The liver as a lymphoid organ annu rev


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The liver as a lymphoid organ annu rev

  1. 1. ANRV371-IY27-06 ARI ANNUAL REVIEWS 16 February 2009 9:31 Further Annu. Rev. Immunol. 2009.27:147-163. Downloaded from by Rutgers University Libraries on 05/24/09. For personal use only. Click here for quick links to Annual Reviews content online, including: • Other articles in this volume • Top cited articles • Top downloaded articles • Our comprehensive search The Liver as a Lymphoid Organ Ian Nicholas Crispe David H. Smith Center for Vaccine Biology and Immunology, Aab Institute for Biomedical Research, University of Rochester Medical Center, Rochester, New York 14642; email: nick Annu. Rev. Immunol. 2009. 27:147–63 Key Words The Annual Review of Immunology is online at antigen presentation, hepatitis, Kupffer cells, innate immunity, sinusoid, stellate cells This article’s doi: 10.1146/annurev.immunol.021908.132629 Copyright c 2009 by Annual Reviews. All rights reserved 0732-0582/09/0423-0147$20.00 Abstract The liver receives blood from both the systemic circulation and the intestine, and in distinctive, thin-walled sinusoids this mixture passes over a large macrophage population, termed Kupffer cells. The exposure of liver cells to antigens, and to microbial products derived from the intestinal bacteria, has resulted in a distinctive local immune environment. Innate lymphocytes, including both natural killer cells and natural killer T cells, are unusually abundant in the liver. Multiple populations of nonhematopoietic liver cells, including sinusoidal endothelial cells, stellate cells located in the subendothelial space, and liver parenchymal cells, take on the roles of antigen-presenting cells. These cells present antigen in the context of immunosuppressive cytokines and inhibitory cell surface ligands, and immune responses to liver antigens often result in tolerance. Important human pathogens, including hepatitis C virus and the malaria parasite, exploit the liver’s environment, subvert immunity, and establish persistent infection. 147
  2. 2. ANRV371-IY27-06 ARI 16 February 2009 9:31 INTRODUCTION sinusoid: thin-walled blood space through which blood passes in the liver Kupffer cell: intravascular macrophage lining the liver sinusoids Annu. Rev. Immunol. 2009.27:147-163. Downloaded from by Rutgers University Libraries on 05/24/09. For personal use only. DC: dendritic cell TLR: Toll-like receptor APC: antigenpresenting cell The liver stands at a hemodynamic confluence. In distinctive, thin-walled vessels termed sinusoids, oxygenated blood from the arterial system mixes with portal venous blood returning from the intestine. The sinusoids contain a diversity of immunologically active cell types, including both lymphocytes and myeloid cells (Figure 1). The Kupffer cells form a large intravascular macrophage bed and, with liver dendritic cells (DCs), come in both immunogenic and tolerogenic forms. The liver also contains diverse lymphocytes, including T cells, natural killer T (NKT) cells, and natural killer (NK) cells. Portal venous blood contains the products of digestion, along with antigens and microbial products that originate from the bacteria in the small and large intestine. Among Hepatocytes HSC mDC/ pDC KC LSEC these bacterial products is lipopolysaccharide endotoxin (LPS), derived from the cell walls of Gram-negative bacteria. Under normal conditions, LPS is undetectable in the systemic circulation, but it is present at up to 1.0 ng/ml in portal venous blood (1). The cells of the hepatic sinusoids express the LPS receptor and effectively remove this molecule so that the systemic circulation is protected from endotoxemia (2). Many cells of the innate immune system express the LPS receptor, which consists of Toll-like receptor-4 (TLR4) together with the molecules CD14 and MD2; engagement of this receptor on most cell types delivers a strong activating signal. However, in the liver these receptors are continuously exposed to low levels of LPS, resulting in altered responsiveness to an LPS challenge. At the same time, the adaptive immune cells of the liver are exposed to food-derived antigens, the majority of which are harmless. The continuous presence, under normal conditions, of both TLR ligands and antigens has resulted in a distinctive set of mechanisms to maintain self-tolerance yet deliver immunity to infection. Liver immunity features a local concentration of overlapping innate immune mechanisms, together with the capacity of unusual cell types to act as antigen-presenting cells (APCs). The liver’s resident immune cells are not passive in the face of continuous exposure to antigens and LPS; instead they exist in a state of active tolerance, which results in liver allograft tolerance (3) but also creates a window of vulnerability for welladapted pathogens. This state of tolerance is metastable; the right combination of signals can reverse tolerance and activate immunity locally. Figure 1 INNATE IMMUNITY IN THE LIVER Antigen-presenting cells (APCs) in liver sinusoids. The liver contains multiple subsets of dendritic cells, including myeloid and plasmacytoid dendritic cells (mDCs and pDCs). There are abundant mononuclear phagocytes in the form of Kupffer cells (KCs), and these can express costimulatory molecules. In addition, there are two additional populations of APCs in the form of liver sinusoidal endothelial cells (LSECs) and hepatic stellate cells (HSCs). The evidence for APC function in each of these cell types is summarized in the text. The LPS from intestinal bacteria is not the only immune stimulus to which the liver is exposed. Pattern-recognition receptors in the liver sense the presence of enteric pathogens. These include cell surface and endosomal TLRs, cytoplasmic nucleotide-binding oligomerization 148 Crispe
