In a recent study published in the journal Nature Microbiology, researchers investigated age-related differences in nasal epithelial cell (NEC) responses to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection across pediatric, adult, and older adult groups.
Study: Age-specific nasal epithelial responses to SARS-CoV-2 infection. Image Credit: Designua / Shutterstock
Background
Despite effective vaccines, age remains the primary risk factor for mortality in coronavirus disease 2019 (COVID-19). SARS-CoV-2 initially infects NECs, which is critical for understanding the progression to severe respiratory issues. Further research is needed to fully understand the mechanisms behind age-related differences in response to SARS-CoV-2, which could lead to targeted therapies and improved outcomes for different age groups.
About the study
Participants for the present study were recruited from five major hospital sites in London, United Kingdom (UK), including the Great Ormond Street Hospital National Health Service (NHS) Foundation Trust, the Royal Free London NHS Foundation Trust, the University College London Hospitals NHS Foundation Trust, and the Whittington Health NHS Trust, between March 2020 and February 2021. All participants consented in writing, and the study was ethically approved through the Living Airway Biobank, managed by the University College London (UCL) Great Ormond Street Institute of Child Health. Individuals excluded from participation were current smokers, those with active cancer or immunodeficiencies, recent recipients of blood transfusions, and those with chronic respiratory conditions such as asthma or cystic fibrosis.
Nasal brushings were collected from healthy donors across three age groups- pediatric (0-11 years), adult (30-50 years), and older adult (≥70 years)- who had tested negative for SARS-CoV-2 and showed no respiratory symptoms in the seven weeks prior to sampling. Samples were collected using cytological brushes and immediately processed to preserve cell integrity, following strict guidelines.
The NECs were cultured at an air-liquid interface, infected with SARS-CoV-2, and monitored for gene and protein expression changes, cell differentiation, and viral replication dynamics.
Schematic of method and model used to study SARS-CoV-2 infection of paediatric (P, <12 years), adult (A, 30–50 years) and older adult (O, >70 years) nasal epithelial cells.
Study results
In examining the cellular composition of NECs across different ages, a dataset of 139,598 cells was analyzed using single-cell Ribonucleic Acid (RNA) sequencing, revealing 24 distinct epithelial cell types or states. These ranged from basal cells, with subpopulations such as cycling basal and basal|epithelial–mesenchymal transition (EMT) cells, to secretory and ciliated cells, each categorized by unique gene expression patterns linked to various functions, including mucosal defense.
The study also noted age-related variations in the distribution of these cell types. Adults exhibited a higher abundance of basal/progenitor cells compared to children, a pattern that aligns with findings from previous nasal epithelial studies. Despite these differences, ciliary activity and expression levels of structural proteins like tubulin showed no significant change with age. However, NEC cultures from older adults were observed to be thicker, suggesting a denser cellular arrangement without impacting the epithelial barrier's integrity.
One striking finding in pediatric cultures was the increased presence of a specific type of goblet cell, indicating a shift in cell state not observed in adults or older individuals. Despite similar total protein levels of SARS-CoV-2 entry factors among all age groups, messenger RNA (mRNA) levels of these proteins were distinctly higher in pediatric goblet cells, hinting at a different susceptibility to viral infection across ages.
When exploring viral replication dynamics, NEC cultures infected with an early-lineage SARS-CoV-2 strain did not show significant differences in the number of viral reads over time across age groups. However, the type of infected cells varied, with younger individuals showing infections concentrated in fewer cell types than adults and older adults. Interestingly, older adults showed a greater viral infection spread within the culture, evidenced by a higher percentage of cells expressing viral RNA.
The study extended to the effects of SARS-CoV-2 on cell phenotypes, revealing that infection led to decreased culture thickness and compromised epithelial integrity in adults and older adults. There was also a noted increase in basal cell mobilization in older adults, suggesting a heightened response to infection involving cellular repair mechanisms and possibly contributing to more severe disease outcomes. Additionally, older adult NECs showed a higher propensity to express markers related to tissue injury and fibrosis, which may explain the more pronounced physical and clinical impacts of COVID-19 observed in older populations.
Conclusions
To summarize, the study analyzed 139,598 NECs across different ages, revealing 24 distinct cell types using single-cell RNA sequencing. It showed age-related differences in cell distribution, with adults having more basal/progenitor cells and older adults displaying thicker NEC cultures. Pediatric cultures had increased specific goblet cells, indicating variable susceptibilities to SARS-CoV-2. While viral replication was consistent across ages, older adults experienced a broader infection spread. Infection altered cell phenotypes, reducing epithelial integrity in adults and increasing tissue injury markers in older adults, underscoring the importance of considering age in COVID-19 strategies.
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