Cathepsin S Orchestrates Trained Immunity via KEAP1/NRF2 axis to Exacerbate Kidney Inflammation and Fibrosis

Trained immunity involves epigenetic and metabolic reprogramming of innate immune cells, leading to an enhanced response upon secondary stimulation and exacerbating chronic inflammation and organ metabolic disorders. In chronic kidney disease (CKD), renal and systemic metabolic disturbances trigger systemic immune dysregulation, sustaining a persistent inflammatory state that promotes kidney injury and fibrosis. Although the mechanisms of trained immunity align closely with CKD pathology, its role in CKD remains unclear. Using a β-glucan–induced trained immunity model, this study investigates whether trained immunity contributes to renal fibrosis via inflammatory regulation and explores the underlying mechanism.

A murine trained immunity model was established using β-glucan. After restoration of immune homeostasis, kidney ischemia-reperfusion injury (uIRI-7d) was induced. Differentially expressed genes from multi-stage transcriptomic data of β-glucan–stimulated murine and human myeloid cells/progenitors were integrated to identify conserved cross-species targets. Wild-type and Ctss‑knockout mice were subjected to the trained immunity model and renal uIRI-7d to systematically analyze the role of Cathepsin S (CTSS) in bone marrow-derived trained immunity and its effect on renal inflammation and fibrosis. ScRNA-seq and ATAC-seq of murine bone marrow myeloid progenitors was performed to elucidate the molecular mechanisms. We analyzed scRNA-seq data from monocyte of CKD patients, and validated the association between CTSS and trained immunity.

Trained immunity induced myeloid differentiation bias in hematopoietic stem and progenitor cells (HSPCs) in the bone marrow, resulting in circulating myeloid-derived monocytes with heightened inflammatory activity. This led to increased infiltration of pro-inflammatory macrophages into the kidney and enhanced IL‑1β production, which aggravated renal fibrosis and histopathological damage. Through cross-species transcriptomic analysis, we identified cathepsin S (CTSS) as a conserved and essential mediator of trained immunity. Genetic deletion of Ctss abolished the training-induced myeloid bias in hematopoiesis, monocyte activation, renal macrophage infiltration, and subsequent fibrosis. Mechanistically, CTSS, via its enzymatic activity, triggered the lysosomal degradation of the repressor KEAP1, leading to nuclear translocation of the transcription factor NRF2. This NRF2 activation drove the expression of cytochrome c oxidase subunit 6C (COX6C), enhancing oxidative phosphorylation and Complex IV activity. This metabolic reprogramming was prerequisite for the establishment of the trained epigenetic state, as CTSS deficiency impaired H3K4me3 methylation at the Il1b promoter and suppressed IL-1β production upon restimulation. Notably, a pro-inflammatory monocyte subset with high CTSS expression was expanded in CKD patients. Circulating monocytes in CKD patients exhibited trained immunity features, characterized by augmented cytokine production and elevated H3K4me3 expression. Moreover, serum CTSS protein level correlated with trained immunity features and fibrosis severity.

Our findings nominate CTSS as a potential biomarker for trained immunity and reveal its targeting as a therapeutic strategy to mitigate trained immunity-driven kidney inflammation and fibrosis.