Immunity to Bacteria and Fungi

Chapter 14 Immunity to Bacteria and Fungi




Summary




Mechanisms of protection from bacteria can be deduced from their structure and pathogenicity. There are four main types of bacterial cell wall and pathogenicity varies between two extreme patterns. Non-specific, phylogenetically ancient recognition pathways for conserved bacterial structures trigger protective innate immune responses and guide the development of adaptive immunity.


Lymphocyte-independent bacterial recognition pathways have several consequences. Complement is activated via the alternative pathway. Release of proinflammatory cytokines and chemokines increases the adhesive properties of the vascular endothelium and promotes neutrophil and macrophage recruitment. Pathogen recognition generates signals then regulate the lymphocyte-mediated response.


Antibody provides an antigen-specific protective mechanism. Neutralizing antibody may be all that is needed for protection if the organism is pathogenic only because of a single toxin or adhesion molecule. Opsonizing antibody responses are particularly important for resistance to extracellular bacterial pathogens. Complement can kill some bacteria, particularly those with an exposed outer lipid bilayer, such as Gram-negative bacteria.


Ultimately most bacteria are killed by phagocytes following a multistage process of chemotaxis, attachment, uptake, and killing. Macrophage killing can be enhanced on activation. Optimal activation of macrophages is dependent on TH1 CD4 T cells, whereas neutrophil responses are promoted by TH17 CD4 T cells. Persistent macrophage recruitment and activation can result in granuloma formation, which is a hallmark of cell-mediated immunity to intracellular bacteria.


Successful pathogens have evolved mechanisms to avoid phagocyte-mediated killing and have evolved a startling diversity of mechanisms for avoiding other aspects of innate and adaptive immunity.


Infected cells can be killed by CTLs. Other T cell populations and some tissue cells can contribute to antibacterial immunity.


The response to bacteria can result in immunological tissue damage. Excessive release of cytokines caused by microorganisms can result in immunopathological syndromes, such as endotoxin shock and the Schwartzman reaction.


Fungi can cause life-threatening infections. Immunity to fungi is predominantly cell mediated and shares many similarities with immunity to bacteria.



Innate recognition of bacterial components


Bacterial infections have had an enormous impact on human society and despite the discovery of antibiotics continue to be a major threat to public health.


Plague caused by Yersinia pestis is estimated to have killed one-quarter of the European population in the Middle Ages, whereas infection with Mycobacterium tuberculosis is currently a global health emergency.


The immune defense mechanisms elicited against pathogenic bacteria are determined by their:




There are four main types of bacterial cell wall


The four main types of bacterial cell wall (Fig. 14.1) belong to the following groups.




The outer lipid bilayer of Gram-negative organisms is of particular importance because it is often susceptible to lysis by complement. However, killing of most bacteria usually requires uptake by phagocytes. The outer surface of the bacterium may also contain fimbriae or flagellae, or it may be covered by a protective capsule. These can impede the functions of phagocytes or complement, but they also act as targets for the antibody response, the role of which is discussed later.



Pathogenicity varies between two extreme patterns


The two extreme patterns of pathogenicity are:




However, most bacteria are intermediate between these extremes, having some invasiveness assisted by some locally acting toxins and spreading factors (tissue-degrading enzymes).


Corynebacterium diphtheriae and Vibrio cholerae are examples of organisms that are toxic, but not invasive. Because their pathogenicity depends almost entirely on toxin production, neutralizing antibody to the toxin is probably sufficient for immunity, though antibody binding to the bacteria and so blocking their adhesion to the epithelium could also be important.


In contrast, the pathogenicity of most invasive organisms does not rely so heavily on a single toxin, so immunity requires killing of the organisms themselves.



The first lines of defense do not depend on antigen recognition


The body’s first line of defense against pathogenic bacteria consists of simple barriers to the entry or establishment of the infection. Thus, the skin and exposed epithelial surfaces have non-specific or innate protective systems, which limit the entry of potentially invasive organisms.


