Hot off the presses! May 01 Immunol Rev

The May 01 issue of the Immunol Rev is now up on Pubget (About Immunol Rev): if you're at a subscribing institution, just click the link in the latest link at the home page. (Note you'll only be able to get all the PDFs in the issue if your institution subscribes to Pubget.)

Latest Articles Include:

• Anatomy of a murder: how cytotoxic T cells and NK cells are activated, develop, and eliminate their targets
  - Immunol Rev 235(1):5-9 (2010)
  
• Inherited defects in lymphocyte cytotoxic activity
  - Immunol Rev 235(1):10-23 (2010)
  Summary: The granule-dependent cytotoxic activity of lymphocytes plays a critical role in the defense against virally infected cells and tumor cells. The importance of this cytotoxic pathway in immune regulation is evidenced by the severe and often fatal condition, known as hemophagocytic lymphohistiocytic syndrome (HLH) that occurs in mice and humans with genetically determined impaired lymphocyte cytotoxic function. HLH manifests as the occurrence of uncontrolled activation of T lymphocytes and macrophages infiltrating multiple organs. In this review, we focus on recent advances in the characterization of effectors regulating the release of cytotoxic granules, and on the role of this cytotoxic pathway in lymphocyte homeostasis and immune surveillance. Analysis of the mechanisms leading to the occurrence of hemophagocytic syndrome designates γ-interferon as an attractive therapeutic target to downregulate uncontrolled macrophage activation, which sustains clinical an! d biological features of HLH.
• Cytotoxic immunological synapses
  - Immunol Rev 235(1):24-34 (2010)
  Summary: One of the most fundamental activities of the adaptive immune system is to kill infected cells and tumor cells. Two distinct pathways mediate this process, both of which are facilitated by a cytotoxic immunological synapse. While traditionally thought of as innate immune cells, natural killer (NK) cells are now appreciated to have the capacity for long-term adaptation to chemical and viral insults. These cells integrate multiple positive and negative signals through NK cell cytotoxic or inhibitory synapses. The traditional CD8+αβ T-cell receptor-positive cells are among the best models for the concept of an immunological synapse, in which vectoral signaling is linked to directed secretion in a stable interface to induce apoptotic cell death in an infected cell. Large-scale molecular organization in synapses generated a number of hypotheses. Studies in the past 5 years have started to provide clear answers regarding the validity of these models. In vivo imagi! ng approaches have provided some hints as to the physiologic relevance of these processes with great promise for the future. This review provides an overview of work on cytotoxic immunological synapses and suggests pathways forward in applying this information to the development of therapeutic agents.
• Perforin: structure, function, and role in human immunopathology
  - Immunol Rev 235(1):35-54 (2010)
  Summary: The secretory granule-mediated cell death pathway is the key mechanism for elimination of virus-infected and transformed target cells by cytotoxic lymphocytes. The formation of the immunological synapse between an effector and a target cell leads to exocytic trafficking of the secretory granules and the release of their contents, which include pro-apoptotic serine proteases, granzymes, and pore-forming perforin into the synapse. There, perforin polymerizes and forms a transmembrane pore that allows the delivery of granzymes into the cytosol, where they initiate various apoptotic death pathways. Unlike relatively redundant individual granzymes, functional perforin is absolutely essential for cytotoxic lymphocyte function and immune regulation in the host. Nevertheless, perforin is still the least studied and understood cytotoxic molecule in the immune system. In this review, we discuss the current state of affairs in the perforin field: the protein's structure ! and function as well as its role in immune-mediated diseases.
• The transcriptional control of the perforin locus
  - Immunol Rev 235(1):55-72 (2010)
  Summary: Natural killer (NK) cells and cytotoxic T lymphocytes (CTLs) use cytotoxic granules containing perforin and granzymes to lyse infected or malignant host cells, thereby providing immunity to intracellular microbes and tumors. Perforin is essential for cytotoxic granule-mediated killing. Perforin expression is regulated transcriptionally and correlates tightly with the development of cells that can exhibit cytotoxic activity. Although a number of genes transcribed by T cells and NK cells have been studied, the cell-specificity of perforin gene expression makes it an ideal model system in which to clarify the transcriptional mechanisms that guide the development and activation of cytotoxic lymphocytes. In this review, we discuss what is known about perforin expression and its regulation, then elaborate on recent studies that utilized chromosome transfer and bacterial artificial chromosome transgenics to define a comprehensive set of cis-regulatory regions that co! ntrol transcription of the human PRF1 gene in a near-physiologic context. In addition, we compare the human and murine Prf1 loci and discuss how transcription factors known to be important for driving CTL differentiation might also directly regulate the cis-acting domains that control Prf1. Our review emphasizes how studies of PRF1/Prf1 gene transcription can illuminate not only the mechanisms of cytotoxic lymphocyte differentiation but also some basic principles of transcriptional regulation.
