Adaptive immunity to SARS-CoV-2 and COVID-19.

The COVID-19 puzzle: deciphering pathophysiology and phenotypes of a new disease entity.

Considerable research effort has been made worldwide to decipher the immune response triggered upon severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections, identify the drivers of severe and fatal COVID-19, and understand what leads to the prolongation of symptoms after disease resolution. We review the results of almost 2 years of COVID-19 immunology research and discuss definitive findings and remaining questions regarding our understanding of COVID-19 pathophysiology. We discuss emerging understanding of differences in immune responses seen in those with and without Long Covid syndrome, also known as post-acute sequelae of SARS-CoV-2. We hope that the knowledge gained from this COVID-19 research will be applied in studies of inflammatory processes involved in critical and chronic illnesses, which remain a major unmet need. Description

The immunology and immunopathology of COVID-19

The Coronavirus Disease 2019 (COVID-19) pandemic has become a global crisis and is more devastating than any other previous infectious disease. It has affected a significant proportion of the global population both physically and mentally, and destroyed businesses and societies. Current evidence suggested that immunopathology may be responsible for COVID-19 pathogenesis, including lymphopenia, neutrophilia, dysregulation of monocytes and macrophages, reduced or delayed type I interferon (IFN-I) response, antibody-dependent enhancement, and especially, cytokine storm (CS). The CS is characterized by hyperproduction of an array of pro-inflammatory cytokines and is closely associated with poor prognosis. These excessively secreted pro-inflammatory cytokines initiate different inflammatory signaling pathways via their receptors on immune and tissue cells, resulting in complicated medical symptoms including fever, capillary leak syndrome, disseminated intravascular coagulation, acute respiratory distress syndrome, and multiorgan failure, ultimately leading to death in the most severe cases. Therefore, it is clinically important to understand the initiation and signaling pathways of CS to develop more effective treatment strategies for COVID-19. Herein, we discuss the latest developments in the immunopathological characteristics of COVID-19 and focus on CS including the current research status of the different cytokines involved. We also discuss the induction, function, downstream signaling, and existing and potential interventions for targeting these cytokines or related signal pathways. We believe that a comprehensive understanding of CS in COVID-19 will help to develop better strategies to effectively control immunopathology in this disease and other infectious and inflammatory diseases.

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The signal pathways and treatment of cytokine storm in COVID-19.

Since the initial reports of a cluster of pneumonia cases of unidentified origin in Wuhan, China, in December 2019, the novel coronavirus that causes this disease — severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) — has spread throughout the world, igniting the twenty-first century’s deadliest pandemic. Over the past 12 months, a dizzying array of information has emerged from numerous laboratories, covering everything from the putative origin of SARS-CoV-2 to the development of numerous candidate vaccines. Many immunologists quickly pivoted from their existing research to focus on coronavirus disease 2019 (COVID-19) and, owing to this unprecedented convergence of efforts on one viral infection, a remarkable body of work has been produced and disseminated, through both preprint servers and peer-reviewed journals. Here, we take readers through the timeline of key discoveries during the first year of the pandemic, which showcases the extraordinary leaps in our understanding of the immune response to SARS-CoV-2 and highlights gaps in our knowledge as well as areas for future investigations. A timeline of the major scientific discoveries during the first year of the COVID-19 pandemic showcases the collaborative efforts that enabled the key aspects of the immune response to SARS-CoV-2 to be reported at unprecedented speed.

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The first 12 months of COVID-19: a timeline of immunological insights.

A central paradigm of immunity is that interferon (IFN)-mediated antiviral responses precede pro-inflammatory ones, optimizing host protection and minimizing collateral damage1,2. Here, we report that for coronavirus disease 2019 (COVID-19) this paradigm does not apply. By investigating temporal IFN and inflammatory cytokine patterns in 32 moderate-to-severe patients with COVID-19 hospitalized for pneumonia and longitudinally followed for the development of respiratory failure and death, we reveal that IFN-λ and type I IFN production were both diminished and delayed, induced only in a fraction of patients as they became critically ill. On the contrary, pro-inflammatory cytokines such as tumor necrosis factor (TNF), interleukin (IL)-6 and IL-8 were produced before IFNs in all patients and persisted for a prolonged time. This condition was reflected in blood transcriptomes wherein prominent IFN signatures were only seen in critically ill patients who also exhibited augmented inflammation. By comparison, in 16 patients with influenza (flu) hospitalized for pneumonia with similar clinicopathological characteristics to those of COVID-19 and 24 nonhospitalized patients with flu with milder symptoms, IFN-λ and type I IFN were robustly induced earlier, at higher levels and independently of disease severity, whereas pro-inflammatory cytokines were only acutely produced. Notably, higher IFN-λ concentrations in patients with COVID-19 correlated with lower viral load in bronchial aspirates and faster viral clearance and a higher IFN-λ to type I IFN ratio correlated with improved outcome for critically ill patients. Moreover, altered cytokine patterns in patients with COVID-19 correlated with longer hospitalization and higher incidence of critical disease and mortality compared to flu. These data point to an untuned antiviral response in COVID-19, contributing to persistent viral presence, hyperinflammation and respiratory failure.

Untuned antiviral immunity in COVID-19 revealed by temporal type I/III interferon patterns and flu comparison.

A central paradigm of immunity is that interferon (IFN) mediated antiviral responses precede the pro-inflammatory ones, optimizing host protection and minimizing collateral damage. Here, we report that for COVID-19 this does not apply. By investigating temporal IFN and inflammatory cytokine patterns in 32 COVID-19 patients hospitalized for pneumonia and longitudinally followed for the development of respiratory failure and death, we reveal that IFN-λ and type I IFN production is both diminished and delayed, induced only in a fraction of patients as they become critically ill. On the contrary, pro-inflammatory cytokines such as TNF, IL-6 and IL-8 are produced before IFNs, in all patients, and persist for a prolonged time. By comparison, in 16 flu patients hospitalized for pneumonia with similar clinicopathological characteristics to COVID-19 and 24 milder non-hospitalized flu patients IFN-λ and type I IFN are robustly induced, earlier, at higher levels and independently of disease severity, while pro-inflammatory cytokines are only acutely and transiently produced. Notably, higher IFN-λ levels in COVID-19 patients correlate with lower viral load in bronchial aspirates and faster viral clearance, and a higher IFN-λ:type I IFN ratio with improved outcome of critically ill patients. Moreover, altered cytokine patterns in COVID-19 patients correlate with longer hospitalization time and higher incidence of critical disease and mortality compared to flu. These data point to an untuned antiviral response in COVID-19 contributing to persistent viral presence, hyperinflammation and respiratory failure.

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Vasiliki Panou

Papers

Untuned antiviral immunity in COVID-19 revealed by temporal type I/III interferon patterns and flu comparison.

Untuned antiviral immunity in COVID-19 revealed by temporal type I/III interferon patterns and flu comparison