Reduced immune effector cell NKG2D expression and increased levels of soluble NKG2D ligands in multiple myeloma may not be causally linked

Background

There is limited understanding of the dysregulation of the innate immune system in multiple myeloma (MM). We analysed the expression of the activating receptor NKG2D on NK cells and T cells of MM patients and investigated the impact of soluble versus membrane-bound NKG2D ligands on the expression of NKG2D.

Design

NKG2D expression on NK cells and CD8+ αβ T cells from patients with MM or monoclonal gammopathy of uncertain significance and healthy controls was examined flow-cytometrically. Sera from patients and controls were analysed for soluble NKG2D ligands (sNKG2D ligands).

Results

Significantly fewer NK cells and CD8+ αβ T cells from patients expressed NKG2D compared to healthy controls (NK cells: median 54% interquartile range (IQR) 32–68 versus 71% IQR 44–82%, P = 0.017, CD8+ αβ T cells: median 63% IQR 52–81 versus 77% IQR 71–90%, P = 0.018). The sNKG2D ligand sMICA was increased in patients [median 175 (IQR 87–295) pg/ml] versus controls [median 80 (IQR 32–129) pg/ml, P < 0.001], but levels of sMICA did not correlate with NKG2D expression on effector cells. To elucidate the mechanism of NKG2D down-regulation, we incubated lymphocytes from healthy donors in the presence of sNKG2D ligands or in co-culture with MM cell lines. sNKG2D ligands in clinically relevant concentrations did not down-regulate NKG2D expression, but co-culture of effector cells with myeloma cells with high surface expression of NKG2D ligands reduced NKG2D expression significantly.

Conclusions

These results indicate that MM is associated with a significant reduction in NKG2D expression which may be contact-mediated rather than caused by soluble NKG2D ligands.     

Inhibitory receptor signals suppress ligation-induced recruitment of NKG2D to GM1-rich membrane domains at the human NK cell immune synapse

NKG2D is an activating receptor expressed on all human NK cells and a subset of T cells. In cytolytic conjugates between NK cells and target cells expressing its ligand MHC class I chain-related gene A, NKG2D accumulates at the immunological synapse with GM1-rich microdomains. Furthermore, NKG2D is specifically recruited to detergent-resistant membrane fractions upon ligation. However, in the presence of a strong inhibitory stimulus, NKG2D-mediated cytotoxicity can be intercepted, and recruitment of NKG2D to the immunological synapse and detergent-resistant membrane fractions is blocked. Also, downstream phosphorylation of Vav-1 triggered by NKG2D ligation is circumvented by coengaging inhibitory receptors. Thus, we propose that one way in which inhibitory signaling can control NKG2D-mediated activation is by blocking its recruitment to GM1-rich membrane domains. The accumulation of activating NK cell receptors in GM1-rich microdomains may provide the necessary platform from which stimulatory signals can proceed.     

Prostate Tumor-Derived Exosomes Down-Regulate NKG2D Expression on Natural Killer Cells and CD8+ T Cells: Mechanism of Immune Evasion

Tumor-derived exosomes, which are nanometer-sized extracellular vesicles of endosomal origin, have emerged as promoters of tumor immune evasion but their role in prostate cancer (PC) progression is poorly understood. In this study, we investigated the ability of prostate tumor-derived exosomes to downregulate NKG2D expression on natural killer (NK) and CD8⁺ T cells. NKG2D is an activating cytotoxicity receptor whose aberrant loss in cancer plays an important role in immune suppression. Using flow cytometry, we found that exosomes produced by human PC cells express ligands for NKG2D on their surface. The NKG2D ligand-expressing prostate tumor-derived exosomes selectively induced downregulation of NKG2D on NK and CD8⁺ T cells in a dose-dependent manner, leading to impaired cytotoxic function in vitro. Consistent with these findings, patients with castration-resistant PC (CRPC) showed a significant decrease in surface NKG2D expression on circulating NK and CD8⁺ T cells compared to healthy individuals. Tumor-derived exosomes are likely involved in this NKG2D downregulation, since incubation of healthy lymphocytes with exosomes isolated from serum or plasma of CRPC patients triggered downregulation of NKG2D expression in effector lymphocytes. These data suggest prostate tumor-derived exosomes as down-regulators of the NKG2D-mediated cytotoxic response in PC patients, thus promoting immune suppression and tumor escape.     

