Immune inhibitory receptor LILRB2 enhances SARS-CoV-2 spike protein-mediated immune inflammation

ABSTRACT

Objective·To explore the possible roles of immune inhibitory receptor leukocyte immunoglobulin-like receptor subfamily B member 2 (LILRB2) in the immune inflammation after severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, and provide a potential therapeutic way for the coronavirus disease 2019 (COVID-19). Methods·The supernatants containing the extracellular domain of spike protein (S-ECD) were collected, and the detection of the protein expression and activity in the conditional medium by Western blotting and flow cytometric analysis was followed by. The binding of S-ECD with LILRB2 was measured by co-immunoprecipitation and flow cytometric analysis. The mRNA expression levels of several inflammation genes in a human mononuclear cell line (THP1) or peripheral blood mononuclear cells (PBMC) were measured after spike protein stimulation for 24 h by quantitative RT-PCR. The protein levels of interleukin-6 (IL-6) and interleukin-1β (IL-1β) in the conditional medium were examined by enzyme-linked immunosorbent assay (ELISA). The siLILRB2 was transferred into CD33+ myeloid cells purified from human peripheral blood with Lipofectamine 3000 reagents. The knockdown efficiency was detected 24 h after transfection by flow cytometric analysis. The difference in the protein levels of IL-6 between the control cells and LILRB2-knocked-down cells after spike protein treatment was evaluated by ELISA. Results·The study established a transfection system with 293T cells by which the SARSCoV-2 S-ECD could be secreted to supernatants with normal biological activities. The interaction and the binding of spike protein with LILRB2 were evaluated by a co-immunoprecipitation assay and flow cytometric analysis, respectively. The mRNA expression levels of IL-6, IL-8, arginase 1 and IL-2 in THP1 cells were significantly up-regulated 24 h after spike protein treatment compared to the control cells (all P<0.05). Consistently, the mRNA levels of IL-6, transforming growth factor-β (TGF-β), IL-8, IL-10 and IL-β in PBMC were notably increased after spike protein stimulation (all P<0.05). In addition, spike protein could also induce the release of IL-6 and IL-1β in PBMC as measured by ELISA (all P<0.05). More importantly, spike protein was able to increase the secretion of IL-1β and IL-6 by CD33+ myeloid cells 24 h after treatment (both P<0.05). LILRB2-overexpressing THP1 cells produced more IL-6 24 h after treatment with spike protein than the control cells (P<0.05). Two siRNAs could efficiently down-regulate the expression of LILRB2 in CD33+ cells as evaluated by flow cytometric analysis. Consistently, spike protein had no effect on the IL-6 secretion to supernatant from LILRB2-knockdown CD33+ myeloid cells. Conclusion·SARS-CoV-2 can induce cytokine release syndrome by inflammatory factors, such as IL-6 and IL-1β, released by myeloid cells through spike protein binding to LILRB2. © 2022 Editorial Department of Journal of Shanghai Second Medical University. All rights reserved.