Specific depletion of preformed IgM natural antibodies by administration of anti-mu monoclonal antibody suppresses hyperacute rejection of pig to baboon renal xenografts

Background, The elimination of circulating antiporcine preformed antibodies is crucial for avoiding hyperacute vascular rejection (HAVR) of primarily vascularized xenograft in discordant pig to baboon model. Previously descri bed methods used for eliminating natural antibodies, however, constantly re moved both anti-porcine IgM and IgG antibodies, as well as often complement proteins. To study specifically the role of preformed anti-porcine IgM ant ibodies, a specific anti-IgM monoclonal antibody (mAb) has been designed an d evaluated in vivo. Methods. Iterative injections of anti-IgM mAb (LOBM2) at high dose (20 mglk g) depleted to undetectable level the circulating IgM and therefore anti-po rcine IgM antibodies but did not change the concentration of anti-pig IgG a ntibodies. The serum concentration of IgM and IgG antibodies was assessed b y ELISA and the level of anti-pig natural IgM and IgG antibodies by flow cy tometry (FC). Anti-rat sensitization was assessed by specific ELISA as well as the serum concentration of LO-BM2. Results, Iterative injections of LO-BM2 allowed to specifically eliminate t he anti-porcine IgM antibodies to undetectable levels at ELISA. Despite a n ormal serum level of anti-porcine IgG and complement proteins, HAVR was avo ided. Without immunosuppression, the specific elimination of preformed anti porcine IgM prolonged the survival of a renal xenograft in baboon up to g d ays, whereas without IgM antibody elimination, the renal xenografts were hy peracutely rejected within hours. The lost of activity of LO-BM2 after 10 d ays was concomitant to an IgM and IgG antibody rebound, which caused an acu te vascular rejection of the xenograft. Conclusion, Specific elimination of natural anti-porcine IgM antibodies all ows to avoid HAVR of a pig to baboon renal xenograft, whereas anti-porcine IgG antibodies and complement proteins were present in the serum, This resu lt confirms previous in vitro reports and demonstrates for the first time i n vivo that preformed IgM antibodies alone are responsible for HAVR, while preformed anti-porcine IgG antibodies are unable alone to cause HAVR, Anti- IgM therapy appears as an important tool to transiently but completely elim inates xeno-IgM antibodies in vivo.