pathogenesis, treatment, and prevention of pneumococcal pneumonia

8/13/2019 pathogenesis, treatment, and prevention of pneumococcal pneumonia

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www.thelancet.com Vol 374 October 31, 2009 1543

Pathogenesis, treatment, and prevention of pneumococcal

pneumoniaTom van der Poll, Steven M Opal

Pneumococcus remains the most common cause of community-acquired pneumonia worldwide.Streptococcus pneumoniaeis well adapted to people, and is a frequent inhabitant of the upper airways in healthy hosts.This seemingly innocuous state of colonisation is a dynamic and competitive process in which the pathogen attemptsto engage the host, proliferate, and invade the lower airways. The host in turn continuously deploys an array of innateand acquired cellular and humoral defences to prevent pneumococci from breaching tissue barriers. Discoveries intoessential molecular mechanisms used by pneumococci to evade host-sensing systems that are designed to contain thepathogen provide new insights into potential treatment options. Versatility of the genome of pneumococci and thebacterias polygenic virulence capabilities show that a multifaceted approach with many vaccine antigens, antibioticcombinations, and immunoadjuvant therapies will be needed to control this microbe.

IntroductionNearly a century ago, Sir William Osler proclaimedStreptococcus pneumoniae (or pneumococcus) as thecaptain of all the men of death.1This statement remainsas true today as it was then. Severe community-acquiredpneumonia is the most common cause of death frominfection in developed countries, and the pneumococcusis the most frequent cause of lower respiratory tractinfection. We can now control many other respiratorypathogens of man reasonably well (eg, influenza,pertussis, tuberculosis, and Haemophilus influenzae), yetpneumococcus remains the main cause of community-acquired pneumonia worldwide.1

Reasons for the success of this obligate human

pathogen are becoming increasingly apparent as thebasic pathogenic mechanisms of pneumococcalpneumonia are discovered. This pathogen causes at least12 million infant deaths every year worldwide.2,3 Theyearly death toll attributable to pneumococci in patientswith AIDS and other immunocompromised states,elderly people, and those with comorbid illnesses isdiffi cult to quantify, but probably exceeds the infantmortality rate.

Advances in comparative genomics and insights intoinnate and acquired immune-signalling mechanismscould provide new treatment options for pneumococcaldisease.28 We focus on the clinical outcomes after theinitial hostpathogen interaction when S pneumoniae

first reaches the airways and invades the lowerrespiratory tract.

EpidemiologyS pneumoniae is a common inhabitant of the upperrespiratory tract, existing mainly as a commensalbacterium along with other co-resident microorganismsidentified on the respiratory epithelium. Aftercolonisation by one of 91 presently recognised serotypes,a new strain eliminates other competing pneumococcalserotypes, and persists for weeks (in adults) or months(in children), usually without any adverse sequelae. Thiscarrier state maintains the organism within human

populations, and induces some acquired B-cell-mediatedimmunity to reinfection.3,4Genetic and epidemiological evidence show that one of

two survival strategies account for the success ofS pneumoniae. Specific clones are selected with either aninvasive pneumococcal diseasephenotype or a persistentcolonisation phenotype with low risk of tissue invasion.Success of the phenotype of invasive pneumococcaldisease depends on its capacity for rapid disease inductionand effi cient person-to-person spread by coughing. Bycontrast, the non-invasive phenotype uses various surfaceadhesins, immune evasion strategies, and secretorydefences such as IgA1 protease and inhibitors ofantibacterial peptides to help with long-term carriage

within the nasopharynx.5

Persistent carriage ofpneumococci allows for low-level and longlastingtransmission, thereby retaining non-invasive strains inhuman populations. Defects in host defences can alterthis hostpathogen interaction and allow strains of lowvirulence to invade the immunocompromised host.57

The pneumococcus is mainly transmitted by directcontact with contaminated respiratory secretions betweenhousehold members, infants, and children. Pneumococciare generally not regarded as highly contagious, andrespiratory isolation of patients who are infected in thecommunity or hospital settings is rarely indicated.However, a large, community-wide outbreak of serotype 5pneumococci in Vancouver, Canada,9 showed that

potential exists for epidemic spread of S pneumoniae

Lancet2009; 374: 154356

See Commentpage 1485

Centre for Infection and

Immunity Amsterdam, Centre

for Experimental and Molecular

Medicine, Academic Medical

Centre, University of

Amsterdam, Amsterdam,

the Netherlands

(Prof T van der Poll MD); and

Infectious Disease Division,

Warren Alpert Medical School,

Brown University, Providence,RI, USA(Prof S M Opal MD)

Correspondence to:

Prof Tom van der Poll, Academic

Medical Centre, University of

Amsterdam, Meibergdreef 9,

1105 AZ Amsterdam,

the Netherlands

[email protected]

Search strategy and selection criteria

We searched Medline and PubMed in English with the search

terms pneumococci, Streptococcus pneumoniae,

pneumococcal pneumonia, community acquired

pneumonia, and pneumococcal pathogenesis for reports

relating to pneumococcal pneumonia published in the past

10 years until January, 2009. We reviewed the publications

and searched the reference lists of identified articles for older

reports we judged to be of major importance.