  3. 3. Annu. Rev. Immunol. 2009.27:147-163. Downloaded from by Rutgers University Libraries on 05/24/09. For personal use only. ANRV371-IY27-06 ARI 16 February 2009 9:31 domain (NOD)-like receptors, and RNA helicases, including retinoic acid inducible protein-I (RIG-I). The TLRs recognize a diverse array of bacterial and viral molecules, including bacterial lipopeptides (heterodimers of TLR1 and TLR2, and of TLR2 with TLR6); LPS and flagellin (TLR4 and TLR5, respectively); and exogenous dsRNA, viral ssRNA, and bacterial unmethylated DNA (TLR3, TLR7, TLR8, and TLR9). The NOD receptors recognize bacterial peptidoglycans, whereas the RIG-I molecules recognize structural features of viral ssRNA. These receptors, their specificity, and their roles in host defense were recently reviewed (4, 5). From the perspective of a global discussion of liver immunity, the key point is that signals from these diverse receptors converge on two signaling pathways. Thus, all of the TLRs except TLR3 transmit signals via the adaptor protein MyD88 (myeloid differentiation factor-88), which results in the activation of the kinases p38, JNK, and IκB kinase, leading to NF-κB activation. NOD receptors also activate NF-κB. The TLR4 receptor complex, in addition to activation via MyD88, recruits the adaptor protein TRIF (TIR-domain containing adaptor recruiting interferon-β), which acts via TBK1 (TRAF family member–associated NFκB activator–binding kinase 1) to cause phosphorylation and nuclear localization of IRF-3 (IFN regulatory factor 3), the transcription factor that drives synthesis of type 1 interferon (IFN). Ligation of TLR3 selectively activates this signaling pathway. Similarly, RIG-I and its homolog MDA-5 promote the activation of mitochondrial IPS-1 (IFN-β promoter stimulator 1), resulting in IRF-3 activation and type 1 IFN secretion. These pathways are optimized such that pattern-recognition receptors engaged by bacterial products generally promote NF-κB activation, whereas those pathways activated by viral infection strongly induce IFN-β. The continuous, low-level stimulation of the former pathway is one of the distinctive features of the liver environment (Figure 2). NK cells are present at higher frequency in the liver than in most tissues. Thus, in human liver leukocytes obtained by elution from donor livers (6), in cell suspensions obtained from human liver tissue (7), and in the cells isolated from the mouse liver by enzymatic digestion (8), NK cells make up as many as 50% of liver lymphocytes. Similarly to NK cells elsewhere, these cells respond both to cytokine activation and to engagement of an excess of activating receptors over inhibitory receptors (9). Once activated, they manifest their function through cytokine synthesis and cytotoxicity. NK cells express two key adaptor molecules: DAP10 and DAP12. DAP10 associates with the activating NKG2D receptor, and the ITAM (immunoreceptor tyrosine-based activation motif)-bearing DAP12 adaptor protein associates with several receptors, including the CD94-NKG2C heterodimer and the Ly49H receptor. Ligands for NKG2D are expressed in the liver under diverse circumstances. Low amounts of NKG2D ligands are expressed constitutively in the liver (20), and these ligands can be upregulated after viral infection or transformation of hepatocytes. The NKG2D ligands include MHC class I–related proteins A and B (MICA and MICB), which are expressed on human hepatocellular cancer cells (10), and mouse retinoic acid early inducible-1 (RAE-1), which is transcriptionally upregulated by cytomegalovirus infection (11). Liver NK cells are induced to synthesize IFN-γ in response to IL-12 (12) and to manifest perforin-dependent cytotoxicity in response to the Kupffer cell–produced cytokine IL-18 (13). They are also cytotoxic owing to the expression of TRAIL (TNF-receptor apoptosis-inducing ligand), which can engage death receptors that are induced on hepatocytes by hepatitis B virus (HBV) (14). The liver lymphocytes also contain an unusually high frequency of NKT cells. These include both canonical NKT cells expressing an invariant T cell receptor (TCR) that binds to CD1d complexed with α-galactosylceramide and the noncanonical NKT cells that recognize other ligands. Liver NKT cells are abundant • The Liver as a Lymphoid Organ HBV: hepatitis B virus 149
  4. 4. ANRV371-IY27-06 ARI 16 February 2009 9:31 Constitutively engaged by gastrointestinal ligands Bacterial peptidoglycans Bacterial lipopeptides TLR2/6 TLR1/2 Constitutively quiescent, activated by virus infection Flagellin LPS TLR5 dsRNA Viral ssRNA TLR4 Annu. Rev. Immunol. 2009.27:147-163. Downloaded from by Rutgers University Libraries on 05/24/09. For personal use only. RIG-1 NOD-1 NOD-2 MyD88 TLR3 TRIF NF-κB IL-10 MDA-1 IPS -1 IRF-3 TNF-α, IL-1, IL-6, IL-12, IL-18 IFN-α IFN-β Figure 2 Effects of microbial products. The model explains the effects of the liver environment on patternrecognition receptors. The presence of detectable LPS in portal blood suggests that other gut-derived microbial molecules may be present also. We therefore propose the model that NOD proteins and the subsets of TLRs that recognize microbial products are constitutively engaged in the liver. In contrast, receptors for viral elements (TLR3, RIG-I, MDA-1) are not engaged under normal conditions. We propose that this changes the balance between NF-κB and IRF-3-dependent signaling pathways. In this diagram, heterodimeric TLRs are indicated as TLR1/2 and TLR2/6. both in mouse (15) and in human (6). These cells were recently filmed patrolling the hepatic sinusoids, based on their expression of a GFP reporter molecule driven by the endogenous CXCR6 promoter (16). Despite their thymic origin (17) and expression of a TCRαβ generated by V(D)J recombination, these CD1d-reactive T cells show evolutionary convergence with leukocytes expressing innate pattern-recognition receptors; their TCRs recognize glycolipid antigens that are conserved features of bacterial cell walls. These include glycosphingolipids from the soil bacterium, Sphingomonas sp. (18), and a diacylglycerol derived from the pathogenic spirochaete Borrelia burgdorferi (19). The NKT cells may also have the potential to respond to the bacterial cell wall components derived from the intestinal bacteria, and this could account not only HCV: hepatitis C virus 150 Crispe for their abundance in the liver, but also for their expression of markers of activation (6). Like many other T cells, the NKT cells express DAP10- and DAP12-associated receptors, and the NKG2D receptors on these cells are implicated in immunopathology in a mouse model of hepatitis B in which RAE-1 is induced to engage these receptors (20). The significance of innate immunity in the defense of the liver is evident from the multiple adaptations through which pathogens subvert it. Thus, HCV RNA interacts with RIG-I to activate NF-κB and IFN-β secretion; however, HCV also subverts this pathway because its NS3/4 protease cleaves the IPS-1 adaptor protein of RIG-I signaling (21). IRF-3 may also be activated through the TLR3 pathway, but HCV NS3/4 also targets this pathway through the cleavage of TRIF (22). Strikingly, this
