Preventing NK cell activation by donor dendritic cells enhances allospecific CD4 T cell priming and promotes Th type 2 responses to transplantation antigens

Recipient natural killer cells alter the course of rejection of allogeneic heart grafts in rats

Rejection of solid organ grafts is regarded to be dependent on T cell responses. Nonetheless, numerous studies have focused on the contribution of NK cells in this process, resulting in contradictory theories. While some conclude that there is no participation of NK cells, others found an inflammatory or regulative role of NK cells. However, the experimental settings are rarely comparable with regard to challenged species, strain combinations or the nature of the graft. Thus, clear definition of NK cell contribution might be impeded by these circumstances. In this study we performed heterotopic heart transplantation (HTx) in rats, choosing one donor-recipient-combination leading to a fast and a second leading to a prolonged course of graft rejection. We intervened in the rejection process, by depletion of recipient NK cells on the one hand and by injection of activated NK cells syngeneic to the recipients on the other. The fast course of rejection could not be influenced by any of the NK cell manipulative treatments. However, the more prolonged course of rejection was highly susceptible to depletion of NK cells, resulting in significant acceleration of rejection, while injection of NK cells induced acceptance of the grafts. We suggest that, depending on the specific setting, NK cells can attenuate the first trigger of immune response, which allows establishing the regulatory activity leading to tolerance of the graft.

Effector mechanisms of rejection

Organ transplantation appears today to be the best alternative to replace the loss of vital organs induced by various diseases. Transplants can, however, also be rejected by the recipient. In this review, we provide an overview of the mechanisms and the cells/molecules involved in acute and chronic rejections. T cells and B cells mainly control the antigen-specific rejection and act either as effector, regulatory, or memory cells. On the other hand, nonspecific cells such as endothelial cells, NK cells, macrophages, or polymorphonuclear cells are also crucial actors of transplant rejection. Last, beyond cells, the high contribution of antibodies, chemokines, and complement molecules in graft rejection is discussed in this article. The understanding of the different components involved in graft rejection is essential as some of them are used in the clinic as biomarkers to detect and quantify the level of rejection.

Donor and recipient HLA/KIR genotypes do not predict liver transplantation outcome

Whether or not Natural Killer (NK) cells affect the immune response to solid organ allografts is still controversial. Main determinants of NK-cell activation are specific HLA/killer-cell immunoglobulin-like receptors (KIR) interactions that, in transplantation, may induce NK-cell alloreactivity. So far, in liver transplantation (LTX) donor-versus-recipient alloreactivity has not been investigated; in addition, studies of predicted recipient-versus-donor NK-cell alloreactivity have led to contradicting results. We typed a cohort of LTX donors and recipients for HLA-C/Bw4 and KIRs. We estimated the effect of NK-cell alloreactivity, as predicted by classically used models, in the donor-versus-recipient direction. The results indicate that HLA/KIR mismatches in the donor-versus-recipient direction do not predict graft rejection nor graft or patient survival, suggesting that donor-derived NK cells do not play a major role in LTX outcome. In addition, when considering predicted NK-cell alloreactivity in the reverse direction (recipient-versus-donor), we first confirmed that donor HLA-C genotype was not associated with acute rejection, graft or patient survival and secondly we found that none of the models describing NK-cell alloreactivity could predict LTX outcome. Overall our observations suggest that, in contrast to what is shown in haematopoietic stem cell transplantation, donor-derived NK cells may not contribute in preventing liver graft rejection, and that recipient-versus-donor NK-cell alloreactivity does not predict LTX outcome.

In vitro expansion of alloantigen-specific regulatory T cells and their use in prevention of allograft rejection

Regulatory T lymphocytes expressing CD4, high levels of CD25, and the transcription factor Foxp3 play a crucial role in the control of immune responses to self and nonself antigens. In contrast to immunosuppressive drugs currently used to treat immunopathology, these cells act in a very specific manner. Consequently, their clinical potential in the treatment of autoimmune disorders, inflammatory diseases, graft-versus-host disease, and allograft rejection is currently extensively studied in experimental animal models as well as in clinical trials. We have previously shown that appropriately in vitro stimulated CD4(+)CD25(high) regulatory T cells can be used to prevent rejection of bone marrow, skin, and heart allografts in the Mouse. We here describe the protocols used in our laboratory to isolate mouse regulatory T cells, to stimulate them in vitro in order to enrich in cells specific for donor-antigens, and to transplant bone marrow under cover of regulatory T cells. Thus, generated hematopoietic chimeras may subsequently be transplanted with solid tissues and organs from the same donor.

