Infections

In the middle of the 19th century,it became clear thatmicro-organisms could cause disease. Effective treat-ment, however, was not possible at that time; preven-tion and spread of infectious diseases dependedsolely on proper hygienic means.At the beginning ofthe 20th century, passive and active vaccination pro-cedures were developed against a number of thesepathogenic micro-organisms in order to prevent thediseases in question (rabies,diphtheria, tetanus,etc.)and due to the discovery of antimicrobial chemicals(Ehrlich) and antibiotics (Fleming), the threat ofinfectious diseases seemed to be minimized. Large-scale vaccination programs against childhood dis-eases (diphtheria, whooping cough and polio) start-ed in the early fifties, giving hope to finally eradicatethese diseases from the planet. This approach wassuccessful for smallpox (1980); however, new infec-tious diseases emerged (e.g., Legionella, HIV, Heli-cobacter, SARS, etc.). New vaccines and antibioticsare needed. Furthermore, due to the intensive med-ical treatment with antibiotics and immunosuppres-sive drugs,hospital infections are a growing problem.Bacteria hitherto deemed harmless are causingopportunistic infections in immunocompromisedpatients.The pathogens develop multiple resistancesto antibiotics and sometimes no effective antibioticsare available to treat those patients.

To make the story evermore serious, man is sur-rounded and populated by a large number of differ-ent non-pathogenic micro-organisms. In the normal– healthy – situation, there is a balance between theoffensive capabilities of micro-organisms and thedefences of the human body. The body’s defencesare based on vital non-specific and specific immuno-logical defence mechanisms. An infection means

that the microorganism has succeeded in penetrat-ing those lines of defence,signaling a partial or com-plete breakdown of the body’s defence system.

Natural resistance

The body’s first line of defence comprises the intactcell layers of skin and mucous membrane, whichform a physical barrier. The skin’s low pH level andbactericidal fatty acids enhance the protection pro-vided by this physical barrier.The defence in the res-piratory tract and the gastrointestinal tract ismucous, the ‘ciliary elevator’ of the epithelium, andthe motility of the small intestine. The presence ofnormal microbial flora (colonization resistance) inthe intestine also plays a role in protection againstcolonization.

The most important humoral natural resistancefactors are complement, lysozyme, INTERFERON, and anumber of CYTOKINES. Lysozyme, which is found inalmost all body fluids, degrades sections of the cellwall of Gram-positive and – in combination withcomplement – Gram-negative bacteria. This causesthe otherwise sturdy cell wall to leak and the bacteri-um to burst.

Interferons are glycoproteins which may inhibitthe replication of viruses. Within several hours afterthe onset of a virus infection, INTERFERONs are pro-duced in the infected cell and help protect theneighbouring unaffected cells against infection.Thisprotection is brief, but high concentrations of INTER-FERONs are produced at a time when the primaryimmunological response is relatively ineffective.

CYTOKINES, such as IL-2 (INTERLEUKIN-2), GM-CSF(granulocyte-macrophage colony-stimulating fac-tor),and TNF-α (TUMOR NECROSIS FACTOR α), stimulatenon-specifically the proliferation, maturation, and

Immune response in human pathology: infections caused by bacteria, viruses, fungi,and parasites Jan Verhoef and Harm Snippe

A7

function of the cells involved in defence (seeChapter A.4).

Innate immune cells recognize microbes by TOLL-LIKE RECEPTORS (see section Pathogenesis of shock),giving rise to the above production of CYTOKINES inthe early phase of the response.

