Our study of pancreas transplant biopsies provides evidence connecting the ductal and beta cell phenotypes in the human pancreas, further supporting the concept that ductal cells may give origin to new beta cells under certain conditions. We demonstrate the existence of insulin+
ductal cells, confirmed by CK-19 co-expression, in the ducts of the transplanted pancreas, specifically in SPK patients with recurrent autoimmunity and diabetes. These cells expressed PDX-1, a transcription factor required for pancreatic development that is expressed in mature beta cells [41
] and in ductal cells that do not express insulin [43
]. The number of insulin+
ductal cells varied between 33 and 90% of the ductal cells examined. Similarly, 17% to 95% of the ducts had insulin+
cells, indicating that these phenomena were quite extensive. The patient with the most severe beta cell destruction and complete loss of C-peptide secretion at the time of biopsy was the one with the highest number of ducts containing insulin+
cells (patient #1). A consistent feature observed in our SPK patients with recurrent autoimmunity who had also progressed to diabetes by the time of biopsy was that most (although not all, ) ductal cells were insulin+
in those ducts that contained insulin+
cells. In contrast, SPK patient #6 who was normoglycemic and without beta cell loss, although with clear evidence of ongoing autoimmunity, had rare cells in the ducts that stained for insulin, CK-19 and PDX-1. Thus, the presence of both hyperglycaemia and autoimmunity may be critical for triggering insulin expression in ductal cells.
By comparison, we rarely observed insulin+
cells in the ducts of non-diabetic organ donors. Importantly, while those cells expressed PDX-1, they did not express CK-19. Similarly, we rarely observed insulin+
cells in the ducts when we examined pancreas transplant biopsies from SPK patients without apparent islet autoimmunity, of whom two were normoglycemic and one had developed type 2 diabetes. Mirroring the observations in non-diabetic pancreas donors, these cells did not stain for CK-19. The findings in these SPK patients without autoimmunity and in the non-diabetic organ donors are consistent with earlier reports of single insulin+
cells in close association with ductal structures in the normal human pancreas [44
Insulin and PDX-1 expression in CK-19+
ductal cells, with a pattern similar to that of our SPK patients with recurrent autoimmunity and diabetes, was noted in human pancreata from patients with autoimmune chronic pancreatitis with diabetes [45
]. This condition is similar to recurrence of autoimmune diabetes in the transplanted pancreas in that both autoimmunity and hyperglycaemia are present. Thus, there are now two studies of human pancreata suggesting that hyperglycaemia and chronic autoimmunity/inflammation may stimulate pancreas remodelling pathways including insulin expression in ductal cells. A similar role for autoimmunity and inflammation in driving beta cell self-replication was suggested by mouse studies [22
]. Moreover, autoimmunity and diabetes have been associated with beta cell and islet neoformation in diabetes prone rats [23
], with regeneration occurring in tubular complexes which resemble the small ductal structures that stained for insulin in the biopsies of patient #1 () and patient # 2 ().
Another study of patients with pancreatitis of unspecified aetiology reported the presence of cells expressing insulin, or glucagon, or PDX-1 in pancreatic ducts [46
]. However, this study did not show co-expression of islet hormones with CK-19 or Ki-67, albeit Ki-67-positive cells were noted in the ducts. Here we find that insulin+
cells can express Ki-67, suggesting that these cells are capable of replication. This was noted in the patient with the most severe beta cell destruction, again suggesting a possible link between the replication of insulin+
ductal cells and the severity of beta cell loss/hyperglycaemia. We did not observe Ki-67+
cells among the surviving beta cells in the islets of any of the SPK patients with recurrent autoimmunity. However, we must recognize the limitations associated with the study of biopsy materials, in particular when assessing direct beta cell replication. Moreover, our SPK patients were immunosuppressed with tacrolimus, which inhibits direct beta cell replication in mice [12
]. On the other hand, the formation of beta cells from ductal cells may be enhanced by chronic immunosuppression if this antagonizes chronic autoimmunity.
Three of our SPK patients with recurrent autoimmunity (patients #1, 2, and 5) also had ductal cells expressing insulin together with CgA, a protein found in the secretory granules of endocrine cells, including pancreatic beta cells [40
]. Importantly, those cells expressing CgA and insulin no longer expressed CK-19, while clearly being present in ductal structures. Various stages of this putative transition were observed in these patients, with some ducts having mixed cell composition (insulin+
; the latter represented the majority). This observation provides further evidence for the differentiation potential of ductal cells towards an endocrine phenotype, accompanied by an apparent loss of ductal cell features. Among these three patients, patients #1 and #2 had undetectable C-peptide levels, even after stimulation, in the days preceding the biopsy. The absence of C-peptide secretion in these patients seems consistent with the rarity of insulin+
ductal cells expressing CgA, despite a high overall proportion of insulin+
ductal cells. Based on the C-peptide data and on the rarity of this putative transition towards a more mature beta cell phenotype, we speculate that if these cells secrete insulin this may be of negligible clinical significance.
In conclusion, our findings suggest that ductal cells participate in beta cell regenerative processes occurring in the transplanted human pancreas, in the context of hyperglycaemia and recurrent autoimmunity, which may be critical stimuli to trigger pancreas remodelling mechanisms in the adult. Dissecting the mechanisms involved in these remodelling processes may potentially lead to therapeutic exploitation.