Whether thymic stromal lymphopoietin (TSLP) directly induces potent human CD4(+) T cell proliferation and Th2 differentiation is unknown. We report that resting and activated CD4(+) T cells expressed high levels of IL-7 receptor a chain but very low levels of TSLP receptor (TSLPR) when compared with levels expressed in myeloid dendritic cells (mDCs). This was confirmed by immunohistology and flow cytometry analyses showing that only a subset of mDCs, with more activated phenotypes, expressed TSLPR in human tonsils in vivo. IL-7 induced strong STAT1, -3, and -5 activation and promoted the proliferation of naive CD4(+) T cells in the presence of anti-CD3 and anti-CD28 monoclonal antibodies, whereas TSLP induced weak STAT5 activation, associated with marginally improved cell survival and proliferation, but failed to induce cell expansion and Th2 differentiation. The effect of TSLP on enhancing strong human T cell proliferation was observed only when sorted naive CD4(+) T cells were cultured with mDCs at levels as low as 0.5%. TSLP could only induce naive CD4(+) T cells to differentiate into Th2 cells in the presence of allogeneic mDCs. These results demonstrate that IL-7 and TSLP use different mechanisms to regulate human CD4(+) T cell homeostasis.