Isolation and Culture of Lung Fibroblasts

Fibroblasts were isolated from RA-ILD or control lung as previously described^{34 (link),35 (link)}. Briefly, lung tissue was minced and digested in collagenase buffer. Cell pellets were resuspended and cultured for 7–10 days. Cells were CD45-depleted using CD45 microbeads (Miltenyi Biotec) to remove hematopoietic cell populations. Fibroblasts were cultured in Dulbecco’s Modified Eagle Medium (Corning) containing 10% fetal bovine serum (Corning), 100 IU of penicillin and 100 μg/ml streptomycin (Corning), 292 μg/ml L-glutamine (Corning), and 100 μg/ml Primocin (InVivoGen) in humidified incubators at 37 °C and 10% CO₂. Cells were periodically tested for Mycoplasma spp. contamination using a commercially available kit (ThermoFisher Scientific).