The role of immunosuppressive regulatory T cells (Tregs) in cancer is well established, yet factors driving Treg accumulation are not well understood. In particular, it is unclear whether direct tumour recognition by T cells and Tregs is required. We have developed a mouse model in which to study CD4 T cell-mediated tumour rejection and/or escape in the absence of direct tumour antigen recognition.
Specifically, B16 (IE-deficient) subcutaneous tumours expressing membrane Hen Egg Lysozyme-Moth Cytochrome C (mHELMCC) as a model antigen were established in RAG-knockout mice. Transfer of naive IE-restricted MCC-specific 5C.C7 TCR-transgenic CD4 T cells efficiently controlled tumour growth in IE+RAG-/- hosts. T cell priming in tumour-draining lymph nodes (TDLNs) did not occur in IE-=>IE+ radiation chimeras, suggesting a requirement for radiosensitive bone marrow-derived IE-expressing cells. Ex vivo analysis of sorted APCs from TDLNs of IE+ mice revealed that CD11b+ and CD11b- subsets of migratory dendritic cells (DCs) could present tumour-derived antigen to 5C.C7 T cells, while Langerhans cells, CD103+ DCs, conventional DCs and naive B cells could not.
During tumour rejection, 5C.C7 cells producing IFNγ and TNFα accumulated in the tumour and TDLNs. Treatment of mice with IFNγ-blocking antibodies did not affect tumour rejection, suggesting that other mechanisms were responsible. Analysis of rejecting tumours revealed massive neutrophilic infiltration and a dramatic reduction in IE+ cells in the tumour bed, suggesting that they may have been killed by tumour-infiltrating 5C.C7 T cells, which were shown to express Granzyme B. Finally, late tumour escape occurred in some mice and correlated with systemic accumulation of 5C.C7 iTregs in all affected animals.
In summary, direct presentation of tumour antigen to CD4 T cells was not required for either tumour rejection or tumour escape via iTreg induction. We are currently investigating factors that drive iTreg development in cancer.