| NK T cells are a lymphocyte lineage that is selected by CD1d and characterized by the rapid secretion of potent immune regulatory cytokines, such as IFN-γ and IL-4. The signaling mechanisms controlling NK T cell development are poorly characterized. Here, we define the signaling requirements for the Src family kinase members Fyn and Lck as well as the Tec kinase family member Itk, during NK T cell ontogeny. Fyn mutant mice contain a block in NK T cell development, with a five to ten fold decrease in the number of cells. In contrast, conventional T cell development occurs normally. Studies using bone marrow chimeras indicate that the defect behaves in a cell autonomous manner. The few NK T cells present in fyn null mice were analyzed to determine whether Fyn is also required during peripheral NK T cell activation. The fyn−/− NK T cells can respond to ligand and produce cytokines, but have depressed proliferative capacity. Transgenic expression of the NK T cell specific TCR α chain, Vα14Jα18, leads to a complete restoration of NK T cell numbers in fyn−/− mice, suggesting that Fyn may have a role prior to α chain rearrangement. NK T cells are thought to develop from a progenitor expressing the Vα14Jα18 TCR α chain, but which are negative for NK cell markers. Fyn deficient mice have decreased numbers of NK1.1− NK T cell progenitors as well as mature NK1.1+ cells, confirming that Fyn is required early during NK T cell ontogeny. The earliest NK T cell progenitor populations (NK1.1−) could proliferate to IL-7, while mature (NK1.1+) NK T cells did not. The NK1.1 − NK T cell progenitors were capable of upregulating NK1.1 when transferred in vivo. Upon stimulation, the NK1.1− populations secrete IL-4, but little IFN-γ. As the cells mature and upregulate NK1.1, they acquire the ability to secrete IFN-γ. Finally, the Tec family tyrosine kinase Itk is necessary for optimal NK1.1 upregulation and hence final maturation of NK T cells. Itk mutant mice also display a progressive decrease in NK T cells in older mice, suggesting a possible further role in peripheral maintenance. |
