Functional assessment of mouse complement pathway activities and quantification of C3b/C3c/iC3b in an experimental model of mouse renal ischaemia/reperfusion injury

The complement system is an essential component of our innate immunity, both for the protection against infections and for proper handling of dying cells. However, the complement system can also contribute to tissue injury and inflammatory responses. In view of novel therapeutic possibilities, there is an increasing interest in measurement of the complement system activation in the systemic compartment, both in the clinical setting as well as in experimental models. Here we describe in parallel a sensitive and specific sandwich ELISA detecting mouse C3 activation fragments C3b/C3c/iC3b, as well as functional complement ELISAs detecting specific activities of the three complement pathways at the level of C3 and at the level of C9 activation. In a murine model of renal ischaemia/reperfusion injury (IRI) we found transient complement activation as shown by generation of C3b/C3c/iC3b fragments at 24 h following reperfusion, which returned to base-line at 3 and 7 days post reperfusion. When the pathway specific complement activities were measured at the level of C3 activation, we found no significant reduction in any of the pathways. However, the functional complement activity of all three pathways was significantly reduced when measured at the level of C9, with the strongest reduction being observed in the alternative pathway. For all three pathways there was a strong correlation between the amount of C3 fragments and the reduction in functional complement activity. Moreover, at 24 h both C3 fragments and the functional complement activities showed a correlation with the rise in serum creatinine. Together our results show that determination of the systemic pathway specific complement activity is feasible in experimental mouse models and that they are useful in understanding complement activation and inhibition in vivo.