defensins in the immunology of bacterial infections

Defensins in the immunology of bacterial infectionsAlfredo Menendez and B Brett Finlay

Defensins are a component of the host response against

bacterial infections. Multiple studies suggest a linked

upregulation of b-defensins and pro-inflammatory cytokines

expression in various tissues, as well as the possibility of

mutual induction. Recent data demonstrate the importance of

nucleotide-binding oligomerization proteins for the expression

of defensins, and associate low levels of a-defensins

expression by intestinal Paneth cells with susceptibility to

Crohn’s disease of the ileum. A novel anti-toxin activity has

been identified for several a- and u-defensins, expanding the

repertoire of the antimicrobial functions of defensins. It has

been shown that bacterial proteins can inactivate the action of

defensins, and that pathogen type III secretion systems (T3SS)

manipulate defensins expression via T3SS-mediated inhibition

of the NF-kB pathway.

Addresses

Michael Smith Laboratories, The University of British Columbia, 301-

2185 East Mall, Vancouver, BC, Canada V6T 1Z4

Corresponding author: Brett Finlay, B. ([email protected])

Current Opinion in Immunology 2007, 19:385–391

This review comes from a themed issue on

Host–pathogen interactions

Edited by Louis Schofield and Ed Pearce

0952-7915/$ – see front matter

# 2007 Elsevier Ltd. All rights reserved.

DOI 10.1016/j.coi.2007.06.008

IntroductionDefensins are one of the two most common types of

antimicrobial peptides (AMPs). The term ‘defensin’ was

coined by Ganz et al. [1] and Selsted et al. [2], to refer to

three short peptides with antimicrobial activity isolated

from human neutrophil granules [1,2]. Since then, a large

number of defensins and defensin-like peptides have

been identified in many organisms. As of February 2007,

304 entries had been recorded in a defensin database

compiled by the Bioinformatics Institute of Singapore

(http://defensins.bii.a-star.edu.sg) [3]. The ubiquitous dis-

tribution of defensins and defensin-like peptides across the

biological world is testimony to their crucial role in protec-

tion against pathogens.

Defensins from vertebrates are small in size, are cationic,

and have three intra-molecular disulfide bonds, mediated

by six conserved cysteines [4�]. There are two main

subfamilies: the a- and the b-defensins, based on the

www.sciencedirect.com

cysteine pairing and the length of the peptide fragments

between the cysteines. All members of these two sub-

families whose structure has been analyzed show a

similar, distinctive ‘defensin fold’ composed of a predo-

minant three-stranded b-sheet [5–7]. A third group of

defensins, the u-defensins are only found in non-human

primates, and are structurally unrelated to the a and b

subfamilies [8,9]. A paradoxical feature of a number of

defensins is the lack of in vitro antimicrobial activity in

physiological conditions or at the observed physiological

concentrations, but in addition to direct microbial killing,

defensins perform other functions related to both the

innate and adaptive immune responses (reviewed in [4�]).

Herein, recent advances in the understanding of the

biology of defensins (mainly human defensins) are

described in the context of bacterial infections. Most of

the work reviewed has been published during the last two

years, and centers on the immunoregulatory functions of

defensins, rather than on their bactericidal properties. We

highlight new findings regarding the expression of defen-

sins as components of the overall innate immune

response, as well as their role in protection from bacterial

infections and their possible connection with suscepti-

bility to disease. In addition, several mechanisms used by

bacteria to antagonize their antimicrobial functions are

also examined.

Defensins expression can be induced inresponse to bacterial infectionDefensins are expressed in multiple tissues in the body,

most notably leukocytes and epithelial surfaces. In

humans, six a- and four b-defensins have been charac-

terized in some detail. The expression and release of

defensins is differentially regulated, depending on the

defensin molecule, the cell type, and the microenviron-

mental stimuli [10–14]. Detection of bacteria may trigger

the induction and release of defensins, mediated either

by the engagement of toll-like receptors (TLR) by their

cognate bacterial pathogen-associated molecular patterns

(PAMP), or by any of several TLR-independent path-

ways (reviewed in [15�]). Bactericidal concentrations of a-

defensins are found in the phagocytic vacuoles of human

neutrophils after being delivered from cytoplasmic gran-

ules, and in the crypts of the small intestine following

their secretion by Paneth cells. There is no indication

thus far that the expression of the myeloid a-defensins

human neutrophil protein (HNP)-1-3 vary in any signifi-

cant way [4�]. Moreover, enteric mouse a-defensins

(cryptdins) levels are generally invariant ([16] and refer-

ences therein), although the normal, constitutive expres-

sion of cryptdin-4 and cryptdin-10 requires signaling


mailto:[email protected]

