M2 Macrophages Attenuate AQP2⁺ Collecting Duct Cell Apoptosis via the TRAF1–TRAF2 Complex to Suppress Randall's Plaque Formation

Calcium oxalate (CaOx) kidney stones are common and recur frequently, yet effective pharmacological prevention is limited. Randall’s plaques (RPs), calcium deposits on renal papillae, act as anchoring sites for CaOx crystal growth, but the cellular origin and immunoregulatory mechanism underlying early calcium deposition remain unclear. Here, we found that early calcium deposition was partially localized to the renal interstitium surrounding aquaporin 2 (AQP2)-labeled collecting duct cells with apoptotic phenotype, and inhibiting high calcium-induced apoptosis of AQP2+ cells markedly reduced cell layer calcium deposition. Given the regulatory role of macrophages in renal stone formation, we revealed that M2 macrophages protected AQP2+ cells by delivering TRAF2 via exosomes. Mechanistically, exosomal TRAF2 interacted with intracellular TRAF1 to form a stable complex, which mutually inhibited their ubiquitin-mediated degradation by excluding the shared E3 ubiquitin ligase CBLC. This complex activated downstream nuclear factor κB1 (NF-κB1) and NF-κB2 signaling. NF-κB1 enhanced TRAF1 transcription, whereas NF-κB2 increased BCL2 expression while suppressing BAX and cleaved-PARP1, thereby limiting apoptosis and subsequent calcium deposition. Based on these findings, we developed M2 exosome-loaded, AQP2+ cell membrane-coated poly (lactic-co-glycolic acid) nanoparticles (Exo@A-P) for renal-targeted delivery. Exo@A-P treatment reduced collecting duct apoptosis and calcium deposition in hypercalciuria and renal calcified mice (Umod−/−) without detectable toxicity, potentially supporting a targeted nanotherapeutic strategy to prevent early RP formation by utilizing TRAF2-rich exosome from M2 macrophages.