• Functional dissection of the granzyme family: cell death and inflammation
  - Immunol Rev 235(1):73-92 (2010)
  Summary: Cytotoxic lymphocytes rapidly respond and destroy both malignant cells and cells infected with intracellular pathogens. One mechanism, known as granule exocytosis, employs the secretory granules of these lymphocytes. These include the pore-forming protein perforin (pfp) and a family of serine proteases known as granzymes that cleave and activate effector molecules within the target cell. Over the past two decades, the study of granzymes has largely focused on the ability of these serine proteases to induce cell death. More recently, sophisticated mouse models of disease coupled with gene-targeted mice have allowed investigators to ask why granzyme subfamilies are encoded on different chromosomal loci and what broader role these enzymes might play in inflammation and immune response. Here, we provide a brief overview of the granzyme superfamily, their relationship to pfp, and their reported functions in apoptosis. This overview is followed by a comprehensive an! alysis of the less characterized and developing field regarding the non-apoptotic functions of granzymes.
• Granzyme A activates another way to die
  - Immunol Rev 235(1):93-104 (2010)
  Summary: Granzyme A (GzmA) is the most abundant serine protease in killer cell cytotoxic granules. GzmA activates a novel programed cell death pathway that begins in the mitochondrion, where cleavage of NDUFS3 in electron transport complex I disrupts mitochondrial metabolism and generates reactive oxygen species (ROS). ROS drives the endoplasmic reticulum-associated SET complex into the nucleus, where it activates single-stranded DNA damage. GzmA also targets other important nuclear proteins for degradation, including histones, the lamins that maintain the nuclear envelope, and several key DNA damage repair proteins (Ku70, PARP-1). Cells that are resistant to the caspases or GzmB by overexpressing bcl-2 family anti-apoptotic proteins or caspase or GzmB protease inhibitors are sensitive to GzmA. By activating multiple cell death pathways, killer cells provide better protection against a variety of intracellular pathogens and tumors. GzmA also has proinflammatory activit! y; it activates pro-interleukin-1β and may also have other proinflammatory effects that remain to be elucidated.
• Cytotoxic and non-cytotoxic roles of the CTL/NK protease granzyme B
  - Immunol Rev 235(1):105-116 (2010)
  Summary: The caspase family of cysteine proteases becomes activated in response to diverse cellular insults and coordinates apoptosis through proteolysis of hundreds of cellular substrates. Cytotoxic lymphocytes are adept at promoting apoptosis of virally infected or transformed cells through delivery of cytotoxic enzymes, such as granzyme B, into target cells via the granule exocytosis pathway. Granzyme B promotes apoptosis of target cells through direct processing of certain caspases, which leads to their autoactivation. Granzyme B can also activate caspases indirectly through proteolysis of Bid, a protein that promotes mitochondrial permeabilization and consequent activation of the apoptosome pathway to caspase activation. Evidence also indicates that granzyme B may contribute to antiviral immunity by directly suppressing viral replication through direct proteolysis of viral proteins that are essential for pathogenicity. Recent reports also suggest that granzyme B m! ay have additional non-cytotoxic roles under certain circumstances and may also function in the extracellular space. Here, we discuss the cytotoxic and putative non-cytotoxic functions of granzyme B within the immune system.
• Orphan granzymes find a home
  - Immunol Rev 235(1):117-127 (2010)
  Summary: Cytotoxic lymphocytes are armed with granules that are released in the granule-exocytosis pathway to kill tumor cells and virus-infected cells. Cytotoxic granules contain the pore-forming protein perforin and a family of structurally homologues serine proteases called granzymes. While perforin facilitates the entry of granzymes into a target cell, the latter initiate distinct apoptotic routes. Granzymes are also implicated in extracellular functions such as extracellular matrix degradation, immune regulation, and inflammation. The family of human granzymes consists of five members, of which granzyme A and B have been studied most extensively. Recently, elucidation of the specific characteristics of the other three human granzymes H, K, and M, also referred to as orphan granzymes, have started. In this review, we summarize and discuss what is currently known about the biology of the human orphan granzymes.
