Immune Evasion by Staphylococci

Abstract

Staphylococci are a common cause of hospital-acquired infections and are

increasingly linked to community-acquired infections. There are two primary human

pathogens in the genus; Staphylococcus aureus and Staphylococcus epidermidis. The

latter is generally non-pathogenic, although it can cause disease in

immunocompromised individuals. In contrast, S. aureus infection is often much more

serious and can, in some cases, be life-threatening. S. aureus possesses a variety of

virulence factors that enable it to evade the host immune system, the majority of

which are not found in S. epidermidis. S. aureus produces a variety of proteins that

perform various functions in evasion of the immune system, but also has certain

structural features that enable it to thwart the host immune response. Understanding

the pathogenesis of this organism is critical to developing effective and sustainable

treatments for S. aureus infections. By elucidating the mechanisms by which the

organism causes disease, it may be possible to identify new drug targets either for

preventative or curative treatments. In addition, there has been interest in exploiting

this knowledge for use in the treatment of autoimmune disorders. Autoimmune

disorders are characterised by an excessive or unwanted immune response, which is

harmful to the patient rather than protective. Therefore, the proteins produced by S.

aureus, which have the effect of interfering with or inhibiting the immune response,

may have use as therapeutic products for these types of illnesses. This review will

focus on the evasion of the innate immune response in humans by S. aureus, namely

evasion of the complement system, phagocytes and antimicrobial peptides, as well as

briefly outlining some of the promising applications of this research in the treatment

of S. aureus infection and auto-immune disorders.

Table of Contents

1. Introduction

1.1. Staphylococcal infection

1.2. Immune mechanisms encountered by staphylococci

1.2.1. Complement

1.2.2. Phagocytes

1.2.3. Antimicrobial peptides

2. Complement evasion

2.1. Staphylokinase

2.2. Protein A

2.3. Staphylococcal complement inhibitor

2.4. Extracellular fibrinogen binding protein

3. Phagocyte evasion

3.1. Inhibition of neutrophil chemotaxis

3.1.1. Chemotaxis inhibitory protein of staphylococci

3.1.2. Extracellular adherence protein

3.2. Resistance to the respiratory burst

3.3. Leukotoxins

4. Antimicrobial peptide evasion

4.1. Secreted proteins

4.1.1. Proteolytic cleavage and production of extracellular CAMP-binding

molecules

4.2. Structural features

4.2.1. Modification of cell surface net charge

4.2.2. Alteration of membrane structure/fluidity

4.2.3. Alteration of peptidoglycan structure

5. Conclusions

6. References

Introduction

Staphylococci are a common cause of nosocomial (hospital-acquired) infections and

are increasingly linked to community-acquired infections (Carleton et al, 2004).

Staphylococcus aureus and Staphylococcus epidermidis are the primary

staphylococcal pathogens in humans. S. epidermidis is generally non-pathogenic,

although it can cause disease in immunocompromised individuals, including the

elderly and those with indwelling medical devices (Foster, 2005). On the other hand,

S. aureus infection is often much more serious and can lead to life-threatening

diseases. This may be due in part to the fact that S. aureus has a variety of virulence

factors which enable it to evade the host immune system that are not present in S.

epidermidis (Foster, 2005). For this reason, S. aureus will be the primary focus of this

review.

S. aureus has a variety of virulence factors, but in this review we will focus only on

those virulence factors that are involved directly in evasion of the immune system.

The majority of immune evasion strategies used by S. aureus relate to evasion of the

innate immune system. The innate immune system consists of three primary

components; complement, phagocytes and antimicrobial peptides. The various

strategies that S. aureus uses to evade these three components will be discussed. It

should be noted that there is a degree of overlap between these three subheadings, e.g.

certain staphylococcal proteins have multiple immune evasion mechanisms. S. aureus

primarily uses an array of secreted, cell wall-anchored and membrane-bound proteins

to defend against the host immune response. However, it also has some structural

features which contribute to its ability to evade destruction by the immune system.

These mechanisms will be outlined and their potential implications for treatment of S.

aureus infection will be discussed.

1.1 Staphylococcal infection

Staphylococci are a genus of coccoid, Gram-positive, facultative aerobes (Casey et al,

2007). They are commonly found in commensal relationships with animals and

humans. They are primarily found on the skin of healthy individuals, particularly in

moist skin areas, such as the nostrils. However, they are also found in lower numbers

in the respiratory tract, the gastrointestinal tract and the urogenital tract (Zecconi and

Scali, 2013).

