Box 1 | Preparation and testing of human islet cells for transplantation

The technologies currently used in human islet-cell processing are based on an automated method that was first introduced in 1986 and that rapidly replaced all previously tested procedures^{8, 9}. The concept introduced by this method was to progressively disassemble the pancreas after injection of an enzyme blend through the pancreatic duct. This process allowed gradual digestion of the organ into fragments of decreasing size, until cell clusters of the volume range of islets are released. A constant flow through the digestion chamber allows the released islets to pass through a screen and to be collected in separate compartments, in which further enzymatic action is blocked by cooling and dilution (see figure, part a). A final purification step is carried out by density-gradient separation of the islets, now carried out using a COBE2991 cell processor (see figure, part b).

Several qualitative and quantitative tests are used to verify the quality of the final human islet-cell product. Pre-transplant criteria that must be met include determination of the total islet-cell number (>5,000 islet equivalents/kg recipient body weight), total pellet volume of the final preparation (<7 ml of tissue) and islet-cell purity (>30% islets). Product release criteria also include negativity of a Gram stain (to detect the presence of contaminating bacteria) and >70% viability as assessed by fluorescent inclusion/exclusion dyes. However, the best predictive test of post-transplant functional competence is the reversal of diabetes after transplantation of an aliquot of islets into an immunodeficient (athymic) mouse¹⁵. At present, research is underway to develop alternative technologies for the assessment of final islet-cell preparations, including assessment of apoptosis, ATP and oxygen consumption, and mitochondrial membrane potentials, towards definition of improved prospective product release tests for the prediction of post-transplant islet-cell function. Images are redrawn with permission from Landes Bioscience from Ref. 80.