Absence of mouse 2B4 promotes NK cell-mediated killing of activated CD8+ T cells, leading to prolonged viral persistence and altered pathogenesis

Viral cross-class serpin inhibits vascular inflammation and T lymphocyte fratricide; a study in rodent models in vivo and human cell lines in vitro

Poxviruses express highly active inhibitors, including serine proteinase inhibitors (serpins), designed to target host immune defense pathways. Recent work has demonstrated clinical efficacy for a secreted, myxomaviral serpin, Serp-1, which targets the thrombotic and thrombolytic proteases, suggesting that other viral serpins may have therapeutic application. Serp-2 and CrmA are intracellular cross-class poxviral serpins, with entirely distinct functions from the Serp-1 protein. Serp-2 and CrmA block the serine protease granzyme B (GzmB) and cysteine proteases, caspases 1 and 8, in apoptotic pathways, but have not been examined for extracellular anti-inflammatory activity. We examined the ability of these cross-class serpins to inhibit plaque growth after arterial damage or transplant and to reduce leukocyte apoptosis. We observed that purified Serp-2, but not CrmA, given as a systemic infusion after angioplasty, transplant, or cuff-compression injury markedly reduced plaque growth in mouse and rat models in vivo. Plaque growth was inhibited both locally at sites of surgical trauma, angioplasty or transplant, and systemically at non-injured sites in ApoE-deficient hyperlipidemic mice. With analysis in vitro of human cells in culture, Serp-2 selectively inhibited T cell caspase activity and blocked cytotoxic T cell (CTL) mediated killing of T lymphocytes (termed fratricide). Conversely, both Serp-2 and CrmA inhibited monocyte apoptosis. Serp-2 inhibitory activity was significantly compromised either in vitro with GzmB antibody or in vivo in ApoE/GzmB double knockout mice. Conclusions The viral cross-class serpin, Serp-2, that targets both apoptotic and inflammatory pathways, reduces vascular inflammation in a GzmB-dependent fashion in vivo, and inhibits human T cell apoptosis in vitro. These findings indicate that therapies targeting Granzyme B and/or T cell apoptosis may be used to inhibit T lymphocyte apoptosis and inflammation in response to arterial injury.

Immunoregulatory natural killer cells suppress autoimmunity by down-regulating antigen-specific CD8+ T cells in mice

Natural killer (NK) cells belong to the innate immune system. Besides their role in antitumor immunity, NK cells also regulate the activity of other cells of the immune system, including dendritic cells, macrophages, and T cells, and may, therefore, be involved in autoimmune processes. The aim of the present study was to clarify the role of NK cells within this context. Using two mouse models for type 1 diabetes mellitus, a new subset of NK cells with regulatory function was identified. These cells were generated from conventional NK cells by incubation with IL-18 and are characterized by the expression of the surface markers CD117 (also known as c-Kit, stem cell factor receptor) and programmed death (PD)-ligand 1. In vitro analyses demonstrated a direct lysis activity of IL-18-stimulated NK cells against activated insulin-specific CD8(+) T cells in a PD-1/PD-ligand 1-dependent manner. Flow cytometry analyses revealed a large increase of splenic and lymphatic NK1.1(+)/c-Kit(+) NK cells in nonobese diabetic mice at 8 wk of age, the time point of acceleration of adaptive cytotoxic immunity. Adoptive transfer of unstimulated and IL-18-stimulated NK cells into streptozotocin-treated mice led to a delayed diabetes development and partial disease prevention in the group treated with IL-18-stimulated NK cells. Consistent with these data, mild diabetes was associated with increased numbers of NK1.1(+)/c-Kit(+) NK cells within the islets. Our results demonstrate a direct link between innate and adaptive immunity in autoimmunity with newly identified immunoregulatory NK cells displaying a potential role as immunosuppressors.

A critical role for Rictor in T lymphopoiesis

Apart from a critical role for Notch and pre-TCR, the signaling pathway required for T lymphopoiesis is largely unknown. Given the potential link between Notch and mammalian target of rapamycin (mTOR) signaling in cancer cells, we used mice with conditional deletion of either Raptor or Rictor genes to determine potential contribution of the mTOR complex I and II in T lymphopoiesis. Our data demonstrated that targeted mutation of Rictor in the thymocytes drastically reduced the thymic cellularity, primarily by reducing proliferation of the immature thymocytes. Rictor deficiency caused a partial block of thymocyte development at the double-negative 3 stage. The effect of Rictor deficiency is selective for the T cell lineage, as the development of B cells, erythrocytes, and myeloid cells is largely unaffected. Analysis of bone marrow chimera generated from a mixture of wild-type and Rictor-deficient hematopoietic stem cells demonstrated that the function of Rictor is cell intrinsic. These data revealed a critical function of mTOR complex 2 in T lymphopoiesis.

