Stem cells could serve as a renewable source of transplantable tissue-specific cells for the treatment of immune disorders. However, the excitement surrounding this potential has been curbed by the absence of a simple in vitro system that supports the differentiation of stem cells into specific lymphoid lineages. Now, Zúñiga-Pflücker and colleagues bring us one step closer to realizing this potential and describe an in vitro co-culture system to induce the differentiation of embryonic stem cells (ESCs) into functional T cells.

The Notch signalling pathway has a crucial role in determining whether lymphocyte progenitors become T or B cells. The authors observed that co-culturing ESCs on the stromal cell line OP9 led to the generation of B cells. However, when co-cultured on OP9 cells expressing the Notch ligand Delta-like 1 (OP9-DL1), the ESCs were induced to differentiate into T cells not B cells. These cells follow the normal programme of T-cell differentiation as seen in vivo, and by day 22 the cultures contained CD8 single-positive T cells. The cultures, however, did not support the development of CD4+ T cells, probably because MHC class II molecules are not expressed by OP9 cells. CD8+ T cells generated on OP9-DL1 cells had a diverse repertoire of expressed T-cell receptors, similar to ex vivo thymocytes, and underwent a coordinated expression programme of lineage-specific genes that are known to be important in T-cell development. In addition, these T cells could proliferate and produce interferon-γ in response to in vitro activation.

The authors next asked whether T cells generated on OP9-DL1 cells were functional in vivo and could reconstitute immunodeficient mice. To approach this, they isolated double-negative T-cell progenitors after 12 days in culture with OP9-DL1 cells and placed them in a lymphocyte-depleted fetal thymic organ culture, which supported the development of both CD4+ and CD8+ T cells. The in vitro-reconstituted thymic lobes were then implanted into immunodeficient mice. They found that the lymphoid organs from these mice were fully reconstituted with the donor T cells, enabling the mice to mount an effective immune response to viral infection.

Armed with this new technique, T-cell biologists are closer to realizing the therapeutic potential of ESCs.