Staphylococcus epidermidis and Staphylococcus aureus are the two species in the

genus that most frequently cause disease in humans. Of these two, S. aureus (Figure

1) is more clinically significant, i.e. it is more frequently associated with disease. S.

epidermidis is generally nonpathogenic, although it can cause disease in patients with

severely compromised immune systems (Foster, 2005).

Figure 1. SEM of Staphylococcus aureus. (Source: Centers for Disease Control and Prevention, Public

Health Image Library)

S. aureus is a common healthcare-associated pathogen and often causes infection in

immunocompromised individuals. The emergence of antibiotic-resistant strains such

as MRSA (Methicillin Resistant Staphylococcus aureus) represents a significant

threat to human health in hospital environments; MRSA is now estimated to cause

44% of all nosocomial infections in Europe every year (Gould et al, 2012). Infection

with S. aureus can cause diseases of the skin, soft tissue, respiratory tract and

endovascular system. Diseases caused by S. aureus infection can range from the

superficial (e.g. boils) to the deeply invasive (e.g. meningitis) and can be life-

threatening. The bacterium has a plethora of virulence factors that enable it to cause

infection, and a subset of these virulence factors relate directly to evasion of the host

immune response.

1.2 Immune mechanisms encountered by staphylococci

Immunity is defined as the ability of the body to resist disease. The immune system in

humans consists of innate and adaptive immunity. The innate immune system

comprises the non-specific aspect of the immune system and is the first line of

defence against invading microorganisms. It consists of various cells and molecules,

including phagocytes, complement, natural killer cells and dendritic cells. The innate

immune system is immediately activated upon infection with a pathogen, as many of

the components of innate immunity are present and fully functional prior to infection.

Components of innate immunity then stimulate the adaptive immune response

(Medzhitov and Janeway, 2000).

Adaptive immunity is the specific response of the immune system to a pathogen.

While the cells and molecules of the innate immune system are designed to defend

against a wide range of pathogens, the cells and molecules of adaptive immunity are

specific to a particular pathogen, with a particular antigen. The main components of

the adaptive immune system are B and T lymphocytes. B cells produce antibodies that

are specific to a particular antigen, while effector T cells carry out a range of

functions in destroying pathogens (Medzhitov and Janeway, 2000). The majority of

staphylococcal immune evasion mechanisms relate to the innate immune response.

The adaptive immunity appears to be ineffective in providing immunological memory

in the case of staphylococcal infection and is generally not sufficient to prevent

recurring infection (Foster, 2005). The innate immune system has three primary

components that combat invading microbes complement, phagocytes and

antimicrobial peptides.

1.2.1 Complement

The complement system consists of a series of proteins that are activated in sequence

in response to an invading pathogen (Dunkelberger and Song, 2010). Complement

proteins circulate in the blood as inactive enzyme precursors until they are activated.

Once the first protein in the sequence has been activated, it becomes capable of

activating the next protein in the sequence by proteolytic cleavage. This protein in

turn activates the next protein in the sequence in the same manner, and so on

(Dunkelberger and Song, 2010). Complement serves two primary functions

opsonisation and direct lysis of pathogens. In the case of Gram-positive bacteria like

staphylococci, their thick cell wall confers resistance to direct complement attack

(Rooijakkers, et al, 2005c). Therefore opsonisation is the main function of the

complement system in the case of S. aureus. Opsonisation is the labelling of

pathogens for destruction by phagocytes. Phagocytes have receptors that enable them

to recognise specific molecules (chemoattractants) that cause migration of phagocytes

to the site of infection. This process is called chemotaxis.

Complement is initially activated by one of three pathways (Figure 2). Two of these

pathways (the mannose-binding lectin pathway and the alternative pathway) are

components of the innate immune response and one (the classical pathway) is part of

the adaptive immune response. Although the pathways are activated in different ways,

they share one common step the production of a C3 convertase that cleaves the

complement protein C3 into the fragments C3a and C3b. C3a acts as a

chemoattractant, while C3b acts as an opsonin. Several other important proteins and

protein fragments are released throughout the complement cascade which are

involved in recruiting phagocytes to the site of infection or in directly destroying the

pathogen (Rooijakkers et al, 2005c). S. aureus produces a variety of proteins that

interfere with this process.

