Complement activation in the kidney after ischemia/reperfusion (I/R) seems to occur primarily via the alternative complement pathway. The ability of an inhibitory mAb to mouse factor B, a necessary component of the alternative pathway, to protect mice from ischemic acute renal failure was tested. Treatment with the mAb prevented the deposition of C3b on the tubular epithelium and the generation of systemic C3a after renal I/R. Treated mice had significantly lower increases in serum urea nitrogen and developed significantly less morphologic injury of the kidney after I/R. For gaining insight into potential mechanisms of protection, the activity of caspases within the kidney also was measured, and it was found that caspases-2, -3, and -9 increased in a complement-dependent manner after renal I/R. Apoptotic cells were detected by terminal deoxynucleotidyl transferase catalyzed labeling of DNA fragments, and mice in which the alternative pathway was inhibited demonstrated significantly less apoptosis than control mice. Thus, use of an inhibitory mAb to mouse factor B effectively prevented activation of complement in the kidney after I/R and protected the mice from necrotic and apoptotic injury of the tubules. T he alternative pathway of complement is activated after ischemia/reperfusion (I/R) of the kidneys in rodents and humans (1,2), and complement deficiency protects mice from the full development of ischemic acute renal failure (ARF) (2,3). Complement activation by the classical and lectin pathways has been demonstrated after I/R of the heart (4), intestine (5,6), and skeletal muscle (7). Mannose-binding lectin is deposited in the postischemic kidney (8), perhaps indicating activation of this pathway. Complement activation after I/R of the kidney is critically dependent on an intact alternative pathway (2,3), however, and does not require the protein C4 (3). Selective inhibition of the alternative pathway, therefore, should confer protection against the development of ischemic ARF while leaving the classical and lectin pathways intact.Renal I/R results in both epithelial cell necrosis and apoptosis (9). Although necrosis is often the more prominent morphologic finding, inhibition of apoptosis has been shown to protect mice from ischemic ARF (10). The caspases are a group of cysteine proteases that execute many of the processes that are necessary for cellular apoptosis (11). Studies have demonstrated that complement activation directly activates caspases within renal cells (12) and may also induce caspase activity within other tissues after I/R (13). Furthermore, the use of an anti-C5 antibody (which prevents formation of C5a and the membrane attack complex) has been shown to prevent the development of apoptosis after renal (14) and cardiac (13) I/R. Thus, there is reason to suspect that complement activation in the kidney after I/R mediates the development of apoptosis through the caspases and that this is an important mechanism of complement-mediated injury to the kidney.We recently developed an inhibitory mAb ...

The complement system is one of the major ways by which the body detects injury to self cells, and the alternative pathway of complement is rapidly activated within the tubulointerstitium after renal ischemia/reperfusion (I/R). In the current study, we investigate the hypothesis that recognition of tubular injury by the complement system is a major mechanism by which the systemic inflammatory response is initiated. Gene array analysis of mouse kidney following I/R initially identified MIP-2 (CXCL2) and keratinocyte-derived chemokine (KC or CXCL1) as factors that are produced in a complement-dependent fashion. Using in situ hybridization, we next demonstrated that these factors are expressed in tubular epithelial cells of postischemic kidneys. Mouse proximal tubular epithelial cells (PTECs) in culture were then exposed to an intact alternative pathway and were found to rapidly produce both chemokines. Selective antagonism of the C3a receptor significantly attenuated production of MIP-2 and KC by PTECs, whereas C5a receptor antagonism and prevention of membrane attack complex (MAC) formation did not have a significant effect. Treatment of PTECs with an NF-κB inhibitor also prevented full expression of these factors in response to an intact alternative pathway. In summary, alternative pathway activation after renal I/R induces production of MIP-2 and KC by PTECs. This innate immune system thereby recognizes hypoxic injury and triggers a systemic inflammatory response through the generation of C3a and subsequent activation of the NF-κB system.

The complement system effectively identifies and clears invasive pathogens as well as injured host cells. Uncontrolled complement activation can also contribute to tissue injury, however, and inhibition of this system may ameliorate many types of inflammatory injury. Several studies have demonstrated that the filtration of complement proteins into the renal tubules, as occurs during proteinuric renal disease, causes tubular inflammation and injury. In the present study, we tested the hypothesis that activation of the complement system in the urinary space requires an intact alternative pathway. Using a model of adriamycin-induced renal injury, which induces injury resembling focal segmental glomerulosclerosis, we examined whether mice deficient in factor B would be protected from the development of progressive tubulointerstitial injury. Complement activation was attenuated in the glomeruli and tubulointerstitium of mice with congenital deficiency of factor B (fB-/-) compared with wild-type controls, demonstrating that complement activation does occur through the alternative pathway. Deficiency in factor B did not significantly protect the mice from tubulointerstitial injury. However, treatment of wild-type mice with an inhibitory monoclonal antibody to factor B did delay the development of renal failure. These results demonstrate that complement activation in this nonimmune complex-mediated model of progressive renal disease requires an intact alternative pathway.

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