Complement-mediated vasoconstriction and graft rejection

Renal Damage in Recurrent Atypical Hemolytic Uremic Syndrome Associated with C3 p.Ile1157Thr Gene Mutation

Patients with atypical hemolytic uremic syndrome (aHUS) associated with a C3 p.Ile1157Thr mutation show a relatively high renal survival and low mortality rates, but renal histopathological findings after recurrence have been rarely reported. A 30-year-old man with a C3 p.Ile1157Thr mutation experienced a third recurrence of thrombotic microangiopathies with neurological and gastrointestinal disorders. A renal biopsy performed during the recovery phase of acute kidney injury revealed collapsed glomeruli and arteriolar vacuolization. Approximately 10% of glomeruli were globally sclerotic, despite the absence of arterio-/arteriolo-sclerosis. These findings suggest substantial progression of irreversible injuries in multiple organs, including kidneys, which occurs in aHUS patients with repeated thrombotic microangiopathies.

Evidence for vascular tone regulation by resident or infiltrating leukocytes

The normal vascular wall contains resident leukocytes, notably tissue macrophages (histiocytes) and mast cells, that confer a rapid, eicosanoid-dependent vasoconstrictor response to agonists typical of leukocytes, such as the complement-derived anaphylatoxin C5a or the formylated peptide f-Met-Leu-Phe (isolated organ methodology). The eicosanoid-dependent vasomotor response is even more intense in pathologies that involve leukocyte infiltration of the blood vessel wall, such as atherosclerosis and serum sickness in the rabbit. The leukocyte compartment of the blood vessel is the likely source of vasoactive mediators (eicosanoids, radicals, cytokines) of physiopathological importance, with possible application in cardiac ischemia, lupus nephritis, vasculitides, and graft rejection. This line of investigation may be compared to the discovery and characterization of endothelium-dependent vasomotor responses. However, the problem is experimentally more demanding: histological correlations, experiments based on leukocyte depletion, reconstitution, and enrichment are useful approaches to document this form of circulatory control.

Effects of systemic complement activation on renal circulation of rats

In a variety of immunopathological diseases activation of the complement cascade occurs either systemically or localized in the kidney. To elucidate the functional impact of complement activation upon the renal microcirculation, we administered cobra venom factor of Naja naja kaouthia (CVF) i.v. into thiobarbital anaesthetized female rats. CVF is a potent activator of the alternative pathway of complement by forming the C3-convertase CVF, Bb which cannot be downregulated by the natural inhibitor factors H and I and thereby leads to generation of the anaphylatoxins C3a and C5a and formation of the membrane attack complex (MAC). We utilized creatinine clearance and flowmeter measurements in the normal kidney and intravital microscopy of the split hydronephrotic rat kidney model to observe the microvascular changes. Bolus injection of CVF (100 U kg-1) resulted in an immediate reduction of RBF (-68% after 10 min), which remained decreased during the entire experiment (90 min). Systemic blood pressure was significantly reduced only in the early period (-23% of control: 126 mmHg after 10 min). After an initial anuric phase of 30 min duration, the glomerular filtration rate was significantly diminished by 47%. White cell count was decreased by about 50% after the experiments. Application of the competitive thromboxane A2-antagonist, BM 13505, reversed all renal and systemic CVF-effects. Continuous infusion of the competitive leukotriene D4-antagonist, ICI 198615, attenuated the late renal CVF-effects (i.e. 30 min after injection of CVF). Depletion of polymorphonuclear cells (PMN) attenuated the CVF-effects similar to BM 13505. Intravenous administration of CVF in the hydronephrotic kidney model resulted in a massive constriction of the interlobar and arcuate artery, with a fall in glomerular blood flow comparable to the reduction of RBF in the normal kidney. Diameters of the afferent arterioles--most sensitive to many vasoconstricting agents--were not significantly altered. Our results suggest that injection of CVF and the liberation of high amounts of the anaphylatoxins, C3a and C5a, induces the release of TXA2, which contributes to the early renal effects and the formation of cysteinyl-leukotrienes which play an important role in the late phase of systemic complement activation. Utilizing the split hydronephrotic kidney model we demonstrated the predominant action of complement activation on the large preglomerular vessels for the first time. PMN are seemingly involved in the liberation of secondary mediators which appear to reduce renal blood flow and glomerular filtration.