Binding of factor H to tubular epithelial cells limits interstitial complement activation in ischemic injury.

Factor H is a regulator of the alternative pathway of complement, and genetic studies have shown that patients with mutations in factor H are at increased risk for several types of renal disease. Pathogenic activation of the alternative pathway in acquired diseases, such as ischemic acute kidney injury, suggests that native factor H has a limited capacity to control the alternative pathway in the kidney. Here we found that an absolute deficiency of factor H produced by gene deletion prevented complement activation on tubulointerstitial cells after ischemia/reperfusion (I/R) injury, likely because alternative pathway proteins were consumed in the fluid phase. In contrast, when fluid-phase regulation by factor H was maintained while the interaction of factor H with cell surfaces was blocked by a recombinant inhibitor protein, complement activation after renal I/R increased. Finally, a recombinant form of factor H, specifically targeted to sites of C3 deposition, reduced complement activation in the tubulointerstitium after ischemic injury. Thus, although factor H does not fully prevent activation of the alternative pathway of complement on ischemic tubules, its interaction with the tubule epithelial cell surface is critical for limiting complement activation and attenuating renal injury after ischemia.

The complement inhibitors Crry and factor H are critical for preventing autologous complement activation on renal tubular epithelial cells.

Congenital and acquired deficiencies of complement regulatory proteins are associated with pathologic complement activation in several renal diseases. To elucidate the mechanisms by which renal tubular epithelial cells (TECs) control the complement system, we examined the expression of complement regulatory proteins by the cells. We found that Crry is the only membrane-bound complement regulator expressed by murine TECs, and its expression is concentrated on the basolateral surface. Consistent with the polarized localization of Crry, less complement activation was observed when the basolateral surface of TECs was exposed to serum than when the apical surface was exposed. Furthermore, greater complement activation occurred when the basolateral surface of TECs from Crry(-/-)fB(-/-) mice was exposed to normal serum compared with TECs from wild-type mice. Complement activation on the apical and basolateral surfaces was also greater when factor H, an alternative pathway regulatory protein found in serum, was blocked from interacting with the cells. Finally, we injected Crry(-/-)fB(-/-) and Crry(+/+)fB(-/-) mice with purified factor B (an essential protein of the alternative pathway). Spontaneous complement activation was seen on the tubules of Crry(-/-)fB(-/-) mice after injection with factor B, and the mice developed acute tubular injury. These studies indicate that factor H and Crry regulate complement activation on the basolateral surface of TECs and that factor H regulates complement activation on the apical surface. However, congenital deficiency of Crry or reduced expression of the protein on the basolateral surface of injured cells permits spontaneous complement activation and tubular injury.