  5. 5. Annu. Rev. Immunol. 2009.27:147-163. Downloaded from by Rutgers University Libraries on 05/24/09. For personal use only. ANRV371-IY27-06 ARI 16 February 2009 9:31 mechanism of innate immune escape is also seen in hepatitis A virus infection, where the protease 3ABC cleaves IPS-1 (also known as MAVS), again resulting in subversion of NF-κB activation (23). This convergence supports the argument that the IRF-3 pathway plays a critical role in antiviral immunity in liver. Apart from inhibiting innate immune signaling pathways within the infected cells, HCV targets NK cells though a completely independent mechanism. The HCV envelope protein, E2, binds to CD81 on human NK cells, and this results in suppression of their cytotoxic function and of IFN-γ synthesis (24). An important feature of HCV is therefore both a concerted attack on IFN-β synthesis and a major cellular source of IFN-γ. Because the pathogen so actively subverts IFN delivery and function, it is reasonable to suppose that IFN-responsive genes play a key role in anti-HCV immunity. It is not, therefore, surprising that the mainstay of treatment is high-dose exogenous IFN-α. Complex protozoan pathogens also manipulate the phagocytic function of Kupffer cells. Thus, the malaria parasites Plasmodium sp. enter Kupffer cells as part of the process by which they cross liver endothelium and gain access to hepatocytes. Evidence in favor of the mechanism comes from direct visualization of malaria sporozoite behavior in vivo (25) and from the observation that parasitization of the liver is reduced in osteopetrotic mice, which lack mature macrophages (26). As part of their interaction, the malaria sporozoites disable the Kupffer cells’ respiratory burst by increasing intracellular cyclin AMP (27). This effect is mediated by an abundant malaria protein, the circumsporite protein (CSP), which binds to a Kupffer cell’s surface receptor, LRP-1 (the low-density lipoprotein receptor-related protein). Thus, the most likely model is that the malaria sporozoite’s CSP engages LRP-1, inducing cyclic AMP and suppressing the Kupffer cell’s normal response to phagocytosed pathogens. This converts the Kupffer cells from effective elements in innate host defense into portals through which the parasite traverses the endothelium (28). WORLD PREVALENCE OF LIVER DISEASE Malaria causes severe disease in 500 million people each year; many more are infected, and 40% of the world population is at risk. There is no effective vaccine. More than 350 million people have chronic infection with HBV, which results in one million deaths per year from cirrhosis and liver cancer. A recombinant subunit vaccine is effective, so this total will probably decline. Around 180 million people are infected with HCV, and, of these, 130 million are chronically infected and at risk for cirrhosis and liver cancer. There is no effective vaccine. THE DIVERSITY OF POTENTIAL ANTIGEN-PRESENTING CELLS The liver contains plasmacytoid DCs (pDCs) and myeloid DCs (mDCs), and pDCs are more abundant than they are in lymphoid tissue. These pDCs are a major source of IFN-α, consistent with the importance of innate immune mechanisms in the liver. But in addition, the mouse liver contains two other identifiable subsets of DCs, the CD8α+ DCs (29) and the less well-defined natural killer DCs (NKDCs) (30), neither of which has yet been identified in humans. In addition to synthesizing IFN-α, pDCs synthesize both IL-10 and IL-12. In LPS-treated mice, liver pDCs synthesized less IL-12 than did splenic pDCs (31) and were poor APCs compared with splenic DCs (32). Conversely, liver mDCs synthesized IL-10, and this was increased in HCV patients (33). As discussed below, diverse other potential APCs in the liver respond to TLR ligation by secreting IL-10. The liver contains a large macrophage population; these are the Kupffer cells. In addition to their role as phagocytes, these cells express MHC and costimulatory molecules, rendering them potential APCs (Figure 3). However, relatively little work has addressed the APC function of Kupffer cells. Early experiments suggested that Kupffer cells were primarily immunosuppressive. Thus, addition of Kupffer cells to a mixed leukocyte reaction performed in the presence of low arginine resulted in • The Liver as a Lymphoid Organ sporozoite: the developmental stage of the malaria parasite that infects liver cells 151
  6. 6. ANRV371-IY27-06 ARI 16 February 2009 9:31 TNF-α HSC PD-L1 KC Annu. Rev. Immunol. 2009.27:147-163. Downloaded from by Rutgers University Libraries on 05/24/09. For personal use only. LPS Proteins Particulates TLR4 IL-10 IL-12/18 IFN-γ T cell NK cell LSEC Figure 3 Immunology of Kupffer cells. These cells can take up LPS, proteins, and particulates from the blood and secrete a number of cytokines, including TNF-α, IL-12, and IL-18, but also IL-10. The balance between IL-12/-18 and IL-10 production regulates NK cell activity. Kupffer cells express PD-L1 and have the capacity to inactivate T cells, a function shared with other liver APCs. Both T cells and NK cells secrete IFN-γ, which powerfully activates Kupffer cells. (Abbreviations: KC, Kupffer cell; HSC, hepatic stellate cell.) LSEC: liver sinusoidal endothelial cell stellate cell: a distinctive liver cell type, located between the liver endothelial cells and the hepatocytes 152 immunosuppression, mediated in part by PGE2 (34). Kupffer cells may also mediate suppression through their synthesis of nitric oxide (35) and respond to TLR4 ligation by secreting IL-10 (36). In a liver transplant model, Kupffer cells expressed FasL, leading to alloreactive CD4+ T cell apoptosis (37). On the basis of such experiments, investigators have invoked Kupffer cells to explain such diverse phenomena as oral tolerance, portal vein tolerance, and liver allograft tolerance. However, Kupffer cells may also act as effective APCs. In HCV infection, human Kupffer cells became MHC I and II high, expressed CD40 and CD80, and formed clusters with CD4+ T cells, consistent with their acting as APCs (38). Perhaps the capacity of Kupffer cells to stimulate or inhibit T cell activation depends on the signals to which these cells have been exposed. In a study of liver NK cell activation by human Kupffer cells, the selective Crispe activation of either the TRIF pathway or the MyD88 pathway of TLR signaling resulted in the predominant expression of either IL-18 or IL-10, leading to higher or lower levels of NK cell activation (39). The liver sinusoidal endothelial cells (LSECs) have been implicated in antigen presentation (Figure 4). These endothelial cells are unusual in several respects: They do not secrete an organized basement membrane, and they are perforated by numerous fenestrations, clustered into sieve plates. These cells express the scavenger receptor, which renders them competent to take up circulating proteins. They also