Micro-organisms that succeed in penetrating thefirst line of defence are ingested, killed, and degrad-ed by phagocytic cells (leukocytes, MONOCYTES,MACROPHAGES), which are attracted to a microbial

infection through chemotaxis. The ingestion byphagocytic cells of the micro-organism is enhancedby serum proteins (opsonins),such as ANTIBODIES andthe C3b component of complement, for which thesephagocytes have a receptor.After ingestion, the parti-cle is surrounded by the membrane of the phago-cyte, forming a vacuole known as a PHAGOSOME .ThePHAGOSOME then fuses with some of the countlesslysosomes in the phagocyte,thus allowing the lysoso-mal microbicidal agents and enzymes to do their

106 Immune response in human pathology: infections caused by bacteria, viruses, fungi, and parasites

BOX 1. CLASSIFICATION OF MICRO-ORGANISMS

Classification of bacteria byGenotypic characteristics: chromosomal DNA fragment analysis, nucleic acid sequence analysis, probesPhenotypic characteristics: morphology, biotyping, serotyping, antibiotic resistanceAnalytic characteristics: cell-wall analysis, lipid and protein analysis, enzyme typing (catalase)Gram staining positive or negativeAerobic, anaerobic: fermentation of different sugars

Naming and classification of viruses according toStructure: size, morphology (naked, enveloped), nucleic acid (RNA, DNA)Molecular aspects: mode of replication, assembly and budding Disease: encephalitis, hepatitisMeans of transmission: droplets, water, blood, insectsHost range: animal, plant, bacteria

Classification of fungi according toStructure: macroscopic morphology of hyphae (mycelium)

microscopic morphology of hyphae, conidophores and conidia (spores)shape and size

Cell features: nucleus, cytosol, plasmalemma (cell membrane which contains cholesterol),physiology, staining properties

Sexual characteristics: sexual and /or asexual reproduction, extended dikaryotic phase, basidium formation

Genotypic characteristics: chromosomal DNA fragment analysis, nucleic acid sequence analysis, probes

Diagnosing of parasites byMacroscopic examinationConcentration of cysts and eggs by microscopic examination Serological diagnosis: antibody responseDetection of parasite by serology and by nucleic acid hybridization: probes and amplification techniques

work.In the case of leukocytes,the formation of toxicoxygen radicals greatly contributes to the killing andelimination of the ingested micro-organism (Fig. 1)(see Chapter A.6).

A special role in cellular natural resistance isreserved for the NK (natural killer) cells, which dis-play considerable cytotoxic activity against virus-infected cells. This NK activity is stimulated by INTER-FERONs and,at a very early stage in the infection,servesto reinforce the non-specific defence mechanism.

Specific resistance

In the specific immune response, elements of thenatural defence mechanism are directed against aspecific enemy. Depending on the micro-organism,either the cellular defence mechanism (tuberculo-sis) or the humoral antibody-dependent defencemechanism (influenza) is of primary importance.In many cases, a joint cellular and humoral

107Specific resistance

BOX 2. SOME EXAMPLES OF IMPORTANT HUMAN PATHOGENS

Species Disease / location Treatment / prevention

Bacterium Streptococcus pneumoniae pneumonia / meningitis antibiotics / vaccinationMycobacterium tuberculosis lung tuberculosis antibioticsVibrio cholerae severe diarrhea antibiotics / liquid

suppletion / sanitationStaphylococcus aureus MRSA wound infection / antibiotics, MRSA not sensitive

hospital infection for standard antibiotics,difficult to treat

Neisseria meningitidis meningitis antibiotics / vaccinationBacillus anthracis systemic infection (sepsis) antibiotics as early as possibleCorynebacterium diphteriae throat / heart anti-serum, vaccinationCampylobacter jejuni intestinal infections hygiene, especially food (chicken)Helicobacter pylori gastritis, ulcer antibiotics

DNA Virus Poxviridae smallpox vaccination, eradicationHerpesviridae Herpes genitalis anti-viral agentsPapavoviridae warts and cervical carcinoma surgeryHepadnaviridae hepatitis B vaccination

RNA Virus Orthomyxoviridae influenza vaccinationCoronaviridae SARS unknownRetroviridae AIDS anti-viral agentsCaliciviridae gastro-intestinal infection sanitation, hygiene

Parasites Plasmodium species malaria prophylactic medication,anti-malarial drugs

Giardia species intestinal tract hygieneTrypanosoma cruzi sleeping sickness anti-parasitic agents

response is needed to provide an effective defence(typhus).

