Researchers at the Francis Crick Institute have used a proteomic approach to study protein levels and enzyme activity in cases of severe COVID-19 and sepsis. The study, published in Immunity, identified a protein signature that was associated with severe infection. This could help identify healthy people who are more susceptible to serious illness before they contract an infection.
Breaking down NETs
Immune cells are activated during infection. Neutrophils are proinflammatory white blood cells that are among the first cells to arrive at sites of infection. As a defence mechanism, these cells can undergo a distinct type of cell death known as NETosis. The cells release neutrophil extracellular traps (NETs), which are a network of fibres and chromatin (a complex molecule of DNA and proteins).
Microorganisms get trapped in NETs and the infection is contained, allowing other immune cells to assist in clearing the infection. However, if chromatin is not cleared efficiently, it can accumulate in the blood and contribute to disease pathology. Chromatin is cleared by enzymes called DNAses, which break down DNA in the NETs.
Researchers at the Crick identified an association between aberrant chromatin release, low NET clearance and increased COVID-19 pathology. Using proteomics, they found that the build-up of chromatin and less active DNases were contributing factors to severe infection.
Dr Iker Valle Aramburu, first author and Postdoctoral Training Fellow at the Crick said, “We know that an accumulation of chromatin can be harmful, but the underpinning mechanisms have been poorly understood. We’ve helped answer this mystery here and also shown that the most severe cases of infection are characterised by both a release of chromatin alongside defective chromatin clearance.”
Developing a proteomic profile
The researchers analysed the plasma proteome of 63 patients with COVID-19 pneumonia and 35 patients with microbial sepsis. They found that patients had a build-up of chromatin. They then studied the underlying mechanism of this accumulation. The team found that the chromatin clearance mechanism was disrupted due to decreased activity of the DNAse enzymes.
Actin, another protein that is abundant in cells and released during cell death, inhibits DNAse activity. The researchers found that patients with high actin levels in their blood also had chromatin build-up. They concluded that DNAse enzymes are less active due to actin inhibition, allowing chromatin to build up and drive disease pathology.
By combining the information on protein levels and enzyme activity, the researchers developed a proteomic profile that could be used to identify severe and high-risk cases of infection.
The study shows that plasma proteomics could make it possible to characterise samples and identify cases of severe infection that would otherwise be challenging to distinguish using standard clinical techniques. Therefore, with additional development, the proteomic profile could be used in primary healthcare to help stratify patients and identify those who may need further treatment.
Dr Venizelos Papayannopoulos, senior author and Principal Group Leader at the Crick said, “Our results might also help identify healthy people who are at higher risk of severe disease before they become infected. We saw that chromatin degrading enzymes are also less active in cases of healthy people who have low levels of inflammation and a high risk for cardiovascular disease. This could put this group at higher risk of massive build-up of chromatin upon infection, as the process of chromatin clearance is already weakened.”