Never make assumptions: the complicated role of complement in urinary tract infections.

Complement activation can cause tissue inflammation and injury, and complement-inhibitory drugs are effective treatments for several inflammatory diseases. The complement cascade is part of the body's defense against bacteria and other pathogens, however, and a major concern regarding inhibition of this system is that it may increase the risk for infection. Now, a study by Choudhry et al. demonstrates that blockade of signaling at one of the C5a receptors (C5a receptor 1 [C5aR1]) reduces renal fibrosis in a mouse model of urinary tract infection with Escherichia coli. Surprisingly, C5aR1 blockade was also associated with faster clearance of the infection. The results of this study demonstrate that C5a-a highly proinflammatory molecule-reduces bacterial killing by macrophages. Other recent studies have also shown that C5a impairs the elimination of tumor cells by the immune system. These data indicate that complement inhibition may have some unexpected benefits. These results also demonstrate, however, that the complement cascade probably has physiologic functions that have yet to be discovered.