The systems biology of infection in animal models bears fruit
The host response to pathogenic bacteria defies simple, general descriptions. The complicated nature of these interactions is a natural consequence of the complexity of the participants: bacteria, which are free-living organisms with genomes encoding thousands of genes, and mammals and plants, which are multicellular organisms with elaborate genomes orders of magnitude larger. The diversity of hosts and bacterial pathogens further complicates a straightforward, comprehensive portrayal of infectious diseases. Despite this complexity, the relative conservation of innate immune responses has provided a basis for our understanding of bacterial diseases that transcends the differences among hosts and infectious agents. These recently identified innate immune response pathways, which regulate the adaptive immune response to infections through the production of biologically active cytokines, offer a valuable framework by which infectious diseases can be classified and defined (1). In the article by Handley et al. in a recent issue of PNAS (2), the combined use of two powerful experimental techniques yielded entirely unexpected information regarding the innate immune response to bacteria by using a well studied host–pathogen model: Yersinia entercolitica infection of susceptible, inbred mice.
The development of mammalian cell culture infection models has improved the throughput of bacterial pathogenesis research and greatly advanced our knowledge of the mechanisms by which bacterial pathogens alter host cells (3). These methods, coupled with advances in genomic and proteomic technologies, provide a powerful means to define responses essential for host defense. However, these model systems do not offer the opportunity to study infections within the context of intact tissues or animals. Animal models can be used to define the interactions of pathogens with complex tissues, such as mucosal surfaces. However, these models also are limited in their reflection of natural disease. For example, different animal models involving inbred mice (frequently with unique innate immune responses) can yield apparently contradictory data, hindering a complete understanding of mechanisms of both bacterial infection and clearance (4, 5). The Yersinia spp. are among the best studied bacterial pathogens, and many virulence mechanisms were defined first by using available animal and tissue culture infection models of infection by the gastrointestinal pathogen Y. enterocolitica (3). Based on these experiments, a model of infection exists in which, after oral ingestion, Y. enterocolitica penetrates the areas of intestinal lymphoid tissue known as Peyer’s patches, eventually spreading to more distal mesenteric lymph nodes (6). This pathogen utilizes a specific receptor-based mechanism to invade host tissues, and it then transports an assortment of bacterial proteins into host immune cells by type III secretion that impair the host innate and acquired immune response (5). Although studies have been performed that helped define the transcriptional profile of host cell types to Yersinia (5), a comprehensive transcriptional profile of lymphoid tissue response in the context of mucosal infection of a whole animal has not yet been performed for this or any bacterial pathogen. Handley et al. (2) report the new identification of a host response by defining the global gene expression profile of whole mouse intestinal lymphoid tissue at various time points after oral infection with Y. enterocolitica. The results confirmed several findings from previous models of infection by using cultured mammalian cells, but they also suggested the involvement of a surprising host molecule that previously was not implicated in Yersinia infections: histamine. This unexpected finding highlights the potential of microarray methodology to identify novel host responses to infection in complex systems, a primary goal of systems biology (7).
The advantage of the authors’ (2) approach was that they made no assumptions regarding the cell type most important to the infection process. For example, host inflammatory cells (and bacteria) likely entered and departed these tissues during the course of infection. Because the lymphoid tissue was analyzed intact, any change in the aggregate tissue gene expression could be due to changes in either cellular content or the gene expression of resident cells. The current approach was advantageous because it afforded a simplified description of physiologically relevant changes in a single tissue.
Using this microarray-based approach, Handley et al. (2) found that the expression of the gene histidine decarboxylase (Hdc) was increased during the entire monitored course of infection. The HDC gene product produces histamine, a potent, bioactive compound perhaps best known for its role in allergy and inflammation (8). Using histochemical analyses, the authors confirmed that histamine was increased in the Peyer’s patches from infected mice, in agreement with their transcriptional data. Furthermore, the authors repeated their experiments, this time by using a collection of specific histamine receptor antagonists and agonists, focusing on the commonly expressed H1 and H2 receptor subtypes. Treatment with the H2 receptor antagonist cimetidine, a compound used frequently to treat humans, led to decreased host survival, increased intestinal pathology, and increased bacterial load at early time points within the mesenteric lymph nodes and Peyer’s patches compared with no cimetidine treatment. The receptor specificity of this histamine response was suggested by the fact that the H1 receptor antagonist pyrilamine had no effects in this model. Conversely, treatment with an H2 receptor agonist, dimaprit, had effects opposite to those of cimetidine. Together, these results suggest that signaling through the H2 receptor is an important regulator of the host response to Y. enterocolitica.