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within urban neighbourhoods. Similar small-scaleoutbreaks have been reported in day-care centres, jails,military bases, and mens shelters.1012Airway colonisationby pneumococci is readily detectable in about 10% ofhealthy adults. 2040% of healthy children are carriers,and more than 60% of infants and children in day-care

settings can be carriers.3,8

Most people colonised with S pneumoniae have onlyone serotype at a time, although simultaneous carriageof more than one serotype is possible.13 Duration ofpneumococcal colonisation in an individual is highlyserotype-specific. 14 Recent acquisition of an invasiveserotype is more important as a determinant ofsubsequent risk of invasive pneumococcal disease thanis duration of colonisation or specific clonal genotype. 15Results of studies of the conjugate seven-valentpneumococcal vaccine in children suggest that thehumoral immune response can reduce risk ofserotype-specific disease, decrease likelihood oftransmission in vaccine recipients, and even reduce

transmission risk between non-immunised siblings andadults (ie, herd immunity).3,16

Incidence of invasive pneumococcal disease(ie, bacterial pneumonia and bacteraemia) varies sub-stantially by age, genetic background, socioeconomicstatus, immune status, and geographical location. In alarge US epidemiological survey in 1999, overallincidence in children older than 5 years was 987 per100 000 per year. By 2005, the same survey showed thatincidence had reduced by 75% to 234 per 100 000 peryear. This change was largely attributable to reducedrates of infection with serotypes contained in thepneumococcal conjugate vaccine17 (figure 1). This

decreased overall incidence was accompanied by a smallrise in invasive pneumococcal disease from non-vaccineserotypes.18This pattern seems to be continuing and hasprompted a renewed interest in other vaccineformulations. Incidence rates can be as high as 441 casesper 100 000 per year in isolated Indigenous populationssuch as Alaskan Eskimos, Aboriginal Australians, andthe Maoris of New Zealand.19

Other recognised risk factors for invasive pneumo-coccal disease in children and adults include asplenia,alcoholism, diabetes mellitus, age greater than 65 years,underlying lung disease, severe liver disease, influenzaand probably other respiratory viruses, immunoglobulinand complement deficiencies, and other immuno-compromised states, including HIV infection. Crowding,recent acquisition of a virulent strain, poverty, cigarette

smoking, proton pump inhibitors,20

and other riskfactors probably contribute to pneumococcal disease(panel).3,14 Antecedent respiratory infections and recentexposure to antibiotics seem to be additional risk factorsfor colonisation of airways and invasive pneumococcaldisease. Antibiotic-induced alterations in competinginhibitory bacteria (-haemolytic streptococci inparticular), release of viral enzymes (eg, influenzaneuraminidase) that promote adherence,3 and hostinflammation-induced expression of pneumococcalinvasion receptors on host cells, such as platelet-activatingfactor receptor21and CD14,22contribute to pneumococcalcolonisation and invasion.

Genetics and virulence of S pneumoniaeKnowledge of genomes of many invasive andnon-invasive strains of S pneumoniae enable detailedcomparative analyses.2329 The pneumococcal genomehas between 2 million and 21 million basepairs,dependent on strain virulence (figure 2). It is a covalentlyclosed, circular DNA structure often accompanied bysmall cryptic plasmids. The guaninecytosine content ofS pneumoniaeDNA (397%) is lower than that of manyother bacterial pathogens. One of the originalpneumococcal genomes to be sequenced, strain TIGR4,23has 2236 open reading frames of which two-thirds haveassigned roles for their predicted gene products. Another16% of open reading frames generate conserved,

hypothetical proteins of unknown function. About20% exist only in S pneumoniae.23

The genome contains a core set of 1553 genes that areessential for viability.2429An additional 154 genes form thecomplement of bacterial genes that collectively contributeto virulence (the virulome), and 176 genes activelymaintain a non-invasive phenotype. Pneumococci havean unusually large number of insertion sequences,accounting for up to 5% of the entire genome. Somestrains contain conjugative transposons within theirgenomes that mediate antibiotic resistance. Muchplasticity exists within the S pneumoniaegenome, with upto 10% variation between strains. The genome is replete

19980

50

100

150

200

250 PCV-7 vaccineintroduced

Age


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with many copies of direct-repeat DNA elements thatprovide recombination hotspots for genetic variability.23

Pneumococci express the most diverse array ofsubstrate transport, and use systems known to humanpathogens.23,24Specific ATP-binding cassette transportersimport carbon or aminoacid substrates and export outersurface adhesins, degradation enzymes, or capsularsynthetic components. These transporters are also

essential for genetic competence (ability to take uphomologous strands of naked DNA), for nutrientacquisition, and as effl ux pumps to resist antibiotics.23Pneumococci have much genetic space for capsularpolysaccharide synthesisthe most important virulencefactor for this organism.5 Phase variation arises withmolecular switches at promoter regions of open readingframes for major surface antigens.23 Pneumococciregulate the amount of capsular material producedduring colonisation and invasion. Transparent (thin)capsules are favoured in early colonisation, whereasopaque (thick) capsules are favoured during invasion toresist complement-mediated opsonophagocytosis.3,5

Pneumococci cluster virulence genes into smalldefined sequences within the genome known as regionsof diversity. These regions distinguish invasive fromnon-invasive strains. At least 13 such regions are identi-

fied in pneumococci, many of which carry the geneticsignatures of pathogenicity islands.25,28 Non-virulentstrains exist even within the same capsular serotypethat contain some but not all elements of the entirepneumococcal virulome.5,2931 Pneumococci have anarray of two-component sensor-kinase signal systemsthat recognise environmental cues (eg, cell density,substrate availability, and microbial competitors) andalter their genetic programmes in response. Thesesystems direct the synthesis of bactericins 32 (bacterialpeptides that kill other related bacteria), competencegenes for genetic recombination, biofilm formation,33and virulence expression.5,33 Pneumococcal virulencehas been acquired by horizontal transmission from

other pathogens during the evolutionary past.34

Evidence of continuing evolution through the acquisi-tion of new antibiotic resistance genes and virulencetraits is shown in pneumococci and other gram-positivebacterial pathogens, such as group A streptococci,35Staphylococcus aureus,36and Clostridium diffi cile.37

Pneumococcal virulence factorsTable 1 shows a summary of virulence traits identified inpneumococci.3849An array of virulence factors needs tobe expressed in a coordinated way for tissue invasion tobe successful (phenotype for invasive pneumococcaldisease). The most important virulence determinants

Panel: Risk factors for pneumococcal pneumonia and

invasive pneumococcal disease

Definite risk factors* (high risk)