express MHC class I and class II and costimulatory molecules including CD40, CD80, and CD86, giving them the surface characteristics of highly active stimulatory APCs, such as DCs (40). However, they respond to TLR4 ligation with the secretion of IL-10, to which they also respond by downregulating their APC functions (41), and their main effect seems to be the induction of T cell tolerance. Thus, when LSECs were isolated from mice that received ovalbumin parenterally or orally, they engaged T cells, resulting in immune deviation to a CD4+ T regulatory phenotype, or in CD8+ T cell tolerance (42, 43). Hepatic stellate cells reside in the subendothelial space of Disse and constitute the primary site for the storage of vitamin A. They regulate hepatic sinusoidal blood flow and can also transdifferentiate into myofibroblasts during the process of liver fibrosis (44). Recent studies suggest that these cells also belong among the liver APCs (Figure 5). Thus, they express MHC class I, MHC class II, and CD1d, and they have the potential to respond to innate immune signals though their expression of TLR4, CD14, and MD2, which renders them LPS responsive (45, 46). Ex vivo, stellate cells can activate NKT cells and classical T cells (47), although their coexpression of the inhibitory molecule PD-L1 also renders them capable of T cell inactivation, leading to tolerance (48). With so many potential liver cell subsets manifesting APC activity ex vivo, the issue of cell purification becomes particularly
  7. 7. Annu. Rev. Immunol. 2009.27:147-163. Downloaded from by Rutgers University Libraries on 05/24/09. For personal use only. ANRV371-IY27-06 ARI 16 February 2009 9:31 acute. Classical experiments by Steinman and colleagues (49) demonstrated that, among spleen cells, the most powerful APC activity was concentrated in a very rare subset of cells, the DCs; indeed, the alleged APC activity of other subsets of spleen cells, including splenic macrophages, could have been attributed to rare, contaminating DCs. This same concern applies to studies that document the APC action of ex vivo liver cell subsets, but now we have more complexity. Which of the APC functions of a culture of stellate cells may be attributed to rare, contaminating DCs or LSECs? And in the case of the LSECs, how many contaminating DCs or Kupffer cells is enough to account for their ability to activate or to silence a T cell response? The optimum reagents for the analysis of the significance of APC function would be a series of transgenic mice in which molecules of interest, such as MHC class I or MHC class II molecules, are expressed or selectively inactivated under the control of promoters of absolute cell-type specificity. These ideal tools are not yet at hand; nevertheless, we can draw some solid conclusions from the extant in vivo experiments. LOCAL PRIMING OF T CELLS IN THE LIVER The presence in the liver of so many distinct subsets of cells with APC function raises the question of whether T cells are in fact activated locally in vivo. The distinctive architecture of the hepatic sinusoids permits circulating T cells to make direct contact with underlying hepatocytes and also with stellate cells, as well as with LSECs and intravascular Kupffer cells. Such interactions have in fact been revealed by electron microscopy (50), providing a structural basis for primary T cell activation by hepatocellular antigens. This idea was first supported by experiments in which CD8+ T cells made a rapid, antigen-driven, local, intrahepatic immune response in transgenic mice expressing the HBV genome in both liver and other tissues (51) and subsequently in transgenic mice expressing nonself MHC class I molecules (52). In addi- TNF-α HSC CD95L TLR4 LPS Proteins MHC I/II CD40/80/86 IL-10 Scavenger-R Mannose-R T Trapping, Tregs, tolerance ICAM-1 VCAM-1 VAP-1 LSEC Transcytosis? Figure 4 Immunology of liver sinusoidal endothelial cells (LSECs). These cells respond to LPS via TLR4 and can acquire circulating proteins via the scavenger receptor and the mannose receptor. LSECs may transport proteins across themselves, and into hepatocytes, a process termed transcytosis. They process and present antigens in association with costimulatory ligands (CD40, CD80, and CD86) but respond to ambient LPS by secreting IL-10, biasing T cells toward tolerance. These cells also express multiple adhesion molecules, including ICAM-1 (intercellular adhesion molecule-1), VCAM-1 (vascular cell adhesion molecule-1), and VAP-1 (vascular adhesion protein-1), all of which are implicated in T cell retention in the liver sinusoids. Thus, these cells can promote immune tolerance, both through the local trapping of activated T cells and the induction of regulatory T cells. This concept is summarized in the diagram as “trapping, Tregs, tolerance.” tion to transgenic antigens, CD8+ T cells made a local response to antigen delivered specifically to hepatocytes using an AAV-2 vector, and this resulted in the subsequent seeding of activated cells to the lymph nodes and the spleen (53). Furthermore, the transplanted mouse liver, depleted of bone marrow–derived APCs, was fully competent to activate naive CD8+ T cells in response to a peptide antigen (54). Abundant evidence therefore supports the concept that the liver is a secondary lymphoid organ, acting as a site of primary T cell activation. In such experiments, it is not always clear which cell population is the main APC. The most compelling evidence that hepatocytes • The Liver as a Lymphoid Organ 153
  8. 8. ANRV371-IY27-06 ARI 16 February 2009 9:31 TLR4 HSC IL-10 CD1d NKT TGF-β1 LPS T Annu. Rev. Immunol. 2009.27:147-163. Downloaded from by Rutgers University Libraries on 05/24/09. For personal use only. KC LSEC Figure 5 Immunology of hepatic stellate cells. Stellate cells perceive LPS via TLR4 and also respond to multiple cytokines, including IL-10 and TGF-β1. They express a high surface density of the nonclassical MHC class I–like molecule CD1d and thereby activate NKT cells. parenchymal cell: the most abundant cell type in the liver, also known as hepatocytes cross-presentation: transfer of an antigen from the antigenexpressing cell to a distinct APC, resulting in T cell engagement 154 (also termed liver parenchymal cells) themselves act as primary APCs comes from parallel studies in vivo and in vitro, using allogeneic MHC antigens. In liver allograft experiments, cytotoxic T cells underwent spontaneous apoptosis in the transplanted liver (55), and both hepatocytes and nonparenchymal cells activated, then caused apoptosis of, the activated T cells in vitro (56). Similarly, both activation and