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Microbiome and innate immunity in the respiratory tract – a primer



The pathogenesis of respiratory infections such as acute otitis media (AOM), sinusitis, and pneumonia involves complex interactions among bacteria, respiratory viruses, and host immune responses.

My clinical and laboratory group and others have described respiratory infections as resulting from the growth of a single otopathogen, such as Streptococcus pneumoniae (Spn), nontypeable Haemophilus influenzae (NTHi), or Moraxella catarrhalis (Mcat) in the nasopharynx (NP) followed by ascension to the middle ear, sinuses, or descent to the lungs. Recent research from my group and others has resulted in a shift from a single pathogen focus toward consideration of respiratory infections as a polymicrobial disease. Bacterial and viral interactions are critical in respiratory infection pathogenesis. Commensal bacteria can alter virulence of bacterial pathogens and interfere with antibiotic treatment.

The traditional view of the immune system is that it is an assembly of human tissues, cells, and molecules that work to eliminate pathogens. Recent discoveries indicate that commensals play a central role in regulating human immune responses. Thus, the key questions in the field are:

1) How do members of the NP microbiome and innate immune responses maintain health in young children over time?

2) Do specific deleterious members of the NP microbiome alter host innate immune responses in a manner that predisposes to respiratory infections?

3) How does the microbiome and innate response in the NP differ when recovery, relapse of infection, or persistent infection occurs?

Virtually all young children are colonized by Spn, NTHi, or Mcat during the first 3 years of life. My group and others have shown that competitive interactions among bacteria influence whether these potential pathogens successfully colonize and cause respiratory infections. Recent studies have demonstrated that hundreds of different bacterial species colonize the upper respiratory tract. Diverse communities have been shown to be more stable and resistant to invasion by foreign species. Data from cross-sectional studies demonstrate that specific commensals, including Dolosigranulum, Corynebacterium, and Lactococcus, are associated with decreased risk of respiratory infections. Prior studies have been limited by the use of culture-based methods or have been cross sectional in design. Therefore, the optimal levels of diversity and NP commensals critical for maintaining health in the upper respiratory tract of children are currently unknown and under study by my group and others. Studies that utilize high-throughput culture-independent molecular detection methods are now used to identify optimal levels of diversity and commensal members of the microbiome critical for maintaining health homeostasis.

The innate immune system constitutes the first line of defense against respiratory pathogen colonization and respiratory virus infection. It relies on pattern recognition receptors on innate immune cells to detect evolutionarily conserved pathogen-associated molecular patterns expressed on pathogen surfaces. Toll-like receptors (TLRs) are crucial in the innate immune response; TLR 3, 7, and 8 recognize respiratory infection-associated viral pathogens. TLR2, 4, and 5 recognize respiratory infection-associated bacterial pathogens, and TLR9 and TLR13 recognize both viral and bacterial pathogens. The activation of TLRs triggers signaling cascades and regulates the expression of a wide range of cytokines leading to antimicrobial and inflammatory responses. Cytokines (there are dozens) associated with the pathogenesis, development, severity, and clinical outcomes of respiratory infections identify hypotheses that our group is exploring to expand our understanding of how innate responses might be manipulated to favor the child host. Importantly, it has already been shown that cytokine profiles differ in the NP depending on the number and type of bacteria and viruses involved.

My group recently has shown that serum IL-10 levels are significantly higher in AOM from Spn than are the levels associated with NTHi and Mcat, suggesting use of detection of this cytokine as a serum biomarker. Others have shown that the levels of IL-1-beta, TNF-alpha, IL-6, IL-8, IL-10, and IL-17a in middle ear fluids from children with recurrent AOM correlate significantly with higher bacterial load (and worse disease). Previous studies on cytokine responses associated with AOM have focused on limited numbers of cytokines and have not examined any relationship with commensals of the NP microbiome. Moreover, the subset of children who experience excessively frequent respiratory infections likely have disturbances in their microbiome (made worse with antibiotics) and innate immune response. Because of our growing knowledge about the microbiome and innate immune response, I see a compelling need to assess interactions of the NP microbiome and innate immune responses in children that are associated with sustained health and control of respiratory infections.

Dr. Pichichero, a specialist in pediatric infectious diseases, is director of the Research Institute, Rochester (N.Y.) General Hospital. He is also a pediatrician at Legacy Pediatrics in Rochester. Dr. Pichichero said the work was supported by a National Institutes of Health grant, and he had no relevant conflicts of interest. E-mail him at [email protected].

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