Intact skin is impenetrable to most bacteria. Additionally, fatty acids produced by the skin are toxic to many organisms. Indeed, the pathogenicity of some strains correlates with their ability to survive on the skin. Epithelial surfaces are cleansed, for example, by ciliary action in the trachea or by flushing of the urinary tract.


Many bacteria are destroyed by pH changes in the stomach and vagina, both of which provide an acidic environment. In the vagina, the epithelium secretes glycogen, which is metabolized by particular species of commensal bacteria, producing lactic acid.



Commensals can limit pathogen invasion


Commensal bacteria have co-evolved with us over millions of years, providing an essential protective function against more pathogenic species by occupying an ecological niche that would otherwise be occupied by something more unpleasant. In fact it has been estimated that the human body contains approximately 10 times more bacterial cells than human cells. This is mostly because of the gut microbiota, made up of perhaps thousands of different bacterial species many of which have not been cultured but identified more recently by high throughput sequencing technology of 16S ribosomal RNA sequences. The precise makeup of this microbiota is different between individuals, with a core of common species together with an additional set that is determined in part by the genetics of the host. The normal flora protect against pathogens by competing more efficiently for nutrients, by producing antibacterial proteins termed colicins and by stimulating immune responses which act to limit pathogen entry.


Maintaining this protective flora without eliciting inflammatory reactions is a delicate and immunologically complicated process as even these bacteria are not immunologically inert. The host attempts to minimize contact between the bacteria and the epithelial cells of the gut lumen by production of mucins, and by effector molecules including antimicrobial peptides and secretory IgA. Nevertheless, some commensal bacteria do penetrate these barriers and are sampled by intestinal dendritic cells, inducing local (but not systemic) immune responses involving CD4 T cells and regulatory T cells.


When the normal flora are disturbed by antibiotics, infections by Candida spp. or Clostridium difficile can occur. Several studies suggest that the reintroduction of non-pathogenic ‘probiotic’ organisms such as lactobacilli into the intestinal tract (or in extreme circumstances even the normal flora from an otherwise healthy person) can alleviate the symptoms, presumably by replacing those killed by the antibiotics.


In practice, only a minute proportion of the potentially pathogenic organisms around us ever succeed in gaining access to the tissues.



The second line of defense is mediated by recognition of bacterial components


If organisms do enter the tissues, they can be combated initially by further elements of the innate immune system. Numerous bacterial components are recognized in ways that do not rely on the antigen-specific receptors of either B cells or T cells. These types of recognition are phylogenetically ancient ‘broad-spectrum’ mechanisms that evolved before antigen-specific T cells and immunoglobulins, allowing protective responses to be triggered by common microbial components bearing so-called ‘pathogen-associated molecular patterns’ (PAMPs), recognized by ‘pattern recognition molecules’ of the innate immune system (see Chapter 6).




Q. List some examples of soluble molecules, cell surface receptors, and intracellular molecules that recognize PAMPs


A. Collectins and ficolins (see Fig. 6.w3)image, the Toll-like receptors (see Fig. 6.21), the mannose receptor (see Fig. 7.11), and the NOD-like receptor proteins (see Fig. 7.13) all recognize PAMPs.


Many organisms, such as non-pathogenic cocci, are probably removed from the tissues as a consequence of these pathways, without the need for a specific adaptive immune reaction. Figure 14.3 shows some of the microbial components involved and the host responses that are triggered.



The immune system has selected these structures for recognition because they are not only characteristic of microbes, but are essential for their growth and cannot be easily mutated to evade discovery (though, as might be predicted, there are increasing examples of pathogen strategies that attempt to subvert this process).


