Revealing the molecular signatures of host-pathogen interactions

Advances in sequencing technology and genome-wide association studies are now revealing the complex interactions between hosts and pathogen through genomic variation signatures, which arise from evolutionary co-existence.     

Metallobiology of host-pathogen interactions: an intoxicating new insight

Iron, zinc and copper, among others, are transition metals with multiple biological roles that make them essential elements for life. Beyond the strict requirement of transition metals by the vertebrate immune system for its proper functioning, novel mechanisms involving direct metal intoxication of microorganisms are starting to be unveiled as important components of the immune system, in particular against Mycobacterium tuberculosis. In parallel, metal detoxification systems in bacteria have been recently characterized as crucial microbial virulence determinants. Here, we will focus on these exciting advancements implicating copper- and zinc-mediated microbial poisoning as a novel innate immune mechanism against microbial pathogens, shedding light on an emerging field in the metallobiology of host-pathogen interactions.     

Caenorhabditis elegans for the study of host-pathogen interactions

The nematode worm Caenorhabditis elegans, for which the complete genome sequence is available, has several other advantages as an experimental system, and has already been widely used as a model for the study of vertebrate biology. Recent investigations have revealed that C. elegans could also be an extremely useful model system in the study of bacterial pathogenesis and have reinforced the notion that common virulence and host defence mechanisms exist.     

Beyond host-pathogen interactions: microbial defense strategy in the host environment

Many extracellular pathogenic bacteria colonize human or animal bodies through evasion of the host immune system, a process called host-pathogen interaction. What happens when other intruders try to invade the same host and try to establish themselves in the same niche is largely unknown. In one well-studied case, Pseudomonas aeruginosa is known to secrete the protein azurin as a weapon against such invaders as cancers, parasites and viruses. The production of such weapons by pathogenic bacteria could provide important insights into how a pathogen responds in the post-colonization state to impede other intruders for its own survival. Moreover, these molecules might find use in the pharmaceutical industry as next-generation therapeutics.     

Models to study ancient host-pathogen interactions: lessons from Crete

The Aegean Conferences' first International Conference on Model Hosts took place on the picturesque Greek island of Crete. This meeting was the first of its kind and gathered together international experts who are using a vast array of hosts as models of infection, including worms, insects, mice, fish, rats, humans, squids, pigs, monkeys, protozoa, amoebae and ticks.     

Animal models in the analysis of Candida host-pathogen interactions

An increasingly diverse array of clinically relevant animal models of candidiasis have been established that mimic both the immune perturbations of the host and tissue-specific features of candidiasis in humans. Cause-and-effect analysis of Candida host-pathogen interactions using these animal models has made a quantum leap forward in the genomic era, with the concurrent construction of C. albicans mutants with targeted mutations of putative virulence factors, the application of microarrays and other emerging technologies to comprehensively assess C. albicans gene expression in vivo, and construction of transgenic and knockout mice to simulate specific host immunodeficiencies. The opportunity to combine these powerful tools will yield an unprecedented wealth of new information on the molecular and cellular pathogenesis of candidiasis.     

Micromanipulation of the Chlamydia pneumoniae inclusion: implications for cloning and host-pathogen interactions

The Chlamydia trachomatis inclusion is fragile, rendering it incompatible to micromanipulation. We show that the Chlamydia pneumoniae inclusion differs, being resistant to micromanipulation as shown by direct microinjection of the infected host cytosol or the inclusion itself. We have used micromanipulation to clone C. pneumoniae and to free it from mycoplasma contamination.     

