ISG15 and interstitial lung disease: Conversely, type I interferon signaling and production was significantly upregulated amongst SPP1hi and FABP4hi macrophages in SSc-ILD (Figures 2E, H), compared to IPF, including expression of multiple interferon-induced transmembrane proteins, IFI35, ISG15, and EGR1 within the SPP1hi macrophages and TBK1, CHUK, and PTPN11, amongst others, within the FABP4hi macrophages.