STAT3- and STAT5-dependent pathways competitively regulate the pan-differentiation of CD34pos cells into tumor-competent dendritic cells.

The clinical outcomes of dendritic cell (DC)-based immunotherapy remain disappointing, with DCs often displaying a tenuous capacity to complete maturation and DC1 polarization in the tumor host. Surprisingly, we observed that the capacity for successful DC1 polarization, including robust IL12p70 production, could be regulated by STAT-dependent events even prior to DC differentiation. Exposure of CD34(pos) cells to single-agent granulocyte-macrophage colony-stimulating factor (GMCSF) induced multilineage, STAT5-dependent differentiation, including DCs that failed to mature in the absence of further exogenous signals. In contrast, Flt3L induced nearly global differentiation of CD34(pos) cells into spontaneously maturing DCs. IL-6 synergized with Flt3L to produce explosive, STAT3-dependent proliferation of phenotypically undifferentiated cells that nevertheless functioned as committed DC1 precursors. Such precursors not only resisted many tumor-associated immunosuppressants, but also responded to tumor contact or TGFbeta with facilitated DC maturation and IL12p70 production, and displayed a superior capacity to reverse tumor-induced T-cell tolerance. GMCSF preempted Flt3L or Flt3L plus IL-6 licensing by blocking STAT3 activation and promoting STAT5-dependent differentiation. Paradoxically, following overt DC differentiation, STAT5 enhanced whereas STAT3 inhibited DC1 polarization. Therefore, nonoverlapping, sequential activation of STAT3 and STAT5, achievable by sequenced exposure to Flt3L plus IL-6, then GMCSF, selects for multilog expansion, programming, and DC1 polarization of tumor-competent DCs from CD34(pos) cells.