Janelle S. Ayres

Two ways to survive infection: what resistance and tolerance can teach us about treating infectious diseases

A host can evolve two types of defence mechanism to increase its fitness when challenged with a pathogen: resistance and tolerance. Immunology is a well-defined field in which the mechanisms behind resistance to infection are dissected. By contrast, the mechanisms behind the ability to tolerate infections are studied in a less methodical manner. In this Opinion, we provide evidence that animals have specific tolerance mechanisms and discuss their potential clinical impact. It is important to distinguish between these two defence mechanisms because they have different pathological and epidemiological effects. An increased understanding of tolerance to pathogen infection could lead to more efficient treatments for infectious diseases and a better description of host–pathogen interactions.

The ubiquitin ligase parkin mediates resistance to intracellular pathogens

Ubiquitin-mediated targeting of intracellular bacteria to the autophagy pathway is a key innate defence mechanism against invading microbes, including the important human pathogen Mycobacterium tuberculosis. However, the ubiquitin ligases responsible for catalysing ubiquitin chains that surround intracellular bacteria are poorly understood. The parkin protein is a ubiquitin ligase with a well-established role in mitophagy, and mutations in the parkin gene (PARK2) lead to increased susceptibility to Parkinson’s disease. Surprisingly, genetic polymorphisms in the PARK2 regulatory region are also associated with increased susceptibility to intracellular bacterial pathogens in humans, including Mycobacterium leprae and Salmonella enterica serovar Typhi, but the function of parkin in immunity has remained unexplored. Here we show that parkin has a role in ubiquitin-mediated autophagy of M. tuberculosis. Both parkin-deficient mice and flies are sensitive to various intracellular bacterial infections, indicating parkin has a conserved role in metazoan innate defence. Moreover, our work reveals an unexpected functional link between mitophagy and infectious disease.

Recognition of Bacteria by Inflammasomes

Inflammasomes are cytosolic multiprotein complexes that assemble in response to a variety of infectious and noxious insults. Inflammasomes play a critical role in the initiation of innate immune responses, primarily by serving as platforms for the activation of inflammatory caspase proteases. One such caspase, CASPASE-1 (CASP1), initiates innate immune responses by cleaving pro-IL-1β and pro-IL-18, leading to their activation and release. CASP1 and another inflammatory caspase termed CASP11 can also initiate a rapid and inflammatory form of cell death termed pyroptosis. Several distinct inflammasomes have been described, each of which contains a unique sensor protein of the NLR (nucleotide-binding domain, leucine-rich repeat-containing) superfamily or the PYHIN (PYRIN and HIN-200 domain-containing) superfamily. Here we describe the surprisingly diverse mechanisms by which NLR/PYHIN proteins sense bacteria and initiate innate immune responses. We conclude that inflammasomes represent a highly adaptable scaffold ideally suited for detecting and initiating rapid innate responses to diverse and rapidly evolving bacteria.

Tolerance of Infections

A host has two methods to defend against pathogens: It can clear the pathogens or reduce their impact on health in other ways. The first, resistance, is well studied. Study of the second, which ecologists call tolerance, is in its infancy. Tolerance measures the dose response curve of a hosts health in reaction to a pathogen and can be studied in a simple quantitative manner. Such studies hold promise because they point to methods of treating infections that put evolutionary pressures on microbes different from antibiotics and vaccines. Studies of tolerance will provide an improved foundation to describe our interactions with all microbes: pathogenic, commensal, and mutualistic. One obvious mechanism affecting tolerance is the intensity of an immune response; an overly exuberant immune response can cause collateral damage through immune effectors and because of the energy allocated away from other physiological functions. There are potentially many other tolerance mechanisms, and here we systematically describe tolerance using a variety of animal systems.

The Role of Anorexia in Resistance and Tolerance to Infections in Drosophila

Infections initiate a signaling loop in which sick animals become anorexic, and the resulting change in diet alters the bodys ability to fight infections in good and bad ways.

A signaling protease required for melanization in Drosophila affects resistance and tolerance of infections.

Organisms evolve two routes to surviving infections—they can resist pathogen growth (resistance) and they can endure the pathogenesis of infection (tolerance). The sum of these two properties together defines the defensive capabilities of the host. Typically, studies of animal defenses focus on either understanding resistance or, to a lesser extent, tolerance mechanisms, thus providing little understanding of the relationship between these two mechanisms. We suggest there are nine possible pairwise permutations of these traits, assuming they can increase, decrease, or remain unchanged in an independent manner. Here we show that by making a single mutation in the gene encoding a protease, CG3066, active in the melanization cascade in Drosophila melanogaster, we observe the full spectrum of changes; these mutant flies show increases and decreases in their resistance and tolerance properties when challenged with a variety of pathogens. This result implicates melanization in fighting microbial infections and shows that an immune response can affect both resistance and tolerance to infections in microbe-dependent ways. The fly is often described as having an unsophisticated and stereotypical immune response where single mutations cause simple binary changes in immunity. We report a level of complexity in the flys immune response that has strong ecological implications. We suggest that immune responses are highly tuned by evolution, since selection for defenses that alter resistance against one pathogen may change both resistance and tolerance to other pathogens.