Human tumor-derived exosomes down-modulate NKG2D expression

NKG2D is an activating receptor for NK, NKT, CD8(+), and gammadelta(+) T cells, whose aberrant loss in cancer is a key mechanism of immune evasion. Soluble NKG2D ligands and growth factors, such as TGFbeta1 emanating from tumors, are mechanisms for down-regulating NKG2D expression. Cancers thereby impair the capacity of lymphocytes to recognize and destroy them. In this study, we show that exosomes derived from cancer cells express ligands for NKG2D and express TGFbeta1, and we investigate the impact of such exosomes on CD8(+) T and NK cell NKG2D expression and on NKG2D-dependent functions. Exosomes produced by various cancer cell lines in vitro, or isolated from pleural effusions of mesothelioma patients triggered down-regulation of surface NKG2D expression by NK cells and CD8(+) T cells. This decrease was rapid, sustained, and resulted from direct interactions between exosomes and NK cells or CD8(+) T cells. Other markers (CD4, CD8, CD56, CD16, CD94, or CD69) remained unchanged, indicating the selectivity and nonactivatory nature of the response. Exosomal NKG2D ligands were partially responsible for this effect, as down-modulation of NKG2D was slightly attenuated in the presence of MICA-specific Ab. In contrast, TGFbeta1-neutralizing Ab strongly abrogated NKG2D down-modulation, suggesting exosomally expressed TGFbeta as the principal mechanism. Lymphocyte effector function was impaired by pretreatment with tumor exosomes, as these cells exhibited poor NKG2D-dependent production of IFN-gamma and poor NKG2D-dependent killing function. This hyporesponsiveness was evident even in the presence of IL-15, a strong inducer of NKG2D. Our data show that NKG2D is a likely physiological target for exosome-mediated immune evasion in cancer.     

Cutting edge: down-regulation of MICA on human tumors by proteolytic shedding

The immunoreceptor NKG2D stimulates tumor immunity through activation of CD8 T cells and NK cells. Its ligand MICA has been shown to be broadly expressed on human tumors of epithelial origin. MICA expression correlates with an enrichment of Vdelta1 T cells in tumor tissue. We report that human tumor cells spontaneously release a soluble form of MICA encompassing the three extracellular domains, which is present at high levels in sera of patients with gastrointestinal malignancies, but not in healthy donors. Release of MICA from tumor cells is blocked by inhibition of metalloproteinases, concomitantly causing accumulation of MICA on the cell surface. Shedding of MICA by tumor cells may modulate NKG2D-mediated tumor immune surveillance. In addition, determination of soluble MICA levels may be implemented as an immunological diagnostic marker in patients with epithelial malignancies.     

Costimulation of multiple NK cell activation receptors by NKG2D

The activation of NK cells is mediated through specific interactions between activation receptors and their respective ligands. Little is known, however, about whether costimulation, which has been well characterized for T cell activation, occurs in NK cells. To study the function of NKG2D, a potential NK costimulatory receptor, we have generated two novel hamster mAbs that recognize mouse NKG2D. FACS analyses demonstrate that mouse NKG2D is expressed on all C57BL/6 IL-2-activated NK (lymphokine-activated killer (LAK)) cells, all splenic and liver NK cells, and approximately 50% of splenic NKT cells. Consistent with limited polymorphism of NKG2D, its sequence is highly conserved, and the anti-NKG2D mAbs react with NK cells from a large number of different mouse strains. In chromium release assays, we show that stimulation of NK cells with anti-NKG2D mAb can redirect lysis. Also, enhanced lysis of transfected tumor targets expressing NKG2D ligand could be inhibited by addition of anti-NKG2D mAb. Interestingly, stimulation of LAK cells via NKG2D alone does not lead to cytokine release. However, stimulation of LAK via both an NK activation receptor (e.g., CD16, NK1.1, or Ly-49D) and NKG2D leads to augmentation of cytokine release compared with stimulation through the activation receptor alone. These results demonstrate that NKG2D has the ability to costimulate multiple NK activation receptors.     