apoptosis of T cells were observed in vivo following the adoptive transfer of alloreactive T cells to transgenic mice expressing a nonself MHC class I molecule on hepatocytes (52, 57). Furthermore, the T cell apoptosis was also manifest in vitro when T cells were cultured with hepatocytes, leading to the hypothesis that a feature of liver tolerance was “death by neglect,” the engagement of T cells by APCs deficient in costimulatory activity (58). In these experiments, the lack of susceptibility to cross-presentation of intact MHC molecules was key in the interpretation of the data and strongly suggested that hepatocytes Crispe themselves were engaging the CD8+ T cells. However, it is also noteworthy that in all these studies the final outcome was T cell inactivation or apoptosis. In a distinct transgenic model, the deliberate priming of antigen-specific T cells at an extrahepatic site was not enough to break tolerance to a transgenic liver antigen expressed in hepatocytes, supporting the argument for an active state of antigen-driven local tolerance (59). The presentation of cell-intrinsic MHC antigens by hepatocytes constitutes direct presentation. Similarly, the in vitro analysis of the APC functions of stellate cells dealt primarily with direct presentation of cellular antigens, or the presentation of exogenous peptides (47). In vivo experiments revealed the capacity of stellate cells to act as immunosuppressive APCs, protecting pancreatic islet allografts against rejection (60). In this capacity, also, stellate cells were presenting their intrinsic antigens. In contrast, we know that LSECs are capable of presenting exogenous antigens encountered in vivo. This is the case for soluble antigens given parenterally or orally (42, 43). The LSECs also appear to be capable of true cross-presentation, in which tumor cell– derived antigen was acquired by LSECs and resulted in CD8+ T cell tolerance (61). The mechanism of this kind of tolerance is likely to involve PD-L1, based on antibody blocking experiments (48); in addition, LSECs deficient in this molecule failed to induce CD8+ T cell tolerance (62). In addition to all these nonclassical APCs, DCs also traffic through the liver. Thus, classical mDC precursors expressing CD11c, but not B220, are recruited to the liver during granulomatous inflammation initiated by Propionibacterium acnes and subsequently detected first in the Disse space, then in granulomas, and subsequently in lymphoid aggregates in the portal tracts (63). These movements were orchestrated by chemokines, with CCL3 driving the initial localization to the granulomas, and CCL21 causing the subsequent relocalization to portal-associated lymphoid tissue (PALT) (64). This certainly suggests, though it does not
  9. 9. Annu. Rev. Immunol. 2009.27:147-163. Downloaded from by Rutgers University Libraries on 05/24/09. For personal use only. ANRV371-IY27-06 ARI 16 February 2009 9:31 prove, that DCs localize to areas of lymphoneogenesis in the inflamed liver and engage T cells there. In humans, the same chemokine, CCL21, is expressed by vascular endothelium in portal areas of the liver, but not in lymph node vessels (65). On this basis, investigators argue that PALT is a distinct immunological compartment; sadly, the acronym lacks distinctiveness because it has been applied to lymphoid tissue in the human prostate (66) and in the chicken pineal gland (67). But the existence of PALT, in the hepatic context, adds to the complexity of liver immunology. In addition to examining diverse APCs resident in the sinusoids, we also need to consider more conventional lymphoid tissue threaded through the liver in the network of portal tracts. THE RISE AND FALL OF LIVER-PRIMED T CELLS Whereas the immune system is competent to eliminate infection with hepatitis A virus, a state of persistent infection is a common outcome in HBV infection and the usual outcome in HCV infection. We may therefore ask why liver immune responses frequently fail. Does this simply indicate the sophistication of several well-adapted pathogens, or, in some sense, is the liver to blame? In the case of chronic HCV infection, antigen-specific CD8+ T cells frequently assume a stunned or exhausted phenotype, in which they express a low level of the IL-7 receptor-α (CD127) and a high level of the inhibitory receptor PD-1 (68). In several infections, including HIV and HCV in humans and lymphocytic choriomeningitis virus in mice, CD8+ T cells with this phenotype are able neither to secrete IFN-γ nor to make IL-2 (69–71). In the case of HCV, another striking feature of chronic infection is selective IL-10 production by CD4+ T cells (72). Furthermore, chronic HCV in humans may be associated with very weak or absent CD4+ T cell responses (73), whereas in chimpanzees, viral escape mutants that prevent CD4+ T cell recognition fa- vor chronicity (74), suggesting that the CD8+ T cells may be incapacitated owing to the lack of CD4+ T cell help, a state termed “helpless” (75). To what extent, then, are stunning, exhaustion, PD-1 expression, IL-10 intoxication, and helplessness all effects of the liver environment? As far as PD-1/PD-L1 interactions are concerned, the liver seems to be a preferential site of action. PD-L1 is expressed on several liver cell types (76). Mice deficient in PD-L1 developed an immunoinflammatory hepatitis caused by CD8+ T cells, suggesting a key role for PD-L1 in regulating both CD8+ T cell abundance and immunopathology in the liver (77). The immunosuppressive effects of IL-10 secreted by LSECs, Kupffer cells, and liver pDCs have already been emphasized, along with the interpretation that this is an effect of low-level TLR4 ligation. Why should this bias exist? One possibility is that the LPS-responsive cells of the liver are manifesting a mechanism that has evolved to suppress chronic immune inflammation. Thus, in an acute immune response, IL-10 synthesis occurs after the peak of proinflammatory cytokines and may help to restore the system to a resting state. Under conditions of chronic activation, this mechanism predominates, limiting tissue injury. In the liver, the continuous presence of low levels of LPS may emulate chronic inflammation, calling forth IL-10 as a regulatory response. Does the liver promote CD8+ T cell helplessness? The liver’s unique vasculature permits circulating T cells to engage with hepatocytes (50), which act as APCs (56, 78). Therefore, we can consider the liver as a tissue that favors CD8+ T cell priming on cells that can neither prime nor be engaged by CD4+ T cells. The helpless phenotype would naturally follow from direct priming of HCV-specific and possibly also HBV-specific CD8+ T cells on hepatocytes. In fact, there is one report of apparently directly primed CD8+ T cells in the context of liver transplantation. A liver expressing HLA-A2 was transplanted into an HLA-A2-negative recipient, who subsequently developed HLA-A2-restricted anti-HCV • The Liver as a Lymphoid Organ 155
