoma cells, tumorigenic human papilloma virus (HPV) 18-positive HeLa cells were transfected with an expression vector harboring the cDNA for the macrophage chemoattractant protein-1 JE (MCP-1). Although the endogenous gene is present and not structurally rearranged, its expression seems to be negatively affected by a still unknown mechanism. Inoculation of JE (MCP-1)-negative HeLa cells into nude mice led to rapidly growing tumors, where macrophage infiltration into the inner tumor mass was not detectable immunohistochemically. The activity that attracted mononuclear cells under both in vitro and in vivo condition was reconstituted in HeLa cells after transfection with the JE (MCP-1) expression vector. Heterotransplantation of those cells into immunocompromised animals resulted in significant growth retardation that was accompanied by a strong infiltration of macrophages. On the other hand, in vivo selection of nonmalignant hybrids made between wild-type HeLa cells and normal human fibroblasts in nude mice resulted in tumorigenic segregants 4 mo after inoculation into the animals. Monitoring JE (MCP-1) expression directly within those nodules, we found that transcription was either absent or only weakly detectable. Recultivation of JE (MCP-1)-positive tissue grafts under in vitro conditions revealed that the gene was only marginally inducible by tumor necrosis factor-alpha, a cytokine that normally induces a very strong activation of transcription in nontumorigenic cells. These findings suggest that functional JE (MCP-1) expression and in turn activated macrophages may play a pivotal role in controlling the proliferation rate of HPV-positive cells in vivo.