Shed NKG2D ligand boosts NK cell immunity

Ligands for natural killer (NK) cell activating receptors can be released from tumor cells and are believed to promote tumor growth by acting as decoys for effector lymphocytes. In a recent paper published in Science, Deng et al. report another scenario in which a shed form of the MULT1 mouse NKG2D ligand boosts NK cell functions.Natural killer (NK) cells are cytolytic and cytokine-producing lymphocytes of the innate immune system that participate in the control of tumor growth and microbial infections¹. NK cell effector activities are tightly controlled by a fine balance of inhibitory and activating signals delivered by surface receptors. Activating receptors can recognize two types of ligands, self-molecules encoded by the host''s own genome whose expression is upregulated upon cellular stress, or exogenous molecules produced by microbes during infection. NKG2D, one of the best characterized activating receptor expressed by NK and T cells, binds to several different ligands in human and mouse². NKG2D ligands are poorly expressed on the vast majority of normal cell surfaces, but are upregulated on tumor and virus-infected cells. In addition, NKG2D ligands can be released by both surface cleavage and exosome excretion. It has been reported that shed ligands can block tumor cell recognition by effector cells by preventing NKG2D interaction with its ligands³. However, several reports do not correlate the presence of soluble ligands with decreased NKG2D expression nor functional activities.Deng et al.⁴ focused their analysis on the NKG2D mouse ligand MULT1, which is commonly overexpressed on primary tumor cells. They first showed that MULT1-transduced fibroblast can cleave MULT1 from the plasma membrane, resulting in a released shed form in the supernatant. Shed MULT1 is of high affinity to NKG2D (∼13 nM) similar to recombinant MULT1. They further reveal the presence of shed MULT1 in the serum of mice developing spontaneous MULT1⁺ tumors. Interestingly, the authors detected a very high concentration of shed MULT1 in the sera of Apoe^−/− mice exhibiting severe atherosclerosis and liver inflammation. Given that these autoimmune injuries observed in this mouse model depend on NKG2D activity⁵, it was unlikely that shed MULT1 exert an inhibitory effect on immunity.Surprisingly, the authors further showed that mouse tumor cells engineered to release a secreted form of MULT1 (secMULT1) similar to the shed MULT1 were rejected when injected into syngenic mice. Tumor rejection is dependent on NK cells as cells grow in NK but not in CD8⁺ T cell-depleted host and requires NKG2D. Importantly, the controlled release of secMULT1 from tumors harboring inducible secMULT1 promotes tumor rejection. To rule out the possibility that tumor cell rejection was due to intrinsic modifications of tumor cells, the author monitored the rejection of a mixture of 9:1 secMULT1⁻: secMULT1⁺ tumor cells and showed an improved antitumoral effect on both secMULT1⁺ and, importantly, secMULT1⁻ tumors. In addition, direct intratumoral injection of recombinant MULT1 promotes tumor rejection. These results suggested that soluble MULT1 mobilizes or activates anti-tumor effector cells. Deng et al. further reported increased frequencies of cytotoxic and IFN-γ-secreting NK cells associated with secMULT1⁺ tumors as compared to control tumor cells. Altogether, these data suggest that a shed NKG2D ligand can promote tumor rejection by boosting NK cell effector functions.Shed MULT1 could crosslink NKG2D and thus activate NK cells. However, shed and secMULT1 are monomeric molecules similar to the recombinant MULT1 which fails to activate NK cells in vitro. Formation of multivalent structures in vivo was not detected. In addition, whereas the transmembrane form of MULT1 can activate NK cells by crosslinking NKG2D and induces NKG2D downregulation, soluble MULT1 