Younger than 2 years or older than 65 years

Asplenia or hyposplenia

Alcoholism

Diabetes mellitus

Antecedent influenza

Defects in humoral immunity (complement or

immunoglobulin)

HIV infection

Recent acquisition of a new virulent strain

Probable risk factors (moderate risk)

Genetic polymorphisms (eg, complement, MBL, IRAK-4,

Mal, MyD88)

Isolated populations

Poverty, crowding, low pneumococcal vaccine use

Cigarette smoking

Chronic lung disease

Severe liver disease

Other antecedent viral infections

Poor mucociliary function

Possible risk factors (low risk)

Recent exposure to antibiotics

Defects in cellular immunity and neutrophil defects

Diminished cough reflex, aspiration pneumonitis

Proton-pump inhibitors and other gastric-acid inhibitors

Large organism burden in upper airways

Childhood day care

MBL=mannose binding lectin. IRAK=interleukin I receptor associated kinase.

Mal=Myeloid differentiation primary response factor 88 adaptor like.*Many clinical

studies. Some clinical and laboratory studies. Few clinical studies.

RDs

700000

800000

9000001000000

11000001200000

1300000

1400000

1500000

1600000

1700000

1800000

19000002

000

000

2100000 1

600000

500000

400000

300000

200000100000

Point of origin ofreplication

GCskew

Distinct tovirulent strains

IS and

repetitiveelements

Figure :Genomic structure of Streptococcus pneumoniaeTIGR4

RDs=regions of diversity. GC skew=difference in guanine (G)cytosine (C) ratio from GC average of pneumococcal

genome. IS=insertion sequence elements. Adapted from Tettelin, et al23with permission from the AAAS.


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include the antiphagocytic and adherence properties ofcapsular polysaccharides, adherence factors, invasiongenes, iron and other heavy-metal transporters, oxidativestress protection, host-defence evasion, pneumolysinproduction, bacteriocin production, and quorum sensingand biofilm formation (figure 3).5,3849

Virulence expression varies with tissue site andpopulation density. Sessile pneumococci residing insidebiofilms are more able to induce meningitis and

pneumonia but have less capacity to disseminate in bloodthan do those unattached and not inside biofilms. Bycontrast, free-living, unattached (or planktonic) bacteriaare more likely to induce bacteraemic infection, but areless successful in causing pneumonia.33Many virulencefactors probably contribute to invasive pneumococcaldisease in people, but unequivocal evidence exists onlyfor pneumococcal capsulesthe target of present vaccineformulations.

Pneumococcal capsular antigen is the most importantvirulence determinant for pneumococci in experimentaland clinical studies.38 The capsule is crucial duringcolonisation, invasion, and dissemination from the

respiratory tract. It prevents mechanical clearance bymucous secretion38 and helps with transit of theorganism to the epithelial surface. Capsularpolysaccharide is highly negatively charged and stericallyinhibits the interaction between phagocytic CR3receptors to iC3b, and between Fc receptors to the Fccomponent of IgG fixed to pneumococci.38,39 Pneumo-coccal capsules also restrict autolysis and reduceexposure to several antibiotics.3,5

Pneumococcal exotoxin-pneumolysin is expressed byalmost all invasive strains of S pneumoniae. Thispore-forming cytotoxin is released during autolysis as asoluble monomer that oligomerises on host membranes.Pneumolysin is lytic to host cells if suffi cient amounts ofit are generated. The toxin has many other pathologicaleffects, including its ability to inhibit ciliary action ofepithelial cells, activate CD4+ T cells, impair respiratoryburst of phagocytic cells, induce production ofchemokines and cytokines, stimulate complementfixation, and activate inflammation.7,40 Pneumolysin-negative mutants of S pneumoniaeare much less likely toproduce lethal pulmonary infections than are wild-type

Mechanism of action Level ofevidence*

Polysaccharide capsule5,6,38,39 Prevents mucosal clearance, antiphagocytic, sterically inhibits complement andimmunoglobulin-binding to host receptors

4+

Pneumolysin7,40 Cytolytic, TLR4 ligand, induces ciliostasis, impairs respiratory burst, activates complement,cytokine, chemokine production

3+

Pneumococcal surface protein A41 Blocks C3b binding to factor B, binds to epithelial membranes 2+

Pneumococcal surface protein C; also known ascholine binding protein A42,43

Binds factor H and blocks C3b fixation, binds to human polymeric immunoglobulinreceptor during invasion

2+

Cell wall polysaccharide5 Activates complement, pro-inflammatory 1+

Pneumococcal surface antigen A5,22,25 Mediates metal ion uptake (zinc and manganese), protects against oxidant stress, bindsto GlcNac-Gal

12+

Lipoteichoic acid44 Binds to PAFR, TLR2 ligand, proinflammatory 2+

Autolysin5 Releases peptidoglycan, teichoic acid, pneumolysin other intracellular contents 2+

Hyaluronate lyase45 Degrades hyaluronan in the extracellular matrix 1+

Enolase5 Binds to fibronectin in host tissues 1+

Sortase A46 Links surface proteins to cell wall 1+

Pili47 Pilion cell surface inhibits phagocytosis, promotes invasion 1+

Pneumococcal adhesion and virulence48 Binds to plasminogen within host tissues 1+

Pneumococcal iron acquisition A and iron uptake A5 ABC transporters that acquire iron for bacterial growth and virulence 1+

Bacteriocin5 Inhibits bacterial competition, might have other cytotoxic actions 2+

Neuraminidase45 Contributes to adherence, removessialic acids on host glycopeptides and mucin to expose binding sites

1+

Biofilm and competence33 Biofilm organisms upregulate CSP; competence genes contribute to pneumonia andmeningitis

2+

IgA protease5 Degrades human IgA1 12+

Phosphorylcholine44 Binds to PAFR on human epithelial cells 2+

Pneumococcal serine-rich repeat protein25 Surface adhesion, binds to platelet surface and promotes tissue invasion 2+

Pneumococcal choline binding protein A49 Low manganese-induced protein, unknown function but important in lungs and blood 12+

1+=one or few animal studies. 2+=some evidence from s everal animal studies. 3+=confirmed in several animal studies. 4+=confirmed in people. TLR=toll-like receptor.

