Isolation and Culture of Lung Spheroid Cells

Lung spheroid cells (LSC) were isolated from lung samples of healthy human obtained from the National Disease Research Interchange and passaged every 3–5 days, which has been conducted in our previous studies^{32 (link)}. After 2–3 passages, LSC were plated on a fibronectin-coated flask and maintained in Iscove’s Modified Dulbecco’s Media (IMDM) containing 20% fetal bovine serum (FBS). Media changes were performed every other day. LSC were allowed to reach 70–80% confluence before generating serum-free secretome (LSC-Secretome). LSC-Secretomes were collected and filtered with a 0.22 μm filter to remove cellular debris. HEK293T cells (CRL-3216) were purchased from ATCC and cultured in Dulbecco’s Modified Eagle Medium (DMEM) with 10% FBS. A549 cells expressing human ACE2 and human TMPRSS2 were purchased from InvivoGen (a549-hace2tpsa) and cultured in DMEM with 10% FBS. All procedures in this study were in accordance with the ethical standards of the institutional research committee and with the guidelines set by the Declaration of Helsinki.