http://dx.doi.org/10.1016/j.coi.2007.06.008

http://defensins.bii.a-star.edu.sg/

386 Host–pathogen interactions

mediated by nucleotide-binding oligomerization domain-

2 (Nod2), as has been demonstrated in Nod2-deficient

mice [17��], indicating some level of regulation mediated

by this PAMP receptor.

The human b-defensin 2 (hBD-2) is a well-known

example of a defensin induced by bacterial products,

and by pro-inflammatory cytokines normally produced

in response to infection [18,19,20�]. Human vaginal

epithelial cells respond to bacterial LPS and peptido-

glycan by inducing the expression of TNF-a and hBD-

2 [21], as do pulmonary epithelial cells upon infection

with live Mycobacterium bovis [22]. Infection of the

intestinal epithelial cell lines Caco-2 and HT-29 with

C. jejuni induces the expression of hBD-2 [23]. Sig-

naling through the IL-1 receptor (activated either by

IL-1a or IL-1b), or treatment with TNF-a, induce the

synthesis of hBD-2 by keratinocytes [24], epidermal

cultures [11], and pulmonary [18,25], uterine [26], gin-

gival [19], intestinal [23] and middle ear epithelial cells

[27]. The NF-kB pathway has been recognized as a

key component in the induction of hBD-2 expression

[18,20�,21,22], however, other studies have observed

induction mediated by the mitogen-activated protein

kinase (MAPK) pathways [19,27]. Thus, increased

expression of hBD-2 in epithelial cells is associated

with the pro-inflammatory response, a concept sup-

ported by the finding that the anti-inflammatory cyto-

kines IL-10 and IL-13 downregulate the synthesis of

hBD-2 in atopic dermatitis [28].

Expression of hBD-3 is induced in gastric epithelial cells

infected with Helicobacter pylori [29�], as well as in

primary human keratinocytes infected with Staphylococ-cus aureus [30] and in epidermal cultures treated with

supernatants from mononuclear blood leukocytes stimu-

lated with LPS or peptidoglycan [11]. The NF-kB path-

way does not appear to play such a prominent role in

promoting the expression of hBD-3, but instead, induc-

tion seems to be mainly mediated by the MAPK/p38 [30]

and the EGFR/ERK pathways [11,29�]. The regulation

of hBD-1 and hBD-4 synthesis is less well understood;

hBD-1 has been largely considered a constitutively

expressed defensin, but a recent observation of a rela-

tively low degree of induction in epidermal cultures,

suggests some sort of regulation by LPS and supernatants

from stimulated mononuclear blood leukocytes [11].

Moreover, hBD-1 upregulation by albumin, arginine

and isoleucine has been observed in human colon tumor

cells, by a pathway involving c-myc [31], and induction

by glucose has been reported in human renal cells [32].

Similar to hBD-2 and -3, hBD-4 expression is induced by

Ca2+, TNFa, IL-1b, phorbol 12-myristate 13-acetate and

heat-killed P. aeruginosa in primary keratinocytes [10].

The above indicates a general trend of b-defensin induc-

tion in response to bacteria, or the inflammatory response

they induce.


Defensins are tough; they can handlebacteriaThe role of defensins in bacterial infections is not limited

to bacterial killing; in fact, the immunomodulatory reper-

toire of defensins is likely as important as their killing

capabilities. It has been demonstrated that physiological

concentrations of b-defensins can increase the expression

of pro-inflammatory cytokines and chemokines by pur-

ified human peripheral blood mononuclear cells (PBMC)

[33�] and by human primary keratinocytes [34], in the

absence of infection or stimulation with bacterial pro-

ducts. In PBMC, hBD-1 or hBD-2 but not hBD-3, were

strong cytokine inducers [33�], whereas in keratinocytes

hBD-2, -3 and -4 but not hBD-1 upregulated most of the

genes tested [34]. Interestingly, hBD-2, -3 and -4 but not

hBD-1 increased phosphorylation of EGFR, STAT1 and

STAT3, and keratinocyte proliferation and migration

[34]. These results strongly suggest that b-defensins

are critical to expand the initial cytokine response of

inflammation and wound healing at sites of injury or

infection (Figure 1). Another recent report showed that

hBD-3 and hBD-4 induced chemotaxis and degranulation

of rat and human mast cells, as well as phosphorylation of

MAPK/p38 and ERK1/2 [35].