The H2 receptor perhaps is best known for its role in regulating acid secretion in the stomach (8). The H2 receptor antagonist cimetidine is one of several drugs used frequently to treat gastric acid-related illnesses. The production of gastric acid is regulated by many signaling pathways, and histamine signaling is one of the most potent of them (8). Blockade of acid production can increase the risk for a number of infections, including pneumonia (9) and gastrointestinal infections (10). However, these effects are thought to be due to the removal of a protective barrier function of gastric acid itself, rather than to H2 signaling, because using inhibitors of acid production that work through a histamine-independent mechanism (proton pump inhibitors) also raises the infection risk (9, 10). To test whether inhibition of gastric acid was involved in the effects of H2 signaling on the mouse response to Y. enterocolitica, Handley et al. (2) compared the effects of cimetidine with the proton pump inhibitor omeprazole in the orally infected mouse model. The authors identified no effect of omeprazole under their conditions, suggesting that gastric acid production is not involved in the effects of cimetidine on Y. enterocolitica infection, but rather that histamine likely plays a role in regulating the innate immune response to infection (Fig. 1).
A model for histamine-regulated host response to Y. enterocolitica infection in the intestinal lymphoid tissue. According to the findings of Handley et al. (2), after oral infection, Y. enterocolitica infects the lymphoid tissues of the small intestine at the Peyer’s patches. These tissues then increase production of histamine, which controls both local infection and dissemination to the mesenteric lymph nodes, as well as the risk of an intestinal complication of infection, intussusception. These effects are mediated by interaction of histamine through an unidentified host cell at the H2 receptor.
In addition to its role in allergy, histamine exerts a multitude of physiological effects, and the diversity of these roles is due, in part, to differences in tissue expression of the four known histamine receptor subtypes (8). Among the more recently described functions of histamine are the alteration of cytokine production (11, 12), immune cell differentiation (11), lymphocyte responsiveness and regulation (13, 14), phagocyte motility (12), and phagocyte killing (11). Consistent with this immune-regulatory role, histamine has been implicated in the host response to several other bacteria, although results vary with infection model. Histamine-deficient mice or those treated with histamine antagonists clear i.p. Escherichia coli infections more rapidly (15). Similarly, histamine-deficient mice infected intragastrically with Helicobacter pylori demonstrated diminished production of the inflammatory cytokines TNF-α and IL-6 and reduced inflammation (16). Conversely, macrophages from histamine-deficient mice infected with the attenuated Mycobacterium bovis strain bacillus Calmette–Guérin demonstrated deficient production of cytokine IL-18 and increased intracellular bacterial survival (11). In contrast with the current report by Handley et al. (2), none of these previous studies involved an infection via a gastrointestinal route with an invasive pathogen. However, it is plausible that the role of histamine is specific for the particular pathogen, host, or tissue studied and that histamine can be either beneficial or detrimental depending on the pathogen interaction with the innate immune system.
A noteworthy contribution of this work by Handley et al. (2) was the application of global transcriptional profiling, by using microarrays, to an entire immune tissue from an intact animal infected by a natural route. This type of methodology has been used only rarely previously, for example, to define changes in gene expression in the spleen or whole blood after malarial infection (17) or of mouse intestine (and then of its individual constituent cells, elegantly isolated through laser-capture microdissection) to a newly introduced commensal bacterium (18). These previous studies have demonstrated the power of the unbiased, global characterization of whole tissues in suggesting previously unknown host responses to microbes, one of the key components of a systems biology approach to forming a comprehensive, integrated model of these complex interactions (7). Studies such as these provide a strong argument for the utility of transcriptional profiling, despite evidence from early global transcriptional profiling studies in yeast that indicated a lack of correlation between transcription and associated function (19). Recently, the systems biology approach also has borne fruit by defining new inhibitory responses important to mouse survival on exposure to lipopolysaccharide, the endotoxin that results in septic shock (20). Systems biology has been slow to gain acceptance because of its exploratory nature and the lack of a linear hypothesis. However, these current studies and those to come will likely demonstrate that, similar to our current molecular marker-driven approach to neoplasms, unique and common signatures of infection with different infectious agents should revolutionize our approach to infectious diseases. Systems biology, essentially a headlong appraisal of haystacks, seems hopelessly complex, but needle-yielding studies such as this one will go a long way toward allaying these concerns within the scientific community.
Footnotes
- ‡To whom correspondence should be addressed. E-mail: millersi{at}u.washington.edu
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Author contributions: L.R.H. and S.I.M. wrote the paper.
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See companion article on page 9268 in issue 24 of volume 103.
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Conflict of interest statement: No conflicts declared.
- © 2006 by The National Academy of Sciences of the USA