C3b=complement component 3b. GlcNac-Gal=N-acetylglucosamine-beta-(1-4)-galactose disaccharide. ABC=ATP-binding cassette. CSP=competence-stimulating peptide.

PAFR=platelet-activating factor receptor.

Table :Known virulence factors in Streptococcus pneumoniaeinfection


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pneumococci.5,40 Figures 3 and 4 show the role of thistoxin and other virulence factors during invasion.

Mechanisms of host recognitionPattern-recognition receptors are key components of theinnate immune system.50,51 They recognise conservedmotifs expressed by pathogens that are referred to aspathogen-associated molecular patterns. Several of thesereceptors contribute to initiation of an effective, innateimmune response to the pneumococcus (figure 4).C-reactive protein, an acute-phase protein, functions as apattern-recognition receptor for S pneumoniae. It bindsphosphorylcholine in the pneumococcal cell wall andactivates complement.52 In animal models,53 humanC-reactive protein protects against lethal infection withS pneumoniaeinfection, and in man probably contributes

to host defence during bacteraemic pneumonia.S pneumoniae uses the host-derived receptor forplatelet-activating factor to cross from lung tissue intoblood. This receptor recognises the phosphorylcholinedeterminant of its natural human ligandplatelet-activating factor. Pneumococci express phosphorylcholinein their cell wall that binds to this receptor, initiatingbacterial uptake.54 In mice deficient in this receptor,pneumococcal growth is impaired, bloodstream invasionis reduced, and survival is improved.21,55 The dendriticcell-specific intercellular adhesion molecule (ICAM-3-grabbing nonintegrin [DC-SIGN] homologueSIGN-related 1 [SIGNR1]) is a C-type lectin implicated incapture of capsular polysaccharides from S pneumoniae

by marginal-zone macrophages.56

Although SIGNR1 isnot usually expressed by alveolar macrophages,SIGNR1-/- mice are highly susceptible to pneumococcalinfection and do not clear S pneumoniaefrom their lungsor blood.57 SIGNR1 probably restricts bacterial growthduring pneumococcal pneumonia with presentation ofpneumococcal antigens to B cells by SIGNR1 onmarginal-zone splenic macrophages. Antiphosphoryl-choline IgM antibodies resulting from this presentationassist in clearance of pneumococci from the lung.57,58

Macrophage receptor with collagenous structure(MARCO) is a class A scavenger receptor expressed onalveolar macrophages that is able to bind and internaliseS pneumoniae in vitro. MARCO-/- mice have a greatly

reduced resistance against pneumococcal pneumonia,with accelerated growth of pneumococci and increasedmortality.59Toll-like receptors (TLRs) have a central role aspattern-recognition receptors in initiation of cellular innateimmune responses because they can detect many microbialpathogens at either the cell surface or in lysosomes andendosomes.50,51TLR2 is generally thought to be the mostimportant pattern-recognition receptor for gram-positivepathogens (ie, peptidoglycan, lipoteichoic acid, andbacterial lipopeptides), but its importance for pneumococciis not fully understood. S pneumoniaecell-wall componentsare recognised by TLR2,6064but TLR4 is the receptor for theproinflammatory effects of pneumolysin.6567Unexpectedly,

when many different types of TLR-deficient mice werestudied to investigate which of these receptors was mostessential to detect and defend against pneumococci, onlyTLR9-/- mice were highly susceptible to lethal infection.TLR9 detects bacterial DNA and seems to be essentialfor effective phagocytosis and killing of pneumococci bylung macrophages.68

Intracellular signalling elements of the TLR system areessential for defence against pneumococcal pneumoniain experimental systems. Mice deficient in the commonTLR-adaptor protein myeloid-differentiation primary-response protein 88 (MyD88) are very susceptible topneumococcal pneumonia, probably in part as a result ofimpaired innate immune activation by interleukin 1 andinterleukin 18.6971The clinical relevance of these findingsaccords with the discovery of children with a geneticdeficiency for MyD88 or interleukin 1 receptor-associatedkinase 4 (IRAK4), a kinase acting directly downstreamfrom MyD88, who are especially susceptible to invasivepneumococcal disease.72,73Additionally, a single nucleotide

IgA1

protease

PavA

Eno

NanA

PsrP

Pneumocin

Extracellular toxins and enzymes

Pneumolysin

Degradesextracellularmatrix

PAF receptorbinding,inflammation

Anticomplement,invasion factors

LTA

PspC

PspA

PsaA

Pili

Metal binding transporters

PiaA/PiuA Intracellular

space

Periplasmicspace

Peptidoglycancell wall

Polysaccharide

capsule

Surfaceadhesins

Cps

Hyl LytA

StrA

Figure :Virulence factors of pneumococcus

PsaA=pneumococcal surface antigen A. PiaA/PiuA=pneumococcal iron acquisition and uptake.

PsrP=pneumococcal serine-rich repeat protein. NanA=neuraminidase. Eno=enolase. PavA=pneumococcal

adhesion and virulence. LytA=autolysin. StrA=sortase A. Hyl=hyluronate lyase. LTA=lipoteichoic acid.

PspC=pneumococcal surface protein C. PspA=pneumococcal surface protein A. Cps=polysaccharide capsule.

PAF=platelet-activating factor.


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polymorphism in the TLR-adaptor-protein Mal affectshost defence against S pneumoniae in people.74CD14 isanother crucially important pattern-recognition receptor.22CD14-/- mice are strongly protected against disseminationof S pneumoniaefrom the respiratory tract,22suggestingthat pneumococci specifically use CD14 in thebronchoalveolar compartment to spread.

