Lipids with overall false discovery rate (FDR) <0.05 were shortlisted and grouped according to major lipid classes for visual clarity in a forest plot (Figure 2 ). We observed reductions in major classes of plasma glycerophospholipids including phosphatidic acids (PAs), phosphatidylinositols (PIs), and PCs, with accompanying increases in their corresponding lysophospholipids (i.e., LPAs, LPIs, and LPCs) that indicate enhanced phospholipase A2 activity in COVID-19 patients. Cytosolic phospholipase A2α (cPLA2α) activation was reported to trigger pulmonary inflammation following pathogen infection (Bhowmick et al., 2017). PUFA-PEs were the only diacyl forms of glycerophospholipids that increased in COVID-19. Changes in phospholipidome (i.e., reductions in PCs and PIs and increases in PEs) observed in COVID-19 corroborated a previous study on plasma lipid alterations in EVD fatalities compared to survivors (Kyle et al., 2019). Among glycerophospholipids, PCs constitute the major membrane components of circulating lipoproteins (Cole et al., 2012), and PC-transfer protein (PC-TP) promotes cellular lipid efflux in nascent high-density lipoprotein (HDL) formation mediated by apolipoprotein A1 (apo-A1) (Baez et al., 2002), the major protein component of HDLs. PI was found to exhibit selective enrichment in plasma HDL fractions and was not detected in plasma lipoprotein-free fractions (Dashti et al., 2011). Changes in phospholipidome therefore suggested reductions in circulating HDLs as COVID-19 progresses, which was in agreement with the observed reductions in apo-A1 (p = 0.0687) as disease severity increases (Figure S1C). As for neutral lipids, marked reductions in several DAGs, with concomitant increases in FFAs (e.g., FFA18:1 and FFA 18:2), were observed in mild and moderate COVID-19 (Figure 2), while TAGs were significantly reduced only in mild cases (Figure S4). Increases in C18-FFAs and diminished TAGs were in agreement with previously reported circulating lipid changes associated with ARDS (Bursten et al., 1996; Maile et al., 2018). In stark contrast to the glycerophospholipid and neutral lipid pathways, sphingolipid classes of SMs and GM3s, e.g., GM3 d18:1/16:0, GM3 d18:0/16:0, GM3 d18:1/25:0, and GM3 d18:0/25:0, displayed progressive increases with increasing severity (Figures 2 and S4), possibly reflecting the augmented secretion of these lipids into the circulation. Of particular interest, lipidomes of exosomal membranes were previously shown to be specifically enriched in SMs (Stoorvogel et al., 2002) with diminished DAGs that cumulatively give rise to enhanced membrane rigidity (Laulagnier et al., 2004). Exosomes from lymphocytes also exhibited cell-type-specific enrichments in GM3s (Wubbolts et al., 2003) and PSs (Brügger et al., 2006). Thus, a gross overview of plasma lipidomic signatures distinctly associated with COVID-19, taking into account baseline cofounders, revealed a close resemblance to that of exosomal lipid compositions (i.e., enriched in SMs and GM3s with reduced DAGs).
Figure 2 Plasma Lipids Associated with Severity of COVID-19
Logistic regression model with covariates BMI, age, and sex was built with each lipid to search for significant variables that could predict disease severity of subjects (i.e., healthy control, and mild, moderate, and severe COVID-19). Only lipids with false discovery rate (FDR) <0.05 were shortlisted and presented. Forest plots illustrate the magnitude of odds ratios with indicator of significance of the estimate in the model; ∗∗∗p < 0.001, ∗∗p < 0.01, ∗p < 0.05. For non-significant lipids, the estimates were plotted as zeros. Lipids were broadly classified according to major classes of neutral lipids, glycerophospholipids, and sphingolipids. See also Figure S4.