Use of the EpiAirway model for characterizing long-term host-pathogen interactions

Nontypeable Haemophilus influenzae (NTHi) are human-adapted Gram-negative bacteria that can cause recurrent and chronic infections of the respiratory mucosa (1; 2). To study the mechanisms by which these organisms survive on and inside respiratory tissues, a model in which successful long-term co-culture of bacteria and human cells can be performed is required. We use primary human respiratory epithelial tissues raised to the air-liquid interface, the EpiAirway model (MatTek, Ashland, MA). These are non-immortalized, well-differentiated, 3-dimensional tissues that contain tight junctions, ciliated and nonciliated cells, goblet cells that produce mucin, and retain the ability to produce cytokines in response to infection. This biologically relevant in vitro model of the human upper airway can be used in a number of ways; the overall goal of this method is to perform long-term co-culture of EpiAirway tissues with NTHi and quantitate cell-associated and internalized bacteria over time. As well, mucin production and the cytokine profile of the infected co-cultures can be determined. This approach improves upon existing methods in that many current protocols use submerged monolayer or Transwell cultures of human cells, which are not capable of supporting bacterial infections over extended periods(3). For example, if an organism can replicate in the overlying media, this can result in unacceptable levels of cytotoxicity and loss of host cells, arresting the experiment. The EpiAirway model allows characterization of long-term host-pathogen interactions. Further, since the source for the EpiAirway is normal human tracheo-bronchial cells rather than an immortalized line, each is an excellent representation of actual human upper respiratory tract tissue, both in structure and in function(4). For this method, the EpiAirway tissues are weaned off of anti-microbial and anti-fungal compounds for 2 days prior to delivery, and all procedures are performed under antibiotic-free conditions. This necessitates special considerations, since both bacteria and primary human tissues are used in the same biosafety cabinet, and are co-cultured for extended periods.     

Divalent-metal transport by NRAMP proteins at the interface of host-pathogen interactions

The NRAMP family of divalent-metal transporters plays a key role in the homeostasis of iron and other metals. NRAMP2 (DMT1) acts as an iron-uptake protein in both the duodenum and in peripheral tissues. NRAMP1 functions as a divalent-metal efflux pump at the phagosomal membrane of macrophages and neutrophils, and mutations in NRAMP1 cause susceptibility to several intracellular pathogens. NRAMP homologues have been identified in bacteria and are involved in acquiring divalent metals from the extracellular environment. Interestingly, bacterial and mammalian NRAMP proteins would compete for the same essential substrates within the microenvironment of the phagosome, at the interface of host-pathogen interactions.     

Exploring host-pathogen interactions at the epithelial surface: application of transcriptomics in lung biology

The epithelial surface of the airways is the largest barrier-forming interface between the human body and the outside world. It is now well recognized that, at this strategic position, airway epithelial cells play an eminent role in host defense by recognizing and responding to microbial exposure. Conversely, inhaled microorganisms also respond to contact with epithelial cells. Our understanding of this cross talk is limited, requiring sophisticated experimental approaches to analyze these complex interactions. High-throughput technologies, such as DNA microarray analysis and serial analysis of gene expression (SAGE), have been developed to screen for gene expression levels at large scale within single experiments. Since their introduction, these hypothesis-generating technologies have been widely used in diverse areas such as oncology and brain research. Successful application of these genomics-based technologies has also revealed novel insights in host-pathogen interactions in both the host and pathogen. This review aims to provide an overview of the SAGE and microarray technology illustrated by their application in the analysis of host-pathogen interactions. In particular, the interactions between epithelial cells in the human lungs and clinically relevant microorganisms are the central focus of this review.     

Caenorhabditis elegans as a host for the study of host-pathogen interactions

Recently, pathogenicity models that involve the killing of the genetically tractable nematode Caenorhabditis elegans by human pathogens have been developed. From the perspective of the pathogen, the advantage of these models is that thousands of mutagenized bacterial clones can be individually screened for avirulent mutants on separate petri plates seeded with C. elegans. The advantages of using C. elegans to study host responses to pathogen attack are the extensive genetic and genomic resources available and the relative ease of identifying C. elegans mutants that exhibit altered susceptibility to pathogen attack. The use of Caenorhabditis elegans as the host for a variety of human pathogens is discussed.     

