XENOTRANSPLANTATION II

K. Yamada, H. Nishimura, J.R. Scalea, A. Griesemer, S. Moran, B. Gillon, S. Robson, J. Fishman, M. Sykes, A. Shimizu, D.H. Sachs
Transplantation Biology Research Center, Harvard Medical School/MGH, Boston/MA/UNITED STATES OF AMERICA

Body: Introduction and Objective: We have recently reported greater than 80 days maximum, and greater than 50 days mean baboon survival with steroid free regimens in a GalT-KO pig to baboon thymokidney (TK) model. T cell depletion (TCD) in the induction period consisted of two doses of rabbit ATG (polyclonal Ab) followed by one or two doses of anti-CD2 mAb (LoCD2) that achieved peripheral blood T cell levels <200/ul in the first two weeks (r-ATG based TCD regimen). In this study, we tested the effect of monoclonal (mAb) based TCD regimens, anti-human CD3 recombinant immunotoxin (CD3 r-IT) based or LoCD2 based; on TCD and graft survival in this life-supporting TK model. Methods: Four baboons received composite TK grafts from GalT-KO miniature swine with monoclonal Ab based-TCD regimens. Two baboons received CD3 r-IT (50ng/kg/dose x 8 doses over 4 days) and single dose of LoCD2 and two received 3 doses of LoCD2 (4, 4 and2mg/kg/day) and a single dose of horse ATG. All other treatments were identical to the r-ATG based regimen. All baboons had similar levels of preformed nonGal NAb (<30% cytotoxicity) to those in baboons treated with the r-ATG based TCD regimen. Renal xenograft function was assessed by plasma creatinine (cre) levels and histology. TCD was followed by FACS analysis. Result: (1) Life-supporting TK graft function: In contrast to baboons treated with ATG, all 4 baboons had unstable renal function. Baboons with LoCD2 based-TCD regimen had stable renal function for the first 9 days, however, creatinine increased sharply thereafter. Both TK grafts were completely rejected at day 14 with cellular infiltrates and interstitial hemorrhage on histology, indicating cellular and humoral xenograft rejection. Baboons with CD3 r-IT had better renal function than LOCD2 treated baboons; however, both died from pneumonia at days 15 and 23, respectively. (2) Development of non-Gal NAb: Baboons with LoCD2 based-TCD developed induced nonGal cytotoxic IgG following TK Tx. Baboons with CD3 r-IT did not develop cytotoxic IgG in serum, although focal IgG deposition was seen in some glomeruli. (3) TCD levels in peripheral blood:All five baboons with r-ATG had stable renal function in the first month and <200 T cells /ul in the first 10 days. One of 5 had prolonged TCD (<50/ul) and developed baboon CMV. In contrast, baboons with LoCD2 rapidly restored T cells by day 7 (>300/ul) which correlated with elevation of cre. Baboons with CD3 r-IT had a slow increase in T cells (150-250 at day 7) but slightly higher numbers than those that recieved r-ATG. Conclusions: Adequate TCD during the induction period (< 2weeks) appears essential for success using the thymo-kidney strategy. These data suggest that the optimal level of TCD, along with other maintenance therapy, is between 50 and 150 T cells/ul during this period. A newly developed CD3 r-IT could be a potentially useful TCD agent, but doses and duration must be further optimized to avoid unacceptable side effects.

Disclosure: All authors have declared no conflicts of interest.