On 2020-06-11 04:56:01, user Sinai Immunol Review Project wrote:
Main findings<br />
Despite similarities in presentation at onset, differences in the underlying immunopathology of SARS-CoV-2 infections and other respiratory infections, like influenza, remain largely unknown. In this pre-print, Mudd et al. performed single-cell RNA sequencing (scRNAseq) of PBMCs from COVID-19 patients and influenza patients, in order to delineate potential key differences between the aforementioned respiratory infections. Analyses were performed using a cohort of 79 COVID-19 patients (n=79; 35 of whom developed acute respiratory failure), 26 influenza patients (n=26; 7 of whom developed acute respiratory failure), and 15 healthy controls (n=15
First, plasma cytokine levels were evaluated in the 79 COVID-19 patients, 26 influenza patients, and 8 of the healthy controls. Cytokine analyses identified a reduced production of GM-CSF, IFN-?. and IL-9 but a significant elevation of IL-6 and IL-8 across all COVID-19 patients, compared to influenza patients. In fact, certain chemokines and others were more up-regulated in influenza patients, as opposed to COVID-19 patients. The authors subsequently performed a computational assessment of whether certain groups (or modules) of cytokines were predictive of one of two infections.
Interestingly, two modules, containing G-CSF, IFN-?, IL-2R, IL-6, IL-8, and MCP-1 (among several others), were inversely correlated with an increased likelihood of being SARS-CoV-2-positive. The authors observe that while higher generalized inflammation is characteristic of influenza patients, COVID-19 patients exhibit a marked elevation of a distinct subset of cytokines. Using intubation status and expiration as end-markers of disease severity, the authors found that IL1-RA and IL-6 were associated with COVID-19 disease severity and predictive of poor outcome, both with and without comparison to influenza patients. Collectively, these results suggest that only a selection of inflammatory cytokines are predictive of disease severity, while cytokine storm syndrome is not necessarily descriptive of all COVID-19 patients; these characterizations distinguish COVID-19 immunopathology from that of influenza.
PBMCs had been collected from 79 COVID-19 patients (n=79; 35 of whom developed acute respiratory failure), 26 influenza patients (n=26; 7 of whom developed acute respiratory failure), and 15 healthy controls (n=15). A comparison of the peripheral immune landscape identified several primary differences. Though both groups of patients exhibited pan-lymphopenia, generally, COVID-19 patients had more antibody-secreting plasmablasts and activated CD4+ T cells than influenza patients or controls. However, COVID-19 patients showed significantly reduced numbers of circulating monocytes, in line with previous reports that inflammatory monocytes are recruited to the lung and reduced in the periphery in COVID-19 patients. Notably, both these peripheral monocytes and CD4+ T cells in COVID-19 patients showed reduced HLA-DR expression, indicative of reduced activation.
A closer interrogation of potential immuno-regulatory cell types (as a compensatory response to the hyper-inflammation observed in COVID-19 patients) via scRNAseq (of 3 COVID-19 patients [n=3], 3 influenza patients [n=3], and 1 healthy control) revealed a significantly suppressed type I interferon (IFN) response among B cells, CD8+ T cells, regulatory T cells, plasmacytoid dendritic cells (pDCs), and especially among monocytes. In contrast, his pathway and its associated downstream cascades were enriched in influenza patients. Notably, pathways enriched in COVID-19 patients were glucocorticoid and metabolic stress pathways across multiple cell types, but most significantly in monocytes.
Limitations<br />
Technical<br />
The limited patient sample size of the scRNAseq analysis should be noted.
Biological<br />
Without additional clinical information, it is difficult to know whether relative time-points (at which blood samples were collected and cytokine analyses were performed) may have been different, so an analysis of patients at different stages of their disease course may be a confounding factor. Indeed, the authors make some reference to this potential limitation, in addition to age, in their linear regression models.
In addition, the authors use HLA-DR expression to evaluate myeloid cell activation; other markers should be used to validate the observation of reduced HLA-DR expression. This reduced activation phenotype, in combination with the fewer number of monocytes in the periphery and down-regulated IFN response, provides the basis for the authors' conclusion that an overall suppressed monocyte response underscores COVID-19 immunopathology, when compared to the immune profile of influenza patients. However, it is important to consider the recruitment of the inflammatory subset of monocytes to the lung or other extrapulmonary organs as a reason for the reduced number in the vasculature.
Significance<br />
Through a much needed comparison, Mudd et al. provide a closer look at the cellular differences between the immune response to COVID-19 and influenza. Using scRNAseq, the authors identify notable changes in monocyte transcriptional activity and number and in cytokine profiles that suggest potential associations with disease severity of COVID-19, but not influenza. In particular, the identification of a glucocorticoid response in monocytes is worth further investigation, given previous claims towards the use of immunosuppressive agents to treat COVID-19.
This review was undertaken by Matthew D. Park as part of a project by students, postdocs and faculty at the Immunology Institute of the Icahn School of Medicine, Mount Sinai.