Abstract
Many tumors down-regulate major histocompatibility complex (MHC) antigen expression to evade host immune surveillance. However, there are very few in vivo models to study MHC antigen expression during tumor spontaneous regression. In addition, the roles of transforming growth factor betal (TGF-beta1), interferon gamma (IFN-gamma), and interleukin (IL)-6 in modulating MHC antigen expression are ill understood. We previously reported that tumor infiltrating lymphocyte (TIL)-derived IL-6 inhibits TGF-beta1 and restores natural killing (NK) activity. Using an in vivo canine-transmissible venereal tumor (CTVT) tumor model, we presently assessed IL-6 and TGF-beta involvement associated with the MHC antigen expression that is commonly suppressed in cancers. IL-6, IFN-gamma, and TGF-beta1, closely interacted with each other and modulated MHC antigen expression. In the presence of tumor-derived TGF-beta1, host IFN-gamma from TIL was not active and, therefore, there was low expression of MHC antigen during tumor progression. TGF-beta1-neutralizing antibody restored IFN-gamma-activated MHC antigen expression on tumor cells. The addition of exogenous IL-6 that has potent anti-TGF-beta1 activity restored IFN-gamma activity and promoted MHC antigen expression. IFN-gamma and IL-6 in combination acted synergistically to enhance the expression of MHC antigen. Thus, the three cytokines, IL-6, TGF-beta1, and IFN-gamma, closely interacted to modulate the MHC antigen expression. Furthermore, transcription factors, including STAT-1, STAT-3, IRF-1, NF-kappaB, and CREB, were significantly elevated after IL-6 and IFN-gamma treatment. We conclude that the host IL-6 derived from TIL works in combination with host IFN-gamma to enhance MHC molecule expression formerly inhibited by TGF-beta1, driving the tumor toward regression. It is suggested that the treatment of cancer cells that constitutively secrete TGF-beta1 should incorporate anti-TGF-beta activity. The findings in this in vivo tumor regression model have potential applications in cancer immunotherapy.