Converging pathways in pulmonary fibrosis and Covid-19 - The fibrotic link to disease severity.

As Covid-19 affects millions of people worldwide, the global health care will encounter an increasing burden of the aftermaths of the disease. Evidence shows that up to a fifth of the patients develop fibrotic tissue in the lung. The SARS outbreak in the early 2000 resulted in chronic pulmonary fibrosis in a subset (around 4%) of the patients, and correlated to reduced lung function and forced expiratory volume (FEV). The similarities between corona virus infections causing SARS and Covid-19 are striking, except that the novel coronavirus, SARS-CoV-2, has proven to have an even higher communicability. This would translate into a large number of patients seeking care for clinical signs of pulmonary fibrosis, given that the Covid-19 pandemic has up till now (Sept 2020) affected around 30 million people. The SARS-CoV-2 is dependent on binding to the angiotensin converting enzyme 2 (ACE2), which is part of the renin-angiotensin system (RAS). Downregulation of ACE2 upon virus binding disturbs downstream activities of RAS resulting in increased inflammation and development of fibrosis. The poor prognosis and risk of developing pulmonary fibrosis are therefore associated with the increased expression of ACE2 in risk groups, such as obesity, heart disorders and aging, conferring plenty of binding opportunity for the virus and subsequently the internalization of ACE2, thus devoiding the enzyme from acting counter-inflammatory and antifibrotic. Identifying pathways that are associated with Covid-19 severity that result in pulmonary fibrosis may enable early diagnosis and individualized treatment for these patients to prevent or reduce irreversible fibrotic damage to the lung.