In vivo correction of complement regulatory protein deficiency with an inhibitor targeting the red blood cell membrane.

Because of the complement system's involvement in many human diseases and potential complications associated with its systemic blockade, site-specific regulation of this effector system is an attractive concept. We report on further developments of such an approach using a single-chain Ab fragment as a vehicle to deliver complement regulatory proteins to a defined cell type. In a model system in which RBCs deficient in complement receptor 1-related gene/protein y (Crry) are rapidly cleared after injection into wild-type animals by a complement-dependent mechanism, we selectively reconstituted these cells with N- and C-terminally targeted recombinant forms of Crry. Transfusion of Crry-coated knockout RBCs into C57BL/6 mice extended their in vivo half-life from <5 min to approximately 2 days. Maintenance of protective levels of Crry (by a combined treatment of donor and recipient RBCs) led to nearly normal RBC survival. Uniform in vitro and in vivo coating of the RBCs and the more efficient complement inhibitory capacity of C-terminally tagged Crry were other interesting features of this experimental system. These results suggest the possibility of using the single-chain Ab fragment-mediated targeting concept of complement regulatory proteins to restrict complement inhibition to the site of its excessive activation.