NK cell activation through the NKG2D ligand MULT-1 is selectively prevented by the glycoprotein encoded by mouse cytomegalovirus gene m145.

The NK cell-activating receptor NKG2D interacts with three different cellular ligands, all of which are regulated by mouse cytomegalovirus (MCMV). We set out to define the viral gene product regulating murine UL16-binding protein-like transcript (MULT)-1, a newly described NKG2D ligand. We show that MCMV infection strongly induces MULT-1 gene expression, but surface expression of this glycoprotein is nevertheless completely abolished by the virus. Screening a panel of MCMV deletion mutants defined the gene m145 as the viral regulator of MULT-1. The MCMV m145-encoded glycoprotein turned out to be necessary and sufficient to regulate MULT-1 by preventing plasma membrane residence of MULT-1. The importance of MULT-1 in NK cell regulation in vivo was confirmed by the attenuating effect of the m145 deletion that was lifted after NK cell depletion. Our findings underline the significance of escaping MULT-1/NKG2D signaling for viral survival and maintenance.

Role of natural-killer group 2 member D ligands and intercellular adhesion molecule 1 in natural killer cell-mediated lysis of murine embryonic stem cells and embryonic stem cell-derived cardiomyocytes.

The transplantation of cardiomyocytes derived from embryonic stem (ES) cells into infarcted heart has been shown to improve heart function in animal models. However, immune rejection of transplanted cells may hamper the clinical application of this approach. Natural killer (NK) cells could play an important role in this process in both autologous and allogeneic settings by eliminating cells expressing low levels of major histocompatibility complex (MHC) class I molecules. Here we characterize embryonic stem cell-derived cardiomyocytes (ESCM) in terms of their sensitivity to NK cells. We show that despite expression of very low levels of MHC class I molecules, murine ESCM were neither recognized nor lysed by activated syngeneic NK cells in vitro. In contrast, undifferentiated ES cells expressing similarly low levels of MHC class I molecules as ESCM were recognized and lysed by NK cells. This differential susceptibility results from the differential expression of ligands for the major activating natural killer cell receptor natural-killer group 2 member D (NKG2D) and intercellular adhesion molecule 1 (ICAM-1) on ES cells versus ESCM. NKG2D ligands and ICAM-1 were expressed on ES cells but were absent from ESCM. Undifferentiated ES cells were lysed by NK cells in a perforin-dependent manner. However, simultaneous blockade of NKG2D and ICAM-1 by antibodies inhibited this killing. These data suggest that in the course of differentiation ESCM acquire resistance to NK cell-mediated lysis by downregulating the expression of ligands required for activation of NK cell cytotoxicity.

DC-NK cell cross talk as a novel CD4+ T-cell-independent pathway for antitumor CTL induction.

It is generally accepted that priming of antitumor CD8+ cytotoxic T lymphocytes (CTLs) needs help that can be provided by CD4+ T cells. We show that interactions between dendritic cells (DCs) and natural killer (NK) cells can bypass the T helper arm in CTL induction. Bone marrow-derived DCs caused rejection of the A20 lymphoma and induced tumor-specific long-term memory, although they were not loaded with tumor-derived antigen. Experiments using CD40(-) knock-out mice and cell depletion showed that this effect did not require CD4+ cells. Both primary rejection and long-term CTL memory were the result of NK cell activation by DCs. NK cytotoxicity, which was necessary for primary rejection, was dependent on expression of natural killer group 2 D (NKG2D) ligands on tumor cells. Blocking of these ligands using NKG2D tetramers abrogated tumor killing in vitro and in vivo. The long-term response was due to CTLs directed against antigen(s) expressed on A20 and in vitro-differentiated DCs. The mechanism leading to CD4+ helper cell-independent CTL responses was elucidated as a cascade that was initiated by NK cell activation. This pathway was dependent on inter-feron-gamma expression and involved priming endogenous DCs for interleukin-12 production. Our data suggest a novel pathway linking innate and adaptive immunity.

NKG2D-independent suppression of T cell proliferation by H60 and MICA.

The activating receptor NKG2D recognizes a wide range of different ligands, some of which are primarily expressed in "stressed" tissues or on tumor cells. Until now, similar stimulatory effects on natural killer and CD8+ T cells have been described for all NKG2D ligands, and the NKG2D receptor/ligand system has therefore been interpreted as a sensor system involved in tumor immune surveillance and activation of immune responses. We show here that the NKG2D ligands H60 and MIC class 1 chain-related protein A (MICA) can also mediate strong suppressive effects on T cell proliferation. Responsiveness to H60- and MICA-mediated suppression requires IL-10 and involves a receptor other than NKG2D. These findings might provide explanations for the observation that strong in vivo NKG2D ligand expression, such as that on tumor cells, sometimes fails to support effective immune responses and links this observation to a distinct subgroup of NKG2D ligands.