Evasion of immune defenses
Having overcome the first layer of host lung defenses, NTHi must also evade innate and adaptive immune responses. Pulmonary phagocytic cells (alveolar macrophages and neutrophils) constantly sense the local microenvironment for pathogen associated molecular patterns (PAMPs), which are molecules with highly conserved motifs associated with human pathogens. PAMPs bind to pattern recognition receptors, such as TLRs, which activate intracellular pathways to secrete chemotaxins, including IL-8 to recruit and upregulate neutrophils to the site of infection, and to initiate intracellular killing of phagocytosed pathogens.64 When phagocytes encounter NTHithere is a sustained release of ROS, both within intracellular phagolysosomes and into the extracellular environment. The generation of ROS is also associated with neutrophil and macrophage extracellular traps (NETs and METs, respectively), which are composed of a mesh of DNA, histones and bactericidal proteins to neutralize invading pathogens. The production of ROS represents the primary mechanism adopted by these cells to eliminate phagocytosed pathogens. Nevertheless, NTHi can neutralize the effects of oxidative stress by producing antioxidants, including catalase, peroxiredoxin-glutaredoxin and superoxide dismutase (SOD), which in addition to forming biofilms is one of the mechanisms it uses to resist NET.
Furthermore, submucosal plasma cells produce dimeric IgA molecules, which bind to an epithelial cell membrane protein, the secretory component, and are transported through AECs to be released as secretory IgA into mucosal secretions.67 IgA aids lung defenses by preventing microbial adherence, neutralizing toxins and boosting adaptive immunity. There are two types of IgA (IgA1 and IgA2) of which IgA1 accounts for more than 90% of the total IgA. However, NTHiproduces an IgA1 protease, IgaA, encoded by the IgaA gene as an extracellular endopeptidase that cleaves IgA1 at the hinge region, disabling its antibacterial functions and aiding adherence to and invasion of AECs. Approximately half the NTHi strains also possess a second IgA1 protease gene, IgaB, that encodes IgaB, which seems necessary for intracellular survival.
The human complement system is also part of the innate defenses against pathogenic bacteria. It has multiple functions and when activated complement mediates the inflammatory response and elimination of pathogens and enhances adaptive immunity. The complement system leads to C3b deposition on the bacterial cell surface with formation of the membrane attack complex (MAC) and bacterial lysis. There are three complement activation pathways, which can all be inhibited byNTHi . The classical and lectin pathways are blocked by the C4b binding protein (C4bP), whereas factor H is believed to be main inhibitor of the alternative pathway. C4bP and factor H are captured by OMP P5 and protectNTHi against opsonization and phagocytosis, while also preventing MAC formation. Vitronectin, a potent negative regulator of the terminal pathway can also be bound by NTHI proteins E and F, further inhibiting the formation of the MAC by NTHi . Moreover, LOS covering the outer cell wall of NTHi can block bactericidal antibodies accessing cell surface structures and activating the classical complement pathway.73 Finally, NTHimay benefit from other co-pathogens, such as Moraxella catarrhalis, and their inhibition of the complement system.74