The size of the Natural Killer (NK) cell pool is maintained through production and subsequently export from the bone marrow, peripheral survival and proliferation, and ultimately cell death. While there exists considerable knowledge about developmental stages of lymphoid, myeloid and erythroid cells, comparably little is known about NK cell intermediates and the genes required for their development. Most of the models of NK cell differentiation have been based on in vitro culture systems where NK cells could be generated from multipotent HSC precursors. However, this approach suffers from the problems inherent to in vitro cell manipulations. We have utilized conditional gene targeting in adult mice to examine the role of the bcl-x gene in the development of NK cells in bone marrow and after their export to the spleen. Bcl-x is an important member of the anti-apoptotic member of the Bcl-2 gene family, and a critical role for this gene in the survival of hematopoietic cells was demonstrated in bcl-x-deficient mice causing embryonic death due to massive apoptosis of immature hematopoietic cells and of neurons. Conditional deletion in erythroid cells lead to hemolytic anemia and extensive splenomegaly. Furthermore, bcl-x is critical for the maturation of pre-B cells to the pro B cell stage, while it is not essential for the development of effector and memory T cells. We have conditionally deleted the gene in the stem cells of adult mice by cross breeding them with the Mxi-cre deleter strain, which allows for induced expression of cre recombinase by injection with pIpC.
As early as 9 days after the first injection of pIpC, the number of NK cells in the bone marrow of mice started to decline as demonstrated by multi-color FACS analysis staining for IL2 Receptor beta (CD122) and NKG2D, among other markers. Cultures of bone marrow and spleen cells in the presence of cytokines to generate lymphokine activated killer cells failed, while no such effect was observed in cultures from Mxi-cre mice that were subjected to pIpC injections and carried along as controls. Analysis of animals after 3 weeks of pIpC administration revealed absence of NK cell precursors in the bone marrow as demonstrated by the lack of CD122+/Lin- negative cells. This phenomenon was accompanied by a reduction in the number of mature NK cells in the spleen.
To date, six stages of NK cell maturation are described with the acquisition of IL2 receptor beta expression marking commitment to the NK-cell lineage. IL2 Receptor beta as well as NKG2D are expressed throughout NK cell development and at all stages. In order to characterize the specific stage at which expression of bcl-x is essential for NK cell maturation, we employed multi-color FACS analysis staining for CD122, NKG2D, CD49b, and the integrins CD43 and Mac-1 after depletion of lineage positive cells, followed by sorting for defined populations. Real Time PCR on sorted cells demonstrated that Bcl-x mRNA is highly expressed throughout all stages of NK cell development. Taken together, the gradual reduction in the number of NK cell precursors eventually leading to complete loss of this lineage in the bone marrow and peripheral sites suggests that bcl-x is indispensable for the development of NK cells presumably from the earliest time point of commitment to this lineage.