Renal Inflammation and Innate Immune Activation Underlie the Transition From Gentamicin-Induced Acute Kidney Injury to Renal Fibrosis

Subjects recovering from acute kidney injury (AKI) are at risk of developing chronic kidney disease (CKD). The mechanisms underlying this transition are unclear and may involve sustained activation of renal innate immunity, with resulting renal inflammation and fibrosis. We investigated whether the NF-κB system and/or the NLRP3 inflammasome pathway remain activated after the resolution of AKI induced by gentamicin (GT) treatment, thus favoring the development of CKD. Male Munich-Wistar rats received daily subcutaneous injections of GT, 80 mg/kg, for 9 days. Control rats received vehicle only (NC). Rats were studied at 1, 30, and 180 days after GT treatment was ceased. On Day 1, glomerular ischemia (ISCH), tubular necrosis, albuminuria, creatinine retention, and tubular dysfunction were noted, in association with prominent renal infiltration by macrophages and myofibroblasts, along with increased renal abundance of TLR4, IL-6, and IL1β. Regression of functional and structural changes occurred on Day 30. However, the renal content of IL-1β was still elevated at this time, while the local renin-angiotensin system remained activated, and interstitial fibrosis became evident. On Day 180, recurring albuminuria and mild glomerulosclerosis were seen, along with ISCH and unabated interstitial fibrosis, whereas macrophage infiltration was still evident. GT-induced AKI activates innate immunity and promotes renal inflammation. Persistence of these abnormalities provides a plausible explanation for the transition of AKI to CKD observed in a growing number of patients.

NF-κB blockade during short-term l-NAME and salt overload strongly attenuates the late development of chronic kidney disease

Nitric oxide synthase inhibition by N^ω-nitro-l-arginine methyl ester (l-NAME) plus a high-salt diet (HS) is a model of chronic kidney disease (CKD) characterized by marked hypertension and renal injury. With cessation of treatment, most of these changes subside, but progressive renal injury develops, associated with persistent low-grade renal inflammation. We investigated whether innate immunity, and in particular the NF-κB system, is involved in this process. Male Munich-Wistar rats received HS + l-NAME (32 mg·kg^-1·day^-1), whereas control rats received HS only. Treatment was ceased after week 4 when 30 rats were studied. Additional rats were studied at week 8 (n = 30) and week 28 (n = 30). As expected, HS + l-NAME promoted severe hypertension, albuminuria, and renal injury after 4 wk of treatment, whereas innate immunity activation was evident. After discontinuation of treatments, partial regression of renal injury and inflammation occurred, along with persistence of innate immunity activation at week 8. At week 28, glomerular injury worsened, while renal inflammation persisted and renal innate immunity remained activated. Temporary administration of the NF-κB inhibitor pyrrolidine dithiocarbamate, in concomitancy with the early 4-wk HS + l-NAME treatment, prevented the development of late renal injury and inflammation, an effect that lasted until the end of the study. Early activation of innate immunity may be crucial to the initiation of renal injury in the HS + l-NAME model and to the autonomous progression of chronic nephropathy even after cessation of the original insult. This behavior may be common to other conditions leading to CKD.

Chronic exposure to hypoxia attenuates renal injury and innate immunity activation in the remnant kidney model

Hypoxia is thought to influence the pathogenesis of chronic kidney disease, but direct evidence that prolonged exposure to tissue hypoxia initiates or aggravates chronic kidney disease is lacking. We tested this hypothesis by chronically exposing normal rats and rats with 5/6 nephrectomy (Nx) to hypoxia. In addition, we investigated whether such effect of hypoxia would involve activation of innate immunity. Adult male Munich-Wistar rats underwent Nx (n = 54) or sham surgery (sham; n = 52). Twenty-six sham rats and 26 Nx rats remained in normoxia, whereas 26 sham rats and 28 Nx rats were kept in a normobaric hypoxia chamber (12% O₂) for 8 wk. Hypoxia was confirmed by immunohistochemistry for pimonidazole. Hypoxia was confined to the medullary area in sham + normoxia rats and spread to the cortical area in sham + hypoxia rats, without changing the peritubular capillary density. Exposure to hypoxia promoted no renal injury or elevation of the content of IL-1β or Toll-like receptor 4 in sham rats. In Nx, hypoxia also extended to the cortical area without ameliorating the peritubular capillary rarefaction but, unexpectedly, attenuated hypertension, inflammation, innate immunity activation, renal injury, and oxidative stress. The present study, in disagreement with current concepts, shows evidence that hypoxia exerts a renoprotective effect in the Nx model instead of acting as a factor of renal injury. The mechanisms for this unexpected beneficial effect are unclear and may involve NF-κB inhibition, amelioration of oxidative stress, and limitation of angiotensin II production by the renal tissue.

