Enhanced renal allograft rejection by inhibitors of nitric oxide synthase: a nonimmunologic influence on alloreactivity.

Clinical and experimental studies indicate that nonimmunologic factors may modulate the alloreactivity of a renal transplant. Nitric oxide (NO) is an essential modulator of endothelial function. It was postulated that, in renal allografts, inhibition of constitutive NO synthase may lead to an aggravation of immunologic damage to endothelia and therefore may enhance dysfunction of the graft. Male Lewis (RT1l) rats received syngeneic or allogeneic Brown Norway (RT1n) renal grafts and were treated with cyclosporin A (CyA) or with CyA and an NO synthase blocker (NOS-B): N omega-nitro-L-arginine (L-NNA) or NG-monomethyl-L-arginine (L-NMMA). CyA was given at a dose of 3.5 mg/kg body weight for 14 days and the NOS-B at a dose of 66 mg/L drinking water for up to 28 days postoperatively. Animals (N = 6/group) were studied at 4 to 7, 14, and 28 days posttransplantation. Four to 5 days posttransplantation, renal blood flow and glomerular filtration rate of allogeneic grafts did not differ between animals treated only with CyA and those treated with CyA and NOS-B. Mean arterial pressure was significantly elevated by NOS-B (CyA+L-NNA: 115 +/- 13 versus CyA: 78 +/- 16 mm Hg). Combined NOS-B and CyA administration led to a pronounced increase in vascular and tubulointerstitial damage. The number of mononuclear cells in vessels, glomeruli, and tubulointerstitium increased significantly in allografts upon treatment with NOS-B. During NOS-B administration, adhesion molecules (intracellular adhesion molecule-1; leukocyte-function-associated molecules-1 alpha and-beta) were strongly expressed in endothelial and leukocytic cells of the allograft. A pronounced positivity for mRNA and protein of cytokines tumor necrosis factor-alpha and transforming growth factor-beta could be demonstrated in the inflammatory infiltrate. With L-NNA treatment, the total vascular injury index was 10-fold higher (14 days posttransplantation, CyA+L-NNA: 59.8 +/- 11.7 versus CyA: 6.0 +/- 1.8; p < 0.05). The tubulointerstitial damage score rose more than 2.5-fold after CyA and L-NNA therapy (28 days posttransplantation: CyA+L-NNA: 83 +/- 1 versus CyA:29 +/- 1). L-NNA was more potent than L-NMMA at the dosages used. Thus, pronounced vascular leukostasis, vasculitis, and T-cell and monocyte infiltration of the tubulointerstitium led to a severe damage of the allograft under therapy with CyA and NOS-B. Inhibition of NO synthesis may aggravate alloreactive immunemediated injury in kidney transplants acting primarily by a disturbance of endothelial function.