Unambiguous proof of specific functions of a particular

defensin in vivo are difficult to obtain, mainly because

of the overlap of antibacterial mechanisms operating

simultaneously in most tissues, and the redundancy of

defensins. Nonetheless, altered levels of defensins at sites

of active infection and inflammation have generally been

interpreted as an indication of their role in antagonizing

bacterial infections in vivo [36–40]. Reduced expression of

Paneth cell a-defensins HD-5 and HD-6 has been

observed in ileal Crohn’s disease [37��], while low hBD-

2 gene copy number resulting from genetic polymorphism

at the defensin locus in chromosome 8, is associated with

Crohn’s disease of the colon [41�]. Although a causal

relation was not established in either of these studies, they

support the interpretation that decreased expression of

defensins affects the innate immunity of the bowel and

the control of the microbiota, thereby contributing to the

establishment and/or maintenance of the chronic inflam-

mation characteristic of Crohn’s disease.

Increasing the levels of porcine (p) BD-1 in the lungs of

piglets (by administration of synthetic pBD-1) provided

protection from challenge with B. pertussis [42]. Over-

expression of rat BD-2 in the lungs of rats has conferred

protection against P. aeruginosa pneumonia and sepsis-

induced lung injury [43]. However, other studies using

BD-1-deficient mice have found only a modest role of

BD-1 in protection, in the form of delayed clearance of H.influenzae from the lungs of knockout mice [44], and

increased numbers of Staphylococcus sp. in the bladder,

without any difference in the clearance of S. aureus from

the lungs of infected animals [45].


Defensins in the immunology of bacterial infections Menendez and Finlay 387

Figure 1

Amplification of b-defensin expression may occur through local autocrine/paracrine loops. At sites of infection, bacteria are detected by

phagocytes (A) and induce the synthesis and secretion of pro-inflammatory mediators (e.g. IL-1 and TNF-a). Infected cells (e.g. a keratinocyte, B)

also detect bacteria through interactions with LPS and peptidoglycan (PG), resulting in synthesis of defensins and more pro-inflammatory

mediators, which can act in an autocrine fashion (dashed black arrows), or in a paracrine fashion (solid black arrows) on neighboring cells (C),

and promote further induction of defensins and cytokines.

Experiments with defensin-deficient animals have

suggested that a-defensins are involved in protection

from enteric bacterial infections in vivo. Loss of Paneth

cell defensins due to a knockout of the processing enzyme

matrilysin (MMP-7) gene improved the survival of

luminal E. coli, and rendered the deficient mice more

susceptible to infection and killing by S. typhimurium [46].

MMP-7-deficient mice also had higher bacterial burden

than controls in the small intestine but not in spleen, lung

and kidneys, in a model of vaginal infection with C.trachomatis, suggesting again that the loss of Paneth cell

defensins has a severe influence on the intestinal innate

immunity [47]. The strongest evidence yet of a defensin-

mediated protection in vivo has been provided by Salz-

man et al. [48], who found that mice transgenic for the

enteric HD-5 were markedly resistant to oral challenge

with S. typhimurium [48].

Recently, a novel toxin inhibition function has been

described for the human a-defensins HNP-1-3


[49��,50��], and for retrocyclins (RC), a group of synthetic

u-defensins [51��]. Lethal toxin (LT) from B. anthracis is a

virulence factor with zinc-dependent metalloproteinase

activity, which targets selected MAPK kinases of host

cells. HNP-1-3 are capable of neutralizing LT and protect

from toxin-induced death in vitro and in vivo [49��]; HNP-

1-3 also inhibit diphtheria toxin (DT) and Pseudomonasendotoxin A (ETA), two bacterial toxins with mono-

ADP-ribosyltransferase activity, and protect cells from

DT-induced or ETA-induced cell death [50��]. More-

over, Wang et al. demonstrated that RC-1, -2 and -3, as

well as HNP-1 not only killed B. anthracis bacilli, but also

inhibited the enzymatic activity of LT [51��].