Pneumococci are slowly killed inside phagolysosomes,especially if poorly opsonised,75and this process mightprovide an opportunity for viable organisms or theirbacterial products to escape from early phagosomes andinvade the cytosol. Within the intracellular space,pneumococci can be recognised by various cytoplasmic

pattern-recognition receptors. Nucleotide-binding oligo-merisation domain protein (NOD2) recognises muramyldipeptide, a common fragment of bacterial peptido-glycan, in the cytosol.50,76Additionally, bacterial infectionleads to activation of caspase-1 by a protein complexcalled inflammasome.77Inflammasome regulates activityof caspase-1, an enzyme responsible for secretion ofthree major host-defence cytokinesinterleukin 1,interleukin 18, and interleukin 33. However, caspase-1-/-mice seem to tolerate pneumococcal pneumonia.68Thus, the clinical implications of NOD2 andinflammasomes in pneumococcal infections in peopleremain unclear.

ImmunologyRespiratory epithelial cells not only provide the mucociliarycarpet to continually remove potential pathogens from thelower airways, but also actively respond to the presence ofpathogens. Respiratory epithelium releases variousmediators such as cytokines, chemokines, and anti-microbial peptides (eg, lysozyme, defensins, andcathelicidins), contributing to innate immunity againstpneumococci.78Transgenic mice that overexpress nuclearfactor inhibitor B- (IB-) and block nuclear factor B(NFB) nuclear translocation within epithelial cells have areduced ability to clear pneumococci from the airways.79Alveolar lining cells produce a pneumococcal-bindingprotein known as surfactant protein-D (SP-D). Micedeficient in this surfactant protein have a decreasedcapacity for clearance of pneumococci and are prone to

disseminated infection.80

Alveolar macrophages represent the first phagocyticdefence in the lungs and can phagocytise and kill lownumbers of pneumococci.75,81When large numbers ofpneumococci are introduced into the lower airways,neutrophils are recruited and they become the mainphagocytic cells in the acutely inflamed lung. Alveolarmacrophages are then relegated to clearing apoptoticneutrophils.82 They also undergo apoptosis duringpneumococcal pneumoniamacrophage apoptosishelps with killing of phagocytised S pneumoniae83andkeeps pneumococcal invasion into the bloodstream toa minimum.66,81

Neutrophils predominate within cellular infiltrates in

pneumococcal pneumonia. The conventional notion ofneutrophil migration with selectin-mediated rolling and2-integrin-mediated tight adhesion to the endotheliumdoes not apply to pneumococcal pneumonia.842-integrin-deficient mice show normal neutrophiltraffi cking into lung tissue after infection withS pneumoniae.85Another host-derived, soluble adhesionmolecule known as galectin-3 has been implicated as amajor neutrophil recruitment signal in pneumococcalpneumonia.86Galectin-3-/- mice with S pneumoniaehaveaccelerated lung infection, with early disseminateddisease.87 Additionally, galectin-3 augments neutrophilphagocytosis and exerts bacteriostatic effects onS pneumoniae.

chemokines also promote an influx of neutrophilsinto lung tissue during pneumococcal pneumonia,88whereas reactive oxygen species derived fromnicotinamide adenine dinucleotide phosphate (NADPH)oxidase restrict neutrophil reflux.89 Moreover, pneumo-cocci produce chemotactic factors such as N-formyl-methionyl-leucyl-phenylalanine and pneumolysin thathelp with neutrophil recruitment.8890The net effect to ahost of neutrophil influx in pneumococcal pneumoniacan be good or bad, dependent on the virulence of thepathogen. Antineutrophil antibody treatment resulted inwidespread infection and increased mortality withserotype 3 pneumococci.91 Depletion of neutrophils

MARCO

PneumolysinCD14

LTA

LTATLR2

ChoP

S pneumoniae

MDP-PG NOD 2

p65 p50

IkB

NFkB

p300p65 p50

PAFR G-protein

Nucle

arme

mbran

e

TLR4

TIR

TLR9

MyD88

Cytokine genes, acute phase proteins

BacterialDNA

EndosomalvacuoleCell

membrane

Figure : Pattern-recognition signalling receptors and pathways in pneumococcal infection

S pneumoniaeis recognised as a pathogen in the lung by several toll-like receptors ( TLRs), including TLR2 (with

pneumococcal lipoteichoic acid [LTA] as its major ligand), TLR4 (recognises pneumolysin), and TLR9 (within

endosomes; interacts with bacterial DNA). Macrophage receptor with collagenous structure (MARCO) expressed

by alveolar macrophages contributes to innate immune response in lungs. PAFR is shown as a pattern-recognition

receptor because it recognises pneumococcal phosphorylcholine and LTA, thereby contributing to tissue invasion.

(Soluble) cluster-determining (CD) 14 probably further helps S pneumoniaeinvade from the airways into blood.

Within cytoplasm, the muramyl dipeptide component of pneumococcal peptidoglycan (MDP-PG) is recognised by

nucleotide-binding oligomerisation domain (NOD-2) and can activate host defence and inflammation.

ChoP=phosphorylcholine. PAFR=platelet-activating factor receptor. TIR=toll-interleukin-1 receptor domain.

MyD88=myeloid differentiation primary response protein-88. NFB=nuclear factor B. IB=inhibitor B.


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worsened outcomes in those with a high infectious doseof serotype 4 S pneumoniae, but not in those with a lowinoculum.92 By contrast, neutrophil depletion in micewith a serotype 8 strain improved survival and resulted inreduced bacteraemia.93 Pneumococcal pneumonia isuncommon in adults with isolated neutropenia andwithout other concomitant immune defects.

