Search

Streptococcus pneumoniae is a leading cause of pneumonia and invasive disease, particularly, in the elderly. S. pneumoniae lung infection of aged mice is associated with high bacterial burdens and detrimental inflammatory responses. Macrophages can clear microorganisms and modulate inflammation through two distinct lysosomal trafficking pathways that involve 1A/1B-light chain 3 (LC3)-marked organelles, canonical autophagy, and LC3-associated phagocytosis (LAP). The S. pneumoniae pore-forming toxin pneumolysin (PLY) triggers an autophagic response in nonphagocytic cells, but the role of LAP in macrophage defense against S. pneumoniae or in age-related susceptibility to infection is unexplored. We found that infection of murine bone-marrow-derived macrophages (BMDMs) by PLY-producing S. pneumoniae triggered Atg5- and Atg7-dependent recruitment of LC3 to S. pneumoniae-containing vesicles. The association of LC3 with S. pneumoniae-containing phagosomes required components specific for LAP, such as Rubicon and the NADPH oxidase, but not factors, such as Ulk1, FIP200, or Atg14, required specifically for canonical autophagy. In addition, S. pneumoniae was sequestered within single-membrane compartments indicative of LAP. Importantly, compared to BMDMs from young (2-mo-old) mice, BMDMs from aged (20- to 22-mo-old) mice infected with S. pneumoniae were not only deficient in LAP and bacterial killing, but also produced higher levels of proinflammatory cytokines. Inhibition of LAP enhanced S. pneumoniae survival and cytokine responses in BMDMs from young but not aged mice. Thus, LAP is an important innate immune defense employed by BMDMs to control S. pneumoniae infection and concomitant inflammation, one that diminishes with age and may contribute to age-related susceptibility to this important pathogen.

Borrelia burgdorferi is one of the few extracellular pathogens capable of establishing persistent infection in mammals. The mechanisms that sustain long-term survival of this bacterium are largely unknown. Here we report a unique innate immune evasion strategy of B. burgdorferi, orchestrated by a surface protein annotated as BBA57, through its modulation of multiple spirochete virulent determinants. BBA57 function is critical for early infection but largely redundant for later stages of spirochetal persistence, either in mammals or in ticks. The protein influences host IFN responses as well as suppresses multiple host microbicidal activities involving serum complement, neutrophils, and antimicrobial peptides. We also discovered a remarkable plasticity in BBA57-mediated spirochete immune evasion strategy because its loss, although resulting in near clearance of pathogens at the inoculum site, triggers nonheritable adaptive changes that exclude detectable nucleotide alterations in the genome but incorporate transcriptional reprograming events. Understanding the malleability in spirochetal immune evasion mechanisms that ensures their host persistence is critical for the development of novel therapeutic and preventive approaches to combat long-term infections like Lyme borreliosis.

Enterohemorrhagic Escherichia coli (EHEC) is an extracellular pathogen that causes hemorrhagic colitis and hemolytic uremic syndrome. The proinflammatory cytokine, interleukin-1β, has been linked to hemolytic uremic syndrome. Here we identify the nucleotide-binding domain and leucine rich repeat containing family, pyrin domain containing 3 (NLRP3) inflammasome as an essential mediator of EHEC-induced IL-1β. Whereas EHEC-specific virulence factors were dispensable for NLRP3 activation, bacterial nucleic acids such as RNA:DNA hybrids and RNA gained cytosolic access and mediated inflammasome-dependent responses. Consistent with a direct role for RNA:DNA hybrids in inflammasome activation, delivery of synthetic EHEC RNA:DNA hybrids into the cytosol triggered NLRP3-dependent responses, and introduction of RNase H, which degrades such hybrids, into infected cells specifically inhibited inflammasome activation. Notably, an E. coli rnhA mutant, which is incapable of producing RNase H and thus harbors increased levels of RNA:DNA hybrid, induced elevated levels of NLRP3-dependent caspase-1 activation and IL-1β maturation. Collectively, these findings identify RNA:DNA hybrids of bacterial origin as a unique microbial trigger of the NLRP3 inflammasome.

