original articleT h e ne w e ngl a nd jou r na l o f m e dic i ne n engl j med 355;13 www.nejm

The cytoarchitecture of human islets has been examined, focusing on cellular associations that provide the anatomical framework for paracrine interactions. By using confocal microscopy and multiple immunofluorescence, we found that, contrary to descriptions of prototypical islets in textbooks and in the literature, human islets did not show anatomical subdivisions. Insulin-immunoreactive ␤ cells, glucagon-immunoreactive ␣ cells, and somatostatin-containing ␦ cells were found scattered throughout the human islet. Human ␤ cells were not clustered, and most (71%) showed associations with other endocrine cells, suggesting unique paracrine interactions in human islets. Human islets contained proportionally fewer ␤ cells and more ␣ cells than did mouse islets. In human islets, most ␤, ␣, and ␦ cells were aligned along blood vessels with no particular order or arrangement, indicating that islet microcirculation likely does not determine the order of paracrine interactions. We further investigated whether the unique human islet cytoarchitecture had functional implications. Applying imaging of cytoplasmic free Ca 2؉ concentration, [Ca 2؉ ]i, we found that ␤ cell oscillatory activity was not coordinated throughout the human islet as it was in mouse islets. Furthermore, human islets responded with an increase in [Ca 2؉ ]i when lowering the glucose concentration to 1 mM, which can be attributed to the large contribution of ␣ cells to the islet composition. We conclude that the unique cellular arrangement of human islets has functional implications for islet cell function.␣ cell ͉ ␤ cell ͉ cytoplasmic free Ca 2ϩ concentration ͉ insulin ͉ glucagon I n the last three decades, hundreds of individuals with type 1 diabetes mellitus have received allogeneic transplants of endocrine pancreas, the islets of Langerhans, to cure their chronic condition. In these patients, diabetes is reversed by transplanting cells capable of physiologically regulating insulin secretion. Determining the quality of islets obtained from cadaveric pancreata should be indispensable in this context. However, it is not known which physiological parameters correlate best with a fully functional islet capable of reversing diabetes after transplantation. There is a wealth of information about the physiology of rodent islets, but the biology of human islets remains poorly understood. As assays for determining islet quality are being developed by many laboratories in the field of islet transplantation, a reassessment of the structure and function of human islets is warranted.The islets of Langerhans are small organs located in the pancreas that are crucial for glucose homeostasis. Islets typically consist of four types of secretory endocrine cells, namely, the insulin-containing ␤ cells, the glucagon-containing ␣ cells, the somatostatin-containing ␦ cells, and the pancreatic polypeptideproducing (PP) cells. In rodent islets, the vastly predominating ␤ cells are clustered in the core of a generally round islet, surrounded by a mantle of ␣, ␦, and PP cells. Thus, ...

Type 1, or insulin-dependent diabetes mellitus (IDDM) is an autoimmune disease associated with loss of tolerance to several pancreatic islet cell molecules, including insulin, glutamic acid decarboxylase (GAD), ICA69 and the tyrosine phosphatase IA-2 (refs 1-3). Among several predisposing loci, IDDM2 maps to the insulin gene (INS) VNTR (variable number of tandem repeats) minisatellite on chromosome 11p15 (refs 4-9). Allelic variation at this VNTR locus correlates with steady-state levels of INS mRNA in pancreas and transfected rodent cell lines, but it is difficult to reconcile the association of lower INS mRNA levels in the pancreas with class III VNTRs that are dominantly protective from IDDM. We show that during fetal development and childhood, mRNAs for insulin and other islet cell autoantigens (GAD, ICA69, IA-2) are expressed at low levels in the human thymus. Critically, we also detect proinsulin and insulin protein. VNTR alleles correlate with differential INS mRNA expression in the thymus where, in contrast to the pancreas, protective class III VNTRs are associated with higher steady-state levels of INS mRNA expression. This finding provides a plausible explanation for the dominant protective effect of class III VNTRs, and suggests that diabetes susceptibility and resistance associated with IDDM2 may derive from the VNTR influence on INS transcription in the thymus. Higher levels of (pro)insulin in the thymus may promote negative selection (deletion) of insulin-specific T-lymphocytes which play a critical role in the pathogenesis of type-1 diabetes.

The chimeric nature of the transplanted liver was first shown in our long-surviving human recipients of orthotopic hepatic allografts in 1969. 1 When liver grafts were obtained from cadaveric donors of the opposite sex, karyotyping studies showed that hepatocytes and endothelium of major blood vessels retained their donor specificity, whereas the entire macrophage system, including Kuppfer cells, was replaced with recipient cells. 2 Where donor cells that had left the liver had gone was unknown, but their continued presence was confirmed by the acquisition and maintenance in recipient blood of new donor-specific immunoglobulin (Gm) types 1,3 and red-blood-cell alloantibodies, if donors with ABO non-identity were used. 4 Davies et al 5 attributed the secretion of new soluble HLA class I antigens of donor type to transplanted hepatocytes. However, these HLA molecules come from bone-marrow-derived macrophages and/or dendritic cells, 6 and probably have the same origin from migrated donor cells as the additional Gm types and red-cell antibodies.Although this early evidence of systemic mixed allogeneic chimerism was circumstantial, we have recently shown with both anatomical and molecular techniques the presence, in clinically stable patients, of peripherally located donor cells many years after liver replacement. For instance, in patients with type IV glycogen storage disease, a disorder in which an insoluble amylopectin-like polysaccharide accumulates throughout the body because of a deficiency in a branching enzyme, we found resorption of extrahepatic amylopectin after liver replacement. 7 This process could not be explained until the migrated donor cells, which had acted as enzyme couriers, were identified by both HLA monoclonal antibodies (fig 1) and polymerase chain reaction (PCR) studies (fig 2) in the biopsied myocardium and skin of 2 patients, 33 and 91 months after hepatic transplantation.Recent experiments in rats have shown the timing and extent of seeding from the hepatic allograft to both non-lymphoid and lymphoid organs (fig 3). 8 A similar pattern of distribution was found after successful rat-to-mouse bone-marrow transplantation. 9 This similarity between liver transplantation and bone-marrow transplantation has not been reported before. The prompt development, and then the persistence, of this systemic chimerism may help to explain the resistance of the liver to cellular 10 and humoral 11 rejection, as well as its tolerogenicity to other organs from the same donor. 12 The chimeric structure of the transplanted liver was thought to be a unique feature of this organ for many years until we identified lymphoid and dendritic cell replacement under FK 506 immunosuppression in rat 13 and human 14 intestinal allografts; a similar finding has been reported in swine. 15 In our experiments with rats, the two-way traffic was the same, irrespective of whether bowel was transplanted alone or as a part of a multivisceral graft that also contained Correspondence to

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