Is the renal subcapsular space the preferred site for clinical porcine islet xenotransplantation? Review article.

It can reasonably be anticipated that, within 5-10 years, islet allotransplantation or pig islet xenotransplantation may be the preferred options for β-cell replacement therapy. The portal vein/liver is currently the preferred clinical site for free islet transplantation, constituting 90% of clinical islet transplants. Despite being the site of choice for rodent and some large animal studies, the renal subcapsular space is rarely used clinically, even though the introduction of islets intraportally is not entirely satisfactory (particularly for pig islet xenotransplantation). We questioned why this might be so. Is it perhaps based on prior clinical evidence, or from experience in nonhuman primates? When we have questioned experts in the field, no definitive answers have been forthcoming. We have therefore reviewed the relevant literature, and still cannot find a convincing reason why the renal subcapsular space has been so relatively abandoned as a site for clinical islet transplantation. Owing to its sequestered environment, subcapsular transplantation might avoid some of the remaining challenges of intraportal transplantation. This may be particularly true when using pig islets for xenotransplantation, which are exceptionally pure in comparison to human islets used in auto- or allo-transplantation. With evidence from the literature, we question the notion that the subcapsular space is inhospitable to islet transplantation and suggest that, when porcine islet transplantation is introduced, this site should perhaps be reconsidered.