Host-Pathogen Interactions Scientists studying pathogenesis have typically focused on single genes or small subsets of genes (or operons) as potential virulence factors. However, microarrays have enabled the exploration of genome-wide expression, uncovering virulence pathways consisting of previously unknown genes. This technique has proven particularly useful when analyzing a pathogen's response to its host environment. By developing model systems of infection, scientists can examine how a pathogen alters gene expression in response to its host, and can then deduce which genes are involved in virulence.
The ubiquitous environmental bacterium Pseudomonas aeruginosa chronically infects the lungs of Cystic fibrosis (CF) patients, where the overproduction and accumulation of viscous respiratory mucus and excessive inflammation represents an ideal environment for P. aeruginosa. To further elucidate the mechanisms behind this capability, Wolfgang et al. used GeneChip® P. aeruginosa Genome Arrays to study the changes in gene expression in P. aeruginosa when exposed to airway liquids from chronically infected cystic fibrosis patients. By examining genome-wide expression, the group noticed that a majority of the repressed genes encoded proteins relating to flagellar biosynthesis, and found that surface flagella were indeed reduced as determined by electron microscopy. Flagella are highly immunogenic and repression of their synthesis is a way for the bacteria to avoid detection by host defense mechanisms, allowing the bacteria to successfully
establish infection in immunocompromised CF patients.
Group A Streptococcus (GAS) is responsible for a broad range of human diseases from mild skin and throat infections to life-threatening diseases such as puerperal sepsis or streptococcal toxic shock syndrome. As GAS transitions from a topical infection to an invasive infection, it must adapt to the changes in the host environment. Graham et al. used GeneChip® CustomExpress Arrays to examine the changes in gene expression in GAS when exposed to human blood. Upon ex vivo culture in human whole blood, substantial changes in the expression of numerous genes were observed. These included genes encoding superantigens and host-evasion proteins, regulated by a multiple gene activator called Mga, which enhance bacterial survival through evasion of immune and innate host defenses. Coordinated expression of genes involved in proteolysis, transport, and catabolism of oligopeptides was also observed, allowing the auxotrophic GAS to obtain free amino acids in this protein-rich host environment.
By understanding the virulence factors and survival systems expressed by different pathogens, researchers are able to identify disease mechanism pathways as targets for potential treatments. However, a complete understanding of infectious disease requires the examination of both the virulence factors expressed by the microbe, as well as the host response mechanisms and host pathways that are subverted by that microbe. To this end, scientists have used GeneChip® microarrays to understand virulence by monitoring changes in host gene expression following challenge with a microbe or with purified virulence factors.
Apidinakis et al. employed GeneChip® Drosophila Genome Arrays to measure gene expression in Drosophila melanogaster infected with either an avirulent (CF5) or a highly virulent (PA14) human strain of P. aeruginosa to identify host genes that play a role in the initiation and progression of infection. Of the 241 differentially expressed genes identified, most were involved in host defense, encoding broad spectrum antimicrobial peptides (AMP) and other defense effectors. Interestingly, infection of D. melanogaster with CF5 prior to infection with PA14 led to heightened defense against PA14, while infection with PA14 alone significantly reduced expression of AMP genes. Repression of antimicrobial peptide gene expression during the initial stages of infection may be a key survival mechanism for P. aeruginosa in CF patients, allowing the bacteria to establish a chronic infection.
Marshall et al. used GeneChip® Mouse Genome Arrays to examine gene expression in influenza virus-specific CD8+ T cells isolated from the respiratory tract as compared to that in T cells isolated from the spleen of mice, following secondary challenge with influenza A. A total of 90 genes were identified that exhibited differential expression between the two lineage-related lymphocyte populations. Of these, 25 were more highly expressed in cells recovered from the respiratory tract with the remaining 65 more highly expressed in cells recovered from spleen. Their observation that the more activated (virus-specific) lymphocytes had fewer highly expressed genes, while the less activated (undifferentiated) spleen cells had more highly activated genes, was unexpected. These findings suggest that more differentiated or terminally differentiated lymphocytes exhibit narrowed or focused gene expression of essential effector molecules as compared to more na?ve lymphocytes.