TGF-β1 down-regulation of NKG2D/DAP10 and 2B4/SAP expression on human NK cells contributes to HBV persistence

The mechanism underlying persistent hepatitis B virus (HBV) infection remains unclear. We investigated the role of innate immune responses to persistent HBV infection in 154 HBV-infected patients and 95 healthy controls. The expression of NKG2D- and 2B4-activating receptors on NK cells was significantly decreased, and moreover, the expression of DAP10 and SAP, the intracellular adaptor proteins of NKG2D and 2B4 (respectively), were lower, which then impaired NK cell-mediated cytotoxic capacity and interferon-γ production. Higher concentrations of transforming growth factor-beta 1 (TGF-β1) were found in sera from persistently infected HBV patients. TGF-β1 down-regulated the expression of NKG2D and 2B4 on NK cells in our in vitro study, leading to an impairment of their effector functions. Anti-TGF-β1 antibodies could restore the expression of NKG2D and 2B4 on NK cells in vitro. Furthermore, TGF-β1 induced cell-cycle arrest in NK cells by up-regulating the expression of p15 and p21 in NK cells from immunotolerant (IT) patients. We conclude that TGF-β1 may reduce the expression of NKG2D/DAP10 and 2B4/SAP, and those IT patients who are deficient in these double-activating signals have impaired NK cell function, which is correlated with persistent HBV infection.

NK cells have been viewed as the most important effectors of the initial antiviral innate immune response. Their activation depends on the integration of signals from “co-activation” receptors, and the cytotoxic effects of NK cells on target cells are tempered by a need for combined signals from multiple activating receptors, such as NKG2D and 2B4. In this study, we showed that NKG2D and 2B4 expression levels were decreased on NK cells from patients in the IT phase of HBV infection. We further demonstrated that lower levels of intracellular adaptor proteins (DAP10 and SAP) were associated with lower surface expression of NKG2D and 2B4. As a result, the synergistically co-activated signalling pathway initiated by NKG2D and 2B4 did not operate properly in IT-phase patients. We demonstrated that high levels of soluble TGF-β1 were associated with the reduction of NKG2D and 2B4 in patients. In addition, we showed that TGF-β1 causes the cell-cycle arrest of NK cells by up-regulating the levels of p15 and p21 in NK cells from IT patients. Collectively, these findings may contribute to our understanding of the immune tolerance mechanism and aid in the development of novel therapeutic methods to clear HBV infection during the initial phase.

Natural killer cell activation enhances immune pathology and promotes chronic infection by limiting CD8+ T-cell immunity

Infections with HIV, hepatitis B virus, and hepatitis C virus can turn into chronic infections, which currently affect more than 500 million patients worldwide. It is generally thought that virus-mediated T-cell exhaustion limits T-cell function, thus promoting chronic disease. Here we demonstrate that natural killer (NK) cells have a negative impact on the development of T-cell immunity by using the murine lymphocytic choriomeningitis virus. NK cell-deficient (Nfil3(-/-), E4BP4(-/-)) mice exhibited a higher virus-specific T-cell response. In addition, NK cell depletion caused enhanced T-cell immunity in WT mice, which led to rapid virus control and prevented chronic infection in lymphocytic choriomeningitis virus clone 13- and reduced viral load in DOCILE-infected animals. Further experiments showed that NKG2D triggered regulatory NK cell functions, which were mediated by perforin, and limited T-cell responses. Therefore, we identified an important role of regulatory NK cells in limiting T-cell immunity during virus infection.

