Murine Natural Killer Cell Cloning from Fetal Thymic Organ Cultures

The relationship of natural killer (NK) cells to T cells has been a longstanding enigma. It is well established that development of NK cells does not require a thymus (because they are present in athymic mice), recombination activating genes (because they are present in RAG-1 and RAG-2 knockout mice) or DNA-dependent protein kinase (because they are present in severe combined immunodeficiency [SCID] mice) (1 ,2 ). NK cells do not express CD3 or TCR on their surface and they are classically defined as CD3–CD56⁺ (in humans) or CD3–NK1.1⁺ (in mice) (3 –5 ). However, there has been some suggestion that NK cells might share a common progenitor with T cells. This has been derived from the observations that some NK cells contain a truncated mRNA for TCR-β (4 ), that activated NK contain cytoplasmic CD3ε and that fetal NK cells also contain cytoplasmic CD3ε (3 –7 ). Several studies demonstrated that fetal thymi contain progenitors that might develop into T cells or NK cells depending on whether they mature within a thymic or extrathymic microenvironment, respectively (7 ). The discovery of a novel lymphocyte subset that expresses markers for both T cells and NK cells, the so-called T/NK lymphocytes, raised further questions about the ontogenic relationship of NK to T cells (8 ,9 ). More recently, Sanchez et al. (7 ) showed that human fetal thymi contain a bipotential progenitor that could develop along either of the T or NK maturation pathways. This led us and others to determine whether NK cells can be demonstrated and cloned from fetal thymi (10 ,11 ). Surprisingly, we found that NK1.1 is among the earliest lymphohematopoietic genes to be transcribed as its mRNA is demonstrable in d 9 fetuses (exact organ distribution not yet determined) and in the earliest of thymic anlages (d 11 of gestation) (11 ). Although T/NK cells can be demonstrated in fetal thymic organ cultures (FTOC) established from day 14 fetal thy mi, the majority of NK1.1⁺ cells obtained from fetal thymi appear to be the classical, CD3– NK1.1⁺ cells (10 ,11 ).