  10. 10. Annu. Rev. Immunol. 2009.27:147-163. Downloaded from by Rutgers University Libraries on 05/24/09. For personal use only. ANRV371-IY27-06 ARI 16 February 2009 9:31 CD8+ T cells (79). This would be consistent with direct priming on newly infected donor hepatocytes. A further test of the inadequacy of liver-primed CD8+ T cells comes from a comparison of the response of TCR-transgenic T cells to a neo-self-MHC antigen expressed either exclusively in the liver or also in lymph nodes. Extrahepatic priming resulted in fully differentiated CD8+ T cells that could localize to the liver and cause autoimmune immunopathology. In contrast, the exclusively liver-primed CD8+ T cells were relatively innocuous (80). In summary, multiple mechanisms account for the rise and fall of T cells specific for liver pathogens, but many of these effects are linked to the liver’s unique immunobiology. This is a clear case of contributory negligence; HBV and HCV are subtle and devious pathogens, to be sure, but the liver offers them opportunities for immune subversion. TGF-β1 IL-10 Trapping, FasL, TRAIL, phagocytosis T T HSC PD-L1 KC LSEC Figure 6 Mechanisms of T cell tolerance in liver. The expression of adhesion molecules facilitates the trapping of activated T cells in liver sinusoids, where they may undergo apoptosis owing to FasL and TRAIL expressed on Kupffer cells and may also be phagocytosed. In addition, T cells that recognize antigen in the liver are exposed to immunosuppressive cytokines, including IL-10 and TGF-β1, and to inhibitory ligands, including PD-L1 (also known as B7-H1). 156 Crispe HOW DOES THE LIVER INDUCE SYSTEMIC TOLERANCE? In view of the bias toward tolerance when T cells encounter antigens in the liver (summarized in Figure 6), it is not surprising that such a large organ can impose systemic immune tolerance. This phenomenon was first recognized in the context of allogeneic liver transplantation. In the classic experiments conducted at the University of Oxford, renal transplants between unrelated pigs were promptly rejected, whereas liver transplants between equally unrelated pigs were generally accepted. Strikingly, the transplantation of a kidney and a liver from the same donor enhanced the survival of the kidney. In the half-century since the description of these “Strange English Pigs”1 (3, 81), this phenomenon has not been fully explained. Many explanations have been considered. For example, serial transplantation experiments implicated passenger leukocytes as playing a role in the induction of kidney allograft rejection and suggested that their loss plays a role in the induction of allograft tolerance (82). However, the loss of passenger leukocytes cannot be implicated in the tolerance associated with liver transplantation because of the abundance of long-lived donor hematopoietic cells within the liver graft (83). It was therefore reasonable to propose that, unlike other passenger leukocytes, those originating in the liver were tolerogenic. Investigators (84, 85) suggested that the detection of a low frequency of graft-derived leukocytes in multiple tissues of a liver transplant recipient (microchimerism) explains liver transplantation tolerance, but the survival of these cells would be equally well explained if the liver were imposing allospecific tolerance by some other mechanism. If recirculating passenger leukocytes were an effect rather than a cause of liver allograft tolerance, what other mechanisms would be candidates? The sessile Kupffer cells, LSECs, and 1 “Strange English Pigs” was the title of an anonymous editorial in The Lancet, November 1, 1969, on the topic of liver allograft tolerance.
  11. 11. Annu. Rev. Immunol. 2009.27:147-163. Downloaded from by Rutgers University Libraries on 05/24/09. For personal use only. ANRV371-IY27-06 ARI 16 February 2009 9:31 stellate cells all have the capacity to present antigens to T cells, along with cosignals including IL-10 and PD-L1 that induce tolerance. Therefore, we may conjecture that liver allograft tolerance is induced by this mechanism, with persistent microchimerism as a by-product. In this model, allospecific precursor T cells would enter the transplanted liver, undergo activation by liver-specific APCs, and then either undergo paralysis or deletion owing to liver-specific local signals. We might consider how this mechanism could account for the effect of the liver in oral tolerance. The delivery of a protein antigen, usually ovalbumin, into the stomach results in systemic tolerance, particularly of Th1 CD4+ T cells and CD8+ T cells. If the venous drainage of the gut is surgically diverted, this oral tolerance is lost (86). In the context of an oral tolerance model, isolated and ex vivo cultured LSECs interacted with ova-specific T cells, activating them but then causing them to deviate their immune function toward an antiinflammatory pattern of cytokine synthesis (43). While the caveats already raised in relation to the absolute purity of cultured liver APCs certainly apply to these experiments, they make a prima facie case for the induction of systemic tolerance by liver APCs. Similarly, systemic immune tolerance can be induced by the injection of APCs into the portal vein (87), although it is unclear whether these APCs take up residence in the liver and become tolerogenic or whether cross-presentation by resident liver APCs accounts for this effect. The induction of systemic tolerance by liver APCs has been attributed both to peripheral deletion and to the induction of antigen-specific Tregs. Supporting the deletion model is the abundant evidence that activated, circulating CD8+ T cells are sequestered in the liver, even in the absence of antigen. This is most strikingly illustrated in the context of influenza infection, where both virus RNA and protein are limited to the respiratory system, but influenza-specific CD8+ T cells are found in the liver, associated with Kupffer cell–rich inflammatory foci and with subclinical hepatocyte damage (88). This sequestration depends on integrin lig- ands, including ICAM-1 (intercellular adhesion molecule-1) and VCAM-1 (vascular cell adhesion molecule-1) (89), and, like other aspects of liver tolerance, it may be driven by LPS acting via TLR4 (90). The evidence that this form of antigen-independent