Understanding acute LRTI

Acute lower respiratory tract infections (LRTIs) are a persistent and pervasive public health problem. They cause a greater burden of disease worldwide than HIV, malaria, cancer or heart attacks.¹ In the US they cause more disease and death than any other infection, and there has been little change in mortality due to respiratory tract infection for more than five decades.^1,2 The outcome of an acute LRTI depends on the virulence of the organism and the inflammatory response in the lung. When small numbers of low-virulence microbes are deposited in the lungs, an effective defence can be mounted by resident innate immune defences, such as the mucociliary escalator, antimicrobial proteins in airway surface liquid and alveolar macrophages. In contrast more virulent microbes elicit an inflammatory response. Although this response serves to reinforce innate immunity and is essential to rid the lungs of microbes, it contributes directly to lung injury and abnormal pulmonary function.

Acute inflammation features the accumulation of neutrophils and a plasma exudate outside of blood vessels. In the pulmonary capillaries of uninfected lungs, these blood contents are normally separated from the alveolar air by less than 1μm, the thinnest interface between the blood and the environment. The trapping of neutrophils in these capillaries, which is the result of geometric and biophysical constraints,³ increases their quantity per volume of blood by approximately 50 times as compared with other blood vessels, forming a marginated pool of neutrophils that is ready to respond when needed.

During pulmonary infection, neutrophils migrate out of the pulmonary capillaries and into the air spaces.⁴ Élie Metchnikoff, the discoverer of phagocytosis, considered neutrophils (or microphages, as he called them) to be “the defensive cells par excellence against microorganisms.”⁵ After phagocytosis, neutrophils kill ingested microbes with reactive oxygen species (eg. hypochlorite), antimicrobial proteins (eg. bactericidal permeability-inducing protein and lactoferrin), and degradative enzymes (eg.  elastase).⁶ An additional microbicidal pathway has also been identified — the neutrophil extracellular trap (NET). Neutrophils extrude NETs composed of a chromatin meshwork containing antimicrobial proteins, and these NETs ensnare and kill extracellular bacteria.⁷ It remains to be determined whether NETs are useful host defence mechanisms against motile microbes in the dynamic and unstructured liquid-filled air spaces of the infected lung.

The content of plasma proteins in the interstitium and air spaces of infected lungs is determined by the combined actions of pericellular bulk flow and transcellular transport by endothelial and epithelial cells. Many plasma proteins, including natural antibodies, complement proteins, C-reactive protein (originally identified in serum from patients with pneumonia), and pentraxin 3 are important in the defence against microbes in the lungs.⁸

Deficits in neutrophil quantity (neutropenia) and defects in quality (eg. chronic granulomatous disease) predispose patients to opportunistic lung infections, as do deficiencies of complement and immunoglobulins. Since neutrophils and plasma proteins mediate innate immune functions and are needed to prevent lung infection, acute inflammation can be considered an essential innate immune response in the lungs.

Microbes must be detected by host cells to initiate inflammation in infected lungs. The identification of microbial invaders relies on pattern-recognition receptors, which bind molecular moieties that are common to microbes.⁹ Discoveries of new families of pattern-recognition receptors, including toll-like receptors, nucleotide-binding and oligomerisation-domain proteins, and caspase recruitment domain helicases, have fuelled research in the biology of innate immunity.

For any one microbe, there are a variety of molecules that can activate many different pattern-recognition receptors. Perhaps for this reason, deficiencies of individual pattern-recognition receptors result in more modest phenotypes during experimentally-induced acute lower respiratory infections than deficiencies of downstream adapter proteins, which signal from multiple pattern-recognition receptors.¹⁰ The intracellular signalling pathways triggered by diverse pattern-recognition receptors converge on signalling hubs, such as transcription factors of the nuclear factor kB (NF-kB) and interferon regulatory factor families. These factors integrate signals from diverse stimuli (interacting with pattern-recognition receptors) and initiate responses. NF-kB mediates the transcription of adhesion molecules, chemokines, colony-stimulating factors, and other cytokines that are necessary for inflammatory responses.¹¹ In mice with bacterial stimuli in the lungs, NF-kB RelA (also known as p65) is required for inducing the production of adhesion molecules and chemokines as well as for initiating neutrophil recruitment and host defence. Interferon regulatory factors mediate the expression of type I interferons and interferon-induced antiviral genes. Interferon regulatory factor 3 influences parainfluenza virus infection in mouse lungs, but the genes and immune functions that require it or other interferon regulatory factors during lung infection remain unknown.¹²

