Developmental ages of the thymic epithelium and of the T cell precursors together determine the proportions of peripheral CD4+ cells.

In earlier studies on chimeric animals, we found that fetal thymocytes produced peripheral T lymphocyte populations depleted of CD4+ cells. This occurred whether the fetal thymocytes matured in the presence of adult or fetal thymic stromal cells. In contrast, fetal liver cells that differentiated in the adult thymus generated normal proportions of peripheral CD4+ cells. Because fetal liver cells are thought to be the immediate precursors to fetal thymocytes, we proposed that fetal thymic stroma would modulate the differentiation of fetal liver cells; specifically, that fetal liver cells maturing in the fetal thymus would resemble fetal thymocytes and produce low levels of peripheral CD4+ cells. To test this hypothesis, fetal thymic lobes were colonized in vitro with fetal liver cells and subsequently transplanted in vivo to Thy-1 congenic hosts. Under these conditions, fetal liver cells produced reduced proportions of CD4+ peripheral progeny. The under-representation of CD4+ peripheral T cells was apparently governed by the thymic epithelium because similar results were obtained with 2-deoxyguanosine-treated fetal thymuses colonized by fetal liver cells. In contrast, adult bone marrow cells made normal levels of CD4+ peripheral T cells whether maturation occurred in the fetal or the adult thymus. Thus, pre-T cells (fetal liver or adult bone marrow) lose the capacity to respond to fetal thymic stromal cells during development. These results indicate that the proportions of CD4+ cells in peripheral tissues are regulated by a combination of the developmental ages of the T cell precursors and the thymic stromal environment.