Lethal inflammasome activation by a multidrug-resistant pathobiont upon antibiotic disruption of the microbiota

The mammalian intestine harbors a complex microbial community that provides numerous benefits to its host. However, the microbiota can also include potentially virulent species, termed pathobiont, which can cause disease when intestinal homeostasis is disrupted. The molecular mechanisms by which pathobionts cause disease remain poorly understood. Here we describe a sepsis-like disease that occurs upon gut injury in antibiotic-treated mice. Sepsis was associated with the systemic spread of a specific multidrug-resistant Escherichia coli pathobiont that expanded markedly in the microbiota of antibiotic-treated mice. Rapid sepsis-like death required a component of the innate immune system, the Naip5-Nlrc4 inflammasome. In accordance with Kochs postulates, we found the E. coli pathobiont was sufficient to activate Naip5-Nlrc4 and cause disease when injected intravenously into unmanipulated mice. These findings reveal how sepsis-like disease can result from recognition of pathobionts by the innate immune system.

Identification of Drosophila Mutants Altering Defense of and Endurance to Listeria monocytogenes Infection

We extended the use of Drosophila beyond being a model for signaling pathways required for pattern recognition immune signaling and show that the fly can be used to identify genes required for pathogenesis and host–pathogen interactions. We performed a forward genetic screen to identify Drosophila mutations altering sensitivity to the intracellular pathogen Listeria monocytogenes. We recovered 18 mutants with increased susceptibility to infection, none of which were previously shown to function in a Drosophila immune response. Using secondary screens, we divided these mutants into two groups: In the first group, mutants have reduced endurance to infections but show no change in bacterial growth. This is a new fly immunity phenotype that is not commonly studied. In the second group, mutants have a typical defense defect in which bacterial growth is increased and survival is decreased. By further challenging mutant flies with L. monocytogenes mutants, we identified subgroups of fly mutants that affect specific stages of the L. monocytogenes life cycle, exit from the vacuole, or actin-based movement. There is no overlap between our genes and the hundreds of genes identified in Drosophila S2 cells fighting L. monocytogenes infection, using genomewide RNAi screens in vitro. By using a whole-animal model and screening for host survival, we revealed genes involved in physiologies different from those that were found in previous screens, which all had defects in defensive immune signaling.

Drosophila eiger Mutants Are Sensitive to Extracellular Pathogens

We showed previously that eiger, the Drosophila tumor necrosis factor homolog, contributes to the pathology induced by infection with Salmonella typhimurium. We were curious whether eiger is always detrimental in the context of infection or if it plays a role in fighting some types of microbes. We challenged wild-type and eiger mutant flies with a collection of facultative intracellular and extracellular pathogens, including a fungus and Gram-positive and Gram-negative bacteria. The response of eiger mutants divided these microbes into two groups: eiger mutants are immunocompromised with respect to extracellular pathogens but show no change or reduced sensitivity to facultative intracellular pathogens. Hence, eiger helps fight infections but also can cause pathology. We propose that eiger activates the cellular immune response of the fly to aid clearance of extracellular pathogens. Intracellular pathogens, which can already defeat professional phagocytes, are unaffected by eiger.

Disease tolerance mediated by microbiome E. coli involves inflammasome and IGF-1 signaling.

The benefits of Escherichia coli Infection and intestinal damage can trigger severe muscle wasting and loss of fat in mice. How this happens is poorly understood. Palaferri Schieber et al. discovered a protective Escherichia coli strain in their mouse colony. Mice intestinally colonized with the E. coli and infected with the food-poisoning bug Salmonella or with the lung pathogen Burkholderia did not waste away. Without the E. coli, similarly infected mice became fatally ill. The protective E. coli stimulated an innate immune mechanism that ensured that muscle-signaling pathways were not damaged by infection. Thus, the friendly E. coli allowed its host to tolerate and survive the pathogens. Science, this issue p. 558 Gut bacteria help the host by promoting tolerance to infection and limiting physiological damage. Infections and inflammation can lead to cachexia and wasting of skeletal muscle and fat tissue by as yet poorly understood mechanisms. We observed that gut colonization of mice by a strain of Escherichia coli prevents wasting triggered by infections or physical damage to the intestine. During intestinal infection with the pathogen Salmonella Typhimurium or pneumonic infection with Burkholderia thailandensis, the presence of this E. coli did not alter changes in host metabolism, caloric uptake, or inflammation but instead sustained signaling of the insulin-like growth factor 1/phosphatidylinositol 3-kinase/AKT pathway in skeletal muscle, which is required for prevention of muscle wasting. This effect was dependent on engagement of the NLRC4 inflammasome. Therefore, this commensal promotes tolerance to diverse diseases.