Up-Regulation of NK Cell Activating Receptors Associated with Elevated IL-15 Levels Post-Transplant.

NK cells express diverse activating and inhibitory receptors, which collectively determine the NK cytotoxic response. Because NK cells can be potent anti-tumor effectors in the post transplant period of minimal residual disease, we investigated the receptor expression on NK cells following non-myeloablative, HLA-matched, allogeneic hematopoietic stem cell transplant (HSCT). We focused on activating receptors, comparing the expression of natural cytotoxicity receptors (NCR) and C-type lectin receptors on circulating NK cells at one, three, six and twelve months following HSCT with that of the donor-derived mobilized stem cell apheresis. During the first post-transplant month, the absolute numbers of NK cells recovered to normal levels in the 14 patients studied, but the subpopulation of CD56++(bright) CD16− NK cells increased disproportionately compared to the more common CD56+(dull) CD16+ NK cells. By six months the proportions of the NK cell subsets normalized to pre-transplant levels. At one month post transplant, the median percentage of NK cells expressing the anti-tumor NCR NKp46 increased from 15 to 64% and that expressing NKp30 increased from 23 to 40% as compared to the donor’s NK cells; expression remained elevated during the first year. Expression of NKG2D, the homodimeric activating C-type lectin receptor, similarly increased post-transplant. CD94 was also upregulated on NK cells, but the activating heterodimeric partner of CD94, NKG2C, did not change significantly. To investigate these shifts in NK populations and receptor expression, we investigated the effect of cytokine milieu post transplant on these shifts in NK populations and receptor expression. We have determined that plasma levels of IL-15, a cytokine critical in NK differentiation and expansion, increase more than 50-fold in these HSCT patients from pretreatment to the day of transplant and decline in the first weeks post transplant, inversely proportional to NK recovery. When NK cells were cultured with rIL-15, we observed a disproportionate expansion of CD56++ NK cells and a rapid up-regulation of the NCR and NKG2D receptors, similar to the changes observed post-transplant. These data suggest that the cytokine milieu of transplant, in particular elevated levels of IL-15, may contribute to anti-tumor efficacy post transplant by affecting the recovery of NK subsets and modulating expression of activating receptors.