The cytolytic function of natural killer (NK) cells is induced by the engagement of a series of activating receptors and coreceptors some of which have recently been identified and collectively termed natural cytotoxicity receptors (NCRs). Here, we analyzed the cytolytic function of NK cells obtained from patients with acute myeloid leukemia (AML). In sharp contrast with healthy donors, in most (16 of 18) patients with AML the majority of NK cells displayed low NCR surface density (NCR^dull). This phenotype correlated with a weak cytolytic activity against autologous leukemic cells that could not be reversed by the monoclonal antibody-mediated disruption of HLA class I/killer immunoglobulinlike receptor interaction. The remaining 2 patients were characterized by NK cells having an NCR^bright phenotype. Surprisingly, although displaying NCR-mediated cytolytic activity, these NCR^bright NK cells were unable to kill autologous leukemic blasts. Importantly, the leukemic blasts from these 2 patients were also resistant to lysis mediated by normal NCR^brightallogeneic NK cells. Our study suggests that in most instances the inability of NK cells to kill autologous leukemic blasts is consequent to low NCR surface expression. In few cases, however, this failure appears to involve a mechanism of tumor escape based on down-regulation of ligands relevant for NCR-mediated target cell recognition.

Natural killer (NK) cells play an important role in tumor-cell clearance, particularly against leukemia, as shown by killer cell inhibitory receptor (KIR)–mismatched allogeneic stem cell transplantation. Analysis of in vitro IL-2–expanded NK cells from patients with myelocytic/monocytic acute myeloid leukemia (AML-NK cells) has revealed poor cytolytic functions because of deficient expression of pivotal activation molecules—the natural cytotoxicity receptors (NCRs) NKp30, NKp44, and NKp46. To exclude the possibility that this observation was caused by the in vitro amplification of a small NCRdull population, we analyzed the AML-NK phenotype directly, without any in vitro expansion. We first confirmed that the NCRdull phenotype was not an in vitro artifact. Moreover, analysis of a large population of AML patients allowed us to demonstrate that phenotype was not restricted to a French-American-British (FAB) subtype and was not associated with a particular cytogenetic abnormality. Our longitudinal study of AML patients showed that the NCRdull phenotype was acquired during leukemia development because we observed its complete (for NKp46) or partial (for NKp30) reversibility in patients achieving complete remission (CR). Reversibility of the NCRdull phenotype after CR suggested that leukemia cells might be involved in NCR down-regulation. In agreement with this hypothesis, direct contact between leukemic blasts and NK cells (but not leukemia-cell supernatants) induced loss or decrease in NKp30 and NKp46 expression while impeding NKp44 induction by IL-2. We excluded the major implication of TGF-β in NCR down-regulation. Although the clinical antitumor value of NK cells is clearly demonstrated in allogeneic stem cell transplantation, the role of NK cells in autologous transplantation is not proved. Interestingly, we observed a correlation between the NCRdull phenotype and poor survival in AML patients, suggesting that NK-deficient activation caused by NCR down-regulation could play a role in patient outcome. The prognostic value of NCR expression is discussed, and pathophysiologic implication of the NCR phenotype will be further investigated in a larger study.

The TNF receptor (TNFR) family plays a central role in the development of the immune response. Here we describe the reciprocal regulation of the recently identified TNFR superfamily member herpes virus entry mediator (HVEM) (TR2) and its ligand LIGHT (TL4) on T cells following activation and the mechanism of this process. T cell activation resulted in down-regulation of HVEM and up-regulation of LIGHT, which were both more pronounced in CD8⁺ than CD4⁺ T lymphocytes. The analysis of HVEM and LIGHT mRNA showed an increase in the steady state level of both mRNAs following stimulation. LIGHT, which was present in cytoplasm of resting T cells, was induced both in cytoplasm and at the cell surface. For HVEM, activation resulted in cellular redistribution, with its disappearance from cell surface. HVEM down-regulation did not rely on de novo protein synthesis, in contrast to the partial dependence of LIGHT induction. Matrix metalloproteinase inhibitors did not modify HVEM expression, but did enhance LIGHT accumulation at the cell surface. However, HVEM down-regulation was partially blocked by a neutralizing mAb to LIGHT or an HVEM-Fc fusion protein during activation. As a model, we propose that following stimulation, membrane or secreted LIGHT binds to HVEM and induces receptor down-regulation. Degradation or release of LIGHT by matrix metalloproteinases then contributes to the return to baseline levels for both LIGHT and HVEM. These results reveal a self-regulating ligand/receptor system that contributes to T cell activation through the interaction of T cells with each other and probably with other cells of the immune system.

LIGHT is a recently identified member of the TNF superfamily that is up-regulated upon activation of T cells. Herpesvirus entry mediator, one of its receptors, is constitutively expressed on immature dendritic cells (DCs). In this report, we demonstrate that LIGHT induces partial DC maturation as demonstrated by Ag presentation and up-regulation of adhesion and costimulatory molecules. LIGHT-stimulated DCs show reduced macropinocytosis and enhanced allogeneic stimulatory capacity but fail to produce significant amounts of IL-12, IL-6, IL-1β, or TNF-α compared with unstimulated DCs. However, LIGHT cooperates with CD154 (CD40 ligand) in DC maturation, with particular potentiation of allogeneic T cell proliferation and cytokine secretion of IL-12, IL-6, and TNF-α. Moreover, LIGHT costimulation allows DCs to prime in vitro-enhanced specific CTL responses. Our results suggest that LIGHT plays an important role in DC-mediated immune responses by regulating CD154 signals and represents a potential tool for DC-based cancer immunotherapy.