Expression of complement regulatory proteins on islets of langerhans: A comparison between human islets and islets isolated from normal and hDAF transgenic pigs'

Background. The expression of regulators of complement activity (RCAs) on i slet cells may be of great importance for protecting them against complemen t-mediated lysis in the immediate posttransplant period after intraportal i slet transplantation. We examined porcine and human islet cells for express ion of RCA. We also examined to what extent human decay accelerating factor (hDAF) is expressed on adult and fetal islet cells isolated from hDAF tran sgenic (TG) pigs having a high transgene expression on endothelial cells. M oreover, the susceptibility of the various types of cells to lysis in human serum and blood was investigated. Methods. Adult human islets (n = 5), normal adult and fetal porcine islets (n = 9 and n = 8, respectively), and islets from adult and fetal hDAF TG pi gs (n = 5 and n = 6, respectively) were examined. With islet single-cell su spensions and flow cytometry, adult human islet cells were examined for exp ression of hDAF (CD55), hCD59, and human membrane cofactor protein (hMCP; C D46), while porcine islet cells were examined for expression of pCD59 and p MCP. Islet cells from hDAF TG pigs were also examined for hDAF expression. Porcine peripheral blood lymphocytes, normal and hDAF TG porcine endothelia l cell lines, a human endothelial cell line, and the human cell line, U937 served as controls. Islet cytotoxicity was assayed after incubation of the islet cells with fresh human serum. Furthermore, adult islets from normal c ontrol pigs and hDAF TG pigs were exposed to fresh human blood in vitro for 60 min, and the inflammatory reaction elicited was compared between the di fferent types of islets. Results. All human islet cell preparations expressed hCD59, two of five exp ressed hMCP, but none expressed hDAF. Porcine islet cells expressed both pC D59 and pMCP. Normal adult porcine islet cells exposed to fresh human serum resulted in 74 +/- 5.4% cell lysis (mean +/- SEM, n = 16). In comparison, only 1.3 +/- 2.8% (n = 20, P < 0.001) of human islet cells were lysed in th e human serum. One islet cell preparation from an hDAF TG pig expressed sma ll amounts of hDAF. This preparation from hDAF TG pigs bound significantly less C3c than did normal control islets (mean fluorescence ratio 16 +/- 2.2 and 58 +/- 4.3, respectively; P = 0.046) and were partially protected from cell lysis in fresh human serum (47 +/- 10% and 78 +/- 18% cell lysis, res pectively; P = 0.046). The other four preparations from hDAF TG pigs were n egative for hDAF and were equally susceptible to lysis as normal control is lets. All fetal pancreatic islet cells from hDAF TG pigs analyzed were nega tive for hDAF expression. When exposed to fresh human blood in vitro, adult and fetal islets from hDAF TG pigs elicited equally strong inflammatory ch anges as did the normal control islets. The inflammatory changes were chara cterized by activation of the complement and coagulation systems, resulting in islet damage with "dumping" of insulin into the blood. Conclusions. Porcine and human islet cells express species-restricted compl ement regulatory proteins, with the human islet cells expressing mainly hCD 59. A low expression of hDAF was detected on islet cells from one of five h DAF TG pigs. These islet cells displayed reduced islet cell cytotoxicity in fresh human serum. We conclude that protection from complement-mediated ly sis will be important in the context of intraportal pig-to-human islet tran splantation, and expression of a human RCA on islet cells should be benefic ial in this context.