Acute LRTIs can be monomicrobial or polymicrobial, with organisms ranging in virulence from commensal to highly pathogenic.¹³ These microbes have mechanisms for counteracting many of the effector and signalling events described above. Microbial subversion of individual pathways may be a selective pressure driving mammalian hosts to have multiple, parallel, sometimes redundant-seeming pathways for innate immunity.

Counteracting effector mechanisms of innate immunity is of obvious advantage to a microbe. Although NETs were discovered only recently,⁷ microbial countermeasures are already recognised. For example, NETs extruded by neutrophils fail to contain and kill pneumococci. A pneumococcal DNase cleaves NETs and frees bacteria. During infection, this DNase is a virulence factor that gives the bacteria a competitive advantage against DNase-mutated strains in the lungs of mice, resulting in increased mortality from pneumonia in such mice.

Preventing the host from detecting pathogens is another strategy often used by microbes. For example, the retinoic acid-inducible gene I intracellular pattern-recognition receptor for viral RNA is bound by an influenza virus protein that prevents downstream signalling, activation of interferon regulatory factor, and expression of type I interferon.¹⁴ Deleting this protein attenuates influenza virus infection, increasing type I interferon in the lungs and decreasing mortality.¹⁵ Many pathogens interrupt proinflammatory signalling pathways or mimic anti-inflammatory signalling pathways.

Not only do lung pathogens interfere with host signalling, they also listen in on these immune conversations and use this information to guide their responses appropriately. For example, P. aeruginosa expresses a receptor that recognises interferon-k, and in the presence of interferon-k, this receptor stimulates gene expression dictating biofilm formation.¹⁶ Since biofilms render bacteria more resistant to both innate immunity and antibiotics, this is probably an adaptive response during infection. In addition, P. aeruginosa and other bacteria respond to TNF-k and other cytokines with increased growth rates. In neutropaenic mice, the ability of TNF-k to increase bacterial growth worsens lung infection. Thus, pathogens sense innate immune signalling and respond in ways that subvert host defence and facilitate infection.

The mechanisms for generating and regulating acute inflammation, described above, determine the outcomes of experimentally-induced lung infections in animals. Deficiencies and polymorphisms in human genes for the factors involved in these mechanisms have been associated with lung infection and its consequences, such as disseminated or invasive infection or acute lung injury. Although the limitations of such genotype-phenotype associations warrant consideration, these data indicate that knowledge of innate immunity and lung infection derived from experiments in animals can apply to humans. Genetic variations in innate-immunity mediators influence the outcome of inevitable exposures of the human lower respiratory tract to microbes.

Another reason why human genotype-phenotype studies are important is that they occur in natural instead of laboratory environments. Infections involve intersections of host and microbe within complex and dynamic ecosystems not mimicked in laboratory studies. For example, patients with a deficiency of IRAK-4 (which signals from multiple pattern-recognition receptors) are susceptible to a narrower spectrum of microbes, over a narrower age range, and with more variation across the population than in vitro experiments with human cells or in vivo experiments with mice would suggest.¹⁷ Patients with an immunodeficiency tend to present with select subgroups of infections (eg. patients with chronic granulomatous disease are especially susceptible to five microbes130).

Environmental and genomic variations result in a range of susceptibility among patients with similar immunodeficiencies. In the future, polygenic analyses may demonstrate that combined polymorphisms in multiple genes influence lung infections more dramatically than monogenic variation, since parallel paths and redundancies are common in innate immunity. An emerging theme is that genetic susceptibility to infection is more common than is now appreciated; susceptibility is probably polygenic, with incomplete penetrance restricted to narrowly defined clinical phenotypes.¹⁸

Innate immune responses to microbes in the lungs determine the outcome of infection; an insufficient response can result in life-threatening infection, but an excessive response can lead to life-threatening inflammatory injury. Further studies will help identify populations that are particularly susceptible to severe lung infection and will guide prophylactic and therapeutic interventions.

References

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