Both T LYMPHOCYTES and MACROPHAGES play a rolein cellular defence. During the first contact with anantigen, MACROPHAGES process the antigen and pres-ent its protein fragments (T-cell epitopes) to T cells,which then proliferate and remain present for yearsin the body as memory cells. When a secondencounter occurs, T cells produce lymphokines,which activate the MACROPHAGES. These activatedMACROPHAGES grow larger, produce more and betterdegrading enzymes, and are now able to eliminatemicro-organisms which otherwise would have sur-vived intracellularly (tuberculosis, typhoid fever).MACROPHAGES from non-immune animals are not ableto eliminate these micro-organisms.

Five different classes of ANTIBODIES can be distin-guished in man, namely, IgG, IgA, IgM, IgD, and IgE.They differ from one another in size, charge, aminoacid composition, and glycosylation (see ChapterA.3, C.2). In principle, the structure of the ANTIBODIES

is the same, i.e., 2 heavy and 2 light chains: it is thevariable part of these chains which recognizes themicro-organism.The biological function (see below)is determined by the constant part (Fc) of the heavy

chain.With the exception of IgD, all these ANTIBODIES

are important in antimicrobial activity.- IgA,which is found in all external secretions,reacts

with the surface of micro-organisms, preventingthem from adhering to sensitive cells and mucousmembranes.

- IgG neutralizes microbial toxins.- IgG, IgM, and C3b serve as opsonins, which pro-

mote PHAGOCYTOSIS.- IgG, IgM, and to a lesser extent IgA activate the

COMPLEMENT SYSTEM after binding to the micro-organism.Activation products C3a and C5a ensurethat the phagocytes are attracted to the inflamma-tory response.

- IgG and IgM, in combination with complementand lysozyme, have a lytic effect on bacteria andenveloped viruses.

- IgG and IgM inhibit the mobility of micro-organ-isms by attaching specifically to the flagellum.When this happens the chance of PHAGOCYTOSIS

increases and the chance of spreading of diseasedecreases.

- IgG, together with the killer or K cells, can elimi-nate infected host cells which carry viral or otherforeign ANTIGENS on their surface.

108 Immune response in human pathology: infections caused by bacteria, viruses, fungi, and parasites

O2-

MPOO2

H O2 2

enzymes

killinguptakechemotaxis

diapedesis

neutrophil

endothelial cells

granulesnucleus

bacterial cell FIGURE 1Schematic representation of the pro-gressive steps of phagocytic endo-cytosis.

- IgE is of importance in parasite infections. At thesite of the infection,mast cells,bearing specific IgE,release large quantities of vasoactive amines,which cause the contraction of smooth muscle tis-sue and increase the permeability of the blood ves-sels. In the intestine, this results in worms beingdetached and eliminated.

Defence against bacteria, viruses,fungi, and parasites

Several non-invasive bacteria, i.e., those that do notinvade the body, cause disease through the produc-tion of exotoxins (tetanus, diphtheria, cholera). Theimmune system neutralizes the toxin with the aid ofANTIBODIES (IgG, IgM). If the individual has not beeninoculated, the toxin will act on certain cells in thebody directly through a receptor. This bond is verystrong (i.e., has a high affinity), and is difficult tobreak by the administration of ANTIBODIES. In practice,if there are clinical symptoms of the disease, thenlarge doses of antitoxins must be administered.If oneis trying to prevent the development of the disease,then the presence of small quantities of specificANTIBODIES (IgG) is sufficient.

The adherence of bacteria to cells is effectivelyblocked by IgA. Oral vaccination against cholera, forexample, is aimed at obtaining sufficient specific IgAin the intestine, so that no colonization of this bac-terium can take place, and the cholera toxin can nolonger adhere to its receptor.

In general, defence against invasive bacteria isprovided by ANTIBODIES (IgG, IgM) that are directedagainst bacterial surface antigens. In many cases,these bacteria have a capsule which interferes witheffective PHAGOCYTOSIS. ANTIBODIES against these cap-sule ANTIGENS neutralize the interference, with subse-quent elimination of the bacteria by phagocytes.ANTIBODIES (IgM, IgG, IgA) in combined action withcomplement kill bacteria by producing holes in thecell wall of the bacterium.