upregulates NKG2D on the NK cell surface. This upregulation is probably due do a decreased downregulation of NKG2D surface expression because no increase in NKG2D mRNA or protein was observed. Based on these findings, the authors hypothesized that NKG2D ligands expressed on non-tumor host cell membrane continuously engage NKG2D on NK cells, leading to NKG2D downregulation and NK cell desensitization, whereas soluble MULT1 blocks these interactions to increase NK cell responsiveness (Figure 1). Along this line, NK cells from mutant mice genetically deficient for the NKG2D ligand expressed by tumor-associated myeloid cells are not desensitized.Open in a separate windowFigure 1Tumor-associated cells express NKG2DL which can desensitize NK cells. Tumor shedding of MULT1 delivers soluble MULT1 that outcompetes for NKG2D binding and prevents NK cell desensitization. Boosted NK cell functions lead to improved tumor cell rejection by other activating receptors.The induction of cell desensitization by a frequent or even constant stimulation is a very common mechanism across living objects. Regarding NK cells, another example of tuning via desensitization resides in the impact of the long lasting absence of MHC class I molecules in their environment. Indeed, NK cells are hyporesponsive in a MHC-I-deficient host⁶. There are accumulating data indicating that in the absence of engagement of inhibitory receptors for MHC class I molecules, NK cells get desensitized due to their chronic interaction with endogenous stimulating ligands⁷. Indeed, in the absence of engagement of this inhibitory pathway, NK cell activation would be unleashed⁸. This scenario is supported by a series of in vitro and in vivo experiments in which NK cells are desensitized following chronic exposure to stimulatory molecules expressed at the surface of interacting cells^9,10. Thus, the induction of MHC class I downregulation or NKG2D ligand upregulation boosts NK cell function, whereas the sustained lack of MHC class I or expression of NKG2D ligands impairs NK cell reactivity. This tuning of immune response as a function of the speed of change of the stimuli detected by lymphocytes is at the center of the recently proposed Discontinuity Theory¹¹.Finally, consistent with their findings with secMULT1 but somewhat counter-intuitively, Deng et al. also show that NKG2D receptor deficiency or blockade using anti-NKG2D monoclonal antibodies mimics the effect of soluble MULT1. Indeed, in both conditions, NK cell effector functions are boosted, resulting in improved tumor rejection. Similarly, blocking other NK activating receptors, such as NKp46, may also lead to NK cell desensitization¹². Checkpoint inhibitory receptors are revolutionizing the treatment of cancers by inhibiting the inhibitory receptors. The findings reported by Deng et al. together with earlier results propose alternative strategies of cancer treatment using antibodies that are directed against activating receptors. In the case of NKG2D, the chronic engagement of NK cells with membrane-bound NKG2D ligand affects not only NKG2D-dependent but also NKG2D-independent signaling pathways⁹. The blockade of NKG2D desensitization by antibodies directed against NKG2D should thus also boost NK cell activation via other pathways, such as antibody-dependent cell cytotoxicity. However, the precise identification of the ligand-receptor pair involved in the control of tumors by NK cells will be a limiting factor to these innovative therapeutic approaches. Indeed, antibodies against activating receptors should be designed to boost NK cell reactivity but should not block the recognition of the tumors by NK cells. Finally, as the tuning of NK cell reactivity by soluble NKG2D ligands depends on their affinity for NKG2D, the pre-clinical development of this new class of drug candidates might reveal novel pharmacokinetics and the pharmacodynamics guidelines.     