Figure 2. Three systems of complement activation. A. The classical pathway: the C1 complex

(containing proteins C1q, C1r and C1s) binds to antibodies bound to the bacterial surface. This leads to

autoactivation of C1r, which in turn activates C1s, which cleaves C4 and C2 to form the C3 convertase,

C4bC2a. Cleavage of C3 produces C3a and C3b. C3a is a chemoattractant, while C3b binds to the C3

convertase to form the C5 convertase, C3bC4bC2a. Cleavage of C5 produces C5a and C5b. C5a is a

chemoattractant and C5b forms the membrane attack complex, which is involved in direct killing of

pathogens. B. The lectin pathway: mannose-binding lectin (MBL) or ficolin binds to carbohydrate

groups on the bacterial cell surface. MBL and ficolin are associated with a variety of MBL-associated

serine proteases; MASP1, MASP2, MASP3 and sMAP (small MBL-associated protein). MASP2

activates C4 and C2 to form the C3 convertase, C4bC2a. MASP1 is also capable of directly cleaving

C3. The pathway then proceeds as in the classical pathway. C. The alternative pathway: low-level

cleavage of C3 by the hydrolysed form of C3 in combination with activated factor B (Bb) gives C3a

and C3b. C3b binds factor B, followed by cleavage of B to Bb by factor D, resulting in the formation

of the C3 convertase C3bBb. The C3 convertase cleaves C3 to C3b and C3a, resulting in an

amplification of the pathway wherein the C3b produced by the C3 convertase feeds back into the

pathway to form more C3 convertases. Source: Foster, 2005.

1.2.2 Phagocytes

Phagocytes are cells that engulf and destroy pathogens (Figure 3). Phagocytes

recognise a variety of chemoattractants including complement fragments C5a and

C3a, and formylated peptides released by bacteria. Neutrophils are the primary

C1r

C1s

Bb

BbB

C2a

C1q

MBL/ficolin

Immunoglobulin

Bacterialantigen

Carbohydrate

Bacterial surface Bacterial surface

Bacterial surface

C3

C2

C4

C3

C3

C3

C3(H2O) C3(H2O)

C3b

C3a

C3bC3b

C3b

C3a

C3a

C3b C3b

C3a

C4b

C2a

C4b

BD

D

MASP2

MASP3

MASP1

sMAP

c Alternative pathway and amplification loop

a Classical pathway b Lectin pathway

?

Figure 1 | Pathways for complement activation. a | The classical pathway is initiated by the binding of the C1 complex to

phagocytes involved in combatting S. aureus infection and can also recognise

immunoglobulin G (IgG) and complement proteins bound to the bacterial surface.

Phagocytes contain a variety of substances which they use to kill pathogens once they

have been ingested, including lysozyme, antimicrobial peptides and reactive oxygen

species (Foster, 2005).

Figure 3. Phagocytosis. The phagocyte migrates to the site of infection by chemotaxis. It then adheres

to the bacterial cell, which may or may not be opsonised. The bacterium is taken up into the cell by

endocytosis. This results in the formation of a phagosome, a membrane-bound vesicle containing the

bacterium. Fusion of the phagosome with lysosomes results in the formation of the phagolysosome.

The lysosome contains toxic substances, such as lysozyme, which act to kill the bacterial cell. Reactive

oxygen species (e.g. OH-, H2O2, and O2-) are also produced in the phagolysosome as a result of the

respiratory burst, requiring the cell to take up O2. In macrophages, the destroyed bacterial cell is then

egested from the cell, while the antigen is processed and presented to lymphocytes. In neutrophils, the

phagocyte itself will die after a period of active phagocytosis.

