Insects depend solely upon innate immune responses to survive infection. These responses include the activation of extracellular protease cascades, leading to melanization and clotting, and intracellular signal transduction pathways inducing antimicrobial peptide gene expression. In Drosophila, the IMD pathway is required for antimicrobial gene expression in response to gram-negative bacteria. The exact molecular component(s) from these bacteria that activate the IMD pathway remain controversial. We found that highly purified LPS did not stimulate the IMD pathway. However, lipid A, the active portion of LPS in mammals, activated melanization in the silkworm Bombyx morii. On the other hand, the IMD pathway was remarkably sensitive to polymeric and monomeric gram-negative peptidoglycan. Recognition of peptidoglycan required the stem-peptide sequence specific to gram-negative peptidoglycan and the receptor PGRP-LC. Recognition of monomeric and polymeric peptidoglycan required different PGRP-LC splice isoforms, while lipid A recognition required an unidentified soluble factor in the hemolymph of Bombyx morii.

Although pneumonic plague is the deadliest manifestation of disease caused by the bacterium Yersinia pestis, there is surprisingly little information on the cellular and molecular mechanisms responsible for Y. pestis-triggered pathology in the lung. Therefore, to understand the progression of this unique disease, we characterized an intranasal mouse model of primary pneumonic plague. Mice succumbed to a purulent multifocal severe exudative bronchopneumonia that closely resembles the disease observed in humans. Analyses revealed a strikingly biphasic syndrome, in which the infection begins with an antiinflammatory state in the first 24 -36 h that rapidly progresses to a highly proinflammatory state by 48 h and death by 3 days. To assess the adaptation of Y. pestis to a mammalian environment, we used DNA microarray technology to analyze the transcriptional responses of the bacteria during interaction with the mouse lung. Included among the genes up-regulated in vivo are those comprising the yop-ysc type III secretion system and genes contained within the chromosomal pigmentation locus, validating the use of this technology to identify loci essential to the virulence of Y. pestis.inflammation ͉ microarray ͉ Yersinia pestis P neumonic plague is the deadliest manifestation of disease caused by the bacterium Yersinia pestis. Although rare compared with the bubonic form of plague, which is acquired by skin penetration, primary pneumonic plague is highly contagious and almost always fatal. The current worldwide incidence of plague is low by historical standards, but the possible combination of widespread aerosol dissemination and rapid disease progression are of particular concern for defense against bioterrorism (1).In cases of primary pneumonic plague in humans, microscopic examination of lung tissue reveals multiple histological patterns, including acute pneumonia, intraalveolar hemorrhage and edema, and the presence of extracellular bacteria in the alveoli but not the interstitium (2). In addition, extensive neutrophilic infiltrate and fibrin deposition have been observed, and in some cases a complete loss of recognizable alveolar architecture results from the infection (3, 4). Studies of experimental primary pneumonic plague in monkeys, mice, and guinea pigs showed similar pathologic effects, including extensive intraalveolar edema, massive bacterial proliferation in the small airways, and numerous neutrophils in the alveoli (5-8).How Y. pestis triggers pulmonary pathology is largely unexplored, as are the bacterial responses to this dramatically changing host environment. DNA microarray technology has been widely used to assess transcriptional changes in bacterial gene expression in vitro, but analyses of the bacterial transcriptome during host infection have been hampered by two significant problems (9). These include the excess amounts of copurified eukaryotic RNA that may produce nonspecific hybridization signals on the microarray, and the oftenlimiting amounts of bacterial RNA extracted from an animal, which w...

Drosophila rely entirely on an innate immune response to combat microbial infection. Diaminopimelic acid-containing peptidoglycan, produced by Gram-negative bacteria, is recognized by two receptors, PGRP-LC and PGRP-LE, and activates a homolog of transcription factor NF-kappaB through the Imd signaling pathway. Here we show that full-length PGRP-LE acted as an intracellular receptor for monomeric peptidoglycan, whereas a version of PGRP-LE containing only the PGRP domain functioned extracellularly, like the mammalian CD14 molecule, to enhance PGRP-LC-mediated peptidoglycan recognition on the cell surface. Interaction with the imd signaling protein was not required for PGRP-LC signaling. Instead, PGRP-LC and PGRP-LE signaled through a receptor-interacting protein homotypic interaction motif-like motif. These data demonstrate that like mammals, drosophila use both extracellular and intracellular receptors, which have conserved signaling mechanisms, for innate immune recognition.

Primary pneumonic plague is transmitted easily, progresses rapidly, and causes high mortality, but the mechanisms by which Yersinia pestis overwhelms the lungs are largely unknown. We show that the plasminogen activator Pla is essential for Y. pestis to cause primary pneumonic plague but is less important for dissemination during pneumonic plague than during bubonic plague. Experiments manipulating its temporal expression showed that Pla allows Y. pestis to replicate rapidly in the airways, causing a lethal fulminant pneumonia; if unexpressed, inflammation is aborted, and lung repair is activated. Inhibition of Pla expression prolonged the survival of animals with the disease, offering a therapeutic option to extend the period during which antibiotics are effective.

Successful infection by fungal pathogens depends on subversion of host immune mechanisms that detect conserved cell wall components such as ␤-glucans. A less common polysaccharide, ␣-(1,3)-glucan, is a cell wall constituent of most fungal respiratory pathogens and has been correlated with pathogenicity or linked directly to virulence. However, the precise mechanism by which ␣-(1,3)-glucan promotes fungal virulence is unknown. Here, we show that ␣-(1,3)-glucan is present in the outermost layer of the Histoplasma capsulatum yeast cell wall and contributes to pathogenesis by concealing immunostimulatory ␤-glucans from detection by host phagocytic cells. Production of proinflammatory TNF␣ by phagocytes was suppressed either by the presence of the ␣-(1,3)-glucan layer on yeast cells or by RNA interference based depletion of the host ␤-glucan receptor dectin-1. Thus, we have functionally defined key molecular components influencing the initial host-pathogen interaction in histoplasmosis and have revealed an important mechanism by which H. capsulatum thwarts the host immune system. Furthermore, we propose that the degree of this evasion contributes to the difference in pathogenic potential between dimorphic fungal pathogens and opportunistic fungi.cell wall ͉ fungal pathogenesis ͉ virulence factor ͉ dectin-1 ͉ macrophage