The greatly increased frequency of pneumococcalpneumonia in patients with AIDS attests to the protectiverole of CD4+ T cells in resistance to pneumococci.S pneumoniae elicits an early accumulation of T cells inlung infection94,95a response that is dependent onpneumolysin. MHCII-/- mice have a pronounced deficiency

in CD4+ T cells and are highly susceptible to pneumococcalpneumonia.95 T cells make up only 2% of circulating Tcells but are present in increased concentrations inrespiratory tissues. Pneumococcal pneumonia is associatedwith an increase in T-cell subsets in lung tissues.96,97V4T cell-/- mice are very susceptible to pneumococci andshow reduced traffi cking of neutrophils into lung tissue.96 T cells also contribute to the resolution phase ofS pneumoniae infection.97 Natural killer T cells have acrucially important role in defence against pneumococcalpneumonia.98Treatment of mice with -galactosylceramide,which specifically activates V14+ natural killer T cells,improves clearance of pneumococci.99

Clearance of pneumococci from the circulation stronglydepends on opsonisation by complement componentsand phagocytosis by myeloid cells.1,3 Disruption of thecommon C3 pathway profoundly impairs resistance topneumococci.85 Of the three complement cascades(classic, alternative, and mannose-binding lectinpathways), the classic pathway has been identified asmost important in host defence against pneumococci. 100Natural IgM antibodies contribute to innate immuneresponses through activation of the classical complementpathway.100Antibody-independent activation of the classicpathway also arises through SIGNR1,101 C-reactiveprotein,52and serum amyloid P.102

Recognition of pneumococci by immune cells withinthe respiratory tract generates an array of proinflammatoryand anti-inflammatory cytokines. Some of these cytokinesplay a pivotal part in innate defence against pneumococci.Tumour necrosis factor (TNF) is of particularimportance because its inhibition or elimination greatlyhelps with growth and dissemination of pneumococci103an effect mainly mediated by the type I TNF receptor. 104Interleukin 1 seems to have a similar yet less important protective role.70Inhibition of both TNF and interleukin 1renders animals very susceptible to pneumococcalpneumonia and other types of bacterial infections.70,105Other proinflammatory cytokines that are important for

Endothelium

Pneumococcus

AMTLRs

MARCOPAFR

CD14

PMN influx

C5a chemokines,galectin-3, Ply, fMLP

Host defenceIL1, TNF, IL18,

C, TF, AMPs, SP-D

A

Lung

interstitium

Air spaces(alveolus) PMN

efflux

sCD14, PAFR

Invasion

IgM, IgG, C,CRP, SAP

H2O

B

Pneumococcus

Capillary

Alveoli

Figure :Major pathological events in invasive pneumococcal pneumonia

(A) Pneumococci that enter lower airways are recognised by pattern-recognition receptors, including toll-like receptors (TLRs; on epithelial cells and alveolar macrophages) and macrophage receptor

with collagenous structure (MARCO; on alveolar macrophages). At low infectious doses, epithelial cells and alveolar macrophages can clear S pneumoniaewithout help from recruited neutrophils, in

part by release of protective inflammatory mediators such as interleukin 1 (IL 1), tumour necrosis factor ( TNF ), interleukin 18 (IL 18), complement products (C), surfactant protein-D (SP-D), and

antimicrobial peptides (AMPs). These mediators continue to have a role after infection with a high infectious dose, whereby polymorphonuclear cells (PMN) are recruited by chemoattraction through

various mediators including C5a, and galectin-3, and pneumococcal products such as pneumolysin (Ply) and formyl-methionine-leucine-phenylalanine (fMLP). (B) If alveolar defence mechanisms are

overwhelmed by multiplication of pneumococci, invasion of S pneumoniaeinto the bloodstream takes place, helped by platelet-activating factor receptor (PAFR) and (soluble) CD14 (sCD14). In the

bloodstream, several host proteins contribute to host defence including natural IgM antibodies, C, C reactive protein (CRP), and serum amyloid (SAP). TF=tissue factor. AM=alveolar macrophage.


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defence against pneumococci are interleukin 6 andinterleukin 18,71,106whereas the anti-inflammatory cytokineinterleukin 10 impairs defence mechanisms during bothprimary and postinfluenza pneumococcal pneumonia.98

Surprisingly, interferon inhibits antibacterial defencein models of either primary or postinfluenza pneumo-coccal pneumonia,91,107 possibly because this cytokinedecreases the capacity of alveolar macrophages to killS pneumoniae.91 Other investigators108,109 have reportedwidely disparate results with blockers of interferon .Interleukin 12, a prominent inducer of interferon , hasbeen reported to either enhance109or have no effect77onhost defence against pneumococcal pneumonia. Together,these studies show that cytokine-mediated enhancementof lung inflammation improves outcomes for animalswith S pneumoniae pneumonia, although interferon might worsen outcomes.

S pneumoniae pneumonia results in activation ofnuclear factor B within the lungs.110 Inhibition ofnuclear factor B activation increases lethality andenhances bacterial growth in animal models of infectionwith S pneumoniae.79,111A role for adequate activation ofthis factor in defence against pneumococci in people issupported by the association between commonpolymorphisms in the inhibitor B genes NFKBIAandNFKBIE, and increased susceptibility to invasivepneumococcal disease.112 Extensive cross-talk existsbetween the coagulation system and the innate immunesystem in response to microbial infection.113 Thesesystems simultaneously activate and collaborate tocontain and eradicate localised infection.114 Pneumo-coccal pneumonia is associated with both intrapulmonaryand systemic activation of the coagulation systema

response initiated by tissue factor.115,116

Anticoagulanttreatment with recombinant tissue-factor pathwayinhibitor, activated protein C, or antithrombin greatlyrestricts lung coagulopathy in pneumococcal pneumoniain animals.117 Infusion of recombinant human-activatedprotein C improves 28 day mortality in patients withsevere sepsis and reduces the high likelihood of dying. 116This beneficial effect was most evident in a subgroup ofpatients who had pneumococcal pneumonia.116Figure 5shows the host responses within the lung induced byS pneumoniae.

