Pro-inflammatory Fibroblasts Drive Tubulointerstitial Secondary Inflammation and Promote AKI-to-CKD Transition

Tubulointerstitial inflammation (TII) is a central driver of the transition from acute kidney injury (AKI) to chronic kidney disease (CKD). While fibroblasts have long been viewed as structural cells maintaining interstitial integrity, emerging evidence indicates that they actively regulate immune responses across multiple pathological settings. However, their specific contribution to TII during AKI–to–CKD progression remains elusive. Here, we define the cellular diversity and immunomodulatory dynamics of renal fibroblasts and uncover their role in shaping the inflammatory remodeling of the tubulointerstitium.

A unilateral renal ischemia-reperfusion injury (uIRI) model was established in mice, and kidney tissues were collected at days 0, 1, 3, 14, and 28 for single-cell RNA sequencing. Dynamic changes in fibroblast and immune cell populations were analyzed through subclustering, marker-based annotation, functional enrichment, and flow cytometry. A previously unrecognized fibroblast subpopulation (termed CD74⁺ iFibro) was identified and validated. To determine its functional relevance, we generated fibroblast-specific CD74 knockout mice. In addition, CellChat analysis predicted amyloid precursor protein (APP) as a potential upstream regulator of the CD74⁺ iFibro phenotype, which was further validated in vivo and in vitro. Finally, the PDGFRβ/CD74 bispecific cytotoxic antibody was developed to selectively eliminate CD74⁺ iFibro, and its therapeutic efficacy in mitigating the chronic progression of injury was assessed. 

Following AKI, the kidney exhibited pronounced infiltration of macrophages, neutrophils, and T cells at days 1 and 14, accompanied by increased expression of chemokines including CCL2, CCL3, CCL5, and CCL7, indicative of a secondary inflammatory insult during the AtoC process. Spatial analysis revealed close proximity and strong interactions between fibroblasts and infiltrating immune cells during the chronic phase at day 14 post-injury. Single-cell transcriptomic profiling identified a pro-inflammatory fibroblast subpopulation, CD74⁺ iFibro, that emerged specifically during this secondary inflammatory phase. CD74⁺ iFibro expressed high levels of CD74, CCL2, CCL7, and CXCL2, suggesting a role in immune cell recruitment and activation. Co-culture of sorted CD74⁺ iFibro with primary bone marrow–derived macrophages markedly enhanced macrophage activation, as evidenced by upregulation of iNOS, IL-6, and CCL2. Mechanistically, activation of the APP–CD74 signaling axis induced the CD74⁺ iFibro phenotype, increasing expression of CD74, CCL2, CCL7, and CXCL2 and activating the NF-κB pathway. Targeted depletion of CD74⁺ iFibro using the PDGFRβ/CD74 bispecific antibody significantly attenuated chronic inflammatory progression and preserved renal function.

We identify CD74⁺ inflammatory fibroblasts (iFibro) as key mediators of secondary inflammation during the AKI–to–CKD transition. Through activation of the APP–CD74 axis, these fibroblasts acquire a pro-inflammatory program that sustains immune cell recruitment and drives maladaptive remodeling. This work uncovers a fibroblast-centered mechanism underlying persistent tubulointerstitial inflammation and nominates CD74⁺ iFibro and the APP–CD74 pathway as actionable targets to intercept chronic progression.