• Non-cytotoxic antiviral activities of granzymes in the context of the immune antiviral state
  - Immunol Rev 235(1):128-146 (2010)
  Summary: Viruses are obligatory intracellular parasites, whose replication depends on components encoded by the virus and pathways and functions of the host cell. In addition to the pathways required for viral synthesis, viruses activate multiple mechanisms to evade immune attack, promoting viral propagation while avoiding or slowing the host immune response. To achieve efficient control of viral infections, the immune system in vertebrates relies on diverse and synergistic antiviral pathways (both at the innate and adaptive immune response), which target and inactivate viral and host components involved both in viral replication and in viral defenses that block host antiviral activities. During this process, the immune system uses mechanisms to slow down viral propagation, while apoptotic pathways are triggered to kill (when possible) the infected cell. Granzymes (granule enzymes) are key components of the immune response that play important roles in eliminating host ! cells infected by intracellular pathogens. Although the induction of target cell death has been considered the central function for these proteases, recent evidence supports that granzymes can achieve direct antiviral activities through the cleavage of viral and host factors required for viral replication and viral defense. In addition, granzyme A can stimulate the production of pro-inflammatory cytokines. The focus of this review is to discuss recent views on antiviral mechanisms involved in controlling viral infections, with special interest in novel and potential non-death-related antiviral functions of the granzymes, and how these unique functions complement and synergize with the 'antiviral state' created by interferons and cytotoxic lymphocytes in response to virus.
• Serine protease inhibitors and cytotoxic T lymphocytes
  - Immunol Rev 235(1):147-158 (2010)
  Summary: Serine proteases control a wide variety of physiological and pathological processes in multi-cellular organisms, including blood clotting, cancer, cell death, osmoregulation, tissue remodeling, and immunity to infection. Cytotoxic T lymphocytes (CTLs) are required for adaptive cell-mediated immunity to intracellular pathogens by killing infected cells and through the development of memory T cells. Serine proteases not only allow a CTL to kill but also impose homeostatic control on CTL number. Serine protease inhibitors (serpins) are the physiological regulators of serine proteases' activity. In this review, I discuss the role of serpins in controlling the recognition of antigen, effector function, and homeostatic control of CTLs through the inhibition of physiological serine protease targets. An emerging view of serpins is that they are important promoters of cellular viability through their inhibition of executioner proteases. This view is discussed in the co! ntext of the T-lymphocyte survival during effector responses and the development and persistence of long-lived memory T cells. Given the important role serpins play in CTL immunity, I discuss the potential for developing new immunotherapeutic approaches based directly on serpins or knowledge gained from identifying their physiologically relevant protease targets.
• Delivery and therapeutic potential of human granzyme B
  - Immunol Rev 235(1):159-171 (2010)
  Summary: Granzyme B (GzmB) is used by cytotoxic lymphocytes as a molecular weapon for the defense against virus-infected and malignantly transformed host cells. It belongs to a family of small serine proteases that are stored in secretory vesicles of killer cells. After secretion of these cytolytic granules during killer cell attack, GzmB is translocated into the cytosol of target cells with the help of the pore-forming protein perforin. GzmB has adopted similar protease specificity as caspase-8, and once delivered, it activates major executioner apoptosis pathways. Since GzmB is very effective in killing human tumor cell lines that are otherwise resistant against many cytotoxic drugs and since GzmB of human origin can be recombinantly expressed, its use as part of a 'magic bullet' in tumor therapy is a very tempting idea. In this review, we emphasize the peculiar characteristics of GzmB that make it suited for use as an effector domain in potential immunoconjugates. W! e discuss what is known about its uptake into target cells and the trials performed with GzmB-armed immunoconjugates, and we assess the prospects of its potential therapeutic value.
