1. Cytokine Storm
The term “cytokine storm” (CS) refers to a spectrum of different immune dysregulation disorders characterized by constitutional symptoms, systemic inflammation and multiorgan dysfunction
[1]. The trigger for the CS is an uncontrolled immune response with the continuous activation and expansion of immune cells, lymphocytes and macrophages, which produce immense amounts of proinflammatory cytokines
[2]. In viral infections, the release of proinflammatory factors leads to apoptosis of lung epithelial cells, and pulmonary alveolar and microvascular endothelial cells, leading to complications such as alveolar edema and hypoxia. The uncontrolled production of proinflammatory factors, containing IL-6, IL-8, IL-1β and GM-CSF, and chemokines such as CCL2, CCL-5, IP-10 and CCL3, along with reactive oxygen species, results in pulmonary fibrosis
[3]. CXCL8 chemokine is also involved in the inflammation and trafficking of immune cells in the context of viral infections, as it has a chemotactic action for neutrophils and monocytes in the respiratory tract
[4]. The clinical picture of the cytokine storm, although variable, generally manifests itself as laboratory “overlap syndrome” with a decreased cell count, a decreased ESR, an increased ferritin level, NK dysfunction and hemophagocytosis
[5]. This clinically results in the increased perivascular infiltration of activated macrophages, neutrophils and fibroblasts, accompanied by extensive fibrin deposition and alveolar collapse, leading to acute respiratory distress syndrome (ARDS)
[6][7].
Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2, previously 2019-nCoV) was registered in the Wuhan city of China for the first time, and since then 445,188 deaths related to the disease have been reported. It is an enveloped, positive-sense single-stranded genomic RNA virus (+ssRNA)
[8][9], and entry of viral RNA into host cells occurs due to the affinity of the SARS-CoV protein S for the ACE-2 receptor
[10]. The common symptoms observed in patients with COVID-19 are fever, cough, severe headache, myalgia and fatigue
[11]. In most cases, SARS-CoV-2 infections occur asymptomatically or cause only mild and less fatal symptoms than MERS-CoV and SARS-CoV infections. However, in 10–20% of cases they can progress to interstitial pneumonia and acute respiratory distress syndrome (ARDS), especially in those with advanced age and associated comorbidities
[12]. Since the COVID-19 pandemic first began in December 2019, SARS-CoV-2 has continuously evolved, with many variants emerging around the world
[13]. These mutations, especially when they occur on the S gene encoding the spike protein (S), can affect both the viral entry into the target cells and the effectiveness of the antibodies
[14]. The result is a greater visual transmissibility and a greater ability to escape the previous immunity
[15]. The variants that attract the most attention as potentially dangerous to public health include B.1.1.7 (UK), P.1 (Brazilian) and B.1.351 (South African). Added to these are also variants of interest that are emerging and expanding in some countries, but are found sporadically in others, such as B.1.427 and B.1.429 (Californians) or B.1.617 (Indian)
[16].
The cytokine storm caused by COVID-19 has been suggested to be associated with COVID-19 severity
[17][18]. The cytokine storm is considered to be the main cause of the disease severity and death in COVID-19 patients, and is related to the high levels of circulating cytokines, severe lymphopenia, thrombosis and massive mononuclear cell infiltration in multiple organs
[19]. The excessive local release of cytokines is considered to be the determinant of pathological alterations and the clinical manifestation of acute respiratory distress syndrome (ARDS).
SARS-CoV-2 infection triggers an immune response producing inflammatory cytokines along with a weak response to interferon (IFN). Thereafter, membrane-bound immune receptors and downstream signaling pathways mediate the proinflammatory immune responses of pathogenic Th1 cells and intermediate CD14 + CD16 + monocytes. This process therefore favors the infiltration of macrophages and neutrophils into the lung tissue, which leads to the cytokine storm
[20].
In particular, SARS-CoV-2 can rapidly activate pathogenic Th1 cells resulting in the secretion of proinflammatory cytokines, including granulocyte-macrophage colony stimulating factor (GM-CSF) and interleukin-6 (IL-6). GM-CSF then activates inflammatory CD14 + CD16 + monocytes to produce IL-6, tumor necrosis factor-α (TNF-α) and other cytokines. Membrane-bound immune receptors, including Fc and Toll-like receptors, might contribute to an imbalanced inflammatory response, whereas weak IFN-γ induction might amplify cytokine production
[21]. Neutrophil extracellular traps (NETs) might also contribute to cytokine release. Overall, the cytokine storm in COVID-19 is characterized by the high expression of IL-6 and TNF-α
[22].