It is interesting to note that the ‘Limulus assay’, which is used to detect contaminating lipopolysaccharide (LPS) in preparations for use in humans, is based on one such recognition pathway found in an invertebrate species. In Limulus polyphemus (the horseshoe crab), tiny quantities of LPS trigger fibrin formation, which walls off the LPS-bearing infectious agent.image




LPS is the dominant activator of innate immunity in Gram-negative bacterial infection


Injection of pure LPS into mice or even humans is sufficient to mimic most of the features of acute Gram-negative infection, including massive production of proinflammatory cytokines, such as IL-1, IL-6, and tumor necrosis factor (TNF), leading to severe shock.



Recognition of LPS is a complex process involving molecules that bind LPS and pass it on to cell membrane-associated receptors on leukocytes, and endothelial and other cells, which initiate this proinflammatory cascade (Fig. 14.4).



Binding of LPS to TLR4 is a critical event in immune activation. TLR4 knockout mice are resistant to LPS-induced shock and there is some evidence that polymorphisms in human TLR4 may influence the course of infection with these bacteria.


The LBP and CD14, which bind LPS, are also involved in recognition of lipid-containing bacterial components from mycoplasmas, mycobacteria, and spirochetes.



Other bacterial components are also potent immune activators


Gram-positive bacteria do not possess LPS yet still induce intense inflammatory responses and severe infection via the actions of other chemical structures such as peptidoglycans and lipotechoic acids of their cell wall, which can be recognized by TLR2, often in cooperation with TLR1 or TLR6.


Most capsular polysaccharides are not potent activators of inflammation (though some can activate macrophages) but they shield the bacterium from host immune defenses.


Other bacterial molecules that trigger innate immunity include lipoproteins (via TLR 2/6), flagellin (via TLR5), and DNA (due to its distinct CpG motifs) via TLR9.


Most pattern recognition receptors are expressed on the plasma membrane of cells, making contact with microbes during the process of binding and/or phagocytosis.


However, others are designed to detect intracellular pathogens and their products inside phagosomes (such as TLR9) or in the cytosol.



Epithelial cells of the gut and lung can have few TLRs on their luminal surface, but can be triggered by pathogens that:



This helps to explain why constant exposure to non-pathogenic microbes in the intestine and airways does not induce a chronic state of inflammation – the host waits until they move beyond the lumen, signifying the presence of a real pathogenic threat.



Lymphocyte-independent effector systems




Release of proinflammatory cytokines increases the adhesive properties of the vascular endothelium


The rapid release of cytokines such as TNF and IL-1 (see Fig. 14.4) from macrophages increases the adhesive properties of the vascular endothelium and facilitates the passage of more phagocytes into inflamed tissue. Combined with the release of chemokines such as CCL2, CCL3, and CXCL8 (see Chapter 6), this directs the recruitment of different leukocyte populations.


Epithelial cells, neutrophils, and mast cells are also important sources of proinflammatory cytokines.


IL-1, TNF, and IL-6 also initiate the acute phase response, increasing the production of complement components as well as other proteins involved in scavenging material released by tissue damage and, in the case of CRP, an opsonin for improving phagocytosis of bacteria.


When NK cells are stimulated by the phagocyte-derived cytokines IL-12 and IL-18 they rapidly release large quantities of interferon-γ (IFNγ). This response happens within the first day of infection, well before the clonal expansion of antigen-specific T cells, and provides a rapid source of IFNγ to activate macrophages. This T cell-independent pathway helps to explain the considerable resistance of mice with SCID (severe combined immune deficiency, a defect in lymphocyte maturation) to infections such as with Listeria monocytogenes. In mice, CD1d-restricted NK T cells also secrete IFNγ in response to IL-12 and IL-18 and other ligands, and help to further activate both NK cells and macrophages.



Pathogen recognition generates signals that regulate the lymphocyte-mediated response


The signals generated following the recognition of pathogens not only generate a cascade of innate immune events, but also regulate the development of the appropriate lymphocyte-mediated response.