But bacteria are also tough, andthey retaliateDefensins, and in general AMPs, are a pervasive threat to

microbes. Thus, not surprisingly, a significant number of

pathogen-encoded virulence mechanisms function to

antagonize the action and effects of AMPs (see Table 1



Table 1

Recently described examples of bacterial antagonism of the antimicrobial actions of defensins

Bacterium Virulence factor Mechanism Reference

Streptococcus pyogenes Streptolysin O Degranulation of neutrophils [70]

Streptococcus pyogenes Streptococal inhibitor of complement (SIC) Inhibition of HNP-1, hBD-1, hBD-2, hBD-3 [52,53]

Streptococcus pyogenes Distantly related to SIC (DRS) Inhibition of HNP-1, hBD-2, hBD-3 [53]

Streptococcus agalactiae Penicillin-binding Protein 1a (PBP1a) Not determined [58]

Salmonella typhimurium Alternative sigma factor sE Induction of alternative electron transport pathway(s) [71]

Listeria monocytogenes VirR (part of the two components

system VirR/VirS)

Positive regulation of mprF and the dlt operon

(involved in cell surface charge modification)

[72]

Listeria monocytogenesa Peptidoglycan N-deacetylase (PgdA) N-deacetylation of peptidoglycan [59]

Bordetella bronchiseptica T3SS Inhibition of the NFk-B pathway [60]

a Effect on defensin expression not experimentally demonstrated, but very likely.

for recently reported examples). Several highly virulent

strains of Streptococcus pyogenes (group A streptococci)

secrete proteins that have a direct effect on the host

immune function. For example, the streptococcal inhibitor

of complement (SIC) binds to and inhibits the antimicro-

bial activities of HNP-1, hBD-1, -2 and -3 in vitro [52,53].

Mutants of SIC have shown significant reduction of throat

colonization in a mouse infection model, suggesting an

important role of SIC on S. pyogenes virulence [54]. Another

S. pyogenes protein, DRS (Distantly Related to SIC) also

inactivates HNP-1, hBD-2 and hBD-3 [53]. In addition, S.pyogenes secretes streptolysin O (SLO), a cholesterol-de-

pendent cytolytic toxin that promotes degranulation of

neutrophils and the release of their a-defensins [55].

Alpha-defensins stored in the azurophil granules of neu-

trophils are normally released into phagosomes, where they

(and other AMPs) reach high concentrations that are effec-

tively antimicrobial. Thus, the SLO-induced premature

release of a-defensins may function as a mechanism to

disarm the neutrophils from a safe distance, much the same

as the staphylokinase of S. aureus [56]. Other cholesterol-

dependent cytolysins are expressed by pathogenic Bacillus,Clostridium and Listeria [57]. It will be interesting to deter-

mine whether their toxins have a similar neutrophil degra-

nulation function.

S. agalactiae (group B streptococci) produces a surface-

associated penicillin-binding protein (PBP1a), which is

essential for bacterial virulence. PBP1a promotes resist-

ance of S. agalactiae to HNP-1 and other AMPs [58] by a

yet unknown mechanism that does not involve cell

envelope charge modification such as those promoted

by dltABCD and mprF . Recently, Boneca et al. demon-

strated that peptidoglycan N-deacetylation functions as

an important virulence mechanism in L. monocytogenesinfections [59�]. This type of cell surface modification

makes Listeria more resistant to killing by lysozyme, and

impairs the detection of peptidoglycan by Nod1 and

Nod2. Since Nod proteins are required for expression

of some cryptdins in the mouse ileum [17��] and for

induction of human b-defensin synthesis [20�,29�], it is

conceivable that peptidoglycan N-deacetylation is also

involved in preventing defensin expression.


T3SS effector-mediated inhibition of the MAPK and NFk-

B pathways is becoming a recognized bacterial strategy

to interfere with the induction of the innate immune

response. A connection between T3SS effectors and

antagonism of defensin function has been recently uncov-

ered [60��]. A T3SS from Bordetella bronchiseptica has been

implicated in inhibiting the expression of the bovine b-

defensin Tracheal Antimicrobial Peptide (TAP), in

primary bovine tracheal epithelial cells [60��]. This down-

regulation occurred through the inhibition of NFk-B-

mediated induction of TAP expression, at some point

along the pathway that leads to the nuclear translocation

of NFk-B [60��]. From these results, it can be predicted

that T3SS effectors recently found to inactivate NFk-B,

such as VP1686 (V. parahaemolyticus), YopJ (Yersinia), AopP

(Aeromonas) and OspF (Shiguella) [61��–65��], may also

mediate the inhibition of defensin synthesis.

