17 Pathologic consequences of infection

Symptoms of infections are produced by the microorganisms or by the host’s immune responses

Symptoms that appear rapidly after the acquisition of an infection are usually due to the direct action of the invading microbe by what it secretes. Thus a virus in a cell may cause metabolic ‘shut-down’ or lyse the cell. Bacteria, however, provoke most of their acute effects by releasing toxins, but may also cause distress by inducing inflammation. The inflammatory response is, of course, an important component of host protection, vascular permeability being vital for the rapid mobilization of cells such as neutrophils, and serum components such as complement and antibody. Inflammation is therefore intrinsically a healthy sign, and it is interesting that some virulent bacteria (e.g. staphylococci) can, to some extent, inhibit the inflammatory response.

Often, however, pathologic changes are secondary to the activation of immunologic mechanisms that are normally thought of as protective. These may involve innate or the adaptive immune system or, more usually, both (Fig. 17.1). Tissue damage resulting from adaptive immune responses is usually referred to as ‘immunopathology’ and is quite common in infectious diseases, particularly those that are chronic and persistent. The immunologic basis of these mechanisms of tissue damage is described in Chapter 11.

Figure 17.1 Pathologic effects of infection: a general scheme. Infectious parasitic organisms can cause disease directly (top) or indirectly via overactivation of various immune mechanisms, either natural (middle) or adaptive (bottom). IFN, interferon; IL, interleukin; Mφ, macrophage; PMN, polymorphonuclear leukocyte; TNF, tumour necrosis factor.

Certain viruses can cause permanent malignant change in cells as a result of direct, indirect and a mixture of both mechanisms. Seven viruses that infect humans cause up to 15% of human cancers around the world. These include HTLV type 1 (lymphomas, leukaemias), Epstein–Barr virus (nasopharyngeal carcinoma and Burkitt’s lymphoma), human papillomaviruses (cervical cancer), hepatitis B and C virus infections (liver cancer), HIV (immunosuppression leads to the development of cancers associated with KSHV and EBV) and Merkel cell polyomavirus (Merkel cell carcinoma of the skin). Co-factors may be involved. Immunization programmes focusing on hepatitis B and human papillomaviruses will reduce the incidence of liver and cervical cancer, respectively.

Pathology caused directly by microorganism

Direct effects may result from cell rupture, organ blockage or pressure effects

Organisms that multiply in cells and subsequently spread usually do so by rupturing the cell. Many viruses and some intracellular bacteria and protozoa behave in this way (Table 17.1). It is important to realize that many others do not. For example, viruses or bacteria may remain latent (e.g. herpes simplex virus and varicella-zoster virus in nerve ganglia, and Mycobacterium tuberculosis in macrophages), and many viruses can bud from a cell without disrupting it. The type of cell infected may also have an influence on survival of the organism. Thus although HIV causes lysis of CD4 T cells, macrophages are more resistant to lysis, perhaps because the virus activates the NFκB pathway, and so virus may persist within intracytoplasmic compartments. Other direct effects include:

• blockage of major hollow viscera by worms

• blockage of lung alveoli by dense growth of, e.g. Pneumocystis

• mechanical effects of large cysts (e.g. hydatid).

Table 17.1 Organisms that directly damage tissue

Organism	Cell or tissue damaged	Mechanism
Viruses
PoliovirusRhinovirusHIVCoxsackievirusRotavirus	NeuronesURT mucosaCD4 T cells, macrophagesPancreatic β cells, heart cellsEnterocytes
Bacteria
Streptococcus mutans	Teeth	Acid production
Mycobacteria	Macrophages	Damaged macrophage releases cytokines
Fungi
Histoplasma	Macrophages	Damaged macrophage releases cytokines
Protozoa
Plasmodium	Erythrocytes	Damaged erythrocyte removed
Helminths
Ascaris	Intestinal occlusion	Mechanical
	Biliary occlusion	Mechanical, inflammation
Echinococcus	Hydatid cyst	Pressure effects

Many organisms directly damage or destroy the tissues they infect. This is especially common with cytopathic viruses. URT, upper respiratory tract.

Exotoxins are a common cause of serious tissue damage, especially in bacterial infection

The parasite may actively secrete ‘exotoxins’ (Table 17.2). In some cases, these are clearly part of its strategy for entry, spread or defence against the host, but sometimes they seem to be of little or no benefit to the parasite.

Table 17.2 Important exotoxins in disease

Most exotoxins are proteins and are often coded not by the bacterial DNA, but in plasmids (e.g. E. coli) or phages (e.g. botulism, diphtheria, scarlet fever). In some cases, they consist of two or more subunits, one of which is required for binding and entry to the cell while the other switches on or inhibits some cellular function.

