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158 NATURE MEDICINE • VOLUME 7 • NUMBER 2 • FEBRUARY 2001

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Shigellae control the Gates of LLPathogenic bacteria use a variety of mechanisms to combat the host immune response. New data indicate that Shigella spp.

make a preemptive strike against the deployment of host antibacterial peptides (pages 180–185).

Humans and other animals have mul-tiple innate mechanisms to combat

microbial incursions, and successfulpathogens have evolved various strate-gies to overcome them. Antibacterialpeptides such as LL-37 and human β-defensins (HBD) are produced by ep-ithelial cells and induced duringinflammation and infection at mucosalsurfaces. In this issue of NatureMedicine, Islam et al.1 offer the firstdemonstration that a pathogen cancombat the host immune response bysuppressing production of these en-dogenous antimicrobial peptides. Theyreport that in vivo enteric infec-tions caused by Shigella dysenteriaetype I or S. flexneri suppressed theability of rectal epithelial cells toproduce LL-37. Moreover, in vitrostudies using human cell lines in-dicate that this effect was mediatedby Shigella plasmid DNA.

Shigella spp. (S. dysenteriae, S.flexneri, S. boydii and S. sonnei) arehighly contagious bacteria thatcause dysentery and are especiallydangerous to infants and children.In spreading throughout thecolonic epithelia, they use a vari-ety of tricks to establish infection2.By injecting host cells with in-vasins via a specialized (type-III)secretory system, they enhancetheir ability to enter these cells.Once inside a host cell, they escapevacuolar confinement to enter the cy-toplasm and proliferate, before hijack-ing the cell’s motor machinery to enterneighboring epithelial cells.3 Finally,their exit strategy—production of an exotoxin that causes copious diarrhea—disseminates multi-tudes of infectious bacteria.

Islam et al. studied children andadults with this form of diarrhea, per-forming serial rectal biopsies through-out the course of their illness1. Forethical reasons, the 15 normal controlbiopsies were obtained only fromadults. Rectal epithelial cells from 13 of15 control specimens expressed LL-37,while 15 out of 15 expressed HBD-1. Incontrast, only 3 of 8 epithelial cell sam-ples from adults with S. dysenteriae in-fection expressed LL-37 on the first day

of illness. In subsequent biopsies, onday 11 none of the samples were posi-tive, and on day 60 only 2 of the sam-ples were positive for LL-37 expression.Thus, interference with epithelial cellproduction of LL-37 was rapid in onsetand surprisingly long in duration.None of the rectal epithelial cells takenfrom taken from children with S. dysen-teriae infection on their 13th day of ill-ness expressed LL-37. The long-lasting

suppressive effect is surprising, sinceinduction of antimicrobial peptidesgenerally occurs within a few hoursafter challenge by living bacteria orlipopolysaccharides.

The study was also performed onchildren with watery diarrhea that wasnot associated with Shigella (orSalmonella) infection. On the first dayof illness, rectal epithelial cells fromonly 1 of 8 biopsies expressed LL-37,indicating that this suppression is notunique to Shigella infection. Since age-matched pediatric controls were un-available, it may be that LL-37expression by rectal epithelial cells isnormally absent in some children, per-haps for reasons related to maturation,nutrition or prior infection. Such possi-bilities merit examination in animal

models, such as in newborn or knock-out mice. Not only do mice express anα-helical cathelicidin (CRAMP) mole-cule that is homologous to human LL-37 (ref. 4), they also expresshomologues of HBD-2 and HBD-3 (refs.5–9).

Many antimicrobial peptides, in ad-dition to LL-37, are present in thehuman digestive tract. Some enter viasecretions from salivary glands (his-tatins), small intestinal Paneth cells (α-defensins HD-5 and HD-6) and thepancreas (HBD-1). Others are expressedby epithelial cells in the mouth and

esophagus (calprotectin), tongue(HBD-1 and HBD-3) and small in-testine (HBD-1 and HBD-2).Intestinal secretions also containantimicrobial proteins, includinglactoferrin, lysozyme (murami-dase), secretory phospholipase A2,secretory leukoprotease inhibitor(SLPI) and so forth. This multiplic-ity of potential effector moleculescomplicates the assessment of datafrom systems that may lack one ortwo of these peptides.