ConclusionsAn increasing number of bacterial virulence factors are

being associated with antagonizing the effects of AMPs; it

is also apparent that some mechanisms of resistance are

shared by pathogens from different taxonomical groups. A

recent report showing acquisition of adaptive resistance to

hBD-1-4 by Porphyromonas gingivalis [66] is especially

disturbing, since such a mechanism could have a pro-

found impact in microbial persistence. The transcription

factor NF-kB is an essential component of the AMP

induction pathways, as well as for many other aspects

of the inflammatory response; therefore, the involvement

of a T3SS in resistance to AMPs via the inhibition of NF-

kB function is not unexpected; in fact, we predict that

more of such examples will be reported shortly.

The findings that hBD-2, -3 and -4 can activate the

MAPK/p38 and EGFR pathways [34,35] suggests the

existence of positive feedback regulation in the control

of defensin expression, since these pathways can also be

involved in the induction of hBD-2 and hBD-3 in various

cell types. In addition, pro-inflammatory cytokines can

induce the synthesis of defensins whereas, defensins in

turn may upregulate the synthesis of cytokines and che-

mokines, and influence the expression of other AMPs



[67]. Such regulatory loops would act in an autocrine or

paracrine manner, and most likely, involve the participa-

tion of several cell types (Figure 1). Little is known about

how defensin expression is turned down and their homeo-

stasis restored, although it may be anticipated that some

of the mechanisms turning off other elements of the

inflammatory response may act directly or indirectly to

downregulate the expression of defensins. This is an

important area to investigate since defensins can reach

cytotoxic levels in vivo [68], and sustained overexpression

of mBD-6 induces muscle degeneration in a transgenic

mouse model [69]. In addition, studies of possible con-

nections between altered defensin expression and chronic

inflammatory conditions will be interesting. In summary,

the expression and functions of defensins are intimately

associated with other aspects of the innate immunity; and

in many respects, defensins emulate the behavior of other

elements of the inflammatory response. As we learn more

about their functions and how their expression is regulated,

it becomes apparent that (at least in vertebrates) defensins

should be considered an integral component of the host

inflammatory response to bacterial infection.

AcknowledgementsThis work was supported by operating grants to B.B.F. from theCanadian Institutes of Health Research (CIHR), the Howard HughesMedical Institute (HHMI) and the Foundation for NIH, as part ofthe Bill and Melinda Gates Grand Challenge program. AM is a recipientof postdoctoral fellowships from the Michael Smith Foundation forHealth Research (MSFHR), and the Natural Sciences and EngineeringResearch Council of Canada. B.B.F. is a CIHR Distinguished Investigator,an HHMI International Research Scholar and the University of BritishColumbia Peter Wall Distinguished Professor. We would like to thankthank M. Chow and members of the Finlay laboratory for critical readingof the manuscript.

References and recommended readingPapers of particular interest, published within the annual period ofreview, have been highlighted as:

� of special interest

�� of outstanding interest

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4.�

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29.�

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33.�

Boniotto M, Jordan WJ, Eskdale J, Tossi A, Antcheva N,Crovella S, Connell ND, Gallagher G: Human beta-defensin 2induces a vigorous cytokine response in peripheral bloodmononuclear cells. Antimicrob Agents Chemother 2006,50:1433-1441.

This paper provides a detailed set of data showing the cytokine synth-esis response of PBMC after exposure to hBD-1, -2 and -3. It alsodelineates differences in the cytokine expression patterns induced bythese three defensins. These are interesting findings that may alsocontribute to explain the diversity of the defensins gene pool withinhumans and mice.

34. Niyonsaba F, Ushio H, Nakano N, Ng W, Sayama K, Hashimoto K,Nagaoka I, Okumura K, Ogawa H: Antimicrobial peptides humanbeta-defensins stimulate epidermal keratinocyte migration,proliferation and production of proinflammatory cytokinesand chemokines. J Invest Dermatol 2007, 127:594-604.

