Role of Pancreatic Polypeptide as a Market of Transplanted Insulin-Producing Fetal Pig Cells

Abstract

Transplantation of insulin-producing fetal pancreatic tissue into diabetic recipients has been shown to normalize blood glucose levels after several months. This time period is required for the growth and maturation of the fetal tissue so insulin levels cannot be used as a marker of graft function while the beta-cell is immature. Therefore, we have examined the use of another pancreatic endocrine hormone, pancreatic polypeptide (PP), to monitor graft function. The cell that produces this hormone has been shown to be the first mature endocrine cell in the fetal pancreas. Fetal pig pancreatic tissue, both in the form of 1 mm3 explants and islet-like cell clusters (ICCs), was transplanted into immunodeficient SCID mice and the levels of PP and insulin were measured in plasma and in the graft for up to 12 weeks. PP was detected in the untransplanted explants (0.58 pmol/mg) and ICCs (0.06 pmol/ICC) and the PP to insulin ratio was 2.7% and 5.8%, respectively. PP (but not porcine C-peptide, a marker of insulin secretion) was detectable in the plasma of SCID mice from 4 days to 3 weeks after transplantation, but not thereafter. The highest values were obtained at 4 days to 1 week. In the grafted tissue PP and insulin were present at all time points and the ratio of PP to insulin was 59%, 87%, 75%, 56%, 7%, 8%, and 7% at 4 days, 1, 2, 3, 6, 9, and 12 weeks, respectively. The decline in PP levels 3 weeks after transplantation was associated with beta-cell development in the graft. PP was also secreted by fetal pig pancreatic explants transplanted into diabetic NOD/SCID mice, with plasma levels measurable in the first week after the tissue was grafted. In immunocompetent BALB/c mice transplanted with the tissue, PP was detectable in plasma for 2 days after transplantation but not at 4 days, when cellular rejection commenced, or thereafter. We conclude that plasma PP levels can be used as a marker of the viability of fetal porcine pancreatic tissue in the first 3 weeks after it is transplanted into mice. These findings may have relevance to fetal pancreatic tissue transplanted into humans if suitable techniques can be developed to separate pig from human PP.

Secretion of Pancreatic Icosapeptide from Porcine Pancreas

“…Levels of PI were highest 4 d after fetal pig pancreatic tissue was transplanted into the mice, but were still measurable for at least the next 17 d (Fig. Our current results are similar to those reported previously when serum levels of PP were used as a marker of graft viability for up to 3 wk after transplantation, but was absent thereafter (11). No attempt was made to measure levels of PI earlier than 4 d, but based on previous experiments it would be expected that PI would be measurable earlier than this (11).…”

“…Previously we have shown that PP secretion could probably be used as a marker of viability at this time (11). Previously we have shown that PP secretion could probably be used as a marker of viability at this time (11).…”

“…As vagal innervation of pancreas transplants does not occur, the secretion of PP (and PI) from such grafts cannot be enhanced by vagal stimulation (17). Exposure of the grafts to a cholinergic agent, carbachol, directly stimulated PP release from the graft (11). Therefore, the same stimulus, carbachol, was used in the current study to measure PI release from the graft.…”

“…We have previously shown that levels of porcine PP can probably be used to monitor the viability of fetal pig pancreatic tissue transplanted into mice (11). Levels of PP were measured by RIA using an antibody that bound more to porcine than to mouse PP; these two peptides are six amino acids different from each other.…”

Characterisation of the Xenogeneic Immune Response to Microencapsulated Fetal Pig Islet-Like Cell Clusters Transplanted into Immunocompetent C57BL/6 Mice

“…This hormone was undetectable at all time points both before and after onset of PFO on day 7; suggesting that immaturity of the tissue [22], rather than blockage of nutrient supply was the reason. The controls, sera from mice transplanted with empty microcapsules, similarly had non-detectable levels of porcine C-peptide.…”