A new preprint deals with the characterization of neutrophil phenotypes in an attempt to provide biomarkers that can predict severe disease in patients hospitalized with coronavirus disease 2019 (COVID-19).
Study: Longitudinal characterization of circulating neutrophils uncovers distinct phenotypes associated with disease severity in hospitalized COVID-19 patients. Image Credit: Kateryna Kon/ Shutterstock
A preprint version of the study is available on the bioRxiv* server while the article undergoes peer review.
Neutrophils have been reported to be hyperactivated in severe COVID-19, and some scientists think this phenomenon indicates that these cells are closely implicated in the underlying disease mechanism.
Neutrophils recognize pathogens via opsonic receptors, including the Fc receptors, indicating that neutrophil responses are affected by the disease. Neutrophils recognize antigen-antibody complexes, which may be important in subsequent effector responses.
Neutrophils can eliminate pathogens bound to antibodies directly in a process called antibody-dependent neutrophil phagocytosis (ADNP). Alternatively, they undergo NETosis, a dedicated program of cell death in which neutrophils release their chromatin extracellularly as a web, modified by anti-microbial and highly reactive proteins such as myeloperoxidase, to form neutrophil extracellular traps (NETs).
The NETosis that occurs in cancers, viral infections, and heparin-induced thrombocytopenia is driven mostly by interactions between antibodies and Fc receptors, modulated by factors such as the isotype of the antibody and the type of sugars attached to the antigen. In COVID-19, NETs may be involved in heart attacks and immunothrombotic events.
Earlier studies indicate that the protein translation patterns and gene expression profiles in hospitalized COVID-19 patients that reflect neutrophil activation and degranulation are mortality risk markers.
The current paper describes a study of blood neutrophils from a large group of such patients using several methods to delineate the dynamic neutrophil response to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).
What did the study show?
The results showed that the absolute neutrophil count (ANC) was related to creatinine, C-reactive protein (CRP), lactate dehydrogenase, and D-dimer, all acute-phase reactants. The ANC was also strongly correlated with intubation risk, increasing from day 0 to day seven.
The virus elicits a robust interferon response, both IFNγ and IFN-α, along with cytokine production. Distinct neutrophil states were found to be linked to severe and non-severe COVID-19.
Activated and inflammatory neutrophils were elevated in samples from severe COVID-19, while in other samples, the activation was reduced, and the number of neutrophils in circulation was decreased by day seven.
From the day of admission onwards, very early neutrophil states allowed the eventual severity of the disease to be predicted with great accuracy. Several metabolic circuits went from being enriched on day 0 in those who would not survive to be enriched on day seven in the eventual survivors.
This included delayed or dysregulated interferon signaling. Some research has shown that this could lead to increased interferon activity, resulting in the activation of immune cells when the lungs are already heavily inflamed. This could promote an increased risk of death.
Interferon-stimulated genes were also enriched early in survivors but late in those who died. The metabolic pathways showed the opposite direction, with fatty acid metabolism, the tricarboxylic acid cycle, and NADP pathways being enriched in fatal cases on day 0, while day seven enrichment corresponded to surviving patients.
Proteins that enhance histone modification can only be detected via proteomics, as they are post-transcriptional modifications. A search for such markers showed them to be associated with disease severity at all time points across neutrophil subtypes.
Cell-free DNA levels were also associated with severity and with ANC, as well as with immature activated neutrophil samples. Moreover, severe disease was associated with neutrophil degranulation and T cell inhibition.
Neutrophils also prevent T cell activation and proliferation. The researchers also found that only IgG:S immune complexes had a strong ADNP effect and induced reactive oxygen species (ROS) to a higher level in severe COVID-19 than non-severe. NETosis from healthy neutrophils was much higher when incubated with IgA than IgG, independent of severity.
Among non-severe patients, MPO was reduced with IgA compared to IgG, indicating that the latter stimulates higher production to accelerate ROS via MPO.
What are the implications?
The researchers found that they could differentiate six neutrophil phenotypes in COVID-19 and other cases. These could differ significantly from severe and fatal COVID-19 patients to non-severe cases. The dysregulation of neutrophils coupled with the changes in phenotypes indicates a common mechanism in COVID-19 and other diseases.
In all cases, an interferon-driven phenotype was seen but went down over time to be replaced by a suppressive signature or a neutrophil progenitor signature in severe and non-severe COVID-19, respectively. Higher interferon levels were found on days three and seven in fatal cases, making this a potential biomarker predicting severe disease.
IgA and IgG have different effects on the neutrophil effector functions, with a high IgA1/IgG1 ratio in fatal cases relative to survivors. This suggests that the antiviral antibodies and neutrophil effector functions mediate severe outcomes in COVID-19. The researchers suggest an initial IgA-driven mucosal response to the entry of the virus, followed by IgA-secreting B cells flooding the bloodstream.
bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.
- LaSalle, T. J. et al. (2021). Longitudinal Characterization of Circulating Neutrophils Uncovers Distinct Phenotypes Associated With Disease Severity In Hospitalized COVID-19 Patients. bioRxiv preprint. doi: https://doi.org/10.1101/2021.10.04.463121.