35. Chen X, Niyonsaba F, Ushio H, Hara M, Yokoi H, Matsumoto K,Saito H, Nagaoka I, Ikeda S, Okumura K et al.: Antimicrobialpeptides human beta-defensin (hBD)-3 and hBD-4 activatemast cells and increase skin vascular permeability.Eur J Immunol 2007, 37:434-444.

36. Harada K, Ohba K, Ozaki S, Isse K, Hirayama T, Wada A,Nakanuma Y: Peptide antibiotic human beta-defensin-1 and -2contribute to antimicrobial defense of the intrahepatic biliarytree. Hepatology 2004, 40:925-932.


37.��

Wehkamp J, Salzman NH, Porter E, Nuding S, Weichenthal M,Petras RE, Shen B, Schaeffeler E, Schwab M, Linzmeier R et al.:Reduced Paneth cell alpha-defensins in ileal Crohn’s disease.Proc Natl Acad Sci U S A 2005, 102:18129-18134.

A clear association between Crohn’s disease of the ileum and a specificdecrease of defensins synthesis had not been previously demonstrated.Here, the levels of HD-5 and HD-6 in intestinal specimens from diseasedhuman subjects were analyzed and found to be lower than in non-diseased controls. The reduction on HD-5 and HD-6 was further exa-cerbated in some specimens also harboring mutations in the NOD2 gene.So far the clearest indication of defensins alteration in Crohn’s. Alsodocuments some fundamental differences between Crohn’s disease ofthe ileum and the colon (see also annotation to Ref. [41]).

38. Rivas-Santiago B, Sada E, Tsutsumi V, Aguilar-Leon D,Contreras JL, Hernandez-Pando R: beta-Defensin geneexpression during the course of experimental tuberculosisinfection. J Infect Dis 2006, 194:697-701.

39. Valore EV, Wiley DJ, Ganz T: Reversible deficiency ofantimicrobial polypeptides in bacterial vaginosis. Infect Immun2006, 74:5693-5702.

40. Yanagi S, Ashitani J, Imai K, Kyoraku Y, Sano A, Matsumoto N,Nakazato M: Significance of human beta-defensins in theepithelial lining fluid of patients with chronic lower respiratorytract infections. Clin Microbiol Infect 2007, 13:63-69.

41.�

Fellermann K, Stange DE, Schaeffeler E, Schmalzl H, Wehkamp J,Bevins CL, Reinisch W, Teml A, Schwab M, Lichter P et al.: Achromosome 8 gene-cluster polymorphism with low humanbeta-defensin 2 gene copy number predisposes to Crohndisease of the colon. Am J Hum Genet 2006, 79:439-448.

A genetic explanation to the hBD-2 deficiency observed in colonicCrohn’s disease.

42. Elahi S, Buchanan RM, Attah-Poku S, Townsend HG, Babiuk LA,Gerdts V: The host defense peptide beta-defensin 1 confersprotection against Bordetella pertussis in newborn piglets.Infect Immun 2006, 74:2338-2352.

43. Shu Q, Shi Z, Zhao Z, Chen Z, Yao H, Chen Q, Hoeft A, Stuber F,Fang X: Protection against Pseudomonas aeruginosapneumonia and sepsis-induced lung injury by overexpressionof beta-defensin-2 in rats. Shock 2006, 26:365-371.

44. Moser C, Weiner DJ, Lysenko E, Bals R, Weiser JN, Wilson JM:beta-Defensin 1 contributes to pulmonary innate immunity inmice. Infect Immun 2002, 70:3068-3072.

45. Morrison G, Kilanowski F, Davidson D, Dorin J: Characterizationof the mouse beta defensin 1, Defb1, mutant mouse model.Infect Immun 2002, 70:3053-3060.

46. Wilson CL, Ouellette AJ, Satchell DP, Ayabe T, Lopez-Boado YS,Stratman JL, Hultgren SJ, Matrisian LM, Parks WC: Regulation ofintestinal alpha-defensin activation by the metalloproteinasematrilysin in innate host defense. Science 1999,286:113-117.

47. Pal S, Schmidt AP, Peterson EM, Wilson CL, de la Maza LM: Roleof matrix metalloproteinase-7 in the modulation of aChlamydia trachomatis infection. Immunology 2005,117:213-219.

