Chapter 14 Immunity to Bacteria and Fungi
• 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
The immune defense mechanisms elicited against pathogenic bacteria are determined by their:
• mechanism(s) of pathogenicity; and
• whether they are predominantly extracellular or also have the ability to survive inside mammalian cells.
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.
Pathogenicity varies between two extreme patterns
The two extreme patterns of pathogenicity are:
The first lines of defense do not depend on antigen recognition
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), 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.
Bacterial PAMPs activate cells via Toll-like receptors
Other important pattern recognition receptors include:
LPS is the dominant activator of innate immunity in Gram-negative bacterial infection
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).
Lymphocyte-independent effector systems
Complement is activated via the alternative pathway
Perhaps more importantly, complement activation releases C5a, which attracts and activates neutrophils and causes degranulation of mast cells (see Chapter 3). The consequent release of histamine and leukotriene (LTB4) contributes to further increases in vascular permeability (see Fig. 14.3).
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.
Pathogen recognition generates signals that regulate the lymphocyte-mediated response
• display of MHC molecule–peptide complexes;
• expression of co-stimulatory molecules (such as CD40, CD80, and CD86); and
Some of this DC activation occurs secondary to their production of cytokines such as type I IFN.
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.
Antibody dependent anti-bacterial defenses
• it neutralizes diphtheria toxin by blocking the attachment of the binding portion of the molecule to its target cells;
• similarly it may block locally acting toxins or extracellular matrix-degrading enzymes, which act as spreading factors.
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).
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).
Fig. 14.6 Effect of antibody and complement on rate or clearance of virulent bacteria from the blood
Pathogenic bacteria may avoid the effects of antibody
Neisseria gonorrhoeae is an example of a pathogenic bacterium that uses several immune evasion strategies (Fig. 14.8) and humans can be repeatedly infected with N. gonorrhoeae with no evidence of protective immunity.