Pathogenic role of innate immunity in a model of chronic NO inhibition associated with salt overload

Nitric oxide inhibition with N^ω-nitro-l-arginine methyl ester (l-NAME), along with salt overload, leads to hypertension, albuminuria, glomerulosclerosis, glomerular ischemia, and interstitial fibrosis, characterizing a chronic kidney disease (CKD) model. Previous findings of this laboratory and elsewhere have suggested that activation of at least two pathways of innate immunity, Toll-like receptor 4 (TLR4)/NF-κB and nucleotide-binding oligomerization domain, leucine-rich repeat, and pyrin domain containing 3 (NLRP3) inflammasome/IL-1β, occurs in several experimental models of CKD and that progression of renal injury can be slowed with inhibition of these pathways. In the present study, we investigated whether activation of innate immunity, through either the TLR4/NF-κB or NLRP3/IL-1β pathway, is involved in the pathogenesis of renal injury in chronic nitric oxide inhibition with the salt-overload model. Adult male Munich-Wistar rats that received l-NAME in drinking water with salt overload (HS + N group) were treated with allopurinol (ALLO) as an NLRP3 inhibitor (HS + N + ALLO group) or pyrrolidine dithiocarbamate (PDTC) as an NF-κB inhibitor (HS + N + PDTC group). After 4 wk, HS + N rats developed hypertension, albuminuria, and renal injury along with renal inflammation, oxidative stress, and activation of both the NLRP3/IL-1β and TLR4/NF-κB pathways. ALLO lowered renal uric acid and inhibited the NLRP3 pathway. These effects were associated with amelioration of hypertension, albuminuria, and interstitial inflammation/fibrosis but not glomerular injury. PDTC inhibited the renal NF-κB system and lowered the number of interstitial cells staining positively for NLRP3. PDTC also reduced renal xanthine oxidase activity and uric acid. Overall, PDTC promoted a more efficient anti-inflammatory and nephroprotective effect than ALLO. The NLRP3/IL-1β and TLR4/NF-κB pathways act in parallel to promote renal injury/inflammation and must be simultaneously inhibited for best nephroprotection.

NLRP3 inflammasome inhibition ameliorates tubulointerstitial injury in the remnant kidney model

Recent studies suggest that NLRP3 inflammasome activation is involved in the pathogenesis of chronic kidney disease (CKD). Allopurinol (ALLO) inhibits xanthine oxidase (XOD) activity, and, consequently, reduces the production of uric acid (UA) and reactive oxygen species (ROS), both of which can activate the NLRP3 pathway. Thus, ALLO can contribute to slow the progression of CKD. We investigated whether inhibition of XOD by ALLO reduces NLRP3 activation and renal injury in the 5/6 renal ablation (Nx) model. Adult male Munich-Wistar rats underwent Nx and were subdivided into the following two groups: Nx, receiving vehicle only, and Nx + ALLO, Nx rats given ALLO, 36 mg/Kg/day in drinking water. Rats undergoing sham operation were studied as controls (C). Sixty days after surgery, Nx rats exhibited marked albuminuria, creatinine retention, and hypertension, as well as glomerulosclerosis, tubular injury, and cortical interstitial expansion/inflammation/fibrosis. Such changes were accompanied by increased XOD activity and UA renal levels, associated with augmented heme oxigenase-1 and reduced superoxide dismutase-2 renal contents. Both the NF-κB and NLRP3 signaling pathways were activated in Nx. ALLO normalized both XOD activity and the parameters of oxidative stress. ALLO also attenuated hypertension and promoted selective tubulointerstitial protection, reducing urinary NGAL and cortical interstitial injury/inflammation. ALLO reduced renal NLRP3 activation, without interfering with the NF-κB pathway. These observations indicate that the tubulointerstitial antiinflammatory and antifibrotic effects of ALLO in the Nx model involve inhibition of the NLRP3 pathway, and reinforce the view that ALLO can contribute to arrest or slow the progression of CKD.