Dysregulation of Pulmonary Responses in Severe COVID-19

Abstract

Patients with coronavirus disease 2019 (COVID-19) predominantly have a respiratory tract infection with various symptoms and high mortality is associated with respiratory failure second to severe disease. The risk factors leading to severe disease remain unclear. Here, we reanalyzed a published single-cell RNA-Seq (scRNA-Seq) dataset and found that bronchoalveolar lavage fluid (BALF) of patients with severe disease compared to those with mild disease contained decreased TH17-type cells, decreased IFNA1-expressing cells with lower expression of toll-like receptor 7 (TLR7) and TLR8, increased IgA-expressing B cells, and increased hyperactive epithelial cells (and/or macrophages) expressing matrix metalloproteinases (MMPs), hyaluronan synthase 2 (HAS2), and plasminogen activator inhibitor-1 (PAI-1), which may together contribute to the pulmonary pathology in severe COVID-19. We propose IFN-I (and TLR7/TLR8) and PAI-1 as potential biomarkers to predict the susceptibility to severe COVID-19.

Keywords: COVID-19; IgA; PAI-1; TH17; type I interferon.

Figure 1

Dysregulation of TH and B cell profiles and IFN-I pathway in BALFs. (A) Frequencies of TH1 (TBX21⁺), TH2 (GATA3⁺), TH17 (RORC⁺ or CCR6⁺) cells and regulatory T cells (FOXP3⁺) in BALF cells on a CD4⁺ CD14⁻ gate. (B) Frequencies of γδT cells (TRDC⁺) on a CD3E⁺ gate. (C) Frequencies of IL22⁺ cells on a KRT18⁻ gate. (D) Frequencies of IgA1 (IGHA1)- and IgG1 (IGHG1)-expressing B cells on a CD19⁺ gate. (E) Frequencies of IFN-I-expressing macrophages on a CD68⁺ gate. (F,G) Abundances of RNA recognition receptors in (F) macrophages and (G) epithelial cells. HC, healthy control (n = 3); M, mild (n = 3); S, severe (n = 6). Mean and s.d. are shown; p values; unpaired t-test.

Figure 2

Expression of tissue factors in BALF cells. (A,B) Frequencies of MMP- and TIMP-expressing epithelial cells (A) and macrophages (B). (C) Abundances of MUCs in epithelial cells. (D) Frequencies of HAS-expressing epithelial cells. (E,F) Frequencies of tPA- and PAI-1-expressing epithelial cells (E) and macrophages (F). (G) Ratios of total tPA vs. PAI-1 and total uPA- vs. PAI-1-expressing cells. HC, healthy control (n = 3); M, mild (n = 3); S, severe (n = 6). Mean and s.d. are shown; p values; unpaired t-test.

Figure 3

Outline of unfavorable conditions and deleterious pulmonary responses. (1) Older age, male gender, underlying conditions (such as hypertension, diabetes, etc.), and unknown factors (including genetic background) impair antiviral immunity (including IFN-I deficiency and decreased CD8⁺ T), leading to higher virus loads and tissue damage/stress. (2) Elevation of humoral responses results in massive immune complexes that activate macrophages and neutrophils. (3) Decreased TH17 cell responses cause overgrowth of commensal bacteria and fungi, which further activate macrophages and neutrophils. (4) TH17 hyperactivation and/or expansion (in the intestine?) cause high levels of serum IL17, which induces G-CSF expression and, in turn, promotes neutrophilia. (5) Hyperactivated macrophages and neutrophils release immense amounts of proinflammatory cytokines, leading to cytokine release syndrome and subsequent ARDS, as well as tissue-destructive products, such as ROS, NO, MMPs, and other enzymes. (6) During ARDS, proinflammatory cytokines act on epithelial cells and induce MMPs, mucins, hyaluronic acids, antimicrobial peptides, and PAI-1 (unfavorable conditions also elevate PAI-1 expression). (7) ROS, NO, MMPs, and other enzymes cause epithelial and endothelial leakage, leading to tissue fluid/plasma accumulation in alveolar spaces. Mucins, hyaluronic acids, and antimicrobial peptides concentrate alveolar fluids and thicken mucosal lining, resulting in edema and even lung failure. Heightened PAI-1 facilitates coagulation and strengthens edema formation (and thrombosis). ARDS also has systemic consequences causing multiorgan injury.