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.

NK cell patrolling and elimination of donor-derived dendritic cells favor indirect alloreactivity

Direct presentation of foreign MHC molecules expressed by donor-derived dendritic cells (DCs) has generally been considered the dominant pathway of allorecognition in acute transplant rejection. However, recent studies implicate preferential activation of the indirect pathway by host DCs. The respective importance of each pathway and the mechanisms that determine their relative contributions remain to be clearly established. In this study, using two-photon microscopy, we visualized host NK cell interactions with syngeneic and allogeneic DCs within intact lymph nodes of mice. Upon contact with allogeneic DCs, NK cells formed prolonged interactions that led directly to target cell lysis. This rapid elimination limited the ability of allogeneic DCs to stimulate primary and recall T cell responses. To discriminate whether donor or host DCs are principally involved in presenting Ag derived from allografts, we used CD11c-diphtheria toxoid receptor mice to conditionally ablate CD11c(+) DCs and to show that direct presentation by donor DCs is alone insufficient to elicit acute allograft rejection. We thus propose that rapid elimination of allogeneic DCs limits direct Ag presentation and thereby favors the indirect pathway of alloreactivity.

Role of NK and NKT cells in solid organ transplantation

In the context of solid organ transplantation, the exact interactions between the innate and adaptive alloimmune response have not yet been fully explored. In this transplant setting, natural killer (NK) cells have emerged as a particular focus of interest because of their ability to distinguish allogeneic major histocompatibility complex (MHC) antigens and their potent cytolytic activity. Based on this observation and its potential clinical relevance, NK cells have recently been shown to participate in the immune response in both acute and chronic rejection of solid organ allografts. Numerous experimental and clinical studies demonstrate that NK cells determine transplant survival by rejecting an allograft not directly but indirectly by providing bystander effects. In addition, NK cells are influenced by immunosuppressive therapies such as calcineurin inhibitors or steroids. As NK and natural killer T (NKT) cells have also been shown to play a profound role in allograft tolerance induction, this review summarizes the major findings to highlight the functional role of these lymphocyte subsets, which may constitute an underestimated mechanism affecting graft outcome in solid organ transplantation.

Killers and beyond: NK-cell-mediated control of immune responses

Effective immunity requires coordinated activation of innate and adaptive immune responses. NK cells are principal mediators of innate immunity, able to respond to challenge quickly and generally without prior activation. The most acknowledged functions of NK cells are their cytotoxic potential and their ability to release large amounts of cytokines, especially IFN-gamma. Recently, it has become clear that NK cells are more than assassins. Indeed, NK cells play critical roles in shaping adaptive immunity.

Recipient Immune Repertoire and Engraftment Site Influence the Immune Pathway Effecting Acute Hepatocellular Allograft Rejection

As novel acute allograft rejection mechanisms are being discovered, determining the conditions that promote or subvert these distinct rejection pathways is important to interpret the clinical relevance of these pathways for specific recipient groups as well as specific tissue and organ transplants. We have employed a versatile hepatocellular allograft model to analyze how the host immune repertoire and immune locale influences the phenotype of the rejection pathway. In addition, we investigated how peripheral monitoring of cellular and humoral immune parameters correlates with the activity of a specific rejection pathway. Complete MHC mismatched hepatocellular allografts were transplanted into immune competent CD4-deficient, CD8-deficient, or C57BL/6 hosts to focus on CD8-dependent, CD4-dependent, or combined CD4 and CD8-dependent alloimmunity, respectively. Hepatocellular allografts were transplanted to the liver or kidney subcapsular space to investigate the influence of the immune locale on each rejection pathway. The generation of donor-reactive DTH, alloantibody, and allospecific cytotoxicity was measured to assess both cellular and humoral immunity. Graft-infiltrating lymphocytes were phenotyped and enumerated in each recipient group. In the presence of CD8+ T cells, cytolytic cellular activity is the dominant mechanism of graft destruction and is amplified in the presence of CD4+ T cells. The absence of CD8+ T cells (CD8 KO) results in potent humoral immunity as reflected by high levels of cytotoxic alloantibody and graft rejection with similar kinetics. Transplant to the liver compared to the kidney site is distinguished by more rapid kinetics of rejection and alloimmunity, which is predominately cell mediated rather than a mix of both humoral and cell-mediated immunity. These studies define several rejection mechanisms occurring in distinct immune conditions, highlighting the plasticity of acute allograft rejection responses and the need to design specific monitoring strategies for these pathways to allow dynamic immune assessment of clinical transplant recipients and targeted immunotherapies.

