The lab of Avinash Bhandoola, Ph.D., professor of Pathology and Laboratory Medicine, has studied the origins of T cells for many years. One protein called Notch, which has well-known roles in the development of multiple tissues, plays an essential role in triggering T-cell development. T cells are immune cells that are made in the thymus, a small organ situated under the breastbone near the heart. However, T cells, like all blood-cell types, originate from blood-producing stem cells in the bone marrow. Immature T-cell progenitors leave the bone marrow, settle within the thymus, and eventually give rise to T cells. With graduate student Maria Elena De Obaldia, Dr. Bhandoola describes, in a November 3, 2013 online article in Nature Immunology, how Notch signaling induces expression of genes that promote the maturation of T cells and discourage alternative cell fates. Deficiency of the Notch target gene Hes1 in blood stem cells results in extremely low T-cell numbers, but the underlying mechanism is unknown. Keeping in mind that Notch signaling gone awry induces leukemia, Dr. De Obaldia notes that "understanding the Notch pathway on a molecular level can shed light on how normal cells are transformed in the context of cancer." The current study describes the mechanism of action of Hes1, a repressor protein that acts in the nucleus of immature T cells in the thymus. Dr. De Obaldia and Ms. Bhandoola found that Hes1 turns off genes such as C/EBPalpha, which promote the myeloid-cell fate and antagonize the T-cell fate. Whereas Hes1-deficient mice show severe T-cell defects, deleting the myeloid gene C/EBPalpha could restore normal T-cell development. This provided evidence that Hes1 keeps immature T cells on track by preventing them from defaulting to a myeloid developmental pathway, which controls non-lymphocyte cell maturation.
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