DISCORDANT ISLET XENOTRANSPLANTATION; LONG-TERM SURVIVAL FOLLOWING SHORT-TERM TREATMENT WITH ANTI-CD45RB AND CYCLOPHOSPHAMIDE

Abstract

125 We have recently reported that a short treatment course of anti-CD45RB mAb (MB23G2) is able to induce tolerance to islet allografts in mice. (Auersvald et al, Transplantation 63: 1355 ). Furthermore, when combined with cyclophosphamide (CTX), anti-CD45RB was able to prolong survival of concordant islet xenografts. In this study, we examine the immunosuppressive effect of anti-CD45RB mAb and CTX in a discordant model of porcine islet transplantation into mice. C57BL/6 (H-2b) streptozotocin-induced diabetic mice were used as recipients. Adult porcine islets (3500 islet equivalents) were transplanted under the left kidney capsule. Recipient mice were either left untreated (n=2) or received a combination of anti-CD45RB mAb (100 mg IV on days -1 to 10, 20, 21) plus CTX (20 mg/kg IV on days -1, 1, 0, 3, 5, 19, 20) (n=11). Results: Untreated mice show rapid onset of hyperglycemia (Median day 4). Treated mice had a median graft survival of 37 days. Significantly, in 4/11 mice (36%), indefinite graft survival (>120 days) was achieved. After nephrectomy, long-term survivors reverted to hyperglycemia. Analysis of splenocytes on day 6 post-transplant, reveals that CTX causes a decrease in B cells and a relative increase in CD4 and CD8 cells. Anti-CD45RB induced a marked decrease in the intensity of CD45RB expression after transplantation. This suggests that this mAb induces a shift from CD45RBHi to CD45RBLo isoforms in xenograft recipients, as previously demonstrated in untransplanted animals. Histologic examination of long-term surviving grafts shows very well preserved islet tissue surrounded by a lymphocytic infiltrate within the renal capsule and in the renal parenchyma immediately underlying the xenograft. The infiltrating lymphocytes stained strongly positive for IL-4 and to a much lesser extent, IL-2, by immunohistochemistry. Within in the islets themselves, there were only rare lymphocytes, staining positive for IL-4, but not IL-2. In conclusion, this study shows that a short course of anti-CD45RB combined with CTX can induce significant prolongation of discordant islet xenografts with indefinite survival in over one third of the recipients.