The authors found that moder-ate concentrations of LL-37 pep-tide could kill greater than 97% ofShigella spp. in vitro within threehours and suggested that early invivo lysis of some Shigella bacteriamight release plasmid DNA thatwould suppress local production of

LL-37, disarming the epithelial cellsand allowing them to be invaded by or-ganisms that came later1. It would behelpful to learn about the expression ofother inducible antimicrobial peptidesin the human colon (such as HBD-2and HBD-3), and determine if Shigellainfections can suppress their expres-sion as well.

Although LL-37 has not been studiedas widely as defensins, it clearly meritsattention as a potent natural antibi-otic. The precursor of LL-37, hCAP-18,contains a highly conserved cathelindomain that is also found in antimicro-bial peptide precursors (known ascathelicidins) from other mammals.hCAP-18 is expressed constitutivelywithin the epididymis and by the bonemarrow precursors of neutrophils, and

ROBERT I. LEHRER

Shigella flexneri escaping from an infected epithelial cell

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NATURE MEDICINE • VOLUME 7 • NUMBER 2 • FEBRUARY 2001 159

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undergoes proteolytic processing thatreleases LL-37 from its carboxy termi-nus. Skin keratinocytes do not expresshCAP-18/LL-37 under normal condi-tions, but only produce it in responseto injury, inflammation or infection10.LL-37 has a strong affinity forlipopolysaccharide, and exhibits po-tent, broad-spectrum antibacterial ac-tivity that is affected little by the ionicconcentrations found in extracellularfluids11.

Limited information exists about thesignals, receptors and transductionmechanisms involved in the inductionof LL-37 and other mammalian catheli-cidins. The promoter region of the LL-37 gene contains potential bindingsites for acute-phase-response factors12.Lipopoly-saccharides, interleukin-6and all-trans retinoic acid were re-ported to increase expression of twoporcine cathelicidins13. Recently, theexistence of murine and human Toll-like receptors (TLR-9) that recognizebacterial CpG DNA was reported14. Themechanism by which Shigella plasmidsmay interact with this receptor or itsregulatory network to prevent hCAP-18

induction remains to be resolved.If the ability of Shigella spp. to pre-

vent hCAP-18/LL-37 production by ep-ithelial cells resides on a plasmid, onewonders if similar plasmids exist inother bacteria or if the plasmid inShigella bacteria can be transferred tothem. Though these and many otherquestions remain to be addressed, thereport by Islam, et al.1 opens a new areain the study of pathogenesis. Futurefindings are awaited with interest.

1. Islam, D. et al. Nature Med. 7,180-185 (2001).2. Nhieu, G.T. & Sansonetti, P.J. Mechanism of

Shigella entry into epithelial cells. Curr. Opin.Microbiol. 2, 51−55 (1999).

3. Rathman, M., de Lanerolle, P., Ohayon, H.,Gounon, P. & Sansonetti, P. Myosin light chainkinase plays an essential role in S. flexneri dissem-ination. J. Cell Sci.113, 3375−3386 (2000).

4. Gallo, R.L. et al. Identification of CRAMP, a cathe-lin-related antimicrobial peptide expressed in theembryonic and adult mouse. J. Biol. Chem. 272,13088−13093 (1997).

5. Morrison, G.M., Davidson, D.J. & Dorin, J.R. Anovel mouse β defensin, Defb2, which is upregu-lated in the airways by lipopolysaccharide. FEBSLett. 442, 112−116 (1999).

6. Bals, R. et al. Mouse β-defensin 3 is an inducibleantimicrobial peptide expressed in the epitheliaof multiple organs. Infect. Immun. 67, 3542−3547 (1999).

7. O’Neil, D.A. et al. Expression and regulation of thehuman β-defensins hBD-1 and hBD-2 in intesti-

nal epithelium. J. Immunol. 163, 6718−6724(1999).

