Expansion of Myeloid Derived Suppressor Cells Contributes to Platelet Activation by L-Arginine Deprivation during SARS-CoV-2 Infection

Abstract

Massive platelet activation and thrombotic events characterize severe COVID-19, highlighting their critical role in SARS-CoV-2-induced immunopathology. Since there is a well-described expansion of myeloid-derived suppressor cells (MDSC) in severe COVID-19, we evaluated their possible role in platelet activation during SARS-CoV-2 infection. During COVID-19, a lower plasmatic L-arginine level was observed compared to healthy donors, which correlated with MDSC frequency. Additionally, activated GPIIb/IIIa complex (PAC-1) expression was higher on platelets from severe COVID-19 patients compared to healthy controls and inversely correlated with L-arginine plasmatic concentration. Notably, MDSC were able to induce PAC-1 expression in vitro by reducing L-arginine concentration, indicating a direct role of PMN-MDSC in platelet activation. Accordingly, we found a positive correlation between ex vivo platelet PAC-1 expression and PMN-MDSC frequency. Overall, our data demonstrate the involvement of PMN-MDSC in triggering platelet activation during COVID-19, highlighting a novel role of MDSC in driving COVID-19 pathogenesis.

Keywords: COVID-19; L-Arginine; MDSC; platelet.

Figure 1

Plasmatic L-arginine level in SARS-CoV-2 patients correlated with PMN-MDSC frequency. (a) Plasmatic Arginine level of SARSCoV-2 ICU (ICU, n = 31) and no ICU (no ICU, n = 31) patients and healthy donors (HD, n = 9). Results are shown as box and whiskers plot. Kruskal-Walliswith Dunn’s post hoc test was applied. (b) Flow-cytometry gating strategy used to identify MDSC among PBMC. Dead cells were excluded by selecting DRAQ7neg cells. One representative COVID-19 patient and one HD are shown. (c) Correlation between plasmatic L-arginine level and PMN-MDSC percentage from SARSCoV-2 infected patients and HD. Non-parametric Spearman correlation was applied. (d) Representative plots of the adopted gating strategy to evaluate platelet activation. Platelets were selected as CD41a+ (SSC/CD41a plot), and PAC-1 expression (mean fluorescence intensity, mfi) was evaluated. (e) PAC-1 expression (mfi) on platelets from ICU (n = 21), no ICU (n = 15) patients, and HD (n = 9). Results are shown as box and whiskers plot. Kruskal-Walliswith Dunn’s post hoc test was applied. (f) Correlation between PAC-1 platelet expression (mfi) and L-arginine plasmatic level.

Figure 2

PMN-MDSC induced platelet activation by reducing L-arginine. (a) Correlation between PAC-1 platelet expression (mfi) and PMN-MDSC percentage from SARSCoV-2 infected patients (ICU n = 21, no ICU n = 15) and HD (n = 9). Non-parametric Spearman correlation was applied. (b) Representative histogram plot showing PAC-1 expression on platelets after culture with or without PMN-MDSC or PBMC depleted from MDSCs (PBMC). (c) PAC-1 expression on platelets from HD (n = 6) after culture with PMN-MDSC or PBMC from SARSCoV-2 infected patients (n = 6). Results are shown as median and IQR. Wilcoxon matched-pairs signed rank test was applied. (d) L-arginine reduction (ΔArg) calculated as the difference between L-arginine concentration in the presence of MDSCs or PBMC and platelets alone. Results are shown as median and IQR. A Wilcoxon matched-pairs signed rank test was applied.