The relationship between the adaptive CD4+ T cell response and human cancer is unclear. The oncofetal antigen 5T4 is expressed on many human carcinomas, including colorectal cancer (CRC) cells, but has limited expression on normal tissues. We previously identified anti-5T4 CD4+ T cells in a proportion of CRC patients, and we extended this study to examine whether the quality or quantity of the T cell response reflects tumor stage. An overlapping peptide library spanning 5T4 was used as a target to enumerate cognate IFN-γ+CD4+ T-cells (measured as spot forming cells [SFC]/105 cultured T cells) in peripheral blood-derived lymphocytes following a 12-day in vitro culture period comparing patients pre-operatively (n = 27) to healthy controls (n = 17). Robust 5T4-specific T cell responses were present in 100% of healthy donors. There was a steady loss of T cell responses with advancing tumors with a significant negative correlation from stage I to III (P = 0.008). The predictability of the decline meant < 200 SFC/105 was only found in subjects with stage III CRC. The mechanism of loss of T cell response is independent of HLA-DR type or patient age, but does correspond to increases in Foxp3+ regulatory T cells (Tregs). Using low-dose cyclophosphamide to reduce the proportion of Tregs in vivo resulted in increased anti-5T4 T cell responses in CRC patients. The selective loss of 5T4-specific IFN-γ+CD4+ T cell responses implies a link between tumor stage and antitumor Th1 effector function; depleting Tregs can enhance such responses.
Keywords: 5T4; CD4+ T cells; colorectal cancer; epitopes; regulatory T cells.