The first defence for bacterial infection in the lungs

By Yuchen Lin

Bacteria is everywhere, and it can be present even inside the human body. Some of the bacteria are beneficial for humans, as they promote cellular activities or immune responses, but others are pathogenic. One of the most common human tissues bacteria reside in is the lung, and Streptococcus pneumoniae is the leading cause of such invasive bacterial infection in children and the elderly. Apart from the infection itself, the host defence strategy can also cause pulmonary diseases. 

Streptococcus pneumoniae is a Gram-positive capsule-forming pathogenic anaerobic bacterium. It can cause pneumonia, as well as diseases like meningitis, endocarditis, and brain abscesses.¹ This bacterium has lancet-shaped cocci and occurs as either diplococci or single cocci in short chains. Upon reaching the lower respiratory tract, bacteria bypass the ciliated upper respiratory epithelial cells unless there is damage to the epithelium. They progress to the alveolus and associate with specific alveolar cells that produce choline-containing surfactants using their surface choline-binding proteins. Normally, a high bacterial load is required to trigger inflammatory responses, but if there is a proinflammatory signal such as previous viral infection, as few as 10 bacteria can ensue the inflammation and lead to considerable tissue damage. 

Neutrophils constitute the first line of defence against bacterial infections. They are short-lived and can be rapidly recruited from capillaries to the site of infection. A hallmark of S. pneumoniae lung infection is a robust proinflammatory response characterized by a massive influx of neutrophils into the alveoli.² Alveolar neutrophil recruitment is essential for reducing S. pneumoniae burden. The neutrophil transmigration across alveolar endothelial and epithelial cell barriers into the airspace is mainly driven by a major virulence factor of S. pneumoniae, pneumolysin. It is a pore-forming exotoxin that plays a prominent role in S. pneumoniae pathogenesis. This toxin is not actively secreted into extracellular alveolar space but released via S. pneumoniae autolysis mediated by pneumococcal enzyme autolysin or antibiotic treatment.¹ It fosters a hepoxilin A3-dependent neutrophil movement in a pore-dependent fashion. Hepoxilin A3 is a potent chemoattractant that is implicated in both intestinal and pulmonary inflammation induced during bacterial infection. The toxin activates phospholipase activity to release arachidonic acid from the plasma membrane and triggers 12-lipoxygenase production from host epithelial cells. They stimulate the metabolism of alveolar epithelial cells to produce hepoxilin A3 that drives neutrophil influx as well as non-resident macrophages into the alveolar space.² The activated alveolar neutrophils perform phagocytic killing or degranulation to release contents such as reactive oxygen species (ROS), proteases, and neutrophil extracellular DNA traps to confront invading bacteria and control bacterial outgrowth during the early stage of infection. 

S. pneumoniae infection activates signals that induce large amounts of cytokine and chemokine production from alveolar macrophages and epithelial cells. Recruited alveolar neutrophils have increased chemokine receptor expression than circulating neutrophils to promote bacterial clearance.³ They eradicate infection via phagocytic killing which is promoted by opsonization of bacterium with IgG antibody specifically against pneumococcal capsular polysaccharides. Adaptor molecule CARD9 plays an essential role in regulating chemokine production and subsequent neutrophil infiltration and accumulation inside alveoli. This is critical in host defence through enhancing neutrophil accumulation and promoting bacteria elimination from infected alveoli. Chemokines like KC and MIP-2 directly facilitate the migration, and chemokine receptor CXCL1 increases neutrophil influx from bone marrow to alveoli in a CD62L- and CD49d-dependent manner. Dectin-2 recognizes pneumococcal polysaccharides and induces dendritic cells to produce IL-12, which activates the CARD9-mediated signalling pathway and induces IFNg synthesis. IFNg promotes the production of IgG3 anti-pneumococcal polysaccharide antibodies that enhance neutrophil phagocytosis capacity.⁴ CXCR2 is necessary for immunity against S. pneumoniae infection. Together with its respective ligands and other receptors including CCR2, they synergistically regulate the overall inflammatory recruitment and local activation of leukocytes such as neutrophils in the infected alveoli.  

