Defending the fortress
Professor Jan Shute, Professor of Respiratory Pharmacology, offers valuable insight into the workings of the body's immune system and the ways in which University of Portsmouth research is informing the development of effective COVID-19 treatment.
Defending the human body against viral intruders
In health, our bodies are fortresses, excluding potential disease-causing bacteria and viruses on a daily basis. The largest surface of the body in contact with the environment is in the lungs, which has a total surface area of approximately 70 m2 or the size of half a tennis court. On a daily basis it is estimated that we inhale approximately 6 x 10⁶ (six million) virus-like particles (20–500 nm, nanometers, in diameter) and the same number of bacteria-like particles (500 nm to 5000 nm in diameter). To put these dimensions into perspective, a human hair is around 75,000 nm in diameter. Since the average human spends 90% of their time indoors under normal circumstances, most of these bacteria and viruses are encountered indoors. Most bacteria are not disease-causing pathogens and humans normally harbour 10¹² microorganisms on their skin and 10¹⁴ microorganisms in their gut. Conversely, most, but not all, viruses cause disease which can range from a mild cold to severe life-threatening viral pneumonia. Viruses cause disease by invading host cells and taking over the cellular machinery to replicate and spread the infection to neighbouring cells.
The human body is normally defended against viral intruders by innate and adaptive immunity, the two arms of the immune system that work together, firstly to exclude disease-causing pathogens and then to eliminate them following an infection. The innate immune system detects viruses by recognizing molecular patterns within viral components, and stimulates the production of powerful antiviral molecules called interferons, so-called because these small peptides interfere with the replication of viruses in neighbouring cells and limit the spread of infection.
The induction of type I interferon (IFN-α and IFN-β) synthesis is deficient or delayed in patients with asthma, chronic obstructive pulmonary disease (COPD) and cystic fibrosis (CF), making these patient groups highly susceptible to virus infections of the lungs. In most cases, an increase in symptoms, for example during an asthma attack, is associated with a viral infection. In addition, elderly individuals do not respond to virus exposure as robustly as the young, making the elderly increasingly susceptible to viral infection, particularly over the age of 70. Impaired innate immunity is therefore a feature of respiratory viral infection in these groups of immunocompromised people. The observation of a deficiency in IFN-β in asthma and COPD led to clinical trials of inhaled IFN-β in asthma and COPD as a novel therapeutic approach aimed at limiting exacerbations of disease.
The recent emergence of the highly infectious and pathogenic severe acute respiratory syndrome (SARS)-corona virus-2 (SARS-CoV-2) has led to the search for an effective anti-viral drug to treat coronavirus, for which none is currently available. The effort is focussed on repurposing available anti-viral drugs, including interferon therapy. The Respiratory Centre at the Queen Alexandra Hospital in Portsmouth, directed by Professor Anoop Chauhan, Honorary Professor of Respiratory Medicine at the University of Portsmouth, is one of ten centres participating in a placebo-controlled clinical trial (the RECOVERY trial) of inhaled IFN-β over 14 days in patients with proven coronavirus infection.
Coronavirus infection stimulates a cascade of immune and pro-inflammatory responses that further enhance the airway inflammation that is already present in people with asthma, COPD and CF. The airway epithelial cells at the site of infection produce the earliest wave of pro-inflammatory cytokines, small peptides which in turn trigger local inflammatory and systemic responses. The first cytokines made in this cascade are associated with the systemic infection symptoms of fever, anorexia, and sleepiness. Some of these cytokines promote the entry of inflammatory white blood cells to the site of infection, and stimulate the local activation of these cells, which function directly to kill the virus and to remove infected host cells. However, collateral damage to bystander cells is a common side effect of inflammatory cell influx and activation.
Thus viral infection induces cell and tissue damage both directly and indirectly, via the inflammatory response, which can result in lung tissue damage and impaired lung function. Viral infection may affect large and small airways and, in the most severe cases, inflammation results in destruction of the air sacs, or alveoli, where gas exchange takes place resulting in pneumonia, as reported for the effect of SARS-CoV-2 in severe cases requiring ventilation.
Anti-inflammatory therapy would seem an appropriate therapeutic approach to preserve lung tissue in the face of virus infection. However, anti-inflammatory corticosteroids may compromise the immune response, and the non-steroidal anti-inflammatory drugs (NSAIDS) have been reported to be associated with respiratory complications.
Researchers in my laboratory in the School of Pharmacy and Biomedical Sciences have been investigating an alternative approach to the treatment of airway inflammation which is based on repurposing heparin, a drug normally used intravenously as an anticoagulant. Heparin is a unique molecule, the most highly negatively charged in biology, with a diverse molecular structure and multiple molecular targets. Our research has identified novel anti-inflammatory and mucus-thinning properties of heparin, effects that are independent of anti-coagulation activity when heparin is inhaled directly into the airways. These observations translated into significant improvements in lung function with the use of inhaled heparin over 21 days of treatment in a clinical trial in patients with moderate to severe COPD. Inhaled heparin may therefore represent an effective add-on therapy in COPD and asthma patient groups. In addition, we propose that the multiple pharmacological effects of inhaled heparin are likely to have therapeutic benefit in patients with inflammatory lung disease in SARS-CoV-2 infected patients.
In further support of this notion, we have identified inhibitory effects of heparin on the mechanisms of viral adhesion to the cells lining the airways, which are the airway epithelial cells. Adhesion is via a viral spike protein, S, binding to a specific protein, the angiotensin-converting enzyme (ACE)-2, expressed in airway epithelial cells. We have confirmed direct binding of heparin, and novel non-anticoagulant derivatives, to the SARS-CoV-2 S protein. We are currently collaborating with the University of Liverpool to investigate the inhibitory effect of these interactions on virus adhesion to human lung epithelial cells. These are important experiments since inhibition of viral adhesion suggests prevention of infection in those at risk is possible.
Other inhibitory effects of heparin on virus attachment are also predicted. Heparan sulphate proteoglycans (HSPG) on cell surfaces play an important role in virus attachment and infection of host cells by viruses including some types of coronavirus. Viral adhesion to HSPG seems in most of the cases to be the first step of the infection process and is followed by interaction of viral proteins with secondary receptors for adhesion and cell entry. In general, viral proteins binding to HSPG contain basic positively charged amino acids that bind to the negatively charged HSPG carbohydrate chain. In most cases viral adhesion to HSPG can be blocked by heparin, which is more highly sulphated and therefore more negatively charged than HSPG. It appears that heparin can displace viruses from the host cell surface at the beginning of the adhesion process, before the viruses have established contact with secondary receptors, such as ACE-2, which would make them resistant to heparin competition.
In patients with airway infection, it was found that sputum samples retain SARS-CoV-2 for 39 days after throat swabs are negative. Therefore the sputum-thinning (mucolytic) effects of inhaled heparin are also likely to improve clearance of virus from the airways. Overall, the evidence is that inhaled heparin will prevent initial attachment of SARS-CoV-2 to airway epithelial cells and promote mucus clearance of the virus. The additional anti-inflammatory properties of inhaled heparin may serve to limit the tissue damage following viral infection and avoid the development of life-threatening pneumonia.
I am currently advising Public Health England on the design of a clinical trial (the ACCORD-2 study) which is testing a number of candidate anti-viral agents, including inhaled heparin in patients hospitalised with SARS-CoV-2.