Inflammation is a double-edged sword. Controlled and modest proinflammatory responses can enhance host immunity against viruses and decrease bacterial colonization and infection, whereas excessive uncontrolled proinflammatory responses may increase the susceptibility to bacterial colonization and secondary infection to facilitate disease pathogenesis. The immune system produces both proinflammatory and anti-inflammatory cytokines and chemokines. It is a balanced response that is key to maintaining good health.
Viral upper respiratory tract infections (URIs) are caused by rhinoviruses, coronaviruses, enteroviruses, respiratory syncytial viruses, influenza A and B viruses, parainfluenza viruses, adenoviruses, and human metapneumoviruses. Viruses are powerful. In the nose, they induce hypersecretion of mucus, slow cilia beating, up-regulate nasal epithelial cell receptors to facilitate bacterial attachment, suppress neutrophil function, and cause increased release of proinflammatory cytokines and chemokines. All these actions by respiratory viruses promote bacterial overgrowth in the nasopharynx and thereby facilitate bacterial superinfections. In fact, progression in pathogenesis of the common bacterial respiratory infections – acute otitis media, acute sinusitis, acute conjunctivitis, and pneumonia – almost always is preceded by a viral URI. Viruses activate multiple target cells in the upper respiratory tract to produce an array of proinflammatory cytokines and chemokines. The symptoms of a viral URI resolve coinciding with an anti-inflammatory response and adaptive immunity.
In recent work, we found a higher frequency of viral URIs in children who experienced more frequent acute otitis media (AOM). We sought to understand why this might occur by comparing levels of inflammatory cytokines/chemokines in the nose during viral URI that did not precipitate AOM versus when a viral URI precipitated an AOM episode. When a child had a viral URI but did not go on to experience an AOM, the child had higher proinflammatory responses than when the viral URI precipitated an AOM. When differences of levels of proinflammatory cytokines/chemokines were compared in otitis-prone and non–otitis-prone children, lower nasal responses were associated with higher otitis-prone classification frequency (Clin Infect Dis. 2018.).
The powerful virus and the inflammatory response it can induce also play a major role in allergy and asthma. Viral URIs enhance allergic sensitization to respiratory viruses, such as influenza and respiratory syncytial virus, cause cytopathic damage to airway epithelium, promote excessive proinflammatory cytokine/chemokine production, and increase the exposure of allergens and irritants to antigen-presenting cells. Viral infections also may induce the release of epithelial mediators and cytokines that may propagate eosinophilia. Viral URIs, particularly with respiratory syncytial virus and rhinovirus, are the most common causes of wheezing in children, and they have important influences on the development of asthma. Studies have shown that viral infections trigger up to 85% of asthma exacerbations in school-aged children.
Because this column is being published during the winter, a brief discussion of influenza as a powerful virus is appropriate. Influenza occurs in winter outbreaks of varying extent every year. The severity of the influenza season reflects the changing nature of the antigenic properties of influenza viruses, and their spread depends on susceptibility of the population. Influenza outbreaks typically peak over a 2-3 week period and last for 2-3 months. Most outbreaks have attack rates of 10%-20% in children. There may be variations in disease severity caused by different influenza virus types. The symptoms are caused by excessive proinflammatory cytokine/chemokine production in the nose and lung.
Influenza and other viruses can precipitate the systemic inflammatory response syndrome (SIRS), a manifestation of extreme immune dysregulation resulting in organ dysfunction that clinically resembles bacterial sepsis. In this syndrome, tissues remote from the original insult display the cardinal signs of inflammation, including vasodilation, increased microvascular permeability, and leukocyte accumulation. SIRS is another example of the double-edged sword of inflammation.
The onset and progression of SIRS occurs because of dysregulation of the normal inflammatory response, usually with an increase in both proinflammatory and anti-inflammatory cytokines and chemokines, initiating a chain of events that leads to organ failure.
Dr. Pichichero is a specialist in pediatric infectious diseases and director of the Research Institute at Rochester (N.Y.) General Hospital. He reported having no conflicts of interest. Email him at [email protected].