Natural killer cells recruited into lymph nodes inhibit alloreactive T-cell activation through perforin-mediated killing of donor allogeneic dendritic cells

Natural killer (NK)-cell alloreactivity is exploited in bone marrow transplantation to improve clinical outcome. Likewise, in solid organ transplantation, it has been recently shown that recipient NK cells may limit alloreactive T-cell responses through their capacity to prevent the persistence of graft-derived allogeneic dendritic cells (DCs). In a model of CD4(+) T cell-mediated allogeneic skin graft rejection, we show that the absence of host NK-cell alloreactivity was characterized by enhanced expansion of alloreactive effector T lymphocytes, including Th2 cells, and massive eosinophilic infiltrates in the rejected tissues. In CD8(+) T cell-deficient C57BL/6 (H-2(b)) recipients injected with allogeneic BALB/c (H-2(d)) DCs, we demonstrated that NK cells expressing the H-2D(d)-specific Ly49D activating receptor were implicated in the regulation of alloreactive CD4(+) T-cell responses. Moreover, we showed that Ly49D(+) CD127(-) NK cells were recruited within DC draining lymph nodes and rapidly eliminated allogeneic H-2(d) DCs through the perforin pathway. In normal mice, we further demonstrated that NK cells by quickly eliminating allogeneic DCs strongly inhibited alloreactive CD8(+) T-cell responses. Thus, NK cells act as early regulators of alloreactive T-cell priming in allotransplantation through their capacity to kill allogeneic DCs in draining lymph nodes.

Natural killer and dendritic cell liaison: recent insights and open questions

The functional links between natural killer (NK) and dendritic cells (DCs) have been widely investigated in the last years and different studies have demonstrated that reciprocal activations ensue upon NK/DC interactions. More recently, the anatomical sites were these interactions take place have been identified together with the related cell subsets involved. Remarkably, as predicted by pioneering studies, there is now "in vivo" evidence that this cellular cross-talk occurring during the innate phase of the immune response can deeply affects the magnitude and the quality of the subsequent adaptive response. Thus, NK cells are not merely cytotoxic lymphocytes competent in containing viral and tumor spreading but can now rather be considered as crucial fine-tuning effector cells. Despite the large mass of information rapidly obtained in this field, several fundamental questions still remain to be addressed. Among them, two central issues require additional consideration: (a) what mediates the activation of NK cell cytotoxicity induced by DCs and (b) what factors are responsible for NK-dependent maturation of DCs. Unexpectedly, for both of these questions insufficient or inconsistent results are so far available; factors either dependent or independent from cell contacts between DCs and NK cells have been convincingly described, and it is likely that several mechanisms, rather than a single one, are responsible for each of these novel innate functions. Understanding the molecular bases of this complex liaison will pave the way to new and more effective immune adjuvants. The most recent advances about NK/DC interplay are here reviewed and possible answers to still open questions in this field are considered.

Prevention of acute and chronic allograft rejection with CD4+CD25+Foxp3+ regulatory T lymphocytes

A major challenge in transplantation medicine is controlling the very strong immune responses to foreign antigens that are responsible for graft rejection. Although immunosuppressive drugs efficiently inhibit acute graft rejection, a substantial proportion of patients suffer chronic rejection that ultimately leads to functional loss of the graft. Induction of immunological tolerance to transplants would avoid rejection and the need for lifelong treatment with immunosuppressive drugs. Tolerance to self-antigens is ensured naturally by several mechanisms; one major mechanism depends on the activity of regulatory T lymphocytes. Here we show that in mice treated with clinically acceptable levels of irradiation, regulatory CD4+CD25+Foxp3+ T cells stimulated in vitro with alloantigens induced long-term tolerance to bone marrow and subsequent skin and cardiac allografts. Regulatory T cells specific for directly presented donor antigens prevented only acute rejection, despite hematopoietic chimerism. By contrast, regulatory T cells specific for both directly and indirectly presented alloantigens prevented both acute and chronic rejection. Our findings demonstrate the potential of appropriately stimulated regulatory T cells for future cell-based therapeutic approaches to induce lifelong immunological tolerance to allogeneic transplants.

