68Ga-FAPI-04 PET/CT Imaging for Assessing Renal Tubulointerstitial Fibrosis in Lupus Nephritis

The objective of this study was to evaluate the feasibility of using ⁶⁸Ga-labeled fibroblast activation protein inhibitor-04 (⁶⁸Ga-FAPI-04) PET/CT imaging as a molecular tracer and noninvasive tool for assessing active renal tubulointerstitial fibrosis in patients with lupus nephritis (LN). Methods: The study included 29 LN patients who underwent ⁶⁸Ga-FAPI-04 PET/CT scanning to quantify ⁶⁸Ga-FAPI-04 uptake. Renal biopsies were performed within a week of scanning. Renal fibrosis levels in biopsy samples were assessed by Masson staining. Immunofluorescence analysis identified the expression of fibroblast activation protein (FAP), E-cadherin, and α-smooth muscle actin in the renal biopsy specimens. FAP expression of healthy controls, LN patients, and patients with minimal change of disease at the messenger RNA level and its association with interferon levels were explored using microarray data from the Gene Expression Omnibus database. Additionally, quantitative polymerase chain reaction and Western blot analyses quantified FAP messenger RNA and protein expression in renal tubular epithelial cells (RTEC) after stimulation with interferon-α in vitro. Results: The study revealed significantly increased renal ⁶⁸Ga-FAPI-04 uptake in LN patients (n = 29) compared with healthy controls (n = 26). Normalized renal ⁶⁸Ga-FAPI-04 uptake positively correlated with disease duration, creatinine levels, chronicity index, and renal tubulointerstitial fibrosis levels. Patients with a chronicity index exceeding 4, indicative of a poorer prognosis, showed markedly higher renal ⁶⁸Ga-FAPI-04 uptake. FAP expression was predominantly localized in RTEC, where increased FAP expression corresponded to reduced E-cadherin expression. Gene set enrichment analysis of GSE112943 and GSE60861 datasets showed positive enrichment of type I interferon signaling gene sets (P < 0.01). Correlation analysis of interferon-α and interferon-γ pathways with FAP expression in these datasets was significant (P < 0.01). In vitro experiments, with interferon-α–stimulated RTEC showed elevated FAP expression. Conclusion: This study demonstrates the feasibility of using ⁶⁸Ga-FAPI-04 PET/CT imaging for noninvasive assessment of active lupus renal tubulointerstitial fibrosis. Elevated FAP expression in LN is primarily expressed by RTEC and contributes to the development of interstitial fibrosis. Type I interferon appears to induce FAP expression. These findings provide insights into the molecular mechanisms underlying renal tubulointerstitial fibrosis in LN and offer a valuable tool for assessing kidney status in lupus.