Analysis of the cellular reaction towards microencapsulated xenogeneic islets after intraperitoneal transplantation

Xenotransplantation of encapsulated islets of Langerhans is a possibility to overcome problems of human organ donor shortage in islet transplantation. Preexisting natural xenoantibodies are known to play a major role in the rejection of vascularized xenografts. Only little is known about the mechanism of rejection of non-vascularized cellular xenotransplants. In this study we introduce a method for the characterization of xenograft rejection of encapsulated islets by FACS analysis of peritoneal cells. Pig islets were transplanted intraperitoneally into non-diabetic Lewis rats either encapsulated or non-encapsulated. Animals receiving empty capsules and sham-operated animals served as controls. After 7 days a peritoneal lavage was performed. The total cell number and the viability of the cells were determined. Cells were analysed after staining with a panel of antibodies for the detection of T-lymphocytes, B-lymphocytes, macrophages, MHC class II molecules. Total cell number was highest after microencapsulated transplantation (149.4±30.1X10⁶) compared with empty capsules (41.4±19.7X10⁶) and non-encapsulated porcine islets (18.1±3.3X10⁶). The percentage of CD 3-positive T-lymphocytes rose to 44.5±11.5% in case of microencapsulated xenografts compared with 19.2±8.2% for non-encapsulated xenografts and 4.9±2.4% for empty controls. B-lymphocytes were detected in only small amounts. MHC class II expression on macrophages as activation marker was significantly increased after encapsulated transplantation (60.2±8.9% vs 15.2±7.0% for free islets and 4.9±1.2% for empty controls). The discrepancy between the macrophage activation due to encapsulated xenogeneic islets in comparison to empty capsules made from the same material clearly indicates that the reaction is not only material related but that a recognition of the en- capsulated islet takes place despite the effective inhibition of a direct cell-to-cell contact. This recognition occurs on a T- cell level as well as on the macrophage level. 7 days after transplantation the reaction towards encapsulated xenografts is even more intense than to non-encapsulated xenografts. This might be due either to the time course of the rejection process or to a prolongation of the activation because antigen elimination is hindered by the capsule.