Tumor Arrests DN2 to DN3 Pro T Cell Transition and Promotes Its Conversion to Thymic Dendritic Cells by Reciprocally Regulating Notch1 and Ikaros Signaling.

Tumor progression in the host leads to severe impairment of intrathymic T-cell differentiation/maturation, leading to the paralysis of cellular anti-tumor immunity. Such suppression manifests the erosion of CD4(+)CD8(+)double-positive (DP) immature thymocytes and a gradual increase in CD4(-)CD8(-)double negative (DN) early T-cell progenitors. The impact of such changes on the T-cell progenitor pool in the context of cancer remains poorly investigated. Here, we show that tumor progression blocks the transition of Lin(-)Thy1.2(+)CD25(+)CD44(+)c-Kit(low)DN2b to Lin(-)Thy1.2(+)CD25(+)CD44(-)c-Kit(-)DN3 in T-cell maturation, instead leading to DN2-T-cell differentiation into dendritic cells (DC). We observed that thymic IL-10 expression is upregulated, particularly at cortico-medullary junctions (CMJ), under conditions of progressive disease, resulting in the termination of IL-10R(high)DN2-T-cell maturation due to dysregulated expression of Notch1 and its target, CCR7 (thus restricting these cells to the CMJ). Intrathymic differentiation of T-cell precursors in IL-10(-/-)mice andin vitrofetal thymic organ cultures revealed that IL-10 promotes the interaction between thymic stromal cells and Notch1(low)DN2-T cells, thus facilitating these DN2-T cells to differentiate toward CD45(+)CD11c(+)MHC-II(+)thymic DCs as a consequence of activating the Ikaros/IRF8 signaling axis. We conclude that a novel function of thymically-expressed IL-10 in the tumor-bearing host diverts T-cell differentiation toward a DC pathway, thus limiting the protective adaptive immune repertoire.