COVID-19 may present with mild, moderate, or severe illness. The severe clinical manifestations include severe pneumonia, Acute Respiratory Syndrome (ARDS), sepsis, and septic shock (Cascella M, Rajnik M, Cuomo A, et al. Features, Evaluation and Treatment Coronavirus (COVID-19) [Updated 2020 Mar 8]. In: StatPearls-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK554776/).
While the virus itself is only moderately pathogenic and about 50% of infected individuals barely notice (“a bad flu”). The others suffer from the direct effect of the virus on lung cells, plus a violent response from the innate immune system, similar to what happens in sepsis. In sepsis, the response is systemic and is called “cytokine storm”. In most covid-19 patients, the response is ARDS, which is more localized to the lungs. In some patients, sepsis occurs. In either case, ARDS or sepsis, the patient requires intensive care, and mortality is high. We thus focus on the covid-19 patients that are more critically ill. In their case, the focus should be on allaying the hyper-inflammatory response.
Our working hypothesis is that the violent inflammatory response to SARS-CoV, linked to ARDS, in some cases to sepsis and leading in any case to high mortality, is caused by the excessive activation of Toll-like Receptor 4 (TLR4), one of the most important receptors expressed on cells of the immune system. TLR4, that normally responds to the presence of gram-negative bacteria endotoxin (mainly lipopolysaccharide, LPS), triggering the inflammatory and innate immunity response, can be also activated by endogenous molecules, called Damage-Associated Molecular Pattern molecules (DAMPs), released by the pulmonary tissue damaged by viral infection. In the case of SARS-CoV2 infection, the DAMPs released as a consequence of the acute lung injury (ALI), encompass oxidized phospholipids and, very likely, the protein HMGB1.
Toll-like Receptor TLR4, mainly expressed on cells of the immune system including monocytes, macrophages and dendritic cells, is the PRR that senses gram-negative bacteria endotoxin (LPS). LPS is a highly potent inflammatory stimulus, and LPS/TLR4 signalling has been strongly implicated in Gram-negative septic shock, including acute respiratory distress syndrome caused by endothelial leak. TLR4 has also been shown to be a sensor for endogenous DAMPs, included HMGB1. Stimulation of TLR4 by PAMPs or DAMPs results in the activation of two different intracellular signal pathways, the myeloid differentiation primary response 88 (MyD88), and/or toll/interleukin-1 receptor (TIR)-domain-containing adapter-inducing interferon-β (TRIF), ultimately resulting in the expression and secretion of pro-inflammatory mediators. Influenza-infected MyD88-/- and MyD88/TRIF doubly deficient mice show a marked reduction of pulmonary cytokine production when compared with WT mice, indicating the important role of these TLR signalling pathways in disease. Chemical or microbial insults to lung tissues, trigger the production of DAMPs such as oxidized phospholipids and HMGB1 protein, starting a common TLR4-, TRIF-, and IL-6-dependent pathway in macrophages that leads to acute lung injury (ALI).
We showed that treatment of influenza-infected mice with synthetic glycolipids projected and produced in our group (FP compounds) with strong activity as TLR4 antagonists, blocked influenza-induced lethality and ALI. We also observed at a molecular level that FP glycolipids block TLR4 activation by occupying MD-2 binding pocket and are able to inhibit HMGB1/TLR4 signalling.
Roche’s biologic drug blocking IL-6 receptor (Actemra or Tocilizumab), first approved by the U.S. FDA in 2010 for rheumatoid arthritis, has been used in China and Italy to treat serious coronavirus patients with lung damage. This antibody blocks the inflammatory action of IL-6, a cytokine produced as a consequence to TLR signalling. We think that blocking the HMGB1 and TLR4 signal upstream to IL-6 production could impact even stronger on pulmonary injury caused by COVID19.
In conclusion, stimulation of TLR4 by DAMPs could be an important molecular mechanism by which SARS-COV2 induce pulmonary damage, strong immune and inflammatory responses, ARDS and sepsis-like syndrome.