Powerful toxins are generally secreted from extracellular microbes. Microbes that multiply in cells cannot afford to cause serious damage at too early a stage, and such toxins therefore tend to be less prominent in intracellular infections due to Mycobacteria, Chlamydia or Mycoplasma. For example, leprosy patients with lepromatous disease can live with huge bacterial loads for many years. Although many toxins can kill host cells, lower concentrations may be important by causing dysfunction in immune or phagocytic cells. For example, concentrations of streptolysin well below the cell-killing level will inhibit leukocyte chemotaxis, and the staphylococcal enterotoxin and epidermolytic toxins also have immunomodulatory activity at exceedingly low (nanogram to picogram) levels.

Inactivation of toxins without altering antigenicity results in successful vaccines

Toxins can often be inactivated (e.g. by formaldehyde) without altering their antigenicity, and the resulting toxoids are among the most successful of all vaccines (see Ch. 34), the classic examples being diphtheria and tetanus toxoids. Toxins are generally more highly conserved in their structure than the surface antigens of the organism secreting them. This allows for more effective cross-immunity and explains, for example, why scarlet fever (caused by streptococcal erythrotoxin) usually occurs only once, while streptococcal infections recur almost indefinitely.

Mode of action of toxins and consequences

These can be considered under five headings (Fig. 17.2).

Figure 17.2 The mode of action of some exotoxins. Bacterial toxins act in a variety of ways. Often the toxin is a two-chain molecule, one chain being concerned with entry into cells while the other has inhibitory activity against some vital function. ACh, acetylcholine; cAMP, cyclic adenosine monophosphate; C, Corynebacterium; Cl, Clostridium; Staph, Staphylococcus; V, Vibrio.

Diphtheria toxin blocks protein synthesis

Diphtheria toxin is synthesized as a single polypeptide and binds by the B subunit to target cells (Fig. 17.2). The polypeptide is partially cleaved and then the entire toxin-receptor complex is internalized. The A subunit then splits off and passes into the cytosol, where it inactivates the transfer of amino acids from transfer RNA to the polypeptide chain during translation of mRNA by ribosomes. It does this by catalysing attachment of adenosine diphosphate (ADP) ribose to the elongation protein (ADP ribosylation), effectively blocking protein synthesis.

Cholera toxin results in massive loss of water from intestinal epithelial cells

Cholera toxin is released as a complex of five B subunits surrounding the A subunit. The latter is cleaved into two fragments: A1 and A2, held by disulfide bonds. The B subunits bind to ganglioside receptors on intestinal epithelial cells, leading to internalization of the A subunits, which then separate from one another (Fig. 17.2). The Al portion then ADP-ribosylates one of the regulatory molecules involved in the production of cyclic adenosine monophosphate (cAMP). As a result, the regulatory molecule is unable to turn off cAMP production. The increased levels of cAMP in the cell change the sodium/chloride flux across the cell membrane, resulting in a massive outflow of water and electrolytes from the cell and causing the profuse diarrhea of cholera. The exotoxins of E. coli and salmonella have similar actions, as does pertussis toxin.

Tetanus and botulinum toxins are among the most potent affecting nerve impulses

These toxins are extremely potent and active at low doses. Tetanus and botulinum toxins have the characteristic A + B structure, the B subunit binding to ganglioside receptors on nerve cells. The internalized A subunit of tetanus is carried by axonal transport from the point of production to the central nervous system (CNS), where it interferes with synaptic transmission in inhibitory neurones by blocking neurotransmitter release. This allows the excitatory transmitter to continuously stimulate the motor neurones, causing spastic paralysis. Botulinum toxin enters the body via the intestine, escaping digestion and crossing the gut wall. The toxin affects peripheral nerve endings at the neuromuscular junction, blocking presynaptic release of acetylcholine. This prevents muscle contraction, causing flaccid paralysis.

Toxins as magic bullets

An interesting offshoot of the two-subunit structure of toxins is that by changing the specificity of the part responsible for attachment, the specificity of the toxin for a particular cell type can be changed. An example is the plant toxin ricin – the A subunit can be attached to a monoclonal antibody to make it a specific poison for tumour cells. The same strategy could obviously be used against parasites if desired.

Diarrhea

Diarrhea is an almost invariable result of intestinal infections

Diarrhea is one of the major causes of death in children worldwide, with rotavirus as the main culprit (see Ch. 22). In industrialized regions, bacterial pathogens such as Campylobacter and non-typhoidal Salmonella are increasingly important, and Clostridium difficile can be a problem in hospitals, particularly in the elderly. Diarrhea can be considered as:

• a means for the host to rid itself rapidly of the infectious organism

• a means for the infection to spread to other hosts.

Diarrhea is a feature of a wide range of organisms, but in only a few cases is the exact mechanism understood. While toxins are often the cause (e.g. cholera, shigella), microbial invasion and damage to epithelial cells may also be important. The pathophysiology, with changes in electron transport or loss of enterocytes, has been elucidated in some cases. Many of the organisms causing diarrhea can be ‘picked up’ from food, but the term ‘food poisoning’ is usually reserved for those cases where toxins are already present in the food rather than being generated during the growth of organisms in the intestine (Fig. 17.3). As would be expected, ‘food poisoning’ causes symptoms earlier – that is, hours after exposure rather than days (Table 17.3). Some viral infections, such as Norovirus, sometimes referred to as causing ‘winter vomiting disease’, cause outbreaks of diarrhea and vomiting, particularly in closed groups or communities – such as in hospitals or on cruise ships; in the UK in 2009/2010 there were 1888 reported hospital outbreaks, of which 1538 led to ward closures.

