Editorial Journal of Innate J Innate Immun 2010;2:505–507 Published online: September 10, 2010 Immunity DOI: 10.1159/000320473 Bacteria and Phagocytes: Mortal Enemies Victor Nizet Department of Pediatrics, School of Medicine and Skaggs School of Pharmacy and Pharmaceutical Sciences, University of California San Diego, La Jolla, Calif. , USA A critical first-line element of mammalian innate im- The research presented in this issue finds historical munity is the function of phagocytic cells, in particular resonance in a classical scientific debate. Russian biolo- neutrophils and macrophages. The effectiveness of these gist Ilya Metchnikoff [1, 2] was awarded the Nobel Prize specialized leukocytes in host defense reflects their ca- in 1908 for the development of the phagocyte theory, the pacity for directed migration, microbial uptake, and in- central tenet of which was that the ‘swallowing and diges- tra- and extracellular microbial killing; the latter is tion’ of bacteria by circulating ‘white corpuscles’ (neutro- achieved through the concerted action of reactive oxy- phils and macrophages), enhanced by prior opsonization, gen species, enzymatic proteolysis, and cationic antimi- provided immunity to infection [1–3] . However, a con- crobial peptides. Stimulated phagocytes also amplify in- temporary of Metchnikoff [1, 2] , the hygienist and bacte- flammatory and immune responses through the release riologist Oskar Bail [4] , who was working at the German of cytokines, nitric oxide, and vasoactive peptides. Their University (today named Charles University) in Prague, general importance is further exemplified by the in- was simultaneously developing a countering ‘aggressin creased susceptibility to invasive bacterial infection in doctrine’. This model cautioned that many pathogens, in- patients whose phagocyte numbers are markedly re- cluding S. aureus , pneumococcus, the tubercle bacillus, duced. However, it is also apparent that several leading or cholera, were able to effectively neutralize phagocytes bacterial pathogens such as Staphylococcus aureus , Strep- by paralysis of function or dissolution of the host cell, all tococcus pyogenes , and others are capable of causing se- through the action of specific ‘aggressin’ molecules in- vere invasive infections even in previously healthy indi- duced and released during the course of infection. Metch- viduals. Such intrinsic disease-producing capacity de- nikoff [5] made a specific point in his Nobel Prize accep- fines a superior ability of these pathogens to resist host tance speech to trivialize Bail’s [4] research: ‘Numerous phagocytic clearance through the coordinated expres- findings, achieved with care over the last few years, clean sion of virulence determinants that interfere with phago- contradict this view. It has been shown that the white cor- cyte trafficking or opsonophagocytosis, or instead to puscles entertain no fear of microbial poisons and are neutralize the molecular effectors of bacterial killing. well fitted to absorb them and make them harmless.’ As This special thematic issue of the Journal of Innate Im- illustrated in this issue, the modern era of molecular mi- munity focuses on the elucidation of new bacterial viru- crobiology has provided genetic approaches to pinpoint lence factors that target phagocyte defense pathways, specific bacterial factors that fulfill the criteria of ‘aggres- pointing out the significant consequences they hold for sin’ proposed by Bail [4] – we can hope to the satisfaction the pathogen-host encounter. rather than the chagrin of Metchnikoff’s [1, 2] legacy. © 2010 S. Karger AG, Basel Prof. Victor Nizet , Department of Pediatrics 1662–811X/10/0026–0505$26.00/0 School of Medicine, Skaggs School of Pharmacy and Pharmaceutical Sciences Fax +41 61 306 12 34 University of California San Diego , 9500 Gilman Drive, Mail Code 0687 E-Mail [email protected] Accessible online at: La Jolla, CA 92093-0687 (USA) www.karger.com www.karger.com/jin Tel. +1 858 534 7408, Fax +1 858 534 5611, E-Mail vnizet @ ucsd.edu After a phagocyte engulfs a bacteria, a critical ensuing An important emerging concept in the field of neutro- step in the killing process is the fusion of the phagosome phil biology is that phagocytosis is not the sine qua with cellular structures such as lysosomes (macrophages) non of bacterial killing. Rather, neutrophils can them- or specific or azurophil granules (neutrophils) which can selves undergo a specialized form of cell death that en- deliver cargos of hydrolytic enzymes, proteases, and cat- hances innate immunity – the formation of neutrophil ionic peptides with potent bactericidal properties. Here extracellular traps or NETs [14] . These structures com- Cederlund et al. [6] provide a sophisticated analysis of