COVID-19 has been described largely as a respiratory disease; indeed, the respiratory tract and alveolus are amongst the primary sites of infection. However, it is also an inflammatory disease where release of inflammatory cytokines is the cause of organ injury and damage. The endothelium is the converging site of the inflammation as its activation (expression of adhesion molecules and cytokines) leads to immune cell recruitment; thus it is reasonable to conclude that COVID-19 is potentially a vascular disease [11, 28, 29]. While this would be an indirect impact of the virus, more recent studies also provide evidence of a direct effect i.e. infection by SARS-CoV-2 virus of endothelial cells . Our inspection of autopsies of the 8 COVID-19 patients showed macro and microthrombi in almost all fields imaged, indicating coagulation pathology. This was not observed in autopsies of non-COVID-19 lung sections. As is well established, coagulation is closely linked to endothelial inflammation signaling; inflammatory moieties on the endothelium increase leukocyte infiltration and alter coagulation control driving a procoagulant direction . Thus, COVID-19 which is increasingly being described as a vascular disease should perhaps be more accurately defined as a pathology which has its origins in “endothelial inflammation” signaling.
Inflammasome activation on the endothelium plays a major part in cell death and injury with inflammation. The NLRP3 inflammasome is a multiprotein complex comprised of three basic components: (1) A sensor such as a NOD-like receptor (NLR) (2) the adaptor protein apoptosis-associated speck-like protein containing a caspase-recruitment domain (ASC) and (3) the inflammatory cysteine aspartase caspase-1. The assembly of this complex leads to release of caspase-1 which then exerts its catalytic activity on the pro-inflammatory cytokines (IL-1β) that after their release perpetuate cell death, specifically inflammation induced cell death or pyroptosis [17, 18].
A recent report showed high levels of NLRP3 inflammasome and caspase-1 in patients with fatal COVID . This is not surprising as increased NLRP3 is associated with various inflammatory lung pathologies including acute lung injury and ARDS [32, 33]. The COVID-19 lung autopsies in this study, showed NLRP3 expression throughout the lung, but intense expression was seen along the lung vessel walls implying inflammasome expression on the endothelium. The downstream effector of NLRP3 inflammasome activation, Caspase-1 was found to be expressed throughout the lungs including in the vascular structures. Caspase-1 is considered as a key pyroptotic mediator; it reportedly drives pulmonary vascular endothelial cell death . Elsewhere too, high caspase-1 expression has been reported with both COVID-19  and with other lung inflammatory pathologies ; however its expression on the endothelium or vascular wall with COVID-19 has not been documented. Possibly the NLRP3-caspase-1 axis can directly (via caspase-1 driven pyroptosis) or indirectly (via NLRP3 driven chemotactic immune cell recruitment ) injure the endothelial layer. This confluence of vascular injury, thrombosis and dysregulated inflammation seems to propagate lung damage with COVID-19 and supports a pivotal role for the pulmonary endothelium in severe and fatal COVID-19. In contrast, non-COVID-19 lungs of subjects that did not have respiratory disease, had significantly lower expression of NLRP3 and caspase-1, indicating that an engagement of the NLRP3 pathway in COVID-19 and in ARDS.
As NLRP3 inflammasome driven pyroptosis is being considered to play a leading role in the pathogenesis of multi-organ failure with COVID-19 , there is some speculation on the mechanisms by which inflammasome activation occurs upon SARS-CoV-2 infection. One possibility is that the SARS-CoV-2 spike protein’s binding to cell surface-expressed angiotensin-converting enzyme 2 (ACE2) directly triggers its enzymatic activation and alters membrane polarity that can result in activation of NLPR3 inflammasome . Or NLRP3 could be activated via Angiotensin II which is reported to facilitate the assembly of the inflammasome. A third possibility could be via interaction of damage associated molecular patterns (DAMPs that are released post infection) and members of the complement cascade with the SARS-CoV-2 virus. Potent cleavage fragments of DAMPs and complement cascade can potentially activate the inflammasome . Yet another possibility is that the stretch from ventilation activates the inflammasome . Once activated around the vascular wall (endothelial layer), the NLRP3 inflammasome would lead to release of caspase-1 and interleukin-1β that would facilitate pyroptosis (cell death) of the endothelium (Schema 1).
To the best of our knowledge, this is the first study on NLRP3 expression in the vascular structures in lungs of fatal cases of COVID-19. The origin of several events that exacerbate inflammation and injury with COVID-19 (such as immune cell aggregation and extravasation, edema, formation of thrombi and leukopenia) possibly lies in pulmonary endothelial inflammasome activation and pyroptotic cell death. Therefore, NLRP3 inhibitors have been suggested for as a potential treatment strategy and are currently being explored for management of moderate COVID-19 symptoms (NCT04540120) [19, 40].
A major drawback of this study is that our sample size is small. Moreover, paraffin based post-mortem samples offer a snapshot of the disease and cannot recreate the evolving disease process. Histology is also impacted with the effects of clinical care and treatment as comorbidities, ventilation and medication pose as challenges in interpretation of results. Nevertheless, this study identifies endothelial NLRP3 inflammation, and documents thrombi and altered vascular structures in the lungs of fatal COVID-19 patients.