Expansion of NK Cells and Reduction of NKG2D Expression in Chronic Lymphocytic Leukemia. Correlation with Progressive Disease

The immune system may mediate anti-tumor responses in chronic lymphocytic leukemia (CLL) which may affect disease progression and survival. In this study, we analyzed the immune characteristics of 99 consecutive previously diagnosed CLL patients and 50 healthy controls. The distribution of lymphocyte subsets at diagnosis was retrospectively analyzed. Compared with controls, leukemia patients showed an expansion of NK and CD8 T cells at diagnosis. The relative number of CD8 T cells at diagnosis was associated with time to treatment, suggesting that CD8 T cells may modify disease progression. The distribution of lymphocyte subsets was analyzed again when patients were enrolled in this study. The median time since these patients were diagnosed was 277 weeks. Compared with diagnosis, the absolute number of CD8 T cells significantly decreased in these patients, reaching similar values to healthy controls; however NK cells kept significantly elevated overtime. Nevertheless, NK cells showed an impaired expression of NKG2D receptor and a defective cytotoxic activity. This down-regulation of NKG2D expression was further enhanced in patients with advanced and progressive disease. Additionally, membrane NKG2D levels significantly decreased on CD8 T cells, but a significant increase of NKG2D+CD4+ T cells was observed in CLL patients. The cytotoxic activity of NK cells was diminished in CLL patients; however the treatments with IL-2, IL-15, IL-21 and lenalidomide were able to restore their activity. The effect of IL-2 and IL-15 was associated with the increase of NKG2D expression on immune cells, but the effect of IL-21 and lenalidomide was not due to NKG2D up-regulation. The expansion of NK cells and the reversibility of NK cell defects provide new opportunities for the immunotherapeutic intervention in CLL.     

Human Herpesvirus 8 (HHV8) Sequentially Shapes the NK Cell Repertoire during the Course of Asymptomatic Infection and Kaposi Sarcoma

The contribution of innate immunity to immunosurveillance of the oncogenic Human Herpes Virus 8 (HHV8) has not been studied in depth. We investigated NK cell phenotype and function in 70 HHV8-infected subjects, either asymptomatic carriers or having developed Kaposi''s sarcoma (KS). Our results revealed substantial alterations of the NK cell receptor repertoire in healthy HHV8 carriers, with reduced expression of NKp30, NKp46 and CD161 receptors. In addition, down-modulation of the activating NKG2D receptor, associated with impaired NK-cell lytic capacity, was observed in patients with active KS. Resolution of KS after treatment was accompanied with restoration of NKG2D levels and NK cell activity. HHV8-latently infected endothelial cells overexpressed ligands of several NK cell receptors, including NKG2D ligands. The strong expression of NKG2D ligands by tumor cells was confirmed in situ by immunohistochemical staining of KS biopsies. However, no tumor-infiltrating NK cells were detected, suggesting a defect in NK cell homing or survival in the KS microenvironment. Among the known KS-derived immunoregulatory factors, we identified prostaglandin E2 (PGE2) as a critical element responsible for the down-modulation of NKG2D expression on resting NK cells. Moreover, PGE2 prevented up-regulation of the NKG2D and NKp30 receptors on IL-15-activated NK cells, and inhibited the IL-15-induced proliferation and survival of NK cells. Altogether, our observations are consistent with distinct immunoevasion mechanisms that allow HHV8 to escape NK cell responses stepwise, first at early stages of infection to facilitate the maintenance of viral latency, and later to promote tumor cell growth through suppression of NKG2D-mediated functions. Importantly, our results provide additional support to the use of PGE2 inhibitors as an attractive approach to treat aggressive KS, as they could restore activation and survival of tumoricidal NK cells.     

Systemic NKG2D down-regulation impairs NK and CD8 T cell responses in vivo

The immunoreceptor NKG2D stimulates activation of cytotoxic lymphocytes upon engagement with MHC class I-related NKG2D ligands of which at least some are expressed inducibly upon exposure to carcinogens, cell stress, or viruses. In this study, we investigated consequences of a persistent NKG2D ligand expression in vivo by using transgenic mice expressing MHC class I chain-related protein A (MICA) under control of the H2-K(b) promoter. Although MICA functions as a potent activating ligand of mouse NKG2D, H2-K(b)-MICA mice appear healthy without aberrations in lymphocyte subsets. However, NKG2D-mediated cytotoxicity of H2-K(b)-MICA NK cells is severely impaired in vitro and in vivo. This deficiency concurs with a pronounced down-regulation of surface NKG2D that is also seen on activated CD8 T cells. As a consequence, H2-K(b)-MICA mice fail to reject MICA-expressing tumors and to mount normal CD8 T cell responses upon Listeria infection emphasizing the importance of NKG2D in immunity against tumors and intracellular infectious agents.     