Dendritic cells (DCs) are crucial for the initial priming of naive T cells specific for bacterial antigens. Contact with bacteria in the periphery induces immature DCs to migrate to the draining lymph nodes and augments their antigen-presenting ability by increasing their:



Some of this DC activation occurs secondary to their production of cytokines such as type I IFN.


Activated macrophages also act as antigen-presenting cells (APCs), but probably function more at the site of infection, providing further activation of effector rather than naive T cells. Following initial T cell activation by dendritic cells, B cells are also able to act as APCs during B cell–T cell cooperation and are essential for the protective action of polysaccharide-conjugate vaccines in children against encapsulated bacteria such as S. pneumoniae and H. influenzae.


Binding of bacterial components to pattern recognition receptors such as TLRs induces a local environment rich in cytokines such as IFNγ, IL-12, and IL-18, which promote T cell differentiation down the TH1 rather than TH2 pathway.


Immunologists have made use of these effects for many decades (even without knowing their true molecular basis) in the use of adjuvants in vaccination. ‘Adjuvant’ is derived from the Latin adjuvare, to help. When given experimentally, soluble antigens evoke stronger T and B cell-mediated responses if they are mixed with bacterial components that act as adjuvants. Components with this property are indicated in Figure 14.1. This effect probably reflects that the antigen-specific immune response evolved in a tissue environment that already contained these pharmacologically active bacterial components.


With the exception of proteins such as flagellin, which itself stimulates TLR5 and is also a strong T cell immunogen, the response to pure bacterial antigens, injected without adjuvant-active bacterial components, is essentially an artificial situation that does not occur in nature.


The best known adjuvant in laboratory use, complete Freund’s adjuvant, consists of killed mycobacteria suspended in oil, which is then emulsified with the aqueous antigen solution.


New-generation adjuvants based on bacterial components (and safe to use in humans, unlike Freund’s adjuvant) include synthetic TLR activators such as CpG motifs and monophosphoryl lipid A (MPL) as well as recombinant cytokines such as IL-12, IL-1, and IFNγ. Identifying the best adjuvant for inclusion in a vaccine is arguably as important as the choice of antigens and is dramatically illustrated in the RTS,S malaria vaccine – a product which was not effective until reformulated with a new MPL based adjuvant.



Antibody dependent anti-bacterial defenses


The relevance to protection of interactions of bacteria with antibody depends on the mechanism of pathogenicity. Antibody clearly plays a crucial role in dealing with bacterial toxins:



Antibody can also interfere with motility by binding to flagellae.


An important function on external and mucosal surfaces, often performed by secretory IgA (sIgA, see Chapter 3), is to stop bacteria binding to epithelial cells – for instance, antibody to the M proteins of group A streptococci gives type-specific immunity to streptococcal sore throats.


It is likely that some antibodies to the bacterial surface can block functional requirements of the organism such as binding of iron-chelating compounds or intake of nutrients (Fig. 14.5).



An important role of antibody in immunity to non-toxigenic bacteria is the more efficient targeting of complement.


Naturally occurring IgM antibodies, which bind to common bacterial structures such as phosphorylcholine, are important for protection against some bacteria (particularly streptococci) via their complement fixing activity.


Specific, high-affinity IgG antibodies elicited in response to infection are most important. This is particularly true for anti-toxin responses where the antibody must compete against the affinity of the toxin receptor on host cells in vivo. Children with primary immune deficiencies in B cell development or in T cell help have increased susceptibility to extracellular rather than intracellular bacteria.


With the aid of antibodies, even organisms that resist the alternative (i.e. innate) complement pathway (see below) are damaged by complement or become coated with C3 products, which then enhance the binding and uptake by phagocytes (Figs 14.6 and 14.7).





The most efficient complement-fixing antibodies in humans are IgM, then IgG3 and to a lesser extent IgG1, whereas IgG1 and IgG3 are the subclasses with the highest affinity for Fc receptors.



Jun 18, 2016 | Posted by in IMMUNOLOGY | Comments Off on Immunity to Bacteria and Fungi

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