Why are levels of maternal microchimerism higher in type 1 diabetes pancreas?

Abstract

Maternal microchimerism (MMc) results from transfer of maternal cells to the fetus in pregnancy. These cells have been shown to persist into adulthood in healthy individuals and an increased frequency of MMc has been associated with autoimmune disease. Female (presumed maternal) islet beta cells have recently been identified at higher levels in pancreas from a child with T1D compared to three controls. There was, however, no evidence that these cells were the targets of autoimmune attack. The aim of this study was to analyze well-characterized T1D pancreases encompassing a spectrum in age at diagnosis, and duration of diabetes, for the presence of maternal microchimerism compared to control pancreases.Pancreas samples were available from six males with T1D and four male controls. Fluorescent-labeled probes were used to detect X and Y chromosomes. At least 1,000 cells, usually 4,000-8,000 cells underwent confocal imaging for each pancreas. The frequency of MMc was higher in T1D pancreases (range 0.31-0.80%, mean 0.58%) than in controls (0.24-0.50%, mean 0.38%) (p = 0.05). Intriguingly, clusters of 2-3 MMc were occasionally found in the pancreases, particularly T1D pancreases, suggesting replication of these cells. Concomitant FISH and immunofluorescence staining for insulin or CD45 was performed to phenotype cells of maternal origin. Insulin positive and insulin negative MMc were identified indicating that MMc contribute to the exocrine and endocrine compartments. No CD45 positive MMc were observed. These data confirm the presence of maternal cells in human pancreas and support previous observations that levels of MMc are higher in T1D pancreas compared to controls. MMc do not appear to be immune effector cells and those that stain positive for insulin within intact islets in T1D tissue appear healthy with no evidence that they are the focus of immune attack. This study adds support to the hypothesis that maternal stem cells have the capacity to cross the placental barrier and differentiate into both endocrine and exocrine cells but more detailed characterization of MMc in the pancreas is required.

Figure 1

Consecutive confocal images of a maternal microchimeric cell (MMc-indicated by the arrow) from a control fresh frozen tissue section (31 years). The X chromosome is shown by a red dot, the Y chromosome by a green dot and the nucleus with DAPI (blue).

Figure 2

MMc frequency (%) in six T1D (closed circles) and four controls (closed squares). The Mean and SD are indicated for each group (p = 0.05).

Figure 3

(A) Two adjacent MMc (indicated by arrows) in pancreatic T1D tissue sample 1 (65 year old with longstanding diabetes); (B) four maternal cells (indicated by arrows) in one field from an 11 year old recent- onset T1D patient (T1D sample 3). The X chromosome is shown by a red dot and the Y chromosome by a green dot. The cytoplasmic green staining represents insulin and the nucleus is stained with DAPI (blue).

Figure 4

(A) a low and high power image of an MMc indicated by the arrow within an insulin stained islet from a healthy control section; (B) two islets from T1D sample 2 (recent onset 89-year-old) with an MMc in each islet; (C) an insulin-ve MMc outside an insulin stained islet. The X chromosome is shown by a red dot and the Y chromosome by a green dot. The cytoplasmic green staining represents insulin and the nucleus is stained with DAPI (blue).

Figure 5

X and Y chromosome FISH with concomitant immunofluorescence for CD45 (FITC) in a control pancreas section. 3 CD45 cells are indicated with a CD45-ve MMc close by.