Natural Killer (NK) Cell Recovery After Bone Marrow Transplantation in Mice: Early Emergence of a Novel Ly49+ Stage in Differentiation Independent of MHC Haplotype.

Abstract 4477

NK cells play critical roles in resistance to viral infections and in cancer. NK cells exist as subpopulations of cells that bear inhibitory and/or activating receptors to MHC. In mice, these are the Ly49+ subsets which have been shown to play a role in the rejection of bone marrow allografts as well as resistance to CMV. These Ly49 receptors also play a central role in the “education”, “arming” or “licensing” of NK cells. NK cell recovery after bone marrow transplantation is thought to proceed quickly but the acquisition of the Ly49 receptors has been incompletely characterized. NK cell differentiation has for the most part been characterized through in vitro models. We therefore examined the recovery of NK cells after syngeneic BMT in mice and assessed the effects of MHC on the recovery of the Ly49+ subsets using congenic recipients. NK cells were rapidly detected in mice as early as 14 days after BMT. Interestingly, the dominant population observed had a mature NK cell phenotype based on the expression of CD94, CD43, CD11b, etc. However, the majority of NK cells had a unique expression of Ly49G2 single positive NK cells with median fluorescence intensity (MFI) approximately twice as high as on NK cells from normal animals. This phenotype was not observed in untreated normal mice and occurred in B6 and B10 (H2b) as well as B10.D2 (H2d) strain mice showing independence from MHC. When these cells were sorted and assessed for cytotoxic activity, the cells had comparable killing efficiency as NK cells from untreated mice. Over time, NK cells with normal Ly49 subset distribution and phenotype were observed with the disappearance of this earlier subset. However, total splenic NK cell numbers remained significantly reduced in the BMT recipients as long as 120 days after BMT and despite normal recovery and numbers of T cells. These results demonstrate the existence of novel stage(s) in NK cell differentiation that occur independently of MHC and that can be characterized by sole expression of Ly49G2 at high levels. However, deficits in NK cell numbers persist in mice for extended periods of time. These findings suggest that NK cell recovery may occur in waves with some inhibitory receptors programmed to repopulate first and that surface expression intensity also needs to be assessed. Furthermore, the long-term deficits in total numbers suggest that overall activity may be compromised for extended periods clinically post-BMT.