NK cells at the interface between innate and adaptive immunity

In recent years a novel concept has emerged indicating that the actual role of natural killer (NK) cells is not confined to the destruction of virus-infected cells or tumors. Indeed, different NK subsets exist that display major functional differences in their cytolytic activity, cytokine production and homing capabilities. In particular, CD56(high) CD16(-) NK cells that largely predominate in lymph nodes, have little cytolytic activity but release high levels of cytokines whereas CD56(low) CD16(+) NK cells that predominate in peripheral blood and inflamed tissues, display lower cytokine production, but potent cytotoxicity. The latter is characterized by granule polarization and exocytosis of various proteins including perforin and granzymes that mediate target cell killing. The recruitment of CD56(low) CD16(+) NK cells into inflamed peripheral tissues is orchestrated by various chemochines including the newly identified Chemerin. At these sites, NK cells, upon engagement of different triggering receptors become activated and upregulate their cytokine production and cytotoxicity after interaction with myeloid dendritic cells (DCs). Importantly, during this interaction NK cells also mediate the 'editing' of DCs undergoing maturation. This process appears to play a crucial role in shaping both innate and adaptive immune responses. Indeed, only DCs undergoing this NK-mediated quality control would become fully mature and capable of inducing priming of protective Th1 responses.

Effector and regulatory events during natural killer-dendritic cell interactions

The different cell types of the innate immune system can interact with each other and influence the quality and strength of an immune response. The cross talk between natural killer (NK) cells and myeloid dendritic cells (DCs) leads to NK cell activation and DC maturation. Activated NK cells are capable of killing DCs that fail to undergo proper maturation ('DC editing'). Encounters between NK cells and DCs occur in both inflamed peripheral tissues and lymph nodes, where both cell types are recruited by chemokines released in the early phases of inflammatory responses. Different NK cell subsets (CD56(bright)CD16(-) versus CD56(+)CD16(+)) differ in their homing capabilities. In particular, CD56(bright)CD16(-) NK cells largely predominate the lymph nodes. In addition, these two subsets display major functional differences in their cytolytic activity, cytokine production, and ability to undergo proliferation. NK cell functions are also greatly influenced by the presence of polarizing cytokines such as interleukin (IL)-12 and IL-4. The cytokine microenvironment reflects the presence of different cell types that secrete such cytokines in response to microbial products acting on different Toll-like receptors (TLRs). Moreover, NK cells themselves can respond directly to microbial products by means of TLR3 and TLR9. Thus, it appears that the final outcome of a response to microbial infection may greatly vary as a result of the interactions occurring between different pathogen-derived products and different cell types of the innate immunity system. These interactions also determine the quality and strength of the subsequent adaptive responses. Remarkably, NK cells appear to play a key role in this complex network.

Multidirectional interactions are bridging human NK cells with plasmacytoid and monocyte-derived dendritic cells during innate immune responses

During innate immune responses, natural killer (NK) cells may interact with both plasmacytoid dendritic cells (pDCs) and monocyte-derived dendritic cells (MDDCs). We show that freshly isolated NK cells promote the release by pDCs of IFN-α, in a CpG-dependent manner, whereas they induce IL-6 production in a CpG-independent manner. In turn pDC-derived IFN-α up-regulates NK-mediated killing, whereas IL-6 could promote B-cell differentiation. We also show that exposure to exogenous IL-12 or coculture with maturing MDDCs up-regulates the NK-cell–dependent IFN-α production by pDCs. On the other hand, NK cells cocultured with pDCs acquire the ability to kill immature MDDCs, thus favoring their editing process. Finally, we show that activated NK cells are unable to lyse pDCs because these cells display an intrinsic resistance to lysis. The exposure of pDCs to IL-3 increased their susceptibility to NK-cell cytotoxicity resulting from a de novo expression of ligands for activating NK-cell receptors, such as the DNAM-1 ligand nectin-2. Thus, different cell-to-cell interactions and various cytokines appear to control a multidirectional network between NK cells, MDDCs, and pDCs that is likely to play an important role during the early phase of innate immune responses to viral infections and to tumors.

Immunologic responses to critical injury and sepsis

Almost 2 million patients are admitted to hospitals in the United States each year for treatment of traumatic injuries, and these patients are at increased risk of late infections and complications of systemic inflammation as a result of injury. Host response to injury involves a general activation of multiple systems in defending the organism from hemorrhagic or infectious death. Clinicians have the capability to support the critically injured through their traumatic insult with surgery and improved critical care, but the inflammatory response generated by such injuries creates new challenges in the management of these patients. It has long been known that local tissue injury induces systemic changes in the traumatized patient that are often maladaptive. This article reviews the effects of injury on the function of immune system cells and highlights some of the clinical sequelae of this deranged inflammatory-immune interaction.