• TIM genes: a family of cell surface phosphatidylserine receptors that regulate innate and adaptive immunity
  - Immunol Rev 235(1):172-189 (2010)
  Summary: The TIM (T cell/transmembrane, immunoglobulin, and mucin) gene family plays a critical role in regulating immune responses, including allergy, asthma, transplant tolerance, autoimmunity, and the response to viral infections. The unique structure of TIM immunoglobulin variable region domains allows highly specific recognition of phosphatidylserine (PtdSer), exposed on the surface of apoptotic cells. TIM-1, TIM-3, and TIM-4 all recognize PtdSer but differ in expression, suggesting that they have distinct functions in regulating immune responses. TIM-1, an important susceptibility gene for asthma and allergy, is preferentially expressed on T-helper 2 (Th2) cells and functions as a potent costimulatory molecule for T-cell activation. TIM-3 is preferentially expressed on Th1 and Tc1 cells, and generates an inhibitory signal resulting in apoptosis of Th1 and Tc1 cells. TIM-3 is also expressed on some dendritic cells and can mediate phagocytosis of apoptotic cells an! d cross-presentation of antigen. In contrast, TIM-4 is exclusively expressed on antigen-presenting cells, where it mediates phagocytosis of apoptotic cells and plays an important role in maintaining tolerance. TIM molecules thus provide a functional repertoire for recognition of apoptotic cells, which determines whether apoptotic cell recognition leads to immune activation or tolerance, depending on the TIM molecule engaged and the cell type on which it is expressed.
• Plasticity in programming of effector and memory CD8+ T-cell formation
  - Immunol Rev 235(1):190-205 (2010)
  Summary: CD8+ T cells (also called cytotoxic T lymphocytes) play a major role in protective immunity against many infectious pathogens and can eradicate malignant cells. The path from naive precursor to effector and memory CD8+ T-cell development begins with interactions between matured antigen-bearing dendritic cells (DCs) and antigen-specific naive T-cell clonal precursors. By integrating differences in antigenic, costimulatory, and inflammatory signals, a developmental program is established that governs many key parameters associated with the ensuing response, including the extent and magnitude of clonal expansion, the functional capacities of the effector cells, and the size of the memory pool that survives after the contraction phase. In this review, we discuss the multitude of signals that drive effector and memory CD8+ T-cell differentiation and how the differences in the nature of these signals contribute to the diversity of CD8+ T-cell responses.
• Once a killer, always a killer: from cytotoxic T cell to memory cell
  - Immunol Rev 235(1):206-218 (2010)
  Summary: The control of the differentiation pathways followed by responding CD8+ T cells to produce protective memory cells has been intensely studied. Recent developments have identified heterogeneity at the effector cytotoxic T-lymphocyte level within which a bona fide memory cell precursor has emerged. The challenge now is to identify the cellular and molecular factors that control this developmental pathway. This review considers aspects of the regulation of the induction of effectors, the transition of effectors to memory cells, and the dynamics of the memory population.
• Generating diversity: transcriptional regulation of effector and memory CD8+ T-cell differentiation
  - Immunol Rev 235(1):219-233 (2010)
  Summary: In response to acute infections or vaccines, naive antigen-specific CD8+ T cells proliferate and differentiate into effector cytotoxic lymphocytes that acquire the ability to kill infected cells. While the majority of differentiated effector cells die after pathogen clearance, a small number evade terminal differentiation, downregulate active effector functions, and survive as long-lived, self-renewing memory T cells. Our understanding of how effector CD8+ T cells adopt these different cell fates has grown greatly in recent years. In this review, we discuss the transcriptional regulators that are known to support general effector differentiation, terminal effector differentiation, and memory cell formation. We propose that the diversity of activated CD8+ T-cell differentiation states is achieved via gradients of activity or expression of transcriptional regulators that are regulated by the level of inflammation and antigenic signaling the T cells experience du! ring infection.
• The role of mTOR in memory CD8+ T-cell differentiation
  - Immunol Rev 235(1):234-243 (2010)
  Summary: The mammalian target of rapamycin (mTOR) is an intracellular kinase that regulates cell growth and metabolism. Its specific inhibitor rapamycin is currently used in transplant recipients as an immunosuppressive drug to prevent allograft rejection. Studies have shown complex and diverse mechanisms for the immunosuppressive effects of rapamycin. The drug has been reported to inhibit T-cell proliferation, induce anergy, modulate T-cell trafficking, promote regulatory T cells, and also prevent maturation of dendritic cells as well as production of type I interferon. However, several other studies have paradoxically demonstrated immunostimulatory effects of rapamycin by improving antigen presentation and regulating cytokine production from macrophages and myeloid dendritic cells. Recently, it has been shown that rapamycin also exhibits immunostimulatory effects on memory CD8+ T-cell differentiation. The drug improved both quantity and quality of memory CD8+ T cells! induced by viral infection and vaccination, showing that mTOR is a major regulator of memory CD8+ T-cell differentiation. These discoveries have implications for the development of novel vaccine regimens. Here, we review the role of mTOR in memory CD8+ T-cell differentiation and compare the effect of rapamycin among CD8+ T cells, CD4+ T cells, and dendritic cells. Also, we discuss potential application of these findings in a clinical setting.
