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Innate Immunity 3 CHAPTER 1 Innate immunity Key topics ◾◾ Knowing when to make an immune response 4 ◾◾ Pattern recognition receptors detect nonself 7 ◾◾ Immune responses are tailored towards particular types of infection 8 ◾◾ Innate versus adaptive immunity 10 ◾◾ External barriers against infection 12 ◾◾ Cells of the immune system 12 ◾◾ The beginnings of an immune response 18 ◾◾ There are several classes of pattern recognition receptors 22 ◾◾ Phagocytic cells engulf and kill microorganisms 29 ◾◾ Phagocytes employ an array of killing mechanisms 30 ◾◾ Complement facilitates phagocytosis and bacterial lysis 34 ◾◾ Humoral mechanisms provide an additional defensive strategy 38 ◾◾ Natural killer cells kill virally infected cells 42 ◾◾ Dealing with large parasites 45 ◾◾ The innate immune system instigates adaptive immunity 45 COPYRIGHTED MATERIAL Roitt’s Essential Immunology, Thirteenth Edition. Peter J. Delves, Seamus J. Martin, Dennis R. Burton, and Ivan M. Roitt. © 2017 John Wiley & Sons, Ltd. Published 2017 by John Wiley & Sons, Ltd. Companion website: www.roitt.com 0002814659.indd 3 10/28/2016 12:46:06 PM 4 / Part 1: Fundamentals of immunology Size (mm) Worms Tapeworm 103 Guineaworm Schistosome 10 Filaria 1 Protozoa 10–1 Amoeba Leishmania Fungi Trypanosome Bacteria 10–2 Aspergillus Candida Mycobacterium Malaria Staphylococcus Rickettsia –3 Viruses 10 Chlamydia Pox Mycoplasma 10–4 Inuenza Polio Figure 1.1 The formidable range of infectious agents that confront the immune system. Although not normally classified as such because of their lack of a cell wall, the mycoplasmas are included under bacteria for convenience. Fungi adopt many forms and approximate values for some of the smallest forms are given. Square brackets with right arrowheads indicate where a range of sizes is observed for the organism(s); square brackets with left arrowheads indicate list of organisms with a definite size. Introduction Knowing when to make an immune We live in a potentially hostile world filled with a bewildering response array of infectious agents (Figure 1.1) of diverse shape, size, The ability to recognize and respond to foreign composition, and subversive character that would very happily entities is central to the operation of the immune use us as rich sanctuaries for propagating their “selfish genes” system had we not also developed a series of defense mechanisms at least their equal in effectiveness and ingenuity (except in the The vertebrate immune system is a conglomeration of cells and case of many parasitic infections in which the situation is best molecules that cooperate to protect us from infectious agents described as an uneasy and often unsatisfactory truce). It is and also provides us with a surveillance system to monitor the these defense mechanisms that can establish a state of immu- integrity of host tissues. Although the immune system is quite nity against infection (Latin immunitas, freedom from) and elaborate, as we shall see, its function can be boiled down to whose operation provides the basis for the delightful subject two basic roles: recognition of foreign substances and organ- called “immunology.” isms that have penetrated our outer defences (i.e., the skin epi- Aside from ill‐understood constitutional factors that make thelium and the mucosal surfaces of the gut and reproductive one species innately susceptible and another resistant to and respiratory tracts) and elimination of such agents by a c ertain infections, a number of relatively nonspecific but diverse repertoire of cells and molecules that act in concert to nonetheless highly effective antimicrobial systems (e.g., neutralize the potential threat. Thus, a critical role of the phagocytosis, production of antimicrobial peptides and reac- immune system is to determine what is foreign (what immu- tive oxygen species) have been recognized that are innate in nologists often call “nonself”) from what is normally present in the sense that they are not affected by prior contact with the the body (i.e., self). As a consequence, the cells and molecules infectious agent and take immediate effect upon encounter that comprise the innate immune system are preoccupied with with anything that our immune systems deem to be an unwel- detecting the presence of particular molecular patterns that come guest. We shall discuss these systems and examine how, are typically associated with infectious agents (Figure 1.2). in the state of adaptive immunity, their effectiveness can be Charlie Janeway dubbed such molecules pathogen‐associated greatly increased though custom tailoring of the response molecular p atterns (PAMPs) and it is these structures that trig- towards microbial intruders. ger activation of the innate immune system. 