As a member of the cholesterol-dependent cytolysin (CDC) family, pneumolysin promotes acute inflammation. It binds free cholesterol and inserts itself into the lipid-rich bilayer of the host cell membrane to assemble into a ring containing 30 to 50 monomers.⁵ This forms a pore on the host cell membrane and increases Ca2+ permeability, inducing profound capillary leakage that is associated with diseases like acute lung injury. Pneumolysin also decreases the bactericidal activity of alveolar neutrophils and leads to greater inflammation and cytotoxic effects. To achieve this, it forms pores on the neutrophil membrane to trigger neutrophil lysis and release of elastase. The elastases impair macrophage phagocytic activity, induce detachment and death of alveolar epithelial cells, and cause macrophages to synthesise neutrophil attractant CXCL8. This forms a positive feedback loop that recruits more neutrophils and results in more alveolar epithelial damage. Pneumolysin induces platelet-activating factor (PAF) and thromboxane A2 (TXA2) synthesis from neutrophils. These two molecules are vasoactive mediators that decrease renal blood flow. TXA2 specifically increases platelet aggregation in the infected spaces.⁶ Together, these activities impair the host first defence against S. pneumoniae.

Meanwhile, the final step of neutrophils across the lung epithelium into the alveoli may disrupt mucosal barrier function, and cause dissemination of S. pneumoniae in the bloodstream and lethal septicaemia. Meanwhile, over-activated neutrophils or sustained neutrophil infiltration can lead to pulmonary oedema and severe lung damage because of their enormous capacity to release ROS and proteolytic enzymes.¹ They disrupt the epithelial barrier and lead to fluid infiltration into the alveolar airspace, causing acute lung injury directly. Recruited neutrophils rapidly undergo apoptosis or necrosis and subsequently contribute to the formation of consolidated infiltrates. This blocks alveolar gas exchange and impairs host defence in the lung.³ Thus, the timing and degree of alveolar neutrophil recruitment are essential. 

Therapeutics are needed to reduce or prevent neutrophilic pulmonary damage in S. pneumoniae infected patients. Inhibitors of chemokines reduce the inflammation but also the neutrophil level, leading to a higher bacterial load in the initial stage of infection. A promising drug target for suppressing alveolar neutrophil-mediated pulmonary damage in bacterial infection has been inositol hexakisphosphate kinase 1 (IP6K1). IP6K1-mediated inorganic polyphosphate production by platelets is essential for facilitating neutrophil accumulation in alveolar spaces. It is also the main enzyme responsible for neutrophil phagocytosis and ROS production capacity.⁷ But IP6K1 mediates bacterial lipopolysaccharide-induced neutrophil-platelet aggregation which block alveolar airspaces. IP6K1 inhibition can reduce aggregate formation. Research showed that disrupting the Ip6k1 gene or pharmacologically inhibiting IP6K1 activity using specific inhibitor TNP can efficiently and effectively enhance neutrophil bactericidal capacity, elevate ROS production, and reduce pulmonary neutrophil accumulation, minimizing lung damage caused by S. pneumoniae.⁸ 

The pneumococcal infection causes millions of deaths worldwide. Although neutrophils respond to the infection rapidly, they can sometimes cause damage to the lungs. There are a variety of treatments against bacterial infection in the lungs, and therapeutics against these neutrophil-mediated alveolar injuries can efficiently reduce the secondary damage raised by the host defence mechanism.

References:

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8. Hou, Q. et al. (2019) Inhibition of IP6K1 suppresses neutrophil-mediated pulmonary damage in bacterial pneumonia. Sci Transl Med. 10(435): eaal4045. Doi: 10.1126/scitranslmed.aal4045