IN BABOON, A COMPLETE ELIMINATION OF THE CIRCULATING IgM BY ANTI-?? MONOCLONAL ANTIBODY ALLOWS A PIG KIDNEY XENOGRAFT TO SURVIVE UP TO SIX DAYS WITHOUT ADDITIONAL IMMUNOSUPPRESSION

Abstract

106 Background: In the pig to baboon xenograft model, the performed IgM xenoantibodies are presumed to be one of the most important factor involved in hyperacute vascular rejection (HAVR). Methods: We therefore developed a rat monoclonal antibody which recognizes human as well as baboon IgM (LO-BM2) in order to deplete "in vivo" the total seric IgM. As assessed by ELISA, two to four consecutive iv injections of LO-BM2 at 15-20 mg/kg totally eliminated the baboon circulating IgM. Results: As control, we transplanted a pig renal xenograft to three baboons without any immunosuppressive therapy and a fourth baboon underwent a splenectomy three days prior to the renal xenograft. All the four animals hyperacutely rejected their xenograft between the first hour after unclamping and the first postoperative day (POD). At histology, signs of HAVR were evidenced. An experimental group of five baboons was then designed to assess the effect of a selective depletion of circulating IgM on the renal xenograft survival. During the preoperative LO-BM2 treatment, the total concentration of seric IgM was assessed daily by ELISA in order to perform the renal xenograft in absence of circulating anti-pig IgM. The first baboon was splenectomized on day -3 and received four injections of LO-BM2 (20 mg/kg/day) prior to grafting. During the following 4 POD, the xenokidney functioned well (creatinine 0.8; 1.0; 1.2 and 2.0mg/dl) and the renal function eventually acutely deteriorated on POD 6 (creatinine 6.4 mg/kg). At this time, the kidney was completely rejected, cyanotic and swollen and the pathology mainly showed signs of vascular rejection. The second animal also underwent a splenectomy on day -3 and a depletion of the circulating IgM by LO-BM2. During three days, the creatinine remained normal and the kidney was acutely rejected on POD4. A third animal was similarly treated by three LO-BM2 injections but without splenectomy. This baboon maintained a normal renal function during 3 days (0.9; 0.8; 0.9mg/kg) and eventually acutely rejected the xenograft on POD4. In these three animals, the circulating seric IgM were undetectable by ELISA up to the rejection time. By Flow Cytometry on porcine aortic endothelial cells specific anti-pig xenoantibodies were detected concomitantly or just after the renal function deterioration. Two additional animals were similarly treated by LO-BM2 prior to transplant but in addition received, in the first case, five injections of DSG at 9mg/kg (from day-2) and, in the second case, 6 days of RS at 250 mg/kg (from day 0). After elimination of the circulating IgM, both animals maintained a normal renal function for 3 and 4 POD but eventually rejected acutely the graft on POD4 and 6, respectively. The lack of prolonged effect of LO-BM2 over five or six days is related to a baboon anti-rat sensitization. Conclusions: These experiments show for the first time in a pig to baboon model that a pig kidney xenograft can survive up to six days without immunosuppression besides the complete elimination of the circulating IgM. The rejection occurred simultaneously to the reappearance of a low level of circulating IgM and was of the vascular type. These results also suggest that neither the alternate pathway of the complement nor the preformed IgG are able in this model to produce alone a HAVR.