Izmailova and colleagues used GeneChip® Human Genome Arrays to study the effects of HIV-1 or Tat protein (a major HIV virulence factor) on immature dendritic cells, which are among the first cells to be infected by retroviruses. By examining genome-wide expression, the researchers were able to identify the induction of a complete interferon pathway. Chemokines are among the molecules induced by this pathway, which in turn recruit macrophages and T cells, which are the ultimate targets of the virus and thus facilitate the expansion of the viral infection. Based on these studies, designing therapies against the Tat protein or against the members of the interferon pathway would produce the combined benefit of limiting viral transcription and also reducing expansion of viral infection into uninfected cell types.
References
·
Wolfgang, M. C. et al.Pseudomonas aeruginosa regulates flagellin expression as part of a global response to airway fluid from cystic fibrosis patients. Proc Natl Acad Sci USA101: 6664-6668 (2004).
·
Graham, M.R., et al. Group A Streptococcus Transcriptome Dynamics during Growth in Human Blood Reveals Bacterial Adaptive and Survival Strategies. American Journal of Pathology166:455-465 (2005).
·
Apidinakis, Y., et al. Profiling early infection responses: Pseudomonas aeruginosa eludes host defenses by suppressing antimicrobial peptide gene expression. Proc Natl Acad Sci USA102: 2573-2578 (2005).
·
Marshall, D.R., et al. Effector CD8+ T cells recovered from an Influenza pneumonia differentiate to a state of focused gene expression. Proc Natl Acad Sci USA102: 6074-6079 (2005).
·
Izmailova, E. et al. HIV-1 Tat reprograms immature dendritic cells to express chemoattractants for activated T cells and macrophages. Nat Med9: 191-197 (2003).
Related Scientific Publications
·
Ng, H., et al. Gene expression profiling of mouse host response to Listeria monocytogenes infection. Genomics86: 657-667 (2005).
·
Comer, J.E. et al. GeneChip Analyses of Global Transcriptional Responses of Murine Macrophages to the Lethal Toxin of Bacillus anthracis. Infection and Immunity, 73: 1879-1885 (2005).
·
Xiang, W., et al. Identification of differentially expressed genes in scrapie-infected mouse brains by global gene expression technology. J. Virol78: 11051-11060 (2005).
·
Peterson, M., et al. The Innate Immune System Is Activated by Stimulation of Vaginal Epithelial Cells with Staphylococcus aureus and Toxic Shock Syndrome Toxin 1. Infect. Immun.73: 2164?2174 (2005).
·
Garner-Hamrick, P.A., et al. Global effects of human papillomavirus type 18 E6/E7 in an organotypic keratinocyte culture system. J Virol.78:9041-9050 (2004).
·
Chun, T-W., et al. Gene expression and viral prodution in latently infected, resting CD4+ T cells in viremic versus aviremic HIV-infected individuals Proc. Natl. Acad Sci. USA100: 1908-1913 (2003).
·
Le Roch, K. G. et al. Discovery of gene function by expression profiling of the malaria parasite life cycle. Science301: 1503-1508 (2003).
Scientists studying pathogenesis have typically focused on single genes or small subsets of genes (or operons) as potential virulence factors. However, microarrays have enabled the exploration of genome-wide expression, uncovering virulence pathways consisting of previously unknown genes. This technique has proven particularly useful when analyzing a pathogen's response to its host environment. By developing model systems of infection, scientists can examine how a pathogen alters gene expression in response to its host, and can then deduce which genes are involved in virulence.
By understanding the virulence factors and survival systems expressed by different pathogens, researchers are able to identify disease mechanism pathways as targets for potential treatments. However, a complete understanding of infectious disease requires the examination of both the virulence factors expressed by the microbe, as well as the host response mechanisms and host pathways that are subverted by that microbe. To this end, scientists have used GeneChip® microarrays to understand virulence by monitoring changes in host gene expression following challenge with a microbe or with purified virulence factors.
