Identification of the earliest natural killer cell committed progenitor in murine bone marrow.

Natural Killer (NK) cells are lymphocytes that provide a wide variety of protective functions against malignancies and viral infections. All lymphocytes, including, B cells, T cells, NK cells, and dendritic cells, are produced from Common Lymphocyte Progenitors (CLP). However, the developmental steps between CLP and the first NK committed cells remains unclear, hampering efforts to understand the mechanisms that regulate the development of this critical cell type. Using 12-color flow cytometry, we mapped the developmental stages between CLP and NK committed progenitors, and identified two major populations. One population is a subfraction of a previously identified progenitor called NKP. NKPs express a receptor for IL-15, a cytokine known to be critical for NK cell development. Our subfraction of NKP expresses several other surface markers, such as CD27, CD244, but does not express Flk2, a receptor expressed in CLP. Upon transplantation, this subfraction contains all the NK progenitor activity within the previously defined NKP population, suggesting that this fraction is the true NKP. We therefore nicknamed this population “rNKP”, for “refined NKP”. The second major population we identified, which we nicknamed “pre-NKP” appears to have a surface expression pattern that places it between CLP and rNKP. For example, it does not express the CLP marker Flk2, but also does not express the NKP-specific IL15 receptor. We show that in vitro, CLP generate NKP through a pre-NKP intermediate. Despite its similarity to CLP, pre-NKP produce only NK cells when transplanted in vivo, demonstrating that pre-NKP are a true NK committed progenitor. Also, because pre-NKP do not express the receptor for IL15, this suggests that IL15 signaling is not a prerequisite to NK commitment. We conclude that pre-NKP is the developmental intermediate that connects CLP to NKP, and thus is the earliest NK committed progenitor identified to date.