0002814659.indd 4 10/28/2016 12:46:07 PM Chapter 1: Innate immunity / 5 Pathogen and associated PAMPS PAMPs PAMPs Soluble Cell- PRRs associated PRRs Signaling Binding of microorganism by Phagocytosis of PAMP and soluble PRR molecules associated microorganism Enhancement of Activation of immune cell phagocytosis of PRR-bound encountering PAMP PAMPs Release of “cytokines” to Proteolytic cascade resulting amplify response in lysis of microorganism Figure 1.2 Pattern recognition receptors (PRRs) detect pathogen‐associated molecular patterns (PAMPs) and initiate immune responses. PRRs can be either soluble or cell‐associated and can instigate a range of responses upon encountering their appropriate ligands. In addition to the fundamental roles of recognition and t rouble as the infection itself. In extreme cases, the immune elimination of infectious agents, it is also very useful to be response can be much more destructive than the agent that able to learn from encounters with pathogens and to main- triggered it (which is what underpins allergy) and in some situ- tain a reserve of cells that are able to respond swiftly to a new ations this can lead to a sustained state of chronic immune infection with a previously encountered microbe. Forewarned activation where the immune system becomes confused is forearmed, and in this situation it may be possible to between what is self and nonself and mounts sustained deliver a decisive blow that ends a nascent infection before it responses against its own tissues (called autoimmunity). Thus, has begun. Fortunately, our immune systems have also there is a cost–benefit analysis that must be conducted during acquired this a bility, which is what our adaptive immune the initial stages of an infection to ascertain the nature of the system excels in, and this property is termed immunological infection, the level of infection, and whether the infectious memory. agent is perturbing tissue function (by triggering cell death for example). immune regulatory Immune responses need to be proportional For these reasons, a number of to the infectious threat mechanisms exist to ensure that immune responses are pro- portional to the level of threat that a particular infectious Having established that recognition, elimination and memory agent poses, as well as to ensure that immune responses are of infectious agents are fundamental to the operation of an not directed against self and that responses directed against effective immune system, there is another important factor, nonself are terminated when the infectious agent has been proportionality, which is key to ensuring that everything runs successfully eliminated from the body. Immune regulatory smoothly and that our immune systems do not lose sight of mechanisms (or immune checkpoints) set thresholds for the their purpose. This is because, as we shall see, the immune sys- deployment of immune responses and are vital to the proper tem can deploy a variety of weapons, each with their own risk operation of the immune system. As we shall see in later of collateral damage, which can sometimes cause as much chapters, many diseases are caused by the failure of immune 0002814659.indd 5 10/28/2016 12:46:08 PM 6 / Part 1: Fundamentals of immunology Physiological stimuli Severe injury or mild injury DAMPs DAMPs Uncontrolled cell Regulated cell death death Necrosis Apoptosis Release of DAMPs DAMPs (danger signals) remain Macrophage hidden Cell-associated Soluble PRRs PRRs Recognition and phagocytosis of apoptotic cell by macrophage Immune system remains Immune response quiescent Figure 1.3 Necrotic cells release danger‐associated molecular patterns (DAMPs), whereas apoptotic cells typically do not. Stimuli that induce necrosis frequently cause severe cellular damage, which leads to rapid cell rupture with consequent release of intracellular DAMPs. DAMPs can then engage cells of the immune system and can promote inflammation. On the other hand, because stimuli that initiate apoptosis are typically physiological and relatively mild, apoptotic cells do not rupture and their removal is coordinated by mac- rophages and other cells of the innate immune system, before release of DAMPs can occur. For this reason, apoptosis is not typically associated with activation of the immune system. checkpoints, leading to conditions such as rheumatoid investigation at present, but molecules such as HMGB1, a arthritis, Crohn’s disease, and even cancer. chromatin‐binding protein, as well as the immunological mes- Tissue damage can also instigate an immune senger proteins interleukin‐1α (IL‐1α) and IL‐33, represent response good candidates. It might seem surprising that the immune system can also be activated by self‐derived molecules, how- Aside from infection, there is a growing recognition that tissue ever, this makes good sense
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