8. Harder, J., Bartels, J., Christophers, E. & Schroder,J.M. Isolation and characterization of Human {β}-Defensin-3, a novel human inducible peptide an-tibiotic. J. Biol. Chem. (Nov 20, 2000, epub aheadof print).

9. Diamond, G., Kaiser, V., Rhodes, J., Russell, J.P. &Bevins, C.L. Transcriptional regulation of β-de-fensin gene expression in tracheal epithelial cells.Infect. Immun. 68, 113−119. (2000).

10. Frohm, M. et al. The expression of the gene cod-ing for the antibacterial peptide LL-37 is inducedin human keratinocytes during inflammatory dis-orders. J. Biol. Chem. 272, 15258−15263 (1997).

11. Turner, J., Cho, Y., Dinh, N.N., Waring, A.J. &Lehrer, R.I. Activities of LL-37, a cathelin-associ-ated antimicrobial peptide of human neu-trophils. Antimicrob. Agts. Chemother. 42,2206−2214 (1998).

12. Gudmundsson, G.H. et al. The human geneFALL39 and processing of the cathelin precursorto the antibacterial peptide LL-37 in granulo-cytes. Eur. J. Biochem . 238, 325−332 (1996).

13. Wu, H., Zhang, G., Minton, J.E., Ross, C.R. &Blecha, F. Regulation of cathelicidin gene expres-sion: induction by lipopolysaccharide, inter-leukin-6, retinoic acid, and Salmonella entericaserovar typhimurium infection. Infect. Immun.68, 5552−5558 (2000).

14. Hemmi, H. et al. A Toll-like receptor recognizesbacterial DNA. Nature 408, 740−745 (2000).

Department of MedicineUCLA School of MedicineLos Angeles, Californiaemail: rlehrer@mednet.ucla.edu

NEWS & VIEWS

The process of neurogenesis, now recog-nized to be an ongoing process in adultmammalian brain, requires a specificmolecular environment that neuroscien-tists are only beginning to understand.Neural stem cells are born in specializedregions of the adult brain such as thesubventricular zone (SVZ). They then mi-grate along specific pathways into theolfactory bulb (where they differentiateinto interneurons), as well as to corticalbrain regions. Little is known about thesignals present in brain regions such asthe SVZ that create these special neuro-genesis ‘niches’. In the December issueof Neuron, Lim et al. (Neuron 28,713–726) report that bone morpho-genetic protein (BMP) and its natural antagonist Noggin, both well-character-ized signaling molecules, are criticalcomponents of these local environmen-tal effects. The authors show that theependymal cells of the adult brain,which lie just adjacent to the SVZ, ex-press Noggin, whereas the SVZ itself ex-presses BMPs and BMP receptors.

The authors use an in vitro model tostudy the mechanisms behind SVZ-de-

rived cell differentiation. The pictureshows a neurogenic colony descendedfrom an adult SVZ precursor cell, cocul-tured with astrocytes. All the cells in thiscolony express the neuron-specific β-tubulin marker Tuj1 (red), and have in-corporated BrdU (green), a nuclearmarker of cell proliferation. The exten-sive area of double stain (yellow) indi-cates the proliferation of these neuronalprecursors. Using this culture system,Lim et al. show that BMPs inhibit neuro-genesis in a cell-autonomous manner,and instead induce the SVC cells to dif-ferentiate into glia. Addition of the BMPantagonist Noggin to SVC cultures, onthe other hand, increased the birth ofneurons by up to 50%. The authors alsoused adenoviral vectors to direct the invivo overexpression of BMP in theependymal cells that lie adjacent to theSVZ. They found that this inappropriateexpression of BMP reduced cell prolifera-tion and halted SVZ neuroblast regener-ation. Conversely, adenovirus-mediatedexpression of Noggin was able to acti-vate neuronal proliferation of SVZ cellstransplanted into a normally non-neuro-

genic region of the brain.The finding that BMP signaling is a key

intrinsic regulator of glial versus neu-ronal fate in proliferating adult neuronsis an important insight into the factorsrequired for adult neurogenesis. Theseresults also have clinical implications forthe development of adult- or fetus-de-rived stem cells to treat neurodegenera-tive disease.

Charlotte Wang

Noggin carves its niche

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