• Heterologous immunity between viruses
  - Immunol Rev 235(1):244-266 (2010)
  Summary: Immune memory responses to previously encountered pathogens can sometimes alter the immune response to and the course of infection of an unrelated pathogen by a process known as heterologous immunity. This response can lead to enhanced or diminished protective immunity and altered immunopathology. Here, we discuss the nature of T-cell cross-reactivity and describe matrices of epitopes from different viruses eliciting cross-reactive CD8+ T-cell responses. We examine the parameters of heterologous immunity mediated by these cross-reactive T cells during viral infections in mice and humans. We show that heterologous immunity can disrupt T-cell memory pools, alter the complexity of the T-cell repertoire, change patterns of T-cell immunodominance, lead to the selection of viral epitope-escape variants, alter the pathogenesis of viral infections, and, by virtue of the private specificity of T-cell repertoires within individuals, contribute to dramatic variations in ! viral disease. We propose that heterologous immunity is an important factor in resistance to and variations of human viral infections and that issues of heterologous immunity should be considered in the design of vaccines.
• Effect of NKG2D ligand expression on host immune responses
  - Immunol Rev 235(1):267-285 (2010)
  Summary: Natural killer group 2, member D (NKG2D) is an activating receptor present on the surface of natural killer (NK) cells, some NKT cells, CD8+ cytotoxic T cells, γδ T cells, and under certain conditions CD4+ T cells. Present in both humans and mice, this highly conserved receptor binds to a surprisingly diverse family of ligands that are distant relatives of major histocompatibility complex class I molecules. There is increasing evidence that ligand expression can result in both immune activation (tumor clearance, viral immunity, autoimmunity, and transplantation) and immune silencing (tumor evasion). In this review, we describe this family of NKG2D ligands and the various mechanisms that control their expression in stressed and normal cells. We also discuss the host response to both membrane-bound and secreted NKG2D ligands and summarize the models proposed to explain the consequences of this differential expression.
• Adaptive immune responses mediated by natural killer cells
  - Immunol Rev 235(1):286-296 (2010)
  Summary: Adaptive immunity has traditionally been considered a unique feature of vertebrate physiology. Unlike innate immune responses, which remain essentially unchanged upon exposure to a recurrent challenge with the same stimulus, adaptive immune cells possess the ability to learn and remember. Thus, secondary adaptive responses to a previously encountered challenge are qualitatively and/or quantitatively distinct from those elicited by a primary encounter. Besides this capacity to acquire long-lived memory, the second cardinal feature of adaptive immunity is antigen specificity. It has been generally believed that only T and B cells can develop antigen-specific immunologic memory, because these lymphocytes uniquely express recombination-activating gene (RAG) proteins, which are necessary for somatic rearrangement of V(D)J gene segments to assemble diverse antigen-specific receptors. However, recent work has uncovered discrete subsets of murine natural killer (NK) c! ells capable of mediating long-lived, antigen-specific recall responses to a variety of hapten-based contact sensitizers. These NK cells appear to use distinct, RAG-independent mechanisms to generate antigen specificity. Murine NK cells have also recently been shown to develop memory upon viral infection. Here, we review recent evidence indicating that at least some NK cells are capable of mediating what appears to be adaptive immunity and discuss potential mechanisms that may contribute to RAG-independent generation of antigenic diversity and longevity.
• Memory-like responses of natural killer cells
  - Immunol Rev 235(1):297-305 (2010)
  Summary: Natural killer (NK) cells are lymphocytes with the capacity to produce cytokines and kill target cells upon activation. NK cells have long been categorized as members of the innate immune system and as such have been thought to follow the 'rules' of innate immunity, including the principle that they have no immunologic memory, a property thought to be strictly limited to adaptive immunity. However, recent studies have suggested that NK cells have the capacity to alter their behavior based on prior activation. This property is analogous to adaptive immune memory; however, some NK cell memory-like functions are not strictly antigen dependent and can be demonstrated following cytokine stimulation. Here, we discuss the recent evidence that NK cells can exhibit properties of immunologic memory, focusing on the ability of cytokines to non-specifically induce memory-like NK cells with enhanced responses to restimulation.