With advancing age, the mammalian thymus undergoes involution, a progressive loss of architectural integrity and lymphoid cellularity that results in reduced T lymphopoiesis. Thymic involution also is associated with extreme malnutrition and states of immune deficiency, such as active HIV infection, after chemotherapy, or during pregnancy. Immune recovery appears to require restoration of normal thymopoiesis. Although several means are known to increase thymic cellularity in the aged, including systemic administration of hormones, androgen ablation, and thymic tissue transplantation, each suffers from specific limitations that prevent widespread application. This paper presents a novel approach to rejuvenate T cell differentiation in the aged that employs intrathymic implantation of engineered stromal cells. Two different proteins have been examined for their impact on thymopoiesis after delivery by somatic cell implantation. Intrathymic injection of IL-7-producing stromal cells enhances the earliest specification steps of T cell development, resulting in the increased representation of pro-T cells in the aged thymus. In contrast, increasing the intrathymic levels of sonic hedgehog diminishes this aspect of T cell poiesis.