Although intracellular bacteria (tuberculosis, lep-rosy, listeriosis, brucellosis, legionellosis, and salmo-nellosis) are ingested by MACROPHAGES, they are ableto survive and multiply. In these cases,cellular immu-

nity alone provides the defence, since ANTIBODIES arenot effective. Only activated MACROPHAGES are capa-ble of killing and degrading these bacteria.

ANTIBODIES neutralize viruses (Fig. 2) directlyand/or indirectly by destroying infected cells thatcarry the virus antigen on their surface.The mecha-nisms of this defence resemble those of humoraldefence against bacterial surfaces. The antibody-dependent cellular cytotoxicity reaction is specificfor the defence against viruses. Cells which carry onthe surface an antigen encoded by the virus areattacked by cytotoxic K cells, bearing ANTIBODIES

which fit the antigen on the target cell (K cells haveFc receptors for IgG).

Not only humoral, but also cellular immunityplays an important role in virus infections. Peoplewith a genetic T-cell deficiency are highly susceptibleto virus infections. In cellular defence, it is primarilythe virus-infected cells which are attacked and elim-inated. Cytotoxic T cells recognize MHC-1-presentedT-cell epitopes on the surface of virus-infected cellsand kill them.

The fungi responsible for human diseases can bedivided into two major groups on the basis of theirgrowth forms or on the type of infection they cause.Pathogens exist as branched filamentous forms or as

109Defence against bacteria, viruses, fungi, and parasites

glycoproteins

nuclear capsidsurroundingthe viral DNA

membrane

FIGURE 2Schematic illustration of an enveloped virus (Herpes sim-plex virus).

yeasts, although some show both growth forms. Thefilamentous types (Trichophyton) form a ‘mycelium’.In asexual reproduction, the fungus is dispersed bymeans of spores; the spores are a common cause ofinfection after inhalation. In yeast-like types (Crypto-coccus), the characteristic form is the single cell,which reproduces by division or budding.Dimorphictypes (Histoplasma) form a mycelium outside, butoccur as yeast cells inside the body. Candida showsthe reverse condition and forms a mycelium withinthe body.

In superficial mycoses, the fungus grows on thebody surface, for example skin, hair, and nails (Epi-dermophyton, Trichophyton), the disease is mild, andthe pathogen is spread by direct contact. In deepmycoses (Aspergillus, Candida, Cryptococcus, Histo-plasma), internal organs are involved and the dis-ease can be life-threatening and is often the result ofopportunistic growth in individuals with impairedimmunocompetence.

Many of the fungi that cause disease are free-liv-ing organisms and are acquired by inhalation or byentry through wounds. Some exist as part of the nor-mal body flora (Candida) and are innocuous unlessthe body’s defences are compromised in some way.The filamentous forms grow extracellularly, whileyeasts can survive and multiply within phagocyticcells. Neutrophils kill yeasts by means of both intra-and extracellular factors. Some yeasts (Cryptococcusneoformans) form a thick polysaccharide capsule inorder to prevent phagocytic uptake. In addition,many cell-wall components of yeasts cause suppres-sion of cell-mediated immune responses.The role ofhumoral and cellular immunity in controlling infec-tions caused by fungi is not yet well defined,but cel-lular immunity is the cornerstone of host defenceagainst (some) fungal infections.As a consequence,HIV infection, which affects the cellular arm of theimmune system, results in previously uncommoninfections such as those caused by C.neoformans.

The immunological defence systems against par-asites are considerably more complex than thoseagainst bacteria and viruses. This is due to variousfactors. In the first place, each parasite has its ownlife cycle, consisting of various stages with specificantigen compositions.Moreover,parasites are able toavoid the host defence system (mimicry), to combat

it (immunosuppression), or to mislead it (antigenicvariation). Both humoral and cellular immunity areimportant for the defence against parasites growingintercellularly, as we have seen in the case of bacte-ria and viruses. ANTIBODY concentrations (IgM, IgG,IgE) are often elevated. IgE also plays a special rolein the removal of parasites (especially worm infec-tions) from the intestine (see above).