Clinical featuresPneumococcal pneumonia usually presents as typical,

acute community-acquired pneumonia. It generallybegins with a mild upper-airway irritation attributableto a respiratory viral infection. When pneumococci aredeposited into the lower airways they are usually expelledby mucociliary clearance, cough, antimicrobial peptides,and local innate immune defences. Should thesesystems fail to eliminate the pathogen, systemicinflammation ensues with characteristic signs andsymptoms of bacterial pneumonia.1,3 Onset of severeillness is abrupt, and develops with a shaking chill,fever, malaise, cough, and dyspnoea. The cough becomesproductive with purulent sputum, sometimes withbrownish or blood-tinged sputum with respirophasicchest pain and progressive dyspnoea. Left untreated,

this toxic illness can progress to acute respiratory failure,septic shock, multiorgan failure, and death withinseveral days from onset.

Figure 6 shows typical radiographic findings in anelderly patient with bacteraemic pneumococcalpneumonia. Resolution of radiographic findings for thisillness is slow, with abnormal findings in half of patientsup to 6 weeks after symptom onset.118 Need for andclinical usefulness of routine follow-up radiographs afteruncomplicated pneumococcal pneumonia is questionableand no longer recommended.119121 Recurrent ornon-resolving pneumonia in the same anatomicallocation is suggestive of a possible endobrochial lesion.

A B

C D

Figure : Typical radiographic findings in pneumococcal pneumonia

This previously healthy woman aged 73 years presented to hospital with bacteraemic multilobar left-sided

pneumococcal pneumonia (A). 5 days later, the infiltrate worsened (B) despite appropriate therapy (ceftriaxone

and clarithromycin) for this penicillin-susceptible strain ofS pneumoniae. By day 10, chest radiograph (C) shows a

parapneumonic effusion, extensive consolidation, and areas of focal cavitation. A chest CT image on day 12 (D)

confirms cavitation and consolidation with air bronchograms. The patient slowly recovered with antimicrobial

therapy alone and had a chest radiogram after 2 months that was clear apart from minor parenchymal scarring in

the left lower lobe.


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Recurrent pneumococcal pneumonia in differentbronchopulmonary segments should prompt a searchfor an underlying immunocompromised state. HIVinfection, congenital or acquired B-cell disorders, andciliary dyskinesia are common concerns in patientsyounger than 18 years, and multiple myeloma and otherymphoreticular disorders are suspected in patients olderthan 65 years.118

Clinical presentation of pneumococcal pneumoniamight not be typical in some vulnerable patients, anddiagnosis can be diffi cult. Invasive pneumococcal diseasecan be subtle in its early phases in neonates, elderlypeople, severely immunocompromised patients, asplenichosts, and in various concomitant co-morbid illnesses.118,122Patients can present with extrapulmonary symptoms(meningitis, overwhelming sepsis, pericarditis, peri-

tonitis, mastoiditis, and endocarditis) before showingevidence of bacterial pneumonia.

DiagnosisDiagnostic methods for pneumococcal pneumonia havenot changed appreciably since Pasteur and Sternbergfirst isolated S pneumoniaein 1881, and Christian Gramused his famous stain to reveal pneumococci under themicroscope in 1886 (figure 7). Pneumococci grow readilyon blood agar plates in a CO incubator at 37C.S pneumoniae colonies are haemolytic and oftenumbilicate because of autolysis. Cultures from sputum,blood, and other tissue sites should be obtained beforeempirical antibiotic therapy is started.118

Two innovations for rapid diagnosis of pneumococcaldisease are now available and can be quite useful. Thefirst is urinary antigen detection of the C polysaccharidefrom the pneumococcal cell wall by an immunochromato-graphy assay. This test compares favourably with cultureand gram stain for detection of invasive pneumococcaldisease.123 The assay is most sensitive in severepneumococcal disease and bacteraemia, and can befalsely negative in early pneumococcal infection. Thisassay is especially useful in patients from whom clinicianshave diffi culty getting an adequate sputum sample, andin those who have already begun antibiotic treatmentbefore cultures were obtained. The urinary antigenremains positive for weeks after onset of severe

pneumococcal pneumonia. Major drawbacks of thisassay are an inability to acquire antimicrobialsusceptibility data from this test, and loss of sensitivitywith mild infections. The second is the many non-cultureassays based on nucleic acids for pneumococcalpneumonia that are now available or in develop-ment. They are rapid and highly specific, withS pneumoniae-specific DNA sequences as targets fordetection assays.124,125 To distinguish pneumococcalcolonisation from infection remains a challenge. Perhapsthese rapid diagnostic techniques will be useful formaking management decisions about patients withpneumococcal infections in the future.

TreatmentThe initial, uniform activity of penicillin againstS pneumoniaeresulted in this antibiotic being treatmentof choice for community-acquired pneumonia since thelate 1940s.1 Penicillin-resistant strains of S pneumoniae

were first noted in the mid 1970s and resistant cloneshave now spread worldwide.1,118,126Penicillin resistance isrelated to structurally modified penicillin-bindingproteins of S pneumoniae. These modified bindingproteins allow peptidoglycan synthesis despite thepresence of penicillin. This resistance mechanism canoften be overcome with high doses of penicillin. Presentclinical laboratory standards that define the minimuminhibitory concentration for susceptibility, intermediateresistance, and high-level resistance to penicillin varysomewhat by region (table 2).118,120,121

Non-meningeal S pneumoniae strains expressingintermediate and even high-level resistance to penicillincan still be treated with high-dose, -lactam antibiotics

(penicillins, or second or third generation cephalo-sporins).121,126,127 Meningitis attributable to S pneumoniaewith even intermediate resistance to pneumococcinecessitates use of other agents to assure a successfuloutcome.118Table 2 provides the present recommendedtreatment regimens and comparisons between theBritish,120 European,121 and American118 guidelines forcommunity-acquired pneumonia and specific types ofS pneumoniaeinfections.

