Host and microbiome features of secondary infections in lethal covid-19

Secondary infections contribute significantly to covid-19 mortality but host and microbial factors driving this sequel remain poorly understood. We performed an autopsy study of 20 covid-19 cases and 14 controls from the first pandemic wave. Autopsies combined with microbial cultivation and deep RNA sequencing (RNAseq) allowed us to define major organ pathologies and specify secondary infections. Lethal covid-19 segregated into two main death causes separating cases with either dominant diffuse alveolar damage (DAD) or secondary infections of lungs. Lung microbiome changes were profound in covid-19 showing a reduced biodiversity and increased presence of prototypical bacterial and fungal pathogens in cases with secondary infections. Deep RNAseq of lung tissues distinctly mirrored death causes and cellular deconvolution stratified DAD cases into subgroups with different cellular compositions. Myeloid cells, including macrophages, and complement C1q activation were found to be strong stratifying factors suggesting a pathophysiological link possibly leading to tolerance in DAD subgroups. Moreover, several signs of immune-impairment were evident in covid-19 lungs including strong induction of inhibitory immune-checkpoints. Thus, our study highlights profound alterations of the local immunity in covid-19, wherein immune-impairment leads to reduced antimicrobial defense favoring the development of secondary infections on top of SARS-CoV-2 infection.

Pre-activated anti-viral innate immunity in the upper airways controls early SARS-CoV-2 infection in children

Children are consistently reported to have reduced SARS-CoV-2 infection rates and a substantially lower risk for developing severe COVID-19. However, the molecular mechanisms underlying protection against COVID-19 in younger age groups remain widely unknown. Here, we systematically characterized the single-cell transcriptional landscape in the upper airways in SARS-CoV-2 negative and age-matched SARS-CoV-2 positive children (n=42) and corresponding samples from adults (n=44), covering an age range of four weeks to 77 years. Children displayed higher basal expression of the relevant pattern recognition receptor (PRR) pathways in upper airway epithelial cells, macrophages, and dendritic cells, resulting in stronger innate antiviral responses upon SARS-CoV-2 infection compared to adults. We further detected distinct immune cell subpopulations with an overall dominance of neutrophils and a population of cytotoxic T cells occurring predominantly in children. Our study provides evidence that the airway epithelial and mucosal immune cells of children are pre-activated and primed for virus sensing, resulting in a stronger early innate antiviral responses to SARS-CoV-2 infection compared to adults.

Cross-talk between the airway epithelium and activated immune cells defines severity in COVID-19

The clinical course of COVID-19 is highly variable, however, underlying host factors and determinants of severe disease are still unknown. Based on single-cell transcriptomes of nasopharyngeal and bronchial samples from clinically well-characterized patients presenting with moderate and critical severities, we reveal the different types and states of airway epithelial cells that are vulnerable for SARS-CoV-2 infection. In COVID-19 patients, we observed a two- to threefold increase of cells expressing the SARS-CoV-2 entry receptor ACE2 within the airway epithelial cell compartment. ACE2 is upregulated in epithelial cells through Interferon signals by immune cells suggesting that the viral defense system may increase the number of potentially susceptible cells in the respiratory epithelium. Infected epithelial cells recruit and activate immune cells by chemokine signaling. Recruited T lymphocytes and inflammatory macrophages were hyperactivated and showed a strong interaction with epithelial cells. In critical patients, increased expression of CCL2, CCL3, CCL5, CXCL9, CXCL10, IL8, IL1B and TNF in macrophages was identified as a likely cause of a hyperinflammatory lung pathology. Moreover, we observed exacerbated epithelial cell death, likely leading to lung injury and respiratory failure in fatal cases. Our study provides novel insights into the pathophysiology of COVID-19 and suggests an immunomodulatory therapy along the CCL2, CCL3/CCR1 axis as promising option to prevent and treat critical course of COVID-19.