Src homology 3 (SH3) domains are globular protein interaction modules that regulate cell behavior. The classic SH3 ligand-binding site accommodates a hydrophobic PxxP motif and a positively charged specificity-determining residue. We have determined the NMR structure of insulin receptor tyrosine kinase substrate (IRTKS) SH3 domain in complex with a repeat from Escherichia coli -secreted protein F-like protein encoded on prophage U (EspF U ), a translocated effector of enterohemorrhagic E. coli that commandeers the mammalian actin assembly machinery. EspF U -IRTKS interaction is among the highest affinity natural SH3 ligands. Our complex structure reveals a unique type of SH3 interaction based on recognition of tandem PxxP motifs in the ligand. Strikingly, the specificity pocket of IRTKS SH3 has evolved to accommodate a polyproline type II helical peptide analogously to docking of the canonical PxxP by the conserved IRTKS SH3 proline-binding pockets. This cooperative binding explains the high-affinity SH3 interaction and is required for EspF U -IRTKS interaction in mammalian cells as well as the formation of localized actin “pedestals” beneath bound bacteria. Importantly, tandem PxxP motifs are also found in mammalian ligands and have been shown to contribute to IRTKS SH3 recognition similarly.

Enterohemorrhagic Escherichia coli O157:H7 translocates 2 effectors to trigger localized actin assembly in mammalian cells, resulting in filamentous actin “pedestals.” One effector, the translocated intimin receptor (Tir), is localized in the plasma membrane and clustered upon binding the bacterial outer membrane protein intimin. The second, the proline-rich effector EspF U (aka TccP) activates the actin nucleation-promoting factor WASP/N-WASP, and is recruited to sites of bacterial attachment by a mechanism dependent on an Asn-Pro-Tyr (NPY 458 ) sequence in the Tir C-terminal cytoplasmic domain. Tir, EspF U , and N-WASP form a complex, but neither EspF U nor N-WASP bind Tir directly, suggesting involvement of another protein in complex formation. Screening of the mammalian SH3 proteome for the ability to bind EspF U identified the SH3 domain of insulin receptor tyrosine kinase substrate (IRTKS), a factor known to regulate the cytoskeleton. Derivatives of WASP, EspF U , and the IRTKS SH3 domain were capable of forming a ternary complex in vitro, and replacement of the C terminus of Tir with the IRTKS SH3 domain resulted in a fusion protein competent for actin assembly in vivo. A second domain of IRTKS, the IRSp53/MIM homology domain (IMD), bound to Tir in a manner dependent on the C-terminal NPY 458 sequence, thereby recruiting IRTKS to sites of bacterial attachment. Ectopic expression of either the IRTKS SH3 domain or the IMD, or genetic depletion of IRTKS, blocked pedestal formation. Thus, enterohemorrhagic E. coli translocates 2 effectors that bind to distinct domains of a common host factor to promote the formation of a complex that triggers robust actin assembly at the plasma membrane.

Host cell binding is an essential step in colonization by many bacterial pathogens, and the Lyme disease agent, Borrelia burgdorferi , which colonizes multiple tissues, is capable of attachment to diverse cell types. Glycosaminoglycans (GAGs) are ubiquitously expressed on mammalian cells and are recognized by multiple B. burgdorferi surface proteins. We previously showed that B. burgdorferi strains differ in the particular spectrum of GAGs that they recognize, leading to differences in the cultured mammalian cell types that they efficiently bind. The molecular basis of these binding specificities remains undefined, due to the difficulty of analyzing multiple, potentially redundant cell attachment pathways and to the paucity of genetic tools for this pathogen. In the current study, we show that the expression of decorin-binding protein (Dbp) A and/or DbpB, two B. burgdorferi surface proteins that bind GAGs, is sufficient to convert a high-passage nonadherent B. burgdorferi strain into one that efficiently binds 293 epithelial cells. Epithelial cell attachment was mediated by dermatan sulfate, and, consistent with this GAG-binding specificity, these recombinant strains did not bind EA-Hy926 endothelial cells. The GAG-binding properties of bacteria expressing DbpB or DbpA were distinguishable, and DbpB but not DbpA promoted spirochetal attachment to C6 glial cells. Thus, DbpA and DbpB may each play central but distinct roles in cell type-specific binding by Lyme disease spirochetes. This study illustrates that transformation of high-passage B. burgdorferi strains may provide a relatively simple genetic approach to analyze virulence-associated phenotypes conferred by multiple bacterial factors.