1.2.3 Antimicrobial peptides

Antimicrobial peptides are a group of molecules that can recognise and kill

pathogens. They generally do this by disrupting the bacterial membrane, causing lysis

of the cell, although variations on this method have been observed. These molecules

are found in high concentrations on the skin, on mucous surfaces and in neutrophils

and platelets (Peschel and Collins, 2001). The majority of antimicrobial peptides are

positively charged, which allows them to bind to the negatively charged bacterial cell

membrane. Therefore, they are referred to as cationic antimicrobial peptides

(CAMPs). A variety of CAMPs are produced within human neutrophils and platelets,

however, S. aureus has developed various ways to evade these molecules. In

particular, S. aureus has been found to thwart the action of defensins, cathelicidins

and thrombocidins (Peschel, 2002). Defensins are short cationic peptides that bind to

the membranes of invading cells and form pores, causing efflux of the cell contents

and, ultimately, cell death. Two classes of defensins are produced in humans; -

defensins and -defensins. -defensins are found primarily in neutrophils while -

defensins are produced by a wide range of leukocytes and epithelial cells (Pescel and

Collins, 2001). Cathelicidins serve many functions in antimicrobial defence, including

leukocyte recruitment and direct killing of antimicrobial cells. Only one cathelicidin

is found in humans, cathelicidin LL-37, which is produced in neutrophils and

epithelial cells (Zanetti, 2004). Thrombocidins are produced by platelets and are

thought to damage the cell membrane of pathogens (Peschel, 2002). Lysozyme is a

bacteriolytic enzyme with a similar function to CAMPs. Lysozyme cleaves

peptidoglycan, a unique component of the Gram-positive cell wall, and irreversibly

damages the cell wall (Peschel, 2002). S. aureus has developed mechanisms of

evading destruction by all of these molecules.

2. Complement evasion

Complement proteins act as opsonins, chemoattractants and effector molecules in

innate and adaptive immunity (Rooijakkers et al, 2005c). S. aureus has developed a

variety of mechanisms to evade and interfere with the complement cascade (Table 1).

Most of these mechanisms take the form of proteins that interfere with complement

(Figure 4).

Table 1: Complement evasion mechanisms of S. aureus

Protein name Protein abbreviation Function Immune evasion

result Reference

Staphylokinase Sak

Converts plasminogen to plasmid

Cleaves complement factors, inhibits complement cascade, inhibits opsonisation

Rooijakkers et al, 2005a

Protein A SpA

Binds IgG Inhibits classical pathway, inhibits Fc-receptor mediated phagocytosis

Atkins et al, 2008; Falugi et al, 2013

Staphylococcal binder of immunoglobulins

Sbi

Binds IgG Inhibits classical pathway, inhibits Fc-receptor mediated phagocytosis

Atkins et al, 2008; Smith et al, 2011

Staphylococcal superantigen-like protein 10

SSL-10

Binds IgG Inhibits classical pathway, inhibits Fc-receptor mediated phagocytosis

Itoh et al, 2010

Staphylococcal complement inhibitor

SCIN

Binds C3 convertases

Inhibits complement cascade, prevents production of opsonins

Rooijakkers et al, 2005b

Extracellular fibrinogen binding protein

Efb Binds to C3 Inhibits complement

cascade, inhibits opsonisation by C3b

Lee et al, 2004a; Lee et al, 2004b

Extracellular complement binding protein

Ecb

Binds to C3 Homolog of Efb, inhibits complement cascade, inhibits opsonisation by C3b

Jongerius et al, 2010

Staphylococcal superantigen-like protein 7

SSL-7 Binds to C5

Inhibits opsonisation by C5a

Langley et al, 2005

2.1 Staphylokinase

Staphylokinase is a secreted protein that interferes with the innate immune response

in several ways, and acts specifically against complement at two different stages. It

does this by converting plasminogen to plasmin, which has the ability to cleave and

inactivate both immunoglobulin G (IgG) and C3b. S. aureus cells have surface-bound

plasminogen receptors that bind plasminogen, which is then converted to the active

form of plasmin by staphylokinase. Activated plasmin then removes the opsonins IgG

and C3b which are bound to the surface of the pathogen, disrupting the complement

cascade at the beginning of the classical pathway (by preventing opsonisation by IgG)

and/or the C3 convertase stage of all pathways. This results in impaired opsonisation

and phagocytosis (Rooijakkers et al, 2005a; Rooijakkers et al, 2005c).

2.2 Protein A

Protein A is a membrane-bound protein which binds to the Fc region of surface-bound

IgG in the classical pathway (Atkins et al, 2008). The Fc region of IgG is the region

that is normally recognised by C1q, the first protein in the classical pathway. Protein

A prevents binding of C1q and the initiation of the classical pathway, and also inhibits

Fc-receptor mediated phagocytosis (Falugi et al, 2013). Staphylococcal binder of

immunoglobulins (Sbi) and staphylococcal superantigen-like protein 10 (SSL10) also

bind IgG in this way (Smith et al, 2011; Itoh et al, 2010).