Pathogenesis of shock

In Gram-negative (Fig. 3) bacterial infections, theinteraction between bacterial endotoxin and varioushost-cell systems has been implicated in the patho-genesis of septic shock. In particular, the release ofTNF-α (also called cachectin) and INTERLEUKIN-1 (IL-1), after the activation of host cells by endotoxin,induces hemodynamic shock.

Several lines of evidence support the currenthypothesis that the monocyte-macrophage is the prin-cipal cellular mediator of endotoxicity. First, C3H/HeJmice carrying a single gene defect are nonresponsiveto lipopolysaccharide (LPS). The transfer of MACRO-PHAGES of a closely related LPS-sensitive strain makesthe mice responsive. Second, when the host is chal-lenged with endotoxin, soluble factors are producedby MACROPHAGES that mediate fever and an acute-phase response. These factors include the proinflam-matory CYTOKINES, IL-1, IL-6, IL-8, and TNF-α. Together,TNF-α and IL-1 stimulate endothelial cells to produceand express proteins on their membrane that haveadhesive properties for leukocytes, promoting themigration and passage of POLYMORPHONUCLEAR LEUKO-CYTES (PMNs) from blood vessels through theendothelial layer, leading to PMN influx into the tis-sue. Adhesion molecules that mediate the binding ofPMNs appear on the endothelium after an inflamma-tory stimulus, followed by molecules that are specificfor adhesion of MONOCYTES or LYMPHOCYTES,which maybe why neutrophils enter before mononuclear cells.Molecules that are currently known to be involved inleukocyte-endothelium interactions belong to threestructural groups: the IMMUNOGLOBULIN gene super-family, the INTEGRIN family, and the selectin family.

Concomitant with cytokine release, LPS inducesthe activation of PMNs,MACROPHAGES,and many other

110 Immune response in human pathology: infections caused by bacteria, viruses, fungi, and parasites

cells,resulting in the release of toxic oxygen radicals,which lead to tissue damage.At the same time,mem-brane-associated phospholipases are activated andproducts of the arachidonic-acid cascade arereleased through the cyclooxygenase and/orlipooxygenase pathways (see Chapter A.6). Platelet-activating factor (PAF) is also generated, partly inresponse to the same signals.All these products con-tribute to a generalized inflammatory state withinflux of PMNs, capillary-leak syndrome, distur-bances in blood coagulation, and myocardial sup-pression.

Endotoxin and TNF-α also produce multipleabnormalities in coagulation and fibrinolysis, lead-ing to microvascular clotting and diffuse intravascu-lar coagulation.They also induce endothelial cells toproduce plasminogen activator and IL-6, which is animportant modulator of the production of acute-phase proteins by the liver. Interestingly, despite hav-ing important structural differences, TNF-α and IL-1have multiple overlapping and few distinct biologi-cal activities,act synergistically,and mimic the wholespectrum of toxicity caused by LPS (see ChapterA.4).IL-8 is an important chemoattractant and activa-tor of neutrophils and is crucial in the early stages ofinflammation.

Infusion of endotoxin in healthy humans leads toan early and transient increase in plasma levels ofTNF-α (detectable after 30 min,peaking after 90–120min, and undetectable after 4–6 h), which coincideswith the development of clinical symptoms andpathophysiological responses encountered in Gram-negative septicemia. TNF-α, IL-1, IL-6, and IL-8 levelsare also increased in patients with sepsis syndrome,with high levels of these CYTOKINES being correlatedwith severity of disease.

All these observations support the concept thatendotoxin largely acts by initiating an inflammatoryresponse through the activation of MONOCYTES-MACROPHAGES and the subsequent release ofCYTOKINES. It also activates the COMPLEMENT SYSTEM

(leading to the generation of C5a, which inducesaggregation of PMNs and pulmonary vasoconstric-tion) and factor XII of the intrinsic coagulation path-way (Hageman factor). Finally, it induces the releaseof ENDORPHINs, which are also involved in the com-plex interactions of the inflammatory response inendotoxic septic shock.