Antimicrobial resistance of pneumococci tomacrolides,128 fluoroquinolones,129vancomycin,126 trime-thoprim, 118 and various other antimicrobial agents isincreasingly recognised worldwide.126,127When feasible,

m

10 200

Figure : Classic gram stain findings in acute pneumococcal pneumonia

Note numerous lancet-shaped gram-positive diplococci and neutrophilic cellular infiltrate. Pneumococci as

diplococci (arrows). (High power magnification under oil immersion 1000.)


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S pneumoniae strains isolated from patients who aresuffi ciently ill to need hospital admission shouldundergo sensitivity testing because susceptibilities to

standard agents are no longer assured.126

This testing isespecially important in assessment of blood orcerebrospinal fluid isolates, and in geographical areaswhere resistance to standard antibiotics is a knownproblem.

Much controversy exists about the advisability of useof combination therapy with a -lactam and eithera macrolide or respiratory fluoroquinolone forbacteraemic pneumococcal pneumonia. Results ofseveral retrospective and small prospective studies130132show a possible survival advantage with combinationtherapy, even when the pneumococcal isolate issusceptible to -lactams. Some form of in-vivo synergymight exist, or combination therapy might treat some

unrecognised co-pathogens not covered by -lactams(eg, Mycoplasma pneumoniae, Chlamydophila spp, orother untypical pathogens). Macrolides and perhapseven fluoroquinolones might limit excessive host-derived inflammatory reactions to severe pneumococcalpneumonia. 130132 In a laboratory study,133 researchersreported improved outcomes and reduced lunginflammation when inhibitors of bacterial proteinsynthesis (clindamycin or azithromycin) were added toampicillin for postinfluenza pneumococcal pneumonia.These protein inhibitors might reduce synthesis ofmicrobial mediators that contribute to lung inflam-mation. Convincing clinical evidence from large

numbers of people to show that combination therapy isbetter than monotherapy is highly desirable. Individualdecisions about dual therapy for severe invasive

pneumococcal disease presently rest on available butinsuffi cient clinical information.Early intervention with effective antimicrobial agents

provides a substantial survival advantage in severepneumococcal pneumonia. Antibiotics should be startedas soon as possible (01 mg/L (S), 0110 mg/L (I), and >14 mg/L

(R); ERS 20 (R); ATS 20 g/mL (S), 24 g/ml (I), 8 g/ml (R). For meningeal isolates: ATS


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compromised patients, and in infants younger thanage 2 yearsprecisely those at greatest risk for severepneumococcal disease.

The covalently linked polysaccharide-protein conjugatepneumococcal vaccine has been in use for almost 10 yearsand was very successful16,17 (figure 1). The seven-valentconjugate vaccine (Prevenar; Wyeth) is approved for usemainly in children younger than 2 years, and for childrenyounger than 5 years with high-risk conditions. Thisvaccine targets the pneumococcal serotypes 4, 6B, 9V, 14,18C, 19F, and 23F. These seven serotypes are responsiblefor 80% of pneumococcal infections in children living indeveloped countries. The vaccine is highly immunogenicin young children because it is T cell-dependent andovercomes the intrinsic loss of immunogenicity topolysaccharide vaccines within the first 2 years of life. 16

This vaccine has reduced incidence of invasivepneumococcal disease in children in the USA youngerthan 1 year by 82%.17One of the most remarkable attributesof the pneumococcal conjugate vaccine has been thedegree of herd immunity it generates. Frequency ofinvasive pneumococcal disease infection in non-vaccinatedsiblings and even adult contacts has declinedsubstantially.16,134,135 Young children are the primaryreservoir for pneumococcal colonisation in people, andthus elimination of the carrier state in children reducesrisk of transmission to the rest of the population.

Despite the effectiveness of the conjugate vaccine,disturbing patterns are developing in the epidemiologyof pneumococcal disease that threaten the long-term

benefits of the vaccine. Invasive disease attributable tonon-vaccine serotypes of S pneumoniae has greatlyincreased.18 Vaccine-induced anticapsular antibodiesrestrict access of vaccine-associated serotypes to thehuman upper airways. This niche is now available tonon-vaccine pneumococcal serotypes or otheroropharyngeal pathogens such as Staphylococcusaureus.1618 Vaccine replacement by non-virulentserotypes would be acceptable, but virulent, non-vaccineserotypes can also replace vaccine serotypes of pneumo-cocci.3,136,137Capsular switching probably arises throughtransformation of capsular genes from one serotype toanother.18,138140 Such events could arm an intrinsicallyvirulent strain of S pneumoniaepreviously encapsulated

with a vaccine-related serotype with a new antigenicpolysaccharide coat to avoid vaccine-induced immunerecognition.

Analysis of patterns of serotype replacement over timeafter widespread institution of the seven-valent conjugatevaccine is needed. An expanded conjugate vaccineincluding serotypes not covered in the present vaccinemight be useful. Serotype 19A S pneumoniaeis a specificconcern.136 This serotype has expanded greatly sinceintroduction of the conjugate pneumococcal vaccine,138,139and has much virulence potential as a pulmonary andextrapulmonary pathogen. Moreover, it has the propensityto acquire multiple drug-resistance genes, complicating

antimicrobial therapy against this organism.139,140 Othervaccine approaches include development of vaccinesagainst many, highly conserved, immunogenic proteinantigenseg, adhesins, pneumolysin, invasion proteins,and transport proteins.5

ConclusionsThe pneumococcus fascinates immunologists, yetfrustrates clinicians and public-health offi cials attemptingto control it. Although the human respiratory tract hasmany local and systemic immune defences, a range ofpneumococcal virulence factors work together to causeinvasive disease. The capacity of S pneumoniaeto resistantimicrobial agents and escape immune defences showsthat control of this pathogen will not be easy to achieve.Thus a multifaceted approach with new generation

vaccines, novel antimicrobial therapies, and improvedadjuvant treatments will be needed.

Contributors

Both authors contributed equally to this Seminar.

Conflicts of interest

We declare that we have no conflicts of interest.

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