COVID-19 Hyperinflammation: What about Neutrophils?

COVID-19 is often related to hyperinflammation that drives lung or multiorgan injury. The immunopathological mechanisms that cause excessive inflammation are under investigation and constantly updated. Here, a gene network approach was used on recently published data sets to identify possible COVID-19 inflammatory mechanisms and bioactive genes. First, network analysis of putative SARS-CoV-2 cellular receptors led to the mining of a neutrophil-response signature and relevant inflammatory genes. Second, analysis of RNA-seq data sets of lung cells infected with SARS-CoV-2 revealed that infected cells expressed neutrophil-attracting chemokines.

FIG 1 (A) ACE2 and 6 related putative SARS-CoV-2 receptors on epithelial cells. ACE2, DPP4, and ANPEP are peptidases. (B) The 7 putative SARS-CoV-2 receptors were inflated in StringDB by adding up to 100 directly interacting proteins with direct association to the 7 input proteins using 1st shell interactions and StringDB default settings. (C and D) The main gene ontology of this inflated network is "Neutrophil Degranulation" (C) (http://amigo.geneontology.org/amigo/ term/GO:0043312) and includes 8 neutrophil-enriched genes isolated in panel D. The gene ontology of these proteins was examined in the Amigo database. (E) One hundred thirteen genes were differentially regulated in RNA-seq data sets of human alveolar adenocarcinoma (A549) and human bronchial epithelial cells infected with SARS-CoV-2 in vitro (study details, data sets, and statistical analysis can be found in reference 14). (F) Following virus infection, most significantly upregulated genes were related to inflammatory and interferon responses. (G and H) Six classic neutrophil chemokines were upregulated in these cells following infection with SARS-CoV-2, depicted in panel G, as well as C3 and related complement pathway genes depicted in panel H. (I) Eighteen neutrophil-enriched and neutrophil chemotaxis genes were upregulated in an RNA-seq data set of BALF cells collected from 2 COVID-19 patients versus 3 healthy BALF donors (study details, data sets, and statistical analysis can be found in reference 16). (J and K) Putative druggable targets with likely neutrophil proinflammatory function derived from the analysis. Approved and experimental drugs with validated pharmacological evidence are presented as interaction (Continued on next page) Opinion/Hypothesis related to the 7 input proteins. Surprisingly, the main ontology of the expanded network ( Fig. 1B) was "Neutrophil Degranulation" (gene ontology [GO] was examined in the Amigo database [http://amigo.geneontology.org/amigo/term/GO:0043312]) with 70 proteins in this category (Fig. 1C). There were also 8 neutrophil-enriched genes (neutrophil gene expression enrichment is available via the Human Protein Atlas, https://www.proteinatlas.org/search/blood_cell_category_rna%3Aneutrophil%3BCell ϩtypeϩenrichedϩANDϩsort_by%3Atissueϩspecificϩscore) including ANPEP, MME, MGAM, CD177, CEACAM1/3, FPR2 and CYSTM1 (Fig. 1D). Thus, coronavirus binding proteins (and directly associated proteins) might be involved in neutrophil or other related classic inflammatory mechanisms. Two of the neutrophil-specific genes, ANPEP and CEACAM1 (Fig. 1D), are coronavirus receptors (Fig. 1A), indicating that neutrophils could be infected by SARS-CoV-2, as they are by influenza virus (7).
The role of neutrophils in viral infections of the upper respiratory tract and their possible importance in therapeutic strategies is not entirely clarified (8). They are involved in early antiviral defense (9), but through degranulation and lysis, they can be cytotoxic during severe pneumonia, including from coronaviruses (10), and can also aggravate lung inflammation caused by influenza virus (11,12). Neutrophil hyperinflammation is also likely in other severe viral infections such as hepatitis (8). In current COVID-19 literature, an increased peripheral neutrophil-to-lymphocyte ratio is observed in severe cases and is likely associated with unfavorable prognosis (13). The mechanisms behind this are not understood, and not much is known regarding neutrophil activity in SARS-CoV-2-infected lungs. COVID-19 lung damage in some patients might involve dysregulated neutrophil activity.
Finally, analysis of a published RNA-seq data set of human bronchoalveolar lavage fluid (BALF) cells from 2 hospitalized COVID-19 patients (16) revealed that 18 (Fig. 1I) neutrophil-enriched genes (PPL, ENCUR, STEAP4, SLP1, MUC21, HEY1, MUC21, and CXCL1) and neutrophil chemotaxis genes (CXCX2, CXCL6, CCL8, CCL2, TGFB2, CCL3L3, and CCL4L4) were upregulated in COVID-19 BALF cells, further supporting likely involvement of neutrophils in COVID-19 lungs. Experimental and statistical details related to this data set are described in reference 16. Caution is needed in assessing the statistical robustness of these data given that BALF from only 2 COVID-19 patients was sequenced in this study, and there was no information on the severity or clinical outcome of these subjects.
Are neutrophils and related inflammatory mechanisms likely targets in COVID-19 complications? This is a difficult question given the complexity of innate immune responses and the importance of neutrophils in early antiviral defense as well as their role in secondary bacterial and fungal infections that are common comorbidities in COVID-19 patients (17). Universal suppression of neutrophils or other myeloid cell types is not trivial, and the clinical evidence on the use of steroids in COVID-19 is inconclusive. Nevertheless, it might be possible to target specific inflammatory mechanisms to prevent lung injury or systemic hyperinflammation and cytokine storm. The target mining presented here includes multiple druggable proteins (Fig. 1J and K) such as neutrophil-attracting chemokine signaling (Fig. 1J), and neutrophil-relevant inflammatory entities (Fig. 1J), as well as SARS-CoV-2 receptors (Fig. 1K). Approved and experimental drug-protein interactions were retrieved from the DGIdb drug-protein interaction database (http://www.dgidb.org/search_interactions) using default filters.
Neutrophil recruitment and related activity might exacerbate COVID-19 immunopathology. In contrast to universal immunosuppression, specific inflammatory pathways could be targeted in COVID-19 hyperinflammation. These approaches can be also combined with antivirals. For instance, oseltamivir (Tamiflu) was recently shown to reduce neutrophil lung recruitment when combined with CXCR2 antagonists in influenza virus-infected mice (19). Moreover, it is perhaps worth noting that some currently investigated COVID-19 therapeutic options include hydroxychloroquine, azithromycin, and colchicine. All three have known antineutrophil effects. Finally, it is important to highlight that this paper utilized publicly available bioinformatics tools to analyze a limited number of high-throughput gene expression data sets from in vitro and in vivo SARS-CoV-2 infections. The likely involvement of neutrophil signatures and related inflammatory pathways presented here is unbiased and should be used with caution to generate hypotheses for future experimental and translational studies. Interestingly, after this manuscript was submitted for consideration, the likely pathological role of lung neutrophil activity (formation of extracellular traps) in COVID-19 clinical complications was published (20). More experimental and clinical work is needed to validate these findings and to inform possible anti-inflammatory interventions for severe cases. The immunopathological mechanisms driving COVID-19 are unclear, are continuously updated, and will evolve drastically in the future.