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The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) outbreak in Wuhan, China has dispersed rapidly worldwide. Although most patients present with mild fever, cough with varying pulmonary shadows, a significant portion still develops severe respiratory dysfunction. And these severe cases are often associated with manifestations outside the respiratory tract. Currently, it is not difficult to find inflammatory cytokines upregulated in the blood of infected patients. However, some complications in addition to respiratory system with the coronavirus disease 2019 (COVID-19) are impossible to explain or cannot be attributed to virus itself. Thus excessive cytokines and their potentially fatal adverse effects are probably the answer to the multiple organ dysfunctions and growing mortality. This review provides a comprehensive overview of the mechanisms underlying cytokine storm, summarizes its pathophysiology and improves understanding of cytokine storm associated with coronavirus infections by comparing SARS-CoV-2 with severe acute respiratory syndrome coronavirus (SARS-CoV) and Middle East respiratory syndrome coronavirus (MERS-CoV)., Highlights • In December 2019, a novel coronavirus of SARS-CoV-2 emerged in Wuhan, Hubei, China, rapidly dispersed and even exported to neighboring countries. • SARS-CoV-2 generally causes a mild lower respiratory tract infection in humans, however, in some sever cases, signs of dysfunctions in multiple organs have been discovered. • Overproduction of cytokines, termed cytokine storm, may involve in disease progression and even be responsible for deaths. • Application of immune-modulators might become a complement to support treatment and flip on the light switch for severe patients.

Background Severe immunopathology may drive the deleterious manifestations that are observed in the advanced stages of coronavirus disease 2019 (COVID-19) but are poorly understood. Objective Our aim was to phenotype leukocyte subpopulations and the cytokine milieu in the lungs and blood of critically ill patients with COVID-19 acute respiratory distress syndrome (ARDS). Methods We consecutively included patients less than 72 hours after intubation following informed consent from their next of kin. Bronchoalveolar lavage fluid was evaluated by microscopy; bronchoalveolar lavage fluid and blood were assessed by 10-color flow cytometry and a multiplex cytokine panel. Results Four mechanically ventilated patients (aged 40-75 years) with moderate-to-severe COVID-19 ARDS were included. Immature neutrophils dominated in both blood and lungs, whereas CD4 and CD8 T-cell lymphopenia was observed in the 2 compartments. However, regulatory T cells and TH17 cells were found in higher fractions in the lung. Lung CD4 and CD8 T cells and macrophages expressed an even higher upregulation of activation markers than in blood. A wide range of cytokines were expressed at high levels both in the blood and in the lungs, most notably, IL-1RA, IL-6, IL-8, IP-10, and monocyte chemoattactant protein-1, consistent with hyperinflammation. Conclusion COVID-19 ARDS exhibits a distinct immunologic profile in the lungs, with a depleted and exhausted CD4 and CD8 T-cell population that resides within a heavily hyperinflammatory milieu., Graphical abstract

Graphical abstract, Highlights • According to WHO, COVID-19 pandemic belongs to very high-risk category. • Potentially lethal manifestations of COVID-19 are due to cytokine storm syndrome. • Cytokine storm in COVID-19 patients with co-morbidities is associated with elevated cytokine synergism. • Exacerbation of ‘cytokine storm’ in high risk patients could be responsible for the high mortality. • Antiviral state imparted by BCG vaccine confers early protection against SARS-CoV2. • Trained immunity of BCG vaccine involves the release of cytokine IL-1β, TNF-α and IL-6 from epigenetically modified monocytes., The seventh human coronavirus SARS-CoV2 belongs to the cluster of extremely pathogenic coronaviruses like SARS-CoV and MERS-CoV, which are established to cause lower respiratory tract infection. The viral infection can be fatal as the disease advances to pneumonia followed by acute respiratory distress syndrome (ARDS). Increasingly higher cytokine concentration on account of over-stimulated immune response against the virus, or the ‘cytokine storm’, is the reason behind the manifestation of lethal clinical symptoms. In this article, we discuss the immune pathogenesis of cytokine storm and its relation with SARS-CoV2/COVID-19 risk factors. People with underlying risk factors are more susceptible to fatal complications of COVID-19 infection leading to poor clinical outcome. The increased pro-inflammatory immune status in patients with risk factors (diabetes, hypertension, cardiovascular disease, COPD) exacerbates the Cytokine-storm of COVID-19 to a Cytokine Super Cyclone. We also overviewed antiviral immune response provided by BCG vaccine involving the IL-1β, IL-6 and TNF-α secretion via ‘trained monocytes’, which confers early protection against SARS-CoV2.