Phenotypic characterisation, differentiation, and tissue distribution of adult human peripheral blood stem cell populations

Abstract

Previous published reports have indicated that adult human peripheral blood could provide a suitable alternative resource of multipotent progenitor or stem cells for the generation of β cells for use in the treatment of type 1 diabetes. Several blood cell populations are reported to either innately produce insulin on culture, or to be amenable to differentiation into pancreatic islet‐ or β‐ like phenotypes. So far though, only limited characterisation of these cell populations has been undertaken, and in each case, only by a single research group. Thus, further research is required in order to better understand these cell populations, and any potential they may offer for diabetes therapy. Towards this end, attempts were made to reproduce the generation of certain adult human peripheral blood‐derived cell populations , namely peripheral blood insulin producing cells (PB‐IPC), f‐macrophages, monocyte‐derived stem cells (MDSC), programmable cells of monocytic origin (PCMO), monocyte‐derived islets (MDI), and NeoIslets, that had been reported to have properties of β cell progenitors, or mature β cells. Each cell population was phenotyped for cell‐lineage markers, in particular those that distinguish hematopoietic cells, stems cells, and pancreatic cells. PB‐IPC and f‐macrophages were subsequently treated with differentiation agents in order to enhance insulin production. In addition, evidence for the presence of insulin‐producing cells outside of the pancreas, which might represent PB‐IPC, was sought by screening different adult human tissues for extrapancreatic sources of insulin and other pancreatic proteins. The current investigation revealed various discrepancies with the literature, most notably the absence of insulin expression in any of the differentiated cell populations (PB‐IPC, MDI and NeoIslets). Intriguingly though, the β cell markers pdx‐1 and GLUT‐2 were expressed by nearly all cell populations examined, including in vitro‐cultured macrophages; and there was evidence of weak expression of certain other endocrine‐pancreas associated markers in some cell populations. Characterisation of the cell surface proteome of PB‐IPC indicated that PB‐IPC bear a strong resemblance to hematopoietic antigen‐presenting cells, and in support they were phagocytic. Surprisingly they expressed the antigen AIRE which plays a key role in maintaining immune tolerance, suggesting that if PB‐IPC were to exist in nature, they may play a role in peripheral tolerance. Attempts to further differentiate PB‐IPC and f‐macrophages to produce insulin were not successful. This study provides the first evidence of insulin mRNA expression outside of the pancreas in humans, in adult duodenum and stomach tissues. These same tissues also expressed other pancreatic endocrine markers, suggesting they contain endocrine‐like tissue. Cursory investigation of these tissues for the expression of insulin protein though was not fruitful, but this was not unexpected as the amount of endocrine tissue could be so rare at these sites that it renders detection of islets of endocrine‐like tissue problematic. Taken together, these data question the ability of peripheral blood‐derived progenitor/stem cell populations to provide a vehicle for insulin production in the context of diabetes therapies. However, PB‐IPC may hold promise for enhancing immune tolerance, which is another obstacle that must be surmounted if β cell replacement therapy is have clinical utility . The origins and physiological function of the insulin mRNA expression recorded in the adult human duodenum and stomach regions remains intriguing; and is something which should be explored further in the future.