Gram-positive bacteria are frequently and in-creasingly cultured from blood obtained frompatients in shock. Unlike the pathophysiology ofshock caused by Gram-negative bacteria,not much is

111Pathogenesis of shock

Gram-positivebacterial cell wall

Gram-negativebacterial cell wall

lipopolysaccharidephospholipid

protein

phospholipidlipoteicoic acid

peptidoglycan

capsule

flagella

FIGURE 3Schematic illustration of the cellenvelope of a gram-negative and agram-positive bacterium.

known about the sequence of events that controlsthe signaling of MONOCYTES and MACROPHAGES thatleads to the release of CYTOKINES. Cell-wall compo-nents, such as peptidoglycan and teichoic acid, areclearly important in the activation of these cells.Exotoxins,however,may also play a role in the patho-genesis of Gram-positive bacterial shock.

CD14 is a cell surface glycoprotein that functionsas a binding receptor for LPS.However its membraneanchoring by a glycosylphosphatidyl inositol (GPI)linkage suggests little signaling and suggests the exis-tence of additional coreceptors. Recent studies indi-cate that innate immune cells recognize conservedpathogen-associated molecular patterns (PAMPs),including LPS, through TOLL-LIKE RECEPTORS (TLRs)(Fig. 4). This family of proteins, that resemble theantimicrobial Toll proteins of Drosophila, has beenidentified in humans and mice.TLR4 was identifiedas the missing link in LPS-induced cell signal trans-duction and responsiveness that is associated withMD-2 and CD14. The TLR family members are cou-pled to a signaling adapter protein (MyD88) andform differential dimers that may explain the dis-crete responses to TLR ligands such as lipoprotens,heat shock proteins, unmethylated CpG DNA, viraldsRNA and bacterial flagellin. Intracellular signalinginvolves several kinases depending on the TLRs

involved and includes the MAP kinase and NF-κBpathways leading to a cellular response. Recentlyother protein families have been identified via genet-ic screening that also participate in direct recogni-tion of pathogens. A new protein was found to beinvolved in resistance to Gram-positive bacterialinfections and recognizes the cell wall componentpeptidoglycan.

Human immunodeficiency virus infection

The human immunodeficiency virus (HIV) is a retro-virus that infects cells bearing the CD4 antigen, suchas T-helper cells (TH), MACROPHAGES, and DENDRITIC

CELLS.The CD4 molecule,together with other receptormolecules, like chemokine receptor 5,acts as a bind-ing site for the gp120 envelope glycoprotein of thevirus. In an attempt to respond to HIV ANTIGENS andconcomitant secondary microbial infections, thesecells are activated, thus inducing the replication ofHIV in the infected CD4 T cells, which are finallydestroyed. In contrast, HIV-1 infection of MACRO-PHAGES is self-sustained and results in an inexorablegrowth of chronic active inflammatory processes in

112 Immune response in human pathology: infections caused by bacteria, viruses, fungi, and parasites

MyD88(Adapter molecule)

IRAK TRAF6

MD-2CD14

TLR4

LPS

LBPTLR1,2,4

MAPK pathway

NF-κB pathway

MD-2

Cellactivation

TLRs

???

FIGURE 4Schematic illustration of cell activa-tion through toll-like receptors(TLRs).

many tissue compartments including the centralnervous system. Infected cells bear the fusion pro-tein gp41 and may therefore fuse with other infectedcells.This helps the virus to spread and accounts forthe multinucleated cells seen in lymph nodes andbrain. As a result of the decreased numbers of CD4-positive T-helper cells and defects in antigen presen-tation, depressed immune responses in thesepatients are observed. During the progression of thedisease, opportunistic infections by otherwise harm-less micro-organisms can occur. These include Can-dida albicans oesophagitis, mucocutaneous herpessimplex, toxoplasma in the central nervous system,and pneumonia caused by toxoplasma and Pneumo-cystis carinii; Kaposi’s sarcoma also occurs frequent-ly in these patients.This has been linked to the pres-ence of a previously unknown type of Herpes virus(HHV-8).This immune deficiency syndrome is called‘acquired immune deficiency syndrome’ (AIDS). Ithas been suggested that infected MONOCYTES/MACRO-PHAGES carry the HIV virus into the brain where itreplicates in microglia and infiltrating MACROPHAGES.As a consequence many AIDS patients develop cog-nitive and motor brain impairments. However, thepicture is complicated by the various persistentinfections already present in these patients, whichgive rise to their own pathology in the brain. These

include Toxoplasma gondii, Cryptococcus neofor-mans and JC virus.