CD8+ T cell sequestration results in systemic tolerance is thin; however, in the context of a CD8+ T cell response to antigen-pulsed DCs, the interruption of the TLR4-dependent element of T cell trapping in the liver resulted in exaggerated systemic immune responses, followed eight weeks later by an enhanced secondary response. On the basis of this experiment, John et al. (91) proposed the model that the liver regulates the magnitude of CD8+ T cells responses. However, it is abundantly clear that deletion is not the fate of every T cell that enters the liver because liver-derived T cells can repopulate systemic memory (92). It has been argued that this observation invalidates the model that the liver acts as a sink for activated CD8+ T cells (93), but this argument fails to take into account the distinction between activated blast T cells and CD8+ memory T cells. The data are best reconciled by a model in which recently activated lymphoblasts preferentially localize to hepatic sinusoids owing to adhesion molecules expressed on LSECs, and then recruit and are killed by Kupffer cells, creating a “sink” for excess T cell blasts. Resting memory cells, in contrast, may be overrepresented in the liver owing to their adhesion receptors, but they do not activate Kupffer cells nor are they phagocytosed. These cells find the liver quite hospitable, as we have argued previously (94). The alternative to a deletion model is the idea of active regulation by suppressor T cells, also known as Tregs. These cells exist in a variety of subsets, some of which arise as Tregs in the thymus, whereas others differentiate from apparently uncommitted peripheral CD4+ and CD8+ T cell precursors. The impact of liver transplantation on these cells is controversial. A perfusate of human liver was enriched in CD4+ 25+ FoxP3+ CD127low cells, and such donor-derived Tregs were found in the circulation of liver transplant • The Liver as a Lymphoid Organ Treg: regulatory T cell 157
  12. 12. ARI 16 February 2009 9:31 recipients, an aspect of the phenomenon of microchimerism (95). Investigators agree that, after liver transplantation in humans, the overall frequency of Tregs in the circulation falls (96, 97). Among subsets of CD4+ 25+ T cells, the number of cells that expressed high amounts of CD127 increased in human liver transplant recipients with stable allografts, whereas the number of CD4+ 25+ 127low cells with regulatory function decreased (98); this is hard to reconcile with the idea that the CD127low regulatory subset was maintaining tolerance. However, in mouse liver transplantation, CD4+ 25+ FoxP3+ CTLA4+ Tregs increased in abundance after liver grafting, and depletion of these cells using an anti-CD25 antibody caused acute rejection of the graft, consistent with an important role for these kinds of Tregs in maintaining liver allograft tolerance and also, by extrapolation, systemic tolerance (99). It does not follow that mouse and human are fundamentally different. The most straightforward resolution of the apparent conflict between mouse and human data is that spontaneous liver allograft acceptance involves CD4+ 25+ FoxP3+ Tregs but that, in human, liver allografts are tolerated because of immunosuppression and despite the depletion of Tregs by immunosuppressive drugs. Annu. Rev. Immunol. 2009.27:147-163. Downloaded from by Rutgers University Libraries on 05/24/09. For personal use only. ANRV371-IY27-06 THE LIVER AS A LYMPHOID ORGAN: CONCLUSIONS The liver is an important site for primary T cell activation, but this takes place in an environment biased toward tolerance. Several mech- anisms contribute to this suppressive milieu. First, the constitutive exposure of liver cells to traces of endotoxin and other microbial products results in the down-modulation of costimulatory molecules and the synthesis of IL-10 by Kupffer cells and LSECs. Second, the open architecture of the liver endothelium results in ready access of naive T cells to diverse subsets of APCs, including hepatocytes. This may result in selective CD8+ T cell priming without concomitant CD4+ T cell activation, resulting in a “helpless” phenotype that leads to longterm CD8+ T cell dysfunction and a lack of immune memory. Third, the liver endothelium expresses adhesion molecules that facilitate the sequestration of circulating activated T cells, particularly CD8+ T cells. This gives the liver a role in systemic immunoregulation. An important epiphenomenon resulting from these mechanisms is liver allograft tolerance. Because of the high threshold for the initiation of an adaptive T cell response in the liver, innate immune mechanisms assume greater significance. Abundant NK cells and NKT cells may be activated by pathogen-associated structures via TLRs, invariant TCRs, or alternative sensing systems such as RIG-I, and by cytokines. Kupffer cells respond to inflammatory cytokines such as IFN-γ by the synthesis of their own inflammatory mediators: TNF-α, IL-12, and IL-18. These in turn deliver positive signals to both adaptive and innate immune cells. Awash with potential activating signals, the liver’s immune system is held in a baseline state of active tolerance, which can be reversed by sufficiently strong pathogen-specific signals. SUMMARY POINTS 1. The liver receives blood from the intestine, which is rich in microbial products. These engage TLRs, which modify innate immunity in the hepatic environment. 2. The liver lymphocytes are enriched in CD8+ T cells, activated T cells, memory T cells, NKT cells, and NK cells. 3. Multiple cell populations can act as APCs in the liver. These include hepatocytes, endothelial cells, and subendothelial stellate cells, as well as several subsets of dendritic cells. 158 Crispe
