Characterization of NK1.1⁺ CD8 T Cells in Allogeneic Hematopoietic Cell Transplantation Recipients

NKT cells from naïve mice are mainly CD4⁺ or CD4−CD8−. However, it has been reported that CD8⁺ NKT cells can be expanded in vitro from splenocytes, bone marrow cells and thymocytes of C57BL/6 (B6) mice by stimulation with anti-CD3 mAb and cytokines, and that the expanded CD8⁺ NKT cells mediate strong graft-vs.-leukemic (GVL) effects without severe GVHD after adoptive transfer into allogeneic mice. We now describe the presence of CD8⁺NK1.1⁺ cells in recipient livers (approximately 2–6%), but not in other tissues (spleen, lung, bone marrow, thymus and PBMC), in various allogeneic hematopoietic cell transplantation (allo-HCT) models. The generation of CD8⁺NK1.1⁺ cells is likely a consequence of alloresponses, as these cells were not detected in the liver of syngeneic HCT controls. Flow cytometric analysis confirmed that these cells are CD1d-independent, TCRαβ⁺ T cells with a memory phenotype (CD44⁺ and CD62L⁻), and do not express CD49b, Ly-49C/I, Ly-49G2, or Ly-49D. In a sublethally (6 Gy)-irradiated B6-to-B6D2F1 allo-HCT model, NK1.1⁺ CD8 T cells became detectable by week 2, increased in number until approximately week 8, and gradually declined thereafter but were still detectable in the liver at day 100 after allo-HCT. By using CD45.1 and CD45.2 congeneic donors, we determined that the majority of NK1.1⁺ CD8 T cells were derived from the donor splenocytes. Furthermore, depletion of CD8⁺, but not NK1.1⁺, cells from the donor splenocytes prior to transplantation prevented the generation of NK1.1⁺ CD8 T cells, indicating that these cells were derived from donor NK1.1⁻CD8⁺ splenic T cells. Our data demonstrate that donor CD8 T cells can acquire NK1.1 expression upon activation in allo-HCT recipients, and that these NK1.1⁺ CD8 T cells maintain a memory phenotype and persist in the recipients with preferential accumulation in the liver. Studies are currently in progress to determine the role of activated donor NK1.1⁺ CD8 T cells in GVHD and GVL effects.