Phosphorylated P70 S6 Kinase and S6 Ribosomal Protein Value as Diagnostic Marker of Antibody-mediated Rejection in Lung Allografts: A Multicenter Study

Abstract

Purpose: Antibody-mediated rejection (AMR) is emerging as an important cause of solid organ transplantation failure. The role of AMR in lung allografts is a source of ongoing investigation, as well as the identification of more sensitive and accurate diagnostic features. The mammalian target of rapamycin (mTOR) pathway, in particular phosphorylated P70 S6 kinase (S6K) and S6 ribosomal protein (S6RP), is reported to be associated with AMR in heart and kidney transplantation. The main goal of the study was to evaluate S6K and S6RP expression in lung transplanted (LT) patients with AMR from different LT centers. Methods: The study included 66 patients: 52 with AMR (according to the recent ISHLT consensus report) and 14 negative for AMR and other posttransplant complications (controls). Immunohistochemistry for S6K and S6RP was performed in transbronchial biopsies (TBB) of patients at time of AMR diagnosis and in TBB of controls roughly matched for time posttransplantation and other donor/recipient characteristics with the AMR biopsies. Analyses (using a score 0-3) were performed distinguishing different cell components (macrophages, endothelium, alveolar epithelium, airway epithelium). Results: S6K showed unreliable immunostaining. S6RP expression was detected in 94% of AMR cases and only in 43% of controls (p< 0.0001). Median values were higher in macrophages, airway epithelium and alveolar epithelium of AMR vs control cases (p< 0.001, p= 0.0005 and p= 0.008 respectively). Interestingly, endothelial S6RP values were more significantly expressed in AMR patients than controls (p= 0.037), showing higher sensitivity than C4d in AMR patients (p= 0.04). The presence of concomitant acute cellular rejection (ACR) in AMR cases was associated with a significant increase in S6RP expression in all cell types (all p< 0.05). Conclusion: In conclusion, our study confirms the role of the mTOR pathway in graft AMR-related injury. S6RP, detected by immunohistochemistry, represents a useful and sensitive tool for diagnosing AMR. The strong immunostaining in non-endothelial cells remains an intriguing aspect to explore.