TLR4 and COVID-19: In addition, elevated levels of circulating TLR4 DAMPs in COVID-19 patients (beta-defensin-3 [94]; fibrinogen [128,129]; heat-shock protein 70 [130]; HMGB1 [96]; syndecan [131]; S100A8/9 [96,100,132]; and surfactant A and D [133]) may be responsible for the feed-forward loop of the persistent inflammation, resulting in cytokine storm.