Comparing functional genomic datasets: lessons from DNA microarray analyses of host-pathogen interactions

Functional genomic technologies such as high density DNA microarrays allow biologists to study the structure and behavior of thousands of genes in a single experiment. One of the fields in which microarrays have had an increasingly important impact is host-pathogen interactions. Early investigations in this area over the past two years not only emphasize the utility of this approach, but also highlight the stereotyped gene expression responses of different host cells to diverse infectious stimuli, and the potential value of broad dataset comparisons in revealing fundamental features of innate immunity. The comparative analysis of recently published datasets involving human gene expression responses to two bacterial respiratory pathogens illustrates many of these points. Comparisons between these large, highly parallel sets of experimental observations also emphasize important technical and experimental design issues as future challenges.     

The pursuit of cryptococcal pathogenesis: heterologous hosts and the study of cryptococcal host-pathogen interactions

Analysis of the molecular mechanisms by which a pathogen interacts with the human host is most commonly performed using a mammalian model of infection. However, several virulence-related genes previously shown to be involved in mammalian infection with Cryptococcus neoformans have also been shown to play a role in the interaction of these pathogens with invertebrates, such as Acanthamoeba castellanii, Caenorhabditis elegans, Dictyostelium discoideum, Drosophila melanogaster and Galleria mellonella. The study of host-pathogen interactions using these model hosts has allowed rapid screening of mutant libraries and can be used for the study of evolutionarily preserved aspects of microbial virulence and host response.     

Energy cost of infection burden: an approach to understanding the dynamics of host-pathogen interactions

A mathematical model of long-term immune defense against infection was used to estimate the energy involved in the principal processes of immune resistance during periods of health and infection. From these values, an optimal level of energy was determined for immune response depending on infection burden. The present findings suggest that weak but prevalent pathogens lead to latent or chronic infection, whereas more virulent but less prevalent pathogens result in acute infection. This energy-based approach offers insight into the mechanisms of immune system adaptation leading to the development of chronic infectious diseases and immune deficiencies.     

Proteome analysis of host-pathogen interactions: Investigation of pathogen responses to the host cell environment

Infectious diseases are still a major health risk, and thus a better understanding of the interaction between human host cells and pathogenic microbes is urgently required. Since the interplay between both partners is highly complex, genome-wide analysis by OMICs approaches will likely make a major contribution to the elucidation of the pathophysiology of infection processes. In the concert of OMICs technologies, proteomics is particularly important because it reveals changes in the active players of the cell and has thus a close relationship to the phenotypic changes observed. While proteomic studies of in vitro-grown microbial pathogens are routinely established in many labs, in vivo proteomic approaches are still rare. Here, we will review the challenges and recent developments of proteomic analysis of microbial pathogens derived from cell culture or in vivo infection settings and summarize some lessons that have been learned from these studies.     

Retinoic acid and host-pathogen interactions: effects on inducible nitric oxide synthase in vivo

Vitamin A and its metabolite retinoic acid modulate the host response to pathogens through poorly characterized mechanisms. In vitro studies have suggested that retinoic acid decreases inducible NO synthase (NOS2, or iNOS) expression, a component of innate immunity, in several cell types stimulated with lipopolysaccharide (LPS) or cytokines. This study investigated the effect of retinoic acid on LPS-stimulated NOS2 expression in vivo. Wistar-Kyoto rats received all-trans retinoic acid (RA, 10 mg/kg) or vehicle intraperitoneally daily for 5 days followed by LPS (4 mg/kg) or saline intraperitoneally and were killed 6 h later. NOS2 activation was estimated by mRNA (RT-PCR) and protein (Western-blot) expression and plasma nitrate/nitrite accumulation. In sharp contrast to previous in vitro study reports, RA significantly enhanced NOS2 mRNA, protein expression, and plasma nitrate/nitrite concentration in LPS-injected rats but not in saline-injected rats. This was associated with increased expression of interleukin-2, interferon (IFN)-gamma and IFN regulatory factor-1 mRNAs in several organs and increased IFN-gamma plasma concentration. RA significantly increased mortality in LPS-injected rats. The NOS inhibitor aminoguanidine (50 mg/kg before LPS injection) significantly attenuated the RA-mediated increase in mortality. These results demonstrate for the first time that RA supplementation in vivo enhances activation of the LPS-triggered NOS2 pathway.