Thymocyte proliferation and differentiation in human granulocyte-macrophage colony-stimulating factor receptor transgenic mice

Thymocytes display varying responses to cytokines depending on their stage of differentiation. Whether these responses are due to stage-specific cytokine receptor expression or to down-stream signaling mechanisms is unknown. We examined the relationship between receptor expression and thymocyte proliferation or differentiation by using thymocytes from transgenic mice that constitutively expressed the human granulocyte-macrophage colony stimulating factor (hGM-CSF) receptor. Transgenic CD4^-CD8^-, CD4⁺CD8^-, and CD4^-CD8⁺ thymocyte populations expressing the hGM-CSF receptor proliferated when cultured with hGM-CSF, whereas CD4⁺CD8⁺ cells failed to proliferate despite expressing this receptor. We next examined the effect of hGM-CSF receptor signaling on thymocyte differentiation in fetal thymic organ culture supporting a full program of T cell development in vitro. Addition of hGM-CSF to the transgenic fetal thymic organ culture resulted in failure of CD4^-CD8^- cells to differentiate into CD4⁺CD8⁺ cells. To investigate this maturational inhibition more closely, we repopulated wild-type fetal lobes with sorted pro-T, pre-T or post pre-T precursor cells from hGM-CSF receptor transgenic mice. In these cultures hGM-CSF blocked both pro-T and pre-T cell differentiation, whereas the more mature post pre-T cells differentiated normally. These results suggest that hGM-CSF receptor signaling during thymocyte differentiation causes stage-specific inhibition of precursor cell maturation. In addition, repopulation studies of transgenic fetal lobes with sorted wild-type thymocyte precursors indicated that hGM-CSF inhibited proliferation and differentiation of the wild-type precursors, suggesting a secondary effect via transgenic stromal cells.