Study: Proteomic analysis of circulating immune cells identifies novel cellular phenotypes associated with COVID-19 severity. Image Credit: CROCOTHERY/Shutterstock

Proteomic analysis identifies novel phenotypes associated with COVID-19 severity in circulating immune cells

In a recent study published on medRxiv* preprint server, researchers used proteomic analysis to identify novel immune cell phenotypes associated with severe coronavirus disease 2019 (COVID-19). Additionally, they compiled the cumulative dysregulation of immune cells in the progressive disease.

Study: Proteomic analysis of circulating immune cells identifies novel cell phenotypes associated with COVID-19 severity. Image Credit: CROCOTHERIE / Shutterstock


Serum proteins, such as C-reactive protein (CRP) and D-dimers, offer nonspecific and limited prognostic information in patients with severe acute respiratory syndrome coronavirus 2 (SARS-CoV- 2). Thus, there is a need for an unbiased analysis of the total and plasma membrane proteomes of peripheral blood mononuclear cells (PBMCs). This exercise could potentially lead to the discovery of new cellular phenotypes associated with the progression of COVID-19. Using these markers, clinicians could assess patients in real time and mitigate patient-specific immunopathology through early therapeutic interventions.

It should be noted that in most cases, the body mounts an effective immune response that achieves clearance of SARS-CoV-2, showing no or mild symptoms. However, those with severe disease develop an overactive immune response characterized by elevated pro-inflammatory factors, such as interleukin-6 (IL-6) and tumor necrosis factor (TNF). Resulting tissue damage and thrombosis lead to acute respiratory distress syndrome, organ failure, and death.

About the study

In the current study, researchers took blood samples from several healthy people and 33 unvaccinated people with acute SARS-CoV-2 infection to collect their peripheral blood mononuclear cells (PBMCs) spanning the spectrum of COVID-19 severity. Additionally, they combined ribonucleic acid sequencing (RNA-seq) and flow cytometry (FC) data from the same donors to define a comprehensive multi-omics profile for each level of COVID-19 severity.

Study results

The main finding of the study was the progressive upregulation of certain proteins expressed by several immune cells as disease severity increased, for example, differentiating T cells (CD4+) and non-classical monocytes. In the present study, the proteome profiles of PBMCs markedly changed during the progression of mild to severe COVID-19, consistent with observations from previous whole blood transcriptome and plasma proteome studies. Additionally, the researchers observed a highly significant enrichment of terms related to microbial defense among upregulated cellular proteins during severe COVID-19.

Patient metadata documented the emergence of a sepsis-like condition resulting in the immunopathology of COVID-19. Additionally, selective upregulation of canonical interferon-stimulated genes (ISGs), such as the IFIT and Mx families, has been observed in patients with mild disease. Notably, the team obtained the study samples from a time-normalized transcriptomic analysis of the larger cohort. Analysis of these samples suggested that PBMCs from critically ill COVID-19 patients collected relatively soon after symptom onset also exhibited up-regulated expression of the interferon-stimulated gene (ISG), which then declined. .

The researchers observed a similar trend for donors sampled in the early stages of infection. Profiling of the plasma membrane PBMC proteome identified up-regulated expression of the following proteins with immunoregulatory functions:

i) members 1, 6 and 8 of the cell adhesion molecules related to carcinoembryonic antigen (CEACAM) family. These immunoglobulin (Ig)-like surface glycoproteins are present in a range of cell types. They form homophilic and heterophile interactions with binding proteins involved in multiple cellular processes (eg cell adhesion).

ii) CD177, a glycosyl-phosphatidylinositol (GPI)-related surface glycoprotein believed to be a neutrophil marker.

iii) CD63, a biomarker of platelet and T cell activation and granulocyte degranulation.

iv) CD89, a crystallizable (Fc) receptor fragment expressed on neutrophils and monocytes that binds to IgA immune complexes and CRP, initiating cell activation and cytokine release. It also fights bacterial sepsis.

Curiously, when they are expressed simultaneously on CD4+ T lymphocytes, the expression of CEACAM 6 and 8, CD177 and CD89, represents a unique phenotype. Expression of CD177 possibly facilitates migration into critical tissues and cell activation during infection. Interestingly, upregulation of CD89 plays a role in bacterial sepsis. Thus, it is possible that a noncanonical but similar phenotype also develops in the CD4+ T cell population during severe COVID-19.

In addition, the researchers observed a clear change in the platelet phenotype in patients with severe COVID-19, with a large increase in activated CD61+CD62P+ and non-activated CD61+CD62P- platelets and upregulated expression of CD63.

The researchers also identified a population of CD62P platelets that co-regulates CD177, CD89 and CEACAM 1, 6 and 8 at high levels. The expression of these factors on platelets, monocytes and neutrophils could represent a mechanism that facilitates cell-cell contacts and the formation of thrombotic aggregates. Moreover, regardless of antigen specificity, the CD4+ T cell population plausibly constitutes another example of immune dysregulation in severe COVID-19. Consistent with previous reports, study data indicated disruptions in the myeloid compartment, the phenotyping of which showed significant upregulation of CEACAMs 1, 6, and 8 on a small population of CD16+CD14- cells and CEACAM8 on CD14+CD16- cells.

Finally, the study data highlighted an increased abundance of low-density immature pro- or pre-neutrophils (LDNs) associated with a dysfunctional immunosuppressive environment during severe COVID-19. LDNs migrate with PBMCs upon density gradient isolation, unlike their mature counterparts. Also, they are usually released during infection, sepsis, and autoimmune conditions via emergency myelopoiesis. Taken together, the observed upregulated expression of CEACAM family members and CD177 on platelets and CEACAM expression on classical and nonclassical monocytes might be representative of another mechanism of pathological aggregate formation. platelets-monocytes.


The study demonstrated remarkable remodeling of circulating immune cell composition during severe COVID-19. Recognition of new cellular phenotypes identified in people with or developing severe COVID-19 could open new avenues to expand understanding of the mechanisms underlying COVID-19 immunopathology. However, more longitudinal studies are needed to determine their prognostic value and potential involvement in long-lasting COVID symptoms.

*Important Notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be considered conclusive, guide clinical practice/health-related behaviors, or treated as established information.

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