Abstract
Haemophilus influenzae is divided into typeable or nontypeable strains based on the presence or absence of a polysaccharide capsule. The typeable strains (such as type b) are an important cause of systemic infection, whilst the nontypeable strains (designated as NTHi) are predominantly respiratory mucosal pathogens. NTHi is present as part of the normal microbiome in the nasopharynx, from where it may spread down to the lower respiratory tract. In this context it is no longer a commensal and becomes an important respiratory pathogen associated with a range of common conditions including bronchitis, bronchiectasis, pneumonia, and particularly chronic obstructive pulmonary disease. NTHi induces a strong inflammatory response in the respiratory tract with activation of immune responses, which often fail to clear the bacteria from the lung. This results in recurrent/persistent infection and chronic inflammation with consequent lung pathology. This review will summarise the current literature about the lung immune response to nontypeable Haemophilus influenzae, a topic that has important implications for patient management.
Highlights
Nontypeable Haemophilus influenzae (NTHi) is a bacterium that is present in the nasopharynx of most healthy adults and in this situation it appears to be a commensal [1]
There has been increasing evidence to show that this bacterium is highly prevalent and pathogenic in a variety of important lower respiratory conditions including chronic obstructive pulmonary disease (COPD), bronchiectasis, cystic fibrosis, and pneumonia
The “British Hypothesis” proposed that bacterial infection had a primary role in the pathogenesis of COPD [19], whilst subsequently the work of Fletcher and Peto found no association [20] and for many years bacteria were regarded as being peripheral to the pathogenesis of COPD by clinicians with a respiratory/pulmonary background despite publications highlighting its potential importance [21, 22]
Summary
Nontypeable Haemophilus influenzae (NTHi) is a bacterium that is present in the nasopharynx of most healthy adults and in this situation it appears to be a commensal [1]. There has been increasing evidence to show that this bacterium is highly prevalent and pathogenic in a variety of important lower respiratory conditions including chronic obstructive pulmonary disease (COPD), bronchiectasis, cystic fibrosis, and pneumonia. It is not clearly understood why NTHi appears to be a commensal in the pharynx but in the lower respiratory tract is an important respiratory mucosal pathogen. The presence of NTHi in the lower respiratory tract induces activation of innate and adaptive immune responses that often fail to clear the bacteria from the lung This results in recurrent/persistent infection and chronic inflammation with consequent lung pathology. Upper respiratory tract disease with this bacterium is beyond the scope of this review
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