The human pathogenic bacterium group A Streptococcus produces an extracellular cysteine protease [streptococcal pyrogenic exotoxin B (SpeB)] that is a critical virulence factor for invasive disease episodes. Sequence analysis of the speB gene from 200 group A Streptococcus isolates collected worldwide identified three main mature SpeB (mSpeB) variants. One of these variants (mSpeB2) contains an Arg-Gly-Asp (RGD) sequence, a tripeptide motif that is commonly recognized by integrin receptors. mSpeB2 is made by all isolates of the unusually virulent serotype M1 and several other geographically widespread clones that frequently cause invasive infections. Only the mSpeB2 variant bound to transfected cells expressing integrin α v β 3 (also known as the vitronectin receptor) or α IIb β 3 (platelet glycoprotein IIb-IIIa), and binding was blocked by a mAb that recognizes the streptococcal protease RGD motif region. In addition, mSpeB2 bound purified platelet integrin α IIb β 3 . Defined β 3 mutants that are altered for fibrinogen binding were defective for SpeB binding. Synthetic peptides with the mSpeB2 RGD motif, but not the RSD sequence present in other mSpeB variants, blocked binding of mSpeB2 to transfected cells expressing α v β 3 and caused detachment of cultured human umbilical vein endothelial cells. The results ( i ) identify a Gram-positive virulence factor that directly binds integrins, ( ii ) identify naturally occurring variants of a documented Gram-positive virulence factor with biomedically relevant differences in their interactions with host cells, and ( iii ) add to the theme that subtle natural variation in microbial virulence factor structure alters the character of host-pathogen interactions.

Lyme disease is a chronic, multisystemic infection caused by the tick-borne spirochete Borrelia burgdorferi. Attachment of the spirochete to host cells via specific receptors is likely to be important in the establishment of infection. B. burgdorferi have previously been shown to bind to a variety of mammalian cells in vitro. Here we demonstrate that binding of B. burgdorferi to human platelets is mediated by the integrin alpha IIb beta 3 (glycoprotein IIb-IIIa), a critical receptor in thrombosis and hemostasis. Functional expression of this receptor requires platelet activation, and binding of the spirochete was observed only to activated platelets. Binding was inhibited by a synthetic Arg-Gly-Asp peptide that blocks ligand interaction with many integrins and by a synthetic peptide based on the gamma chain of fibrinogen that blocks binding to alpha IIb beta 3. In addition, attachment of the spirochete to platelets was inhibited by monoclonal antibodies directed against alpha IIb beta 3 that are known to block ligand-receptor interaction. No inhibition was seen with control peptides or with antibodies directed against other platelet receptors. B. burgdorferi bound efficiently to purified alpha IIb beta 3 but did not bind to platelets deficient in this integrin. Efficient platelet binding was displayed by a cloned, infectious B. burgdorferi strain, whereas a cloned noninfectious strain did not bind to platelets. Binding to integrins may be important for the ability of B. burgdorferi to establish infection in the diverse tissues affected by Lyme disease.

The expression of invasin, the product of the Yersinia pseudotuberculosis inv gene, allows enteric bacteria to enter cultured mammalian cells. The ability of invasin to bind animal cells and the potential significance of this interaction in the entry process were investigated. It was found that HEp-2 cells could attach to surfaces coated with bacterial membranes containing invasin. By fractionating bacterial membrane proteins on NaDodSO4/polyacrylamide gels and transferring the protein to filters, we demonstrated that the cell-binding component of the membranes comigrated with invasin. Mutations that changed the electrophoretic mobility of the protein also caused a corresponding shift in the migration of the cell-binding activity, showing that the comigrating protein was indeed invasin. Monoclonal antibodies directed against invasin that blocked invasin-HEp-2 cell interaction also inhibited bacteria from penetrating HEp-2 cells, indicating that interaction of this protein with animal cells is critical for cellular penetration.