Lectins for electromagnetic purification of islets from humans and large mammals.

One prerequisite for unimpaired islet graft function is the separation from the presumably cytotoxic and highly immunogenic exocrine tissue (Ulricks and MuUer-Ruchholtz: cf. page 123 in this book; Gray 1989). Three different principles of islet purification are used: (A) "Hand-picking" is a method eSUlblished for purification of islets from the rodent pancreas (Finke, Lacy and Ono 1979). But for the high number of islets needed for human islet transplantation this method is too time-consuming and far too laborious. (B) Density gradient centrifugation is dependent on the different densities of exocrine and endocrine tissue. Large scale purification by this method presently achieves purity degrees of approx. 40%-70% (Lake, Bassett, Larkins, Revell, Walczak. Chamberlain. Rumford. London. Veitch. Bell and lames 1989; Ricordi. Lacy and Scharp 1989; Warnock, Ellis. Cattral. Untch. Kneteman and Rajotte 1989). This is partly explained by the difficulties to control the collagenase digestion process. (C) The principle of electromagnetic cell separation (EMS; Kandzia. Scholz. Anderson and Muller-Ruchholtz 1984; Muller-Ruchholtz. Kandzia and Leyhausen 1987) was further developed for large scale islet purification (Muller-Ruchholtz. Leyhausen. Petersen. Schubert and Ulricks 1987). Magnetic microspheres (MMS) were coupled to lectins with binding specificity for exocrine tissue. UEA-I was found to be a highly selective lectin for rat (Muller-Ruchholtz et al. 1987) and WFA for human (Winoto-Morbach. Ulricks. Leyhausen and Muller-Ruchholtz 1989; Winoto-Morbach. Leyhausen. Schunke. Ulrichs and Muller-Ruchholtz 1989) exocrine tissue. Lectincoupled MMS can trap the latter in an electromagnetic field, thus providing effluent islets of a high degree of purity. Functional integrity of EMS-separated rat islets was tested in vivo (Muller-Ruchholtz et al. 1987) and in vitro (Winoto-Morbach et al. 1989). Because of the difficulties to obtain sufficient human material we propose other species (cattle, pig and dog) to perform extensive preclinical studies and to collect data in view of the possibility of future islet xenotransplantation (Reemtsma 1989). This has the advantage of unlimited access to material for large scale islet separation. But for each new species used as islet donor, a particular lectin has to be found to make this approach effective for either experimental or clinical purposes. Therefore our interest was focussed on lectins with adequate specificity for pancreatic exocrine tissue of cattle, pig and dog. The characteristics of suitable lectins are the following: (a) high degree of binding specificity for the adequate mammalian tissue, (b) non toxicity to endocrine cells, and (c) maintenance of binding specificity for exocrine cells after coupling to microspheres.