So far a cure for HIV infection has not beenachieved. The main effort in the prevention of HIVinfection lies in mass public education programmes.Treatment of infected individuals is possible butexpensive. At this moment a triple therapy is beingprescribed in the Western countries (two reversetranscriptase inhibitors and one protease inhibitor,Fig. 5), each of which interfere with specific steps inthe process of HIV replication. One major problemthat has arisen is the increasing resistance to thesedrugs. Blocking of the chemokine receptor 5, arecently described co-receptor on CD4 cells for HIV,may be an alternative treatment for infected persons.This notion is supported by a recent finding that ahomozygous defect in this chemokine receptoraccounts for resistance of multiple-exposed individ-uals to HIV-1 infection.

Vaccines and vaccinations

Pasteur and Koch triggered the stormy developmentof vaccines (anthrax, rabies, cholera) at the end ofthe 19th century. While Pasteur remained faithful tothe principle of attenuated micro-organisms in

113Vaccines and vaccinations

0

20

40

60

80

100CD4 count (%)

1

10

100virus load

months - yearsweeks

starttherapy

HIV, single

CD4, triple

CD4, single

HIV, triple

FIGURE 5The effect of single and triple thera-py on viral load and CD4 cells overtime in HIV-infected individuals.

preparing his vaccines, Koch employed killed germs(cholera) as a vaccine. Since diphtheria and tetanuscause disease by means of toxins, the next logicalstep in the development of vaccines was the use ofdetoxified toxins to induce protection against thesediseases (diphtheria, Von Behring and tetanus,Kitasato).Von Behring and Kitasato were the first todemonstrate that the source of the protective activityinduced by vaccines was present in blood serum.Von Behring was also the first to prove that protectiveimmunity could be passed on via serum.The devel-opment of new vaccines had its ups (yellow fever)and downs (tuberculosis).With the arrival of antibi-otics, all work on new bacterial vaccines was sus-pended or severely curtailed, although someresearchers continued to work on viral vaccines,such as rubella, measles, polio, and mumps.

Since it has proved difficult to consistentlydevelop new antibiotics to combat antibiotic-resist-ant bacteria, interest in vaccines has graduallyincreased over the last 15 years (see Chapter C.1).Today, thanks to new insights into the immune sys-tem and modern molecular biological and chemi-cal techniques of analysis and synthesis, it is possi-ble to produce well-defined vaccines.These containonly those determinants of the pathogenic micro-organism which induce protection (epitopes).These epitopes are usually short peptide oroligosaccharide chains, which can be producedsynthetically or by means of recombinant DNAtechniques. The immunogenicity of these productscan be enhanced by coupling them to a carrier(tetanus toxoid, liposomes) and/or by adding anadjuvant (a substance which strengthens theimmune response non-specifically). The recombi-nant DNA technique can also be used to obtainattenuated strains of micro-organisms, which arefully immunogenic and thus provide protection,butwhich are no longer virulent.One example of this isthe development of a new cholera vaccine basedon a bacterium which has all the characteristics ofa virulent strain,except the toxin.The bacterium hasretained all its adherence factors, which allow it toadhere to the intestinal mucosa; the length of timeit spends in the intestine is sufficient to stimulatethe local immune system.The newest trend in vacci-nology is immunization by introducing plasmid

DNA into the host.Success has been attained by thismethod for hepatitis B vaccination.

Not only are new vaccines being developed, butit is also possible to heighten natural resistance forlonger or shorter periods.Various INTERLEUKINS (IL-2,GM-CSF) and INTERFERONs are being studied in orderto use them to combat infectious diseases.Monoclonal ANTIBODIES (ANTIBODIES with one speci-ficity) directed against the endotoxin of Gram-nega-tive bacteria are now being administered to patientswith severe Gram-negative sepsis (serum therapy).More work is still necessary, however, to refine thistechnique, as the therapeutic effect is still limited.

