The timing of TGF-[beta] inhibition affects the generation of antigen-specific CD8+ T Cells

Transforming growth factor (TGF)-[beta] is a potent immunosuppressive cytokine necessary for cancer growth. Animal and human studies have shown that pharmacologic inhibition of TGF-[beta] slows the growth rate of established tumors and occasionally eradicates them altogether. We observed, paradoxically, that inhibiting TGF-[beta] before exposing animals to tumor cells increases tumor growth kinetics. We hypothesized that TGF-[beta] is necessary for the anti-tumor effects of cytotoxic CD8.sup.+ T lymphocytes (CTLs) during the early stages of tumor initiation. BALB/c mice were pretreated with a blocking soluble TGF-[beta] receptor (sTGF-[beta]R, TGF-[beta]-blockade group, n=20) or IgG2a (Control group, n=20) before tumor inoculation. Tumor size was followed for 6 weeks. In vivo lymphocyte assays and depletion experiments were then performed to investigate the immunological basis of our results. Lastly, animals were pretreated with either sTGF-[beta]R (n=6) or IgG2a (n=6) prior to immunization with an adenoviral vector encoding the human papillomavirus E7 gene (Ad.E7). One week later, flow cytometry was utilized to measure the number of splenic E7-specific CD8.sup.+ T cells. Inhibition of TGF-[beta] before the injection of tumor cells resulted in significantly larger average tumor volumes on days 11, 17, 22, 26 and 32 post tumor-inoculation (p < 0.05). This effect was due to the inhibition of CTLs, as it was not present in mice with severe combined immunodeficiency (SCID) or those depleted of CD8.sup.+ T cells. Furthermore, pretreatment with sTGF-[beta]R inhibited tumor-specific CTL activity in a Winn Assay. Tumors grew to a much larger size when mixed with CD8.sup.+ T cells from mice pretreated with sTGF-[beta]R than when mixed with CD8.sup.+ T cells from mice in the control group: 96 mm.sup.3 vs. 22.5 mm.sup.3, respectively (p < 0.05). In addition, fewer CD8.sup.+ T cells were generated in Ad.E7-immunized mice pretreated with sTGF-[beta]R than in mice from the control group: 0.6% total CD8.sup.+ T cells vs. 1.9%, respectively (p < 0.05). These studies provide the first in vivo evidence that TGF-[beta] may be necessary for anti-tumor immune responses in certain cancers. This finding has important implications for our understanding of anti-tumor immune responses, the role of TGF-[beta] in the immune system, and the future development of TGF-[beta] inhibiting drugs.