TLR4 activation mediates kidney ischemia/reperfusion injury

TLDR

Ischemia/reperfusion injury can activate innate immunity through endogenous ligands engaging TLRs, and kidney‑expressed TLR4 is a key mediator of this inflammatory response. The study aimed to determine whether TLR4 expressed by kidney parenchymal cells or leukocytes contributes more to damage during ischemia/reperfusion injury by generating chimeric mice. Chimeric mice and TLR4(-/-)/MyD88(-/-) mouse models were employed to dissect TLR4 signaling in parenchymal versus leukocyte compartments and to assess kidney function, histology, cytokine production, and TEC apoptosis. TLR4 and MyD88 signaling are essential for kidney IRI, as knockout mice exhibit reduced dysfunction, inflammation, and TEC apoptosis, and chimeric studies show that TLR4 in intrinsic kidney cells drives most damage, with HMGB1, hyaluronan, and biglycan likely serving as activating ligands.

Abstract

Ischemia/reperfusion injury (IRI) may activate innate immunity through the engagement of TLRs by endogenous ligands. TLR4 expressed within the kidney is a potential mediator of innate activation and inflammation. Using a mouse model of kidney IRI, we demonstrated a significant increase in TLR4 expression by tubular epithelial cells (TECs) and infiltrating leukocytes within the kidney following ischemia. TLR4 signaling through the MyD88-dependent pathway was required for the full development of kidney IRI, as both TLR4(-/-) and MyD88(-/-) mice were protected against kidney dysfunction, tubular damage, neutrophil and macrophage accumulation, and expression of proinflammatory cytokines and chemokines. In vitro, WT kidney TECs produced proinflammatory cytokines and chemokines and underwent apoptosis after ischemia. These effects were attenuated in TLR4(-/-) and MyD88(-/-) TECs. In addition, we demonstrated upregulation of the endogenous ligands high-mobility group box 1 (HMGB1), hyaluronan, and biglycan, providing circumstantial evidence that one or more of these ligands may be the source of TLR4 activation. To determine the relative contribution of TLR4 expression by parenchymal cells or leukocytes to kidney damage during IRI, we generated chimeric mice. TLR4(-/-) mice engrafted with WT hematopoietic cells had significantly lower serum creatinine and less tubular damage than WT mice reconstituted with TLR4(-/-) BM, suggesting that TLR4 signaling in intrinsic kidney cells plays the dominant role in mediating kidney damage.