  13. 13. ANRV371-IY27-06 ARI 16 February 2009 9:31 4. The liver is rich in immunosuppressive cytokines including IL-10, and several liver cell subsets express the inhibitory ligand PD-L1. The consequence of this is that many encounters between T cells and liver APCs end in immune tolerance. FUTURE ISSUES Annu. Rev. Immunol. 2009.27:147-163. Downloaded from by Rutgers University Libraries on 05/24/09. For personal use only. 1. The mechanisms of antigen presentation and T cell activation by the many subsets of liver APCs must be clarified. In particular, do these cell types form distinctive synapses with T cells? Which of them engage in cross-presentation of hepatocellular antigens? What mechanisms promote such cross-presentation? 2. In hepatitis B, hepatitis C, hepatocellular cancer, and the liver stage of malaria, how are hepatocellular antigens presented to the immune system? 3. Which immune mechanisms favor the elimination of hepatocellular antigens? Which mechanisms cause liver immunopathology? How can we promote the former, while limiting the effects of the latter? DISCLOSURE STATEMENT The author is not aware of any affiliations, memberships, funding, or financial holdings that might be perceived as affecting the objectivity of this review. LITERATURE CITED 1. Lumsden AB, Henderson JM, Kutner MH. 1988. Endotoxin levels measured by a chromogenic assay in portal, hepatic and peripheral venous blood in patients with cirrhosis. Hepatology 8:232–36 2. Catala M, Anton A, Portoles MT. 1999. Characterization of the simultaneous binding of Escherichia coli endotoxin to Kupffer and endothelial liver cells by flow cytometry. Cytometry 36:123–30 3. Calne RY, Sells RA, Pena JR, Davis DR, Millard PR, et al. 1969. Induction of immunological tolerance by porcine liver allografts. Nature 223:472–76 4. Thompson AJ, Locarnini SA. 2007. Toll-like receptors, RIG-I-like RNA helicases and the antiviral innate immune response. Immunol. Cell Biol. 85:435–45 5. Abreu MT, Fukata M, Arditi M. 2005. TLR signaling in the gut in health and disease. J. Immunol. 174:4453–60 6. Tu Z, Bozorgzadeh A, Crispe IN, Orloff MS. 2007. The activation state of human intrahepatic lymphocytes. Clin. Exp. Immunol. 149:186–93 7. Norris S, Collins C, Doherty DG, Smith F, McEntee G, et al. 1998. Resident human hepatic lymphocytes are phenotypically different from circulating lymphocytes. J. Hepatol. 28:84–90 8. Crispe IN. 1996. Isolation of mouse intrahepatic lymphocytes. Curr. Protoc. Immunol. 3:22–28 9. Lanier LL. 2008. Up on the tightrope: natural killer cell activation and inhibition. Nat. Immunol. 9:495–502 10. Jinushi M, Takehara T, Tatsumi T, Kanto T, Groh V, et al. 2003. Expression and role of MICA and MICB in human hepatocellular carcinomas and their regulation by retinoic acid. Int. J. Cancer 104:354–61 11. Lodoen M, Ogasawara K, Hamerman JA, Arase H, Houchins JP, et al. 2003. NKG2D-mediated natural killer cell protection against cytomegalovirus is impaired by viral gp40 modulation of retinoic acid early inducible 1 gene molecules. J. Exp. Med. 197:1245–53 12. Nguyen KB, Salazar-Mather TP, Dalod MY, Van Deusen JB, Wei XQ, et al. 2002. Coordinated and distinct roles for IFN-αβ, IL-12, and IL-15 regulation of NK cell responses to viral infection. J. Immunol. 169:4279–87 • The Liver as a Lymphoid Organ 3. The discovery of liver tolerance, leading to the recognition of the liver as a distinct immunological site. 159
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  18. 18. AR371-FM ARI 16 February 2009 15:37 Annual Review of Immunology Annu. Rev. Immunol. 2009.27:147-163. Downloaded from by Rutgers University Libraries on 05/24/09. For personal use only. Contents Volume 27, 2009 Frontispiece Marc Feldmann p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p x Translating Molecular Insights in Autoimmunity into Effective Therapy Marc Feldmann p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p1 Structural Biology of Shared Cytokine Receptors Xinquan Wang, Patrick Lupardus, Sherry L. LaPorte, and K. Christopher Garcia p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 29 Immunity to Respiratory Viruses Jacob E. Kohlmeier and David L. Woodland p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 61 Immune Therapy for Cancer Michael Dougan and Glenn Dranoff p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 83 Microglial Physiology: Unique Stimuli, Specialized Responses Richard M. Ransohoff and V. Hugh Perry p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p119 The Liver as a Lymphoid Organ Ian Nicholas Crispe p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p147 Immune and Inflammatory Mechanisms of Atherosclerosis Elena Galkina and Klaus Ley p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p165 Primary B Cell Immunodeficiencies: Comparisons and Contrasts Mary Ellen Conley, A. Kerry Dobbs, Dana M. Farmer, Sebnem Kilic, Kenneth Paris, Sofia Grigoriadou, Elaine Coustan-Smith, Vanessa Howard, and Dario Campana p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p199 The Inflammasomes: Guardians of the Body Fabio Martinon, Annick Mayor, and Jürg Tschopp p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p229 Human Marginal Zone B Cells Jean-Claude Weill, Sandra Weller, and Claude-Agn` s Reynaud p p p p p p p p p p p p p p p p p p p p p p267 e v
  19. 19. AR371-FM ARI 16 February 2009 15:37 Aire Diane Mathis and Christophe Benoist p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p287 Regulatory Lymphocytes and Intestinal Inflammation Ana Izcue, Janine L. Coombes, and Fiona Powrie p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p313 The Ins and Outs of Leukocyte Integrin Signaling Clare L. Abram and Clifford A. Lowell p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p339 Annu. Rev. Immunol. 2009.27:147-163. Downloaded from by Rutgers University Libraries on 05/24/09. For personal use only. Recent Advances in the Genetics of Autoimmune Disease Peter K. Gregersen and Lina M. Olsson p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p363 Cell-Mediated Immune Responses in Tuberculosis Andrea M. Cooper p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p393 Enhancing Immunity Through Autophagy Christian Munz p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p423 ¨ Alternative Activation of Macrophages: An Immunologic Functional Perspective Fernando O. Martinez, Laura Helming, and Siamon Gordon p p p p p p p p p p p p p p p p p p p p p p p p451 IL-17 and Th17 Cells Thomas Korn, Estelle Bettelli, Mohamed Oukka, and Vijay K. Kuchroo p p p p p p p p p p p p p p485 Immunological and Inflammatory Functions of the Interleukin-1 Family Charles A. Dinarello p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p519 Regulatory T Cells in the Control of Host-Microorganism Interactions Yasmine Belkaid and Kristin Tarbell p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p551 T Cell Activation Jennifer E. Smith-Garvin, Gary A. Koretzky, and Martha S. Jordan p p p p p p p p p p p p p p p591 Horror Autoinflammaticus: The Molecular Pathophysiology of Autoinflammatory Disease Seth L. Masters, Anna Simon, Ivona Aksentijevich, and Daniel L. Kastner p p p p p p p p p621 Blood Monocytes: Development, Heterogeneity, and Relationship with Dendritic Cells Cedric Auffray, Michael H. Sieweke, and Frederic Geissmann p p p p p p p p p p p p p p p p p p p p p p p p669 Regulation and Function of NF-κB Transcription Factors in the Immune System Sivakumar Vallabhapurapu and Michael Karin p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p693 vi Contents