Role of natural killer cell function in dendritic cell-based vaccines

Recent studies have elucidated the functional links between natural killer (NK) cells and, demonstrating the reciprocal activation of these cell types through NK-DC interactions. The subsets of cells and molecular pathways involved in such interactions have been defined, and the possible anatomical sites of these interactions have also been reported. Murine experiments have demonstrated that injection of mature DCs induces rapid recruitment of NK cells to lymph nodes and that these NK cells provide interferon-gamma for Type 1 priming. Thus, there is an increasing body of in vivo evidence indicating that NK-DC interactions during the early phase of innate immunity can impact the quality and magnitude of the subsequent adaptive immune response. Importantly, these studies imply that NK cells might not serve merely as cytotoxic lymphocytes combating viral pathogens and malignant tumors, but must also be considered as important immunoregulatory cells with a significant influence on adaptive immunity. In contrast to the large volume of knowledge obtained through basic research, there is a relative paucity of information regarding NK cell function in adaptive immunity from clinical trials, as few DC vaccine studies have attempted to evaluate the nonspecific, yet potentially clinically relevant, NK response to immunization. In this article, the authors will review studies focusing on NK-DC interactions and highlight the most recent clinical findings relating to the potential role of NK cells in DC-based vaccine therapy.

Natural killer-dendritic cell cross-talk in cancer immunotherapy

Natural killer (NK) cells and dendritic cells (DCs), two important components of the immune system, can exchange bidirectional activating signals in a positive feedback. Myeloid DCs, the cell type specialised in the presentation of antigen and initiation of antigen-specific immune responses, have recently been documented to be involved in supporting innate immunity, promoting the production of cytokines and cytotoxicity of NK cells, and enhancing their tumouricidal activity. Natural interferon-producing cells/plasmacytoid DCs (IPCs/PDCs) play an additional role in NK cell activation. Reciprocally, NK cells, traditionally considered to be major innate effector cells, have also recently been shown to play immunoregulatory 'helper' functions, being able to activate DCs and to enhance their ability to produce pro-inflammatory cytokines, and to stimulate T helper (Th) 1 and cytotoxic T lymphocyte (CTL) responses of tumour-specific CD4+ and CD8+ T cells. Activated NK cells induce the maturation of myeloid DCs into stable type-1 polarised DCs (DC1), characterised by up to a 100-fold enhanced ability to produce IL-12p70 in response to subsequent interaction with Th cells. In addition, the ability of NK cells to kill tumour cells may facilitate the generation of tumour-related antigenic material, further accelerating the induction of tumour-specific immunity. DC1, induced by NK cells or by NK cell-related soluble factors, are stable, resistant to tumour-related suppressive factors, and demonstrate a strongly enhanced ability to induce Th1 and CTL responses in human in vitro and mouse in vivo models. Compared with the standard mature DCs that are used in clinical trials at present, human NK cell-induced DC1s act as superior inducers of anticancer CTL responses during in vitro sensitisation. This provides a strong rationale for the combined use of NK cells and DCs in the immunotherapy of patients with cancer and patients with chronic infections that are resistant to standard forms of treatment. Stage I/II clinical trials that are being implemented at present should allow evaluation of the immunological and clinical efficacy of combined NK-DC therapy of melanoma and other cancers.

The changing role of natural killer cells in solid organ rejection and tolerance

Natural killer (NK) cells have emerged as a particular focus of interest in transplantation due to their ability to distinguish allogeneic major histocompatibility complex (MHC) antigens and their potent cytolytic effector mechanisms. Once relegated to the field of bone marrow transplantation, NK cells have recently been shown to participate in the immune response against solid organ allo- and xenografts. These new findings suggest that the role of NK cells in solid organ rejection and tolerance needs to be reexamined.

CD8+ T-cell-mediated killing of donor dendritic cells prevents alloreactive T helper type-2 responses in vivo

