In Vivo Development of Murine NK Cells Early after Congeneic BMT: Effects of MHC Molecules, and the Administration of hIL-15.

Natural Killer (NK) cells are lymphocytes of the innate immune system that are able to kill a variety of tumors and pathogen-infected cells without prior sensitization. NK cells are the first lymphoid cells to repopulate following bone marrow transplantation (BMT). The present study shows that murine NK cells developing in vivo after BMT are phenotypically and functionally distinguishable from mature adult NK cells. Using a congeneic BMT mouse model (CD45.1/H-2^b → C57BL/6, CD45.2/H-2^b) we demonstrated here that donor derived BM-NK progenitors (CD122⁺NK1.1-CD3-)were present in the BM and periphery (spleen), at day 14 after BMT, and developed into NK cells expressing high levels of CD122, NKG2D, CD94, DX5 and CD43. However, surface expression of Ly49 inhibitory receptors was altered with Ly49C/I being under-expressed (9.8 %± 3.2 %), and Ly49G2 being highly expressed (62 %± 3.1 %) when compared to NK cells from non-transplanted adult mice (Ly49C/I, 43.1 %± 7.6 % and Ly49G2, 44.3 %± 13.5 %). At 28 days after BMT, there was a normalization with regard to Ly49 phenotype. The developing NK cells manifested functional deficits compared to non-transplanted adult NK, even when stimulated in vitro with Interleukin (IL)-2, as evidenced by decreased interferongamma (IFN-g) production and cytolytic activity. In order to examine the effects of major histocompatibility complex (MHC) molecules on NK development independently of allelic variation of Ly49s, we performed syngeneic BMT using B10D2 mice (H-2^d). We observed the same pattern of Ly49s expression during NK development demonstrating that the acquisition of these receptors early after BMT is not determined by the MHC haplotype. Because IL-15 plays a crucial role in NK cell development, we administrated human (h) IL-15, via hydrodynamic injection of the respective gene, at day 11 after BMT, to accelerate NK maturation. This method resulted in a high but transient production of hIL-15 (736 ± 41 pg/ml) in the serum for up to 2 weeks. A subsequent but temporary increase in the numbers of donor derived NK cells (3 fold) occurred within 72 hours, in both BM and periphery, following hIL-15 gene delivery. Despite these augmenting effects, the developing NK cells still exhibited poor functionality compared to non-transplanted adult mice. These studies describe the possibility of NK development in the spleen, and indicate that normal surface expression of Ly49C/I and Ly49G2 early after BMT is time dependent, but MHC independent. Moreover, administration of hIL-15 is not sufficient to induce NK functional maturation early after BMT despite increased NK numbers.