2.3 Staphylococcal complement inhibitor

Staphylococcal complement inhibitor (SCIN) is a secreted protein that binds to the C3

convertases in all three complement activation pathways (C4b2a in the classical and

mannose-binding lectin pathways and C3bBb in the alternative pathway). Binding of

SCIN to the C3 convertases inactivates them, thereby blocking the complement

pathway at this point. The opsonin C3b and the chemoattractant C3a are not

produced, resulting in reduced phagocytosis. In addition, binding of SCIN stabilises

these complexes, which are usually unstable. The C3 convertases normally dissociate

after a short time, leaving C4b and C3b bound to the bacterial cell membrane to form

more C3 convertases. By preventing this dissociation, SCIN prevents the formation of

more convertases and the amplification of the complement cascade (Rooijakkers et al,

2005b).

2.4 Extracellular fibrinogen binding protein

Extracellular fibrinogen binding protein (Efb) and its homolog Ecb bind to C3 and

prevent it from binding to the bacterial cell surface. This prevents opsonisation and

the formation of the C3 and C5 convertases, thereby further reducing phagocytosis

(Lee et al, 2004a; Lee et al, 2004b; Jongerius et al, 2010).

Figure 4. Staphylococcal proteins interfere with the complement cascade. Staphylococcal proteins are

shown in red.

3. Phagocyte evasion

Phagocytes play a key role in innate immune defence against bacterial pathogens. S.

aureus is resistant to direct complement attack, therefore the innate immune defence

is largely dependent on the action of phagocytic cells, particularly neutrophils

(Rooijakkers et al, 2005c). S. aureus has developed three main mechanisms of

evading neutrophil attack interference with neutrophil chemotaxis, resistance to the

respiratory burst and the production of pore-forming leukotoxins which destroy

leukocytes (Table 2).

Table 2. Neutrophil evasion strategies of S. aureus

Factor Name Abbreviation Function Immune evasion effect Reference

Inhibition of neutrophil chemotaxis Chemotaxis inhibitory protein of S. aureus

CHIPS Binds to C5a receptor and FPR

Prevents chemotaxis

Haas et al, 2004; Haas et al, 2005; Postma et al, 2004

FPR-like inhibitory protein

FLIPr Binds to FPR-like 1 receptor (FPRL1) and FPR

Inhibits chemotaxis

Prat et al, 2006; Stemerding et al, 2013

FLIPr-like N/A Binds to FPRL1 receptor and FPR

Inhibits chemotaxis

Prat et al, 2009; Stemerding et al, 2013

Extracellular adherence protein

Eap Binds to ICAM-1 in endothelial cells, prevents binding of LFA-1

Prevents neutrophil adhesion and extravasation

Chavakis et al, 2002; Haggar et al, 2004

Resistance to respiratory burst in phagosomes Staphyloxanthin N/A Anti-oxidant Scavenges free

radicals and inhibits production of reactive oxygen species

Clauditz et al, 2006; Liu et al, 2005

Catalase CatA Degrades hydrogen peroxide

Inactivates reactive species

Cosgrove et al, 2007

Alkylhydroxide reductase

AhpC Catalase activity Inactivates reactive species

Cosgrove et al, 2007

Thioredoxin N/A Reducing agent Inactivates reactive oxygen species

Chavakis et al, 2007; Roos et al, 2007

Superoxide dismutases

N/A Catalyses dismutation of O2

- Inactivates superoxide radical

Karavolos et al, 2003

Leukotoxins -haemolysin Hld Neutrophil binding Leukolysis Peacock et al,

2002; Somerville et al, 2003

Bicomponent pore-forming leukotoxins -haemolysin Hlg Pore-forming Leukolysis Peacock et al,

2002; Yoong and Torres, 2013

Panton-Valentine leukocidin

PVL Pore-forming Leukolysis Aman et al, 2010; Prevost et al, 1995

Leukotoxin DE LukED Pore-forming Leukolysis Gravet et al, 1998; Yoong and Torres, 2013

Leukotoxin AB LukAB Pore-forming Leukolysis, may facilitate release of bacterium from inside the phagosome

