Introduction: Type 1 Diabetes (T1D) is an autoimmune disorder characterized by the loss of pancreatic beta cells, causing chronic hyperglycemia. While whole pancreas and islet transplantation can restore normoglycemia, the shortage of donors and the recurrent autoimmunity present challenges for the application of this therapy. Multiple research groups have developed different protocols to generate islet cells from human pluripotent stem cells (hPSCs), but unless transplanted, these cells only recapitulate some features of their mature counterpart. This underlines the importance of the microenvironment in shaping endocrine cell maturation. While the crosstalk between pancreatic epithelium and surrounding cell types, such as mesenchymal and vascular populations, has been extensively studied, little attention has been placed on understanding whether the immune cells support pancreatic organogenesis.  Additionally, during fetal development, immune cells migrate and colonize the fetal organs to establish peripheral tolerance, a process that is impaired in many autoimmune diseases, such as type 1 diabetes. Here, we investigated the phenotype and the role played by immune cells during human pancreas development.
Methods: To this aim, we molecularly characterized the hematopoietic cells present in the developing pancreas by performing single nuclei RNA sequencing (snRNAseq) during the second trimester of human gestation. This analysis led to the identification of 30 individual cell clusters describing the pancreatic epithelium and its microenvironment and included 13 distinct hematopoietic cell types. To investigate whether fetal-like myeloid cells play a role during human pancreatic development, we established a human pluripotent stem cell (hPSC)-derived co-culture system to study potential interactions between pancreatic cells and myeloid cells. Specifically, by modeling Yolk Sac Hematopoiesis and pancreatic development, we generated hPSC-derived embryonic myeloid cells and pancreatic endoderm, which were bench-marked in cellular composition and molecular profiles to the fetal dataset.
Results: We determined that hPSC-derived myeloid cells improved the development and viability of hPSC-derived endocrine cells when compared to standard growth conditions. Finally, we found that transplantation of myeloid-endocrine co-cultures improved graft vascularization, insulin secretion, and the frequency of hormone+ cells in the murine subcutaneous space.
Conclusions: This comprehensive interrogation of the hematopoietic cells within the pancreatic niche and its applications could pave the way for novel stem cell-based transplantation modalities and tissue engineering strategies for diabetes.