Infections in the new millenium

As outlined above for a number of bacteria andviruses, effective vaccines have been developed andapplied worldwide.The eradication of smallpox (Var-iola major) virus in the seventies of the last centurywas a milestone for the World Health Organization.The next goal of the WHO is to eradicate thepoliovirus in the coming years.Major problems to bedealt with are the distribution of these vaccines, thecosts involved, the registration and the complianceof the vaccinees and molecular techniques to tracethe final bug. Meanwhile new unexpected microbio-logical threats come into focus. Hospital infectionscaused by multiple resistant micro-organisms due tothe abundant use of antibiotics and exchange ofgenetic material between micro-organisms imposemajor problems on patients and healthcare workers.New antibiotics and/or vaccines should be devel-oped and new strategies employed to contain theseinfections.Due to crowding and high mobility of theworld population, old and new pathogens, e.g.,influenza and SARS, threaten our society. On top ofthis terrorists might intentionally use micro-organ-isms (Smallpox, Anthrax, Plague etc.), or bacterialtoxins (Botulism) to cause death and disease inhumans or animals in a civilian setting.The recogni-tion that an event was caused by a biologicalweapon presents a severe challenge to be preparedfor such an attack, especially for medical careproviders and public health officials. Strategies tocombat bioterrorism have to be worked out but with

114 Immune response in human pathology: infections caused by bacteria, viruses, fungi, and parasites

the experience of 100 years of combating micro-organisms with hygiene measures, vaccination,antibiotic and anti-viral treatment, there must be away out.

Summary

Despite the introduction of effective health measure-ments, vaccination and antimicrobial therapy infec-tious diseases continue to threaten human life. Thereasons are numerous and diverse: antibiotic resist-ance, hospital-invading pathogens, new emerginginfectious diseases, bioterrorism, biological warfare.This chapter is an introduction to several aspects ofinfectious diseases viewed from the host as well asfrom the pathogen (bacterium, virus and parasite).Furthermore the basic principles of INNATE and ADAP-TIVE IMMUNE RESPONSES, especially in debilitatedpatients, are described. Detailed information is givenon the pathogenesis of septic shock,AIDS and vacci-nation strategies.

Acknowledgments

We thank Dr. C.P.M. van Kessel of The Eijkman-Wink-ler Center for the design and layout of the artwork.

Selected readings

1 Jawetz E, Melnick JL,Adelberg EA, Brooks GF, Butel JS,Ornston LN (eds.) (2001) Medical microbiology. Pren-tice-Hall International, London

2 Mims CA, Playfair JHL, Roitt IM,Wakelin D,Williams R,Anderson RM (eds.) (1998) Medical microbiology.Mosby, London

3 Roitt I, Brostoff J, Male D (eds.) (2001) Immunology.Mosby, London

4 Silverstein AM (ed.) (1988) A history of immunology.Academic Press, San Diego

5 Snippe H,Willers JMN (1992) Attack and Defence. In:JJ van Everdingen (ed.): The beast in man: microbesand macrobes as intimate enemies.Part II.The battle ofbugs. Belvedere, Overveen,The Netherlands

6 Janeway CA,Travers P,Walport M, Shlomchik M (eds.)

(2001) Immunobiology, The immune system in healthand disease. Garland Science Publishing, New York

7 Prescott L, Harley J, Klein D (eds.) (2003) Microbiolo-gy. Mc Graw Hill, New York

Recommended websites

Scientific Research:American Society for Microbiologists: http://www.asm.orgAmerican Association of Immunology:http://www.aai.orgEuropean Federation of Immunological Societies: http://

www.efis.orgFederation of European Microbiological Societies: http://

www.fems-microbiology.orgNational Library of Medicine: http://www.ncbi.nlm.nih.gov

Outbreaks of Infectious Diseases:Centers for Disease Control and Prevention: http://www.

cdc.govInternational Society for Infectious Diseases: http://www.

isid.orgDaily update: http://www.promedmail.orgWorld Health Organization: http://www.who.int(All accessed March 2005)

115Summary