DuMont et al, 2011; Ventura et al, 2010

3.1 Inhibition of neutrophil chemotaxis

3.1.1 Chemotaxis Inhibitory Protein of S. aureus

Binding of chemoattractants to receptors on the neutrophil surface stimulates

chemotaxis. The chemotaxis inhibitory protein of S. aureus (CHIPS) has two distinct

binding domains one which can bind to the neutrophil C5a receptor and one which

can bind to the neutrophil formyl peptide receptor (FPR), thus preventing binding of

the true ligand and inhibiting chemotaxis (Haas et al, 2004). Formyl peptide receptor-

like 1 inhibitory protein (FLIPr) and its homolog FLIPr-like also bind to the FPR and

to the formyl peptide receptor-like 1 protein, (a homolog of the FPR, which is also

found on neutrophils) further inhibiting chemotaxis (Stemerding et al, 2013).

3.1.2 Extracellular adherence protein

The extracellular adherence protein (Eap) recognises many ligands, including the

intercellular adhesion molecule (ICAM-1), which is found on the surface of the

endothelial cells that form the lining of blood and lymph vessels (Chavakis et al,

2002). Neutrophils have lymphocyte-function-associated antigen (LFA-1) on their

surface, which binds to ICAM-1, allowing the neutrophils to adhere to the endothelial

cells and migrate to the site of infection (Chavakis et al, 2002). S. aureus releases

Eap, which binds to ICAM-1, blocking neutrophil adhesion and preventing

extravasation of neutrophils (Chavakis et al, 2002; Haggar et al, 2004).

3.2 Resistance to the respiratory burst

The respiratory burst takes place inside phagocytes once a pathogen has been

engulfed. During the respiratory burst, reactive oxygen species (such as hydrogen

peroxide and superoxide) are produced inside the acidic environment of the

phagolysosome (Robinson, 2008). These compounds destroy invading organisms by

oxidising components of the bacterial cell, e.g. lipids and proteins. S. aureus has

multiple ways of defending against these toxic substances. Staphyloxanthin, a yellow

carotenoid pigment found in S. aureus, has potent antioxidant activity, i.e. it inhibits

the production and action of reactive oxygen species by scavenging (i.e. inactivating)

free radicals (Clauditz et al, 2006). A variety of enzymes, including catalase,

alkylhydroxide reductase, superoxide dismutases and thioredoxin are also produced

by the bacterium to inactive reactive oxygen species (Table 2).

3.3 Leukotoxins

S. aureus produces four bi-component, pore-forming leukotoxins which directly act to

disable leukocytes (Yoong and Torres, 2013). These toxins consist of two subunits

that form a monomer and assemble together with other bi-component monomers to

form oligomeric pores in the membrane of leukocytes (Aman et al, 2010). This leads

to osmotic dysregulation of the leukocyte and lysis of the cell (Yoong and Torres,

2013). Toxins are often produced in different amounts in different areas of the body,

indicating that the environment of the bacterium may dictate which toxins are

produced and in what quantities (Yoong and Torres, 2013).

4. Antimicrobial peptide evasion

S. aureus has multiple mechanisms of anti-microbial peptide evasion, comprising a

mixture of secreted proteins and structural features of the bacterial cell.

4.1 Protein secretion

4.1.1 Proteolytic cleavage and production of extracellular CAMP-binding

molecules

S. aureus secretes extracellular proteins which either cleave cationic antimicrobial

peptides (CAMPs) or chelate them, rendering them inactive in both cases (Kraus and

Peschel, 2008). Aureolysin and serine-protease V8 cleave the cathelicidin LL-37

(Sieprawska-Lupa et al, 2004). Staphylokinase, as previously discussed, activates

plasminogen, but it also has a second binding domain for -defensins (Jin et al,

2004).

4.2 Structural features

4.2.1 Modification of cell surface net charge

S. aureus is capable of modifying the charge at its cell surface to repel antimicrobial

peptides (Figure 5). Antimicrobial peptides are generally cationic, while the bacterial

cell surface generally has a net negative charge, due to the presence of molecules such

as peptidoglycan, teichoic acids and phospholipids (Kraus and Peschel, 2008). This

makes antimicrobial peptides highly selective for bacterial cells, because human cells

are usually uncharged. S. aureus can incorporate D-alanine into its teichoic acids

(Collins et al, 2002). This introduces a positively charged amino group into the

peptidoglycan structure. S. aureus can also incorporate L-lysine into

phosphatidylglycerol, a membrane phospholipid (Kristian et al, 2003). This causes the

cell surface net charge to become positive, thus repelling the cationic antimicrobial

peptides and preventing them from binding to the bacterium.

