Distinct roles for C3a and C5a in complement-induced tubulointerstitial injury

To prevent injury to host tissues, complement activation is regulated by a number of plasma and membrane-associated proteins, most of which limit C3 and C5 activation. An influx of circulating C3 from a syngeneic host into donor kidneys deficient in Crry (a membrane protein that reduces C3 convertase activity) causes spontaneous complement activation, primarily in the tubulointerstitum, leading to renal failure. To determine the roles of the C3a and C5a anaphylatoxins in tubulointerstitial inflammation and fibrosis, kidneys from Crry −/− C3 −/− mice were transplanted into hosts lacking the C3a and/or C5a receptor. While unrestricted complement activation in the tubules was not affected by receptor status in the transplant recipient, C3a receptor deficiency in the recipients led to significantly reduced renal leukocyte infiltration and the extent of tubulointerstitial inflammation and fibrosis, all of which led to preserved renal function. The absence of C5a receptors in recipients was not only inconsequential, but the protective effect of C3a receptor deficiency was also eliminated, suggesting distinct roles of C3a and C5a receptor signaling in this model. There was significant infiltration of the tubulointerstitum with 7/4 + F4/80 + CD11b + myelomonocytic cells and Thy1.2 + T cells along injured tubules, and interstitial collagen I and III deposition, all of which were C3a receptor dependent. Thus, blockade of C3a receptor signaling is a possible treatment to reduce renal inflammation and preserve renal function associated with complement activation.