Accumulating evidence indicates that, in absence of CD8+ T-cell activation, CD4+ T-cell-mediated allograft rejection is associated with a dominant Th2-cell response and eosinophil infiltrates. In this study, we analyzed the mechanisms by which CD8+ T cells regulate alloreactive CD4+ T-cell priming and differentiation into interleukin 4 (IL-4)-producing cells. We showed that interferon gamma (IFN-gamma) production by CD8+ T cells was dispensable for the inhibition of Th2-cell development, as well as tissue eosinophilia and type 2 cytokine production in the rejected grafts. Since we noticed that CD8+ T cells not only suppressed Th2 differentiation, but also down-modulated the overall priming of alloreactive CD4+ T cells, we evaluated whether CD8+ T cells act by limiting the accumulation of donor-derived dendritic cells (DCs) in lymph nodes. We found that indeed, alloreactive CD8+ T cells rapidly eliminated allogeneic DCs from T-cell areas of draining lymph nodes, through a perforin-dependent mechanism. Thus, our data demonstrate that cytotoxic T lymphocyte (CTL)-mediated clearance of allogeneic DCs is a negative feedback mechanism that limits the duration of alloantigen presentation in draining lymph nodes, thereby modulating the amplitude and polarization of the primary alloreactive CD4+ T-cell responses.

TWEAK Attenuates the Transition from Innate to Adaptive Immunity

Innate immunity is the first line of defense against infection, protecting the host during the development of adaptive immunity and critically affecting the nature of the adaptive response. We show that, in contrast to tumor necrosis factor alpha (TNF-alpha), the related protein TWEAK attenuates the transition from innate to adaptive mechanisms. TWEAK-/- mice had overabundant natural killer (NK) cells and displayed hypersensitivity to bacterial endotoxin, with their innate immune cells producing excess interferon (IFN)-gamma and interleukin (IL)-12. TWEAK inhibited stimulation of the transcriptional activator STAT-1 and induced p65 nuclear factor (NF)-kappaB association with histone deacetylase 1, repressing cytokine production. TWEAK-/- mice developed oversized spleens with expanded memory and T helper 1 (TH1) subtype cells upon aging and mounted stronger innate and adaptive TH1-based responses against tumor challenge. Thus, TWEAK suppresses production of IFN-gamma and IL-12, curtailing the innate response and its transition to adaptive TH1 immunity.

Early liaisons between cells of the innate immune system in inflamed peripheral tissues

The crosstalk between natural killer (NK) cells and myeloid dendritic cells (DCs) results in NK-cell activation and DC maturation. Activated NK cells acquire the ability to kill DCs that have failed to undergo complete maturation ('DC editing'). Recent studies have revealed that this crosstalk can be promoted by pathogen-derived products that activate different innate immune cell types directly and simultaneously through their Toll-like receptors (TLRs). These cells include NK cells and DCs, as well as plasmacytoid DCs (PDCs) and mast cells. This crosstalk can have a great impact on the quality and strength of the subsequent adaptive immune response. Thus, NK cells have an important role in the defense against pathogens, acting as regulatory cells as well as effector cells.

CD4+ T-cell-dependent immune damage of liver parenchymal cells is mediated by alloantibody

BACKGROUND

Allogeneic hepatocytes initiate both CD4- and CD8-dependent rejection responses. The current studies address the hypothesis that acute damage of allogeneic liver parenchymal cells by the CD4-dependent pathway is alloantibody-mediated and examines immune conditions which promote activation of this pathway.

METHODS

The role of alloantibody in CD4-dependent hepatocyte rejection was evaluated by assessing hepatocyte (FVB/N, H-2q) survival in CD8-depleted B-cell knockout (KO) (H-2b) recipients and by monitoring hepatocyte survival in C57BL/6.SCID (H-2b) recipients transfused with donor-reactive alloantibody. The development of donor-reactive alloantibody in C57BL/6 (H-2b), CD8-depleted C57BL/6, CD8 KO (H-2b), IFN-gamma KO (H-2b), perforin KO (H-2b), and FasL mutant gld/gld (H-2b) hepatocyte recipients was assessed.

RESULTS

Hepatocyte rejection in B-cell KO mice was significantly delayed by CD8+ T-cell depletion (median survival time [MST], 35 days) when compared to untreated (MST, 8 days) and CD4-depleted (MST, 10 days) recipient mice. Transfusion of donor-reactive alloantibody into SCID recipients with functional hepatocellular allografts was sufficient to precipitate rejection in a dose-dependent fashion. Donor-reactive alloantibody was minimal in the serum of C57BL/6 hepatocyte recipients, but was produced in significant quantities in hepatocyte recipients genetically deficient in or depleted of CD8+ T cells and in recipients with impaired cytotoxic effector mechanisms. In addition, recipients with defects in Th1 immunity, such as IFN-gamma KO recipients, also produced readily detectable alloantibody.

CONCLUSIONS

Collectively, these data support the hypothesis that acute immune damage of allogeneic hepatocytes by the CD4-dependent pathway is mediated by alloantibody and that this pathway is favored when Th1- or cell-mediated cytotoxic effector immune mechanisms are impaired.

