The crucial role of RIG-I in upregulating neutrophil CXCR2 expression through the MAPK signaling pathway in viral pneumonia.

BACKGROUND

Dysregulated neutrophilic inflammation is a hallmark of severe COVID-19, but the mechanisms driving human peripheral neutrophils (HPNs) recruitment remain incompletely defined. C-X-C motif chemokine receptor 2 (CXCR2), a key chemokine receptor on HPNs, is essential for HPNs recruitment during viral pneumonia. However, the role and underlying regulatory mechanisms of CXCR2 on HPNs during viral pneumonia remain unclear.

METHODS

We enrolled 66 COVID-19 patients (30 mild, 33 severe) and 31 healthy controls (HCs). The content of HPNs and the CXCR2 expression on the HPNs were tracked using flow cytometry, RT-qPCR, and ELISA. Transcriptomic sequencing identified differential pathways in the patients. In vitro, HPNs and lung epithelial cells (A549, BEAS-2B) were stimulated with the viral mimic poly(I: C) and treated with pathway inhibitors (CXCR2: SB225002; RIG-I: BX795; MEK: PD98059). In vivo, a poly(I: C)-induced acute lung injury mouse model was used to confirm the roles of RIG-I and CXCR2 in lung injury progression.

RESULTS

Severe patients exhibited a higher HPN percentage and CXCR2 expression. Higher CXCR2 expression was correlated with poor prognosis and elevated inflammatory cytokines (TNF-α, IL-6, and IL-8). In vitro, poly(I: C) triggered lung epithelial cells to produce CXCR2 ligands (CXCL1/2/8), increase pro-inflammatory factors, and promote the HPNs migration to epithelial cells. However, CXCR2 antagonists could reduce the HPNs recruitment and inflammatory factors in HPNs supernatants. Mechanistically, poly(I: C)-mediated activation of intracellular RIG-I upregulated CXCR2 expression on HPNs via the MAPK pathway. In vivo, inhibiting CXCR2, RIG-I, or depleting neutrophils significantly alleviated poly(I: C)-induced inflammatory lung injury, inflammation, and neutrophil infiltration.

CONCLUSION

Our study identifies the RIG-I-MAPK-CXCR2 axis as a novel pathogenic signaling cascade that drives dysregulated neutrophilic inflammation in COVID-19, providing a basis for novel therapeutic strategies against respiratory viral infections.