the prise a backbone lattice of nuclear or mitochondrial DNA activities of several such bactericidal effectors, including decorated with antimicrobial peptides, histones, and lysosomes, ␣ -defensins, azurocidin, cathelicidin LL-37, granule proteases that can ensnare and kill bacterial calprotectin, and lactotransferrin, showing that specific- pathogens [15, 16] . Berends et al. [17] report that S. au- ity in the killing of different bacterial pathogens is pro- reus can thwart this extracellular killing mechanism by vided by distinct repertoires of these bioactive peptides. the elaborat ion of secreted nucleases that degrade the One way for a bacterial pathogen to resist phagocyte kill- DNA backbone to dissolve the NETs. S. pyogenes strong- ing is thus to prevent phagosome fusion with lysosomes ly stimulates NET production by the proinflammatory and granules. Huynh et al. [7] demonstrate that the op- action of its M1 protein in complex with fibrinogen [18] , portunistic pathogen Burkholderia cepacia is able to im- yet it can resist entrapment and killing by nuclease pro- pair the activation of Rab7, a small GTPase that plays a duction [19] or binding and inactivation of the embedded key role in lysosome biogenesis and fusion with late en- cathelicidin antimicrobial peptides mediated by the M1 docytic structures [8] . For the invasive M1 serotype strain protein [20] . In the present issue, Pence et al. [21] use tar- of the pathogen S. pyogenes , Hertzén et al. [9] show that geted mutagenesis and heterologous gene expression to the surface-expressed M1 protein prevents the fusion of show that the streptococcal inhibitor of complement the phagosome with the lysosome and provides the bac- (SIC), expressed by M1 strains of S. pyogenes , also inhib- teria with an intracellular safe haven, creating a reservoir its cathelicidin killing to promote phagocyte resistance of surviving bacteria that can subsequently egress the and systemic virulence. SIC, nuclease, capsule, and other macrophage to reinfect new cells. neutrophil resistance factors of M1 S. pyogenes are up- S. aureus , a leading cause of serious bacterial infec- regulated in vivo as the result of selective pressure for tions and increasing antibiotic resistance, is capable of mutations in the covRS 2-component regulator. Maamary promoting the rapid elimination of human neutrophils. et al. [22] show that a reduced propensity for covRS muta- One key mechanism appears to be the action of stapho- tion predicts the increased neutrophil susceptibility and pain B, a cysteine protease that selectively cleaves CD11b lower invasive disease potential of non-M1 S. pyogenes on phagocytic cells; this is a ‘cell death signal’ which pro- strains. motes rapid clearance of the affected neutrophils by mac- More than a century ago, Bail [4] foresaw that the neu- rophages thus reducing the number of phagocytes at the tralization of bacterial ‘aggressins’ could represent an im- site of infection [10, 11] . In this issue, 2 articles explore portant aspect of infectious disease therapy. While some rapid neutrophil lysis induced by the epidemic USA300 of the virulence mechanisms discussed herein (e.g. S. au- clone of methicillin-resistant S. aureus (MRSA). Pang et reus ␣ -hemolysin or staphopain B) do lead to the aggres- al. [12] report that a global regulator of virulence factors, sive destruction of the phagocytic cell, others (e.g. S. pyo- the agr system, is activated upon phagocytosis of USA300 genes M1 protein or SIC and S. aureus nuclease) simply MRSA leading to the upregulated expression of the pore- allow the pathogen to withstand the antimicrobial killing forming ␣ -hemolysin within the phagosome and con- mechanisms deployed by the phagocyte and continue its tributing to neutrophil destruction in as little as 2 h. Ko- replication in vivo (Author’s note: perhaps virulence fac- bayashi et al. [13] use live-cell imaging, electron micro- tors in the latter category might lightheartedly be referred copy, and whole-genome expression analysis to reveal to as ‘passive aggressins’). that both low- and high-hemolysin-producing strains The life-or-death battles between bacterial pathogens can induce such rapid neutrophil death. In these studies, and host phagocytes will remain a critical determinative neutrophil phagosome membranes appear to remain in- factor dictating infectious disease resolution or progres- tact right up to the point of cell lysis, suggesting that sion. Identification of the molecular effectors of bacterial USA300 MRSA triggers a novel form of programmed cell phagocyte resistance can thus provide the novel oppor- necrosis, a signature phenotype for its hypervirulence. tunities for therapy suggested by Bail [4] . In this context, the therapeutic agent is not a classical