Figure 5. Electrostatic repulsion of CAMPs by modification of cell surface net charge. Adapted from

Peschel, 2002.

4.2.2 Alteration of membrane structure or fluidity

Modification of bacterial membrane structure has been shown to confer resistance to

antimicrobial peptides (Bayer et al, 2000). Bacteria contain multidrug resistance

exporters which can transport antimicrobial peptides out of the cell. One of these, the

QacA pump, increases the resistance of S. aureus to certain antimicrobial peptides,

but experiments suggest that this resistance is actually due to the effect of QacA on

membrane structure rather than its function as an efflux pump (Bayer et al, 2006). S.

aureus can also alter the fluidity of its membrane by introducing more long-chain

unsaturated fatty acids. This change in fluidity causes increased resistance to

antimicrobial peptides (Bayer et al, 2000).

4.2.3 Alteration of peptidoglycan structure

Lysozyme cleaves peptidoglycan, a unique component of the Gram-positive cell wall,

between the N-acetylmuramic acid and N-acetylglucosamine residues. The oatA gene

in S. aureus causes O-acetylation of the N-acetylmuramic acid residue, preventing

recognition by lysozyme and conferring absolute resistance (Bera et al, 2005).

Conclusions

In this review, the primary mechanisms used by S. aureus to evade the host immune

system have been outlined. It is clear that S. aureus possesses a diverse array of

immune evasion mechanisms which contribute significantly to its virulence,

particularly as a nosocomial pathogen which frequently infects individuals who are

already immunocompromised. The emergence of MRSA has increased interest in a

possible vaccine or preventative treatment against S aureus but attempts thus far have

not been successful.

The apparent inability of the immune system to stimulate an effective and lasting

adaptive immune response poses a difficulty for development of a prophylactic S.

aureus vaccine. However, knowledge of the immune evasion strategies of the

bacterium will enable us to identify specific aspects of its pathogenesis that can be

targeted for use in a therapeutic context. Kim et al found that when mice were

inoculated with a non-functioning variant of staphylococcal Protein A, antibodies

were raised against wildtype S. aureus and the mice became resistant to virulent

strains of MRSA, indicating the potential use of Protein A as a possible vaccine

antigen (Kim et al, 2010). The S. aureus -haemolysin has also been identified as a

possible antigen that could be used in vaccination (Wardenburg and Schneewind,

2008).

Beyond vaccination, the proteins used by S. aureus to evade the immune system show

promise as anti-inflammatory agents for use in autoimmune disorders. In particular,

Eap has been identified as a potential anti-inflammatory agent that could be used

against autoimmune disorders such as multiple sclerosis and rheumatoid arthritis

(Chavakis et al, 2007). Eap demonstrates potent anti-inflammatory activity by binding

antagonistically to ICAM-1 on endothelial cells, preventing the transmigration of

leukocytes across the endothelium and into tissues (Chavakis et al 2002). Studies on

the effect of Eap on the mouse equivalent of multiple sclerosis, experimental

autoimmune encephalomyelitis, indicated that administration of Eap prevented the

development of the disease and even reversed the symptoms of the disease in mice

that had already developed it (Xie et al, 2006). Similarly, CHIPS could potentially be

used to treat sepsis, as blockade of the C5a receptor (a function of CHIPS) has been

shown to reduce mortality in sepsis (Czermak et al, 1999; Guo and Ward, 2006).

Furthermore, the staphylococcal protein FLIPr binds to FPRL1 (Table 2), which has

systemic amyloidosis (Chavakis et al, 2007). FLIPr and its homolog FLIPr-like may

have potential uses in the treatment of these diseases, as they bind antagonistically to

FPRL1 (Prat et al, 2006; Prat et al, 2009). Further research in these areas will be

required to ascertain the therapeutic usefulness of these proteins, but it is possible that

certain proteins of the pathogen, which are intended to cause infection, may in fact be

used to our advantage.

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