Complement (C) inhibition alone using a recombinant soluble form of complement receptor type 1 (sCR1) prevents hyperacute rejection but not subsequent irreversible accelerated acute rejection of discordant pig-to- cynomolgus monkey cardiac xenografts, which occurs within 1 week. To inhibit accelerated acute rejection, which is associated with a rise in serum xenoreactive antibody (Ab) and a cellular infiltrate, triple therapy with standard immunosuppressive agents (cyclosporine, cyclophosphamide, and steroids [CCS]) was combined with continuous C inhibition using sCR1. Each of two monkeys that received sCR1 + CCS showed minimal evidence of rejection when killed on days 21 and 32 in comparison to a monkey that received sCRI + subtherapeutic CCS (rejected at 11 days) and a control that received CCS alone (rejected at 38 min). Prolonged xenograft survival was associated with low Ab levels and a minimal cellular infiltrate, suggesting that combined inhibition of C, xenoreactive Ab responses, and cellular immunity may be a useful approach in overcoming the immune barriers to discordant xenotransplantation.
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