Modulation of NKG2D-mediated cytotoxic functions of natural killer cells by viral protein R from HIV-1 primary isolates

HIV-1 viral protein R (Vpr) from laboratory-adapted virus strains activates the DNA damage/stress sensor ATR kinase and induces cell cycle arrest at the G(2)/M phase through a process that requires Vpr to engage the DDB1-CUL4A (VprBP/DCAF-1) E3 ligase complex. Activation of this DNA damage/stress checkpoint in G(2) by Vpr was shown to modulate NKG2D-dependent NK cell effector functions via enhancing expression of NKG2D ligands, notably ULBP2. However, it is unknown whether Vpr from HIV-1 primary isolates (groups M, N, O, and P) could modulate NKG2D-mediated cytotoxic functions of NK cells. Here, we report that Vpr from most HIV-1 primary isolates can upregulate ULBP2 expression and induce NKG2D-dependent NK cell killing. Importantly, these activities were always accompanied by an active G(2) cell cycle arrest function. Interestingly, Vpr variants from group P and a clade D isolate of group M were defective at enhancing NKG2D-mediated NK cell lysis owing to their inability to augment ULBP2 expression. However, distinct mechanisms were responsible for their failure to do so. While Vpr from group P was deficient in its ability to engage the DDB1-CUL4A (VprBP/DCAF-1) E3 ligase complex, the Vpr variant from group D was unable to properly localize to the nucleus, underlining the importance of these biological properties in Vpr function. In conclusion, the ability of Vpr from HIV-1 primary isolates to regulate NK cell effector function underscores the importance of this HIV-1 accessory protein in the modulation of the host's innate immune responses.     

Cancer immunoediting of the NK group 2D ligand H60a

Cancer immunoediting describes the process whereby highly immunogenic tumor cells are removed, or edited, from the primary tumor repertoire by the immune system. In immunodeficient mice, the editing process is hampered, and "unedited" tumor cells can be recovered and studied. In this study, we compared unedited and edited tumors for their expression of NK group 2D (NKG2D) ligands, a family of surface proteins expressed on tumor cells that can activate NK cell cytotoxic activity. We found that the expression of the NKG2D ligand H60a was more heterogeneous in groups of unedited 3'-methylcholanthrene sarcoma cell lines compared with that in edited 3'-methylcholanthrene sarcoma cell lines (i.e., some unedited cell lines expressed very high levels of H60a, whereas other unedited and edited cell lines expressed very low levels). We also found that some highly immunogenic cell lines displayed a bimodal distribution consisting of H60a-hi and H60a-lo cells. In one of these cell lines, the H60a-hi cells could be removed by passaging the cells through RAG2(-/-) mice, resulting in edited cell lines that were poor targets for NK cells and that displayed progressive tumor growth. This editing of H60a-hi cells required NK cells and NKG2D. Our studies show that the expression of H60a on tumors cells can be actively modulated by the immune system, thereby implicating this NKG2D ligand in tumor immunosurveillance.     

Expression of ERp5 and GRP78 on the membrane of chronic lymphocytic leukemia cells: association with soluble MICA shedding

MICA is a ligand of the activating receptor NKG2D, expressed by NK and T cells. MICA expression is induced in cancer cells favoring their elimination by the immune system; however, many advanced tumors shed soluble MICA (sMICA), which impairs NKG2D-mediated cytotoxicity. ERp5 and GRP78 are endoplasmic reticulum-resident proteins that are translocated to the surface of epithelial tumor cells where they interact with MICA and are involved in sMICA shedding. In this study, we analyze the role of ERp5 and GRP78 in sMICA shedding in chronic lymphocytic leukemia (CLL). Immunofluorescence and flow cytometry analyses showed that ERp5 and GRP78 were significantly expressed on the surface of B cells and leukemia cells, but they were not expressed on T cells. The expression of ERp5 and GRP78 was significantly higher in leukemia cells than in B cells from controls. ERp5 and GRP78 co-localized with MICA on the surface of leukemia cells and the levels of expression of ERp5 and GRP78 correlated with the level of expression of membrane-bound MICA in CLL patients. Associated with higher expression of membrane-bound ERp5 and GRP78, serum sMICA levels were approximately threefold higher in patients than in controls. Elevated sMICA levels in CLL patients were associated with the down-modulation of NKG2D surface expression on CD8 T cells. Finally, pharmacological inhibition of B cell lines and stimulated leukemia cells showed that ERp5 activity is involved in sMICA shedding in CLL. In conclusion, these results uncover a molecular mechanism which regulates MICA protein shedding and immune evasion in CLL.     