Natural killer cells act as rheostats modulating antiviral T cells

Antiviral T cells are thought to regulate whether hepatitis C virus (HCV) and human immunodeficiency virus (HIV) infections result in viral control, asymptomatic persistence or severe disease, although the reasons for these different outcomes remain unclear. Recent genetic evidence, however, has indicated a correlation between certain natural killer (NK)-cell receptors and progression of both HIV and HCV infection, implying that NK cells have a role in these T-cell-associated diseases. Although direct NK-cell-mediated lysis of virus-infected cells may contribute to antiviral defence during some virus infections--especially murine cytomegalovirus (MCMV) infections in mice and perhaps HIV in humans--NK cells have also been suspected of having immunoregulatory functions. For instance, NK cells may indirectly regulate T-cell responses by lysing MCMV-infected antigen-presenting cells. In contrast to MCMV, lymphocytic choriomeningitis virus (LCMV) infection in mice seems to be resistant to any direct antiviral effects of NK cells. Here we examine the roles of NK cells in regulating T-cell-dependent viral persistence and immunopathology in mice infected with LCMV, an established model for HIV and HCV infections in humans. We describe a three-way interaction, whereby activated NK cells cytolytically eliminate activated CD4 T cells that affect CD8 T-cell function and exhaustion. At high virus doses, NK cells prevented fatal pathology while enabling T-cell exhaustion and viral persistence, but at medium doses NK cells paradoxically facilitated lethal T-cell-mediated pathology. Thus, NK cells can act as rheostats, regulating CD4 T-cell-mediated support for the antiviral CD8 T cells that control viral pathogenesis and persistence.

Transplantation of CD6-depleted peripheral blood stem cells after DLA-haploidentical bone marrow transplantation contributes to engraftment and tolerance in a preclinical model of stem cell transplantation

Human leukocyte antigen (HLA)-haploidentical stem cell transplantation is an opportunity for nearly all patients lacking an HLA matched stem cell donor. However, graft rejection and graft-versus-host disease (GvHD) as well as infectious complications still result in high treatment-related mortality. Here, we used the dog as a preclinical model for the study of tolerance induction with the aim to optimize and to improve a clinical protocol of haploidentical stem cell transplantation. For this purpose CD6-depleted peripheral blood stem cells (PBSCs) were transfused 6d after transplantation of unmodified bone marrow from dog leukocyte antigen (DLA)-haploidentical littermate donors in order to induce immune tolerance. Besides hematopoietic stem cells CD6-depleted PBSC contain, NK cells and a minority of suppressive CD8-positive cells that may suppress activated T lymphocytes. Recipients were conditioned with, cyclophosphamide and antithymocyte globulin (ATG) preceded by a transfusion of donor buffy coat and either 1, 2 or 3 × 3.3 Gy total body irradiation (TBI). Postgrafting immunosuppression was limited to 30 d of cyclosporine and methotrexate. The additional administration of CD6-depleted PBSCs after unmodified marrow could not prevent GvHD, but it may improve engraftment and chimerism after conditioning with 2 × 3.3 Gy TBI. Reasons for incomplete suppression and possible improvements for clinical applications are discussed.

Tight regulation of memory CD8(+) T cells limits their effectiveness during sustained high viral load

To design successful vaccines for chronic diseases, an understanding of memory CD8(+) T cell responses to persistent antigen restimulation is critical. However, most studies comparing memory and naive cell responses have been performed only in rapidly cleared acute infections. Herein, by comparing the responses of memory and naive CD8(+) T cells to acute and chronic lymphocytic choriomeningitis virus infection, we show that memory cells dominated over naive cells and were protective when present in sufficient numbers to quickly reduce infection. In contrast, when infection was not rapidly reduced, because of high antigen load or persistence, memory cells were quickly lost, unlike naive cells. This loss of memory cells was due to a block in sustaining cell proliferation, selective regulation by the inhibitory receptor 2B4, and increased reliance on CD4(+) T cell help. Thus, emphasizing the importance of designing vaccines that elicit effective CD4(+) T cell help and rapidly control infection.

SLAM family receptors and SAP adaptors in immunity

The signaling lymphocyte activation molecule (SLAM)-associated protein, SAP, was first identified as the protein affected in most cases of X-linked lymphoproliferative (XLP) syndrome, a rare genetic disorder characterized by abnormal responses to Epstein-Barr virus infection, lymphoproliferative syndromes, and dysgammaglobulinemia. SAP consists almost entirely of a single SH2 protein domain that interacts with the cytoplasmic tail of SLAM and related receptors, including 2B4, Ly108, CD84, Ly9, and potentially CRACC. SLAM family members are now recognized as important immunomodulatory receptors with roles in cytotoxicity, humoral immunity, autoimmunity, cell survival, lymphocyte development, and cell adhesion. In this review, we cover recent findings on the roles of SLAM family receptors and the SAP family of adaptors, with a focus on their regulation of the pathways involved in the pathogenesis of XLP and other immune disorders.

