Emergence of immunosuppressive LOX-1+ PMN-MDSC in septic shock and severe COVID-19 patients with acute respiratory distress syndrome

Abstract

Myeloid-derived suppressor cells (MDSC) are a heterogeneous population of immature myeloid cells with immunosuppressive properties. In cancer patients, the expression of lectin-type oxidized LDL receptor 1 (LOX-1) on granulocytic MDSC identifies a subset of MDSC that retains the most potent immunosuppressive properties. The main objective of the present work was to explore the presence of LOX-1+ MDSC in bacterial and viral sepsis. To this end, whole blood LOX-1+ cells were phenotypically, morphologically, and functionally characterized. They were monitored in 39 coronavirus disease-19 (COVID-19, viral sepsis) and 48 septic shock (bacterial sepsis) patients longitudinally sampled five times over a 3 wk period in intensive care units (ICUs). The phenotype, morphology, and immunosuppressive functions of LOX-1+ cells demonstrated that they were polymorphonuclear MDSC. In patients, we observed the significant emergence of LOX-1+ MDSC in both groups. The peak of LOX-1+ MDSC was 1 wk delayed with respect to ICU admission. In COVID-19, their elevation was more pronounced in patients with acute respiratory distress syndrome. The persistence of these cells may contribute to long lasting immunosuppression leaving the patient unable to efficiently resolve infections.

Keywords: HLA-DR; flow cytometry; immunosuppression; sepsis.

FIGURE 1

Lectin‐type oxidized LDL receptor 1 (LOX‐1) neutrophils characteristics. (A) Representative example of CD45^dim LOX‐1 phenotypic characterization in septic shock patient. Polymorphonuclear cells (PMN) were first gated out from other cells on a biparametric CD45/side scatter histogram. Then, eosinophils were excluded from gated PMN on the expression of CRTH2. LOX‐1 expression was finally evaluated on whole neutrophil population (positivity threshold was defined based on isotype values set up at 0.5%). LOX‐1 expression is either represented among CD15+ cells or CD45+ cells. Red cells represent CD45^dim LOX‐1+ cells gated on the biparametric CD45/LOX1 dotplot. CD10 and CD16 expressions gated on the CD45^dim LOX‐1 cells is also represented to illustrate the immature nature of these cells. (B) LOX‐1+ MDSC (myeloid‐derived suppressor cells) immunosuppressive function on T cell IFNg release. IFN‐γ concentrations in the supernatants of T cells purified from healthy volunteers (HV) were measured (n = 3 independent experiments). T cells were either incubated alone, with stimulation beads, or cocultured at 1:1 ratio with CD45+ LOX‐1‐ or CD45^dim LOX‐1+ MDSC purified from 3 septic shock patients. Data are presented as means ± sd

FIGURE 2

Monitoring of lectin‐type oxidized LDL receptor 1 polymorphonuclear myeloid‐derived suppressor cells (LOX‐1+ PMN‐MDSC) overtime in septic shock and coronavirus disease‐19 (COVID‐19) patients. Percentages of LOX‐1+ PMN MDSC among total neutrophils were measured in peripheral blood from 22 healthy volunteers (HV) and (A) 48 septic shock patients (day 0/2, n = 37; day 3/5, n = 44; day 6/9, n = 24; above day10, n = 9), or (B) 39 COVID‐19 ICU patients (day 0/2, n = 29; day 3/5, n = 18; day 6/9, n = 12; above day10, n = 15). Missing values corresponded to patients who died or left ICU before the end of the study and to missing samples during the weekends for which staining was not possible since lab was not operating 24/7. Data are presented as individual values and Tukey boxplots. (C) Acute respiratory distress syndrome (ARDS) in COVID‐19. For COVID‐19 ICU patients with paired samples at day 0/2 and day 3/5, kinetics of CD45dim LOX1+ percentage in patients with (n = 10) or without (n = 8) ARDS was evaluated. Nonparametric Mann‐Whitney test was used to assess differences between HV and patients. The nonparametric Wilcoxon paired test was used to assess variations between patients themselves at day 0/2 and day 3/5; *P < 0.05, **P < 0.01