PROJECT SUMMARY/ABSTRACT Therapeutic strategies to halt antibody-mediated rejection (AMR) following lung transplantation are lacking, due in large part to the complex pathophysiology of the immune process and the multi-faceted approach that must be taken to establish a defined diagnosis. It is becoming increasingly clear that AMR has many endotypes, with donor-specific antibodies mediating a host of complement dependent and independent functions. Thus, the development of strategies to stratify patients based upon endotype has the potential to revolutionize the treatment of AMR. Pre-clinically, fluorescence imaging has been used to investigate a wide array of diseases and disorders. In transplantation, the focus has primarily been T cell-mediated rejection, detecting leukocyte trafficking to the graft via phagocyte-specific approaches or fluorogenic probes for inflammatory signatures, such as cathepsin B. In our own previous work, we developed a fluorogenic probe specific to granzyme B and reported its utility in monitoring therapeutic intervention in a murine model of myocarditis, a surrogate for cardiac transplant rejection. As such, there is a great unmet need to develop agents capable of non-invasively diagnosing or monitoring AMR. To this end, we have developed novel fluorogenic probes adept at detecting complement activation in real time, via the examination of complement convertase activity, as opposed to deposition products such as C4d, which persist for weeks at the site of deposition and can only be confirmed by invasive biopsy procedures. Thus, the goals of this proposal are to develop non- or minimally invasive techniques to image ongoing complement activation in-vivo. To accomplish this, we will use murine models of transplant rejection in concert with an assortment of complement inhibitors that act at different points in the complement activation pathway to characterize the extent to which these probes can determine complement activation. We will concom