Cutting edge: an NK cell-independent role for Slamf4 in controlling humoral autoimmunity

Several genes within a syntenic region of human and mouse chromosome 1 are associated with predisposition to systemic lupus erythematosus. Analyses of lupus-prone congenic mice have pointed to an important role for the signaling lymphocyte activation molecule family (slamf)6 surface receptor in lupus pathogenesis. In this article, we demonstrate that a second member of the Slamf gene family, Slamf4 (Cd244), contributes to lupus-related autoimmunity. B6.Slamf4(-/-) mice spontaneously develop activated CD4 T cells and B cells and increased numbers of T follicular helper cells and a proportion develop autoantibodies to nuclear Ags. B6.Slamf4(-/-) mice also exhibit markedly increased autoantibody production in the B6.C-H-2bm12/KhEg → B6 transfer model of lupus. Although slamf4 function is best characterized in NK cells, the enhanced humoral autoimmunity of B6.Slamf4(-/-) mice is NK cell independent, as judged by depletion studies. Taken together, our findings reveal that slamf4 has an NK cell-independent negative regulatory role in the pathogenesis of lupus a normally non-autoimmune prone genetic background.

The human decidual NK-cell response to virus infection: what can we learn from circulating NK lymphocytes?

NK cells present in the peripheral blood respond rapidly to pathogens or pathogen-infected cells by various means including cytotoxicity and production of cytokines. Whether decidual NK (dNK) cells are able to play a similar role when the pregnant uterus is infected by viruses is still largely unknown. Decidual NK cells are generally considered as poorly cytotoxic when compared to their peripheral blood counterparts. However, we have recently demonstrated that freshly isolated dNK cells from healthy early pregnant uterus do have a cytotoxic potential mediated by the specific engagement of NKp46 activating receptor. We further found that the co-engagement of CD94/NKG2A inhibiting receptor drastically inhibits the cytolytic function of dNK. This latter observation suggests that in situ the CD94/NKG2A receptor interaction with its HLA-E specific ligand is a dominant negative regulatory mechanism that prevents unwanted dNK cell cytotoxicity in non-infected pregnant uterus. How do dNK cells behave when they are activated by virus-infected cells present at the maternal-fetal interface? Largely based on data obtained from circulating NK cells, this review briefly discusses the following questions: Does uterine viral infection promote decidual NK cell proliferative capacity in situ? Are dNK cells able to kill virus-infected autologous decidual target cells and thus limit the virus spreading to the fetus? Which viral-mediated signal(s) and molecular interactions may subvert inhibition of dNK cytotoxic potential? Does uterine viral infection promote decidual NK cell secretion of cytokines and chemokines that boost the anti-viral immune response?

NK cells: elusive participants in transplantation immunity and tolerance

NK cells constitute an innate MHC class I-reactive lymphoid population that rapidly responds to infection, injury, or cell distress. In the transplant field, NK cells have most often been associated with pro-inflammatory immunity resulting in the exacerbation of allograft injury. Despite this general view of NK cell reactivity, it has been challenging to assign unambiguous obligate roles for NK cells in the allograft response. While recent reports continue to provide evidence supporting a role for NK cells in promoting both acute and chronic rejection, there are also a growing number of studies that illustrate an alternative role for NK cells in promoting allograft survival and tolerance. This review addresses the plasticity of NK responses in transplantation by suggesting specific 'checkpoints' whereby NK cells can either enhance or inhibit the allograft response in vivo.

Mechanism of induction of NK activation by 2B4 (CD244) via its cognate ligand

We have previously shown that coincubation of purified B cells with IL-2-propagated NK cells can result in the induction of IL-13 mRNA and that the induction requires the presence of CD48 on B cells and 2B4 on NK cells. Because both of these molecules are expressed on NK cells, it is surprising that very little IL-13 mRNA can be detected in the absence of B cells. We have now found that incubation of NK cells on plates containing immobilized anti-CD48 Abs results in the clustering of CD48 and colocalization with 2B4 on the same cell. This colocalization, together with the requirement for SAP, the signal transducer for 2B4, is necessary for the induction of IL-13 mRNA expression. Activation of NK cell via CD48 on another cell may require a similar ability to alter the configuration of 2B4 to activate downstream signaling. By the use of double CD2/2B4 knockout mice, we have also shown that the induction of NK cell activation by anti-CD48 or by B cells is not due to the release of inhibitory effects of 2B4.