Biphasic increase in circulating and renal TNF-α in MRL-lpr mice with differing regulatory mechanisms

Abstract

Tumor necrosis factor (TNF)-α contributes to expansion of lymphocytes in neonatal mice and can accelerate renal injury. T cells induced by the lpr gene promote renal injury. However, the lpr gene alone is insufficient to cause renal damage, since MRL-lpr, but not C3H-lpr mice develop lupus nephritis. In this study, we examined the temporal expression of TNF-α in the kidney and circulation of mice (MRL and C3H) with the lpr gene and their congenic counterparts (+ +). We measured a bioactive TNF-α using L929 cells and tissue expression with an avidin-biotin immunoperoxidase method. A biphasic increase in circulating TNF-α in MRL-lpr mice was detected. There was an initial peak in neonatal mice (703 ± 208 pg/ml) which normalized by two months of age (87 ± 13 pg/ml) and reascended proportional to the severity of renal injury (non-proteinuric 570 ± 87, proteinuric; 1255 ± 135 pg/ml). In addition, there was only a single peak in neonatal C3H-lpr mice (1270 ± 318 pg/ml) with a nadir by six weeks of age (434 ± 52 pg/ml). In contrast, serum TNF-α was low in MRL-++ and C3H-++ mice (80 ± 3 and 95 ± 30 pg/ml), respectively. TNF-α expression in kidneys paralleled the serum pattern in MRL-lpr mice. Enhanced TNF-α expression was restricted to tubular epithelial cells (TEC) in neonatal MRL-lpr and C3H-lpr mice, and not detected in congenics. In adult mice, intrarenal TNF-α expression was more ubiquitous and was detected in glomeruli, vascular smooth muscle and perivascular infiltrating cells as well as TFC. In addition, TNF-α expression intensified in the kidneys in proportion to the severity of proteinuria. TNF-α was absent in age matched C3H-lpr, C3H-++ and MRL-++ mice. Additional studies indicated that: (1) neither MRL-lpr or C3H-++ TEC constitutively secreted substantial amounts of TNF-α and required a cytokine stimulation; and (2) the clearance of TNF-α via TNF-α receptors was similar in MRL-lpr, MRL-++ and C3H-++ mice, suggesting the increase of serum TNF-α was not a result of a defect in clearance. Thus, these results indicate two distinct mechanisms of TNF-α regulation in MRL-lpr mice: (1) neonatal up-regulation related to the lpr gene; and (2) an increase in mature mice proportional to the severity of lupus nephritis.