Abstract
INTRODUCTION: TGF-β is an important target for many cancer therapies under development. In addition to suppressing anti-tumor immunity, it has pleiotropic direct pro- and anti- tumor effects. The actions of increased endogenous TGF-β production remain unclear, and may affect the outcomes of anti-TGF-β cancer therapy. We hypothesize that tumor-derived TGF-β (td-TGF-β) plays an important role in maintaining tumor remission by controlling tumor proliferation in vivo, and that decreasing td-TGF-β in the tumor microenvironment will result in tumor progression. The aim of this study was to examine the effect of TGF-β in the tumor microenvironment on the balance between its anti-proliferative and immunosuppressive effects. METHODS: A murine BALB/c spontaneous colon adenocarcinoma cell line (CT26) was genetically engineered to produce increased active TGF-β (CT26-TGF-β), a dominant-negative soluble TGF-β receptor (CT26-TGF-β-R), or the empty neomycin cassette as control (CT26-neo). In vitro proliferation rates were measured. For in vivo studies, the three cell lines were injected into syngeneic BALB/c mice, and tumor growth was measured over time. Immunodeficient BALB/c nude mice were used to investigate the role of T and B cells. RESULTS: In vitro, CT26-TGF-β-R and CT26-TGF-β cells showed increased and suppressed proliferation, respectively, compared to control (CT26-neo), confirming TGF-β has direct anti-tumor effects. In vivo, we found that CT26-TGF-β-R cells displayed slower growth compared to control, likely secondary to reduced suppression of anti-tumor immunity, as this effect was ablated in immunodeficient BALB/c nude mice. However, CT26-TGF-β cells (excess TGF-β) exhibited rapid early growth compared to control, but later failed to progress. The same pattern was shown in immunodeficient BALB/c nude mice, suggesting the effect on tumor growth is direct, with minimal immune system involvement. There was minimal effect on systemic antitumor immunity as determined by peripheral antigen-specific splenocyte type 1 cytokine production and tumor growth rate of CT26-neo on the contralateral flank of the same mice. CONCLUSION: Although TGF-β has opposing effects on tumor growth, this study showed that excessive td-TGF-β in the tumor microenvironment renders the tumor non-proliferative. Depleting excess td-TGF-β may release this endogenous tumor suppressive mechanism, thus triggering the progression of the tumor. Therefore, our findings support cautions against using anti-TGF-β strategies in treating cancer, as this may tip the balance of anti-immunity vs. anti-tumor effects of TGF-β, leading to tumor progression instead of remission.