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In this study we investigated the impact of several factors on the expansion of natural regulatory T (nTreg) cells by tumours, including antigen specificity, transforming growth factor-β (TGF-β) signalling and the antigen-presenting cell subsets responsible for expansion. We found that antigen non-specific expansion of nTreg cells is tumour cell line-dependent. Although both antigen-specific and non-specific pathways can contribute to expansion, the migration of activated nTreg cells to tumour tissues is strictly antigen-dependent. Intact TGF-β signalling on nTreg cells is also essential for tumour-induced expansion. Finally, for stimulation of resting antigen-specific CD4 T cells, CD11c cells purified from tumour-draining lymph nodes were more potent than CD11b cells, suggesting that dendritic cells are the key antigen-presenting cell subset involved in cross-presentation of tumour antigens. This study not only provides an in vivo system in which cross-talk between nTreg cells and tumours can be explored but also reveals novel aspects of tumour immune evasion. [ABSTRACT FROM AUTHOR]