An expected source of neutrophils in Covid19
One recent triumph in field of bioengineering is the development of methods to sequence RNA in large number of single cells. As we’ve discussed previously, determining the RNA code of individual cells is useful because this genetic material will be used to produce proteins that carry out the cell’s functions. Several techniques have been established to profile hundreds of thousands of cells at once, with the potential to scale ever higher cell-number barriers as the technology develops further. The single cell RNAseq (scRNAseq) revolution has revealed the heterogeneity of cellular transcriptomic states within tissues, improving on methods that profile gene expression in lysates of whole tissue or purified cellular fractions in bulk.
As sequencing has evolved, tools have also been developed to analyze the RNA beyond simply the genes, or areas which make protein. One such tool is RNA velocity, which takes advantage of what is known about RNA processing. By quantifying the proportion of spliced and unspliced transcripts across all genes, an estimate of developmental directionality within a scRNAseq sample can be determined. The principle is that a set of cells can be defined by progression along a transitional trajectory if there is a net increase in the proportion of spliced genes, since mRNA is increasingly spliced with time. While not formally tracing developmental lineage, these approaches can generate hypotheses regarding the existence of cellular intermediates in development or disease.
RNA Velocity was used by Wilk et al. to detect a surprising developmental connection between plasmablasts and neutrophils in the peripheral blood mononuclear cells (PBMCs) of patients with ARDS from Covid19. ARDS (acute respiratory distress syndrome) is defined by lung edema that results from vascular leak in the setting of systemic inflammation. Common causes are pneumonia, sepsis, trauma, and severe pancreatitis. ARDS is highly lethal and has received attention this year because of COVID19, where patients have had a high rate of pulmonary edema, hypoxemia, and ARDS resulting from infection with SARS-CoV-2. The dataset from Wilk et al. included PBMCs from 6 healthy volunteers and 7 patients with Covid19, and the authors’ analysis renders many insights about multiple leukocyte lineages. Of note, compared with Covid19 patients without ARDS and with healthy controls, ARDS patients with Covid19 had an unexpected population of neutrophils that were developmentally linked to plasmablasts. Plasmablasts traditionally are thought to give rise to antibody-producing plasma cells, and thus the connection to neutrophils, important drivers of inflammation and lung injury in ARDS, is surprising. As pointed out by the authors, the finding is supported by a preclinical literature where more formal lineage tracing was possible.
By using RNA velocity, the authors achieve a level of biological insight not previously possible with clinical samples, detecting a novel cellular state of potential importance to disease outcomes. Many questions remain. What is the functional significance of these plasmablast-derived neutrophils? Are they present in other inflammatory diseases? Do they make their way to the lung? Future studies can address these questions by sampling the lung lavage in patients or by targeting a cognate cell type in ARDS mouse models with genetic or cytoablative approaches.
By Mallar Bhattacharya
https://bhattacharyalab.ucsf.edu
Read the original article at: https://www.nature.com/articles/s41591-020-0944-y.
On the left, RNA velocity analysis from Wilk et al showing plasmablasts giving rise to developing neutrophils in COVID19+ patients. On the right, a traditional understanding of immune cell lineage, from Biorender.com. Red arrow in both diagrams showing the unexpected derivation of neutrophils from plasma blasts.