NK cells promote islet allograft tolerance via a perforin-dependent mechanism

Although major histocompatibility complex (MHC) class II-restricted CD4 T cells are well appreciated for their contribution to peripheral tolerance to tissue allografts, little is known regarding MHC class I-dependent reactivity in this process. Here we show a crucial role for host MHC class I-dependent NK cell reactivity for allograft tolerance in mice induced through either costimulation blockade using CD154-specific antibody therapy or by targeting LFA-1 (also known as CD11a). Tolerance induction absolutely required host expression of MHC class I, but was independent of CD8 T cell-dependent immunity. Rather, tolerance required innate immunity involving NK1.1(+) cells, but was independent of CD1d-restricted NKT cells. Therefore, NK cells seem to be generally required for induction of tolerance to islet allografts. Additional studies indicate that CD154-specific antibody-induced allograft tolerance is perforin dependent. Notably, NK cells that are perforin competent are sufficient to restore allograft tolerance in perforin-deficient recipients. Together, these results show an obligatory role for NK cells, through perforin, for induction of tolerance to islet allografts.

Engraftment of human T, B and NK cells in CB.17 SCID/beige mice by transfer of human spleen cells

Models of severe combined immuno-deficient (SCID) mice reconstituted with a competent human immune system represent a valuable tool for the study of human immune responses in vivo. Reconstitution with human cells can be achieved using large numbers of peripheral blood lymphocytes, but levels of engraftment are poor and graft versus host disease (GVHD) frequently occurs. SCID/beige mice are at the same time deficient for adaptive and innate immunity and the objective of this study was to develop a safe and efficient way to achieve human lymphocyte engraftment in these mice using human spleen cells. After institutional authorisations and informed consent of relatives, a piece of spleen was obtained from cadaveric organ donors and the splenocytes were isolated and cryopreserved for later use. Single intraperitoneal injections of 5-100 x10(6) splenocytes were performed into SCID/beige mice. Reconstitution of a human immune system was monitored weekly by the presence of human cells and IgG in peripheral blood. The mice were sacrificed 4 weeks after the injection and the engraftment in lymphoid organs was studied. A reproducible reconstitution was obtained with intraperitoneal injection of 30-40 x10(6) spleen cells. Human T, B and NK cells as well as human IgG were present in peripheral blood. In lymphoid tissues, the same lymphocytic subpopulations were detected and in addition some antigen presenting cells. The reconstitution was functional because graft rejection was observed after transplantation of human allogeneic tissues. When less than 30 x10(6) cells were injected, the reconstitution was variable. When more than 40 x10(6) cells were injected, GVHD occurred with increasing frequency. In conclusion, we show that intraperitoneal injection of 30-40 x10(6) human splenocytes into SCID/beige mice induces a quick and functional engraftment of human T, B and NK cells with no risk of GVHD. This model may be used to study human transplantation immunobiology in vivo.

Consensual immunity: success-driven development of T-helper-1 and T-helper-2 responses

Non-germline-encoded T- and B-cell receptors allow humans to effectively deal with rapidly mutating pathogens. Here, we argue that, in addition to determining the antigenic specificity of immune responses, the same receptor systems can also regulate the T-helper-1/T-helper-2 profile of immunity. Such a mechanism--based on feedback from distinct effector cells to dendritic cells, rather than on instruction from pathogens--uses the effectiveness of particular effector cells at targeting and destroying a pathogen as a reliable, experience-based criterion to induce and maintain the appropriately polarized response.

IL-12 or IL-4 Prime Human NK Cells to Mediate Functionally Divergent Interactions with Dendritic Cells or Tumors

In the course of inflammatory responses in peripheral tissues, NK cells may be exposed to cytokines such as IL-12 and IL-4 released by other cell types that may influence their functional activities. In the present study we comparatively analyzed purified human peripheral blood NK cells that had been exposed to either IL-12 or IL-4 during short (overnight) incubation. We show that although IL-12-cultured NK cells produced abundant IFN-gamma, TNF-alpha, and GM-CSF in response to stimuli acting on the NKp46-activating receptor, IL-4-cultured NK cells did not release detectable levels of these cytokines. In contrast, IL-4-cultured NK cells produced significant levels of TNF-alpha and GM-CSF only when stimulated with PMA and ionomycin. In no instance could the production of IL-5 and IL-13 be detected. Importantly, IL-12-cultured, but not IL-4-cultured, NK cells displayed strong cytolytic activity against various tumor cells or immature dendritic cells (DCs). Moreover, only NK cells that had been cultured in IL-12 were able to induce substantial DC maturation. Our data suggest that NK cells exposed to IL-12 for a time interval compatible with in vivo responses may favor the selection of appropriate mature DCs for subsequent Th1 cell priming in secondary lymphoid organs. On the contrary, NK cells exposed to IL-4 do not exert DC selection, may impair efficient Th1 priming, and favor either tolerogenic or Th2-type responses.