Figure 17.3 Outbreak of bloody diarrhea caused by Verocytotoxin-producing E. coli 0157 in South Wales, in 2005. The first cases had all eaten school dinners containing cooked meats from a single supplier. Of the total 157 reported cases, 65% were in school-aged children. Thirty-one people were admitted to hospital and one child died. NPHS National Public Health Service

(Redrawn from: The Public Inquiry into the September 2005 Outbreak of E. coli O157 in South Wales. Chairman H. Pennington, March 2009. http://wales.gov.uk/ecolidocs/3008707/reporten.pdf?skip=1&lang=en)

Table 17.3 Infectious causes of diarrhea

	Onset	Source
Food poisoning (due to pre-formed toxin in food)
Staphylococcus aureus	1–6 h	Cream, meat, poultry
Clostridium perfringens	8–20 h	Reheated meat
Clostridium botulinum	12–36 h	Canned food
Bacillus cereus	1–20 h	Reheated foods
Intestinal infections
Rotavirus	2–5 days	Contact
Norovirus	1–2 days	Contact (faecal–oral)
Salmonella	1–2 days	Eggs
Clostridium difficile	1–2 days	Faecal–oral
Shigella	1–4 days	Faecal–oral
Campylobacter	1–4 days	Poultry, domestic animals
Vibrio cholerae	2 days	Faecal–oral
Escherichia coli	1–4 days	Food
Yersinia enterocolitica	days–weeks	Pets (e.g. dogs)
Giardia lambliaEntamoeba histolytica	1–2 weeksdays–weeks
CryptosporidiumIsospora belli

Worldwide, infectious diarrhea is the major cause of infant mortality.

Pathologic activation of natural immune mechanisms

Overactivity can damage host tissues

The very potent natural immune mechanisms discussed in Chapter 14 have inbuilt safety as far as specificity is concerned. They have had to evolve in the constant presence of the host’s ‘self’ antigens, to which they do not therefore respond. However, they are not so well controlled quantitatively, and there are many cases when overactivity not only damages an invading parasite, but also damages innocent host tissues. The expression of natural immunity often causes a certain amount of inflammation – and this can be severe, with tissue damage. Complement, polymorphs and tumour necrosis factor (TNF) play important roles.

Microbial endotoxin activates the immune system and induces cytokines, causing a bewildering variety of biologic effects (Fig. 17.4). At the clinical level, it can be responsible for septic shock.

Figure 17.4 The many activities of bacterial endotoxin. Lipopolysaccharide (LPS) activates almost every immune mechanism as well as the clotting pathway and, as a result, LPS is one of the most powerful immune stimuli known. DIC, disseminated intravascular coagulation; IFN, interferon; IL, interleukin; LBP, LPS binding protein; Mφ, macrophage; PMN, polymorphonuclear leukocyte; TNF, tumour necrosis factor.

Endotoxins are typically lipopolysaccharides

‘Endotoxins’ of bacteria and other microorganisms have a deceptively similar name to exotoxins, but are profoundly different in their significance. Unlike exotoxins, these are integral parts of the microbial cell wall and are normally released only when the cell dies. Endotoxins are particularly characteristic of Gram-negative bacteria. A typical lipopolysaccharide (LPS) endotoxin is composed of:

• a conserved lipid portion (lipid A) inserted into the cell wall, responsible for much of the toxic activity

• a conserved core polysaccharide

• the highly variable O-polysaccharide, responsible for the serologic diversity which is a feature of organisms such as salmonellae and shigellae.

LPSs stimulate an extraordinary range of host responses – or perhaps one should say a wide range of responses have evolved to respond to LPSs. These include LPS binding protein (the LPS–LPS binding protein complex then binds to CD14 on macrophages and dendritic cells) and TLR4 (see Ch. 9). In the words of Lewis Thomas, ‘when we sense lipopolysaccharide, we are likely to turn on every defence at our disposal’ (Fig. 17.4). Evidently, the body needs to be aware of invading Gram-negative bacteria at the earliest possible stage.

Clinically, the most important effects of LPS are:

• fever

• vascular collapse (or shock).

As mentioned in Chapter 14, fever may benefit host or parasite, or both, and is currently considered to be mainly due to the action of two cytokines, interleukin 1 (IL-1) and tumour necrosis factor (TNF), on the hypothalamus. Both these cytokines are produced by macrophages in response to LPS (and to analogous molecules from other organisms, see below and Box 17.1).

Box 17.1 Lessons in Microbiology