48. Salzman NH, Ghosh D, Huttner KM, Paterson Y, Bevins CL:Protection against enteric salmonellosis in transgenic miceexpressing a human intestinal defensin. Nature 2003,422:522-526.

49.��

Kim C, Gajendran N, Mittrucker HW, Weiwad M, Song YH,Hurwitz R, Wilmanns M, Fischer G, Kaufmann SH: Human alpha-defensins neutralize anthrax lethal toxin and protect againstits fatal consequences. Proc Natl Acad Sci U S A 2005,102:4830-4835.

In this series of excellent papers (Refs. [49��–51��]), a novel biologicalfunction of a- and u-defensins is characterized. These defensins showedefficient inhibition of the enzymatic activity of two different types ofbacterial toxins, and by virtue of it, protection from the toxins lethaleffects, in vitro and in vivo. Breakthrough papers that expand the knownantimicrobial repertoire of defensins.

50.��

Kim C, Slavinskaya Z, Merrill AR, Kaufmann SH: Humanalpha-defensins neutralize toxins of the mono-ADP-ribosyltransferase family. Biochem J 2006, 399:225-229.



51.��

Wang W, Mulakala C, Ward SC, Jung G, Luong H, Pham D,Waring AJ, Kaznessis Y, Lu W, Bradley KA et al.: Retrocyclins killbacilli and germinating spores of Bacillus anthracis andinactivate anthrax lethal toxin. J Biol Chem 2006,281:32755-32764.

52. Binks MJ, Fernie-King BA, Seilly DJ, Lachmann PJ, Sriprakash KS:Attribution of the various inhibitory actions of thestreptococcal inhibitor of complement (SIC) to regions withinthe molecule. J Biol Chem 2005, 280:20120-20125.

53. Fernie-King BA, Seilly DJ, Lachmann PJ: Inhibition ofantimicrobial peptides by group A streptococci: SIC and DRS.Biochem Soc Trans 2006, 34:273-275.

54. Lukomski S, Hoe NP, Abdi I, Rurangirwa J, Kordari P, Liu M,Dou SJ, Adams GG, Musser JM: Nonpolar inactivation of thehypervariable streptococcal inhibitor of complement gene(sic) in serotype M1 Streptococcus pyogenes significantlydecreases mouse mucosal colonization. Infect Immun 2000,68:535-542.

55. Nilsson M, Sorensen OE, Morgelin M, Weineisen M, Sjobring U,Herwald H: Activation of human polymorphonuclearneutrophils by streptolysin O from Streptococcus pyogenesleads to the release of proinflammatory mediators. ThrombHaemost 2006, 95:982-990.

56. Jin T, Bokarewa M, Foster T, Mitchell J, Higgins J, Tarkowski A:Staphylococcus aureus resists human defensins byproduction of staphylokinase, a novel bacterial evasionmechanism. J Immunol 2004, 172:1169-1176.

57. Palmer M: The family of thiol-activated, cholesterol-bindingcytolysins. Toxicon 2001, 39:1681-1689.

58. Hamilton A, Popham DL, Carl DJ, Lauth X, Nizet V, Jones AL:Penicillin-binding protein 1a promotes resistance of group Bstreptococcus to antimicrobial peptides. Infect Immun 2006,74:6179-6187.

59.�

Boneca IG, Dussurget O, Cabanes D, Nahori MA, Sousa S,Lecuit M, Psylinakis E, Bouriotis V, Hugot JP, Giovannini M et al.: Acritical role for peptidoglycan N-deacetylation in Listeriaevasion from the host innate immune system. Proc Natl AcadSci U S A 2007, 104:997-1002.

Description of an essential virulence function in Listeria, mediated by abacterial peptidoglycan modification that confers the ability to resistkilling by lysozyme, and escape detection by Nod proteins. Even thougha connection with the expression or functions of defensins was not tested,it is likely to exist since there is evidence that Nod proteins may regulatethe expression of defensins (see Refs. [17��,20�,29�]).

60.��

Legarda D, Klein-Patel ME, Yim S, Yuk MH, Diamond G:Suppression of NF-kappaB-mediated beta-defensin geneexpression in the mammalian airway by the Bordetella type IIIsecretion system. Cell Microbiol 2005, 7:489-497.