Distinct roles of IL-12 and IL-15 in human natural killer cell activation by dendritic cells from secondary lymphoid organs

Dendritic cells (DCs) are known to induce the growth and function of natural killer (NK) cells. Here, we address the capacity of DCs to interact with NK cells in human lymphoid organs and identify the role of specific DC-derived cytokines. We demonstrate that DCs colocalize with NK cells in the T cell areas of lymph nodes. In culture, DCs from either blood or spleen primarily stimulate the CD56(bright)CD16- NK cell subset, which is enriched in secondary lymphoid tissues. Blocking of IL-12 abolished DC-induced IFN-gamma secretion by NK cells, whereas membrane-bound IL-15 on DCs was essential for NK cell proliferation and survival. Maturation by CD40 ligation promoted the highest IL-15 surface presentation on DCs and led to the strongest NK cell proliferation induced by DCs. These results identify secondary lymphoid organs as a potential DC/NK cell interaction site and identify the distinct roles for DC-derived IL-12 and IL-15 in NK cell activation.

Critical role for CD8 T cells in allograft acceptance induced by DST and CD40/CD154 costimulatory blockade

Donor-specific transfusion (DST) and CD40/CD154 costimulation blockade is a powerful immunosuppressive strategy which prolongs survival of many allografts. The efficacy of DST and anti-CD154 mAb for prolongation of hepatocellular allograft survival was only realized in C57BL/6 mice that have both CD4- and CD8-dependent pathways available (median survival time, MST, 82 days). Hepatocyte rejection in CD8 KO mice which is CD4-dependent was not suppressed by DST and anti-CD154 mAb treatment (MST, 7 days); unexpectedly DST abrogated the beneficial effects of anti-CD154 mAb for suppression of hepatocyte rejection (MST, 42 days) and on donor-reactive alloantibody production. Hepatocyte rejection in CD4 KO mice which is CD8-dependent was suppressed by treatment with DST and anti-CD154 mAb therapy (MST, 35 days) but did not differ significantly from immunotherapy with anti-CD154 mAb alone (MST, 32 days). Induction of hepatocellular allograft acceptance by DST and anti-CD154 mAb immunotherapy was dependent on host CD8(+) T cells, as demonstrated by CD8 depletion studies in C57BL/6 mice (MST, 14 days) and CD8 reconstitution of CD8 KO mice (MST, 56 days). These studies demonstrate that both CD4(+) and CD8(+) T-cell subsets contribute to induction of hepatocellular allograft acceptance by this immunotherapeutic strategy.

Dendritic cells mediate NK cell help for Th1 and CTL responses: two-signal requirement for the induction of NK cell helper function

Early stages of viral infections are associated with local recruitment and activation of dendritic cells (DC) and NK cells. Although activated DC and NK cells are known to support each other's functions, it is less clear whether their local interaction in infected tissues can modulate the subsequent ability of migrating DC to induce T cell responses in draining lymph nodes. In this study, we report that NK cells are capable of inducing stable type 1-polarized "effector/memory" DC (DC1) that act as carriers of NK cell-derived helper signals for the development of type 1 immune responses. NK cell-induced DC1 show a strongly elevated ability to produce IL-12p70 after subsequent CD40 ligand stimulation. NK-induced DC1 prime naive CD4+ Th cells for high levels of IFN-gamma, but low IL-4 production, and demonstrate a strongly enhanced ability to induce Ag-specific CD8+ T cell responses. Resting NK cells display stringent activation requirements to perform this novel, DC-mediated, "helper" function. Although their interaction with K562 cells results in effective target cell killing, the induction of DC1 requires a second NK cell-activating signal. Such costimulatory signal can be provided by type I IFNs, common mediators of antiviral responses. Therefore, in addition to their cytolytic function, NK cells also have immunoregulatory activity, induced under more stringent conditions. The currently demonstrated helper activity of NK cells may support the development of Th1- and CTL-dominated type 1 immunity against intracellular pathogens and may have implications for cancer immunotherapy.