Elevated TGF-beta1 secretion and down-modulation of NKG2D underlies impaired NK cytotoxicity in cancer patients

NK cell function in cancer patients is severely impaired, but the mechanism underlying this impairment is not clearly understood. In this study we show evidence that TGF-beta1 secreted by tumors is responsible for the poor NK lytic activity via down-regulating an NK-activating receptor, NKG2D. The plasma level of TGF-beta1 in human lung cancer or colorectal cancer patients was elevated compared with that in normal volunteers, and this elevation was inversely correlated with surface expression of NKG2D on NK cells in these patients. Incubation of NK cells with plasma obtained from cancer patients specifically down-modulated surface NKG2D expression, whereas addition of neutralizing anti-TGF-beta1 mAbs completely restored surface NKG2D expression. Likewise, incubation of NK cells and lymphokine-activated killer cells with TGF-beta1 resulted in dramatic reduction of surface NKG2D expression associated with impaired NK cytotoxicity. Modulation of NKG2D by TGF-beta1 was specific, as expression of other NK receptors, CD94/NKG2A, CD44, CD16, 2B4, or CD56, was not affected by TGF-beta1. Impaired NK cytotoxicity by TGF-beta1 was not due to alteration of lytic moieties, such as perforin or Fas, or apoptotic pathway, but, rather, appeared to be due to lack of NKG2D expression. Taken together, our data suggest that impaired NK function in cancer patients can be attributed to down-modulation of activating receptors, such as NKG2D, via secretion of TGF-beta1.     

Dynamic shift from CD85j/ILT-2 to NKG2D NK receptor expression pattern on human decidual NK during the first trimester of pregnancy

During the first trimester of human pregnancy, Natural Killer (NK) cells of the maternal uterine mucosa (e.g. decidua) have a unique phenotype and are involved in crucial physiological processes during pregnancy. We investigated whether modifications of the NK receptor repertoire occur during the first trimester of pregnancy. We found significantly decreased expression of KIR2DL1/S1 and KIR2DL2/L3/S2 receptors, NKp30 and NKp44 activatory receptors, and the CD85j (ILT-2) inhibitory receptor. We also observed significantly increased expression of the NKG2D activatory receptor at the decidual NK cell surface. By flow cytometry, we further highlighted an evolution of NK subsets between 8 and 12 weeks of gestation, with a shift from the KIR2DL1/S1⁺/KIR2DL2/L3/S2⁺ subset towards the double negative subset, coupled with a decrease of the CD85j⁺/NKG2D⁻ subset in favour of the CD85j⁻/NKG2D⁺ subset. Furthermore, cell surface expression of NK receptor ligands, including CD85j and NKG2D ligands, has been characterized by flow cytometry on decidual immune CD14⁺ and CD3⁺ cells. HLA-G, the high affinity ligand of CD85j, was detected on both cell types. In contrast, NKG2D ligands ULBP-2 ULBP-3 and MICA/B were not expressed on CD14⁺ and CD3⁺ cells, however a variable expression of ULBP-1 was observed. The ligand expression of KIR2DL1/S1 and KIR2DL2/L3/S2 was also analyzed: the HLA-C molecule was expressed at a low level on some CD14⁺ cells whereas it was not detected on CD3⁺ cell surface. NK receptor ligands are known to be also expressed on the invading placental trophoblast cells. Thus, the phenotypic evolutions of decidual NK cells described in this present study may preserve their activation/inhibition balance during the first trimester of pregnancy.