This is the only report (in a surprisingly unexplored area) that associatesthe presence of a functional T3SS with a decreased ability to producedefensins (or any other AMPs, for that matter). This suggests thateffectors translocated to the host cell can block the inducible AMPscomponent of the host response, a very attractive virulence trait, whichwill be probably found on many pathogens. This study brings togetherthe facts that NFk-B is a key player in the expression of some AMPs,and the inhibition of this pathway by T3SS effectors. We are onlybeginning to understand how these effectors do it (see Refs. [61��–65��]).


61.��

Bhattacharjee RN, Park KS, Kumagai Y, Okada K, Yamamoto M,Uematsu S, Matsui K, Kumar H, Kawai T, Iida T et al.: VP1686, aVibrio type III secretion protein, induces toll-like receptor-independent apoptosis in macrophage through NF-kappaBinhibition. J Biol Chem 2006, 281:36897-36904.

The mechanism(s) by which T3SS effectors inactivate the NFk-B andMAPK pathways were not characterized, until now. As is demonstrated inthis series of papers (Refs. [61��–65��]), bacteria have evolved severalways to do it. The end point is the suppression of the pro-inflammatorygene expression, whether is achieved by inactivation of MAPKs throughacetylation or dephosphorylation, or direct interaction of the T3SS effec-tor with NFk-B. In any case, these are novel functions described for T3SSeffectors, which likely are a common virulence trait (see also annotationfor Ref. [60��]).

62.��

Mukherjee S, Keitany G, Li Y, Wang Y, Ball HL, Goldsmith EJ,Orth K: Yersinia YopJ acetylates and inhibits kinase activationby blocking phosphorylation. Science 2006, 312:1211-1214.

63.��

Mittal R, Peak-Chew SY, McMahon HT: Acetylation of MEK2 and Ikappa B kinase (IKK) activation loop residues by YopJ inhibitssignalling. Proc Natl Acad Sci U S A 2006, 103:18574-18579.

64.��

Fehr D, Casanova C, Liverman A, Blazkova H, Orth K,Dobbelaere D, Frey J, Burr SE: AopP, a type III effector protein ofAeromonas salmonicida, inhibits the NF-kappaB signallingpathway. Microbiology 2006, 152:2809-2818.

65.��

Arbibe L, Kim DW, Batsche E, Pedron T, Mateescu B, Muchardt C,Parsot C, Sansonetti PJ: An injected bacterial effector targetschromatin access for transcription factor NF-kappaB to altertranscription of host genes involved in immune responses. NatImmunol 2007, 8:47-56.

66. Shelburne CE, Coulter WA, Olguin D, Lantz MS, Lopatin DE:Induction of beta-defensin resistance in the oral anaerobePorphyromonas gingivalis. Antimicrob Agents Chemother 2005,49:183-187.

67. Stroinigg N, Srivastava MD: Modulation of toll-like receptor 7and LL-37 expression in colon and breast epithelial cells byhuman beta-defensin-2. Allergy Asthma Proc 2005, 26:299-309.

68. Wencker M, Brantly ML: Cytotoxic concentrations of alpha-defensins in the lungs of individuals with alpha(1)-antitrypsindeficiency and moderate to severe lung disease. Cytokine2005, 32:1-6.

69. Yamaguchi Y, Nagase T, Tomita T, Nakamura K, Fukuhara S,Amano T, Yamamoto H, Ide Y, Suzuki M, Teramoto S et al.: {beta}-defensin overexpression induces progressive muscledegeneration in mice. Am J Physiol Cell Physiol 2007,292:C2141-C2149.

70. Kraus D, Peschel A: Molecular mechanisms of bacterialresistance to antimicrobial peptides. Curr Top MicrobiolImmunol 2006, 306:231-250.

71. Crouch ML, Becker LA, Bang IS, Tanabe H, Ouellette AJ, Fang FC:The alternative sigma factor sigma is required for resistanceof Salmonella enterica serovar Typhimurium to anti-microbialpeptides. Mol Microbiol 2005, 56:789-799.

72. Mandin P, Fsihi H, Dussurget O, Vergassola M, Milohanic E,Toledo-Arana A, Lasa I, Johansson J, Cossart P: VirR, a responseregulator critical for Listeria monocytogenes virulence. MolMicrobiol 2005, 57:1367-1380.


defensins in the immunology of bacterial infections

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